Modelling study of the acute cardiovascular response to hypocapnic hypoxia in healthy and anaemic subjects.

The present study analyses the cardiovascular response to acute hypocapnic hypoxia (simulating the effect of respiration at high altitude) both in healthy, unacclimatised subjects and in subjects with moderate anaemia, by means of a mathematical model of short-term cardiovascular regulation. During severe hypoxia, cardiac output and heart rate (HR) exhibit a significant increase compared with the basal level (cardiac output: +90%; HR: +64%). Systemic arterial pressure remains quite constant or shows a mild increase. Coronary blood flow increases dramatically (+200%), thus maintaining a constant oxygen delivery to the heart. However, blood oxygen utilisation in the heart augments, to fulfil the increased power of the cardiac pump during hypoxia. Cerebral blood flow rises only at very severe hypoxia but, owing to the vasoconstrictory effect of hypocapnia, its increase (+80%) is insufficient to maintain oxygen delivery to the brain. The model suggests that a critical level for the aerobic metabolism in these organs (heart and brain) is reached at an oxygen partial pressure in arterial blood (PaO2) of approximately 25 mmHg. Moderate anaemia during normoxia is compensated by an increase in cardiac output (+22%), a decrease in total peripheral resistance (-30%) and an increase in O2 extraction from blood (+40%). As cardiovascular regulation mechanisms are already recruited in anaemic subjects at rest, their action soon becomes exhausted during hypocapnic hypoxia. Critical levels for vital functions are already reached at a PaO2 of approximately 45 mmHg.