Oxygen delivery and oxygen return in humans exercising in acute normobarichypoxia

At a given steady O-2 consumption ((V)over dotO(2)) in normoxia, cardiac ou tput ((Q)over dot) is inversely proportional to arterial O-2 concentration (CaO2), so that O-2 delivery (Q)over dotO(2)=QCaO(2)) is kept constant and adapted to (V)over dotO(2). The matching between (Q)over dotaO(2) and (V)ov er dotO(2) keeps O-2 return ((Q)over dot (v) over bar aO(2)=(Q)over dotO(2) -VO2) constant and independent of (V)over dotO(2) and haemoglobin concentra tion ([Hb][). This may not be so in hypoxia: in order for (over dot)Q (v) o ver barO(2) to be independent of the inspired O-2 fractions (FIO2), the slo pes of the (Q)over dot versus (V)over dotO(2) lines should be greater the l ower the CaO2, which may not be the case. Thus, we tested the hypothesis of constant (Q)over dot (v) over barO(2) by determining (Q)over dotaO(2) and (Q)over dot (v) over barO(2) in acute hypoxia. Thirteen subjects performed steady-state submaximal exercise on the cycle ergometer at 30, 60, 90 and 1 20 W breathing FIO2 of 0.21, 0.16, 0.13, 0.11 and 0.09. (V)over dotO(2) was measured by a metabolic cart. (Q)over bot by CO2 rebreathing, [Hb] by a ph otometric technique and arterial O-2 saturation (SaO(2)) by infrared oximet ry. CaO2 was calculated from [Hb], SaO(2) and the O-2 binding coefficient o f haemoglobin. The (V)over dotO(2) versus power relation was independent of FIO2. The relations between (Q)over dot and (V)over dotO(2) were displaced upward and had higher slopes in hypoxia than in normoxia. However, the (Q) over dot changes did not compensate for those in CaO2. The slopes of the (Q )over dotaO(2) versus (V)over dotO(2) lines tended to decrease in hypoxia. (Q)over dot (v) over barO(2) was lower the lower the FIO2. A significant re lationship was found between Q (v) over barO(2) and SaO(2) (Q ($) over barO (2) = 1.442 SaO(2)+0.107, r=0.871, n=24, P < 10(-7)), which confutes the hy pothesis of constant (Q)over dotvO(2) in hypoxia.