Bradycardias (also known as bradyarrhythmias) are rhythm disorders characterised by a heart rate lower than 60 beats per minute.
Causes include atrioventricular block, bundle branch block and sick sinus syndrome.
First degree heart block
Delay in the conduction of impulses from the sinoatrial node to the ventricles. May be idiopathic or associated with heart disease. No treatment is usually necessary.
ECG findings: Prolonged PR interval (>200 ms)
Second degree heart block
Intermittent failure of conduction of impulses through the atrioventricular node or bundle of His (dropped beats). Dropped beats have a P wave but no QRS complex.
- Wenckebach (Mobitz I): progressive PR prolongation with beat until a P wave fails to conduct.
- Mobitz II: constant PR interval but intermittent failure of P wave conduction. Usually due to cardiac pathology
- 2:1 block: consistent failure of P wave conduction such that there are 2 P waves for every 1 QRS complex. It is impossible to classify this as either Wenckebach or Mobitz II as the PR interval cannot be evaluated as progressively prolonging nor constant.
Mobitz II has a high risk of progressing to third degree heart block, so treatment is usually indicated with pacemaker implantation. Wenckebach rhythms are often asymptomatic and do not usually require treatment.
Third degree heart block (complete heart block)
All atrial activity fails to conduct to the ventricles. Escape rhythms are responsible for continuing ventricular activity, so the heart rate is usually much slower.
Escape rhythms can be from the bundle of His (giving narrow QRS complexes) or at/below the Purkinje fibres (giving wide QRS complexes).
ECG findings: no relationship between P waves and QRS complex;
P waves and QRS complexes have different rates. Additionally, ventricular foci of depolarisation produce wide QRS complexes.
Patients should receive pacing, be it temporary (if presents acutely) or permanent (chronic block), to avoid the risk of developing asystole. Atropine can also be used acutely for symptomatic bradycardia.
Bundle branch blocks
A bundle branch block is a conduction block in the right and/or left bundles of His. This produces widening of the QRS complex. A block in both branches produces third degree heart block as there is no communicating pathway between the atria and ventricles.
ECG findings of left bundle branch blocks (LBBB) and right bundle branch blocks (RBBB) can be remembered with the mnemonic WiLLiaM MaRRoW, respectively (see below).
Right bundle branch block
A block in the right bundle of His. Therefore, impulses travel correctly to the left ventricle via the left bundle of His before spreading to the right ventricle. It is commonly present in healthy people and has a good prognosis.
ECG findings: Broad QRS. Upright RSR’ pattern in V1 (M-shaped)
and deep S wave in V6 (W-shaped)
Left bundle branch block
A block in the left bundle of His. Therefore, impulses travel correctly to the right ventricle via the right bundle of His before spreading to the left ventricle. It may indicate heart disease e.g. myocardial infarction, ventricular hypertrophy, cardiomyopathy.
It should be noted that the left bundle has 2 fascicles: anterior and posterior. A block in a single fascicle produces axis deviation but not QRS widening.
ECG findings: Broad QRS. Deep S wave in V1 (W-shaped)
and tall R waves in V6 (M-shaped)
Left anterior fascicular block
A block in the left anterior fascicle only. This changes the overall direction of depolarisation as the impulses travel inferiorly from the anterior fascicle.
ECG findings: Left axis deviation
Left posterior fascicular block
A block in the left posterior fascicle only. This changes the overall direction of depolarisation as the impulses travel superiorly from the posterior fascicle.
ECG findings: Right axis deviation
A RBBB in combination with left anterior or left posterior fascicular block.
ECG findings: RBBB features with left (if left anterior fascicle block) or right axis deviation (if left posterior fascicle block)
A RBBB in combination with first degree heart block and either left anterior or left posterior fascicular block.
ECG findings: RBBB features, prolonged PR interval and left (if left anterior fascicle block) or right axis deviation (if left posterior fascicle block)
Sick sinus syndrome
Impaired impulse formation due to irreversible dysfunction of the sinoatrial node. Causes include fibrosis, ischaemia and degenerative changes.
ECG findings: intermittent and variable ECG changes. Includes sinus bradycardia, sinoatrial block and bradycardia-tachycardia syndrome (the heart alternates between bradycardia and tachycardia).
Bradycardias can be managed with pacemakers. Consider antiarrhythmic drugs for tachycardia.