Decrease in peak heart rate with acute hypoxia in relation to sea level V̇O_2max

The aim of this study was to evaluate the influence of arterial oxygen saturation (SaO₂) on maximal heart rate during maximal exercise under conditions of acute hypoxia compared with normoxia. Forty-six males were divided into three groups depending on their sea level maximal oxygen consumption (V̇O_2max): high [GH, V̇O_2max=64.2 (3.3) ml.min⁻¹.kg⁻¹], medium [GM, 50.8 (3.9) ml.min⁻¹.kg⁻¹] and low [GL, 41.0 (1.9) ml.min⁻¹.kg⁻¹]. All subjects performed a maximal exercise test in two conditions of inspired oxygen tension (PIO₂, (149 mmHg and 70 mmHg). Among the GM group, seven subjects performed five supplementary incremental exercise tests at PIO₂ 136, 118, 104, 92, and 80 mmHg. Measurements of V̇O_2max and SaO₂ using an ear-oxymeter were carried out at all levels of PIO₂. The decrease in SaO₂ and peak heart rate (HR_peak) with PIO₂ became significant from 104 and 92 mmHg. SaO₂ correlated with the decrease in HR_peak. For PIO₂=70 mmHg, the decrease in V̇O_2max, SaO₂ and HR_peak was, respectively, 44%, 62%, and 17.0 bpm for GH, 38%, 68%, and 14.7 bpm for GM, and 34%, 68%, and 11.8 bpm for GL. During maximal exercise in hypoxia, SaO₂ was lower for GH than GM and GL (p<0.01). Among subjects in GH, five presented exercise-induced hypoxemia (EIH) when exercising in normoxia. The EIH group exhibited a greater decrement in HR_peak than the non-EIH group at maximal hypoxic exercise (21.2 bpm vs. 15.0 bpm; p<0.05). When subjects are exposed to acute hypoxia, the lower SaO₂, due either to lower PIO₂ or to training status, is associated with lower HR_peak.