INOM
EXAMENSARBETE
MEDICINSK TEKNIK,
AVANCERAD NIVÅ, 30 HP
,
STOCKHOLM SVERIGE 2019
Investigating the transmission of
pressure to the heart when using
extended-coverage pneumatic
anti-G suit
ELIN ANDERSSON
KTH
Investigating the transmission of pressure
to the heart when using extended-coverage
pneumatic anti-G suit
Unders¨
okande av tryck¨
overf¨
oring till hj¨
artat
vid anv¨
andning av helt¨
ackande
pneuma-tisk anti-G dr¨
akt
Elin Andersson
Master of Science in Medical Engineering Advanced Level, 30 credits
Supervisor: Roger K¨oleg˚ard Reviewer: Ola Eiken
Examiner: Christer Fuglesang TRITA-SCI-GRU 2019:216
KTH Royal Institute of Technology
Abstract
In normal flight of high-performance aircraft used in the military, the G-suit worn by the pilot is inflated with a “ready-pressure” of about 10 mmHg, allowing for quick inflation at onset of +Gz load. However, it is unknown how the ready-pressure a↵ects the cardiovascular system, and how the pressure from the G-suit is transmitted to the veins of the pilot. Previous studies have sug-gested that these pressures may have profound e↵ect on the cardiovascular system, so the current experimental study was performed to investigate how inflation pressures in the G-suit of ready-pressure size impacts the cardiovascular system and the venous return to the heart. The study also aimed to propose a model describing the pressure transmission a↵orded by the G-suit and how it a↵ects the cardiovascular system.
The experimental protocol was performed on ten healthy individuals, six males and four females, at low to moderate inflation pressures in the G-suit of 0, 5, 10, 20 and 40 mmHg, in four di↵erent body positions; supine, seated, and two head-up tilt (HUT) positions in a 60 -angle from the horizontal; leaning on the back, and leaning on the left side of the body. Each pressure level lasted for four minutes, allowing sufficient stabilisation of measurement variables and register sonographic data of the heart and vessels. In supine position, two zero-pressure test situations were examined with the G-suit first un-zipped and then zipped, measuring the e↵ect of closing the G-suit on the cardiovascular system. Variables collected were heart rate, arterial pressures, pressure transmission, supplied G-suit pressure, tissue-impedance, respiratory variables and cardiac variables using sonographic imaging.
The experimental study implies that inflation pressure of ready-pressure size have a profound e↵ect on the cardiovascular system. The stroke volume (SV) increased 5-13% whereas the cardiac output (CO) showed a slight decrease at low inflation pressures which was restored at the higher inflation pressures. The change of pressure in vena cava inferior did not a↵ect the SV and CO to the same extent as previous data have implied. The study found that the pressure transmission of these relatively low inflation pressures in the G-suit was about 75% and upward, rather than 100% that has been shown in previous studies performed with greater inflation pressures. The study showed no significant e↵ect on the cardiovascular system upon closure of the G-suit at zero inflation pressure.
The pressure supplied to the G-suit is distorted, and not fully transmitted. The G-suit seems to transmit to a greater extent over the upper leg, somewhat less over the lower leg, and quite badly over the abdomen. In the discussion, a model describing the pressure transmission a↵orded by the anti-G suit to the skin and veins at onset of ready-pressure in the G-suit has been suggested.
Keywords: LBPP, HUT, anti-G suit, cardiac output, body positions, acceleration, heart rate, pressure transmission, human physiology, cardiovascular function
Sammanfattning
Vid normal flygning i milit¨ara h¨ogpresterande flygplan anv¨ander piloten en anti-G byxa med ett f¨orfyllnadstryck kring 10 mmHg som m¨ojligg¨or snabb tryck¨okning vid ¨okad +Gz acceleration. Dock ¨
ar det ok¨ant hur detta tryck p˚averkar det kardiovaskul¨ara systemet, och hur trycket fr˚an G-byxan transmitteras till venerna hos piloten. Tidigare studier har f¨oreslagit att detta tryck kan ha en uttalad e↵ekt p˚a det kardiovaskul¨ara systemet, och d¨arf¨or utf¨ordes denna experimentella studie f¨or att unders¨oka hur G-byxans f¨orfyllnadstryck p˚averkar det kardiovaskul¨ara systemet och det ven¨osa ˚aterfl¨odet till hj¨artat. Studien ¨amnar f¨oresl˚a en modell som beskriver trycktransmissionen fr˚an G-byxan och hur den p˚averkar det kardiovaskul¨ara systemet.
Den experimentella studien utf¨ordes p˚a tio friska individer, sex m¨an och fyra kvinnor, vid l˚aga till moderata inflationstryck i G-byxan p˚a 0, 5, 10, 20 och 40 mmHg, i fyra olika betingelser; liggandes, sittandes och tv˚a st˚aende positioner i en 60-gradig vinkel fr˚an det horisontella; lutandes p˚a rygg och lutandes p˚a den v¨anstra sidan av kroppen. Varje tryckniv˚a varade i fyra minuter f¨or att till˚ata tillr¨acklig stabilisering av m¨atvariabler och registrering av sonografisk data av hj¨artat och k¨arl. I liggande position utf¨ordes tv˚a m¨atsituationer med noll inflationstryck, f¨orst med G-byxan uppkn¨appt och sedan kn¨appt, f¨or att unders¨oka e↵ekten p˚a det kardiovaskul¨ara systemet av att kn¨appa G-byxan. Variablerna som samlades var hj¨artfrekvens, art¨artryck, trycktransmission, givet tryck till G-byxan, v¨avnadsimpedans, respiratoriska variabler och variabler om hj¨arta och k¨arl fr˚an sonografiska m¨atningar.
Studien talar f¨or att inflationstryck av f¨orfyllnadstrycks-storlek har en uttalad e↵ekt p˚a det kardiovaskul¨ara systemet. Slagvolymen (SV) ¨okar 5-13%, medans hj¨artminutvolymen (CO) visade en minskning vid l˚aga inflationstryck f¨or att sedan ˚aterh¨amta sig vid de h¨ogre inflationstrycken. F¨or¨andringen av tryck i vena cava inferior p˚averkade inte SV och CO till samma grad som tidigare data visat p˚a. Studien fann att trycktransmissionen av dessa relativt l˚aga inflationstryck i G-byxan var kring 75%, snarare ¨an 100% som tidigare studier p˚ast˚att, dock d¨ar h¨ogre inflationstryck har anv¨ants. Studien visade ingen signifikant e↵ekt p˚a det kardiovaskul¨ara systemet vid kn¨appning av G-byxan vid noll inflationstryck.
Det givna trycket till G-byxan ¨overf¨ors inte fullt. G-byxan verkar transmittera till en st¨orre grad vid l˚aret, n˚agot mindre ¨over smalbenet och ganska d˚aligt ¨over magen. I diskussionen f¨oresl˚as en modell f¨or att beskriva trycktransmissionen som ges av anti-G byxan till huden och vidare till venerna vid ans¨attning av f¨orfyllnadstryck i anti-G byxan.
Keywords: LBPP, HUT, anti-G dr¨akt, minutvolym, betingelser, acceleration, hj¨artfrekvens, tryck¨overf¨oring, fysiology, kardiovaskul¨ar funktion
Acknowledgement
Firstly, I would like to thank my supervisor Roger K¨oleg˚ard for guidance and expertise in the area of human physiology, and also for dedicating a tremendous amount of time and support throughout the process. I would also like to thank Mikael Gr¨onkvist along with Bj¨orn Johannes-son for invaluable support in the technical aspects of the experimental set-up, and support during the experimental study. Thank you to Christina DaSilva for ultrasonic imaging and analysis, Ola Eiken for reviewing and valuble notes on the report, and to the entire department of Environmental Physiology at KTH for the opportunity to carry out my master thesis and making me feel welcome. I would also like to thank Eddie Bergsten for the customized analysis programs, and a big thank you to all of the individuals participating as subjects in the study.
I am grateful for the experience of writing my master thesis at the department of Environmental Physiology where I have been able to combine my knowledge about the human physiology, along with technical knowledge and physics that my five-year long education has provided me.
