In a patient with a pacemaker, does a left bundle branch block (LBBB) pattern on an electrocardiogram (ECG) without preceding pacing spikes represent intrinsic ventricular depolarisation rather than a paced rhythm?

LBBB Pattern Without Pacing Spikes Indicates Intrinsic Rhythm

In a patient with a pacemaker, an LBBB pattern on ECG without preceding pacing spikes represents intrinsic ventricular depolarization, not paced rhythm. The absence of pacing spikes is the definitive indicator that the ventricular activation is occurring through the patient's native conduction system rather than through pacemaker-induced depolarization.

Key Distinguishing Features

Paced Rhythm Characteristics

• Paced ventricular beats always produce visible pacing spikes preceding the QRS complex on standard ECG, which represent the electrical artifact from the pacemaker's impulse delivery 1
• Ventricular pacing typically creates a wide QRS complex with LBBB morphology when the pacing lead is positioned in the right ventricle, as the electrical activation spreads from right to left ventricle through myocardial tissue rather than the His-Purkinje system 1
• The Sgarbossa criteria apply equally to both native LBBB and ventricular paced rhythms when evaluating for acute myocardial infarction, confirming that paced rhythms produce similar QRS morphology but with identifiable pacing artifacts 1

Intrinsic LBBB Without Pacing Spikes

• Absence of pacing spikes definitively indicates intrinsic conduction through the patient's native AV node and ventricular conduction system, even if that conduction is abnormal due to left bundle branch pathology 1
• The LBBB pattern reflects delayed or blocked conduction in the left bundle branch, causing the left ventricle to depolarize later than the right ventricle through myocardial cell-to-cell conduction 2, 3
• Complete LBBB requires QRS duration ≥120 ms with broad notched or slurred R waves in leads I, aVL, V5, and V6, absent q waves in these leads, and R peak time >60 ms in V5-V6 1

Clinical Implications and Diagnostic Approach

Immediate Assessment

• Verify pacemaker function by checking the programmed settings and interrogating the device to confirm whether it is sensing appropriately or if there is undersensing leading to failure to pace 1
• Examine the ECG systematically for any subtle pacing artifacts that might be obscured by baseline artifact or low amplitude 1
• Consider that intermittent LBBB has the same clinical and prognostic significance as stable LBBB, so rate-dependent or intermittent block patterns still warrant full evaluation 4, 5

Underlying Pathophysiology Considerations

• Recent intracardiac mapping studies demonstrate that surface ECG LBBB patterns are heterogeneous, with only 64% showing complete conduction block in the proximal left conduction system, while 36% maintain intact Purkinje activation with conduction delay in working myocardium 2
• The site of block can be at the left His bundle level (72% of cases) or proximal left bundle branch (28%), which has implications for potential His bundle pacing correction 2
• LBBB is extremely rare in healthy individuals and almost always indicates underlying structural heart disease including ischemic heart disease, cardiomyopathy, hypertensive heart disease, or diffuse myocardial pathology 4, 5

Mandatory Evaluation

• Transthoracic echocardiography is indicated to exclude or characterize structural heart disease in patients with newly detected LBBB 5
• Comprehensive cardiology workup should include exercise stress testing to detect exercise-induced arrhythmias or conduction worsening, 24-hour Holter monitoring for intermittent conduction abnormalities, and imaging to assess for cardiomyopathy or ischemic disease 4
• If the patient has syncope with LBBB pattern, electrophysiology study may be warranted to evaluate for HV intervals ≥70 ms or infranodal block, which would indicate need for permanent pacing 1

Common Pitfalls to Avoid

• Do not assume pacemaker malfunction simply because LBBB is present—the pacemaker may be functioning perfectly while sensing intrinsic rhythm that happens to have LBBB morphology 1
• Do not overlook the possibility of alternating bundle branch block, which would show QRS complexes alternating between LBBB and RBBB morphologies and represents a Class I indication for permanent pacing due to high risk of complete heart block 1, 4
• Do not delay evaluation for acute coronary syndrome in patients with LBBB and chest pain—apply Sgarbossa criteria (ST elevation ≥1 mm concordant with QRS, ST depression ≥1 mm in V1-V3, or ST elevation ≥5 mm discordant) to identify acute MI 1
• Be aware that bifascicular blocks (LBBB or RBBB with fascicular block) indicate extensive conduction system involvement and carry increased risk of progression to complete AV block 1, 4