THE EFFECT OF OPIOIDS ON INSPIRATORY MUSCLE FATIGUE DURING INSPIRATORY RESISTIVE LOADING

The effect of opioids on inspiratory muscle function under high mechan ical load is still unknown. Even less clear is the extent to which opi oids influence the shift of the electromyographic power spectrum of th e inspiratory muscles to lower frequencies during ventilatory stress. We studied seven healthy subjects breathing against high inspiratory t hreshold loads until exhaustion while keeping the minute ventilation c onstantly high. We compared runs with and without administration of 0. 2 mg kg-1 of morphine sulphate intramuscularly; two subjects were give n 30 mg morphine sulphate so that we could study the effect of higher opioid concentration. The endurance time (T(lim)), the diagraphragmati c electromyogram (EMG), the transdiaphragmatic pressures (Pdi) and the ventilatory effort sensation were analyses. Morphine did not have any effect on T(lim) or on the effort sensation elicited by the inspirato ry resistance in both concentrations. Analysing the spectral shifts of the diaphragmatic EMG, we did not find any significant difference in the decrease of the centroid frequency between drug and control runs. Furthermore, the activation pattern of the diaphragm and the intercost al muscles. evaluated from the percentage contribution of oesophageal and gastric pressures on the transdiaphragmatic pressure swings, did n ot change following the administration of morphine. Our study shows th at morphine does not change the function of the inspiratory muscles du ring high-resistive breathing. Morphine does not affect the electromyo graphic power spectrum of the diaphragm during those resistive breathi ng runs, either. This points out that during stressful ventilatory sit uations, the shift of the electromyographic power spectrum is attribut ed to a peripheral (muscular) event consequent to muscle fatigue and n ot to the elaboration of endogenous opioids.