Guidelines recommend urgent reperfusion for patients with new left bundle branch block (LBBB), similar to patients with ST-segment elevation myocardial infarction (STEMI). However, there are limited contemporary data comparing these 2 groups of patients. Patients presenting with acute STEMI or presumed new LBBB (nLBBB) enrolled in the Acute Coronary Treatment and Intervention Outcomes Network (ACTION) Registry–Get With the Guidelines (GWTG) from January 2007 to March 2009 were evaluated for clinical characteristics, treatment patterns, and outcomes. Logistic generalized estimating equation modeling was used to examine associated risk-adjusted mortality. Of 46,006 patients with either STEMI or nLBBB, 44,405 (96.5%) had STEMI, and 1,601 (3.5%) had nLBBB. Overall, patients with nLBBB had more baseline co-morbidities compared to those with STEMI. Compared to patients with STEMI, those with nLBBB were less likely to receive acute reperfusion (93.9% vs 48.3% p <0.0001) and were less likely to have door-to-balloon times ≤90 minutes (76.8% vs 34.5%, p <0.0001). Mortality rates were higher for patients with nLBBB compared to those with STEMI (13.3% vs 5.6%, p <0.0001). After multivariate adjustment, nLBBB was not associated with an increased risk for in-hospital mortality (odds ratio 0.91, 95% confidence interval 0.75 to 1.12, p = 0.38). In conclusion, patients with nLBBB were clinically different from those with STEMI, with significantly more co-morbidities, and were less likely to receive emergent reperfusion therapy. Despite these differences, adjusted mortality rates were similar between patients with nLBBB and those with STEMI.
Left bundle branch block (LBBB) has traditionally been regarded as an ST-segment elevation myocardial infarction (STEMI) equivalent in the diagnosis of acute transmural myocardial infarction (MI) in the appropriate clinical setting. Studies have shown that when LBBB is present in the setting of an acute MI, it is associated with increased risk for death and complications. Our aims in this study were to use the National Cardiovascular Data Registry (NCDR) Acute Coronary Treatment and Intervention Outcomes Network (ACTION) Registry–Get With the Guidelines (GWTG) to (1) examine the prevalence of presumed new LBBB (nLBBB) in patients who present with acute MI, (2) compare the clinical characteristics and determine the treatment patterns in patients with acute MI presenting with nLBBB compared to those with persistent ST-segment elevation, and (3) compare the risk for adverse in-hospital cardiovascular outcomes in the 2 groups of patients.
Methods
The ACTION Registry–GWTG and quality improvement program began January 1, 2007. Each participating institution provided approval by its institutional review board. From January 1, 2007, and March 31, 2009, 343 ACTION Registry–GWTG hospitals enrolled 117,781 patients with acute coronary syndromes presenting within 24 hours of symptom onset. We excluded the following patients: those with non–ST-segment elevation MI (n = 71,536), those with missing electrocardiographic findings or isolated posterior MI (n = 160), and those with multiple admissions (only the index admission was included in the analysis; n = 79). This provided a total of 46,006 patients with STEMI (defined as persistent ST-segment elevation or nLBBB) from 333 sites for the analysis. Patients were then analyzed according to whether they presented with STEMI without LBBB or nLBBB. For the purposes of this report henceforth, the term “STEMI” refers to STEMI without LBBB.
Data were abstracted by a trained data collector at each hospital. Variables collected included patient demographics, prehospital data, electrocardiographic findings, medical history, treatments and procedures administered, associated major contraindications to evidence-based therapies, and in-hospital outcomes (all-cause death, stroke, reinfarction, cardiogenic shock, and major bleeding). Presumed new LBBB was defined as LBBB that was not known to be old on initial electrocardiography. In the present study, “LBBB” refers to all occasions of LBBB, “nLBBB” refers to new or presumed new LBBB, and “preexisting LBBB” refers to occasions on which LBBB is known to be previously present. ACTION Registry–GWTG does not collect angiographic data on the culprit lesion or vessel implicated in an MI. To account for different reference normal limits used for cardiac injury markers (troponin and the MB fraction of creatine kinase), cardiac injury markers are reported as the ratio of the measured value to the upper limit of normal for that institution.
ACTION Registry–GWTG collected data on eligibility for and contraindications to primary percutaneous coronary intervention (PCI) or thrombolysis for analysis of patients who underwent reperfusion therapy. Only those who were eligible and did not have contraindications were included in those specific analyses. The proportion of patients taking medications was calculated after excluding those who had contraindications to the specific therapies.
Patients were categorized as having either ST-segment elevation or nLBBB on their qualifying electrocardiograms. Demographic and clinical characteristics, reperfusion strategies, treatment patterns, and in-hospital outcomes were summarized as percentages for categorical variables and as median (first quartile, third quartile) for continuous variables. Comparisons of categorical variables used Mantel-Haenszel chi-square tests, and Wilcoxon’s rank-sum tests were used to compare continuous variables between patients in 2 groups.
A generalized estimating equations logistic regression model with a compound symmetric working correlation matrix and empiric (sandwich) standard error estimates was used to evaluate the multivariate association between nLBBB (vs STEMI) and in-hospital mortality, adjusting for within-site clustering (i.e., statistical dependence) of observations from the same site and the following patient risk factors: age; initial serum creatinine; systolic blood pressure; baseline troponin ratio to upper limit of normal; heart failure only, shock, or shock and heart failure; heart rate (linear spline with knot at 70 beats/min); and previous peripheral arterial disease. These variables were previously identified from a model constructed using ACTION Registry–GWTG data set to independently predict hospital mortality. Transfer-out patients (n = 2,848) were excluded from risk adjusted analysis of mortality. P values are reported without adjustment for multiple comparisons. An α level of 0.05 was used to assess the statistical significance of variables. All analyses were conducted using SAS version 9.2 (SAS Institute, Inc., Cary, North Carolina).
