Low ventilatory response to carbon dioxide not associated with increased depression by morphine

Ventilation (Ve), end-tidal (Pe' CO2), mixed venous PCO2 (PVCO2) and the ventilatory response to carbon dioxide (deltaVE/deltaPCO2) were measured before and within 90 min after morphine 0.15 mg kg-1 i.m. given to 17 adult patients undergoing elective surgery under general anaesthesia. The hypothesis that patients with a low ventilatory response to carbon dioxide are more susceptible to the ventilatory depressant effects of morphine was tested. Morphine induced increases in PE'CO2 and PVCO2 were not correlated with either the slope or the position of hte preinjection response to carbon dioxide. Mean delta Ve/deltaPCO2 was depressed after morphine (P less than 0.05), but individual responses varied widely. Seven patients whose control delta Ve/PCO2 was 9.9 litre min-1 kPa-1 or less decreased delta Ve/deltaPCO2 after morphine. In four patients, delta Ve/deltaPCO2 increased after morphine; however, in each case, PE'CO2 and PVCO2 increased also. Morphine displaced the carbon dioxide response to the right (P less than 0.001) but no correlation was found betwen either the magnitude of the displacement or change in slope and control delta Ve/deltaPCO2. The results suggest that patients with a low value for delta Ve/deltaPCO2 are not more susceptible to the ventilatory depressant action of morphine.