Effect of fentanyl on cardiac automaticity and conduction: direct or mediated action?

The effects of fentanyl at a dose of 100 micrograms.kg-1 on cardiac automaticity, refractoriness and conduction have been studied in a model of heterotopic heart transplantation which combines a denervated heart (donor) and a non-denervated heart (recipient) in each of 20 dogs employed. Once the surgical procedure had been completed, cycle length, sinoatrial conduction time, antegrade and retrograde A-V node block points, cycle length at which 1:1 conduction ceases to exist and the antegrade effective refractory period of the A-V node and ventricles were measured. These parameters were determined in hearts both in the basal situation and 10 min after fentanyl injection. In the recipient organs, fentanyl produced statistically significant prolongation of cycle length, sinoatrial conduction time, antegrade block point and antegrade effective refractory periods of the A-V node and ventricles, with respect to the basal situation (P less than 0.01 for all these parameters); in 20% of cases, a complete A-V block was produced. In the donor hearts (denervated), prolongations of cycle length (P less than 0.01) and the antegrade block point (P less than 0.05), as compared with the basal situation, were recorded, but no other modifications were detected. To test whether these effects were due to the action of fentanyl, 10 animals were subsequently injected with naloxone, an agent which reverted the reported changes, although in the recipient organs, there persisted a slight prolongation of the cycle length with respect to basal measurements (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)