Effects of morphine on the ventilatory response to exercise

The total (steady state) ventilatory response to exercise involves fast and slow components for which both non chemoreceptor and chemoreceptor related mechanisms have been proposed. The authors studied the effects of morphine on the ventilatory response to exercise because its depressant effect on O₂ and CO₂ chemoreception provided a means of selective evaluation of these mechanisms. In 6 subjects ventilatory responses to CO₂ (rebreathing method), hypoxia (isocapneic steady state method, calculated as parameter 'A') and 2 levels of treadmill exercise, were determined before and after administration of 15 mg morphine. This dosage resulted in significant reductions of responses to CO₂ (from 3.63 to 1.76 l/min/mmHg) and hypoxia (from 122 to 52.8 lmmHg/min) in all subjects. Steady state ventilation for a given level of work was always reduced following morphine (av: to 76% of control) but this was accompanied by an equivalent reduction in O₂ consumption so that the relationship between ventilation and O₂ consumption was unchanged. The fast component (<10 sec) of the response to exercise was unaffected by morphine but the transitional response, calculated as the ratio of mean ventilation between 10 and 60 sec to the steady state ventilation was reduced (av: from .90 to .78) in each study. It is concluded that (1) morphine reduces the O₂ cost of treadmill exercise; (2) both the initial ventilatory response and the linkage between ventilation and metabolic rate in the steady state are independent of the functional integrity of the O₂ and CO₂ chemoreceptors; however, (3) the rate at which steady ventilation is reached may be chemoreceptor dependent.