Management of Patients With LBB and Suspected MI
Given the substantial gap between recent evidence and current recommendations, new diagnostic strategies are needed to guide the selection of appropriate patients with suspected AMI and LBBB for urgent reperfusion therapy. The clinical need is greater in centers without on-site primary PCI, because the implications of false catheterization laboratory activation are not as significant as unnecessary administration of fibrinolytic therapy. Potential strategies include differential algorithms for transfer to primary PCI centers for patients with LBBB compared with those who have ST-segment elevation and normal conduction; use of more specific ECG criteria; increased use of cardiac biomarkers, including sensitive assays for cardiac troponins; and bedside echocardiography to improve diagnostic accuracy and timely intervention.
Transfer for primary PCI in patients with STEMI at presentation is equivalent or superior to fibrinolytic therapy as long as the overall first medical contact-to-balloon time is within 120 minutes. In general, if the anticipated time to reperfusion will exceed this threshold, on-site fibrinolytic therapy is recommended. However, because a substantial proportion of patients with LBBB do not have a STEMI-equivalent AMI, and patients with LBBB have an overall higher risk of bleeding (more likely female, older, and with pre-existing cardiovascular disease, hypertension, and congestive heart failure), transfer for primary PCI may be the preferred strategy, with judicious use of on-site fibrinolysis reserved for patients highly likely to have a STEMI-equivalent AMI when PCI is not available. This approach seems particularly attractive for most LBBB patients, because the risk-to-benefit calculation in such patients may favor primary PCI even when the delay associated with transfer for primary PCI exceeds 90 min. Additionally, as prehospital triage of chest pain patients by emergency medical services evolves, patients with LBBB may be recognized immediately and routed preferentially to a PCI-capable hospital, as long as delays are within acceptable time limits.
The use of cardiac biomarkers, specifically the cardiac troponins I and T, holds additional promise in the diagnosis of AMI with LBBB. In recent years, the analytic sensitivity for detection of cardiac troponins has improved 100-fold. Newer assays have improved precision as well, which enables 2 cardiac troponin values with a difference as small as a few picograms per milliliter to be reliably differentiated. This is important because although small cardiac troponin elevations can be measured in many chronic cardiac and noncardiac conditions, and thus lack specificity for AMI, a rise in absolute levels of troponin strongly supports the diagnosis of an evolving AMI. A rapid rise in troponin on serial measurement in a patient with LBBB, especially in the setting of ongoing chest discomfort, could represent a masked STEMI and may prompt additional diagnostic testing such as bedside echocardiography (see below), an invasive angiographic approach, or administration of fibrinolytic therapy if PCI is not available. In contrast, a more gradual rise and lower peak in troponin levels may signal an NSTEMI (in which case, transfer to a PCI-capable facility still would be recommended typically), whereas a static troponin level would suggest a non-ACS cause. Although few data are available, it is becoming increasingly more feasible to accelerate the timing of serial biomarker assessment, such that measurements are performed every 15 min, rather than every 60 to 90 min, in patients with LBBB and suspected AMI. Such an approach would minimize reperfusion delays in those ultimately determined to have STEMI equivalents. Assessment of rapid biomarker measurements, ideally performed at the point of care, should be the focus of additional study in patients with suspected AMI and LBBB.
Finally, bedside echocardiography may be used as an adjunctive measure in clinically complex situations to gain additional insight as to whether a new or presumed new LBBB is the result of a STEMI-equivalent ACS presentation. Point-of-care pocket-size echocardiographic devices can be used by emergency medicine physicians and cardiologists and have been shown to have high feasibility and reliability in the assessment of cardiac structure and function. The presence of significant cardiac chamber dilatation, wall thinning, or chronic valvular dysfunction in the absence of an acute anterior wall motion abnormality suggests that the LBBB is more likely secondary to chronic cardiac disease than AMI. Alternatively, evidence of a hypokinetic or akinetic segmental wall motion abnormality in the anterior wall, in the absence of evidence of a prior infarction (wall thinning, chamber dilatation), may represent a STEMI-equivalent AMI and should prompt emergent coronary angiography. In fact, the use of portable echocardiography to clarify the diagnosis of STEMI, especially if confounded by LBBB, has a class IIa indication in the most recent American College of Cardiology/American Heart Association guidelines. Thus, rapid beside echocardiography represents another potential strategy for improved clinical decision making in patients with suspected AMI and LBBB and may be used in conjunction with the other methods described above.
