The effect of fentanyl on immobility after noxious stimulation in isoflurane-anaesthetized pigs: Exploring the role of the serotonergic system.

OBJECTIVE

To investigate if fentanyl induces immobility through activation of the serotonergic 5HT1A receptor, by using the 5HT1A-antagonist robalzotan.

STUDY

DESIGN

A prospective, blinded, randomized, two-group study.

ANIMALS: A group of 12 mixed-breed pigs aged 71-79 days.

METHODS

The motor response to clamping a claw was assessed in isoflurane-anaesthetized pigs at baseline, then fentanyl was infused intravenously (IV) for 40 minutes and clamping was repeated. The infusion started at 20 μg kg-1 hour-1 and was increased by 60% until fentanyl produced immobility, defined as no motor response for 60 seconds. Subsequently, either robalzotan (1 mg kg-1) or the same volume of saline was injected IV and clamping was repeated. The change in response was compared with Fisher's exact test. Mean arterial blood pressure (MAP) and heart rate (HR) were extracted for 2 minutes before and after 60 seconds of clamping, and the differences compared with a Wilcoxon signed-rank test. Dynamic respiratory compliance was calculated at baseline and after fentanyl; p < 0.05.

RESULTS

Baseline clamping produced a motor response within 5 seconds. This was abolished by fentanyl. Robalzotan or saline did not alter this (p = 0.45). As a response to clamping, MAP and HR changed with median (range) -0.5 (-4.4 to 22.2) mmHg and -1 (-7 to 1.5), respectively, where HR changed significantly (p = 0.039). The 95% confidence interval for the effect size of fentanyl upon dynamic compliance was -3.25 to -1.65 mL cmH2O-1.

CONCLUSIONS AND CLINICAL RELEVANCE: No indication was found for the 5HT1A receptor to be involved in fentanyl-induced reduction of the motor response to claw clamping. The decreased compliance after fentanyl could suggest onset of chest wall rigidity.