OBJECTIVES

Both anaesthesia and surgery influence the living organism causing neurovegetative and hormonal responses that are usually known as stress responses. Heart rate variability is known to represent sympathetic and parasympathetic activity in each individual. This variability can be influenced both by surgery and anaesthetic drugs administration. A new monitor, known as Anemon I ® (Medical Control SA Switzerland), has recently been introduced into clinical practice. This system can evaluate autonomous nervous system reactivity in real time, by analyzing the fractal index of heart rate variability. The system shows a simple, easily interpreted quantitative index (0-200), the Anemon Index (AI), that can give information on patient's protection regarding surgical injury. Higher Anemon Index values represent more sympathetic activity. On the other hand, the bispectral index (BIS), a parameter derived from the EEG, has been previously reported to reflect the sedative component of anaesthetic depth during sevoflurane anaesthesia in men (0-100).

The importance of carrying out this study is to establish the usefulness of these monitoring techniques in dogs undergoing major surgery and subjected to sevoflurane and buprenorfine anaesthesia, assessing the anaesthetic technique used.

MATERIALS

Ten dogs had been evaluated in this study. Animals were referred to the Veterinary Hospital of the Extremadura University for different diseases requiring major surgery, and all were anaesthetized with sevoflurane. Five animals underwent orthopaedic surgery and the other five soft tissue surgery. Animals were premedicated with acepromazine (0.04 mg/kg) buprenorfine (0.01 mg/kg) and atropine (0.04 mg/kg). Anaesthetic induction was performed administering 7-8% sevoflurane in 100% oxygen, at a fresh gas flow of 10 L/min by facial mask. Intubation was performed and sevoflurane administered at approximately 1 CAM (CAM = 2.36 %). Analgesia was ensured by premedicating with buprenorfine. Different parameters were registered in order to evaluate cardiovascular and respiratory stability in the anaesthetized animals. (Heart rate, pulse oximetry, arterial blood pressure, EtCO2 and exhaled anaesthetic agent). Anaesthetic depth was monitored using BIS, and sympathetic activity using the Anemon Index. Results are given as means ± standard error of the mean. Statistically significant differences associated with the surgical technique or the anesthetic duration were studied using two ways analysis of variance, where weight was used as covariable.

RESULTS

A significant (p<0.001) decrease in bispectral index was seen after anaesthetic induction, from 97.4±1.9 to 71.3±16.2, without further significant changes being observed during maintenance. During this time, BIS values were between 71.3.2±16.2 and 61.3±18.1. This values are higher than those described in human anaesthesia (Bis values between 40 and 60 during surgical anaesthetic plane). On the other and, the Anemon Index increased significantly (p<0.001) after intubation, from 82.7±11.0 to 107.5±11.1. No significant changes were seen in AI during incision or during the performance of the higher nociceptive stimulus through the course of the surgical procedure.

No significant changes in heart rate were seen in this study, although a non-significant increase in this parameter was seen after tracheal intubation. Mean arterial blood pressure did not change significantly. No significant changes were thus seen on cardiovascular parameters in this study. Respiratory depression and apnea were seen in one animal after 100 minutes of anaesthesia, so intermittent controlled positive pressure ventilation was used in this case, setting a respiratory rate of 10 breaths per minute with a tidal volume of 15 ml/kg. It is important to state that both Herat and respiratory rate were significantly (p<0.05) higher in animals undergoing orthopaedic surgery. No significant changes in BIS or AI were seen in relationship with the surgical procedure performed.

CONCLUSION

 A significant increase in the Anemon Index values was seen during intubation. Tracheal intubation represents a stressful event during general anesthesia. Sevoflurane anesthetic induction does not prevent the sympathetic stimulus caused by tracheal intubation in dogs.

 The anaesthetic technique used in this study is useful in protecting the patient from the nociceptive stimulus caused by the surgical procedure.

 Bispectral index values seen in this study were higher than previously reported values. Further studies are warranted to determine clinical efficacy of this monitoring in dogs.