Myocardial sparing effect of fentanyl during halothane anaesthesia in dogs

The cardiovascular effects of fentanyl (Sublimaze) were studied in seven mongrel dogs. A constant infusion of the drug was used to produce an apnoeic end-point while concomitant cardiovascular changes were monitored. After one hour the infusion was repeated while ventilation was controlled and atropine was given to eliminate the haemodynamic effect of changes in PaCO2 and heart rate. When PaCO2 was allowed to increase fentanyl reduced heart rate and mean arterial pressure markedly while left ventricular dp/dt was depressed moderately at the time of respiratory arrest. When heart rate and PaCO2 were kept constant there was little variation in the cardiovascular measurements during fentanyl infusion. We conclude that fentanyl allows relative myocardial sparing in dogs when used at clinically effective concentrations.