Cardiac output, oxygen consumption and muscle oxygen delivery in submaximal exercise. Normal and low O2 states.

Cardiac output (Q) changes linearly with oxygen consumption (VO2) in normal subjects undertaking submaximal exercise (Q = A + B x VO2 where A is the y intercept and B the slope). If (hypothesis 1) the increase in cardiac output above the resting state represents the blood flow to exercising muscle (qm) and the increase in VO2 represents the oxygen consumption of exercising muscle (VO2m) then, where CaO2 is the arterial oxygen content, oxygen extraction, Em = 1/(B x CaO2). Secondly, exercising muscle venous oxygen content, CvO2m = CaO2 - 1/B. Limiting the hypothesis just to the calculation of VO2m (hypothesis 2) allows calculation of qm if CaO2 and CvO2m are available. From Koskolov et al. (Am. J. Physiol.: Heart and Circ. Physiol. 273, H1787-H1793, 1997), exercising muscle blood flow (qm) is equal to the increment in cardiac output when CaO2 is normal but exceeds it when CaO2 is low. Muscle Oxygen extraction (Em) is found to be 68% in submaximal exercise. Hence, muscle oxygen delivery (DaO2m) for a given metabolic rate is sustained in low O2 states (at 1.48 ml DaO2m per ml VO2m), confirmed by analysis of Roach et al. (Am. J. Physiol.: Heart and Circ. Physiol. 276, H438-H445, 1999).