Experimental pain stimulates respiration and attenuates morphine-induced respiratory depression: a controlled study in human volunteers

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Abstract
Although evidence is lacking, clinical experience suggests that pain stimulation acts as a respiratory stimulant and antagonises opioid-induced respiratory depression. The present study examined the effects of experimental pain stimulation on the ventilatory response to CO2 and morphine-induced respiratory depression. Pain was induced by a modification of the Tourniquet Pain Technique and changes in ventilatory parameters were registered through monitoring of the CO2 response of tidal volume, minute ventilation, respiratory rate and mouth occlusion pressure. The ventilatory parameters were obtained before and during pain stimulation and repeated after the administration of morphine and finally after naloxone. In the present investigation experimental pain-stimulated respiration and attenuated morphine-induced respiratory depression. Only changes in the intercept values of the CO2 response were observed. The slopes of the CO2-response curves were not affected. These observations suggested that both pain stimulation and morphine administration altered the threshold of the respiratory centre to CO2 stimulation. Naloxone administration was the only intervention altering the sensitivity of the respiratory centre to CO2. These results suggest that pain stimulates respiration and attenuates morphine-induced respiratory depression.