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).

Type II sino-atrial exit block

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

 

ECG Examples

Example 1

Type I SA block:

  • This pattern of grouped beating is characteristic of type I SA block.
  • There is progressive shortening of the P-P interval, followed by an absent P wave-QRS complex.

 

Example 2

Type II SA block:

  • Arrows indicate the presumed timing of each sinus impulse.
  • The blue arrows represent normally transmitted impulses, i.e. resulting in P waves.
  • The black arrows represent blocked sinus impulses (dropped P waves).
  • The pauses around the dropped P waves (2.1 seconds) are exactly double the preceding P-P interval (1.05 seconds)
Also note:
  • The 4th QRS complex is a junctional escape beat followed by a non-conducted P wave (occurring just prior to the T wave).
  • The 8th QRS complex is a junctional escape beat. The following P wave is conducted to the ventricles, albeit with an extremely long PR interval (400ms).
This second ECG example is reproduced from Dr Steve Smith’s excellent ECG Blog. Click here for a more in-depth analysis of this fascinating ECG.

 

Related Topics

Further Reading

Author Credits

References

  • Phibbs BP. Advanced ECG: Boards and Beyond (second edition). Elsevier 2006.
  • Saperia GM. Sinoatrial nodal pause; arrest; and exit block [internet article]. Up To Date 2009. Accessible at www.uptodate.com [subscribers only].
  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
  • Ufberg JW. Sinoatrial exit block [chapter]. In: TC Chan, WJ Brady, RA Harrigan, JP Ornato and PR Rosen (editors). ECG in Emergency Medicine and Acute Care. Elsevier 2005. Chapter 19, pp83-85.
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