Effects of fentanyl, nitrous oxide, or both, on baroreceptor reflex regulation in the cat

We have investigated the effects of fentanyl, nitrous oxide, or both, on carotid sinus baroreceptor reflexes in cats during basal chloralose anaesthesia. The bilaterally isolated carotid sinuses were perfused at prevailing systemic arterial pressure or at predetermined levels of pump-controlled pulsatile pressures of 50-200 mm Hg in steps of 25 mm Hg. Other major baroreceptor sites were denervated by bilateral vagotomy. Fentanyl decreased arterial pressure dose-dependently when the carotid sinuses were perfused at prevailing systemic arterial pressure and when the perfusion pressure was controlled artificially. High-dose fentanyl reduced significantly baroreceptor reflex responses in the sinus perfusion pressure range 50-125 mm Hg. Nitrous oxide increased arterial pressure in the carotid sinus perfusion range 75-125 mm Hg. There was no interaction between nitrous oxide and fentanyl for baroreceptor reflex responses. Our results indicated that baroreceptor reflexes, with and without nitrous oxide, were well preserved by moderate doses of fentanyl while high doses of fentanyl depressed baroreceptor reflexes.