List of Figures
1. Positioning of the pressure balloons for measurement of pressure transmission, and electrodes for tissue-impedance measurements
2. Condition II, III and IV depicted
3. Visualization of the pressure levels used in the experimental study by the LabView program used
4. The tilt table used for condition I, II and III 5. The G-suit pressure-regulator
6. The needle-valve used for manual increase of inflation pressure to the G-suit 7. Pressure balloon used for measurement of pressure transmission
8. Visualization of the pressure levels used in the experimental study along with eventmarks 9. Diagram showing the mean pressure transmission for each condition measured by the five
pressure balloons
10. Diagram showing the mean pressure transmission for each pressure balloon separately over all four conditions
11. Result of heart rate (HR)
12. Result of mean arterial pressure (MAP) 13. Result of systolic arterial pressure (SAP) 14. Result of diastolic arterial pressure (DAP) 15. Result of tissue-impedance
16. Suggested model for the supine position 17. Suggested model for the seated position 18. Suggested model for the standing positions
19. Experimental protocol, information and condition I 20. Experimental protocol for conditions II, III and IV
21. Screen seen during experiment to observe the signals measured 22. Calibration window for experimental signals
List of Tables
1. The order by which the four conditions were performed, for each subject 2. Equivalents of kPa versus mmHg
3. Intended and measured pressure supplied to the G-suit
4. Measured mean supplied pressure to the G-suit for each condition
5. The di↵erence in intended and measured G-suit pressure increase measured by the pressure balloons
6. Transmitted pressure measured by pressure balloon 1, placed on the abdomen
7. Transmitted pressure measured by pressure balloon 2 and 3, placed on the front and back of the thigh
8. Transmitted pressure measured by pressure balloon 4 and 5, placed on the shin and on the calf of the lower leg
9. Pressure transmission a↵orded by the G-suit to the surface of the skin 10. Registered mean values of HR
11. Results from sonographic imaging on stroke volume (SV) 12. Results from sonographic imaging on cardiac output (CO) 13. Results of tissue-impedance in percent
14. Results from zipping the suit on MAP, SAP, DAP and HR
15. Results from zipping the suit on tissue-impedance, both in Ohm and percent change from the baseline, and G-suit pressure
16. Results from zipping the suit on pressure balloons 1 through 5 17. Results from respiratory variables
Contents
1 Introduction 1
1.1 Background . . . 1
1.2 Background Physiology and Sonographic Imaging . . . 2
1.3 Aims . . . 2
2 Method 3 2.1 Subjects and Ethical Consideration . . . 3
2.2 Experimental Protocol . . . 3
2.3 Measurements and Equipments . . . 5
2.3.1 Pressurization of the anti-G suit . . . 5
2.3.2 Pressure Balloons . . . 5
2.3.3 Lower-leg fluid change . . . 6
2.3.4 Arterial Blood Pressure and Heart Rate . . . 6
2.3.5 Sonographic Imaging . . . 7
2.3.6 Respiratory variables . . . 7
2.3.7 Data Acquisition . . . 7
2.3.8 Calibrations . . . 8
2.4 Analysis and Statistics . . . 9
2.4.1 Analysis . . . 9 2.4.2 Statistics . . . 10 3 Result 11 3.1 Pressure Transmission . . . 11 3.2 Heart Rate . . . 15 3.3 Arterial Pressures . . . 15 3.4 Sonographic Imaging . . . 17
3.5 Lower-leg fluid change . . . 18
3.6 The E↵ect of Zipping the G-suit . . . 18
3.7 Respiratory variables . . . 19
4 Discussion 20 4.1 Pressure transmission and the e↵ect on the cardiovascular system . . . 20
4.2 Suggested model for pressure transmission . . . 21
4.3 Limitations . . . 25 4.4 Future work . . . 25 4.5 Conclusion . . . 25 5 References 26 Appendices 28 A 28 A.1 Experimental Protocol . . . 28
A.2 Program used during experiment . . . 30
1
Introduction
1.1
Background
The human body is continuously acted upon by the Earth’s force of gravity, and is adapted for existence in this environment. During flight in high-performance aircraft, pilots are exposed to forces many times greater than that of the Earth’s gravity, combined with inertial forces generated by radial acceleration, causing a centrifugal force. These forces, greater than the gravity’s force, have profound e↵ect on the cardiovascular system [1]. Both military pilots and aerobatic flyers are exposed to G-forces larger than the acceleration of the gravity. Military pilots on the other hand, usually experience the forces for a longer period of time. The high acceleration forces generating G-forces, restrict the movement of the limbs of the pilot [1].
Acceleration, denoted a, is a vector quantity having both magnitude and direction, where radial acceleration is produced by a change in direction without the change in speed. In aerospace medicine, when talking about acceleration, the variable commonly referred to is G, a dimensionless unit, denoting acceleration in relation to the gravity,
G =a
g (1)
where g is the gravitational acceleration on Earth, 9.81 m/s2, and G is therefore multiples
of g. For example, a pilot accelerated at 49 m/s2 would be exposed to 5 G, an acceleration five
times greater than the force of gravity. Pilots in todays high-speed aircraft experience the vector of acceleration in the head-to-foot direction when conduction a co-ordinated turn. The standard aerospace medical terminology describing the sum of the inertial force and Earth’s force of gravity in the head-to-foot direction is +Gz [1, 2].
Throughout the body, columns of blood form in the field of gravity in both arteries and veins creating what is called the hydrostatic pressure [1]. The hydrostatic pressure, p, is a product of h, height of the column, ⇢, density of the blood, and g, the acceleration experienced. So if the acceleration is increased twofold to 2 G, the hydrostatic pressure gradient will be increased twofold. To this day, the limiting factor in high-G flight is the human physiology and a decrease in arterial pressure of the brain, inducing a risk of loss of consciousness [1, 2]. With the increased force gradient in head-to-foot direction, the drop in arterial pressure between the head and heart will be exaggerated and the diaphragm will be displaced downwards [2], which will increase the distance between head and heart, further increasing the hydrostatic pressure di↵erence, creating a fall in arterial pressure at head level [1]. To reduce the risk of loss of consciousness, methods have been applied to increase blood pressure in order for pilots to remain conscious at onset of high accelerations.
One of the methods used to increase the blood pressure is the anti-G suit, which was developed to counteract the fluid shift downward and keep pilots conscious [1]. In the early 1930’s, the US Navy started developing and testing inflatable abdominal belts to increase blood pressure in pilots to prevent ’blackout’. These proved useful and was further developed [3]. Today the extended-coverage pneumatic G-suit used in the Swedish airforce allows for good protection against +Gz load [4], by linear inflation of the bladder covering the legs and the abdomen at increasing acceleration from 10 mmHg at +1-2 Gz to 500 mmHg at +9 Gz [1, 4]. The bladder in the G-suit covers the legs and abdomen except the groin and buttocks area, providing a uniform, homogeneous pressure over the area a↵ected by the counterpressure from the G-suit. The G-suit regulates inflation pressure by a pressure-regulator. Inflation of the bladder at increased G-load will increase the peripheral blood-flow resistance and prevent the shift of fluids downward by a lower body positive pressure (LBPP) a↵orded by the G-suit, and force blood upwards to the central part of the body [2], securing venous return to the heart [5]. Other methods used to increase the blood pressure of pilots, are the anti-G straining manoeuvre (AGSM) and positive pressure breathing for G-protection (PBG) [2]. AGSM is a technique which the pilots perform without help from any equipment through muscular activation and contractions in the majority of the skeletal muscles, combined with repetitive short-duration forced exhalations. The breathing pattern applied is usually a forced breathing against a closed glottis. The principle of PBG is delivery of breathing air with higher pressure than normal, where there are no straining maneuvers. The main purpose of these methods are the increased peripheral blood-flow resistance a↵orded the pilot, and an increased intra-thoracic pressure a↵orded by pilot AGSM and by the PBG.
When the Swedish 39 Gripen pilots are flying, the G-suit is constantly supplied with a ”ready-pressure” of about 10 mmHg (1.33 kPa) in the anti-G suit, to reduce the inflation time upon high-G exposure. The pressure a↵orded the pilot by the PBG starts at +4 Gz and increases linearly to +9 Gz where it assumes a value that is a tenth of the supplied pressure to the G-suit, i.e. 50 mmHg.
