Cardiac output and oxygen release during very high-intensity exercise performed until exhaustion

Our objectives were firstly, to study the patterns of the cardiac output (Q(.)) and the arteriovenous oxygen difference [(a-nu(-))O(2)] responses to oxygen uptake (V(.)O(2)) during constant workload exercise (CWE) performed above the respiratory compensation point (RCP), and secondly, to establish the relationships between their kinetics and the time to exhaustion. Nine subjects performed two tests: a maximal incremental exercise test (IET) to determine the maximal V(.)O(2) (V(.)O(2)peak), and a CWE test to exhaustion, performed at p Delta50 (intermediate power between RCP and V(.)O(2)peak). During CWE, V(.)O(2) was measured breath-by-breath, Q(.) was measured beat-by-beat with an impedance device, and blood lactate (LA) was sampled each minute. To calculate ( a-nu(-)O(2), the values of V(.)O(2) and Q(.) were synchronised over 10 s intervals. A fitting method was used to describe the V(.)O(2), Q(.) and ( a-nu(-))O(2) kinetics. The ( a-nu(-)O(2) difference followed a rapid monoexponential function, whereas both V(.)O(2) and Q(.) were best fitted by a single exponential plus linear increase: the time constant (tau) V(.)O(2) [57 (20 s)] was similar to tau ( a-nu(-)O(2), whereas tau for Q(.) was significantly higher [89 (34) s, P0.05] (values expressed as the mean and standard error). LA started to increase after 2 min CWE then increased rapidly, reaching a similar maximal value as that seen during the IET. During CWE, the rapid component of V(.)O(2) uptake was determined by a rapid and maximal ( a-nu(-)O(2) extraction coupled with a two-fold longer Q(.) increase. It is likely that lactic acidosis markedly increased oxygen availability, which when associated with the slow linear increase of Q(.), may account for the V(.)O(2) slow component. Time to exhaustion was larger in individuals with shorter time delay for ( a-nu(-)O(2) and a greater tau for Q(.).