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HannuParikka

Bundle Branch Blocks in an ECG

Essentials

  • Left and right bundle branch block (LBBB and RBBB) may be present in the absence of significant heart disease. In middle aged and older patients an underlying cardiac pathology is, however, often present.
  • LBBB is more commonly associated with heart disease than RBBB.
  • If a patient with a newly diagnosed bundle branch block is asymptomatic, has no signs suggestive of heart disease and echocardiographic findings are normal it may be concluded that no cardiac pathology is present. Nevertheless, the continuing monitoring of these factors is important.
  • RBBB without a cardiac disease does not affect prognosis, but LBBB is associated with increased cardiac morbidity and mortality.
  • The significance of left anterior hemiblock (LAHB) is slight as far as prognosis is concerned.
  • A peripheral conduction disturbance is associated with increased risk of sudden cardiac death and the development of cardiac failure.
  • Bifascicular and trifascicular block, even if asymptomatic, are an indication for follow-up observation with clinical check-ups and ECG recordings.

Intraventricular conduction disturbances in general

  • Bundle branch block develops when the electrical impulse is blocked in the intraventricular conduction system after the bundle of His.
  • The intraventricular conduction system consists of
    • the bundle of His which is a continuation of the atrioventricular (AV) node and
    • the division of the bundle of His into
      • the right bundle branch
      • the left bundle branch which divides further into an anterior and posterior fascicle.
  • In peripheral conduction disturbances the electrical impulse is blocked at the periphery of the conduction system in poorly defined areas (i.e. the criteria for specific bundle branch blocks are not met [IVCD = intraventricular conduction defect, nonspecific intraventricular block]).
  • Bundle branch block is associated with structural causes.
    • Most commonly as a result of heart disease
    • Degeneration of the conduction pathways in the absence of other heart disease
  • Bundle branch block may also be functional.
    • Aberrancy signifies functional bundle branch block.
    • Rate dependent bundle branch block occurs when the heart rate exceeds a heart rate specific for the person during
      • sinus tachycardia
      • sudden onset supraventricular tachycardia
      • atrial fibrillation.
  • The characteristic picture of bundle branch block in an ECG is a broad M-shaped QRS wave.

Right bundle branch block (RBBB)

  • See pictures 1 2 3

Causative factors

  • Encountered occasionally even in young patients in the absence of significant heart disease.
  • In middle aged and older patients RBBB may be caused by a variety of diseases.
    • Ischaemic heart disease
    • Acute myocardial infarction (MI)
    • Carditis, cardiomyopathies
    • Pulmonary heart disease (cor pulmonale)
    • Pulmonary embolism (transient)
    • Many congenital heart defects as well as surgery to correct such defects
    • Sequela of surgery to correct acquired heart disease.

Principal ECG features

  • Wide M-shaped QRS in the right chest leads V1 and V2.
    • rsR' or rSR' pattern
    • The pattern resembles rabbit or dog ears with the right ear being bigger.
  • QRS duration is at least 0.12 seconds.
  • A secondary repolarisation change in leads V1 and V2 (ST segment depression and T wave inversion).
  • A wide S wave in leads I, aVL, V5 and V6.
  • The morphology may vary slightly if the block is caused by heart disease.

Incomplete RBBB (partial RBBB, pRBBB)

  • The same QRS morphology as in RBBB, but QRS duration < 0.12 seconds
  • The S waves in leads I, aVL and V5-V6 are not deep and wide.
  • A common, benign finding in young endurance athletes
  • Incomplete RBBB is not actually a conduction disturbance as such, and the pattern rSR' can be explained by slight right-sided strain.

