Evaluation of the nociception-antinociception balance using the Parasympathetic Tone Activity index in dogs anaesthetized for castration.

Objective: To evaluate the performance of the Parasympathetic Tone Activity (PTA) index in assessing the nociception-antinociception balance in anaesthetized dogs undergoing castration.

Study Design: Prospective clinical study.

Animals: A group of 22 healthy client-owned dogs.

Methods: The dogs underwent general anaesthesia, with continuous monitoring of mean and instantaneous PTA (PTAm, PTAi), mean arterial pressure and heart rate. The values of these variables were divided according to the occurrence or absence of a haemodynamic reaction (HDR) at different time points: during surgical preparation, cutaneous incision, testicles extraction, cutaneous suture, after fentanyl administration, and after dexmedetomidine administration during recovery. Data were collected initially and 1, 3 and 5 minutes after each time point. The performance of the dynamic variation of the PTA (ΔPTA) to predict HDR or its resolution within 3 or 5 minutes was assessed using receiver operating characteristic (ROC) curves analysis. A p value < 0.05 was considered significant.

Results: During HDR, a decrease in PTAi (-34% and -31%) and PTAm (-26% and -30%) occurred at 3 (p = 0.005; p = 0.004) and 5 minutes (p = 0.001), respectively. After fentanyl administration, a decrease in haemodynamic variables occurred with a 45% increase in PTAi (p = 0.004). The ROC curve analysis of pooled data of the ΔPTAi for the prediction of HDR within 3 minutes indicated an area under the curve (AUC) of 0.70 (p = 0.0016) (threshold value: -16%). After fentanyl administration, the ROC curve analysis of ΔPTAi for the prediction of resolution of HDR within 3 minutes indicated an AUC of 0.69 (threshold value: +12%).

Conclusions And Clinical Relevance: The PTAi appears to be an interesting tool to assess the nociception-antinociception balance. However, further studies with a variety of clinical scenarios and anaesthesia protocols are required to conclude on its performance.