Investigators have recently proposed changes to the current American College of Cardiology/American Heart Association guidelines for the management of patients with suspected ACS and LBBB, based on the limited randomized trial data demonstrating efficacy of reperfusion therapy in this subgroup. These include demotion of the Class I, Level of Evidence: A recommendation that new or presumed new LBBB be treated as a STEMI equivalent and a proposal that most patients with LBBB be evaluated for ST-segment concordance on ECG and positive cardiac biomarkers to determine the need for urgent reperfusion therapy. We also propose an algorithm for the diagnosis and management of these patients (Figure 3) that includes a rapid clinical and ECG assessment. Clinically or hemodynamically unstable patients with possible AMI and LBBB should be considered for immediate reperfusion therapy. Among stable patients, ECG assessment should be performed to determine the presence or absence of ST-segment concordance criteria in both new and chronic LBBB. Patients meeting ST-segment concordance criteria should be treated as having STEMI-equivalent disease and should receive urgent reperfusion therapy. If concordance criteria are absent, rapid serial cardiac biomarker testing, bedside echocardiography, or both should be considered. If biomarkers increase in the presence of ongoing symptoms or a large wall motion abnormality in the anterior wall is seen, emergent angiography should be performed, recognizing that many of these individuals will still have NSTEMI—rather than STEMI—equivalents, because our current clinical tools are insufficient to distinguish between the 2 entities when evidence of infarction is present. Given the low positive predictive value of LBBB, we recommend that patients with LBBB and possible ACS be transferred routinely to a PCI-capable hospital; fibrinolytic therapy should be considered only if the diagnosis is highly likely (i.e., meeting Sgarbossa criteria) or the patient is hemodynamically unstable and transfer to a PCI-capable facility would result in marked treatment delay. This algorithm would ensure that urgent reperfusion is delivered to the highest-risk patients with the greatest likelihood of acute thrombotic coronary occlusion while taking a more deliberate approach to diagnosis among individuals in whom the clinical picture is less clear. Ideally, such a strategy would result in only small treatment delays in patients with STEMI equivalents and would be balanced by less exposure to risk for the larger proportion of patients with non–ST-segment elevation ACS or non-ACS diagnoses.
(Enlarge Image)
Figure 3.
Proposed Diagnostic Algorithm for Suspected Myocardial Infarction and LBBB
ACS = acute coronary syndrome; NSTEMI = non–ST-segment elevation myocardial infarction; PCI = percutaneous coronary intervention; STEMI = ST-segment elevation myocardial infarction; UA = unstable angina; other abbreviations as in (Figure 1) and (Figure 2).
Given the significant advancements in efficacy and safety of AMI treatments and the lack of contemporary evidence for the existing approach to LBBB, more research encompassing additional diagnostic and therapeutic strategies is needed. Because of the difficulty of diagnosing AMI resulting from acute coronary vessel occlusion, inclusion of LBBB in overall STEMI quality reporting (e.g., door-to-balloon times and other metrics) also should be reconsidered in light of the low specificity and positive predictive value of LBBB for STEMI-equivalent AMI. Removal of LBBB as an automatic STEMI equivalent would provide more specific and accurate evaluation of systems of care and quality of outcomes in STEMI. Therefore, as we evolve toward improved quality of care and better outcomes for patients, management of patients with LBBB and suspected AMI should evolve as well to reflect changing epidemiology, new observational and clinical trial data, advances in technology, and continued high-quality evidence-based research.