1.2
Background Physiology and Sonographic Imaging
The body contains two system which circulates blood, the venous system and the arterial system. The venous system leads blood from the body to the heart whereas the arterial system leads blood from the heart to the body. The cardiac output (CO) is the volume of blood pumped from the heart during a minute (L/min) and is the product of heart rate (HR) and the cardiac stroke volume (SV), where SV is the volume of blood pumped from the left ventricle of the heart per beat (mL) [2, 6]. The blood pressure regulates according to the situation which the body is in, and this regulation is essential for proper blood flow through organs [2].
Normally, the lungs have a total capacity of 5-6 liters of air with a normal breathing volume (tidal volume) of 0.5 liters at rest [6]. The lungs allow gas exchange to the blood through alveoli to lung capillaries. Sonographic imaging is a commonly used technique when examining soft tissue and measuring flow of fluids inside arteries or veins. When examining the heart, both imaging and measurement of flow velocities is used to localize specific vessels or the area of the heart to be imaged [6]. One technique is to use pulsed Doppler, which transmit acoustic wave pulses by which echoes are created when reaching a di↵erent media. From the detected echoes, velocity profiles are generated along the organ cross sectional area, and by moving the sonographic probe around, two-dimensional images are obtained from the velocity distribution [6]. Tissue Doppler uses suitable amplification and filtration to enhance the sonographic signals with high amplitude but low velocity, generated by the tissue, e.g. the myocardium [6]. When the ultrasonic wave goes from soft tissue to air, the reflection coefficient is close to 1, meaning that almost all energy from the wave is reflected and imaging will not be possible [6]. Therefore, when imaging the heart, certain breathing patterns need to be implemented for a proper acoustic window to be obtained. This however will distort the normal ventilation pattern.
1.3
Aims
It has been shown that a small change in pressure on the venous side has a great e↵ect on venous return to the heart and therefore arterial pressure regulation [7]. It is unknown how an altered externally applied pressure to the lower half of the body a↵ects the venous return and its e↵ects on cardiovascular regulation. It is also unknown how much of the pressure applied to the G-suit is transmitted to the underlying tissues including arterial and venous vessels. Therefore, the study aims to investigate the e↵ect of low to moderate absolute pressure changes in the anti-G suit on the cardiovascular circulation, using non-invasive measuring methods. The study also aims to suggest a model which describes the pressure transmission from the G-suit, to the skin and to the vascular system. The following questions will be attempted to be answered:
1. Is the pressure applied in the anti-G suit transmitted undistorted to the skin?
2. Is the applied pressure to the anti G-suit evenly distributed to di↵erent regions of the lower body?
3. Does just zipping the anti-G suit a↵ect the cardiovascular function?
4. How is the stroke volume (SV) influenced by externally applied pressures of 0-40 mmHg (0-5.33 kPa) to the lower body?
2
Method
In this chapter, the experimental set-up and experimental protocol is presented and explained, along with equipment used in the study and the means of which analysis was made.
2.1
Subjects and Ethical Consideration
Ten healthy subjects with no cardiovascular or pulmonary diseases were recruited to the study, six males and four females. No food-restriction was given to the subjects; however, ca↵eine and nicotine/tobacco was restricted in a four-hour period before the experiment. The individuals were not taking any medication and were non-smokers. The subjects had a mean (range) height, weight and age of 177.5 (164–193) cm, 75.5 (61.3–96.7) kg, and 25.1 (22 – 29) years respectively. Before participating, each individual gave their informed consent and were familiarized with the study and set-up. The experimental protocol was approved by the Swedish Ethical Review Authority (Dnr: 2019-01317).
2.2
Experimental Protocol
The subjects arrived separately in a three-day period at the department of Environmental Phys-iology at KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH) located at KI Campus in Solna, and shown to a changing room where they put on shorts. The females wore sports-bra or bra, and socks were allowed. Thereafter, the experimental set-up and experimental procedures were explained in the experimental room at ambient temperature of 24 ± 1 C. The age, height and weight of each subject was recorded. Once the subjects received all information, they signed their informed consent and the anti-G suit used in the Swedish fighter 39 Gripen was tightly fitted to the subjects by lacing. Depending on body constitution of each subject, a range of G-suit sizes had been prepared. Once the G-suit had been properly fitted, it was un-zipped and the subject was positioned standing on a ledge of the tilt table (Figure 1), and tilted down to horizontal, supine position.
In the supine position, electrodes were placed to measure fluid displacement by tissue-impedance on the left leg (Figure 2). Five pressure balloons measuring the pressure transmission from the G-suit bladder to the skin were positioned using medical grade tape on the right leg, two on the upper front and back of the thigh, two on the lower front and back of the leg, and one placed on the lower part of the abdomen. Additional electrodes for measuring electrocardiogram was attached to the chest, face mask used for respiratory variables was fitted, the front-end unit of the Portapres for arterial blood pressure and heart rate measurements were attached to the subjects’ right arm on the middle phalanx of the third finger. Once all measuring apparatus were in place, the G-suit was buttoned, but not zipped.
The experimental study consisted of four conditions, each containing an incremental increase in inflation pressures of 0, 0.67, 1.33, 2.67 and 5.33 kPa (0, 5, 10, 20, and 40 mmHg respectively), visualized in Figure 3. Each pressure level was sustained for approximately 4 minutes, and the conditions were as follows:
I Supine position, horizontally on the tilt table
II Standing a an 60 head-up tilt (HUT) of the tilt table from the horizontal, with the back leaning against the table (Figure 4, to the left)
III Standing a an 60 HUT from the horizontal, leaning on the left side against the table (Figure 4, in the middle)
IV Seated on a chair, with torso and legs at approximately 90 angles (Figure 4, to the right)
All subjects started in the supine position, condition I, to achieve a common baseline, and the three following conditions (conditions II-IV) were randomized in a balanced manner. The data was recorded during all four minutes of the pressure levels where the last two minutes were used for data analysis. In Table 1, the order of the conditions for each subject can be seen.
In condition I, the initial pressure level of 0 kPa with the G-suit un-zipped lasted about five minutes. This was regarded as the baseline. The G-suit was zipped and four minutes of additional
Figure 1: Tilt table. The table stands on wooden boards to protect the floor. Made from a steel frame with padded material for comfort. The ledge on which the subjects were standing on can be seen in the lower part of the figure, and the hole in the table allows for sonographic images to be taken without disturbing the position of the subject. A handle on the back of the table was used for manual tilt.
Table 1: The order of which the four conditions was performed by each subject, all starting with condition I, supine position for a common baseline.
Condition I Condition II Condition III Condition IV
Subject 1 1 2 3 4 Subject 2 1 3 2 4 Subject 3 1 2 3 4 Subject 4 1 3 2 4 Subject 5 1 2 4 3 Subject 6 1 3 4 2 Subject 7 1 4 2 3 Subject 8 1 4 3 2 Subject 9 1 4 3 2 Subject 10 1 2 4 3
rest at 0 kPa was recorded, and data collected. During the two initial segments, the pressures from the five balloons were used to measure the e↵ect of zipping the G-suit, and to detect if zipping allows for a sufficiently large pressure-transmission to provide higher pressure to the heart. Thereafter, the incremental pressure levels commenced of 0.67, 1.33, 2.67 and 5.33 kPa, each of four minutes. Table 2 shows the equivalent of mmHg versus kPa for the inflation pressures used in the study.
The three conditions following supine position each commenced with a four-minute period at 0 kPa, with the G-suit zipped, followed by the incremental pressures. Before commencing the two standing conditions (II and III), a brief period was spent in supine position to get a steady baseline. During each condition, a protocol of the procedure was followed that made data-collection accurate, Appendix A.1. At the end of each condition, following the last pressure-level at 5.33 kPa, the pressure in the G-suit was released by removing the regulator and needle-valve allowing for quick deflation of the G-suit bladder. The subject was re-positioned to the next condition and the procedure was repeated until all four conditions were completed.
Figure 2: Positioning of the electrodes for measurement of the fluid displacement by tissue-impedance on left leg. Right leg showing placement of some of the balloons used to measure the pressure transmission from the suit to the skin. On the right and left bottom part of the G-suit, buttons can be seen, and also the zipper of the pants. All tubing and cords from balloons and electrodes were exited from the suit in the opening of the right groin area.
Table 2: Equivalents of kilo-Pascal (kPa) versus millimeters of mercury (mmHg) for the inflation pressures used in the G-suit.
kPa mmHg 0 0 0.67 5 1.33 10 2.67 20 5.33 40
2.3
Measurements and Equipments
The study contained technical elements that were modified from their original form, or entirely custom made. The tilt table used, on which the subjects was positioned on made it possible to be in a horizontal position, or in a tilted angle of 60 from the horizontal (Figure 1). In condition IV, a regular chair was used for all subjects and foot support were provided for shorter subjects to keep feet from dangling.
