Variations in the deviations of S-T in anterior wall infarction

Abstract

Reciprocal deviations of S-T in chest leads from right- and left-side positions occur in clinical cases of anterior wall infarction. They conform to changes observed in experimentally produced ischemia of the heart muscle. When myocardial infarction is of anteroseptal location, elevation of S-T in leads from the right side of the precordium may be accompanied by depression of S-T in chest leads from the left side. Then Lead I may also show depression of S-T, and the resulting features may resemble those sometimes seen in the presence of lateral wall infarction. When myocardial infarction is of lateral location, elevation of S-T in leads from the left side of the precordium may be associated with depression of S-T in leads from the right side. However, when infarction fails to include the superficial muscle layers, or when it is located close to the base of the heart, depression instead of elevation of S-T may be present in leads from the left side of the precordium at the customary level. There is great variability in the deviations of S-T in limb and chest leads in clinical cases of myocardial infarction. One determining factor is the site and extension of infarction. Another important factor is that the the chest leads, unlike direct leads used in the experimental animal, record changes of potential from a far greater area than that covered by an electrode placed directly upon the myocardium, that is, from an area which may include damaged and undamaged myocardium. Thus, positive and negative changes of potential may be mixed and the resultant deviation of S-T will depend on whether the positive or negative potential variation prevails. Therefore, reciprocal deviations of S-T in the chest leads are not invariably observed with anteroseptal or lateral wall infarction. Also, in limb leads, in the presence of anterior wall infarction, elevation of S-T may be inconspicuous in Lead I while marked depression of S-T is noted in Leads II and III. On the other hand, in infarction of posterior site, positive and negative changes in S-T potential which originate in damaged and undamaged parts of the posterior wall may neutralize each other. Thus, Lead III may show no deviation of S-T, while marked depression of S-T is present in Lead I.