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

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Abstract
To determine the optimal chest leads for exercise testing and ambulatory monitoring the projection of ST segment alterations on to the front of the chest in 200 patients were studied. In 20 of these patients recordings during exercise were analyzed to study the pattern of changes of the ST segment during and after exercise. The ECG 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 the standard chest leads of the 12 lead ECG were seen to represent only 41% of the total projection. ST segmental depression was usually first recorded in 1 or 2 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 be recorded using a standard 3 channel ECG machine. This will provide important directional information about the severity of myocardial ischemia when assessing the effects of interventions that would not otherwise by available using conventional ECG lead systems. Failure to record ST segment changes using ambulatory monitoring in patients complaining of chest pain may mean that the wrong site was selected for placing the exploring electrode. Moving 1 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.