Heart Left Bundle Branch Block Research Articles

Left bundle branch area pacing results in more physiological ventricular activation than biventricular pacing in patients with left bundle branch block heart failure.

Biventricular pacing (Biv) and left bundle branch area pacing (LBBAP) are methods of cardiac resynchronization therapy (CRT). Currently, little is known about how they differ in terms of ventricular activation. This study compared ventricular activation patterns in left bundle branch block (LBBB) heart failure patients using an ultra-high-frequency electrocardiography (UHF-ECG). This was a retrospective analysis including 80 CRT patients from two centres. UHF-ECG data were obtained during LBBB, LBBAP, and Biv. Left bundle branch area pacing patients were divided into non-selective left bundle branch pacing (NSLBBP) or left ventricular septal pacing (LVSP) and into groups with V6 R-wave peak times (V6RWPT) < 90 ms and ≥ 90 ms. Calculated parameters were: e-DYS (time difference between the first and last activation in V1-V8 leads) and Vdmean (average of V1-V8 local depolarization durations). In LBBB patients (n = 80) indicated for CRT, spontaneous rhythms were compared with Biv (39) and LBBAP rhythms (64). Although both Biv and LBBAP significantly reduced QRS duration (QRSd) compared with LBBB (from 172 to 148 and 152 ms, respectively, both P < 0.001), the difference between them was not significant (P = 0.2). Left bundle branch area pacing led to shorter e-DYS (24 ms) than Biv (33 ms; P = 0.008) and shorter Vdmean (53 vs. 59 ms; P = 0.003). No differences in QRSd, e-DYS, or Vdmean were found between NSLBBP, LVSP, and LBBAP with paced V6RWPTs < 90 and ≥ 90 ms. Both Biv CRT and LBBAP significantly reduce ventricular dyssynchrony in CRT patients with LBBB. Left bundle branch area pacing is associated with more physiological ventricular activation.

Left bundle branch pacing.

Chronic right ventricular (RV) pacing has been associated with significant electrical and mechanical dyssynchrony leading to increased risk for recurrent heart failure hospitalizations and atrial arrhythmias. His bundle pacing (HBP) is an effective alternative to RV pacing as it is physiological and provides synchronized contraction of both ventricles. But there are limitations to HBP, which include lead stability, rise in threshold, early battery depletion and longer learning curve. Huang etal. recently reported anovel technique to directly capture the left bundle branch (LBB) by deep septal pacing. Subsequently, many studies have demonstrated the feasibility, safety and efficacy of left bundle branch pacing (LBBP). This has the potential to overcome the limitations of HBP and provide asafe technique to capture the conduction system in patients with distal His bundle and proximal bundle branch disease. The criteria for LBB capture and the methodology to perform LBBP are discussed in detail in this review. The Medtronic SelectSecure®3830 pacing lead is used along with afixed-curve C315His® or adeflectable C304His® sheath. LBBP providessafe and low threshold compared to HBP. Left bundle potential should be demonstrable in all patients except in those with infrahisian complete heart block (CHB) and complete left bundle branch block (LBBB), wherein antegrade activation of the left bundle will not occur. LBBP has the potential to be an effective alternative to biventricular pacing or HBP in patients with left ventricular dysfunction, LBBB and recurrent heart failure. Long-term safety and clinical outcomes compared to traditional pacing need to be carefully studied in randomized clinical trials.

Left ventricular hemodynamic forces as a marker of mechanical dyssynchrony in heart failure patients with left bundle branch block

Left bundle branch block (LBBB) causes left ventricular (LV) dyssynchrony which is often associated with heart failure. A significant proportion of heart failure patients do not demonstrate clinical improvement despite cardiac resynchronization therapy (CRT). How LBBB-related effects on LV diastolic function may contribute to those therapeutic failures has not been clarified. We hypothesized that LV hemodynamic forces calculated from 4D flow MRI could serve as a marker of diastolic mechanical dyssynchrony in LBBB hearts. MRI data were acquired in heart failure patients with LBBB or matched patients without LBBB. LV pressure gradients were calculated from the Navier-Stokes equations. Integration of the pressure gradients over the LV volume rendered the hemodynamic forces. The findings demonstrate that the LV filling forces are more orthogonal to the main LV flow direction in heart failure patients with LBBB compared to those without LBBB during early but not late diastole. The greater the conduction abnormality the greater the discordance of LV filling force with the predominant LV flow direction (r2 = 0.49). Such unique flow-specific measures of mechanical dyssynchrony may serve as an additional tool for considering the risks imposed by conduction abnormalities in heart failure patients and prove to be useful in predicting response to CRT.

