Örebro University School of Medicine Degree project, 30 ECTS May 23, 2017
Goal-directed fluid therapy in major head and
neck surgery with free flap reconstruction
Version 1
Author: Fanny Bergman, Bachelor of medicine Supervisors: Stephan Schwager, MD
ABSTRACT
Introduction: Head and neck free tissue transfer surgeries are often long and
complex. Inadequate perioperative fluid administration may lead to postoperative complications, organ dysfunction, prolonged hospital stay and increased mortality. In goal-directed fluid therapy (GDT), a minimally invasive cardiac output-monitor can be used to guide administration of intravenous fluids and vasopressors to optimize tissue perfusion.
Aim: To evaluate if the introduction of goal-directed fluid therapy has affected the
amount of intravenous fluids administered perioperatively in head and neck free tissue transfer surgery. We also wanted to investigate the impact of GDT on postoperative complications and ICU length of stay.
Material and methods: The study is a retrospective review of medical records,
including all patients who underwent head and neck surgery with free flap
reconstruction at Örebro University Hospital between 2008 and 2016. 239 patients’ medical records were reviewed and preoperative, perioperative and postoperative variables were registered and analysed.
Results: 96 patients had received standard fluid therapy and 143 patients had received
goal-directed fluid therapy. Patients in the GDT group received less mean volume of both total intravenous fluids (4266±1231 vs. 4915±1197 ml; p <0.001) and colloids (261±422 vs. 1228±503 ml; p <0.001). Vasoactive drugs were administered more frequently in the GDT group (74.1 vs. 49%; p <0.001). The number of postoperative surgical complications and ICU length of stay did not differ significantly between the two groups.
Conclusion: This study suggests that patients receive less intravenous fluids and
more vasoactive drugs perioperatively when goal-directed fluid therapy is practiced. These results may however be affected by the on-going trend towards less intravenous fluids and a more liberal view on vasoactive drugs in this patient population. More studies are needed to further investigate the impact of GDT on head and neck free tissue transfer patients.
TABLE OF CONTENTS
ABSTRACT ... 2
INTRODUCTION ... 4
OBJECTIVE ... 5
MATERIAL AND METHODS ... 6
DESIGN ... 6 PATIENT POPULATION AND SELECTION ... 6 STUDY VARIABLES ... 6 GOAL-DIRECTED FLUID THERAPY ... 6 ETHICAL CONSIDERATIONS ... 7 STATISTICAL ANALYSES ... 7 RESULTS ... 7 DISCUSSION ... 10 LIMITATIONS ... 13 CONCLUSION ... 14 ACKNOWLEDGEMENTS ... 14 REFERENCES ... 15
INTRODUCTION
Maintaining hemodynamic stability and attaining adequate tissue oxygen perfusion during surgery is an essential part of anaesthesiology practice. The major cause of hemodynamic instability during anaesthesia is hypovolemia, [1] but there is also evidence that excessive volume administration causing hypervolemia is dangerous. [2] Inadequate perioperative fluid administration may lead to postoperative
complications, organ dysfunction, prolonged hospital stay and increased mortality. [3,4] Traditionally, fluid management has been guided by assessment of standard vital parameters such as blood pressure, heart rate and urine output. However, these
endpoints are insensitive to small changes in hemodynamic stability and may be associated with a state of unnoticed hypoperfusion. [5] In recent years, individualized goal-directed therapy (GDT) using minimally invasive hemodynamic monitors has been introduced to overcome this problem. GDT is based on the Frank Starling mechanism, which declares that the contractile power of the heart increases with greater diastolic filling. Cardiac output is thus increased when the total circulating blood volume is increased by administration of intravenous fluids. This augmentation is however reaching a plateau, after which cardiac output decreases if more fluid is administered. [6] In GDT, administration of vasopressors, intravenous fluids and inotropic therapy is guided by real-time measurements of stroke volume (SV) and stroke volume variation (SVV) to predict fluid responsiveness and optimize cardiac output (CO). SVV is the change in SV in one respiratory cycle. If a patient is suffering from hypovolemia, the SVV is exaggerated, and the patient is likely to respond to fluid administration. [7] Many studies have shown that GDT improves patient outcome by reducing postoperative complications and length of hospital stay compared to traditional fluid management. [8-12]
Major head and neck surgery is often associated with complex reconstructions and long surgical procedures, and achieving appropriate perioperative fluid balance is therefore of utmost importance. Free tissue transfer (FTT) is a standard reconstruction method using a free flap to restore extensive defects after tissue resection due to cancer, osteoradionecrosis or trauma. [13] Both negative and positive fluid balance can seriously harm the flap as a result of insufficient tissue perfusion or oedema. The flap is at risk of developing oedema due to denervation and lack of lymphatic
