Simultaneous and accurate recording of arterial blood pressure and ST segment changes is fraught with technical difficulties. We have developed a new system to enable accurate reproduction of the electrocardiogram and intra-arterial blood pressure, using a transducer/perfusion unit conventionally used to study hypertensive subjects, linked to a frequency modulated tape recorder. Detailed methods of digital analysis have been developed to process the data. This system has been used to study 22 patients with arteriographically proven severe obstructive coronary artery disease who suffered frequent attacks of angina. Control data from quantified dynamic exercise in the laboratory were used for comparison with the effects of normal daily activities outside the hospital and to test the hypothesis that "double product" (heart rate X systolic blood pressure) is relevant to the onset of angina in such patients. The most important finding was that both angina and asymptomatic episodes of ST segment depression were invariably accompanied by an increase in heart rate, whereas there was considerable variation in blood pressure changes ranging from an increase to a substantial fall. This suggests that heart rate changes are more important in determining ischaemic episodes than blood pressure. Furthermore, the "double product" was not reproducible during repeated episodes of angina and asymptomatic ischaemia and did not appear to have an important role in the pathogenesis of intermittent myocardial ischaemia in this group of patients.
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