Increasing maximal heart rate increases maximal O2 uptake in rats acclimatized to simulated altitude

Maximal exercise heart rate (HRmax) is reduced after acclimatization to hypobaric hypoxia. The low HRmax contributes to reduce maximal cardiac output ((Q) over dot (max)) and may limit maximal O-2 uptake ((V) over dot O-2max) The objective of these experiments was to test the hypothesis that the reduction in (Q) over dot (max) after acclimatization to hypoxia, due, in part, to the low HRmax, limits (V) over dot O-2max. If this hypothesis is correct, an increase in (Q) over dot (max) would result in a proportionate increase in (V) over dot O-2max. Rats acclimatized to hypobaric hypoxia [inspired PO2 (PIO2) = 69.8 +/- 3 Torr for 3 wk] exercised on a treadmill in hypoxic (PIO2 = 71.7 +/- 1.1 Torr) or normoxic conditions (PIO2 = 142.1 +/- 1.1 Torr). Each rat ran twice: in one bout the rat was allowed to reach its spontaneous HRmax, which was 505 +/- 7 and 501 +/- 5 beats/min in hypoxic and normoxic exercise, respectively; in the other exercise bout, HRmax was increased by 20% to the preacclimatization value of 600 beats/min by atrial pacing. This resulted in an similar to 10% increase in (Q) over dot (max), since the increase in HRmax was offset by a 10% decrease in stroke volume, probably due to shortening of diastolic filling time. The increase in (Q) over dot (max) was accompanied by a proportionate increase in maximal rate of convective O-2 delivery ((Q) over dot (max) x arterial O-2 content), maximal work rate, and (V) over dot O-2max in hypoxic and normoxic exercise. The data show that increasing HRmax to preacclimatization levels increases (V) over dot O-2max, supporting the hypothesis that the low HRmax tends to limit (V) over dot O-2max after acclimatization to hypoxia.