Alternating bundle branch block on ECG

What it looks like on ECG

Within a single tracing or when comparing multiple tracings, the morphology of the wide QRS complex is seen to change: alternating between a complete right bundle branch block (RBBB) pattern — QRS ≥ 120 ms, rsR' or wide notched R in V1–V2, deep terminal S in I and V6; and a complete left bundle branch block (LBBB) pattern — QRS ≥ 120 ms, wide notched R in V5–V6, I, aVL without normal q waves in these leads, and deep QS or rS in V1–V3. Diagnosis of each phase is established by the full set of criteria for the respective block, rather than a single isolated feature — otherwise, non-specific intraventricular conduction delay or artifact can easily be mistaken for alternation. During sinus rhythm, the P wave generally maintains its shape and relationship with QRS — this helps confirm that the rhythm has not changed, but it is not a strict diagnostic criterion by itself: the diagnosis is based on the alternation of two distinct ventricular complex morphologies.

The transition may occur:

• from complex to complex within a single tracing (rare, but most demonstrative);
• between series of complexes on a single long tracing or Holter monitor;
• between different ECGs of the patient recorded on different days or even hours.

Prolongation of the PR interval and other signs of conduction system disease, including episodes of high-grade AV block, may coexist; their presence increases risk, but their absence does not rule out alternating bundle branch block.

Mechanism

Normally, an impulse from the AV node descends through the bundle of His and propagates down the right and left bundle branches almost simultaneously. Alternating bundle branch block is a marker of severe, frequently bilateral disease of the His-Purkinje system, in which conduction reliability along both branches is unstable over time and depends on heart rate, fiber refractoriness, autonomic influences, ischemia, or degenerative changes. At any given moment, conduction is usually preserved through only one bundle branch: when conduction is impaired in the right branch, the QRS assumes an RBBB configuration; when conduction is impaired in the left, the morphology changes to LBBB. Transitions may reflect differing pathway refractoriness or rate-dependent block of one of the branches; involvement is not necessarily symmetrical. Importantly, the mere shift in block type indicates disease of the conduction pathways on both sides, rather than isolated damage to a single branch.

Causes

• Degenerative sclerosis of the conduction system (Lenègre–Lev disease)
• Coronary artery disease, including prior myocardial infarction involving the interventricular septum; in acute coronary syndrome with ischemic symptoms, a new or alternating bundle branch block requires urgent evaluation according to the ACS protocol (symptoms, ECG dynamics, troponin) — the block itself does not prove myocardial infarction independently
• Cardiomyopathies (dilated, hypertrophic)
• Myocarditis
• Infiltrative myocardial diseases (sarcoidosis, amyloidosis)
• Valvular annular calcification extending into the conduction system
• Status post cardiac surgery, trauma to the conduction system during catheter procedures
• Severe hyperkalemia and other toxic/metabolic effects on the myocardium

Differential diagnosis

• Rate-dependent bundle branch block (aberrant conduction) — QRS morphology changes depending on cycle length (e.g., one type of block appears when the heart rate exceeds a certain threshold). Important: rate-dependent block can underlie alternating bundle branch block — different RR intervals can block different bundle branches. However, if a change in heart rate produces a block in only one bundle branch without ever triggering the morphology of the other branch, this is not alternating bundle branch block, but an isolated rate-dependent block.
• Misplaced limb or chest leads — especially important to rule out when morphology differs between separate tracings: incorrect electrode placement produces a persistent pattern rather than a true beat-to-beat alternation. In intra-trace alternation, artifacts, extrasystoles, pre-excitation, and aberrant conduction must additionally be ruled out.
• Premature ventricular complexes with varying morphology (multifocal) — assess prematurity of the complex, presence of a compensatory pause, P–QRS sequence (retrograde P waves or fusion complexes are possible), and morphology across all leads; the absence of a P wave before the wide complex and coupling interval variability are not mandatory features and should not be used as the sole criteria for differentiation.
• WPW syndrome with variable pre-excitation — look for signs of pre-excitation (delta wave, short PR) in all available complexes; with intermittent or minimal pre-excitation, these features may be subtle, and varying degrees of pre-excitation can produce pseudo-alternation of QRS. Expert ECG review is recommended in case of doubt.
• Intermittent bifascicular block — a stable form of one type of block (e.g., RBBB) with episodic complete block of the other fascicle, rather than pure alternation of two different bundle branch blocks. Isolated involvement of a single fascicle of the left bundle branch (anterior or posterior fascicle) can alter the electrical axis and the appearance of a bifascicular block, but does not create the pattern of true RBBB/LBBB alternation and is evaluated as a separate differential diagnosis.

Clinical significance and management

Alternating bundle branch block is one of the most alarming findings among intraventricular conduction disturbances. It serves as a marker of severe, frequently bilateral involvement of the His-Purkinje system and is considered a predictor of high risk for developing complete AV block and Stokes-Adams attacks, sometimes sudden. Unlike stable isolated RBBB or LBBB, this condition involves not an alternative backup pathway, but profound conduction instability affecting both bundle branches, even if only one functions at any given moment.

Upon detecting this pattern, the physician should:

• immediately assess the patient's hemodynamics and symptoms (syncope, presyncope, weakness, pauses);
• ensure continuous rhythm monitoring and consultation with an arrhythmologist/cardiologist;
• search for an acute reversible cause (acute coronary syndrome, myocarditis, hyperkalemia, drug toxicity) and treat it; in the case of an acute reversible cause, correct it first and maintain monitoring with preparedness for temporary pacing as indicated;
• in the event of syncope, documented pauses, high-grade AV block, or hemodynamic instability — hospitalize the patient with readiness for temporary cardiac pacing;
• Holter monitoring is acceptable in a stable outpatient if the diagnosis or frequency of episodes is not documented, but it must not replace urgent evaluation in the presence of symptoms or instability;
• confirmed alternating RBBB/LBBB after ruling out reversible causes is an indication for permanent cardiac pacing regardless of symptoms, due to the high risk of progression to complete block.

Common interpretation errors

• Mistaking QRS morphology changes for electrode placement artifacts without correlating with the P wave and clinical presentation
• Confusing it with rate-dependent bundle branch block and failing to assess the relationship between transitions and cycle length (RR), assuming that the absence of a clear heart rate threshold rules out rate-dependency
• Fail to notice concomitant PQ prolongation or episodes of AV block that sharply increase the clinical significance of the finding
• Delay urgent evaluation and monitoring, limiting intervention to outpatient Holter monitoring in a symptomatic or unstable patient
• Confuse it with polymorphic ventricular extrasystoles due to a superficial assessment of only the P wave before the complex, without checking the prematurity and sequence of P–QRS

Frequently asked questions

Related terms

More in the section "Bundle Branch and Fascicular Blocks"

Entire section

Sources

• Hampton J.R. The ECG in Practice
• Surawicz B., Knilans T.K. Chou's Electrocardiography in Clinical Practice
• Orlov V.N. Guide to Electrocardiography
• AHA/ACCF/HRS Recommendations for the Standardization of the ECG, 2009

Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.