The cardiovascular sparing effect of fentanyl and atropine, administered to enflurane anesthetized dogs

Cardiovascular effects of high dose opioid together with low dose inhalant were compared with inhalant alone to determine whether opioid/inhalant techniques were less depressant on the cardiovascular system. The effects of positive pressure ventilation and increasing heart rate to a more physiological level were also studied. Cardiovascular measurement recorded during administration of enflurane at 1.3 minimum alveolar concentration (MAC; 2.89 ± 0.02%) to spontaneously breathing dogs (time 1) and during controlled ventilation [arterial carbon dioxide tension at 40 ± 3 mmHg (time 2)] were similar. At time 2, mixed venous oxygen tension and arterial and mixed venous carbon dioxide tensions were significantly decreased, while arterial and mixed venous pH were significantly increased compared to measurements at time 1. After administration of fentanyl to achieve plasma fentanyl concentration of 71.7 ± 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, and systemic vascular resistance index increased compared to measurements taken at times 1 and 2. pulmonary arterial occlusion pressure was significantly increased compared to measurements taken at time 2. After administration of atropine until heart rate was 93 ± 5 beats/min (plasma fentanyl concentration 64.5 ± 13.5 ng/mL) heart rate, mean arterial pressure, cardiac index, oxygen delivery index, and venous admixture increased significantly and oxygen utilization ratio decreased significantly compared to values obtained at all other times. Central venous and pulmonary arterial occlusion pressures and systemic vascular resistance index were significantly decreased compared with measurements taken prior to atropine administration, but were not significantly different to those recorded at times 1 and 2. We conclude that, in healthy dogs, an anesthetic technique utilizing fentanyl/enflurane offers considerable cardiovascular sparing compared to enflurane alone, provided an anticholinergic is administered to prevent bradycardia.