2.3.1 Pressurization of the anti-G suit
The pressure of the G-suit was regulated with a modified AGA flight-regulator (modified MF41, Interspiro, Liding¨o, Sweden) that was connected to a hose in the G-suit, Figure 5 (1). The regulator was modified to enable manual pressure regulation of the G-suit. In Figure 5 (2), the needle-valve used to adjust the supplied pressure to the G-suit can be seen, and in more detail in Figure 6.
From the regulator, an absolute pressure transducer (ADZ NAGANO GmbH, Germany) was connected in a coupling to a unit measuring the pressure in kPa (DPI-700, Druck Limited, Leicester, England). From a gas tank, a connection was made to the flight-regulator, supplying the G-suit with pressurized gas at 15 bars. This allowed for a constant flow of compressed air, and a constant monitoring of the pressure supplied to the G-suit.
2.3.2 Pressure Balloons
Pressure transmission from the anti-G-suit to the skin was measured using five esophageal balloon catheters (Viasys Healthcare GmbH, H¨ochberg, Germany), Figure 7. The reason for using these balloons was the length of the surface on which the pressure could be distributed on, just above
Figure 3: Incremental pressures increase of 0, 0.67, 1.33, 2.67, and 5.33 kPa, taken from Subject 9, condition II. It can be seen that the adjustment of the pressure by the needle valve is sensitive and will experience some overshoot before finding a stable pressure level corresponding to the correct segment pressure. The x-axis showing time in minutes (0 to 23 minutes) and y-axis showing the registered G-suit pressure in kPa (-0.4 to 5.8 kPa).
10 cm. The air distributed within the balloon from the internal perforated tube going through the whole length, with perforations at centimeter 2, 3, 4 and 6, 7, 8. If the balloon was tweaked, the perforated tube still allowed detection of pressure.
Placing of the five balloons was chosen based on physiological and anatomical aspects; 1) across the abdomen a few centimeters below the umbilicus, 2) front thigh, running along the large muscle outwards from the medial line, 3) back thigh, in the medial line at the same height as the balloon on the front thigh, 4) on the muscle of the shin, outwards from the medial line, 5) in the medial line of the calf, at the largest circumferential, at the same height as the balloon on the shin. Pressure balloons 3 and 5 experienced a counterpressure from the table underneath, whereas pressure balloons 2 and 4 only experienced pressure from the inelastic G-suit fabric.
The balloons were extended using silicone tubing and inserted to a piezoresistive transducer, a type of pressure sensor (MPX5010, Freescale Semiconductor Inc.) using air as the pressure media, converting pressure to an analog output signal. It was then connected to an AD-card (USB-6215, National Instruments) for further conversion to a digital signal to the computer for monitoring and collection.
2.3.3 Lower-leg fluid change
Relative changes in fluid displacement to and from the lower-leg were estimated from tissue-impedance measurements using tetrapolar constant-current tissue-impedance system (Minnesota Impedance Cardiograph Model 304 B, Minneapolis, USA). Two pairs of pre-gelled surface electrodes were ap-plied to the skin, one pair of reference electrodes, and one pair of measuring electrodes. The reference electrodes were placed above the knee and below the ankle, and the measuring electrodes were placed below the knee and above the ankle. The outer reference electrodes were supplied with constant current, allowing the inner pair of measuring electrodes to register the change in voltage. By utilizing Ohms law, the resistance was derived, the change of which is assumed to be inversely proportional to the change in lower-leg fluid. The tissue-impedance was calculated in percentage, using the first pressure-level as reference, for each individual. This meant that for condition I, the comparison was made from the 0 kPa pressure with the G-suit un-zipped, whereas for the other conditions (II-IV) it was calculated with the 0 kPa pressure for zipped G-suit.
2.3.4 Arterial Blood Pressure and Heart Rate
Throughout the experiment, heart rate (HR), systolic, diastolic and mean arterial pressures (SAP, DAP, and MAP) were measured continuously using a volume-clamp technique (Portapres, TNO, Amsterdam, The Netherlands), with the pressure cu↵ placed around the middle-phalanx of the third finger on the right hand. The height-correction transducer, a column of fluid compensating
Figure 4: To the left: Condition II, subject standing on the ledge in a 60 HUT with the back against the table. In the middle: Condition III, subject standing, leaning on the left side with parallel feet in a 60 HUT, with the arm in shoulder-height to allow for sonographic imaging. To the right: Condition IV, seated position with approximately 90 angles between torso and legs. Pillows were provided for the subjects (can be seen in left and middle image) for comfort. The pressure regulator was placed on the ledge or on the floor.
for the di↵erence in distance between hydrostatic pressure components, was fixed in position close to the heart.
2.3.5 Sonographic Imaging
Two-dimensional (2D) ultrasound and Doppler image acquisition of the heart was performed using an ACUSONSC2000 (Siemens, Germany) ultrasound system, with phased-array 4V1 transducer. All recordings and analyses were conducted by the same sonographer o✏ine. Left ventricular stroke volume (SV) was calculated by multiplying the area of the left ventricular outflow tract (LVOT) with mean flow velocity at LVOT. Heart rate (HR) was derived from electrocardiographic recordings with electrodes in a precordial one-lead position. All recordings were obtained at the end of a normal expiration, or when the best acoustic window occurred.
2.3.6 Respiratory variables
The subjects wore an orofacial mask (Vmask, Hans Rudolph Inc., Shawnee, OK,USA), breathing room air through a respiratory valve (Model 2, 700 T-Shape; Hand Rudolph Inc., Shawnee, OK, USA), the expiratory side of which was connected to a metabolic cart (Quark PFT; Cosmed, Italy) via respiratory tubing. The turbine, used for volume calculations, was calibrated before the experiment using a 3-liter syringe. Expiratory minute ventilation, tidal volume and breathing frequency was measured during the experiment breath-by-breath.
2.3.7 Data Acquisition
All data was collected on one of three devices. Data from sonographic imaging and respiratory variables were collected on separate devices. The remaining signals were collected and stored on a computer using a 16-bit AD-card (USB-6215, National Instruments) that converted analog signals to digital signals for interpretation. The signals were recorded in a customized program based on commercially available LabView software (National Instruments), Figure 21 in Appendices A.2,
Figure 5: The pressure regulator connecting to the anti-G suit. (1) The connection for the anti-G suit, (2) The needle-valve connection in the regulator, regulating the G-suit-pressure manually, (3) The connection and tube that allows for control of face-mask pressurization, however, this was not used in this study and was disconnected. The top right corner of the image shows a trans-parent/white tube which was connected to the absolute pressure transducer and to the measuring unit DPI-700. The blue tube below connected to the gas tank, feeding the G-suit with pressurized gas.
Figure 6: The needle-valve used to regulate and increase the pressure of the G-suit, markings in millimeters can be seen. The needle-valve was connected to the G-suit flight-regulator.
where the variables could be continuously monitored throughout the experiment. All variables were sampled at 100 Hz.
2.3.8 Calibrations
All apparatuses used in the study were calibrated and controlled before the experiment commenced. The absolute pressure transducer for the G-suit was calibrated before each subjects arrival, using two-point calibration, to ensure a true zero-value of indicated pressure by releasing all pressure to 0 kPa, and then increasing pressure to 5 kPa, since this pressure was a↵ected by ambient pressure. The pressure sensor, piezoresistive transducer, taking in the signals from the pressure balloons were calibrated before each subject’s arrival, and between all conditions for the subjects. This was done to control that same amount of air was used at all times. Calibration of the pressure transducer was made by two-point calibration where the initial zero pressure indicate the o↵set, then supplied by a predetermined pressure to both the pressure transducer in parallel coupling to a reference sensor in order to obtain the gain value for a proper pressure value from the voltage input from the pressure transducer. Using a three-way-coupling, the air was released from the balloons, and by using a 5-ml syringe, 2 ml air was inserted to each pressure balloon during the baseline segment of each condition. The use of 2 ml of air in balloons was predetermined, allowing a sufficient
Figure 7: The pressure balloon used for measuring pressure transmission from the G-suit to the skin. About 10 cm long with a perforated tube running along the whole length for constant pressurization. The perforation in the tube were positioned at centimeter 2, 3, 4 and 6, 7, 8. inflation to detect and register the pressure transmission from suit onto the skin, without causing any tension in the pressure balloon material.
