Effect of Fentanyl Infusion on Heart Rate Variability and Anaesthetic Requirements in Isoflurane-Anaesthetized Horses

Abstract

Controversy continues to surround the use of opioids in equine anaesthesia, with variable effects reported. This blinded clinical study aimed to investigate the influence of a low-dose fentanyl continuous rate infusion (CRI) on isoflurane requirements, parasympathetic tone activity (PTA), and anaesthetic parameters in horses during general anaesthesia. All of the twenty-two horses included in the research underwent a standard anaesthetic protocol. Eleven horses in the fentanyl group (Group F) received a loading dose of fentanyl at 6 µg/kg, followed by a CRI of 0.1 µg/kg/min during anaesthesia. A further 11 horses in the control group (Group C) received equivalent volumes of normal saline. Anaesthetic parameters and PTA index were recorded during anaesthesia. The achieved mean fentanyl plasma concentration was 6.2 ± 0.83 ng/mL. No statistically significant differences between groups were found in isoflurane requirements, MAP values, and mean dobutamine requirements. However, horses in Group F required a significantly lower dose of additional ketamine to maintain a sufficient depth of anaesthesia. Significantly higher PTA values were found in the fentanyl group. Further research is warranted to determine the limitations of PTA monitoring, and the influence of various anaesthetics on its values.

Parasympathetic Tone Changes in Anesthetized Horses after Surgical Stimulation, and Morphine, Ketamine, and Dobutamine Administration

“…Changes in the PTAm during this period (i.e., light depth of anesthesia) should be carefully used to assess nociception, although the sympathetic effect of ketamine at the administered dose would last for a short period as suggested in humans [15]. Similar results were found in anesthetized horses with ketamine administered under similar conditions to our study, and they could not exclude the sympathomimetic effect of ketamine [14].…”

“…In humans, it has been reported that fentanyl increases vagal tone and decreases sympathetic nervous system activity [21], and remifentanil increases parasympathetic tone activity, observed as an increase in ANI [1]. Similarly, it has been shown that fentanyl CRI increases parasympathetic tone activity by increasing PTAm in anesthetized horses [14]. The use of morphine has been poorly documented, but is suggested to alter HRV in humans [22].…”

“…It was able to differentiate three drug regimens related to analgesia in pigs [11]. In horses that received a continuous rate infusion (CRI) of fentanyl, the PTAm was higher than in horses that did not receive it during surgery [14]. This monitor also showed lower values of PTAm in horses undergoing colic surgery compared to elective surgeries [9].…”

Evaluation of the averaged parasympathetic tone activity and its dynamic variation to assess intraoperative nociception in relation to hemodynamic changes in dogs

“…A study showed that fentanyl constant rate infusion increased PTAm values in anesthetized horses (Dmitrović et al. 2021 ). However, PTAm seems to be a poor indicator of sympathetic activation after surgical incision in anesthetized horses (Ruíz-López et al.…”