Identify changes in blood oxygen and carbon dioxide content responsible for the observed phase I and II gas exchange characteristics at the start of moderate constant work rate (CWR) exercise, and identify the mechanisms accounting for the transient decrease in R at the start of phase II.
We studied 5 healthy, active young males (mean VO2/kg = 53 ml/kg/min) while performing moderate constant work rate exercise (80% LAT, WR = 139 W) on a cycle ergometer. Blood samples were obtained every 4 seconds during the exercise transition from the pulmonary artery (PA) and inferior vena cava (IVC). Samples were analyzed for PO2, PCO2, pH, oxygen saturation, and lactate. O2 and CO2 contents were calculated.
1) PA and IVC PO2 and oxygen saturations fell abruptly (within 4 seconds) at exercise onset, decreasing further at 20 seconds (s) and reaching their nadir by 60 s. 2) IVC oxygen saturation remained approximately 10-12% lower than PA throughout exercise with identical PO2’s. 3) IVC blood initially had a lower pH relative to PA. IVC blood then underwent alkalinization which reached a peak at 20 s, then became indistinguishable from PA pH. 4) PA and IVC PCO2 became stable after 120 s of exercise. 5) Lactate increase was very small (0.6 mEq/L to 1.4 mEq/L), the increase began at 20 s of exercise, and was complete by 60 s of exercise. There was no difference in PA and IVC lactate concentration.
Desaturated blood from the IVC reaches the lungs within 4 seconds of exercise (phase I) apparently due to rapid increase in venous return. Further desaturation related to muscle metabolism begins approximately 20 seconds into exercise (start of phase II).
The fall in R at the start of phase II is related to the more rapid time course of blood O2 content decrease relative to CO2 content increase from muscle metabolism, and IVC alkalinization which parallels maximal phosphocreatine hydrolysis.
W.W. Stringer, None.