Incidence, Clinical Characteristics, and Prognostic Significance of Right Bundle-Branch Block in Acute Myocardial Infarction
A Study in the Thrombolytic Era
Background Whereas the significance of right bundle-branch block (RBBB) in acute myocardial infarction was extensively studied in the prethrombolytic era, a possible change in the overall incidence and meaning of RBBB as a consequence of thrombolytic therapy is not well known.
Methods and Results A multicenter, prospective study of 1238 patients consecutively diagnosed with acute myocardial infarction and admitted to three coronary care units was conducted. ECGs during the acute phase and clinical events until discharge and 1-year follow-up were monitored. In the 135 (10.9%) patients in whom RBBB was found, there were 51 (37.8%) new cases, 46 (34.1%) old cases, and 38 (28.1%) cases with an indeterminate time of origin. New RBBB was permanent in 26 and transient in 25 patients. RBBB was isolated in 76 (56%) and bifascicular in the remaining 59 (44%) patients. The following complications were more frequently associated with RBBB than non-RBBB patients: heart failure, 24% versus 46% (P<.001); use of pacemaker because of atrioventricular block, 3.6% versus 11% (P<.001); and 1-year mortality, 17.6% versus 40.7% (P<.001). Early mortality was significantly higher for new RBBB (43.1%, P<.001) than for old (15.5%) and indeterminate (15.3%) RBBB. These figures for 1-year mortality were 58.8% (P<.001), 35.5 (P<.01), and 23% (NS), respectively. Permanent and transient RBBB had different mortality rates: early mortality, 76% versus 8%, and 1-year mortality, 84% versus 32% (P<.001 for both). For isolated RBBB versus bifascicular block, early mortality was 14.4% versus 40.6%, and 1-year mortality was 30.2% versus 54.2% (P<.05 for both). Multivariate analysis showed an independent prognostic value of RBBB for early and 1-year mortality.
Conclusions The overall meaning of RBBB in acute myocardial infarction has not changed in the thrombolytic era, although a higher rate of new and transient RBBB and a lower rate of bifascicular block may represent a beneficial effect of thrombolytic therapy.
Various studies from the prethrombolytic era have reported the incidence of RBBB in AMI to be between 3% and 29%.1 2 3 4 5 6 Considering the anatomy and vascular supply of the conduction system,5 7 RBBB is usually the manifestation of large infarctions that are often accompanied by heart failure, complete AV block, arrhythmias, and a high mortality rate.1 2 3 4 5 6 7 8 9 10 11 12 The classification of RBBB according to timing, duration, and association with fascicular block is of clinical importance.9 10 13 However, many studies of RBBB in AMI took place in the prethrombolytic era. Thrombolytic treatment limits infarct size,14 15 improves ventricular morphology and function,16 and decreases mortality.17 18 19 20 Moreover, studies have connected the reversibility of conduction disturbances with coronary reperfusion,21 22 which suggests that thrombolytic treatment can prevent the appearance or limit the duration of bundle-branch blocks. Thus it is probable that the current wide use of thrombolytic therapy may have changed the overall incidence and significance of RBBB in AMI and therefore it is reasonable to reanalyze its meaning in the thrombolytic era.
Based on these considerations, the aim of the present study was to determine the current overall incidence and significance of RBBB complicating AMI and to compare our results with those reported by others.
Between June 1992 and January 1994, we conducted a prospective study of 1238 consecutive patients admitted to the coronary care units of three hospitals in the Murcia region of Spain for AMI <24 hours from the onset of symptoms. Diagnosis was established by the presence of typical precordial pain lasting at least 30 minutes, an increase or decrease of >0.1 mV in ST displacement in more than two leads, and an increase in CK of double the normal value (190 U/L). Routine clinical and historical data as well as infarct complications were recorded for all patients.
A standard 12-lead ECG was recorded on admission and every 8 hours and for patients who had received thrombolytic therapy, at 2, 4, 6, 8, and 12 hours and then daily. Blood samples were taken on admission and every 8 hours during the first day to determine CK values.
Location and Extent of Infarction
AMI was classified as Q or non-Q depending on the presence or absence, respectively, of pathological Q waves in any of the 12 ECG leads. Transmural infarction was considered as anterior or inferior when Q waves appeared in any lead from V1 to V6 or II, III, and aVF, respectively. As indexes of infarct size we used the peak CK, the number of leads with elevated ST segments, and the LVEF. The latter was measured in a subgroup of 724 patients before hospital discharge (on days 7 and 9) by using radionuclide ventriculography or echocardiography.
