Canadian Journal of Anesthesia, Vol 7, 21-31, Copyright © 1960 by Canadian Anesthesiologists' Society

Electroencephalographic Monitoring during Cardiac Surgery: Observations on the EEG Response to Severe Hypotension

¹ Department of Anaesthesia, University of Western Ontario, London, Ontario

The electrical activity of the brain-represents a function of the cortex and changes of these cortical potentials or their disappearance indicate a disturbance severe enough to alter or abolish the function of cerebral cells. Changes of the normal EEG pattern occur. when the O₂ supply to the brain has reached a critically low level, owing to either an insufficient blood flow or a low O₂ saturation. Although several factors can produce a similar sequence of events in the EEG, it is possible in most instances to exclude, a number of these variables and thus determine the most likely cause for the altered EEG pattern.

In cardiac surgery, where episodes of reduced blood flow frequently occur, the EEG is a useful instrument to warn the anaesthetist that a vital function is threatened, providing its limitations, which have been discussed, are taken into account. This warning comes late; only a severe degree-of hypotension or hypoxia will produce EEG manifestations. But these signs occur some time before it is too late to prevent irreversible damage and they may be the only indication to the anaesthetist of the critical state of his patient. During extracorporeal circulation, where many of the usual vital signs fail, the ECG appears to be the most valuable indicator of an adequate cerebral circulation, which can be readily observed and recorded.

In the presented series of cases, two types of EEG responses to hypotension have been observed: (i) a pattern of large slow waves, leading to depression of the amplitude and flattening of the EEG; and (ii) a rapid depression of the normal waves, leading to flattening without the appearance of slow waves.

Observations have been presented which seem to indicate that the suddenness of onset of hypoxia or hypotension is responsible for the type of response. There may also be a difference depending on whether such a disturbance occurs in a conscious or anaesthetized patient.

The sequence of events during a mitral valvulotomy was used to demonstrate the time relation between the onset of hypotension and the appearance of EEG signs. The delay of about ten seconds is probably determined by the circulation time and by the existing state of oxygenation.

During a period of hypotension in a patient undergoing a mitral valvulotomy electrocardiographic signs of myocardial ischaemia were seen without EEG changes. Although the brain is usually more sensitive to hypoxia than the heart, this observation demonstrates the markedly reduced tolerance to hypotension of the diseased heart. It is, therefore, concluded that a degree of hypotension that fails tp produce EEG changes cannot generally be regarded as safe for the organism.