The Pharmacokineties of Alfentanil in Healthy Cats

SUMMARY Alfentanil (50 μg/kg) was administered intravenously, over 1 minute, to 6 healthy cats. Blood samples were collected from a preplaced arterial catheter at 0.625, 1.25, 2.5, 5, 10, 15, 20, 30, 45, 60, 90, 120, 180, 240, 360, and 480 minutes after the end of the alfentanil injection. A radio-immunoassay technique was used to measure alfentanil concentrations in plasma obtained from these samples. Arterial blood pressure was measured at 0.625, 1.25, 2.5, 5, 10, 15, 20, and 30 minutes and pH and blood gas measurements were carried out at 5, 10 and 30 minutes after the alfentanil. Analgesia, tested by placing a clamp on the base of the tail for 5 seconds, was assessed at each blood sampling time until analgesia was absent. The cats were observed for behavioural changes at each sample time. Alfentanil caused a transient increase in blood pressure and respiratory and metabolic acidosis. Following alfentanil all cats became excited and showed pupillary dilation. Analgesia was present for 21.7 ± 14 minutes. The plasma concentration-time data for 5 cats were best described by a 3 compartment open model with the curve fitting the equation: C to = 162.6e −0.2062t + 60.le −0.041t + 13.2e −0.0062t The harmonic means for the half lives of the rapid distribution, the slow distribution and the elimination phases were 4.12, 18.8, 119.2 minutes respectively.