Cerebral ischemia occurs when blood flow, after loss of autoregulation, falls below 25 to 30 per cent of normal levels. An altered level of consciousness, ranging from mild confusion to coma, represents the most important clinical finding of cerebral ischemia. For the heart, the catecholamine release associated with hypotension not only supports myocardial contractility and induces tachycardia, but also increases oxygen demand and the risk of arrhythmia. Hypotension reduces coronary artery perfusion pressure and blood flow, and therefore may lead to myocardial ischemia, particularly in the presence of coronary heart disease or pre-existing cardiac dysfunction. Concomitant metabolic acidosis is followed by a decrease in the pH of myocardial cells and a decrease in contractility.
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