2.4
Analysis and Statistics
2.4.1 Analysis
The data obtained during the experiments by the LabView program (Appendix A.2) was converted to Excel-files for convenience. In total, 40 files were generated, one for each condition and subject. To analyze these data, a second program based on LabView, Appendix A.3, was used.
In this program (Appendix A.3), filtration was made based on measurement periods, and a mean-value for that region of interest was computed and presented by the program. The region of interest was selected with cursors placed 30 seconds apart. In Figure 8, all even eventmarks show the start of a measurement period. All odd eventmarks show the start of a new pressure-segment which is easily seen by the step and slight overshoot.
The mean-values collected from the program was gathered in a separate Microsoft Excel-file (Version 15.37, Microsoft) where the collective means and standard deviations were calculated by inherent functions in Excel, for each condition and each pressure-level. The standard deviation was calculated by the “n-1 ”-method, STDAV which use the following formula:
r P
(x x)2
n 1 (2)
where x is the sampled mean value, n is the sample-size, and x is the mean of the sample. When developing results graphically, Standard Error of the Mean (SEM) is presented which is the SD divided by the square-root of the sample size (n=10).
Figure 8: Pressure-levels of condition II for subject 9. X-axis showing time in minutes (0 to 23 minutes), and y-axis showing pressure in kPa (-0.4 to 5.8 kPa). The image shows each eventmark (EM) numbered. All odd eventmarks show the start or increase of or to the next pressure-level. The even eventmarks mark the start of the measurement-period, where data was sampled. The eventmarks are not exactly 2 minutes apart since the sonographic imaging in some cases took more than 2 minutes.
2.4.2 Statistics
Two types of statistical analysis methods was utilized depending on the data sets. If analysis of two data set was to be compared for statistical significant di↵erence, a two-tailed Student’s t-test assuming unequal variance was applied in Excel. For more than two data sets, analysis of variance followed by Tukey’s HSD post hoc tests were performed using one-way ANOVA (web-based software, VassarStats.net, Vassar Collage, New York, US) for correlated samples. The significance level was set to p<0.05 in both methods.
3
Result
3.1
Pressure Transmission
In Table 3, the mean values from measured pressure supplied to the G-suit during the experiment is presented in relation to the intended inflation pressure as mean kPa± SD. There was no significant di↵erence (p>0.05) when comparing the measured and the intended inflation pressure of the G-suit. Table 4 present the measured mean values of G-suit pressure for each condition. The only statistical di↵erence is between conditions I and IV (supine and seated), (p<0.05).
Table 3: Intended G-suit pressure presented in kPa. The G-suit pressure supplied measured during the experiment presented in kPa± SD (n=10). No significant di↵erence between the two.
Intended G-suit Pressure 0 0.67 1.33 2.67 5.33 Measured G-suit Pressure -0.02 0.63 1.27 2.59 5.24 ± 0.03 ± 0.05 ± 0.02 ± 0.05 ± 0.04
Table 4: Mean values of the G-suit pressure presented for each condition over the pressure segments in mean kPa± SD (n=10). Condition I: supine, condition II: HUT on the back, condition III: HUT leaning on the left side, condition IV: seated on a chair.
0 0.67 1.33 2.67 5.33
Condition I 0.00± 0.12 0.70± 0.13 1.29± 0.11 2.62± 0.10 5.29± 0.09 Condition II 0.01± 0.07 0.60± 0.08 1.25± 0.11 2.61± 0.11 5.24± 0.13 Condition III -0.02± 0.08 0.61± 0.09 1.29± 0.09 2.60± 0.13 5.20± 0.12 Condition IV -0.05± 0.09 0.59± 0.14 1.26± 0.18 2.52± 0.13 5.23± 0.16 In the following section, the balloons used to measure pressure transmission from the G-suit to the skin is presented. Figure 9 show result from each pressure balloon separately over the pressure-levels, where the bars represents the results from the four conditions. The values presented are mean values and it can be noted that the two highest measured pressures are in pressure balloons 3 and 5, placed on the back of the thigh and on the calf. The lowest measured pressure, at the highest inflation pressure was encountered in pressure balloon 1, placed on the abdomen. It can also be noted that pressure balloon 2 and 4, front thigh and shin, follow the same approximate measured values over the pressure-levels.
Figure 10 present the mean values of all conditions for the five pressure-levels for the separate pressure balloons. At the highest inflation pressures in the G-suit, pressure balloon 3 and 5 obtains the highest pressures, whereas pressure balloon 1 obtains the lowest pressure.
Table 5 show the intended pressure increase when going from one pressure-level to the next, in relation to the actual pressure increase measured by the pressure balloons during the experiment. The di↵erence between the intended value and the measured value of the pressure increase is denoted by and were on average 0.24 kPa lower than the intended pressure increase. However, there were no statistical di↵erence between intended and measured pressures, (p=0.73).
Table 5: The mean pressure-increase from the balloons presented in mean kPa for all conditions along with the intended pressure-increase and what the di↵erence between these two are, .
0 to 0.67 0.67 to 1.33 1.33 to 2.67 2.67 to 5.33 Intended Increase [kPa] 0.67 0.66 1.34 2.66 Mean Increase [kPa] 0.36 0.55 1.16 2.32
Figure 9: The five plots show mean values measured by each balloon in mean kPa± SEM (n=10). The bars indicate mean values for the four conditions. Top right: pressure balloon 1 (abdomen), top left: pressure balloon 2 (front thigh), middle left: pressure balloon 3 (back thigh), middle right: pressure balloon 4 (shin), bottom: pressure balloon 5 (calf). Condition I: supine, condition II: HUT on the back, condition III: HUT leaning on the left side, condition IV: seated on a chair.
Figure 10: The bars present mean kPa± SEM over all conditions for the pressure levels. The figure allows for comparison of pressure balloons 1 through 5.
The succeeding tables, Table 6-8, present the mean-values measured from pressure balloons where the abdominal bladder is presented separately, the two pressure balloons from the upper leg (2 and 3) collectively, and the two pressure balloons from the lower leg (4 and 5) collectively. Table 6: Mean values presented in mean kPa± SD (n=10) for the five pressure-levels and four conditions, for pressure balloon 1, placed on the abdomen.
0 0.67 1.33 2.67 5.33
Condition I 1.04± 1.53 1.65± 0.57 2.28± 0.61 3.26± 0.70 4.47± 1.02 Condition II 1.96± 0.42 2.15± 0.41 2.78± 0.40 3.98± 0.42 5.65± 0.82 Condition III 2.04± 0.74 2.20± 0.50 2.55± 0.52 3.70± 0.78 5.52± 1.11 Condition IV 1.83± 0.50 1.96± 0.43 2.44±0.45 3.35± 0.58 5.21± 1.02
Table 7: Mean values presented in mean kPa± SD (n=10) for the five pressure-levels and four conditions, for pressure balloon 2 and 3, placed on the front and back of the thigh
0 0.67 1.33 2.67 5.33
Condition I 1.64± 0.83 2.15± 0.75 2.66± 0.68 3.96± 0.74 6.72± 0.96 Condition II 1.35± 0.58 1.93± 0.51 2.69± 0.60 4.08± 0.74 6.75± 1.03 Condition III 1.23± 0.90 1.92± 0.84 2.68± 0.82 4.12± 1.01 6.80± 1.23 Condition IV 2.61± 1.02 2.56± 0.94 2.86± 0.82 3.62± 0.74 5.86± 0.70
Table 8: Mean values presented in mean kPa± SD (n=10) for the five pressure-levels and four conditions, for pressure balloon 4 and 5, placed on the shin and calf of the lower leg.
0 0.67 1.33 2.67 5.33
Condition I 2.57± 1.78 2.93± 1.62 3.29± 1.44 4.28± 1.07 6.45± 0.60 Condition II 2.40± 1.69 2.62± 1.37 3.10± 1.12 4.19± 0.84 6.48± 0.74 Condition III 1.33± 0.83 1.86± 0.70 2.61± 0.76 3.96± 0.71 6.50± 0.86 Condition IV 2.40± 1.26 2.57± 1.02 2.86± 0.89 4.17± 0.92 6.78± 1.18 In the above tables, the pressure transmission can be calculated by deriving the mean pressure increase from one level to the next by first calculating means of each pressure level, i.e. the means of the columns. By doing so, the results presented in Table 9 is obtained showing the pressure transmission between pressure levels for the separate pressure-increases at the bottom, as well as the mean pressure transmission for the entire body part to the right. The total pressure transmission, was obtained to be 75% of the total supplied pressure to the G-suit.