drainage. Risk of flap related postoperative complications such as wound dehiscence and flap failure has been shown to increase with administration of large amount of intravenous fluids perioperatively [14-17] and long duration of surgery (anaesthesia time of eight hours or more). [18]
To maintain blood pressure without giving excessive amounts of fluids, the
anaesthesiologist may use vasopressors such as epinephrine and phenylephrine. The use of these vasoactive drugs has long been discouraged in this patient population due to concerns that they may provoke spasm in the anastomosis leading to reduction in graft perfusion and, ultimately, to flap failure. [19] However, recent studies have shown no significant correlation between the use of vasopressors and incidence of complications and flap failure [20,21] and according to current guidelines they are now considered safe to use. [22] Certain inotropes, such as dobutamine, have been proven to increase blood flow in the anastomosis, [23] and can be used as an alternative to vasoconstrictors. [22]
Although goal-directed fluid therapy has been proven to be very effective in
intermediate to high-risk surgery, [8,10,12] not many studies have been done in head and neck free tissue transfer surgery. However, a recent prospective randomized trial indicates that GDT in this patient population decreases ICU length of stay and is not associated with an increased complication rate. [24] GDT is now recommended as part of the ERAS (enhanced recovery after surgery) guidelines for major head and neck cancer surgery with free flap reconstruction. [22]
Objective
The main objective of this study was to evaluate if the practice of goal-directed fluid therapy has affected the amount of intravenous fluids administered perioperatively in head and neck free tissue transfer surgery at the Örebro University Hospital since the introduction in November 2012. We also wanted to investigate the impact of GDT on postoperative flap-related complications and ICU length of stay.
MATERIAL AND METHODS
DesignThe study is a retrospective observational study in which the patients’ written and electronic medical records were reviewed.
Patient population and selection
All patients who underwent head and neck surgery with free tissue transfer at Örebro University Hospital (USÖ) during the time period 2008-01-01 to 2016-12-31 were included in the study. These patients’ data were acquired from the head and neck surgery register at the Centre for Head and Neck Oncology at the
Otorhinolaryngology department at USÖ. A total of 258 patients were identified. Out of these, 19 patients were excluded due to the following reasons: anaesthesiology records were missing in 3 patient journals and for 16 patients written medical records could not be found. Ultimately, 239 patients with head and neck free tissue transfer surgery were included. Reviews of the patients’ medical records were conducted in February 2017. The individual performing the study digitalized all data in de-identified form.
Study variables
Demographic variables included in the study were age, gender, weight, height and body mass index. Clinical variables included were: reason for the flap, donor site and medical comorbidity of importance to fluid therapy (diabetes, cardiovascular disease, stroke, asthma, chronic obstructive pulmonary disease, sleep apnoea and renal
disease). Perioperative variables were gathered from intraoperative anaesthetic
records and included use of a non-invasive CO monitor, length of anaesthesia, volume and type of intravenous fluid administered, blood transfusion, estimated blood loss, urine output, and whether vasoconstrictors or inotropes were given or not.
Postoperative variables were ICU length of stay, 30-day survival and flap-related complications (flap dehiscence, donor site complications, need for reoperation and flap failure) occurring during the hospital stay. A flap-related complication was defined as major if the patient needed reoperation.
Goal-directed fluid therapy
When GDT was performed, the FloTrac/Vigileo system (Edwards Lifesciences LCC, Irvine, CA) was used for measurements of hemodynamic parameters to direct
treatment of hypotension. Vigileo is a monitor connected to a transducer in a radial arterial line that continuously analyses the arterial pressure waveform to calculate SV, SVV, CO and cardiac index (CI; CO / total body surface area). [25] If the patient was hypotensive or had a low CI, SVV were assessed. In case of low CI and SVV >13% the patient was given an intravenous fluid bolus of approximately 3 mL/kg of bodyweight. In case of low CI and SVV <13%, vasoactive drugs (norepinephrine, phenylephrine, ephedrine or dobutamine) were administered.
