Sinoatrial Exit Block

• Sino-atrial exit block is due to failed propagation of pacemaker impulses beyond the SA node.
• The sino-atrial node continues to depolarise normally.
• However, some of the sinus impulses are “blocked” before they can leave the SA node, leading to intermittent failure of atrial depolarisation (dropped P waves).

Anatomical Basis

The SA node consists of two main groups of cells:

• A central core of pacemaking cells (“P cells”) that produce the sinus impulses.
• An outer layer of transitional cells (“T cells”) that transmit the sinus impulses out into the right atrium.

Sinus node dysfunction can result from either:

• Failure of the P cells to produce an impulse. This leads to sinus pauses and sinus arrest.
• Failure of the T cells to transmit the impulse. This leads to sino-atrial exit block.

Patterns of conduction

• The patterns of conduction in SA exit block are identical to the different types of AV block.
• However, as the initial sinus impulse is not visible on the ECG, the relationship between impulse generation and transmission must be inferred from the P waves alone (analogous to examining only the R waves in AV block).
• Only second degree SA block (types I and II) can be diagnosed from the 12-lead ECG.

First Degree SA block

= Delay between impulse generation and transmission to the atrium.

• This abnormality is not detectable on the surface ECG.

Second Degree SA block, Type I (Wenckebach)

= Progressive lengthening of the interval between impulse generation and transmission, culminating in failure of transmission.

• The gradually lengthening transmission interval pushes successive P waves closer together.
• This results in grouping of the P-QRS complexes.
• Pauses due to dropped P waves occur at the end of each group.
• The P-P interval progressively shortens prior to the dropped P wave.
• This pattern is easily mistaken for sinus arrhythmia.

Second Degree SA block, Type II

= Intermittent dropped P waves with a constant interval between impulse generation and atrial depolarisation.

• This pattern is the equivalent of Mobitz II.
• There is no clustering of P-QRS complexes.
• Intermittent P waves “drop out” of the rhythm, while subsequent P waves arrive “on time”.
• The pause surrounding the dropped P wave is an exact multiple of the preceding P-P interval.

Third Degree SA Block

= None of the sinus impulses are conducted to the right atrium.

• There is a complete absence of P waves.
• The onset of 3rd degree SA block may produce long sinus pauses or sinus arrest (may lead to fatal asystole).
• Rhythm may be maintained by a junctional escape rhythm.
• Third degree SA exit block is indistinguishable from sinus arrest due to pacemaker cell failure. It can only be diagnosed with a sinus node electrode during electrophysiological evaluation.

Causes

• Sick sinus syndrome
• Increased vagal tone (athletes)
• Vagal stimulation (surgery, pain)
• Inferior myocardial infarction
• Myocarditis
• Drugs: digoxin, beta-blockers, calcium channel blockers, amiodarone.

 

ECG Examples

Example 1

Type I SA block:

 

Example 2

Type II SA block:

 

Related Topics

• AV block: 1st degree
• AV block: 2nd degree, Mobitz I (Wenckebach)
• AV block: 2nd degree, Mobitz II
• AV block: 3rd degree (complete heart block)

Further Reading

• ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
• ECG CLINICAL CASES — Your favourite ECG’s placed in clinical context with a challenging Q&A approach
• ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
• ECG Exam Template — a framework for the FACEM part 2 exam.
• ECG Reference Sites on the WEB — the best of the rest

Author Credits

References