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Acute myocardial infarction in patients with left bundle branch block
  1. Matt Shepherd1,
  2. Richard Hardern2
  1. 1Registrar
  2. 2Consultant, Acute Medical Assessment Area, The General Infirmary, Great George Street, Leeds LS1 3EX

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    Editor,—We read with interest the paper about the electrocardiographic diagnosis of acute myocardial infarction (AMI) in patients with left bundle branch block (LBBB).1 It emphasises the difficulties many have had with electrocardiogram (ECG) interpretation in this situation and explains clearly how to use the criteria of Sgarbossa et al.2 It concludes that these criteria can be used to identify patients with LBBB and AMI.

    It is essential that accident and emergency staff recognise this group of patients so that thrombolysis is delivered promptly. Shlipak et al reviewed patients presenting with LBBB and an acute cardiopulmonary history and assessed the usefulness of the Sgarbossa criteria.3 They found that these criteria had a sensitivity of 10% and a specificity of 100%. Although an ECG that satisfies the criteria is almost certainly indicative of AMI, most (90%) patients with AMI will not meet the criteria. If thrombolytics were to be withheld unless the criteria were met, few patients in this high risk group would receive appropriate treatment.

    Rather than relying on the Sgarbossa criteria, we feel it would be more appropriate to thrombolyse all patients (except those with contraindications) who have a history suggestive of AMI and LBBB. This policy is supported by the data of Shlipak et al.

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    The authors reply

    We read with interest the comments of Shepherd and Hardern concerning our article. In large part, we agree with their thoughts. In our report, we stressed several points, including (1) the confounding effect of LBBB pattern on the electrocardiographic diagnosis of AMI; (2) the “normal” or expected findings of LBBB; and (3) additional electrocardiographic strategies to assist in identifying the patient with a potential AMI. Several electrocardiographic strategies are available to the clinician to assist in this endeavour such as comparison with old ECGs, examination of serial ECGs, and a sound understanding of the anticipated ST segment changes resulting from LBBB. These strategies may be supplemented by the clinical decision rule developed by Sgarbossa et al.1

    Since our report was published, recent literature2, 3 has suggested that the Sgarbossa et al clinical prediction rule is less useful than reported. The first such investigation,2 not noted by Shepherd and Hardern, which applied the Sgarbossa et al criteria to patients with chest pain and LBBB in the emergency department of a North American hospital, found much less promising results—a very low sensitivity coupled with poor interobserver reliability. And, as noted by Shepherd and Hardern, a second study3 investigated the diagnostic and therapeutic impact of this criteria—none effectively distinguished the patients who had AMI from those patients with non-coronary 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. As we stated, even if the Sgarbossa et al clinical prediction rule is found to be less useful in the objective evaluation of the ECG in the patient with LBBB, the report has merit—it has forced the clinician to review the ECG in detail and cast some degree of doubt on the widely taught belief that the ECG is invalidated in the search for AMI in the LBBB patient.

    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 new LBBB pattern represents another electrocardiographic criterion for such treatment. Shepherd and Hardern suggest that all appropriate 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 clinical information—in other words, an analysis of the patient's history and physical examination. Physicians, however, may be uncomfortable administering a thrombolytic agent under such circumstances; in fact, patients with electrocardiographic LBBB and AMI less often receive thrombolysis despite an increased risk of poor outcome1, 4 and the potential for significant benefit.5 The clinician must realise 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 such patients with LBBB and presumed AMI will subject a number of non-infarction patients to the not insignificant 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 which will reliably reveal AMI in timely fashion. Our article was intended to review the appropriate principles of electrocardiography in the LBBB pattern in the hopes that the emergency practitioner would be better versed in interpretation of these complicated ECGs and therefore offer the AMI patient the correct treatment in rapid order.

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