Impaired reflex vasoconstriction in chronically hypoxemic patients

Acute hypoxia impairs vasoconstrictor responses in normal men. The present study was done to determine whether reflex vasoconstriction is impaired in chronically hypoxemic patients and whether correction of hypoxemia in these patients improves their cardiovascular reflexes. In eight chronically hypoxemic patients, arterial P(O2) was increased from an average of 45 mm Hg while breathing room air to 161 mm Hg while breathing 40-100% oxygen, with minimal changes in arterial P(CO2) or pH. Correction of hypoxemia did not cause changes in resting arterial pressure or in forearm vascular resistance, but it caused a small increase in resting heart rate. Reflex responses to lower body negative pressure, which causes pooling of blood in the lower part of the body, were observed. When the patients were hypoxemic, lower body negative pressure caused a fall in arterial pressure, slight constriction of forearm vessels, and a small increase in heart rate. When hypoxemia was corrected, the same intervention caused marked vasoconstriction and a greater increase in heart rate, and there was no decrease in arterial pressure. The results indicate that reflex vasoconstrictor responses are depressed in chronic hypoxemia, indicating that adaptive mechanisms which occur in chronic hypoxemia do not include preservation of sympathetic reflexes.