Predicting Acute MI Despite LBBB: The Controversy Continues
abstract & commentary
Source: Edhouse JA, et al. Suspected myocardial infarction and left bundle branch block: electrocardiographic indicators of acute ischemia. J Accident Emerg Med 1999;16:331-335.
Edhouse and associates retrospectively investigated the diagnosis and management of the chest pain patient with suspected acute myocardial infarction (AMI) and electrocardiographic left bundle branch block (LBBB). In this study, they explored the use of thrombolytic agents in these patients, as well as the ability to diagnose AMI electrocardiographically using the clinical decision rule developed by Sgarbossa and co-workers.1
Seven hundred ninety-seven patients with suspected AMI were analyzed; of these patients, 50 (6%) instances of LBBB were noted. Using either serum marker elevations or postmortem examination findings as the standard for diagnosis of AMI, myocardial infarction was noted in only 26 (52%) patients in this initial group. Thirty-three patients with LBBB were thrombolysed for suspected AMI; ultimately, AMI was diagnosed in only 17 (51%). In retrospective fashion, the clinical decision rule developed by Sgarbossa et al was applied. All patients without AMI had clinical prediction rule scores not suggestive of AMI; approximately 80% of those patients with biochemically proven AMI had scores supporting the diagnosis of AMI. Edhouse et al concluded that the diagnosis of AMI in the chest pain patient with LBBB is difficult; they noted that the clinical decision rule was quite useful in the suspected AMI patient with LBBB.
Comment by William J. Brady, MD
Recent literature has addressed this issue,2,3 suggesting that the Sgarbossa et al clinical prediction rule is less useful than reported. The first such investigation,2 which applied the Sgarbossa et al criteria to patients with chest pain and LBBB, found much less promising results — a very low sensitivity coupled with poor interobserver reliability. A second study3 investigated the diagnostic and therapeutic impact of these criteria, finding them ineffective in distinguishing patients with AMI from those with noncoronary diagnoses. The authors concluded that 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. This report, in contradistinction, suggests that the Sgarbossa et al clinical decision rule has value and should be employed in the patient with suspected AMI and LBBB.
Traditional criteria for administration of thrombolytic agents in the AMI patient most often involve electrocardiographic ST segment elevation in an anatomic distribution; the presence of a new LBBB pattern represents another electrocardiographic criterion for such therapy. Many authorities suggest that all patients with LBBB pattern — presumably regardless of its chronicity — and a history suggestive of AMI receive a thrombolytic agent. Such an approach is perhaps reasonable if the physician has a high suspicion of AMI and is comfortable initiating thrombolysis based solely on information from the history and physical examination. Physicians, however, may be 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 outcome1,4 and the potential for significant benefit.5 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 myocardial infarction.1 Treating all patients with LBBB and presumed AMI will subject a number of non-infarction patients to the risks and expense of thrombolysis.
The chest pain patient with LBBB represents a significant challenge to the emergency practitioner. Currently, no single or combination diagnostic approach exists that will reliably reveal AMI in timely fashion. Even if the Sgarbossa et al clinical prediction rule is found to be less useful for interpreting the ECG in the patient with LBBB, this initial report has merit. It has forced the clinician to review the ECG in detail and casts some degree of doubt on the widely taught belief that the ECG is invalidated in the search for AMI in the LBBB patient.
References
1. Sgarbossa EB, et al. Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle branch block. N Engl J Med 1996;334: 481-487.
2. Shapiro NI, et al. Validation of electrocardiographic criteria for diagnosing acute myocardial infarction in the presence of left bundle branch block. Acad Emerg Med 1998;5:508 (abstract).
3. Shlipak MG, et al. Should the electrocardiogram be used to guide therapy for patients with left bundle branch block and suspected acute myocardial infarction? JAMA 1999;281:714-719.
4. Rogers WJ, et al. Treatment of myocardial infarction in the United States (1990 to 1993): Observations from the National Registry of Myocardial Infarction. Circulation 1994;90:2103-2114
5. ISIS-2 (Second International Study of Infarct Survival) Collaborative Group. Randomized trial of intravenous streptokinase, oral aspirin, both, or neither among 17, 187 cases of suspected acute myocardial infarction: ISIS-2. Lancet 1988;2:349-360.
12. In the study by Edhouse et al on left bundle branch block (LBBB) and suspected acute myocardial infarction (AMI):
a. LBBB occurred in 52% of patients with AMI.
b. 51% of patients thrombolysed who had LBBB ultimately had AMI.
c. LBBB occurred in 52% of thrombolysed patients.
d. 33% of patients with AMI had LBBB.
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