Feature Ar cle Pu ng awareness to bed: improving depth of anaesthesia monitoring Louise Kostos Fourth Year Medicine (Undergraduate) Monash University
AM S J
Louise is currently in her fourth year of the Monash MBBS course. She has always had a keen interest in anaesthe cs and is looking forward to undertaking her elec ve in this area next year in South America.
Intraopera ve awareness and subsequent explicit recall can lead to prolonged psychological damage in pa ents. There are many methods currently in place to prevent this poten ally trauma c phenomenon from occurring. Such methods include iden fying haemodynamic changes in the pa ent, monitoring vola le anaesthe c concentra on, and various electroencephalographic algorithms that correlate with a par cular level of consciousness. Unfortunately none of these methods are without limita ons.
Introduc on Intraopera ve awareness is defined by both consciousness and explicit memory of surgical events. [1] There are a number of risk factors that predispose pa ents to such a phenomenon, both surgical and pa entrelated. Procedures where the anaesthe c dose is low, such as in caesarean sec ons, trauma and cardiac surgery, have been associated with a higher incidence. Likewise pa ents with low cardiac reserve or resistance to some agents are prominent a ributable factors. [2] A small number of cases are also due to a lack of anaesthe st vigilance with administra on of incorrect drugs or failure to recognize equipment malfunc on. [2] Ul mately it is largely an iatrogenic complica on due to administra on of inadequate levels of anaesthe c drugs. Most cases of awareness are inconsequen al, with pa ents not experiencing pain but rather having auditory recall of the experience, which is usually not distressing. [3] In some cases, however, pa ents experience and recall pain, which can have disastrous, long-term consequences. Awareness has a high associa on with post-opera ve psychosoma c dysfunc on, including depression and post-trauma c stress disorder, [4] and is a major medico-legal liability. Though the incidence of awareness is infrequent, es mated to occur in 1-2 cases per 1000 pa ents having general anaesthesia in developed countries, [1] the sequelae of experiencing such an event necessitates the development and implementa on of a highly sensi ve monitoring system to prevent it from occurring.
Measuring depth of anaesthesia: 1. Monitoring clinical signs Adequate depth of anaesthesia occurs when the administra on of anaesthe c agents are sufficient to allow conduct of the surgery whilst ensuring the pa ent is unconscious. There are both subjec ve and objec ve methods of monitoring this depth. [5] Subjec ve methods rely primarily on the pa ent’s autonomic response to a nocicep ve s mulus. [5] Signs such as hypertension, tachycardia, swea ng, lacrima on and mydriasis indicate a possible lightening of anaesthesia. [5] Such signs however are not specific as they can be the result of other factors that cause haemodynamic changes, such as haemorrhage. Addi onally, pa ent body habitus, autonomic tone and medica ons (in par cular beta-adrenergic blockers and calcium channel antagonists) can also haemodynamically affect the pa ent. [5] Consequently the pa ent’s autonomic response is a poor indicator of depth of anaesthesia, [6] and the presence of haemodynamic change in response to a surgical incision does not indicate awareness, nor does the absence of autonomic response exclude it. [5] Pa ent movement remains an important sign of inadequate depth of anaesthesia, however is o en suppressed by administra on of neuromuscular blocking drugs. [1] This consequent paralysis can be
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overcome with the ‘isolated forearm technique’. In this technique, a tourniquet is placed on an arm of the pa ent prior to administra on of a muscle relaxant and inflated above systolic pressure to exclude the effect of the relaxant and retain neuromuscular func on. The pa ent is then instructed to move their arm during the surgery if they begin to feel pain. [5] Though this technique is effec ve in monitoring depth of anaesthesia, it has not been adopted into clinical prac ce. [7] Furthermore, pa ent movement and autonomic signs may reflect the analgesic rather than hypno c component of anaesthesia and thus are not an accurate measure of consciousness. [8]
2. Minimum Alveolar Concentra on (MAC) The unreliable nature of subjec ve methods for assessing depth of anaesthesia has seen the development and implementa on of various objec ve methods which rely on the sensi vity of monitors. The measurement of end- dal vola le anaesthe c agent concentra on to determine the MAC has become a standard component of modern anaesthe c regimens. MAC is defined as the concentra on of inhaled anaesthe c required to prevent 50% of subjects from responding to noxious s muli. [9] It is recommended that administra on of at least 0.5 MAC of vola le anaesthe c should reliably prevent intra-opera ve awareness. [10] Unfortunately the MAC is affected by a number of factors and thus it is difficult to determine an accurate concentra on that will reliably prevent awareness. Pa ent age is the major determinate of the amount of inhala on anaesthesia required, as are altered physiological states such as pregnancy, anaemia, alcoholism, hypoxaemia and temperature of the pa ent. [11] Most importantly, the administra on of opioids and ketamine, both commonly included in the anaesthe c regimen, severely curtail the ability of the gas analyser to determine the MAC. [12] Further, the MAC is a reflec on of inhala onal anaesthe c concentra on, not effect. The suppression of response to noxious s muli whilst under vola le anaesthesia is mediated largely through the spinal cord, and thus does not accurately reflect cor cal func on and the penetra on of the anaesthe c into the brain. [13] Another major limita on to using gas analysers is that they have limited reliability when intravenous anaesthesia is used. Simultaneous administra on of intravenous anaesthe c agents is extremely common and in many cases total intravenous anaesthesia is used; in such cases the use of the MAC is not applicable.