Dissociation between the effects of fentanyl and alfentanil on spontaneous and reflexly evoked cardiovascular responses in the dog.

Observations were made for 3 h in 10 anaesthetized dogs on the effects of alfentanil 500 micrograms kg-1 and fentanyl 100 micrograms kg-1 on resting heart rate (HR) and mean arterial pressure (MAP), and on the changes evoked in heart rate and mean arterial pressure (delta HR and delta MAP) by stimulation of a cutaneous nerve. Both drugs caused similar decreases in the resting HR (43-44%) and MAP (28-31%), which returned to baseline within 180 min following alfentanil but not fentanyl. Fentanyl decreased the somato-cardiovascular reflexes by 73% and 82% compared with a significantly smaller reduction of 54% and 55% (P less than 0.05) with alfentanil. Following alfentanil, delta MAP had recovered by 15 min and delta HR by 70 min compared with 70 and 90 min respectively for fentanyl. In conclusion, there was a dissociation between the maximum effect of alfentanil and fentanyl on the resting circulation and on the evoked cardiovascular reflexes. With both drugs a dissociation was observed between the durations of their effect on the resting cardiovascular system and on the evoked cardiovascular responses.