Changes in Rib Cage and Diaphragm Contribution to Ventilation after Morphine

Abstract

Ventilatory response to CO2 was measured before and after i.v. morphine, 0.15 mg/kg, in 13 healthy adult human subjects. Tidal volume response was partitioned into that component due to rib cage (RC) and that due to abdomen-diaphragm motion (AD). The response was assessed also in terms of inspiratory and expiratory durations (Ti, Tc), the ratio of Ti to total cycle duration (Ti/Ttot) and mean inspiratory flow (VT/Ti). The ventilatory response to CO2 was displaced to the right after morphine; this was due to a decrease in both the tidal volume and breathing frequency responses. The displacement of the tidal volume response was due largely to a similar displacement of the RC response; there was no significant change in the AD response. The decrease in the frequency response after morphine was manifest by increases in both inspiratory and expiratory durations at equivalent CO2 pressures during rebreathing; these increases occurred to equivalent extents such that no change of Ti/Ttot was found during rebreathing after morphine. Mean inspiratory flow per breath was significantly reduced during rebreathing after morphine. When RC, AD, Ti, Te, VT/Ti and Ti/Ttot during rebreathing were plotted as functions of tidal volume and expressed as a percent of vital capacity, no effect was found after morphine. The marked depression of the RC response and the relative stability of the AD response after such dissimilar ventilatory depressants as morphine and halothane may represent characteristic responses of the separate groups of intercostal and phrenic motoneurones to descending influences determined by the direct action of these drugs on respiratory neurons in the medulla.