Effects of chemical and mechanical stimulation on laryngeal motion during anaesthetic induction with alfaxalone, thiopentone or propofol in healthy dogs

Abstract

Objective: To compare the effect of chemical and mechanical stimulation on arytenoid cartilage motion in healthy dogs during anaesthetic induction with alfaxalone, thiopentone or propofol for induction of anaesthesia. Study design: Blinded, randomised, crossover study Animals: Eight adult beagle dogs with median (range) weight and age 15.2 (12.2-19.8) kg and 14 (13-16) months, respectively. Methods: Anaesthesia was induced with thiopentone (7.5 mg kg-1), propofol (3 mg kg-1) or alfaxalone (1.5 mg kg-1) intravenously (IV), which were concurrently paired with either chemical (doxapram hydrochloride at 2.5 mg kg-1 IV) or mechanical (gentle pressure to the corniculate process of the right arytenoid cartilage using a cotton bud) stimulation for enhanced assessment of laryngeal motion, in random order, with a one-week wash-out period between treatments. If deemed inadequately anaesthetised, supplemental boli (25% of induction bolus) of thiopentone (1.8 mg kg-1), propofol (0.75 mg kg-1) or alfaxalone (0.4 mg kg-1) were administered. The calculated induction bolus for each dog was administered over a 60 second period intravenously via a syringe driver and allowed to take effect for 10 seconds. The anaesthetic depth was determined by evaluating jaw tone and palpebral reflexes. Laryngeal examination was performed by the primary investigator (SL) who was blinded to the treatments. Assessment of number of arytenoid motions and vital breaths, among others, began immediately after induction. Chemical and mechanical stimulation were begun 2 minutes after anaesthetic induction (time period 1). Data were collected at 2, 3 and 5 minutes (time period 2) after anaesthetic induction and the Friedman rank sum or repeated measures ANOVA (Analysis of variance) tests were used, when applicable, for statistical analysis. Results: Duration of examination times were significantly different among treatments (p = 0.01). Significant differences were observed regarding the number of arytenoid motions during thiopentone induction combined with chemical stimulation (doxapram hydrochloride) in comparison to alfaxalone (p = 0.0086), thiopentone (p = 0.0108) and propofol (p = 0.0086), when combined with mechanical stimulation at 3 minutes after induction. The laryngeal function score was significantly higher during time period 1 compared to time period 2 for induction with alfaxalone (p = 0.0007), thiopentone (p < 0.0001), and propofol (p = 0.0013) combined with chemical stimulation. No significant differences were observed among treatments or time periods for number of vital breaths recorded during the 3 different time periods, jaw tone, laryngospasm, breath scores, swallowing score or paradoxical motion score. Conclusion and clinical relevance: Doxapram hydrochloride, combined with thiopentone, is the most effective means of stimulating arytenoid motion among the regimens for assessing laryngeal motion in the present study and could improve accuracy of diagnosis of laryngeal paralysis in dogs. Time period 2 (2-5 minute after conclusion of induction) is the optimal time period for laryngeal evaluation. Misdiagnosis of laryngeal paralysis can be avoided by identifying the ideal time period for evaluation. Induction with thiopentone combined with doxapram hydrochloride facilitated increased respiratory efforts, ample arytenoid motions, and adequate arytenoid exposure conducive to laryngeal function evaluation in healthy non-premedicated beagle dogs. The dosages and administration rates used in the present study were effective in achieving adequate depth of anaesthesia for laryngeal function evaluation, without inducing respiratory depression, confirming viable application for diagnosis of laryngeal paralysis.