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
The QRS has the same morphology as in LBBB, but its duration is < 0.12 seconds.
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 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
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
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.