CHAPTER 11 Use of the Electrocardiogram in Acute Myocardial Infarction

Inferior Myocardial Infarction

In 80% of cases, the culprit vessel in inferior myocardial infarction (MI) is the right coronary artery. The 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.¹ 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. 11-1). The added findings of electrocardiogram (ECG) evidence of right ventricle MI increases the specificity for the right coronary artery, and localizes the occlusion to a proximal location.²

Conversely, the 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.³^–⁵ An isoelectric or depressed ST segment with a negative T wave in lead V_4R is very specific, but insensitive for proximal circumflex artery occlusion.^6,⁷ ST segment depression in leads V₁ and V₂ has been reported to be specific for the circumflex artery, although a dominant right coronary artery can produce similar findings. The presence of ST depression in leads V₁ and V₂ with a prominent R wave in lead V₂ 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 (V₇-V₉) can show a primary posterior wall injury pattern with ST segment elevation. A localization schema for inferior MI is summarized in Table 11-1.

Right Coronary Artery	Circumflex Artery
ST segment elevation III > II	ST segment elevation II ≥ III
ST segment depression >1 mm I, avL	ST segment elevation I, avL, V₅-V₆
ST segment elevation V_4R or V₁	ST segment depression V_4R

Right Ventricle Myocardial Infarction

In the setting of inferior MI, right-sided precordial lead recordings are strongly indicated. The presence of right ventricular involvement portends a worse prognosis and enables the clinician to identify a subgroup of inferior MI with a propensity toward hemodynamic instability and shock leading to increased in-hospital mortality.⁸ Right ventricle MI is always associated with a proximal occlusion of the right coronary artery, before the takeoff of the right ventricular marginal branches. The most sensitive sign is 1 mm of ST segment elevation in lead V_4R.⁹ This sign is not fully specific for right ventricle MI, however, because this can be seen in acute pulmonary embolus, anteroseptal MI, and pericarditis. ST segment elevation in lead V₁ in association with elevation in leads II, III, and aV_F is highly correlated with the presence of right ventricular infarction.^2,¹⁰ Isolated right ventricle infarction, although rare, can be easily confused with anterior wall infarction, owing to the anterior location of the right ventricle, with ST segment elevation manifest only in the early precordial leads (V₁-V₃).¹¹