Ethical considerations
The head of the department of Anesthesiology at USÖ have approved analysis of the medical records before the start of the study. This study was carried out as a quality audit with the aim to improve perioperative care amongst the free tissue transfer surgery patient population, and the results will not be published. Therefore, neither patient consent nor information about the quality program were gathered or given to the patients. The patients have nonetheless been anonymized and assigned a study number.
Statistical analyses
Statistical analyses were performed with SPSS version 22. Categorical variables were analysed with chi-square tests. Independent sample t-test was used for normally distributed continuous data and Mann Whitney U test was used for nonparametric and ranked data. Multivariable logistic regression analyses were performed in order to investigate the following variables further: total intravenous fluids, colloids, vasoactive drugs, fluid balance, red blood cell transfusion, age, sex, BMI, surgery duration and patient comorbidity. All p values ≤ 0.05 were considered statistically significant.
RESULTS
Data from a total of 239 patients who had undergone free tissue transfer surgeries were reviewed. Out of these, 96 patients had received standard fluid therapy and 143 patients had received goal-directed fluid therapy with the Vigileo monitor.
Table 1. Patient characteristics and surgical information.
Variables Non-GDT group
(n=96) GDT group (n=143) P-value Age, y 62.8 (±13.9) 65.6 (±12.2) 0.095 Sex Male Female 49 (51.0) 47 (49.0) 77 (53.8) 66 (46.2) 0.67 0.67 Weight, kg 72 (±15.8) 75.0 (±17.2) 0.228 BMI 24.8 (±4.3) 25.6 (±5.0) 0.168 Patient comorbidity* 0 1 2-3 4-7 51 (53.1) 24 (25.0) 18 (18.8) 3 (3.1) 74 (51.7) 48 (33.6) 19 (13.3) 2 (1.4) 0.706 Surgery duration, h 11.4 (±2.3) 10.7 (±2.9) 0.061 Flap indication
Head and neck cancer Osteoradionecrosis Osteomyelitis Benign tumour Trauma Scar contracture 86 (89.6) 1 (1.0) 3 (3.1) 3 (3.1) 2 (2.1) 1 (1.0) 124 (86.7) 14 (9.8) 0 5 (3.5) 0 0 Flap type Radial forearm Fibula ALT Latissimus dorsi Jejunum Fibula + ALT
Fibula + radial forearm
60 (62.5) 26 (27.1) 4 (4.2) 5 (5.2) 1 (1.0) 0 0 68 (47.6) 57 (39.9) 15 (10.5) 0 1 (0.7) 1 (0.7) 1 (0.7)
Abbreviations: BMI, body mass index; ALT, anterolateral thigh
Continuous variables are reported as mean (SD) and categorical variables are reported as number (%). *Number of chronic diseases: diabetes, cardiovascular disease, stroke, asthma, chronic obstructive pulmonary disease, sleep apnoea and renal disease.
There were no statistically significant differences between the groups in terms of age, sex, weight, BMI, comorbidity and surgery duration, although the GDT group had a tendency towards higher age (65.6 vs. 62.8 years, p =0.095) and shorter surgery duration (10.7 vs. 11.4 h, p =0.061). Most free tissue transfers in both groups were undertaken due to head and neck cancer. Other indications included
osteoradionecrosis, osteomyelitis, benign tumours, trauma and scar contracture. The radial forearm was the most common donor site in both groups followed by the fibula, with a tendency towards more use of the fibula and less use of the radial forearm in the GDT group compared to the non-GDT group.
Table 2 illustrates perioperative variables associated with each group. Patients in the GDT group received less mean volume of both total intravenous fluids (4266±1231 vs. 4915±1197 ml; p <0.001) and colloids (261±422 vs. 1228±503 ml; p <0.001). The mean average fluid administration rate was also lower in the GDT group (5.67 vs. 6.28 mL/kg/h; p =0.005). Red blood cell transfusions were given less frequently to the GDT group compared to the non-GDT group (24.5 vs. 41.7%; p =0.005). There was no significant difference in estimated blood loss between the both groups. Vasoactive drugs were administered more frequently in the GDT group (74.1 vs. 49%; p <0.001). Mean net fluid balance were 2823±1056 ml in the GDT group and 3242±1052 ml in the non-GDT group (p =0.003).
