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By J. F. Nunn

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Saidman and Eger (1964) have made an arbitrary but highly 35 CONTROL OF BREATHING practical decision to use lack of motor response to skin incision as the indication of'anaesthesia', for which any one of a dozen different end-points might have been chosen. The minimal anaesthetic concentration (MAC) of an anaesthetic is defined as the minimal end-expiratory concentration which will prevent gross movement following skin incision. Dosage of anaesthetic agents may then be expressed as M A C 1, M A C 2, M A C 3, etc.

Ether spares ventilation to an even greater extent. Ventilatory effects of inhala tional anaesthetic agents are unlikely to be simply a matter of 'central depression of respiratory neurones'. Ether and cyclopropane are known to raise the level of circulating catecholamines, and noradrenalin is known to increase the slope of the Pco 2/ventilation response curve (Cunningham and his colleagues, 1963; Dejours, 1966). Ether, furthermore, has a curare-like effect upon neuromuscular transmission in high dosage and may also stimulate ventilation as a result of the development of an arousal state in response to its irritant vapour.

Historical developments up to 1930 have been well reviewed by Perkins (1964). The Peripheral Chemoreceptors The carotid and aortic bodies have a metabolism which, in proportion to their weight, is considerably higher than that of the cerebral cortex. Their perfusion, however, is about ten times as great as their metabolic rate would appear to require, so that their arterial/venous blood gas content difference is extremely small (Daly, Lambertsen and Schweitzer, 1954). Arterial/ venous shunts exist which can substantially reduce the perfusion of the sinusoids.

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