Arterial Oxygen Saturation Variability Increases During Stepwise Steady State Inspired Hypoxia

BACKGROUND: The magnitude of the hypoxic ventilatory response (HVR) elicited during exposure to hypoxia is highly variable between individuals. This may result in high variability in the pressure of arterial oxygen (PaO2) and O2 saturation (SaO2), particularly with more extreme hypoxia. The inter-individual variability in PaO2 and SaO2 during stepwise inspired hypoxia has not been well-characterized. PURPOSE: To assess the inter-individual variability in alveolar ventilation (V̇A), PaO2, and SaO2 during incremental stepwise steady-state inspired hypoxia at rest. METHODS: 28 healthy participants (10F, 18 M) were exposed to five stepwise plateaus of reduced inspired O2 with incremental reductions in the fraction of inspired O2 (FIO2) during supine rest, including 0.21 (ambient air), 0.18, 0.16, 0.14, and 0.12. Five serial blood samples were drawn through an indwelling radial arterial catheter over two minutes during each inspired O2 step, and V̇A (calculated from PaCO2), PaO2 and SaO2 values were averaged within each FIO2 stage for each participant. Levene’s tests were used to assess the differences in the variances across hypoxic stages (α = 0.05). RESULTS: Variances in V̇A and PaO2 were unchanged with hypoxia (p = 0.984 and p = 0.327, respectively), yet SaO2 variances were significantly different between FIO2 stages (F(4,135) = 11.91, p < 0.001), with increases in variance as FIO2 levels decreased from ambient air: 0.9, 2.7, 7.4, 19.1, and 37.1 SaO2% for FIO2 levels of 0.21, 0.18 (p = 0.148), 0.16 (p = 0.012), 0.14 (p = 0.01), and 0.12 (p = 0.009), respectively (Figure 1). CONCLUSIONS: The inter-individual variability in SaO2 responses increased with decreased levels of FIO2 at rest, despite unchanged variability in V̇A and PaO2. These data highlight the inherent inter-individual variability of integrated cardiorespiratory and blood gas responses during the physiological transport and uptake of O2 during exposure to acute stepwise hypoxia in healthy humans.NSERC Discovery Grant