In the above tables, with the exception from the abdominal balloon at the highest inflation pressure (Table 6), the balloon pressures are always greater than the supplied pressure to the G-suit. When the inflation pressure is increased, the measured pressure from the balloons increase so that it is always greater than the supplied pressure to the G-suit.
Table 9: The pressure transmission from the G-suit to the surface of the skin in percentage change from the baseline, obtained between each pressure level. P1 presents the abdominal pressure balloon, P2-P3 presents pressure balloons placed on the upper leg, P4-P5 presents pressure balloons placed on the lower leg. The right column presenting total pressure transmission for the respective body part, the bottom row presenting the total pressure transmission at each inflation pressure. The bottom, right box, presents the total pressure transmission a↵orded by the G-suit.
0-0.67 0.67-1.33 1.33-2.67 2.67-5.33 Mean %
P1 40 79 79 62 65
P2-P3 64 88 92 98 85.5
P4-P5 48 71 88 90 74.3
3.2
Heart Rate
Table 10 presents mean heart rate (HR) in beats per minute (bpm)± SD. In the supine position, the HR was not a↵ected by the pressure in the G-suit (p=0.86). In both HUT conditions (II and III), the HR decreased with increasing pressure in the G-suit significantly (p<0.05), comparing initial and final pressure segments where the HR decreased by 16% and 11% respectively. For condition IV, seated on a chair, the HR decreased by 5.5% at increasing G-suit pressures (p<0.05), (Table 10 and Figure 11).
Table 10: Presented in the table is mean HR for each condition and pressure level in mean bpm± SD (n=10). 0 0.67 1.33 2.67 5.33 Condition I 65.30± 7.04 65.80± 7.50 64.90± 8.09 65.50± 8.24 65.30± 7.89 Condition II 80.80± 10.50 81.90± 9.84 80.80± 8.99 74.30± 9.82 68.00± 9.43 Condition III 79.40± 12.62 80.00± 11.87 78.30± 9.88 75.80± 10.60 70.70± 11.04 Condition IV 72.90± 10.30 72.10± 9.93 72.20± 10.69 68.80± 9.31 68.90± 9.74
Figure 11: Mean HR± SEM (n=10) can be seen for the four di↵erent conditions, where condition I is the lowest graph and condition II starts out the highest. Condition I does not change over the pressure-levels, whereas conditions II, III and IV decrease by 16, 11 and 5.5 % respectively.
3.3
Arterial Pressures
The arterial pressures were relatively una↵ected by inflation pressures up to 2.67 kPa. At the highest pressure in the G-suit the arterial pressures were slightly higher. When comparing the arterial pressure level of 5.33 kPa in the G-suit with 0 kPa, the SAP was higher in all conditions by 5% to 11%, MAP by 2% to 7% and DAP varied between -1% to 8% (p<0.05) (Figures 12-14). In supine position (condition I), the mean values of MAP showed no significant change. The two HUT conditions and seated condition all showed significant di↵erence at 5.33 kPa inflation pressure compared to 0 kPa. It can be noted that the two standing positions, condition II and III are similar.
In Figure 13, showing results for SAP, there were no change in supine position (p=0.11). Condition II, III and IV showed significant di↵erence in SAP at the highest inflation pressures compared to 0 kPa (p<0.01).
For DAP, there were no change in the two HUT conditions (p=0.61 and p=0.17 respectively), whereas supine position showed significant di↵erence between the two highest inflation pressures compared to zero pressure (p=0.012). In seated position, condition IV, there was a significant di↵erence in the 0.67 kPa to 5.33 kPa.
Figure 12: MAP for each condition over the pressure-levels are presented separately in mean mmHg ± SEM (n=10).
Figure 13: SAP for each condition over the pressure-levels are presented separately in mean mmHg ± SEM (n=10).
Figure 14: DAP for each condition over the pressure-levels are presented separately in mean mmHg ± SEM (n=10).
3.4
Sonographic Imaging
In supine position, condition I, the SV was una↵ected by zipping the G-suit, and inflation by 0.67 kPa. When the inflation pressure was increased to 1.33 kPa, the SV tended to increase by 5% from 0 kPa zipped (p=0.08). At 2.67 kPa inflation pressure the SV increased from 0 kPa zipped by 7% (p=0.02, Table 11). In seated, condition IV, the SV was 21% lower than in supine position 0 kPa (p<0.001) and when the G-suit pressure was increased to 1.33 kPa, the SV increased by 14%. In standing position, condition III, there was a similar, but aggravated, e↵ect on SV as in seated. At 0 kPa inflation of the G-suit SV was reduced by 36% (p<0.001) compared to supine position. When the G-suit was inflated by 1.33 kPa, the SV was 13% (p=0.02) greater than at zero-pressure. At the highest inflation pressure, the SV was still 11% lower than in supine position with the suit un-zipped (p=0.04).
In supine position, the CO was relatively una↵ected by inflation pressures of the G-suit. How-ever, there were a slight increased CO from 0 kPa to 0.67 kPa by about 5-6% (p<0.07, Table 12). In both seated and standing positions, the CO was reduced at lower G-suit pressures, but was restored at the highest pressure to the same level as in supine position. As an e↵ect of increased pressure in the G-suit by about 1.33 kPa the CO increased by 5% when seated and by 9% in standing position.
Table 11: Stroke volume (SV) from sonographic imaging analysis, presented as mean volume (mL) ± SD (n=10) exiting the heart from each heart beat, from the left ventricle. Missing data from condition II. 0 un-zipped 0 zipped 0.67 1.33 2.67 5.33 Condition I 83± 16 84± 16 85± 17 87± 19 89± 18 84± 15 Condition II -Condition III - 54± 9 56± 9 61± 10 67± 10 74± 8 Condition IV - 66± 14 70± 14 75± 16 79± 15 81± 12
Table 12: Cardiac output (CO) from sonographic imaging analysis, presented as mean volume (L/min)± SD (n=10) exiting the heart each mimute, from the left ventricle. Missing data from condition II. 0 un-zipped 0 zipped 0.67 1.33 2.67 5.33 Condition I 5.3± 0.7 5.3± 1.0 5.6± 1.1 5.5± 1.2 5.6± 1.0 5.3± 0.8 Condition II -Condition III - 4.4± 0.5 4.5± 0.6 4.8± 0.7 5.0± 0.6 5.2± 0.5 Condition IV - 4.8± 0.8 5.0± 0.9 5.0± 1.0 5.1± 0.8 5.3± 0.7
3.5
Lower-leg fluid change
In Table 13, the mean values in percentage change from the baseline± SD are presented for the tissue-impedance. In Figure 15, these data are presented graphically where condition I is seen to increase with increasing inflation pressure in the G-suit, whereas the three other conditions initially decrease to later increase to various degrees.
There was a significant di↵erence in supine position between pressure levels 0 kPa and 2.67 kPa, and between 0 kPa and 5.33 kPa (p<0.01).
In condition II, HUT on the back, the tissue-impedance can be seen to first decrease to later increase, however always on the negative side of the axis. The di↵erence in zero-pressure level compared to the following four pressure levels were significant (p<0.05). Condition III, HUT leaning on the side showed statistically significant di↵erence in pressures 1.33 kPa and above, compared to 0 kPa.
In the seated position, condition IV, the tissue-impedance initially decreases to later increase to the positive side of the axis (Figure 15). However, there were no statistical change between the 0 kPa inflation pressure compared to any of the other four pressure levels.
