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Nitric oxide and cardiopulmonary hemodynamics in Tibetan highlanders

¹Department of Medicine, University Hospitals of Cleveland and Case Western Reserve University; ³Departments of Pathobiology and Pulmonary and Critical Care Medicine, Cleveland Clinic Foundation; ⁴Medical Service, Louis Stokes Cleveland Department of Veterans Affairs Medical Center, Department of Medicine, and ⁵Department of Anthropology, Case Western Reserve University, Cleveland, Ohio; and ²Department of Medicine, University of California, San Diego, School of Medicine, San Diego, California

Submitted 22 February 2005 ; accepted in final form 12 July 2005

When O₂ availability is reduced unavoidably, as it is at high altitude, a potential mechanism to improve O₂ delivery to tissues is an increase in blood flow. Nitric oxide (NO) regulates blood vessel diameter and can influence blood flow. This field study of intrapopulation variation at high altitude tested the hypothesis that the level of exhaled NO (a summary measure of pulmonary synthesis, consumption, and transfer from cells in the airway) is directly proportional to pulmonary, and thus systemic, blood flow. Twenty Tibetan male and 37 female healthy, nonsmoking, native residents at 4,200 m (13,900 ft), with an average O₂ saturation of hemoglobin of 85%, participated in the study. The geometric mean partial pressure of NO exhaled at a flow of 17 ml/s was 23.4 nmHg, significantly lower than that of a sea-level reference group. However, the rate of NO transfer out of the airway wall was seven times higher than at sea level, which implied the potential for vasodilation of the pulmonary blood vessels. Mean pulmonary blood flow (measured by cardiac index) was 2.7 ± 0.1 (SE) l/min, and mean pulmonary artery systolic pressure was 31.4 ± 0.9 (SE) mmHg. Higher exhaled NO was associated with higher pulmonary blood flow; yet there was no associated increase in pulmonary artery systolic pressure. The results suggest that NO in the lung may play a key beneficial role in allowing Tibetans at 4,200 m to compensate for ambient hypoxia with higher pulmonary blood flow and O₂ delivery without the consequences of higher pulmonary arterial pressure.

high altitude; oxygen delivery; oxygen availability; hypoxia

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