Future Strategies
Given the substantial gap between recent evidence and current recommendations, new diagnostic strategies are needed to guide the selection of appropriate patients with suspected AMI and LBBB for urgent reperfusion therapy. The clinical need is greater in centers without on-site primary PCI, because the implications of false catheterization laboratory activation are not as significant as unnecessary administration of fibrinolytic therapy. Potential strategies include differential algorithms for transfer to primary PCI centers for patients with LBBB compared with those who have ST-segment elevation and normal conduction; use of more specific ECG criteria; increased use of cardiac biomarkers, including sensitive assays for cardiac troponins; and bedside echocardiography to improve diagnostic accuracy and timely intervention.
Transfer for primary PCI in patients with STEMI at presentation is equivalent or superior to fibrinolytic therapy as long as the overall first medical contact-to-balloon time is within 120 minutes. In general, if the anticipated time to reperfusion will exceed this threshold, on-site fibrinolytic therapy is recommended. However, because a substantial proportion of patients with LBBB do not have a STEMI-equivalent AMI, and patients with LBBB have an overall higher risk of bleeding (more likely female, older, and with pre-existing cardiovascular disease, hypertension, and congestive heart failure), transfer for primary PCI may be the preferred strategy, with judicious use of on-site fibrinolysis reserved for patients highly likely to have a STEMI-equivalent AMI when PCI is not available. This approach seems particularly attractive for most LBBB patients, because the risk-to-benefit calculation in such patients may favor primary PCI even when the delay associated with transfer for primary PCI exceeds 90 min. Additionally, as prehospital triage of chest pain patients by emergency medical services evolves, patients with LBBB may be recognized immediately and routed preferentially to a PCI-capable hospital, as long as delays are within acceptable time limits.
The use of cardiac biomarkers, specifically the cardiac troponins I and T, holds additional promise in the diagnosis of AMI with LBBB. In recent years, the analytic sensitivity for detection of cardiac troponins has improved 100-fold. Newer assays have improved precision as well, which enables 2 cardiac troponin values with a difference as small as a few picograms per milliliter to be reliably differentiated. This is important because although small cardiac troponin elevations can be measured in many chronic cardiac and noncardiac conditions, and thus lack specificity for AMI, a rise in absolute levels of troponin strongly supports the diagnosis of an evolving AMI. A rapid rise in troponin on serial measurement in a patient with LBBB, especially in the setting of ongoing chest discomfort, could represent a masked STEMI and may prompt additional diagnostic testing such as bedside echocardiography (see below), an invasive angiographic approach, or administration of fibrinolytic therapy if PCI is not available. In contrast, a more gradual rise and lower peak in troponin levels may signal an NSTEMI (in which case, transfer to a PCI-capable facility still would be recommended typically), whereas a static troponin level would suggest a non-ACS cause. Although few data are available, it is becoming increasingly more feasible to accelerate the timing of serial biomarker assessment, such that measurements are performed every 15 min, rather than every 60 to 90 min, in patients with LBBB and suspected AMI. Such an approach would minimize reperfusion delays in those ultimately determined to have STEMI equivalents. Assessment of rapid biomarker measurements, ideally performed at the point of care, should be the focus of additional study in patients with suspected AMI and LBBB.
Finally, bedside echocardiography may be used as an adjunctive measure in clinically complex situations to gain additional insight as to whether a new or presumed new LBBB is the result of a STEMI-equivalent ACS presentation. Point-of-care pocket-size echocardiographic devices can be used by emergency medicine physicians and cardiologists and have been shown to have high feasibility and reliability in the assessment of cardiac structure and function. The presence of significant cardiac chamber dilatation, wall thinning, or chronic valvular dysfunction in the absence of an acute anterior wall motion abnormality suggests that the LBBB is more likely secondary to chronic cardiac disease than AMI. Alternatively, evidence of a hypokinetic or akinetic segmental wall motion abnormality in the anterior wall, in the absence of evidence of a prior infarction (wall thinning, chamber dilatation), may represent a STEMI-equivalent AMI and should prompt emergent coronary angiography. In fact, the use of portable echocardiography to clarify the diagnosis of STEMI, especially if confounded by LBBB, has a class IIa indication in the most recent American College of Cardiology/American Heart Association guidelines. Thus, rapid beside echocardiography represents another potential strategy for improved clinical decision making in patients with suspected AMI and LBBB and may be used in conjunction with the other methods described above.