Reservations about the Selvester QRS score in left bundle branch block — Experience in patients with transcatheter aortic valve implantation

BackgroundThe Selvester QRS score (S-score) estimates myocardial scar using electrocardiographic criteria. We evaluated the S-score for left bundle branch block (LBBB). Material and methodsStudied were 36 patients who developed persistent LBBB upon transcatheter aortic valve implantation (TAVI, TAVI-LBBB group) and 36 matched patients with persistent narrow QRS (TAVI-nQRS group). Electrocardiograms were recorded before and briefly after TAVI and during ~6months follow-up. S-score was calculated using criteria for hypertrophic (in absence of LBBB) or LBBB hearts. ResultsIn TAVI-LBBB patients correlation between S-scores pre-TAVI and post-TAVI was absent (R2=0.023). High S-scores post-TAVI occurred in patients with low pre-TAVI scores. Pre-post TAVI scores correlated weakly in TAVI-nQRS (R2=0.182), indicating a possible influence of ventricular unloading by TAVI. In both groups S-scores at post-TAVI and follow-up compared reasonably (R2=0.389 and R2=0.386), indicating reproducibility in more stable conditions. ConclusionThis study indicates that the use of the LBBB S-score criteria overestimates scar size and that caution is recommended in the use of the score in patients with LBBB.

Does cardiac resynchronization therapy benefit patients with right bundle branch block: cardiac resynchronization therapy has a role in patients with right bundle branch block.

Does cardiac resynchronization therapy benefit patients with right bundle branch block: cardiac resynchronization therapy has a role in patients with right bundle branch block.

Interplay of Electrical Wavefronts as Determinant of the Response to Cardiac Resynchronization Therapy in Dyssynchronous Canine Hearts

Background— The relative contribution of electromechanical synchronization and ventricular filling to the optimal hemodynamic effect in cardiac resynchronization therapy (CRT) during adjustment of stimulation-timings is incompletely understood. We investigated whether optimal hemodynamic effect in CRT requires collision of pacing-induced and intrinsic activation waves and optimal filling of the left ventricle (LV). Methods and Results— CRT was performed in dogs with chronic left bundle–branch block (n=8) or atrioventricular (AV) block (n=6) through atrial (A), right ventricular (RV) apex, and LV-basolateral pacing. A 100 randomized combinations of A-LV/A-RV intervals were tested. Total activation time (TAT) was calculated from &gt;100 contact mapping electrodes. Mechanical interventricular dyssynchrony was determined as the time delay between upslopes of LV and RV pressure curves. Settings providing an increase in LVdP/dt max (maximal rate of rise of left ventricular pressure) of ≥90% of the maximum LVdP/dt max value were defined as optimal (CRT opt ). Filling was assessed by changes in LV end-diastolic volume (EDV; conductance catheter technique). In all hearts, CRT opt was observed during multiple settings, providing an average LVdP/dt max increase of ≈15%. In AV-block hearts, CRT opt exclusively depended on interventricular-interval and not on AV-interval. In left bundle–branch block hearts, CRT opt occurred at A-LV intervals that allowed fusion of LV-pacing–derived activation with right bundle–derived activation. In all animals, CRT opt occurred at settings resulting in the largest decrease in TAT and mechanical interventricular dyssynchrony, whereas LV EDV hardly changed. Conclusions— In left bundle–branch block and AV-block hearts, optimal hemodynamic effect of CRT depends on optimal interplay between pacing-induced and intrinsic activation waves and the corresponding mechanical resynchronization rather than filling.

