Projection of ST segment changes on to the front of the chest. Practical implications for exercise testing and ambulatory monitoring.

Abstract

In order to determine the optimal chest leads for exercise testing and ambulatory monitoring we have studied the projection of ST segment alterations on to the front of the chest in 200 patients. In 20 of these patients recordings during exercise were analysed to study the pattern of changes of the ST segment during and after exercise. The electrocardiogram was recorded using 16 unipolar chest leads equally spaced over the left hemithorax. ST segment elevation and depression projected to the front of the chest in a pattern that was individual to each patient and indeed the standard chest leads of the 12 lead electrocardiogram were seen to represent only 41% of the total projection. ST segment depression was usually first recorded in one or two leads and the number of leads involved increased as exercise progressed, returning to normal after exercise in a reciprocal fashion; the leads that first showed ST segment depression were almost invariably those with the most severe depression and the last in which it disappeared. Analysis of the precordial projection of the ST segment has allowed a 12 lead precordial matrix to be designed that can be recorded using a standard three channel electrocardiographic machine. This will provide important directional information about the severity of myocardial ischaemia when assessing the effects of interventions that would not otherwise be available using conventional electrocardiographic lead systems. It is also clear from these data that failure to record ST segment chatiges using ambulatory monitoring in patients complaining ofchest pain may mean that the wrong site was selected for placing the exploring electrode. Moving one or both of the exploring electrodes to other positions of the precordial matrix in subsequent recordings may then provide diagnostic information that otherwise would have been missed.