Table 2. Perioperative totals.
Abbreviations: RBC, red blood cell transfusion; FFP, fresh frozen plasma transfusion; EBL, estimated blood loss. Continuous variables are reported as mean (SD) and categorical variables are reported as number (%).
* Calculation of net fluid balance, including following positive variables: i.v fluids, RBC and FFP, and negative variables: urine output and EBL.
The association between total volume of intravenous fluids and practice of GDT remained significant after controlling for age, sex, BMI, surgery duration and patient comorbidity (p <0.001). Also, the association between GDT and the variables:
volume of colloids (p <0.001); net fluid balance (p =0.005); red blood cell transfusion (p =0.018); and vasoactive drugs (p <0.001) remained significant after controlling for the same variables.
Variables Non-GDT group
(n=96) GDT-group (n=143) P-value i.v fluids, mL 4915 (±1197) 4266 (±1231) <0.001 Colloids, mL 1228 (±503) 261 (±422) <0.001
Average fluid administration rate, ml/kg/h 6.28 (±1.63) 5.67 (±1.63) 0.005 RBC, % yes 40 (41.7) 35 (24.5) 0.005 FFP, % yes 7 (7.3) 7 (4.9) 0.439 Urine output, mL 1223 (±591) 953 (±626) 0.001 EBL, mL 733 (±628) 635 (±464) 0.169
Net fluid balance*, mL 3242 (±1052) 2823 (±1056) 0.003
Figure 1. Boxplot showing total intravenous fluids in both groups.
The number of postoperative surgical complications did not differ significantly between the two groups. Patients in the GDT group showed a trend towards shorter mean ICU length of stay (1.09 vs. 1.3 days), but the association did not reach statistical significance (p= 0.095). Postoperative data are shown in Table 3. Table 3. Postoperative patient characteristics and outcome.
Variables Non-GDT group
(n=96)
GDT-group (n=143)
P-value
ICU length of stay, d 1.3 (±0.93) 1.09 (±0.29) 0.095
Patients staying >1 day at ICU 15 (15.6%) 13 (9.1%) 0.124
Surgical complications
Minor 5 (5.2%) 6 (4.2%) 0.714
Major 10 (10.4%) 18 (12.6%) 0.609
30-day mortality 0 0
DISCUSSION
The main finding in this review is that patients undergoing head and neck surgery with free flap reconstruction receive less intravenous fluids perioperatively when goal-directed fluid therapy are practiced.
It is however possible that the decrease in administered fluids seen in our study would have occurred even without the practice of GDT, considering the trend towards a more restrictive fluid management advocated by many studies [15-18,26,27] and guidelines. [28] The greatest contribution to the decrease in total intravenous fluids in the present study is the decrease in administered colloids. There has long been a controversy on the subject of colloid versus crystalloid solutions in fluid resuscitation, and in recent years, crystalloids have become the preferred fluids. A large meta-analysis by Perel et al. 2013 concludes that colloids should not be used in clinical practice since they do not reduce mortality, some of them might even increase mortality, and are considerably more expensive than crystalloids. [29] This trend is more likely to explain the large decrease in administered colloids in this study than the practice of GDT, although it cannot explain the decrease in total fluid volume. If the avoidance of colloid solutions was the only explanation, a corresponding increase in administered crystalloids would have been observed. Total intravenous fluid volume would have remained the same or even increased, since greater fluid volumes of crystalloids than colloids are required to meet the same haemodynamic targets. [30]
The increase in use of vasoactive drugs in the GDT group logically correlates with the decrease in administered fluids, since vasopressors are the alternative to fluid boluses in order to increase cardiac output. The use of these drugs was long discouraged in free tissue transfer patients, but lately the trend has been towards a more liberal approach and studies have demonstrated that they are safe to use. [20,21] It is likely that the increase in vasoactive drugs seen in our study is partly due to this more liberal attitude towards them in this patient population.
