Myocardial Infarction + LBBB

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Diagnosis and reasoning

This elderly gentleman has presented with classical ischemic chest pain; this should be considered to be an acute coronary syndrome (ACS) until proven otherwise. However, his ECG reveals the presence of a left-bundle branch block (LBBB), resulting in a diagnostic conundrum. Until recently, the American College of Cardiology Foundation/American Heart Association (ACCF/AHA) guidelines recommended that patients with suspected ischemia, and a new or presumably new LBBB be considered as having a ST-elevation myocardial infarction (STEMI). However, the 2013 STEMI guidelines of the AHA/ACC no longer recommend this, possibly because coronary angiography has shown that only between 14% to 44% of such patients actually have an acute arterial occlusion. This is a diagnostic dilemma - although the majority of these patients do not have an acute myocardial infarction (MI), a non-significant percentage of them do. As withholding reperfusion therapy to this population may be potentially lethal, what can be done? Fortunately, there are several non-invasive diagnostic tools which can be used to accurately identify patients presenting with LBBB who do have acute coronary occlusion; the Sgarbossa ECG criteria are the most validated of these. The Sgarbossa tool consists of 3 criteria, each with a different score; a total score of 3 or more (out of 10) is over 95% specific for the diagnosis of acute MI; note that the score is discussed in more detail in the "discussion" section of this case. For purposes of diagnosing this patient, note that the first Sgarbossa criterion is "ST-segment elevation ≥1 mm concordant with the QRS complex in any lead (5 points)". One key characteristic of an uncomplicated LBBB is the presence of "ST-T discordance"; in other words, if the QRS complex is mainly directed upwards (as in the lateral leads), the ST-segment will be depressed, and T-wave inverted; conversely, if the QRS vector is mainly directed downwards (as in the right precordial leads) the ST-segment will be elevated, and the T-wave prominently positive. Loss of this characteristic pattern (i.e. the QRS complex and ST-T segment and T wave all being concordant) is strongly specific for an acute MI; note that this is present in this patient; according to the Sgarbossa criteria, this gives us a score of 5 points which is sufficient to diagnose the presence of MI. While cardiac troponins are highly specific for myocardial injury, and are the preferred biomarkers for diagnosis of MI, it should be appreciated that these are ideally estimated at least 6 hours following symptom onset; assays performed earlier may be misleadingly low. In addition, while an echocardiogram may demonstrate segmental hypokinesis supportive of an acute MI, this is not essential in the presence of a definitive electrocardiographic diagnosis, and should never delay reperfusion therapy. Note that a chest x-ray can be rapidly performed in the emergency department and is routinely recommended in all patients with chest pain; this is to exclude aortic dissection, which itself can give rise to a secondary MI. As he has presented within 12 hours of symptom onset, the 2013 ACCF/AHA guidelines recommend he should receive primary percutaneous coronary intervention (PCI). Antacids, placement of a nasogastric (NG) tube, or aortic surgery are not indicated in this patient.


Discussion

Acute myocardial infarction (MI) is one of the most common diseases encountered in the emergency department; it is also full of diagnostic and management pitfalls, failure to appreciate which may result in considerable mortality and morbidity, as well as legal consequences to the treating clinician. One key pitfall is the diagnosis of acute MI in the presence of patients with left bundle branch block (LBBB). The 1996 and 2004 American College of Cardiology Foundation/American Heart Association (ACCF/AHA) guidelines on the management of ST-elevation myocardial infarction (STEMI) recommended that patients with suspected ischemia, and a new or presumably new LBBB be considered as having a ST-elevation myocardial infarction (STEMI). These guidelines were principally based on early fibrinolytic trials, where the diagnosis of acute MI was confirmed via the presence of positive cardiac biomarkers. However, the increasing use of angiography and percutaneous coronary intervention (PCI) revealed the fact that only between 14% to 44% of such patients actually had an acute arterial occlusion. This is presumably because LBBB is often a chronic manifestation of hypertension and myocardial dysfunction rather than an acute abnormality; as it is impossible to definitively determine if an LBBB is 'new' unless a recent previous ECG is available, many such patients referred for reperfusion probably had an 'old' LBBB. Note also that the risk of adverse effects of thrombolysis (including including intracranial bleeding) is overall higher in patients with LBBB, as they are more likely to be female, older, and have both hypertension and cardiovascular disease. Thus, the 2013 STEMI guidelines of the AHA/ACC no longer recommend that patients with LBBB routinely receive reperfusion therapy. Unfortunately, the conceptual change induced by the new guidelines creates an entirely new issue: while many of these patients do not have an acute myocardial infarction (MI), a not-insignificant percentage of them still do; given the high mortality of MI, delaying reperfusion therapy in this population could be fatal. However, there are some clinical observations as well as non-invasive diagnostic tools which may help with the diagnosis of acute MI in this population. One important clinical point is that patients with a new-onset LBBB due to acute MI are highly likely to be hemodynamically unstable, as very proximal coronary occlusion is required to involve the septal perforating arteries that supply the proximal left bundle branch. Thus, the presence of hemodynamic instability or acute heart failure in patients with an LBBB tracing is suspicious of acute MI; primary PCI or thrombolytic therapy should be considered in this population. The Sgarbossa ECG criteria are the oldest and most validated tool for diagnosis of acute MI in patients with an LBBB; they are as follows: - ST-segment elevation ≥1 mm concordant with the QRS complex in any lead (+ 5 points) - ST-segment depression ≥1 mm in lead V1, V2, or V3 (+ 3 points) - ST-segment elevation ≥5 mm discordant with the QRS complex in any lead (+ 2 points) A score of 3 points or more is over 95% specific for the diagnosis of acute MI in this population; unfortunately the sensitivity is low, ranging from 20% to 79%. Thus, the Sgarbossa criteria are more useful for ruling in the diagnosis of acute MI, than ruling it out. Smith et al, and colleagues proposed a modification to the above criteria; they found out that replacing the 3rd Sgarbossa criterion with a ST/S ratio (the ratio of ST-segment elevation measured at the J point to the R or S wave, whichever is most prominent) of -0.25 or less greatly increased the diagnostic sensitivity of the criteria from 52% to 91%, while maintaining a specificity of 90%. However, it should be noted that at the time of writing, the modified Sgarbossa criteria are yet to be verified in an independent study. If both the normal and modified Sgarbossa criteria are negative, but there is strong clinical suspicion of an acute MI, such patients should receive further evaluation via serial ECGs, serial cardiac biomarker assays, and bedside echocardiography. The presence of dynamic ECG changes, rapidly increasing biomarker levels or development of regional wall motion abnormalities or a reduced cardiac ejection fraction should spur referral to a cardiac catheterization laboratory for further evaluation and possibly reperfusion therapy.


Take home messages

  1. LBBB is often a chronic manifestation of hypertension and myocardial dysfunction rather than an acute abnormality.
  2. The longstanding recommendation that patients with suspected ischemia and a new or presumably new LBBB be treated as having an STEMI has ceased with the 2013 STEMI guidelines of the AHA/ACC.
  3. The Sgarbossa criteria are specific, but not sensitive for detection of STEMI in patients with LBBB.
  4. Use of the ST/S ratio along with the Sgarbossa criteria improves the sensitivity of detection of STEMI in patients with LBBB, while maintaining a high specificity.

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