Assessment of the respiratory actions of intramuscular morphine in conscious dogs

Abstract

The actions on the respiratory system of 0·25, 0·5 and 1·0 mg kg–1morphine given intramuscularly were studied in conscious dogs. Dogs breathed oxygen with 0, 2 and 4 per cent CO2, in that order, through a mask attached to a flow sensor and connected to a respiratory mechanics monitor. When a steady state period of respiration was reached breathing pure oxygen, respiratory rate, tidal volume, respiratory minute volume, peak expiratory flow rate and end tidal CO2(PetCO2) were measured. The respiratory minute volume and PetCO2were measured when the dogs breathed 2 and 4 per cent CO2in oxygen, the points plotted onto a graph and the gradient of the line, describing the PCO2/ventilation response, plus the intercept with the y-axis were determined. Measurements for each morphine dose were taken before injection and at 30 minutes, 1, 2, 3, 4, 6 and 8 hours post injection.The incidence of panting after morphine was dose related and it occurred in all dogs given the high dose. Morphine reduced the gradients of the PCO2/ventilation response lines and raised the intercept. Other changes were increased respiratory minute volume and peak expiratory flow and decreased PetCO2and tidal volume.