Results
Of 46,006 patients with either STEMI or nLBBB, 44,405 (96.5%) had STEMI, and 1,601 (3.5%) had nLBBB. Overall, patients with nLBBB had more baseline co-morbidities compared to patients with STEMI ( Table 1 ). There were more patients in the STEMI group with left ventricular ejection fractions ≥50% compared to the nLBBB group (47.9% vs 27.2%) and fewer patients with left ventricular ejection fractions <25% (4.8% vs 17.4%, p <0.0001). The median peak troponin and creatine kinase-MB levels, reported as multiples of the reporting institutions’ upper limits of normal, were higher in patients with STEMI compared to those with nLBBB (131.9 vs 32.3, p <0.0001, and 21.8 vs 6.0, p <0.0001, respectively). In addition, more patients in the nLBBB group had maximum creatine kinase-MB levels values less than the reporting institutions’ upper limits of normal compared to the STEMI group (10.1% vs 6.9%, p <0.0001). Initial median serum B-type natriuretic peptide was significantly higher in patients with nLBBB compared to those with STEMI (605 vs 144 pg/mL, p <0.0001), but this was reported in only 41% of patients with nLBBB and 23% of those with STEMI.
| Variable | STEMI (Without LBBB) (n = 44,405) | nLBBB (n = 1,601) | p Value |
|---|---|---|---|
| Age (years) | 60.0 (51.0, 71.0) | 74.0 (63.0, 82.0) | <0.0001 |
| Men | 70.3% | 55.1% | <0.0001 |
| Body mass index (kg/m 2 ) | 28.1 (25.0, 32.0) | 27.4 (24.2, 31.6) | <0.0001 |
| Current/recent smoker (<1 year) | 43.8% | 22.2% | <0.0001 |
| Previously diagnosed or treated hypertension | 60.4% | 77.5% | <0.0001 |
| Previously diagnosed or treated dyslipidemia | 48.6% | 56.7% | <0.0001 |
| Previously diagnosed or treated diabetes mellitus | 21.9% | 40.4% | <0.0001 |
| Renal failure on dialysis | 0.9% | 2.6% | <0.0001 |
| Previous MI | 18.8% | 28.4% | <0.0001 |
| Previous congestive heart failure | 4.4% | 23.9% | <0.0001 |
| Previous PCI | 19.0% | 22.2% | 0.0013 |
| Previous coronary artery bypass graft surgery | 6.5% | 19.2% | <0.0001 |
| Previous stroke | 4.8% | 11.6% | <0.0001 |
| Previous peripheral arterial disease | 5.3% | 14.2% | <0.0001 |
| Initial eGFR (ml/min), nondialysis patients only | 86.0 (62.0, 112.4) | 56.8 (37.3, 81.3) | <0.0001 |
| Baseline hemoglobin (g/dl) | 14.3 (13.1, 15.4) | 13.3 (12.0, 14.7) | <0.0001 |
| Heart rate (beats/minute) | 78 (65, 92) | 91 (74, 109) | <0.0001 |
| Systolic blood pressure (mm Hg) | 138 (118, 158) | 140 (119, 162) | 0.0017 |
| Signs of congestive heart failure at presentation | 11.0% | 43.8% | <0.0001 |
| Cardiogenic shock at presentation | 5.7% | 7.3% | 0.0052 |
Table 2 lists the acute medication use (defined as within the first 24 hours) in the 2 groups of patients. With the exception of heparin, the use of all key acute medications, including antiplatelet agents and statins, was significantly lower in the nLBBB group. Table 3 lists reperfusion therapy characteristics between the 2 groups of patients. Table 4 lists procedural findings between the 2 groups of patients. Of note, there were more patients with no significant coronary artery disease in the nLBBB group (7.3% vs 2.6%) compared to the STEMI group.
| Medication | STEMI (Without LBBB) (n = 44,405) | nLBBB (n = 1,601) | p Value |
|---|---|---|---|
| Aspirin | 98.4% | 95.0% | <0.0001 |
| Clopidogrel | 87.4% | 58.0% | <0.0001 |
| Any oral antiplatelet agent | 98.9% | 94.8% | <0.0001 |
| β blockers | 95.3% | 91.8% | <0.0001 |
| Statins | 68.0% | 55.0% | <0.0001 |
| Glycoprotein IIb/IIIa inhibitors | 73.1% | 38.3% | <0.0001 |
| Any heparin (unfractionated and low molecular weight) | 89.6% | 88.7% | 0.21 |
| Bivalirudin | 13.3% | 8.4% | <0.0001 |
| Variable | STEMI (Without LBBB) (n = 44,405) | nLBBB (n = 1,601) | p Value |
|---|---|---|---|
| Overall reperfusion (primary PCI and thrombolytic) | 93.9% | 48.3% | <0.0001 |
| Thrombolytic therapy | 14.7% | 5.5% | 0.0035 |
| Time to thrombolytic (minutes) | 25.5 (15, 43) | 30.0 (17.5, 49.5) | 0.64 |
| Time to thrombolytic ≤30 minutes (among direct arrivals) | 53.3% | 44.4% | 0.63 |
| Primary PCI | 81.1% | 43.6% | <0.0001 |
| Primary PCI stented (among those with primary PCI) | 91.2% | 89.5% | 0.86 |
| Time to primary PCI, direct arrivals only (minutes) | 70.0 (54, 87) | 103.0 (76, 159) | <0.0001 |
| Time to primary PCI ≤90 minutes (among primary PCI, direct arrival) | 76.8% | 34.5% | <0.0001 |
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