Experimental study on the effects of isoflurane with and without remifentanil or dexmedetomidine on heart rate variability before and after nociceptive stimulation at different MAC multiples in cats

BACKGROUND: Heart rate variability (HRV) provides information about autonomic nervous system (ANS) activity and is therefore a possible tool with which to assess anaesthetic depth. The aim of the present study was to evaluate the effects of isoflurane, remifentanil and dexmedetomidine on HRV before and after nociceptive stimulation at different anaesthetic depths. Seven healthy domestic short-hair cats were used, and each cat was anaesthetized three times – group I with isoflurane alone, group IR with isoflurane and a constant rate infusion (CRI) of remifentanil (18 μg/kg/h), and group ID with isoflurane and a CRI of dexmedetomidine (3 μg/kg/h). Minimum alveolar concentration (MAC) values were determined via electrical supramaximal nociceptive stimulation for each treatment group. Nociceptive stimulation was repeated at 3 different MAC multiples (0.75, 1.0 and 1.5 MAC), and electrocardiographic recordings were performed for 3 min before and after stimulation. Only the 1 min epochs were used for further statistical analysis. Electrocardiographic data were exported for offline HRV analysis. RESULTS: The mean isoflurane MAC ± standard deviation (SD) was 1.83 ± 0.22 vol% in group I, 1.65 ± 0.13 vol% in group IR and 0.82 ± 0.20 vol% in group ID. Nociception was indicated by several HRV parameters, however, with high variability between treatments. The best correlation with MAC was found for the SD of heart rate (STD HR) in group I (rₛ = − 0.76, p = 0.0001, r² = 0.46). STD HR was also able to distinguish 0.75 MAC from 1.5 MAC and 1.0 MAC from 1.5 MAC in group I, as well as 0.75 MAC from 1.5 MAC in group ID. CONCLUSIONS: The choice of anaesthetic protocol influences the HRV parameters in cats. Frequency domain parameters respond to nociception at lower MAC levels. The STD HR has the potential to provide additional information for the assessment of anaesthetic depth in isoflurane-anaesthetized cats. The utility of HRV analysis for the assessment of anaesthetic depth in cats is still questionable.