Intermittent vs continuous hypoxia: effects on ventilation and erythropoiesis in humans

Recently, we showed that 5 days of normobaric intermittent hypoxia at rest (IH; 2 hours daily at 3,800 m simulated altitude; partial pressure of inspired oxygen 90 torr) can induce an increase in the isocapnic hypoxic ventilatory response (HVR) and blood reticulocyte count. The purpose of the present study was to compare these data with continuous exposure to the same hypoxic level.

Four of the same subjects were exposed, a year later, to 2 days of continuous hypoxia (CH), and 4 different subjects were exposed to 8 weeks of CH, both at the White Mountain Research Station (3,800 m altitude, barometric pressure approximately 489 torr). Inspired minute ventilation (VI), end-tidal partial pressure of carbon dioxide, arterial oxygen saturation (SaO2[sat]), hematocrit, and hemoglobin concentration were measured at different times during the continuous exposures. The HVR was expressed as the increase in V1 per 1% decrease in SaO2.

The HVR showed no significant difference in the control values 1 year apart (IH, 0.06 +/- 0.03; CH2d (2 days' continuous hypoxia), 0.19 +/- 0.07 L x min(-1) x %sat(-1); means +/- SE), and the HVR values were similar after 2 days of IH compared to CH (0.42 +/- 0.26 and 0.51 +/- 0.22 L x min(-1) x %sat(-1), respectively). On the new subjects after 2 weeks of CH, the HVR showed a maximum increase, similar to the increase observed after only 5 days of IH, hemoglobin concentrations and hematocrit were significantly increased (45.0 +/- 2.7% vs 51.5 +/- 3.0% and 14.5 +/- 0.7 vs 17.2 +/- 1.0 g x dL(-1), respectively). The HVR did not change significantly from week 2 to 8 of CH, whereas hematological data were still increasing at the end of the 8 weeks.

Changes in ventilatory oxygen sensitivity induced by IH and CH are similar in magnitude but occur with different time courses. The effects of IH on erythropoiesis are significant but fewer than on CH.