Mechanical discoordination increases continuously after the onset of left bundle branch block despite constant electrical dyssynchrony in a computational model of cardiac electromechanics and growth

To test whether a functional growth law leads to asymmetric hypertrophy and associated changes in global and regional cardiac function when integrated with a computational model of left bundle branch block (LBBB). In recent studies, we proposed that cardiac myocytes grow longer when a threshold of maximum fibre strain is exceeded and grow thicker when the smallest maximum principal strain in the cellular cross-sectional plane exceeds a threshold. A non-linear cardiovascular model of the beating canine ventricles was combined with the cellular growth law. After inducing LBBB, the ventricles were allowed to adapt in shape over time in response to mechanical stimuli. When subjected to electrical dyssynchrony, the combined model of ventricular electromechanics, haemodynamics, and growth led to asymmetric hypertrophy with a faster increase of wall mass in the left ventricular (LV) free wall (FW) than the septum, increased LV end-diastolic and end-systolic volumes, and decreased LV ejection fraction. Systolic LV pressure decreased during the acute phase of LBBB and increased at later stages. The relative changes of these parameters were similar to those obtained experimentally. Most of the dilation was due to radial and axial fibre growth, and hence altered shape of the LVFW. Our previously proposed growth law reproduced measured dyssynchronously induced asymmetric hypertrophy and the associated functional changes, when combined with a computational model of the LBBB heart. The onset of LBBB leads to a step increase in LV mechanical discoordination that continues to increase as the heart remodels despite the constant electrical dyssynchrony.

Vectorcardiography as a Tool for Easy Optimization of Cardiac Resynchronization Therapy in Canine Left Bundle Branch Block Hearts

Background— In cardiac resynchronization therapy (CRT), optimization of left ventricular (LV) stimulation timing is often time consuming. We hypothesized that the QRS vector in the vectorcardiogram (VCG) reflects electric interventricular dyssynchrony, and that the QRS vector amplitude (VA QRS ), halfway between that during left bundle branch block (LBBB) and LV pacing, reflects optimal resynchronization, and can be used for easy optimization of CRT. Methods and Results— In 24 canine hearts with LBBB (12 acute, 6 with heart failure, and 6 with myocardial infarction), the LV was paced over a wide range of atrioventricular (AV) delays. Surface ECGs were recorded from the limb leads, and VA QRS was calculated in the frontal plane. Mechanical interventricular dyssynchrony (MIVD) was determined as the time delay between upslopes of LV and right ventricular pressure curves, and systolic function was assessed as LV dP/dt max . VA QRS and MIVD were highly correlated ( r =0.94). The VA QRS halfway between that during LV pacing with short AV delay and intrinsic LBBB activation accurately predicted the optimal AV delay for LV pacing (1 ms; 95% CI, –5 to 8ms). Increase in LV dP/dt max at the VCG predicted AV delay was only slightly lower than the highest observed ∆LV dP/dt max (–2.7%; 95% CI, –3.6 to –1.8%). Inability to reach the halfway value of VA QRS during simultaneous biventricular pacing (53% of cases) was associated with suboptimal hemodynamic response, which could be corrected by sequential pacing. Conclusions— The VA QRS reflects electric interventricular dyssynchrony and accurately predicts optimal timing of LV stimulation in canine LBBB hearts. Therefore, VCG may be useful as a reliable and easy tool for individual optimization of CRT.

Asymptomatic Left Bundle Branch Block Predicts New-Onset Congestive Heart Failure and Death From Cardiovascular Diseases.

BackgroundLeft bundle branch block (LBBB) has been proposed as a risk factor for cardiovascular morbidity and mortality. We sought to characterize the strength of these associations in a population without preexisting clinical heart disease.MethodsThe association between LBBB and new-onset congestive heart failure (CHF) or death from cardiovascular diseases was examined in 1,688 participants enrolled in the SPPARCS study who were free of known CHF or previous myocardial infarction. SPPARCS is a community-based cohort study in residents of Sonoma, California that are > 55 years. Medical history and 12-lead ECGs were obtained every 2 years for up to 6 years of follow-up. LBBB at enrollment or year 2 was considered “baseline” and assessed as a predictor of CHF and cardiovascular death ascertained at years 4 and 6.ResultsThe prevalence of LBBB at baseline was 2.5% (n = 42). During 6 years of follow-up, 70 (4.8%) people developed new CHF. Incidence of CHF was higher in patients with LBBB than in participants without LBBB. This association persisted after controlling for potential confounders (odds ratio (OR): 2.85; 95% confidence interval (CI): 1.01 - 8.02; P = 0.047). A higher mortality from cardiovascular diseases was also found in participants with LBBB after adjusting for potential confounders (OR: 2.35, 95%CI: 1.02 - 5.41; P = 0.044).ConclusionsLBBB in the absence of a clinically detectable heart disease is associated with new-onset CHF and death from cardiovascular diseases. Further study is warranted to determine if additional diagnostic testing or earlier treatment in patients with asymptomatic LBBB can decrease cardiovascular morbidity or mortality.

