Abrupt changes in mixed venous blood gas composition after the onset of exercise

Publisher Website
Google Scholar
Abstract
It has been assumed that increases in both O2 uptake and ventilation occurring within the first few seconds after the onset of exercise cannot be the result of changes in blood gas composition reaching the central circulation because of the circulatory delay from the exercising limbs (A. Krogh and J. Lindhard, J. Physiol. Lond. 42: 112-136, 1913). We sought to validate this assumption by measuring the time course of pulmonary arterial blood gases during the transition from rest to exercise. Six healthy men underwent pulmonary arterial catheterization and then performed transitions from rest to moderate cycle ergometer exercise. An anaerobic sampling manifold withdrew 19 samples of blood during the rest-to-exercise transition; sampling interval was usually 4 s. Blood gas analysis showed that, on average, from rest-to-steady-state exercise, O2 saturation (S.hivin.vO2) fell from 71 to 41% and mixed venous PCO2 (P.hivin.vco2) rose from 42 to 59 Torr. Contrary to our expectations, S.hivin.vo2 decreased and P.hivin.Vco2 increased with no discernible latency after exercise onset (by 10% and 2 Torr, respectively, within 6 s). The half time for the S.hivin.vo2 decrease was 32 s, whereas for the P.hivin.vco2 increase it was 80 s. The time course of superior vena cava blood gas composition was determined in several experiments; no rapid changes after exercise onset were found. We conclude that at exercise onset there is a rapid fall in S.hivin.vco2 well in advance of arrival of blood produced by exercising legs. We postulate that the mechanism of these rapid changes could possibly involve a surge of blood (formed at rest), which ascends from the inferior vena cava and which is more hypoxic and hypercapnic than resting superior venous return. These changes must be considered when interpreting ventilation and gas exchange immediated after exercise onset.