To examine the hypothesis that the human pulmonary vascular response to hypoxia has a component with a slow time course, we measured pulmonary vascular resistance (PVR) in six healthy adult males during 8 h of isocapnic hypoxia. A balloon-tipped pulmonary artery catheter with thermistor was introduced via a forearm vein and used to derive PVR. The subjects were seated in a chamber in which the oxygen and carbon dioxide concentrations were adjusted to maintain an end-tidal PO2 of 50 Torr and an end-tidal PCO2 equal to the subject's normal prehypoxic value. PVR was measured before and at 0.5-h intervals during 8 h of hypoxia, the following 3 h of isocapnic euoxia (end-tidal PO2 100 Torr), and a subsequent 1-h reexposure to hypoxia. PVR rose from 1.23 ± 0.26 (SE) Torr · min · l-1 under euoxia [time (t) = 0] to 1.77 ± 0.21 Torr · min · l-1 at t = 0.5 h, reached a maximum at 2 h (2.91 ± 0.33 Torr · min · l-1), and remained fairly constant between 2 and 8 h. Restoration of euoxia at 8 h led to a reduction in PVR with a slow component. Reexposure to hypoxia at 11 h resulted in a greater increase in PVR than at 1 h. Systemic vascular resistance had a similar slow component to its response, falling from 18.6 ± 1.3 Torr · min · l-1 at t = 0 to 17.3 ± 1.4 Torr · min · l-1 at t = 0.5 h, 14.4 ± 0.6 Torr · min · l-1 at t = 4 h, and 13.8 ± 0.8 Torr · min · l-1 at t = 8 h. The human pulmonary and systemic vascular responses to hypoxia extend over at least several hours.
American Journal of Physiology - Heart and Circulatory Physiology