Effects of isoflurane, remifentanil and dexmedetomidine on selected EEG parameters derived from a Narcotrend Monitor before and after nociceptive stimulation at different MAC multiples in cats.

BACKGROUND: The aim of this prospective and complete cross-over study was to evaluate the effects of isoflurane, remifentanil and dexmedetomidine on EEG parameters derived from the Narcotrend® Monitor before and after nociceptive stimulation at different isoflurane MAC (minimal alveolar concentration) multiples. Seven adult European Domestic Short Hair cats were used. Each cat went through 3 experimental treatments. Group I received isoflurane, group IR received isoflurane and a constant rate infusion (CRI) of remifentanil (18 µg/kg/h IV), and group ID received isoflurane and a CRI of dexmedetomidine (3 µg/kg/h IV). The isoflurane MAC in each group was determined via supramaximal electrical stimulation. The EEG parameters were derived by a Narcotrend Monitor at specific time points before and after nociceptive stimulation at 0.75, 1.0 and 1.5 MAC. The depth of anaesthesia was also assessed by a clinical score. RESULTS: The mean MAC sparing effects in group IR and group ID were 9.8 and 55.2%, respectively. The best correlation of EEG and MAC multiples was found for the Narcotrend Index (NI) in group I (r = - 0.67). The NI was also able to differentiate between 0.75 MAC and 1.5 MAC in group IR. Spectral edge frequency had a lower correlation with MAC multiples in group I (r = - 0.62) but was able to differentiate between 0.75 MAC and 1.5 MAC in groups I and IR, and between 1.0 MAC and 1.5 MAC in group IR. Narcotrend Index, SEF 95 and MF increased significantly after nociceptive stimulation at 1.0 MAC in group I, and SEF 95 increased significantly at 0.75 MAC in group ID. The clinical score correlated closer than any of the EEG parameters with MAC in all groups, with highest correlation values in group I (r = - 0.89). Noxious stimulation led to a significant increase of the clinical score at 0.75 MAC and 1.0 MAC in group I. CONCLUSIONS: The EEG parameters derived from the Narcotrend Monitor show correlation to isoflurane MAC multiples in cats, but the anaesthetic protocol and especially the addition of dexmedetomidine have great influence on the reliability. The Narcotrend Monitor can be used as an additional tool to assess anesthetic depth in cats.

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.