Endocardial Left Ventricular Pacing Improves Cardiac Resynchronization Therapy in Chronic Asynchronous Infarction and Heart Failure Models

Studies in canine hearts with acute left bundle branch block (LBBB) showed that endocardial left ventricular (LV) pacing improves the efficacy of cardiac resynchronization therapy (CRT) compared with conventional epicardial LV pacing. The present study explores the efficacy of endocardial CRT in more compromised hearts and the mechanisms of such beneficial effects. Measurements were performed in 22 dogs, 9 with acute LBBB, 7 with chronic LBBB combined with infarction (embolization; LBBB plus myocardial infarction, and concentric remodeling), and 6 with chronic LBBB and heart failure (rapid pacing, LBBB+HF, and eccentric remodeling). A head-to-head comparison was performed of the effects of endocardial and epicardial LV pacing at 8 sites. LV activation times were measured using ≈100 endocardial and epicardial electrodes and noncontact mapping. Pump function was assessed from right ventricular and LV pressures. Endocardial CRT resulted in better electric resynchronization than epicardial CRT in all models, although the benefit was larger in concentrically remodeled LBBB plus myocardial infarction than in eccentrically remodeled LBBB+HF hearts (19% versus 10%). In LBBB and LBBB+HF animals, endocardial conduction was ≈50% faster than epicardial conduction; in all models, transmural impulse conduction was ≈25% faster when pacing from the endocardium than from the epicardium. Hemodynamic effects were congruent with electric effects. Endocardial CRT improves electric synchrony of activation and LV pump function compared with conventional epicardial CRT in compromised canine LBBB hearts. This benefit can be explained by a shorter path length along the endocardium and by faster circumferential and transmural impulse conduction during endocardial LV pacing.

Bundle branch block old and new in myocardial infarction

When left bundle branch block (LBBB) is associated with acute myocardial infarction (AMI) as many as 39.2% to 46.9% of patients may have old LBBB, as opposed to new-onset LBBB. In a typical presentations of AMI, however, the prevalence of old LBBB relative to new-onset LBBB is known, and may well be higher, given the association of old LBBB and 3-vessel coronary heart disease even in the absence of AMI.

Quantitative comparison of 2D and 3D circumferential strain using MRI tagging in normal and LBBB hearts

The response to cardiac resynchronization therapy (CRT), which is applied to patients with heart failure (HF) and left bundle-branch block (LBBB), can be predicted from the mechanical dyssynchrony measured on circumferential strain. Circumferential strain can be assessed by either 2D or 3D strain analysis. In this study was evaluated the difference between 2D and 3D circumferential strain using MR tagging with high temporal resolution (14 ms). Six healthy volunteers and five patients with LBBB were evaluated. We compared the 2D and 3D circumferential strains by computing the mechanical dyssynchrony and the cross correlation (r) between 2D and 3D strain curves, and by quantifying the differences in peak circumferential shortening, time to onset, and time to peak of shortening. The obtained maximum r(2) values were 0.97 +/- 0.03 and 0.87 +/- 0.16 for the healthy and LBBB populations, respectively, and thus showed a good similarity between 2D and 3D strain curves. No significant difference was observed between 2D and 3D in time to onset, time to peak, or peak circumferential shortening. Thus, to measure dyssynchrony, 2D strain analysis will suffice. Since 2D analysis is easier to implement than 3D analysis, this finding brings the application of MRI tagging and strain analysis closer to the clinical routine.

P3-92: Acute hemodynamic improvement predicts reverse remodeling during cardiac resynchronization in canine LBBB hearts

In patients with heart failure and left bundle branch block (LBBB) the occurrence of reverse remodeling after biventricular pacing (BVP) correlates with long term outcome. We investigated whether acute hemodynamic improvement upon BVP predicts reverse remodeling in canine hearts with chronic LBBB.

