IV. Correlation of electrocardiographic and pathologic findings in infarction of the interventricular septum and right ventricle

Abstract Infarction of the interventricular septum was demonstrated pathologically in 102 cases, which represents and incidence of 63 per cent in a series of 161 cases. The findings in thirteen cases of localized anteroseptal infarction were analyzed in a previous report. The present study is concerned with a correlation of electrocardiographic and pathologic findings referable to the septal lesion in the remaining eighty-nine cases. These cases were classified into three groups, according to the distribution of the lesion at autopsy: Group A, infarction primarily in and largely confined to the septum in six cases; Group B, septal extension of large anterior or anteroposterior infarction in fifty-nine cases; Group C, septal extension of posterior infarction in twenty-four cases. The following electrocardiographic patterns could be correlated directly with the septal infarct found at autopsy: 1.1. Complete A-V block was observed as a manifestation of extension of an acute posterior infarct into the base of the septum in two cases. 2.2. A QRS interval of 0.12 second or more, a prominent late R wave, and a delayed intrinsicoid deflection in leads from the right precordium were found in fourteen cases and were attributable to septal infarction in thirteen of the group because of the presence of a distinct Q wave and/or abnormally elevated RS-T junction in these leads. The infarct was confined to the apical one-half to two-thirds of the septum in five of the cases and probably caused delay in right ventricular activation by interruption of conduction through the right Purkinje system, rather than the right bundle branch. Since the electrocardiographic findings in these cases were similar to those in other cases with infarction reaching the anatomic site of the bundle of His, the customary term, "right bundle branch block," was retained to designate the conduction defect. The abnormal Q wave in right ventricular leads constituted the chief distinguishing feature from uncomplicated right bundle branch block and was recorded because of the preponderance of negative potentials transmitted from the left ventricular cavity through the infarcted septum to the right precordium over reduced positive potentials coming from activation of intact remnants of septum. The differentiation of infarcts limited to the septum from those continuing into the anterior wall of the left ventricle depended upon the QRS pattern in leads to the left of the transitional zone and was rendered difficult in three of the cases of right bundle branch block by displacement of the transitional zone into the left axilla. The recognition of extension of a septal infarct into the posterior wall of the left ventricle was possible from Lead aV F in intermediate to vertical cardiac position, but not in transversely placed hearts, since reference of the potential variations of the right side of the septum to the left leg, as a result of horizontal position, produced patterns in Leads aV F , II, and III which simulated those caused by posterior infarction. The standard limb leads did not reveal diagnostic evidence of septal infarction in any of the thirteen cases. 3.3. A QRS interval of 0.12 second or more, an initial upstroke in all leads facing the left ventricle, and an abnormally delayed intrinsicoid deflection in left axillary leads were found in four cases and were attributed to left bundle branch block independent of the septal infarct in three of these. In the remaining case, autopsy revealed an acute infarct limited to the left side of the apical two-thirds of the septum and the subendocardial layer of the anterior and posterior walls of the left ventricle, and the pattern was attributed to septal activation by impulses distributed through the right Purkinje plexus. 4.4. Patterns characterized by a QRS interval of 0.12 second or more, an initial Q wave, and a late intrinsicoid deflection in precordial leads over the left ventricle were found in ten cases and were definitely attributable in three of these to an infarct of the free wall that was dense in the subendocardial layer and patchy in the more superficial portion of the myocardium. This explanation was favored in the other seven cases, but the alternative possibility of left bundle branch block due to extensive septal infarction could not be positively excluded. 5.5. A triphasic QRS complex of normal duration, characterized by a small Q wave, a small R wave, and a deep S wave, was found in right ventricular Leads V 1 and/or V 2 in four cases and was well correlated with the distribution of the septal infarct at autopsy. These findings may be considered diagnostic of septal infarction, provided right ventricular hypertrophy can be excluded. A similar pattern was found in Lead aV F in three other cases and could be correlated with infarction of the posterior part of the septum. The potential variations of the right side of the septum were transmitted to the left leg in these cases because of horizontal position of the heart. 6.6. A monophasic QS complex of normal duration, found in Leads V 1 and V 2 or in V 2 in twenty-four cases, was regarded as a manifestation of infarction, rather than as a normal variant, because of the presence of one or more of the following findings: an abnormal upward displacement of the associated RS-T segment, a normal initial R wave in leads farther to the right, or abnormal Q waves in leads farther to the left. The abnormal QS complexes in Leads V 1 and V 2 in most of the cases were attributable to infarction of the septum, rather than infarction of the free anterior wall of the left ventricle, because of the presence of an intrinsicoid deflection in the accompanying P wave, indicating that the electrode was in the vicinity of the right atrium and thus faced the right side of the septum and right ventricle. The replacement of the initial R wave by a QS complex could be correlated with septal infarction that permitted transmission of left ventricular cavity potentials to the right precordium. An abnormal QS deflection in Lead aV F , found in four patients with horizontally placed hearts, was believed referable to septal infarction. 7.7. Abnormal RS-T displacement, consistent with recent infarction, was found in right ventricular Leads V 1 and/or V 2 in association with normal initial R waves in seven cases. This finding could only be regarded as suggestive of septal infarction during life, but was believed referable to the recent septal infarct found at autopsy in each case. Diagnostic or suggestive evidence of septal infarction was found in all six cases of primary septal infarction. Direct electrocardiographic evidence of the septal lesion was absent in sixteen of the twenty-four cases of septal extensions from posterior infarction, principally because of limitation of the lesion to the posterior one-third to one-half of the septum; and absent in nineteen of the fifty-nine cases of septal extension from anterior or anteroposterior infarction. The diagnostic failures were attrible to limitation of the infarct to the apical one-third of the septum in six of the latter group, to left bundle branch block in two, to a very recent septal lesion in two, and, in the remainder, to marked reduction in opposing negative potentials transmitted to the right precordium as a result of extensive infarction of the free wall of the left ventricle. In nine of the nineteen cases without direct electrocardiographic evidence of septal infarction, leads from the precordium and left leg revealed signs of anteroposterior infarction, which could be regarded as indirect evidence, presumptive of the presence of infarction in the intervening septum. Isolated right ventricular infarction was not found in any case. Left ventricular infarcts continued across the septum into the posterior wall of the right ventricle in thirteen cases and into the anterior wall of the right ventricle in six others, but were not manifested by electrocardiographic signs distinctive of the right ventricular involvement in any case.