Effects of differences of oxygen affinity on circulatory response to hypoxia

Studies were made on eighteen male patients with chronic obstructive pulmonary disease (COPD) to investigate whether differences of P50 affect the circulatory response to acute hypoxia as a model of acute exacerbation. Subjects were divided into two groups according to low (less than 26.6 torr) or high (greater than 26.6 torr) P50. Isocapnic hypoxia was induced progressively (final PaO2 = 45 torr) and maintained for 10 minutes. Blood gases and hemodynamic parameters were measured before and after hypoxia. Results before and after hypoxia and results between groups were compared. The low P50 group includes eleven subjects with a mean P50 of 25.8 +/- 0.2 torr. The high P50 group includes seven subjects with a mean P50 of 27.5 +/- 0.2 torr. Comparison between groups showed no significant differences. In the high P50 group cardiac output and heart rate increased, whereas cardiac output did not change in the low P50 group. We conclude that circulatory response to hypoxia is well preserved in the high P50 group but deteriorated in the low P50 group, and this suggests that patients with low P50 cannot compensate adequately to hypoxia during acute exacerbation.