Effect of fentanyl on ventilatory resistances during barbiturate general anaesthesia

Fentanyl has been shown to increase the overall resistance to inspiratory flow of the ventilatory system (Rmax). Rmax is the sum of the airway resistance (Raw) and of the non-Newtonian resistance (delta R) which may result from the viscoelastic properties of the thoracic tissues, from inequalities of the regional time constants within the lung, or from both. A bronchoconstrictor challenge may increase the magnitude of variation in regional time constants. Thus, in order to describe the effect of fentanyl on the two components of Rmax, this study was performed, with the end-inflation occlusion method, during paralysis and mechanical ventilation in 10 normal men undergoing barbiturate anaesthesia for minor urological procedures. The patients were anaesthetized with methohexitone and paralysed with vecuronium. Before administration of fentanyl, delta R accounted for 56% of Rmax. Fentanyl 5 micrograms kg-1 elicited a significant increase in Rmax (+34.5%; P = 0.005) and a parallel increase in both Raw (+35.2%, P = 0.017) and delta R (+33.5%, P = 0.005). The increase in Raw, but not in delta R, was reversed by atropine, suggesting that the increase in these two components of Rmax was not linked. Thus fentanyl increased both components of Rmax, but the effects of fentanyl on Raw and delta R seemed to depend on different mechanisms.