Diagnostic Criteria for RBBB
RBBB and fascicular block were defined by using standard ECG criteria23 ; a QRS duration of ≥120 ms was required. RBBB was classified according to the time of appearance: new if the RBBB appeared after admission or was present on admission but was not recorded on an ECG within the previous 6 months; old if the RBBB was present on admission and documented on a previous ECG; and of indeterminate age if the RBBB was present on admission and no previous ECG was available. According to the presence or absence of an associated hemiblock, RBBB was classified as bifascicular or isolated. New RBBBs were classified as transient when there was no RBBB present at the time of hospital discharge and permanent when the patient either died or was discharged with RBBB present. The diagnosis of left anterior and left posterior hemiblock was made by using the criteria of Rosenbaum.24 Left anterior hemiblock required a leftward shift of the QRS axis to ≤−30° and left posterior hemiblock a rightward shift to ≥120°. BB was defined as RBBB with left anterior or left posterior hemiblock. Blocks that were found in the terminal phase of cardiogenic shock were excluded from analysis.
Thrombolytic therapy was based on clinical criteria and administered by the attending physician. Either streptokinase (1.5 million U IV over 1 hour) or recombinant human tissue plasminogen activator (100 mg IV over 90 minutes) was used according to standard dosing protocols.25
IRC were considered in the presence of two or more of the following: (1) CK peak <14 hours from the onset of pain26 ; (2) 50% decrease in the ST segment 2 hours after the start of thrombolysis compared with the baseline value27 ; or (3) early relief of pain associated with reperfusion arrhythmias (thrombolysis-related accelerated idioventricular rhythm or sinus bradycardia).
All the patients were followed up from the time of admission to the coronary care unit to either death or 1 year. Early mortality was defined as death that occurred during the hospital stay and total mortality as death that occurred between admission and 1 year of follow-up. The in-hospital follow-up was accomplished in 100% of the patients and the 1-year follow-up in 96.5% of all those discharged alive from the hospital.
A descriptive analysis was done on all variables to obtain a frequency distribution. For the quantitative variables we also determined the mean, median, SD, and range. The relationships between categorical variables were assessed by using contingency tables, the χ2 test, and an analysis of residues to test the trend of associations or dependence. Between-group comparisons were studied by using ANOVA complemented with means equality contrasts using Student’s t test. The level of significance was P<.05. Several variables with influence on mortality were included in a multiple logistic regression analysis. The F ratios presented indicate the relative importance of each factor selected after adjusting for all other factors. The statistical package used was BMDP.28
Among 1238 patients, 681 (55%) were treated with thrombolytic agents. The mean time between the onset of symptoms and admission was 5.5±6.8 hours for all patients. RBBB was documented in 135 patients (10.9%). The incidence of the different blocks according to timing or association with fascicular defects is shown in Table 1⇓. New RBBBs were present on admission or during the first hour in 64% of the cases and in 80% during the first 24 hours. There were 51 cases of new RBBB. Of these, 25 (49%) were transient, lasting <6 hours in 17 patients.
Clinical Characteristics of RBBB Patients
The clinical characteristics and indexes of extent of infarction according to timing or association with hemiblock are shown in Tables 2⇓ and 3⇓. Compared with the patients without RBBB, those with old RBBB were older and had a higher prevalence of history of diabetes, angina, MI, and heart failure. New RBBB was more frequently associated with female gender, anterior location, and extensive infarction. Permanent new RBBBs were often associated with female gender, a history of diabetes, and fewer IRC (Table 4⇓).
RBBB-Associated Complications and Mortality
Compared with non-RBBB patients, RBBB was associated with a greater incidence of heart failure (46% versus 24%, P<.001) and the need for a pacemaker (11% versus 3.6%, P<.001). New RBBBs and BBs were associated with a greater incidence of ventricular fibrillation, cardiogenic shock, complete AV block, and the use of pacemakers (Table 5⇓).
The early and total mortality rates of patients with and without RBBB are shown in Table 4⇑. Compared with patients without RBBB, early mortality was significantly greater only for new RBBB and BB patients, whereas differences in total mortality reached significance for new and old but not for RBBB of indeterminate duration.
Patients with new but permanent RBBB demonstrated greater early and total mortality rates than those with transient new RBBB (early, 76% versus 8%; total, 84% versus 32%; P<.001 for both) (Table 6⇓).
Age, female gender, previous MI, previous heart failure, infarct location, in-hospital heart failure, and RBBB were included in a multiple logistic regression analysis. RBBB was an independent predictor of both early and total mortality (Table 7⇓). Similar results were obtained when only new-onset RBBBs were considered in this analysis.
