5. Depth of Anaesthesia Monitoring

Clinical signs Sympathetic stimulation Evoked potentials (EPs): Compressed spectral array Spectral edge Median frequency Respiratory sinus arrhythmia and R–R interval variation Isolated forearm technique EEG: Cerebral function monitor (CFM Cerebral function analyzing monitor (CFAM Oesophageal contractility Frontalis (scalp) electromyogram (EMG):

Clinical signs: In the spontaneously breathing patient who is not paralyzed, awareness may be manifest by purposeful movement. Movement is a reliable indicator of light anaesthesia, although a patient may have no recall.

the main clinical signs are tachycardia, hypertension, diaphoresis lachrymation. Attempts have been made to quantify these objectively by using the PRST scoring system (blood Pressure, heart Rate, Sweating, Tear formation), but without any real evidence of its benefit. In the absence of other causes, sympathetic signs may be reliable if present, but the main difficulty is that their absence does not exclude awareness.

Evoked potentials (EPs): visual, somatosensory and auditory EPs have been investigated as indicators of the depth of anaesthesia. The few microvolts that are generated by each potential have to be separated from the overall electrical noise that is produced by the brain as a whole. Auditory EPs appear to be the most effective because they are the last to disappear and are the best indicators of anaesthetic depth. The patient’s auditory system is stimulated by repetitive clicks at around 6–10 Hz. The electroencephalogram (EEG) is recorded immediately after each stimulus and is amplified, before the auditory EPs are extracted by taking the average of a large number of responses. This is covered in greater detail in ‘Evoked Potentials’ in the next section.

Compressed spectral array: this is a method of simplifying the EEG in which the signals are subjected to Fourier analysis. Fourier transformation is the mathematical technique whereby complex waveforms are analyzed into their simpler sine wave components. Spectral analysis calculates the total power contained within the different frequencies of cerebral activity over a period of time (known as an epoch). The graph of power against frequency forms a spectral array. As an anaesthetic continues or deepens, each linear plot obtained during successive epochs can be superimposed to give the typical peak and trough, or ‘hill and valley’, display. This compressed display is what constitutes compressed spectral array. In an anaesthetized patient, power shifts to the lower frequencies

Respiratory sinus arrhythmia and R–R interval variation: this method does have promise, although it is only useful in the presence of an intact autonomic nervous system and healthy myocardial conducting system. Its value is greatly restricted in patients, for example, who are being treated with β-adrenoceptor blockers, who have autonomic neuropathy or dysfunction (common in the elderly), sepsis or who have cardiac conduction abnormalities. It provides a measure of brain stem function, which decreases with increasing depth of anaesthesia.

EEG: the formal EEG is a highly complex monitor, usually with multiple channels, which is generally regarded as producing too much data to be of any practical use in theatre. The raw EEG demonstrates differing patterns in response to different anaesthetic agents and changes in response to events such as hypoxia and hypercarbia. It also processes a lot of information from the cerebral cortex, which may not in fact be the area most appropriate for examining depth of anaesthesia

Oesophageal contractility: the amplitude and frequency of contractions of lower oesophageal smooth muscle reduce with increasing depth of anaesthesia. The technique is of limited value because of the high rate of false positive and false negative results.

Frontalis (scalp) electromyogram (EMG): this technique measures the amplitude of the EMG, which decreases with increasing depth of anaesthesia. It is of very restricted benefit if for no other reason than it cannot be used in paralyzed patients.

(NICE Diagnostics Guidance [DG6]. Depth of anaesthesia monitors. 2012). ‘the use of EEG-based depth of anaesthesia monitors is recommended as an option during any type of general anaesthesia in patients considered at higher risk of adverse outcomes. This includes patients at higher risk of unintended awareness and patients at higher risk of excessively deep anaesthesia. The Bispectral Index (BIS) depth of anaesthesia monitor is therefore recommended as an option in these patients. ‘although there is greater uncertainty of clinical benefit for the E-Entropy and Narcotrend-Compact M monitors than for the BIS monitor . . . they are broadly equivalent.’ The report also recommended depth of anaesthesia monitoring in all patients receiving total intravenous anaesthesia (TIVA).

modification of the EEG, in which there is analysis of the phase and power relationships between the numerous frequencies. is a number generated from these phased and power frequencies that are the components of the EEG, and in essence compares frequency harmonics in the frontal EEG. dimensionless scale of 0 (cortical electrical silence) to 100 (normal cortical electrical activity) has been derived from EEG recordings in volunteers and patients undergoing transitions between consciousness and unconsciousness A bispectral index score of between 40 and 60 suggests surgical anaesthesia. The details of the calculation algorithm remain a commercial secret. The device has a variable response time, with a lag that is usually around 25 seconds, but which can extend for as long as 4 minutes The BIS number will reduce if a patient is given muscle relaxants. This is usually attributed to their effect on the EMG, but it is possible that deafferentation may also contribute, with some loss of proprioceptive and muscle input through the reticular activating system. BIS has no predictive value for any particular individual’s threshold for loss of consciousness.