Evaluation of left bundle branch block as a reversible cause of non-ischaemic dilated cardiomyopathy with severe heart failure. A new concept of left ventricular dyssynchrony-induced cardiomyopathy

We sought to determine if amelioration of left bundle branch block (LBBB)-induced contraction disturbances achieved by left ventricular (LV)-based pacing could result in sustained reversal of severe LV dysfunction in certain patients with chronic heart failure due to non-ischaemic cardiomyopathy. It has been shown that LBBB induces asynchronous contraction of LV. However, whether such a functional contraction disturbance, if present for an extended period of time, could account for a dilated cardiomyopathy remains unknown. The study population comprised 29 patients with dilated cardiomyopathy, sinus rhythm, LBBB and severe heart failure (14 patients in New York Heart Association (NYHA) class III and 15 in class IV). Patients were followed prospectively after resynchronization therapy. LV function was considered to be normalized when ejection fraction (EF) was >50% at 1 year. Five among the 29 patients (17%: group 1) demonstrated both complete normalization of LV function following resynchronization therapy (EF: from 19+/-6 to 55+/-3%, P = 0.001) and clinical improvement (mean NYHA class: 3.4+/-0.5 to 1.8+/-0.4, P = 0.02; 6-min walk distance: 300+/-136 to 444+/-75 m, P = 0.12; peak VO2: 11.9+/-4 to 15.8+/-2 ml/min/kg, p = 0.03). Among the remaining 24 patients (83%: group 2) EF improved but did not normalize (from 21+/-8 to 23+/-11%, ns). Baseline clinical features could not predict which patients would exhibit the reversal of LV dysfunction. Normalization of LV function 1 year after resynchronization therapy in a small but important number of patients suggests that long-standing LBBB may be a newly identified reversible cause of cardiomyopathy.

Left bundle branch block induces ventricular remodelling and functional septal hypoperfusion

Left ventricular (LV) dilatation, hypertrophy, and septal perfusion defects are frequently observed in patients with left bundle branch block (LBBB). We investigated whether isolated LBBB causes these abnormalities. In eight dogs, LBBB was induced by radio frequency ablation. Two-dimensional echocardiography showed that 16 weeks of LBBB decreased LV ejection fraction (by 23+/-14%) and increased LV cavity volume (by 25+/-19%) and wall mass (by 17+/-16%). The LV septal-to-lateral wall mass ratio decreased by 6+/-9%, indicating asymmetric hypertrophy. After onset of LBBB, myocardial blood flow (MBF, fluorescent microspheres) and systolic circumferential shortening [CS(sys), magnetic resonance (MR) tagging] decreased in the septum to 83+/-16% and -11+/-20% of baseline, respectively, and increased in LV lateral wall to 118+/-12% and 180+/-90% of baseline, respectively. MBF and CS(sys) values did not change over 16 weeks of LBBB. Changes in external mechanical work paralleled those in CS(sys). Glycogen content was not significantly different between septum and LV lateral wall of LBBB hearts (16 weeks) and control samples, indicating absence of hibernation. The asynchronous ventricular activation during LBBB leads to redistribution of circumferential shortening and myocardial blood flow and, in the long run, LV remodelling. Septal hypoperfusion during LBBB appears to be primarily determined by reduced septal workload.

Intra-ventricular resynchronization for optimal left ventricular function during pacing in experimental left bundle branch block

ObjectivesWe sought to investigate to what extent intra-ventricular asynchrony (intraVA) and inter-ventricular asynchrony (interVA) determine left ventricular (LV) function in canine hearts with left bundle branch block (LBBB) during ventricular pacing. BackgroundPacing therapy improves LV pump function in patients with heart failure and abnormal ventricular conduction supposedly due to resynchronization. However, the relationship between LV pump function and measures of asynchrony is not well established. MethodsIn 15 experiments, LV (various sites) and biventricular (BiV) pacing was performed at atrioventricular (AV) delays of 20 to 140 ms. Measured were the maximum rate of increase (dP/dtmax) of LV pressure and LV stroke work (SW) (conductance catheter), interVA (time delay between the upslope of LV and RV pressures), and intraVA (from endocardial electrical activation maps). ResultsInduction of LBBB increased interVA (−6.4 ± 8.6 to −28.4 ± 8.5 ms [RV earlier]) and intraVA (4.9 ± 2.4 to 18.0 ± 3.3 ms), whereas LV dP/dtmaxand SW decreased (−13 ± 18% and −39 ± 24%, respectively). During LBBB, LV and BiV pacing increased LV dP/dtmaxand SW (mean increases 14% to 21% and 11% to 15%, respectively) without changing diastolic function or preload. Optimal improvement in LV function was obtained consistently when intraVA returned to pre-LBBB values, while interVA remained elevated. Normalization of intraVA required AV delays shorter than the baseline PQ time during LV apex and BiV pacing, thus excluding endogenous LV activation, but AV delays virtually equal to the baseline PQ time (difference 4 ± 9 ms, p = NS) during pacing at (mid)lateral LV sites to obtain fusion between pacing-induced and endogenous activation. ConclusionsIn LBBB hearts, optimal restoration of LV systolic function by pacing requires intra-ventricular resynchronization. The optimal AV delay to achieve this depends on both the site of pacing and baseline PQ time.

