Unexpected cardiovascular response
during ultra-endurance exercise
SUMMARY AND CONCLUSION
During ultra-endurance exercise the drift in
heart rate does not progress continuously, but
is instead reversed when exercise duration
exceeds 6 hours.
Concomitantly oxygen uptake increases,
leading to an increment in oxygen pulse,
which is explainable by elevated
arteriovenous oxygen difference.
INTRODUCTION
It is well established that during exercise at fixed work rate heart rate (HR) increases slowly with concomitant decrease in stroke volume (SV) in order to maintain cardiac output.
METHODS
Nine well-trained male athletes performed 24-h exercise in a controlled laboratory setting with altering blocks of kayaking, running and cycling. Each block consisted of 110 min of exercise and 10 min of rest.
Measurements of HR and VO2
were conducted during cycling
at fixed work rate every 6thhour.
The average work intensity was
55 % of respective VO2peak.
RESULTS 24-h
HR was, as expected, increased with 13 % compared to initial values, but thereafter the drift was reversed.
VO2was increased with 10 % at
6 h and 17 % at 12 h compared to initial values, and then remained stable.
This implies an increased
oxygen pulse (VO2/HR) with
approx. 10 % during the later half of the exercise.
PILOT STUDY
In a recent pilot study (4 males, 12-h exercise) cardiac output was measured using acetylene
rebreathing-method, with the purpose to evaluate if the increased oxygen pulse should be ascribed increased stroke volume (SV) and/or
arteriovenous oxygen difference.
RESULTS PILOT
HR, VO2and oxygen pulse
changed in similar ways as in the 24-h exercise. SV decreased at 4 h but then normalized. Hence, the increased oxygen pulse was due to increased a-v difference.
Björn Ekblom Professor
Åstrand Laboratory of Work Physiology phone: +46 8 16 14 53 GIH - The Swedish School of Sport and Health Sciences e-mail: Mikael.Mattsson@gih.se Stockholm, Sweden webb: www.gih.se/multisport
CM. Mattsson, JK. Enqvist, T. Brink-Elfegoun, PH. Johansson, L. Bakkman and B. Ekblom
Jonas Enqvist Research assistant C. Mikael Mattsson PhD-student 133 148 142 138 100 110 120 130 140 150 H R ( b p m ) 2,53 2,66 2,76 2,84 2,20 2,30 2,40 2,50 2,60 2,70 2,80 V O2 ( l/m in ) 0,00 0,31 0,35 0,38 0,22 0,00 0,10 0,20 0,30 0,40 0,50 0,60 D e lt a V O 2 ( l/ m in ) 1,9 2,4 1,1 -0,6 0,0 -1,0 0,0 1,0 2,0 3,0 4,0 5,0 0 6 12 18 24 D e lt a o x y g e n p u ls e (m l O 2 /H R ) 0 5 4 13 16 0 5 10 15 20 25 D e lt a H R ( b p m ) * * * * * * * * Sample time (h) Sample time (h) * * * * * * * Values: mean ± SD. * P < 0.05 vs initial values.
Values: mean. * P < 0.05 vs initial values.
123 126 124 135 115 120 125 130 135 0 4 8 12 C al cd . A -V d if f (m l O 2 /l b lo o d ) 154 144 156 152 135 140 145 150 155 C al cd . S V ( m l)