Differential diagnosis of RBBB

Clinical significance of RBBB

  • The second most common intraventricular conduction disturbance
  • Prognosis depends on the underlying disease.
    • RBBB has no influence on prognosis if encountered in a young asymptomatic, otherwise healthy patient.
    • RBBB occurring at middle-age may be predictive of later AV block.
    • RBBB with heart disease will increase mortality.
  • When diagnosed for the first time, the presence of heart disease should be considered (symptoms, clinical cardiac signs, echocardiography).
    • If no heart disease is identified, occasional clinical follow-up observation and ECG recordings are indicated.
  • When appearing in an acute MI
    • RBBB with LAHB in an acute anteroseptal MI is suggestive of extensive damage and poor prognosis.
  • Complicates ECG diagnosis.
    • RBBB may mask the ECG findings of posterior MI (but not those of other MIs) because
      • the tall R waves in leads V2-V3 are not clearly shown
      • in an MI, the increased R wave amplitude involves the first rabbit ear (the first positive deflection of the M-pattern).
  • May complicate the evaluation of LVH.
    • Lack of S waves in the septal leads
    • Lateral R wave amplitudes are increased in LVH as usual.
    • Normal lateral R waves do not exclude the possibility of LVH.
  • Impairs the evaluation of RVH.
    • RVH increases the amplitude of the right-sided rabbit ear.
    • In RVH, the frontal plane axis deviates to the right.

Left bundle branch block (LBBB)

  • See picture 4.
  • Can occasionally be encountered even in young patients in the absence of significant heart disease, but less frequently than RBBB.
  • In middle aged and older patients LBBB may be caused by a variety of diseases.
    • Ischaemic heart disease
    • Acute MI
    • Carditis, cardiomyopathies
    • Factors causing LVH (hypertension)
    • Valvular defects (aortic valve disease in particular)
    • Congenital heart disease
    • Sequela of open heart surgery (aortic valve surgery)
    • Degeneration of conduction pathways in the absence of other heart disease

Principal ECG features

  • Wide M-shaped QRS in the lateral chest leads V5 and V6 as well as in leads I and aVL.
    • rsR' or rSR' pattern
    • The pattern resembles rabbit or dog ears with the right ear being bigger.
  • QRS duration is at least 0.12 seconds.
  • No Q wave in leads I, V5 or V6
  • Due to a secondary repolarisation change the ST segment and T wave are deflected opposite to the QRS complex in leads V5 and V6.
  • In leads V1-V2 the S wave is deep and wide with an rS or QS pattern.
  • The morphology may change, if other conditions causing ECG changes coexist.

Incomplete LBBB (partial LBBB, pLBBB)

  • The same QRS morphology as in LBBB, but its duration is < 0.12 seconds
  • Usually caused by LVH
  • Causes are often the same as in complete LBBB.

Differential diagnosis of LBBB

Clinical significance of LBBB

  • The third most common intraventricular conduction disturbance
  • Prognosis depends on the underlying disease.
    • LBBB has little, albeit not completely insignificant, influence on prognosis if encountered in a young asymptomatic, otherwise healthy patient.
    • LBBB occurring at middle-age may be predictive of later AV block and cardiac failure.
    • LBBB with heart disease will significantly increase mortality.
  • When diagnosed for the first time
    • Consider the presence of a heart disease: symptoms, clinical assessment, echocardiography as primary investigations, specialist consultation according to findings.
    • If no heart disease is identified, ECG and clinical follow-up observation are still necessary.
    • In middle aged patients frequently a sign of heart disease
  • If LBBB occurs in association with an acute MI
    • It usually signifies extensive myocardial damage and poor prognosis.
    • New LBBB with symptom(s) of an MI is an indication for angiography.
  • In heart failure (NYHA 2-3)
    • Further worsens the left ventricular pumping function
    • Should be corrected by inserting a pacemaker that synchronises cardiac function (biventricular pacemaker, heart failure pacemaker).
  • An existing LBBB complicates ECG diagnosis.
    • Acute lateral MI is difficult to see (the development of Q waves confirms diagnosis).
    • Septal and inferior wall damage may be seen.
    • New LBBB may be a sign of an MI.
    • During an exercise stress test the ST segment will depress even in healthy individuals due to secondary repolarisation changes brought on by LBBB. The diagnosis of ischaemia necessitates nuclear stress testing, CT scanning of the coronary arteries or coronary angiography.
  • LBBB is suggestive of LVH, the interpretation of which is complicated by the presence of LBBB Assessment of Ventricular Hypertrophies from an ECG.
  • LBBB is sometimes encountered without other identifiable heart defects; a small proportion of these cases may progress into complete AV block, and hence clinical follow-up monitoring (including ECG) is necessary.
  • It is essential to exclude heart disease when LBBB is diagnosed for the first time, even if the patient is asymptomatic.

