The influence of PaO2, pH and SaO2 on maximal oxygen uptake

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Abstract
Influence of arterial oxygen pressure (PaO2) and pH on haemoglobin saturation (SaO2) and in turn on O2 uptake (VO2) was evaluated during ergometer rowing (156, 276 and 376 W; VO2max, 5.0 L min−1; n = 11). During low intensity exercise, neither pH nor SaO2 were affected significantly. In response to the higher work intensities, ventilations (VE) of 129 ± 10 and 155 ± 8 L min−1 enhanced the end tidal PO2 (PETO2) to the same extent (117 ± 2 mmHg), but PaO2 became reduced (from 102 ± 2 to 78 ± 2 and 81 ± 3 mmHg, respectively). As pH decreased during maximal exercise (7.14 ± 0.02 vs. 7.30 ± 0.02), SaO2 also became lower (92.9 ± 0.7 vs. 95.1 ± 0.1%) and arterial O2 content (CaO2) was 202 ± 3 mL L−1. An inspired O2 fraction (FIO2) of 0.30 (n = 8) did not affect VE, but increased PETO2 and PaO2 to 175 ± 4 and 164 ± 5 mmHg and the PETO2–PaO2 difference was reduced (21 ± 4 vs. 36 ± 4 mmHg). pH did not change when compared with normoxia and SaO2 remained within 1% of the level at rest in hyperoxia (99 ± 0.1%). Thus, CaO2 and VO2max increased to 212 ± 3 mL L−1 and 5.7 ± 0.2 L min−1, respectively. The reduced PaO2 became of importance for SaO2 when a low pH inhibited the affinity of O2 to haemoglobin. An increased FIO2 reduced the gradient over the alveolar-arterial membrane, maintained haemoglobin saturation despite the reduction in pH and resulted in increases of the arterial oxygen content and uptake.