Table 13: Results of tissue-impedance presented in percentage change from the baseline ± SD (n=10) for the di↵erent G-suit pressures. Condition I has an additional pressure-level where the suit is un-zipped. 0 0 0.67 1.33 2.67 5.33 un-zipped zipped Condition I 0± 0 0.18± 0.56 0.80± 0.78 0.98± 1.08 1.45± 1.15 1.87± 1.65 Condition II - 0 ± 0 -0.86± 0.48 -1.38± 0.54 -1.22± 0.62 -0.53± 0.65 Condition III - 0 ± 0 -0.99± 1.07 -1.55± 1.37 -1.74± 1.41 -1.54± 1.83 Condition IV - 0 ± 0 -0.85± 1.09 -1.08± 1.13 -0.35± 1.30 0.94± 2.09
Figure 15: The relative tissue-impedance change in mean % ± SEM presented for the four con-ditions over the pressure levels. Condition I, supine position, does not start at 0% since it is compared to the zero-pressure state with the suit un-zipped.
3.6
The E↵ect of Zipping the G-suit
In the following tables (Table 14-16), the mean values± SD are presented for the zero-pressure state with un-zipped suit compared to the zipped suit, in the supine position. Zipping the G-suit did not a↵ect the arterial pressures, HR, tissue-impedance or the G-G-suit pressure significantly (p>0.05). It did impact three out of the five pressure balloons, namely pressure balloons 2, 3 and 4 (p=0.003, p=0.004, p=0.007) placed on the front thigh, back thigh and shin, but not on pressure balloon 1 or on pressure balloon 5 (p>0.05) placed on the abdomen and calf. It can be noted that zipping the G-suit resulted in a mean increase of measured transmitted pressures by the pressure balloons, by about 0.63 kPa, calculated from Table 16.
Table 14: Results from the zero-pressure state with un-zipped G-suit and zipped G-suit. MAP, SAP and DAP presented in mean mmHg± SD, and HR presented in mean bpm ± SD.
MAP SAP DAP HR
un-zipped 74.30± 13.73 117.50± 17.63 57.60± 11.57 68.60± 8.70 zipped 72.40± 13.06 113.60± 17.61 56.20± 10.78 65.30± 7.04
Table 15: Results from the zero-pressure state with un-zipped G-suit and zipped G-suit. The tissue-impedance is presented in both Ohm and %± SD, and the suit pressure is the mean kPa ± SD.
Tissue-imp. (Ohm) Tissue-imp. (%) Suit Pressure un-zipped 68.74± 13.54 0 ± 0 -0.04± 0.08
zipped 68.61± 14.20 0.18± 0.56 0 ± 0.12
Table 16: Results from the zero-pressure state with un-zipped G-suit and zipped G-suit. The pressures presented are the mean kPa± SD measured by the five respective pressure balloons.
Pressure 1 Pressure 2 Pressure 3 Pressure 4 Pressure 5 un-zipped 0.74± 0.53 0.27± 0.15 1.03± 0.40 0.27± 0.25 3.98± 2.35
zipped 1.04± 0.53 1.35± 0.87 1.93± 0.72 1.33± 0.97 3.80± 1.54
3.7
Respiratory variables
The results from the respiratory variables showed no significant di↵erence between the G-suit-pressures. It should be noted that when sonographic recordings were obtained, the normal respi-ratory pattern was interrupted.
Table 17: E↵ects of body positions and pressures in a pneumatic anti-G suit that acts as counter pressure to the lower body, on the respiratory ventilation. The values are in mean L/min± SD (n=10). 0 (un-zipped) 0 (zipped) 0.67 1.33 2.67 5.33 Condition I 9.11± 0.76 9.23± 0.82 9.04± 1.09 8.82± 0.91 8.98± 1.18 8.53± 1.12 Condition II - 10.12± 1.06 10.31± 1.41 10.09± 1.22 10.04± 1.43 10.42± 1.27 Condition III - 10.33± 1.37 10.36±1.08 10.21± 1.19 10.45± 1.90 10.55± 2.11 Condition IV - 9.82± 1.54 9.17± 1.31 9.38± 0.86 9.21± 1.07 9.68± 1.58
4
Discussion
The presented experimental study utilized low inflation pressures in the pneumatic anti-G suit, in four di↵erent body positions to suggest a model for the pressure transmission to the cardiovascular system. The main findings indicate that the inflation of the G-suit is transmitted to the surface of the skin, to the underlying tissues, and to the venous system and further on to the heart, thus having a significant impact on the cardiovascular system.
4.1
Pressure transmission and the e↵ect on the cardiovascular system
The supplied inflation pressure to the G-suit did not show any significant di↵erence compared to the intended inflation pressure (Table 3). However, the pressure transmitted from the G-suit to the surface of the skin, measured by the pressure balloons showed a smaller pressure transmission than the intended (Table 5). This finding is not in accordance with previous studies [8], where the pressure transmission was found to be 100% for a correctly fitted G-suit. In the present study, the transmitted inflation pressure was on average 0.24 kPa lower than the intended (Table 5), and the total pressure transmission to the surface on the skin was about 75% of the supplied inflation pressure to the G-suit (Table 9). The fact that an elevation of the pressure in the G-suit only induced a 75% increase in the balloon pressures can be multifactorial and will not be discussed in detail in this report. However, it is most likely due to the fabric in the G-suit, pressure gradient from the tissue to the skin, or due to that the pressure in balloons were increased already at zero pressure in the G-suit.
The G-suit provide the lower body with a uniform pressure, although, it is a matter of specu-lation how the applied pressure of the G-suit will propagate to the underlying tissues and to the vessels of the lower body. Based on speculation and measured variables, when the pressure in the G-suit is increased the pressure gradient from underlying tissues and the skin surface is maintained and move inwards, increasing the pressure outside the venous vessels. The pressure on the surface of the skin without the G-suit is zero. To maintain blood flow through the veins, the intravenous pressure needs to increase to overcome the elevated tissue pressure outside the veins. It is therefore assumed that the pressure applied to the skin by the G-suit will to the same extent increase the pressure in the veins [9, 10, 11].
In supine position, the pressures in the lower leg tissue and the veins are roughly the same, about 10 mmHg [10, 11]. Venous pressure in the legs while in seated or standing position, with the influence of gravity, is more complex. Due to venous valves throughout the legs which act to hinder retrograde blood flow, the hydrostatic column will be interrupted by activation of skeletal muscles and thus, reducing the hydrostatic pressure components.
When changing body position from the horizontal to the head-up position, a steady state of the venous pressure is leveled out one to four minutes after tilt [12]. In the present study, data was collected between minutes two and four, allowing for the venous pressure to reach steady state sufficiently. This means that after two to four minutes, the venous pressure is equal to the height di↵erence between the heart and where the intravenous pressure is to be determined [12]. During the present experimental study, the subjects were seated and standing as still as possible to avoid any contraction of the skeletal muscles, which would break the hydrostatic column of blood in the veins. The pressure in the dependent veins is about 50 mmHg in the seated position and 80 mmHg in the HUT position [12], estimated to the middle of the shin from the above statement that the venous pressure is equal the height di↵erence. The venous pressure is transmitted to vena cava inferior, which enter the right atrium of the heart, and the di↵erence in pressures a↵orded between vena cava inferior and the right atrium will govern the venous return, and thus the diastolic filling of the right cardiac ventricle and therefore the cardiac output of the heart [13]. In supine position, the pressure is about 8 mmHg in vena cava inferior and about 4 mmHg in the right atrium [13, 14]. This means that the venous pressure decreases from 10 mmHg to 8 mmHg in vena cava inferior, referred to as the venous resistance [13]. In standing position, the venous pressure is about 80 mmHg and drops to about 3 mmHg in vena cava inferior, and to 0 mmHg in the right atrium, the drop being mainly due to the waterfall phenomenon [22], hydrostatic pressure gradient, but also due to gravity and to a small extent to the venous resistance [13].
As depicted in the physical model, in the section further below, when the pressure in the G-suit is increased to 10 mmHg (1.33 kPa) in the supine position, the pressures in the underlying tissues and veins are increased by about 8 mmHg, since 75% of the supplied pressure to the G-suit is
which according to previous data [15], suggesting that the pressure in vena cava inferior increase by 25%. However, in the present study, the elevated pressure in vena cava inferior did not increase SV or CO by the same extent, which is according to the data in the present study, where the SV increased by about 5% and the CO did not change. The reason why the SV and CO did not increase as much as the assumed pressure in vena cava inferior, must be due to that the venous pressure was also transmitted to the right atrium. In fact, the pressure gradient between vena cava inferior and right atrium must be the same since the CO was the same. The same calculations have been made for seated and HUT positions, Table 11 and 12, and depicted in the suggested model.