Left anterior hemiblock

  • LAHB = left anterior hemiblock; LAFB = left anterior fascicular block
  • See picture 5.

Main features

  • Frontal plain axis shows marked left deviation.
    • Negative QRS in leads II and III (S deeper than R) gives immediate diagnosis.
  • Deep S wave of the rS pattern in leads II, III and aVF
  • qR pattern in leads I and aVL
  • Minor widening of QRS (< 0.12 seconds)
  • Secondary repolarisation abnormalities are absent (cf. RBBB, LBBB).
  • Additional criteria that support the diagnosis:
    • a small R wave in lateral chest leads (V5-V6)
    • a deep S wave in the lateral chest leads (V5-V6).

Differential diagnosis

  • LVH (left axis deviation)
  • Anterior MI (poor R wave progression in leads V2-V4)

Clinical significance

  • The most common intraventricular conduction disturbance
  • Has little influence on prognosis.
    • The left anterior fascicle is fragile and easily damaged.
    • Damage to the left anterior fascicle is not an indication of extensive myocardial damage.
    • A benign finding in a young asymptomatic patient
    • LAHB at middle-age is predictive of heart disease, which will not necessarily be serious.
  • Complicates ECG diagnosis.
    • LAHB caused by anterior MI may mask signs of a previous inferior MI.
    • In emphysema the heart ”descends”, and the ECG may resemble that of LAHB (corrected by recording the ECG from one intercostal space lower than normally).

Left posterior hemiblock

  • LPHB = left posterior hemiblock; LPFB = left posterior fascicular block

Main features

  • Right axis deviation
  • QRS complex direction opposite to that in LAHB

Clinical significance

  • Rare
  • Usually associated with extensive myocardial damage
  • Differential diagnosis: RVH, old inferior or lateral MI

Bifascicular blocks

  • See picture 1.
  • A bifascicular block refers to a combination of RBBB and either LAFB or LPFB.

RBBB + LAHB

  • More common than RBBB + LPHB, which is rare
  • ECG features are the same as in RBBB. In addition, the frontal plane axis of the initial QRS complex shifts to the left (-30°- -90°): rS in leads II, III and aVF.

Clinical significance

  • In asymptomatic patients the prognosis is often good
    • Follow-up observation with clinical check-ups and ECG is indicated.
  • If associated with myocardial diseases, the condition may gradually progress to trifascicular block or to complete AV block.
  • RBBB + LAHB in association with an MI is a sign of extensive myocardial damage and predicts complete AV block with an overall worsening of the prognosis.
  • A pacemaker may be indicated if RBBB+LAHB is associated with an acute MI.
    • No pacing is required if the patient remains asymptomatic.
  • If the PR interval is normal or slightly prolonged, a beta blocker may be used in most cases.

Trifascicular blocks

  • See picture 1.
  • A trifascicular block refers to a combination of bifascicular block and
    • 1st degree AV block, i.e. a prolonged PR interval; in a more severe form the PR interval is very prolonged suggesting a defective bundle of His
    • alternating LBBB and RBBB: predictive of imminent complete AV block.
  • The most common combination is RBBB + LAHB + prolonged PR interval.
  • In most cases, a structural heart disease (the same as in bundle branch blocks) can be diagnosed as the underlying cause.
  • The risk of complete AV block is increased.
  • Follow-up observation is necessary in asymptomatic trifascicular block.
  • Drugs that prolong the PR interval (beta blockers, verapamil) must not usually be used, even if the patient is asymptomatic.
  • A pacemaker is indicated for symptomatic patients (syncope, presyncope with no other causes).
  • Regardless of treatment, the prognosis is fairly poor.