Objective—To determine whether moderate
hypothermia during 4 hours of anesthesia with isoflurane
substantially affects serum concentrations of
transdermally administered fentanyl in the perianesthetic
period in cats.
Animals—7 healthy mature cats.
Procedure—A fentanyl patch (25 µg/h) was applied to
the shaved thorax 24 hours before induction of anesthesia.
Anesthesia was induced at time 0. Each cat
received 2 treatments in a random order. Treatments
were isoflurane anesthesia with normothermia and
isoflurane anesthesia with hypothermia. Cats were
intubated, connected to a nonrebreathing circuit, and
maintained at 1.3X minimum alveolar concentration
for 4 hours. Cats in the hypothermia treatment groups
were actively cooled to 35°C following the induction
of anesthesia. Serum fentanyl analysis was performed
at –24, –12, 0, 1, 2, 3, 4, 4.5, 5, 6, 7, 8, 9, 10,
12, and 24 hours.
Results—Mean ± SEM serum fentanyl concentration
(SFC) for the hypothermia treatment group (0.598 ±
0.3048 ng/mL) was significantly lower than the baseline
concentration (1.834 ± 0.6393 ng/mL) at 1 hour.
This significant reduction persisted for the duration of
anesthesia for the hypothermia treatment group.
Serum fentanyl concentrations returned to baseline
values within 1 hour of the end of anesthesia, regardless
of body temperature.
Conclusions and Clinical Relevance—Hypothermia
during inhalant anesthesia induced a significant
reduction in SFC obtained with transdermal administration.
The impact of this reduction in SFC on the
contribution of transdermally administered fentanyl to
any reduction in the need for inhalant anesthesia
remains to be determined. (Am J Vet Res 2003;64:1557–1561)
Procedure—Dogs received each of 4 treatments in random order. Following induction of anesthesia, normothermia was maintained in dogs that were treated with a fentanyl patch (F-NORM) or sham patch (C-NORM), or hypothermia was maintained in dogs that were treated with a fentanyl patch (F-HYPO) or sham patch (C-HYPO). The appropriate patch was applied 24 hours prior to induction of anesthesia. Anesthesia was induced with isoflurane in oxygen; the dogs were intubated and mechanically ventilated. Target esophageal temperatures were maintained within 1°C of baseline values (normothermia) or at 34.5°C (94.1°F; hypothermia) for 1 hour prior to starting MAC determinations. Supramaximal stimulation was achieved with an electrical stimulator attached to needle electrodes placed in the buccal mucosa of the lower jaw of the dog.
Results—Mean MAC ± SEM of isoflurane during C-NORM, C-HYPO, F-NORM, and F-HYPO treatments were 1.20 ± 0.17, 0.89 ± 0.18, 0.76 ± 0.10, and 0.81 ± 0.17, respectively. The mean MAC during C-NORM was significantly higher than values for the other treatments. There was no significant difference in mean MAC among the C-HYPO, F-NORM, and F-HYPO treatments.
Conclusions and Clinical Relevance—Data suggest that transdermal administration of fentanyl significantly reduces isoflurane requirements in normothermic dogs. The isoflurane MAC-sparing effects of transdermal fentanyl are not apparent in hypothermic dogs.
Objective—To compare plasma fentanyl concentrations
and analgesic efficacy during full or partial exposure
to 25-μg/h transdermal fentanyl patches (TFPs) in
cats undergoing ovariohysterectomy.
Design—Randomized controlled clinical trial.
Animals—16 client-owned cats.
Procedure—Cats were randomly assigned to receive
full or partial exposure to a TFP; patches were applied
approximately 24 hours prior to ovariohysterectomy.
Rectal temperature, heart rate, respiratory rate, blood
glucose concentration, and blood pressure were measured
and pain severity was assessed periodically for
72 hours after patch application. Venous blood samples
were collected for determination of plasma fentanyl
concentration 0, 6, 12, 18, 24, 36, 48, 60, and 72
hours after patch application.
Results—Mean ± SD steady state plasma fentanyl
concentration in cats in the full TFP exposure group
(1.78 ± 0.92 ng/mL) was significantly greater than concentration
in cats in the partial exposure group (1.14 ±
0.86 ng/mL). Steady state plasma fentanyl concentrations
were evident between 18 and 72 hours after
patch application. Subjective scores used to evaluate
analgesic efficacy were not significantly different
between treatment groups.
Conclusions and Clinical Relevance—Results suggest
that delivery of fentanyl from TFPs can be
reduced by decreasing the amount of exposed surface
area. In cats weighing < 4 kg (9 lb), exposure to half a
25-μg/h TFP appears to provide adequate analgesia following
ovariohysterectomy. (J Am Vet Med Assoc