SYNOPSIS Eleven cardiac operations are reported in which there was electroencephalographic and/or clinical evidence of seizure activity during cardiopulmonary bypass (CPB). In four patients seizure activity appeared after acute episodes of cerebral ischaemia resulting from either hypotension or pump-generated emboli occurring at the beginning of CPB, or from air embolism occurring at the end of CPB when the myocardium was closed and defibrillated. In the remaining seven patients the seizures appeared to result from the synergistic action of a toxic substance in the perfusate with pre-existing or CPB-induced alterations in cerebral physiology.While numerous reports have appeared over the last 15 years documenting the depression of electrocortical activity which frequently accompanies cardiopulmonary bypass (CPB) (Theye et al., 1957;Silverstein et al., 1960;Lorenz and Hehrlein, 1970), excitatory or epileptiform electroencephalographic (EEG) activity associated with CPB has rarely been observed. The lack of data on epileptic phenomena during CPB may partly reflect the relative infrequency with which the intraoperative EEG is recorded and the use of neuromuscular blocking agents during CPB which would mask any clinical manifestations of seizures. Although probably very rare, seizure activity during CPB is of clinical importance inasmuch as it represents a potentially fatal complication of open-heart surgery. The dangers of having a violent motor seizure during a critical portion of the surgical procedure are obvious. Even if complete neuromuscular blockade is present at the time that electrical seizure activity initially develops, the later unmasking of motor convulsions as the muscle relaxants wear off could pose an unallowable circulatory stress (Storm van Leeuwen et al., 1961). The possibilities of progression to status epilepticus (Seamans et al., 1968;Radvanyi et al., 1970) or of 'consumption anoxia' of the cerebral cortex resulting from the greatly increased oxygen demands of the discharging neurones (Davies and 181 Remond, 1947;Meyer and Waltz, 1961) further suggest the desirability of controlling even a purely electrographic seizure during CPB.The present study investigates the electroencephalographic and clinical circumstances of a number of epileptic seizures occurring during extracorporeal circulation in an attempt to elucidate underlying mechanisms and appropriate therapy of these phenomena and their possible relevance to clinical epilepsy. METHODEleven electrographic seizures during CPB are reported which occurred in a combined series of 280 patients undergoing cardiac surgery with CPB at our hospitals between 1965 and

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