Diminished Ventilatory Response to Hypoxia and Hypercapnia after Morphine in Normal Man

Abstract

Although morphine depresses respiration the mechanism of this depression remains unknown. Accordingly, ventilatory responses to hypoxia and to hypercapnia were measured before and after administration of 7.5 mg of morphine sulfate subcutaneously in six normal subjects. This procedure produced resting hypoventilation manifested as a peak rise in alveolar carbon dioxide tension from 42.9 ± 1.7 to 45.4 ± 1.5 mm Hg ( ±S.E.M.) at 30 minutes (p < 0.01). Hypoxic ventilatory drive, measured by an index of the relation between ventilation and hypoxia (parameter A), decreased from a control of 108 ± 17.6 to 42.8 ± 5.3 at 60 minutes after morphine (p < 0.01). Hypercapnic ventilatory drive, measured as the slope of the ventilatory response to hypercapnia, also decreased from 1.69 ± 0.24 to 0.98 ± 0.20 (p < 0.01) 75 minutes after morphine. Decreased responsiveness to the chemical stimuli to breathing may contribute to the ventilatory depression frequently seen after administration of morphine. (N Engl J Med 292:1103–1106, 1975)