Chronic intermittent hypercapnic hypoxia increases pulmonary arterial pressure and haematocrit in rats.

Sleep-disordered breathing is associated with pulmonary hypertension and raised haematocrit. The multiple episodes of apnoea in this condition cause chronic intermittent hypoxia and hypercapnia but the effects of such blood gas changes on pulmonary pressure or haematocrit are unknown. The present investigation tests the hypothesis that chronic intermittent hypercapnic hypoxia causes increased pulmonary arterial pressure and erythropoiesis. Rats were treated with alternating periods of normoxia and hypercapnic hypoxia every 30 s for 8 h per day for 5 days per week for 5 weeks, as a model of the intermittent blood gas changes which occur in sleep-disordered breathing in humans. Haematocrit, red blood cell count and haemoglobin concentration were measured each week and systemic and pulmonary arterial blood pressure and heart weight were measured after 5 weeks. In relation to control, chronic intermittent hypercapnic hypoxia caused a significant increase in systemic (104.3+/-4.7 mmHg versus 121.0+/-10.4 mmHg) and pulmonary arterial pressure (20.7+/-6.8 mmHg versus 31.3+/-7.2 mmHg), right ventricular weight (expressed as ratios) and haematocrit (45.2+/-1.0% versus 51.5+/-1.5%). It is concluded that the pulmonary hypertension and elevated haematocrit associated with sleep-disordered breathing is caused by chronic intermittent hypercapnic hypoxia.