COMPARISON OF PLASMA FENTANYL CONCENTRATIONS BY USING 3 TRANSDERMAL FENTANYL PATCH SIZES IN DOGS

Objective - To compare plasma fentanyl concentrations attained after t he application of three transdermal fentanyl patch sizes (50, 75, and 100 mu g/hour) in dogs. Design - Repeated Latin square controlled stud y. Animals - Six intact, mixed-breed adult dogs (2 males, 4 females) w eighing 19.9 +/- 3.4 kg. Methods - Each dog was randomly assigned to r eceive each of three treatments: 50 (P50), 75 (P75), or 100 (P100) mu g/hour transdermal patches. Patches were left in place for 72 hours. J ugular venous blood was collected at 1, 2, 4, 8, 12, 24, 36, 48, 60, a nd 72 hours after patch application and for 1, 2, 4, 8, and 12 hours a fter patch removal. Plasma fentanyl concentrations were measured using a radioimmunoassay technique. After a 96-hour washout period, each do g was moved to another treatment group and received a different patch size. Results - The following results were obtained (mean +/- SD): ave rage plasma fentanyl concentration from 24 to 72 hours, 0.7 +/- 0.2 ng /mL (P50), 1.4 +/- 0.5 ng/mL (P75), 1.2 +/- 0.5 ng/mL. (P100); the tot al area under the concentration versus time curve (0 hours to infinity ), 46 +/- 12.2 ng/h/ml (P50), 101.2 +/- 41.4 ng/h/ml (P75), 80.4 +/- 3 8.3 ng/h/ml (P100); and the apparent elimination half-life, 3.6 +/- 1. 2 hours (P50), 3.4 +/- 2.7 hours (P75), and 2.5 +/- 2.0 hours (P100). There was a high degree of variability in plasma fentanyl concentratio ns achieved. Plasma fentanyl concentrations declined rapidly after pat ch removal. Conclusions - The attainment of steady-state plasma concen trations takes up to 24 hours, and there is a great deal of variabilit y in the final concentrations reached in different individuals. In thi s study, the 100 mu g/hour patches did not provide statistically incre ased plasma concentrations when compared with the 50 mu g/hour patches . Clinical Relevance - Because of the interindividual and intraindivid ual variation in plasma fentanyl concentrations, patches should be app lied 24 hours before the anticipated time that analgesia will be requi red. Adequacy of analgesia and potentially deleterious side effects, s uch as sedation and respiratory depression, should be monitored while the patches are in place. Skin reactions may occur, and-the patches sh ould be removed if such skin irritation is seen. After the patch is re moved, it is expected that analgesia will wane rapidly because of the brief elimination half-life. (C) Copyright 1998 by The American Colleg e of Veterinary Surgeons.