Cardiovascular effects of a low and a high dose of fentanyl in the isoflurane anesthetized dog: the influence of the anesthetic-sparing effect and the correction of bradycardia
Williamson, Allan John
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Fentanyl has historically been used to reduce inhalant anesthetic requirements in the dog, with the end goal of reducing detrimental cardiovascular effects seen with their use. While fentanyl has been investigated in this context with the older agent enflurane, this agent is no longer in common use. In the current literature, no studies exist that compare the effects of low and high doses of fentanyl on cardiovascular function in dogs anesthetized with isoflurane. In previous literature, a high dose of fentanyl improved cardiovascular function in enflurane anesthetized dogs only following correction of bradycardia associated with its use. The objective of this study was to evaluate the effect of two doses of fentanyl on isoflurane requirement in the dog, followed by an evaluation of cardiovascular function in the isoflurane-anesthetized dog at equivalent depth of anesthesia. The hypothesis was that fentanyl would reduce inhalant requirements in a dose dependent fashion, and that cardiovascular function would increase with fentanyl administration only following correction of bradycardia. A total of 8 healthy adult male beagle dogs were enrolled in this study, which was performed in a randomized cross-over design. Minimum Alveolar Concentration (MAC) was determined in these dogs via a 30 mA electric stimulation both before and after administration of a low (loading dose 30 µg/kg, continuous rate infusion (CRI) of 0.2 µg/kg/minute) or high (loading dose 90 µg/kg, CRI 0.8 µg/kg/min) dose of fentanyl. A 7-day washout was observed between experimental days. Following MAC determination, in a subsequent anesthetic episode animals were placed at a MAC multiple of 1.3 and cardiovascular and blood gas parameters were evaluated before and after each fentanyl dose in the presence and absence of bradycardia. Fentanyl decreased MAC in a dose-dependent fashion (p < 0.001), with the low dose reducing MAC by about 42% and the high dose by about 77%. MAC reduction, however, did not translate into improvement in cardiovascular function, with a significant reduction in cardiac index and oxygen delivery noted with both doses (p < 0.01) that was not different between treatments. Normal mean arterial pressures were maintained with both treatments despite these effects. Only with the high dose, however, correction of bradycardia caused an increase in both cardiac index and oxygen delivery (p < 0.02) when compared to isoflurane alone. In clinically healthy dogs, administration of a high dose of fentanyl increased cardiac function following correction of bradycardia, but a decrease was observed when bradycardia went uncorrected. Further studies are needed in order to evaluate these effects in clinical patients.
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