No authoritative body has yet endorsed the routine use of the processed EEG (pEEG), probably because the evidence base is not sufficiently robust. The B-Aware randomized controlled trial in 2,463 patients reported an 82% reduced risk of awareness in a population identified as being at high risk

B-unaware study of 2,000 patients showed no difference between subjects monitored with BIS and with those managed according to a MAC-based protocol.

The same primary investigator headed up the BAG-RECALL trial which compared BIS of 40–60 to age-adjusted MAC values of 0.7–1.3 and in the 6,041 patients found no difference

E-entropy. In the context of depth of anaesthesia monitoring, entropy is defined as a measure of irregularity in a signal. In the awake subject the EEG shows very irregular patterns which become more ordered as anaesthesia deepens. The E-entropy device is another form of processed EEG, but it also incorporates signals from the frontalis muscle EMG

Awareness, or accidental awareness during general anaesthesia (AAGA) to give it its full title, is an enduring concern both of anaesthetists and patients. The fifth National Audit project of the Royal College of Anaesthetists and Association of Anaesthetists of Great Britain and Ireland (NAP 5) reported an overall incidence of awareness of 1 in 19,000 but a much higher incidence in selected groups; 1 in 8,000 patients given neuromuscular blockers and a disconcertingly high 1 in 670 patients undergoing caesarean section. It therefore remains a problem.

Definitions: awareness can be ‘explicit’ or ‘implicit’. Explicit awareness is defined by spontaneous or prompted recall of intraoperative events, which may or may not include pain. Implicit awareness is a less specific concept and is defined by some degree of cortical perception but without conscious recall

Causes: its causes lie in equipment and its (mis)use, in pharmacology and its application and, very rarely, in the physiology of patients

Equipment and apparatus: awareness may result from a failure of the apparatus to deliver adequate concentrations of anaesthetic agent. The anaesthetic machine must deliver an accurate fresh gas flow via an appropriate breathing system using a vaporizer. For patients receiving TIVA, an accurate syringe driver is required, together with a reliable system of infusion tubing. Awareness may result if there are failures in any part of these systems. Such failures include leaks, faulty or empty vaporizers, a misconnected or disconnected breathing system, inaccurate pumps, misplaced venous cannulae and occluded infusion tubing.

awareness may result from a failure of the anaesthetist to use the equipment properly. Circle systems can present a particular difficulty.

: failure to monitor the concentrations of inspired and expired volatile agents may result in inadequate anaesthetic agent being delivered. TIVA is more difficult to monitor in this respect because the effect site concentrations are calculated rather than measured directly

: awareness, by definition, results from inadequate anaesthesia. The dose of induction agent may have been inadequate, as may be the alveolar concentration (it is important to remember that the MAC value that is quoted is only the MAC50) or the computed blood concentration in target-controlled infusion (TCI). Awareness is not prevented by hyperventilation, by the use of nitrous oxide and oxygen alone, nor by the use of opioids. Muscle relaxant drugs are not anaesthetics, and anaesthesia should not be discontinued until their effects have been reversed fully.

: very rarely, a patient may be ‘resistant’ to anaesthetic agents. Alcohol and other drugs of abuse are convenient scapegoats, but the evidence is unconvincing. Similarly, high anxiety is frequently cited as the reason that some patients may need larger than normal induction doses. In any of these situations the anaesthetist should be alert to the clinical signs indicative of inadequate anaesthesia. On occasion, a patient may be so moribund (or so inadequately resuscitated) that adequate anaesthesia may be incompatible with maintaining cardiac function.

Airway problems and bronchoscopy: during a difficult intubation, the effects of the induction agent may wear off before those of the muscle relaxant. Awareness is also a potential problem during some anaesthetic techniques for rigid bronchoscopy (such as those using insufflation or injectors). Cardiac bypass is another technique in which the likely incidence of awareness is higher.