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1 Department of Medicine, University Hospitals of Cleveland, Cleveland, OH, USA
2 Department of Medicine, Case Western Reserve University, Cleveland, OH, USA
3 Department of Pathobiology, and Pulmonary and Critical Care Medicine, Cleveland Clinic Foundation, Cleveland, OH, USA
4 Medicine, Louis Stokes Cleveland DVA Medical Center, Cleveland, OH, USA
5 Department of Anthropology, Case Western Reserve University, Cleveland, OH, USA
* To whom correspondence should be addressed. E-mail: bdh6{at}cwru.edu.
When oxygen availability is reduced unavoidably as it is at high altitude, a potential mechanism to improve oxygen delivery to tissues is to increase 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 men and 37 women, healthy, non-smoking, native residents at 4200m (13,900'), with an average oxygen saturation of hemoglobin of 85%, participated. The geometric mean partial pressure of NO exhaled at a flow of 17 mL/s was 23.4 nm Hg, significantly lower than 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 vasodilating the pulmonary blood vessels. The mean pulmonary blood flow (measured by cardiac index) was 2.7 ± 0.1 (SEM) L/min and the mean pulmonary artery systolic pressure was 31.4 ± 0.9 (SEM) mmHg. Higher exhaled NO levels were 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 4200m to compensate for ambient hypoxia with higher pulmonary blood flow and oxygen delivery without incurring the costs of higher pulmonary artery pressure.
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