ABSTRACT NO: 029
The concept of MAC (minimum alveolar concentration) was developed by Merkel and Eger in 1961 as a means to allow comparisons between volatile anaesthetic agents. MAC was defined as that agent concentration at one atmosphere, which, when equilibrated in the body, prevented purposeful (nocifensive) movements in the extremities in 50% of a test population. Because blockade of somatic motor activity was then considered the hallmark of general anaesthesia, knowing this ED50 concentration for different agents allowed comparison of haemodynamic or other side effects between agents. Using MAC as a guide, the potencies of volatile anaesthetics were found to be invariant with gender, length of exposure, moderate acid-base disturbances, etc., but they change according to subject age, temperature or severe challenges to homeostasis. MAC provides a rationale basis for the clinical administration of anaesthetics.
MAC has also been used as a tool in elucidating the mechanisms of general anaesthesia. For example, the discovery that MAC, as a quantitative index of potency, is inversely proportional to the oil/gas partition coefficient strongly suggests that the mechanism of anaesthetic action requires lipophilicity.
Until recently, the site at which anaesthetics act to produce surgical immobility had been believed to be in the rostral CNS-most likely the cerebral cortex. However, the EEG, an indicator of cerebral activity, is at best a poor predictor of movement in response to pain. Even subjects with anaesthetic-induced burst suppression are capable of complex, purposeful movement. These observations suggested that a functional cortex was not a mandatory component of the 'MAC response' and provoked a series of experiments that first disconnected the cerebrum, and then the midbrain and brainstem from the spinal cord. These lesions did not alter MAC or the nature of the motor response. Additional support for these observations comes from the isolated perfusion studies of Antognini, and the cortical ischaemic lesions of McFarlane.
Therefore, MAC appears to measure anaesthetic action within a spinal reflex. Rather than diminish its value, MAC provides a useful tool to study a clinically relevant anaesthetic effect in a relatively tractable spinal model system.
The publication of this supplement has been supported by the sponsors of the Third ISMG Meeting: Abbott, AGA, AstraZeneca, Dräger, Janssen, Medex Medical, Messer Austria, Ohmeda, Pharmacia & Upjohn, Scott Medical Products, Siemens