Isolated RBBB Versus BB
Compared with isolated RBBB, BB was more often associated with anterior location and large infarctions as reflected by the number of affected ECG leads or the CK level (Table 3⇑). BB patients also had more clinical complications, including early (40% versus 14.3%, P<.01) and total (54.2% versus 30.2%, P<.05) mortality (Table 5⇑).
Incidence and Clinical Characteristics of RBBB
The overall rate of RBBB in our study (10.9%) is within the range of previous series.1 2 3 4 5 6 7 8 9 10 11 12 13 According to our criteria, we considered RBBB as new in a percentage (38%) that was similar to that of Hindman et al9 but far below the 64% found by Ricou et al,13 who conducted their study only in anterior infarction patients, who are more often complicated with RBBB.10 11 12 New RBBBs could have been underestimated in the study of Hindman et al9 because patients who presented with RBBB in the first ECG, with infarction being absent in ECGs taken within the two previous years, were classified as RBBB of indeterminate age. In contrast, this pattern of appearance of RBBB was classified as new both in the study by Ricou et al13 and the present study. Half of our new RBBBs were transient, rather more than the 30% and 27% reported by Lie et al10 and Ricou et al,13 respectively. The duration of the transient blocks was >6 hours in only 24% of our cases, almost half the 47% reported by Lie et al.10
RBBB was most frequently associated with the anterior site and extensive necrosis.2 3 9 29 30 31 32 33 The type of RBBB corresponded to certain specific characteristics. New RBBB was more common in women and almost exclusively associated with the anterior site and a greater extent of infarction, whereas old RBBB affected patients who were older and had a more prevalent history of diabetes and CVDs.
The RBBBs of indeterminate duration showed an extent of infarction that was intermediate to the new and old RBBBs, suggesting that this group included “true old” or “true new” RBBBs, in which this conduction disturbance was present on admission without there existing, in either case, a previous ECG tracing for a true classification. Patients with RBBB of indeterminate age very infrequently had a cardiovascular history, and they had a lower mortality than one would expect if this group were the sum of new and old RBBBs. Our data suggested that old RBBBs misclassified as indeterminate would constitute a low-risk population with few or no previous symptoms and therefore with few prior ECGs.
Isolated RBBB Versus BB
We found RBBB associated with fascicular block (44%) to a lesser extent than others: 72% in Hindman et al,9 an average of 67% in the review by Klein et al,8 and 61% in Ricou et al.13 Our new RBBBs were more often accompanied by fascicular block, whereas Hindman et al9 report a similar incidence in new (82%), old (68%), and indeterminate (83%) RBBB. Compared with isolated RBBBs, our BBs were often associated with the anterior site and a greater extent of infarction; the patients affected had a more prevalent history of heart failure and diabetes.
Prognostic Significance of RBBB
RBBB in the prethrombolytic era was associated with a high incidence of heart failure, ventricular arrhythmias, and advanced AV block.1 2 3 4 5 6 7 8 9 10 11 12 In line with the findings of clinicopathological studies,33 34 these complications seem to be a consequence of large infarcts associated with RBBB, although ventricular fibrillation could also be favored by the existence of nonuniform areas of block in Purkinje tissue.11
Our RBBB patients had an in-hospital mortality rate almost three times greater than that of patients without RBBB (25.9% versus 9.9%), a difference similar to that found in studies by Hindman et al9 (28% versus 12%) and Ricou et al13 (32% versus 8%), whereas Klein et al8 report an in-hospital mortality rate for isolated RBBB of 35%.
New RBBB was accompanied by a higher mortality rate than old RBBB, a finding that contrasts with the similar rates reported by Hindman et al9 (33% and 29%, respectively). These discrepancies may be due to the different definition of new RBBB employed in the present study and that of Hindman and colleagues. Ricou et al,13 who used a definition similar to ours, also found a different mortality rate for new and old RBBB (34% versus 14%).
A striking find is the high mortality rate associated with new permanent RBBB. Among the 22 deaths in this group, 20 occurred during hospitalization (in-hospital mortality rate, 77%). These early deaths should be related almost entirely to severe heart failure since 20 of those patients, with new permanent RBBB, were in Killip class ≥III; 16 of these died in cardiogenic shock. Hindman et al9 found that mortality associated with RBBB occurred in spite of a large percentage of patients being fitted with a pacemaker,9 13 whereas advanced AV block did not represent an additional mortality in Killip class III or IV patients.9 However, as our multivariate analysis showed (in accordance with the results of Ricou et al13 ), RBBB is an independent predictor for mortality and therefore represents an additional contribution to the poor outcome.