Investigators have recently proposed changes to the current American College of Cardiology/American Heart Association guidelines for the management of patients with suspected ACS and LBBB, based on the limited randomized trial data demonstrating efficacy of reperfusion therapy in this subgroup. These include demotion of the Class I, Level of Evidence: A recommendation that new or presumed new LBBB be treated as a STEMI equivalent and a proposal that most patients with LBBB be evaluated for ST-segment concordance on ECG and positive cardiac biomarkers to determine the need for urgent reperfusion therapy. We also propose an algorithm for the diagnosis and management of these patients (Figure 3) that includes a rapid clinical and ECG assessment. Clinically or hemodynamically unstable patients with possible AMI and LBBB should be considered for immediate reperfusion therapy. Among stable patients, ECG assessment should be performed to determine the presence or absence of ST-segment concordance criteria in both new and chronic LBBB. Patients meeting ST-segment concordance criteria should be treated as having STEMI-equivalent disease and should receive urgent reperfusion therapy. If concordance criteria are absent, rapid serial cardiac biomarker testing, bedside echocardiography, or both should be considered. If biomarkers increase in the presence of ongoing symptoms or a large wall motion abnormality in the anterior wall is seen, emergent angiography should be performed, recognizing that many of these individuals will still have NSTEMI—rather than STEMI—equivalents, because our current clinical tools are insufficient to distinguish between the 2 entities when evidence of infarction is present. Given the low positive predictive value of LBBB, we recommend that patients with LBBB and possible ACS be transferred routinely to a PCI-capable hospital; fibrinolytic therapy should be considered only if the diagnosis is highly likely (i.e., meeting Sgarbossa criteria) or the patient is hemodynamically unstable and transfer to a PCI-capable facility would result in marked treatment delay. This algorithm would ensure that urgent reperfusion is delivered to the highest-risk patients with the greatest likelihood of acute thrombotic coronary occlusion while taking a more deliberate approach to diagnosis among individuals in whom the clinical picture is less clear. Ideally, such a strategy would result in only small treatment delays in patients with STEMI equivalents and would be balanced by less exposure to risk for the larger proportion of patients with non–ST-segment elevation ACS or non-ACS diagnoses.
(Enlarge Image)
Figure 3.
Proposed Diagnostic Algorithm for Suspected Myocardial Infarction and LBBB
ACS = acute coronary syndrome; NSTEMI = non–ST-segment elevation myocardial infarction; PCI = percutaneous coronary intervention; STEMI = ST-segment elevation myocardial infarction; UA = unstable angina; other abbreviations as in (Figure 1) and (Figure 2).
Given the significant advancements in efficacy and safety of AMI treatments and the lack of contemporary evidence for the existing approach to LBBB, more research encompassing additional diagnostic and therapeutic strategies is needed. Because of the difficulty of diagnosing AMI resulting from acute coronary vessel occlusion, inclusion of LBBB in overall STEMI quality reporting (e.g., door-to-balloon times and other metrics) also should be reconsidered in light of the low specificity and positive predictive value of LBBB for STEMI-equivalent AMI. Removal of LBBB as an automatic STEMI equivalent would provide more specific and accurate evaluation of systems of care and quality of outcomes in STEMI. Therefore, as we evolve toward improved quality of care and better outcomes for patients, management of patients with LBBB and suspected AMI should evolve as well to reflect changing epidemiology, new observational and clinical trial data, advances in technology, and continued high-quality evidence-based research.
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