In the supine position, the HR and MAP was una↵ected by increased inflation pressure in the G-suit. It seems that in supine position, at least during G-suit pressures of 5 to 20 mmHg, the increased pressure in vena cava inferior did not a↵ect the cardiovascular system (Figure 11-14). At the highest pressure in the G-suit (40 mmHg), the DAP was higher and usually corresponded to a higher vascular resistance on the arterial side of the circulation, which also has been seen by others [15]. In seated and standing positions, the HR was decreased at G-suit pressures 20 and 40 mmHg, most obvious during standing where HR was reduced from about 80 bpm at G-suit pressure of 0 to 10 mmHg to about 70 bpm when the G-suit pressure was 40 mmHg (Figure 11). Also, the arterial pressures were, or tended to be, higher during G-suit pressure of 40 mmHg than at G-suit pressure of 0 mmHg (Figure 12-14). This means that in seated and standing positions, an increase in the G-suit pressure by 20 to 40 mmHg improved the cardiovascular circulation. A possible physiological mechanism underlying the e↵ects on the cardiovascular system when the pressure in the G-suit increases is the cardiopulmonary baroreflex. This low pressure baroreflex is mechanoreceptors in the atria producing at least a slight modulation of the vasomotor tone but usually lack of control of heart rate [11], in contradiction, the present study showed substantial change in HR.
If there are changes in the tissue-impedance, it indicates that there in fact are some fluid moving to or from the lower body, causing a shift in volume. Tissue-impedance measurements are not accurate, and can only be assumed to in a qualitative manner reflect di↵erences in the magnitude of fluid displacement. From the experimental data, it is assumed that there is some shift in fluids occurring in the supine position, and in the two HUT conditions. In supine position, the shift is registered at the second to highest inflation pressure, while in HUT on the back, it is registered in all inflation pressures compared to 0 mmHg inflation pressure, whereas it in the seated position there were no change in the tissue-impedance.
Previous studies have shown that counterpressures to the lower body below 20 mmHg does not a↵ect the larger arteries or the peripheral resistance in supine position [15, 16]. Inflation pressures above 20 mmHg a↵ects these arteries, with an increased arterial pressure. The e↵ect of the applied counterpressure is thus thought of as two separate phases, the first in 0-20 mmHg range only a↵ecting veins, and the second phase of pressures higher than 20 mmHg also a↵ecting the arterial side.
When the G-suit was fitted to the subjects the pressure on the surface of the skin was increased, and even higher after closure of the G-suit by zipping (Table 13), but the zipping of the G-suit did not a↵ect the arterial pressures or the HR. The e↵ect of zipping the G-suit elevated the pressure in three out of five pressure balloons significantly. One can argue that these nonsignificant results from pressure balloons 1 and 5 are valid since the added pressure created by closure of the G-suit, does not exceed that of the counterpressure experienced by the calf balloon, balloon 5, from the underlying table. The abdominal pressure balloon, balloon 1, may not be a↵ected since the abdominal part of the G-suit is not zipped per se, unlike the parts covering the legs, the e↵ect may occur at fitting of the suit.
4.2
Suggested model for pressure transmission
The following model is a suggestion of the pressure transmitted from the G-suit to the surface of the skin, to the underlying tissues and vessels of the legs, and onto the heart. The dimensions of the suggested model are exaggerated and not to scale, since it is a mere suggestion based on data from the present study, literature and speculations. This model does not take into account other possible physical mechanisms that could e↵ect the pressure transmission from the legs into the heart. For example, the abdominothoracic pump and the vascular waterfall phenomenon that most likely occur above the G-suit when the caval vein transverse the abdomen and enter the thorax have not
been taken into account [22]. The respiratory variations in pressure of the thoracic and abdominal cavities induce fluctuations in venous return to the right heart. The waterfall phenomenon, a fall in intravenous pressure in encapsulated veins by the G-suit to veins above the G-suit, have an e↵ect on the pressures in vena cava inferior and right atrium. However, it is unclear if and how much the waterfall phenomenon a↵ect venues return. It is also known that the pattern of the venous circulation is di↵erent when the veins are distended and not compliant [17, 18, 19, 20]. During the experiments it was obvious that there was a down regulation of HR reducing the e↵ects on CO and therefore the pressures in vena cava inferior and right atrium, and the pressure gradient between them, most likely due to the cardiopulmonary baroreflex [11].
In supine position when the G-suit is zipped the mean balloon pressures were about 14 mmHg (1.9 kPa, Table 16) and the average tissue and venous pressures were assumed to be 10 mmHg without the G-suit [10, 12], Figure 16. The pressure transmission from the skin to the underlying tissues was assumed to be 100% [9, 10, 12] and the venous pressure is increased to overcome the tissue pressure. Therefore, the tissue and the venous pressures about 24 mmHg. At heart level the pressure in vena cava inferior is about 8 mmHg and the pressure in the right atria is about 4 mmHg [13]. The pressure di↵erence between vena cava inferior and right atrium is then 8-4=4 mmHg, and is according to [13] the driving pressure for venous return and CO.
When the pressure in the G-suit is increased by 10 mmHg the balloon pressures are increased by about 75% (8 mmHg). The venous pressure is therefore assumed to be about 32 mmHg. Based on experiments performed by others [15], it can be assumed that the pressure in the vena cava inferior has been increased by 2 mmHg to 10 mmHg. Since the CO (venous return) only showed a small increase by about 5% it must be assumed that the pressure di↵erence between vena cava inferior and right atrium is only slightly higher than at zero pressure in the G-suit. The right atrial pressure is therefore increasing more or less as much as the pressure in vena cava inferior. The increased SV by about 5% may be due to the slight increased pressure di↵erence between vena cava inferior and right atrium and/or the Starling mechanism (increased filling of the ventricle enhances SV), [23].
In seated position, when the G-suit is zipped, the mean balloon pressures were 14 mmHg and the average tissue pressure is about 15 mmHg without the G-suit [10, 12] and the venous pressure is, by calculations of the height di↵erence between heart and the mid part of the lower leg, assumed to be 64 mmHg [10, 12], Figure 17. In seated position the pressures in vena cava inferior and right atrium are, based on experiments by others [21], and on CO and SV data from present study, assumed to be 6 mmHg and 3 mmHg, respectively. The reduced pressure di↵erence (3 mmHg) between vena cava inferior and right atrium cause the reductions in CO and SV.
When the pressure in the G-suit is increased by 10 mmHg the balloon pressures were increased by about 8 mmHg. The venous pressure is therefore assumed to be about 72 mmHg, based on calculations of the height di↵erence between heart and the mid part of the lower leg [10, 12], and the counter pressure from the tissue.
In standing position when the G-suit is zipped the mean balloon pressures were 14 mmHg and the average tissue pressure is 34 mmHg due to the counterpressure from the G-suit and by the tissue pressure of 20 mmHg without the G-suit including the filled venous system by about 10 mmHg, Figure 18. The venous pressure is assumed to be 94 mmHg, based on calculations of the height di↵erence between heart and the mid part of the lower leg [10, 12] and the counter pressure from the tissue. In standing position the pressures in vena cava inferior and right atrium is assumed to be 3 mmHg and 0 mmHg, respectively [13].
When the pressure in the G-suit is increased by 10 mmHg the balloon pressures were increased by about 8 mmHg. The venous pressure is therefore assumed to be about 102 mmHg, based on calculations of the height di↵erence between heart and the mid part of the lower leg [10, 12], and the counter pressure from the tissue. The pressures in vena cava inferior and right atrium, based on CO and SV data from present study are assumed to be 4.5 mmHg and 1.5 mmHg, respectively, since the CO (venous return) showed an increase by about 9% and SV by 14%.
Figure 16: Suggested model for the supine position. The top figure showing pressures at 0 kPa inflation pressure, and the bottom figure showing the resulting pressure transmission by an inflation pressure a↵orded the G-suit of 1.33 kPa, the so-called ready-pressure.
Figure 17: Suggested model for the seated position. The top figure showing pressures at 0 kPa inflation pressure, and the bottom figure showing the resulting pressure transmission by an inflation pressure a↵orded the G-suit of 1.33 kPa, the so-called ready-pressure.
Figure 18: Suggested model for the standing position. The top figure showing pressures at 0 kPa inflation pressure, and the bottom figure showing the resulting pressure transmission by an inflation pressure a↵orded the G-suit of 1.33 kPa, the so-called ready-pressure.