Even though our results might be influenced by the above-mentioned variables, we believe that the use of a CO-monitor has had an impact on perioperative fluid
administration, not least by giving the anaesthesiologist the confidence and assurance to be more restrictive with the fluid therapy without putting the haemodynamic stability at risk. To our knowledge, there is only one published study investigating the impact of GDT on head and neck free tissue transfer patients. The results of a
prospective randomized trial by Hand et al. were similar to ours, showing that patients in the GDT group received significantly more vasoactive drugs, and demonstrated a
The second finding in our study is that ICU length of stay was not significantly affected by the introduction of GDT. These results differ from the study by Hand et al., who showed an average of 24.6 hours shorter ICU length of stay in the GDT group compared to the control group. The authors postulate that this is due to better postoperative physiologic function, resulting in less time in the ICU before being considered stable for discharge to the regular ward. In accordance with this assumption, Hand et al. found that patients receiving GDT required significantly shorter duration of mechanical ventilation compared to the control group. [24]
Although not significant, we saw a tendency towards shorter ICU length of stay in the GDT group in our study. The lack of statistical significance may be affected by the low resolution of data since ICU length of stay was measured in days and not in hours. All patients in our hospital were admitted to the ICU after head and neck free tissue transfer surgery. This means that, for example, a patient might only stay for 5 hours, but would not be differentiated from someone who required ICU care for 23 hours.
In line with Hand et al.’s results, we found no significant difference in the frequency of flap related complications between the two groups, although in their study these complications occurred less frequently in the GDT group. [24] To date, there have been no studies investigating the impact of GDT on non-flap related complications in head and neck free tissue transfer patients. There is however one study on head and neck cancer patients from 2016 that showed that patients receiving >4000 mL of intravenous fluids within the day of surgery (24 hours) had significantly more major cardiac and cerebrovascular events than patients receiving <4000 mL. [31] In our study, the mean volume of intravenous fluids in both groups exceeded this amount, although the GDT group were closer to 4000 mL than the non-GDT group (4266 mL vs. 4915 mL). It is thus possible that the practice of GDT could have an impact on reducing medical complications in head and neck free tissue transfer patients. Several studies on GDT on other patient populations have shown reduced frequency of medical complications [9-12] and it would have been interesting to see if this also applied to our patients. However, it was not possible to include this variable in our retrospective study because of incomplete data in the medical records.
An incidental finding in this study is that fewer patients in the GDT group received red blood cell (RBC) transfusions compared to the non-GDT group, even though estimated blood loss did not differ significantly between the two groups. The need of RBC transfusion is based on either anaemia or major operative bleeding. We have no data on haemoglobin values but it is not likely that the difference in intravenous fluids administered has diluted the haemoglobin values significantly, since the fluids were given during several hours. It is more likely that the threshold for RBC transfusion were higher in the non-GDT group. Perioperative RBC transfusion have been shown to be associated with higher risk of cardiovascular and wound complications in head and neck surgery [31,32] and higher risk of 30-day mortality in major vascular surgery. [33] It is possible that the decrease in RBC transfusions seen in our study reflects a more restrictive transfusion policy that aims to reduce postoperative complications.
Limitations
There are several limitations of this retrospective study. The greatest limitation is that we had to collect data spanning several years. In our hospital, there are approximately 26 head and neck free tissue transfer surgeries per year, and therefore we had to choose a long time period to get a large enough sample size. It is possible that pre- and postoperative care, trends in anaesthesiology and surgery techniques have changed during this time period, affecting the results. In addition, the introduction of electronic medical records occurred during this time and there is a great variety in both quality and quantity of the medical notes. As in all retrospective reviews, our study relies on the accuracy of written records, which is a major disadvantage compared to prospective studies. Another limitation is the lack of a determined algorithm for the goal-directed fluid therapy, since the exact volume of the fluid boluses and type of vasoactive drug have been decided by the attending
anaesthesiologist and may vary between individuals. To eliminate all these
confounding factors and generate reliable conclusions on the subject, a prospective randomized trial with a predetermined GDT algorithm is needed.
CONCLUSION
This study suggests that patients receive less intravenous fluids and more vasoactive drugs perioperatively when goal-directed fluid therapy is practiced. These results may however be affected by the general on-going trend towards less intravenous fluids and a more liberal view on the use of vasoactive drugs within this patient population. In our study, we did not find statistically significant differences in the frequency of flap related complications nor ICU length of stay. More research is needed to further investigate the impact of GDT on head and neck free tissue transfer patients.
ACKNOWLEDGEMENTS
I would like to thank my supervisors Stephan Schwager and Rebecca Ahlstrand for good support and stimulating discussions throughout the work with this study. I also would like to express my gratitude to my friends Julia König and Sam de Boise for proofreading the manuscript and giving constructive criticism. Thank you.