Considering that the different clinical courses between the permanent and transient RBBBs in the present study were observed in spite of a similarly depressed ventricular function, it could be that as well as in long-term studies of survival, those patients with poor ejection fractions but larger ventricular volumes had the worst prognosis. Moreover, RBBB may be a consequence of acute right ventricular dilatation,36 which seems to be probable in those patients with severe heart failure complicating AMI. Therefore, the appearance of new permanent RBBB in AMI might be interpreted as an ECG expression of an important short-term ventricular enlargement, although unfortunately we did not determine ventricular volumes to confirm this hypothesis.
Prognostic Differences Between Isolated RBBBs and BBs
Patients with BBs are reported to have higher short-term and 1-year mortality than those with isolated RBBBs.5 8 9 Following our data, poor prognosis of hemiblock associated with RBBB may be the consequence either of larger infarcts or the more prevalent history of diabetes and heart failure seen in these patients compared with those with isolated RBBB. In addition, more than half of our BBs were new, whereas isolated RBBBs were more often classified as old or indeterminate. Thus, part of the higher mortality associated with BBs may also be related to the high incidence of new RBBB rather than only to the association with left fascicular block.
Possible Influences of Thrombolytic Treatment
In order to address whether the current and wide use of thrombolytic treatment would have changed the incidence and meaning of RBBB, a comparison between our and previous findings was considered an appropriate approach according to the following premises. First, all previous studies that focused on the incidence and meaning of RBBB1 2 3 4 5 6 9 10 11 12 13 were conducted in the prethrombolytic era, except for a small percentage of patients in one study.13 Second, the usual indications guiding fibrinolysis often bias the selection of a patient,37 which precludes determining whether possible differences found with our nonthrombolyzed patients could be related to this treatment. Third, a study based on a more reliable comparison between patients randomized to be treated or not treated with thrombolytics should be rejected for ethical reasons due to the great benefit associated with the former.17 18 19 20
Although the incidence of new RBBB seems to have decreased, the most notable differences between our results and earlier studies are a lower frequency of bifascicular disturbances and a greater incidence but shorter duration of transient RBBB. Rapid resolution of RBBB in AMI has been described9 13 and more recently has been linked with thrombolytic therapy.21 22 Experimental and clinical evidence suggests that transient conduction disturbances may be related to extensive but reversible ischemia and inflammatory responses surrounding the specialized conduction tissue.33 38 39
The short duration of RBBB in our study should be related to successful fibrinolysis since transient RBBB had a higher rate of IRC. However, both transient and permanent RBBB showed similar large infarcts. Therefore, differences in short-term outcome between transient and permanent RBBB, including a thenfold higher early mortality in the latter, seem to exceed the apparent myocardial salvage related to thrombolytic therapy. Considering that thrombolysis has greater effects on attenuating ventricular dilatation than preserving systolic function,16 it is possible that the short duration of new RBBB represents patients with similar large infarcts but lesser short-term ventricular enlargement than those with permanent RBBB.
It is of clinical interest to remark that the good short-term outcome associated with transient RBBB was followed by a higher 1-year mortality. Poor long-term prognosis associated with RBBB of short duration after thrombolysis may be due to severe residual stenosis of the left ventricular descending artery and the depressed systolic function often found in those patients.21 22 Thus, our results seem to confirm that transient RBBB should be considered as a risk marker, and we advocate an invasive strategy to identify those patients for whom revascularization might improve prognosis.
Compared with the thrombolytic era, the incidence and prognostic significance of RBBB associated with AMI has not changed considerably in the population currently admitted to our coronary units. However, there seems to be a decrease in BB and an increase in transient new RBBB, which could be attributed to the administration of fibrinolytic therapy to a higher percentage of patients. The classification of RBBB according to timing, duration, and association with fascicular disturbances of the left bundle branch is of practical interest since it corresponds to clinical characteristics and prognoses. The greatest risk is associated with BB and new RBBB, especially permanent ones, with a mortality rate of 84% in the latter. Transient new RBBB has a favorable short-term prognosis, similar to that of patients without RBBB, but it is accompanied by a high mortality rate during follow-up that calls for an invasive evaluation of these patients in order to establish a possible benefit from revascularization.
Selected Abbreviations and Acronyms
|AMI||=||acute myocardial infarction|
|IRC||=||indirect reperfusion criteria|
|LVEF||=||left ventricular ejection fraction|
|RBBB||=||right bundle-branch block|
We gratefully acknowledge Dr G.A. Lamas for the preparation of the manuscript. We thank Professor M. Canteras for his assistance in preparing the statistics.
- Received November 26, 1996.
- Revision received March 25, 1997.
- Accepted March 26, 1997.
- Copyright © 1997 by American Heart Association
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