THE CARDIOVASCULAR SPARING EFFECT OF FENTANYL AND ATROPINE, ADMINISTERED TO ENFLURANE ANESTHETIZED DOGS

Cardiovascular effects of high dose opioid together with low dose inha lant were compared with inhalant alone to determine whether opioid/inh alant techniques were less depressant on the cardiovascular system. Th e effects of positive pressure ventilation and increasing heart rate t o a more physiological level were also studied. Cardiovascular measure ments recorded during administration of enflurane at 1.3 minimum alveo lar concentration (MAC; 2.89 +/- 0.02%) to spontaneously breathing dog s (time 1) and during controlled ventilation [arterial carbon dioxide tension at 40 +/- 3 mmHg (time 2)] were similar. At time 2, mixed veno us oxygen tension and arterial and mixed venous carbon dioxide tension s were significantly decreased, while arterial and mixed venous pH wer e significantly increased compared to measurements at time 1. After ad ministration of fentanyl to achieve plasma fentanyl concentration of 7 1.7 +/- 14.4 ng/mL and reduction of enflurane concentration to yield 1 .3 MAC multiple (0.99 +/- 0.01%), heart rate significantly decreased, while mean arterial pressure, central venous pressure, stroke index, a nd systemic vascular resistance index increased compared to measuremen ts taken at times 1 and 2. Pulmonary arterial occlusion pressure was s ignificantly increased compared to measurements taken at time 2. After administration of atropine until heart rate was 93 +/- 5 beats/min (p lasma fentanyl concentration 64.5 +/- 13.5 ng/mL) heart rate, mean art erial pressure, cardiac index, oxygen delivery index, and venous admix ture increased significantly and oxygen utilization ratio decreased si gnificantly compared to values obtained at all other times. Central ve nous and pulmonary arterial occlusion pressures and systemic vascular resistance index were significantly decreased compared with measuremen ts taken prior to atropine administration, but were not significantly different to those recorded at times 1 and 2. We conclude that, in hea lthy dogs, an anesthetic technique utilizing fentanyl/enflurane offers considerable cardiovascular sparing compared to enflurane alone, prov ided an anticholinergic is administered to prevent bradycardia.