Ventricular tachycardias arising from the aortic sinus of valsalva: an under-recognized variant of left outflow tract ventricular tachycardia

OBJECTIVESTo describe a normal heart left bundle branch block, inferior axis ventricular tachycardia (VT), that could not be ablated from the right or left ventricular outflow tracts.BACKGROUNDWhether these VTs are epicardial and can be identified by a specific electrocardiographic pattern is unclear.METHODSTwelve patients with normal heart left bundle branch block, inferior axis VT and previously failed ablation were included in this study. Together with mapping in the right and left ventricular outflow tracts, we obtained percutaneous epicardial mapping in the first five patients and performed aortic sinus of Valsalva mapping in all patients.RESULTSNo adequate pace mapping was observed in the right and left ventricular outflow tracts. Earliest ventricular activation was noted in the epicardium and the aortic cusps. All patients were successfully ablated from the aortic sinuses of Valsalva (95% CI 0% to 18%). The electrocardiographic pattern associated with this VT was left bundle branch block, inferior axis and early precordial transition with Rs or R in V2or V3. Ventricular tachycardia from the left sinus had rS pattern in lead I, and VT from the noncoronary sinus had a notched R wave in lead I. None of the patients had complications and all remained arrhythmia-free at a mean follow-up of 8 ± 2.6 months.CONCLUSIONSNormal heart VT with left bundle branch block, inferior axis and early precordial transition can be ablated in the majority of patients from either the left or the noncoronary aortic sinus of Valsalva.

Prevalence of left bundle branch block in women with ischemic heart disease.

The controversy about the sex distribution in Left Bundle Branch Block (LBBB) led us to study what occurred in 56 hospitalized patients affected by LBBB and ischemic heart disease (IHD). In the group of acute myocardial infarction (AMI), 26 patients, with a high incidence of pump faiture (73%) an a mortality of 33%, the male/female (M/F) ratio was clearly reduced in relation to the global AMI M/F ratio (1.9:1 against 3.5:1). In the group without AMI, there was a female predominance and, namely in cases with unstable angina, the M/F ratio was reversed (1:2 against 2:1 in the general group with unstable angina). Hypertension did not seem responsible for the absence of male prevalence in patients with IHD. The reasons of this phenomenon - the relative female prevalence of LBBB in patients with AMI, and the absolute female predominance in cases without AMI - seem to be obscure at the moment. It is possible that, for the majority of patients with JHD and LBBB, LBBB does not have a simple etiology. The hypothesis of the coexistence of IHD with a primary degenerative disease of the conduction system is admitted.

Effect of intermittent left bundle branch block on left ventricular performance in the normal heart

The effect of left bundle branch block on left ventricular performance was studied in a patient with rate-dependent left bundle branch block and an otherwise normal heart. The timing of left heart events, hemodynamic pressures and indexes of left ventricular contractility were determined during normal intraventricular conduction and during left bundle branch block. The results indicate that in the normal heart left bundle branch block has no significant effect on left ventricular performance.

Bundle-Branch Block Complicating Acute Myocardial Infarction

Nineteen patients with right bundle-branch block (RBBB) and 20 with left bundle-branch block (LBBB) were treated in a coronary care unit for acute myocardial infarction during a 4 1/2-year period. Twenty-six percent of patients with RBBB developed cardiogenic shock, 32% had complete atrioventricular block, and 26% had ventricular asystole. Mortality was 37%, primarily from myocardial failure rather than asystole or arrhythmia. Pacemaker therapy was generally unsuccessful in reducing mortality. In contrast, 70% of the group with LBBB suffered life-threatening ventricular arrhythmia during or in close proximity to the acute phase of myocardial infarction. Mortality was 35%. Electrophysiologic mechanisms may influence ventricular irritability in patients with LBBB and ischemic heart disease. Consideration should be given to prophylactic antiarrhythmic therapy in these patients at least during hospitalization for acute myocardial infarction.