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Populärvetenskaplig sammanfattning
Målstyrd vätsketerapi vid stor huvud-halskirurgi
Cirka 1400 patienter blir diagnostiserade med cancer inom huvud-halsområdet i Sverige varje år. Kirurgi är ofta förstahandsvalet vid dessa diagnoser, och ofta krävs att man tar bort mycket vävnad för att få bort hela tumören och bota patienten. För att bibehålla funktion och utseende krävs komplicerade rekonstruktionsoperationer där man tar vävnad från andra delar av kroppen och flyttar till ansikte och hals. Ett vadben kan till exempel bli en ny käke och en muskel i underarmen kan bli del av en ny tunga. Dessa operationer tar ofta mellan 10 och 20 timmar och det är väldigt viktigt att patienten får rätt mängd intravenös vätska under den här tiden. Får
patienten för lite vätska kan det leda till blodtrycksfall och skador på inre organ. För mycket vätska kan i sin tur leda till att patienten får vätska i lungorna, samt kirurgiska komplikationer som att den flyttade vävnaden inte överlever. För att ge exakt rätt mängd vätska till varje patient finns nu en ny metod som kallas målstyrd vätsketerapi, där narkosläkaren tar hjälp en monitor för att beräkna vätskebehovet.
En journalstudie med 239 deltagare har nyligen genomförts på Örebro
Universitetssjukhus för att ta reda på hur mycket vätska som ges till patienterna under operation sedan den nya metoden infördes i november 2012. Resultaten visar att patienterna får mindre intravenös vätska med den nya metoden, något som forskarna tror är positivt eftersom det kan minska de kirurgiska komplikationerna. Detta är den första studien i Sverige som undersöker vad målstyrd vätsketerapi har för effekter vid stor huvud-halskirurgi. Fler studier behövs för att kunna dra några säkra slutsatser på området.
COVER LETTER
May 9th, 2017 Örebro University Dear Editor,
I am writing to submit our manuscript entitled “Goal-directed fluid therapy in major head and neck surgery with free flap reconstruction” for consideration for publication. This is a retrospective record study, investigating the impact of goal-directed fluid therapy on head and neck free tissue transfer patients.
In this manuscript, we show that free tissue transfer patients received significantly less mean volume of both total intravenous fluids and colloids and that vasoactive drugs were administered more frequently when goal-directed fluid therapy was practiced, compared to conventional fluid therapy. The frequency of postoperative surgical complications and ICU length of stay did not differ significantly between the two groups.
Goal-directed fluid therapy has been thoroughly investigated in both abdominal and heart surgery, where it has been shown to improve patient outcome by reducing postoperative complications and length of hospital stay compared to traditional fluid management. The experience of goal-directed fluid therapy on head and neck free tissue transfer patients is still very limited and we believe that our study is of importance to improve perioperative care in this patient population.
We confirm that this manuscript has not been published elsewhere and is not under consideration by any other journal.
Thank you for receiving our manuscript and considering it for review. We appreciate your time and look forward to your response.
Ethical considerations
In all medical research, the integrity and safety of the patients participating must be a priority. The basics in medical ethics constitutes of four moral principles: autonomy, beneficence, nonmaleficence and justice.
In retrospective studies, it might be difficult to fulfil the autonomy principle that involves informed consent, respecting privacy and confidentiality. Informed consent might be impossible to collect retrospectively due to a high rate of mortality in the patient population or difficulties in tracing individuals. However, it would be
regrettable not to use the huge amount of information that is collected in the hospitals’ medical records for medical research in order to improve patient care. Even if the patients participating in the study will not benefit from the research themselves, it hopefully helps to improve treatment of future patients. Saying this, it is still of utmost importance to protect patient confidentiality by anonymizing the patients. This means assigning each patient a study number in the data capture sheet or database and keep the study code in a separate file and restrict the access to this information to members of the research team. The results will be presented at a group level so that no patient can identify themselves reading the manuscript.
The principles of nonmaleficence and justice are more easily fulfilled in retrospective reviews. There is no intervention that can cause adverse effects, and since the data are extracted from already existing charts there is no interference in patients’ personal lives. Furthermore, there is no risk of discrimination between the study groups since it is an observational study.
