Pulmonary capillary pressures during hypoxia and hypoxemia: experimental and clinical studies.

Unlike the systemic circulation, the pulmonary vasculature constricts in response to hypoxia to divert blood flow to better-ventilated segments. The site of this response, the hypoxic pulmonary vasoconstriction, has been reported as precapillary in numerous experimental models of isolated animal lungs. In the present study, the response of intact chest dog and human lungs to hypoxia and hypoxemia, respectively, was also precapillary vasoconstriction. In dogs, hypoxia in the ipsilateral lung attenuated the normal vertical blood flow gradient. Contralateral hypoxia did not alter pulmonary regional blood flow, precapillary (Ra), postcapillary, or total pulmonary vascular resistance. In patients, an elevated alveolar-arterial oxygen pressure gradient of 50 to 150 torr resulted in significantly increased Ra. Further hypoxemia did not increase this response. In addition, the effective pulmonary capillary pressure did not bear a constant relationship to the pulmonary artery occlusion or wedge pressure (WP). Therefore, in patients in respiratory failure, WP does not reliably estimate hydrostatic pressure at the pulmonary capillaries.