Use of the Electrocardiogram in Acute Myocardial Infarction

Introduction

The term electrocardiogram was first coined by Einthoven at the Dutch Medical Meeting of 1893. In 1901, he successfully developed a new string galvanometer with very high sensitivity, which he used in his electrocardiograph. His device weighed 600 pounds ( Fig. 10.1 ). Sir Edward Schafer of the University of Edinburgh was the first to buy a string galvanometer electrograph for clinical use in 1908. The first electrocardiogram (ECG) machine was introduced to the United States in 1909 by Dr. Alfred Cohn at Mt. Sinai Hospital, New York. In 1924, Einthoven was awarded the Nobel Prize in physiology and medicine for the invention of the electrocardiograph. By 1930, the importance of the ECG in differentiating cardiac from noncardiac chest pain was well recognized; in fact, some patterns were considered so characteristic that the ECG alone could be used to confirm the diagnosis of myocardial infarction (MI). This chapter reviews the contemporary use of the ECG in the diagnosis of acute MI.

Inferior Myocardial Infarction

In 80% of cases, the culprit vessel in inferior MI is the right coronary artery. The left circumflex artery is the culprit vessel in all other cases, with the rare exception of a distally extending inferoapical “wraparound” left anterior descending artery, which is suggested when there is concomitant ST segment elevation in the precordial leads. ST segment elevation in lead III that exceeds the magnitude of elevation in lead II with reciprocating ST segment depressions in I and aVL of greater than 1 mm strongly suggests the right coronary artery as the culprit over the circumflex artery. The ST segment vector is directed toward the right when the right coronary artery is involved, which accounts for the elevation in lead III greater than lead II ( Fig. 10.2 ). The added findings of ECG evidence of right ventricle (RV) MI increases the specificity for the right coronary artery and localizes the occlusion to a proximal location.

Conversely, the left circumflex artery is suggested when ST segment elevation in lead III is not greater than lead II and by the absence of ST segment depression in leads I and aV _L . An isoelectric or depressed ST segment with a negative T wave in lead V _4R is very specific but insensitive for proximal left circumflex artery occlusion. ST segment depression in leads V ₁ and V ₂ has been reported to be specific for the left circumflex artery, although a dominant right coronary artery can produce similar findings. The presence of ST depression in leads V ₁ and V ₂ with a prominent R wave in lead V ₂ can be nonspecific and can suggest involvement of the left ventricular posterior wall or concomitant disease in the left anterior descending artery. Performing an ECG with posterior leads (V ₇ –V ₉ ) can show a primary posterior wall injury pattern with ST segment elevation. A localization schema for inferior MI is summarized in Table 10.1 .