Abstract
The monitoring of anaesthetic depth is usually based on the subjective assessment of the patient. An objective assessment of anaesthesia has only recently become possible. The auditory-evoked response has predictable changes in response to increasing doses of anaesthetic agents. Recent advances have brought about a regression model with exogenous input of the auditory-evoked response, the A-line ARX-Index (AAI Index). The AAI Index is a dimensionless number between 0 and 100. This technology has been incorporated into the AEP (auditory-evoked potential) monitor that is utilised to assess anaesthetic depth in humans. This study was undertaken to determine if the AEP monitor was useful in dogs. Ten dogs were enrolled in the study. After a full clinical and otoscopic examination, dogs were premedicated with acetylpromazine and morphine. Anaesthesia was induced with thiopentone and maintained with halothane. End-tidal carbon dioxide, temperature, pulse oximetry, blood pressure and the electrocardiogram were monitored and recorded every 5 minutes. Anaesthetic depth was assessed as either being adequate or inadequate by the anaesthetist during surgery. An AEP monitor was attached to the patient and automatically collected AAI Index data. The anaesthetist was blinded to the AEP monitor. Following the completion of the surgical procedure, the patient was allowed to wake up with the AEP monitor attached. The AAI Index was analysed to compare adequate with inadequate anaesthesia during the period of surgery and awake with sleep data during recovery. All AAI Index values associated with inadequate anaesthesia were greater than 31 while adequate values were less than 35. The difference between the groups was statistically significant and the power was 0.97. Statistically, the awake and sleep values were significantly different with a power of 0.99. From this study it can be concluded that the AAI Index shows good prospect for the evaluation of anaesthetic depth in dogs undergoing surgery. A larger study is needed to confirm these results.
Highlights
The evaluation of anaesthetic depth in anaesthesia commenced with the landmark paper of John Snow in 1847 describing the planes of anaesthesia[8]
Reliable, repeatable, dose-dependent anaesthetic depth monitor, reflecting the state of arousal induced by surgical stimuli and the depression induced by anaesthetic agents, that is consistent between anaesthetic techniques is needed
This study showed that the AAI Index has the ability to indicate the adequacy of anaesthesia in dogs
Summary
The evaluation of anaesthetic depth in anaesthesia commenced with the landmark paper of John Snow in 1847 describing the planes of anaesthesia[8]. Awareness during anaesthesia has been based on sympathetic, haemodynamic, respiratory, ocular (mydriasis, myosis, ‘fish eye‘) and neurological reflex (palpebral, pedal and corneal reflex) responses. Objective criteria for the assessment of anaesthetic depth are lacking[5]. Anaesthetic agent requirements vary between patients dependent on age, gender, cardiac output, clinical condition, serum proteins, preanaesthetic drug. It is necessary to individualise the administration of anaesthetic agents according to the unique characteristics of each patient. Awareness under anaesthesia occurs in spite of the monitoring of anaesthetic depth[8]. Reliable, repeatable, dose-dependent anaesthetic depth monitor, reflecting the state of arousal induced by surgical stimuli and the depression induced by anaesthetic agents, that is consistent between anaesthetic techniques is needed
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