Cardiopulmonary effects of fentanyl in conscious dogs and dogs sedated with a continuous rate infusion of medetomidine.

OBJECTIVE: To determine the hemodynamic consequences of the coadministration of a continuous rate infusion (CRI) of medetomidine with a fentanyl bolus in dogs. ANIMALS: 12 healthy sexually intact male dogs weighing 30.3 -/+ 4.2 kg (mean +/- SD). PROCEDURE: Dogs received either fentanyl alone (15.0 microg/kg, i.v. bolus) or the same dose of fentanyl during an 11-hour CRI of medetomidine (1.5 microg/kg/h, i.v.). Prior to drug administration, dogs were instrumented for measurement of cardiac output, left atrial pressure, and systemic arterial blood pressures. Additionally, blood samples were collected from the pulmonary artery and left atrium for blood gas analysis.
RESULTS: Medetomidine infusion reduced the cardiac index, heart rate, and O2, delivery while increasing left atrial pressure. Subsequent fentanyl administration further decreased the cardiac index. The Pao2 was not significantly different between the 2 treatment groups; however, fentanyl transiently decreased Pao2 from baseline values in dogs receiving a CRI of medetomidine. CONCLUSIONS AND CLINICAL RELEVANCE: Because of the prolonged hemodynamic changes associated with the CRI of medetomidine, its safety should be further evaluated before being clinically implemented in dogs.