Left Bundle Branch Block and the Electrocardiographic Diagnosis of AMI (2001)

A 63-year-old male with a past history of coronary artery disease and myocardial infarction presented to the ED with substernal chest pain of five hours duration. Examination revealed an ill-appearing patient with diaphoresis. A 12-lead ECG (Figure 1) demonstrated NSR with left bundle branch block (LBBB); the LBBB pattern was known to be pre-existing. The patient received nitrates and aspirin with continued chest discomfort.Figure 1: Normal sinus rhythm with LBBB pattern. The ST segment abnormalities in leads V2 to V6 are not appropriate for the LBBB configuration and suggest AMI. The ST segment elevation in leads V2 to V4, though discordant with the terminal portion of the QRS complex, was worrisome for AMI. In fact, greater than 5 mm of such elevation is suggestive of AMI; such is the case in these leads. The ST segment elevation in leads V5 and V6, concordant with the QRS complex, was felt to be diagnostic for AMI. The Sgarbossa et al criteria report that this electrocardiographic finding is diagnostic for AMI in the LBBB patient.Further review of the ECG revealed ominous findings. The ST segment elevation in leads V2 to V4, though discordant with the terminal portion of the QRS complex, was worrisome for acute myocardial infraction (AMI). The ST segment elevation in leads V5 and V6, concordant with the QRS complex, was felt to be diagnostic for AMI. With the information from the ECG, the emergency physician proceeded with thrombolysis. The patient had an uneventful recovery from the myocardial infarction that was confirmed by elevated cardiac serum markers. The usually accepted electrocardiographic criteria for administration of thrombolytic therapy for AMI include ST segment elevation in two or more anatomically contiguous standard leads and new — or, not known to be old — left bundle branch block. Current evidence shows that patients with new LBBB and a clinical presentation consistent with AMI should be treated in the same fashion as chest pain patients presenting with the “typical” ST segment elevation lacking the LBBB confounder. T Waves In the patient with LBBB,1 the anticipated or expected ST segment-T wave configurations are discordant, directed opposite from the terminal portion of the QRS complex and called QRS complex-T wave axes discordance best described by the rule of appropriate discordance. As such, inferior and right/mid-precordial leads (V1 to V4) with either QS or rS complexes may have markedly elevated ST segments, mimicking AMI (Figure 2, left). The lateral leads, with the large monophasic R wave, demonstrate ST segment depression (Figure 2, right).Figure 2: The normal, or expected, ST segment/T wave morphologies for LBBB: In the left example, the major, terminal portion of the QRS complex (“A”) is located on the opposite side of the baseline from the initial section of the ST segment (“B”). This relationship is appropriate for the LBBB pattern, and is termed discordant ST segment elevation. The right example reveals another form of ST segment change, which is appropriate for LBBB. In this example, the major, terminal portion of the QRS complex is above the baseline (“A”); as such, the initial portion of the ST segment is located below the baseline (“B”), illustrating discordant ST segment depression.The T wave, especially in the right to mid-precordial leads, has a convex upward shape or a tall, vaulting appearance, similar to the hyper-acute T wave of early AMI. The T waves in leads with the monophasic R wave are frequently inverted. Loss of this normal QRS complex-T wave axes discordance in patients with LBBB may imply an acute event, such as AMI. A clinical rule has been developed by Sgarbossa et al to assist in the electrocardiographic diagnosis of AMI in LBBB using specific ECG findings;2 they identified three criteria suggestive of AMI. The criteria suggesting such a diagnosis, ranked using a scoring system based on the probability of AMI, include: ST segment elevation greater than 1 mm, which was concordant with the QRS complex (Figure 3 A, a score of 5).Figure 3: ST segment morphologies suggestive of AMI according to Sgarbossa and colleagues (see text for description).ST segment depression greater than 1 mm in leads V1, V2, or V3 (Figure 3 B, a score of 3). ST segment elevation greater than 5 mm, which is discordant with the QRS complex (Figure 3 C, score of 2). A total score of three or more suggests that the patient is likely experiencing an AMI based on the electrocardiographic information. With a score less than three, the electrocardiographic diagnosis is less assured, requiring additional evaluation. Unfortunately in clinical practice, it is the uncommon patient who will present with LBBB and electrocardiographically obvious AMI. Essentially, one in 10 patients with AMI who have the LBBB pattern will demonstrate ST segment abnormalities beyond those considered appropriate for the altered pattern of ventricular conduction. Prediction Rule Subsequent literature has suggested that the Sgarbossa et al clinical prediction rule2 is less useful than reported. The first such investigation3 which applied the Sgarbossa et al criteria2 to patients with chest pain and LBBB in the ED, found much less promising results, a very low sensitivity coupled with poor interobserver reliability. A second study4 investigated the diagnostic and therapeutic impact of these criteria. None effectively distinguished the patients who had AMI from those patients with noncoronary diagnoses; the authors concluded that these electrocardiographic criteria are poor predictors of AMI in LBBB situations and suggested that all patients suspected of AMI with LBBB should be considered for thrombolysis. The third study5 applied the criteria in retrospective fashion, noting that only concordant STE was associated with a reasonable likelihood of AMI. They also noted that among patients with LBBB, the chronicity of the pattern was associated with AMI only if it was determined to be new. Indeterminant and old LBBB patterns were less often associated with AMI. A fourth investigation6 followed just this recommendation — using a thrombolytic agent in all patients with LBBB presumed to have AMI — and reported an alarmingly high rate of inappropriate thrombolysis in chest pain patients with LBBB and presumed AMI — 49 percent of such patients did not experience acute infarction. These authors also retrospectively investigated the impact of the Sgarbossa et al criteria2 on diagnosis and management. These investigators, in contrast to the previously noted reports, found significant accuracy using the Sgarbossa et al criteria,2 noting an approximate 80 percent rate of correct diagnosis using the prediction rule. Had the clinical prediction rule been employed, the authors suggest that inappropriate thrombolysis would have been avoided in many instances. Regardless of these reports, the Sgarbossa et al criteria2 nonetheless still have merit, if only that they have cast some degree of doubt that the ECG is entirely invalidated as an evaluation tool in the chest pain patient at risk for AMI. Traditional criteria for administration of thrombolytic agents in the AMI patient most often involves electrocardiographic ST segment elevation situated in an anatomic distribution. The presence of a presumably new, or at least not known to be old, LBBB pattern represents another electrocardiographic criterion for such therapy. This second criterion suggests that patients with LBBB and a history suggestive of AMI receive a thrombolytic agent if not otherwise contraindicated. This approach is perhaps reasonable if the physician has a high suspicion of AMI, and is comfortable initiating thrombolysis based solely on clinical grounds; in other words, an analysis of the patient's history and physical examination with little electrocardiographic support. Physicians, however, are uncomfortable administering a thrombolytic agent under such circumstances. In fact, patients with electrocardiographic LBBB and AMI receive thrombolysis less often despite an increased risk of poor outcome2,7 and the potential for significant benefit.8 The clinician must realize that of all patients with chest pain, electrocardiographic LBBB pattern without obvious infarction, and clinically presumed AMI, only a minority will actually be experiencing acute infarction. Treating all such patients with LBBB and presumed AMI will subject a number of non-infarction patients to the not insignificant risks and expense of thrombolysis as seen in the Edhouse et al study.6 Ultimately in this complicated scenario, the clinician must rely on his interpretation of the early ED evaluation (i.e., history, examination, and ECG) to determine the need for thrombolysis in the patient with a presumably new LBBB pattern.