Sinoatrial Exit Block

SA Exit Block Overview

Sinoatrial exit block is due to failed propagation of pacemaker impulses beyond the SA node.

• The sinoatrial 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 of sinoatrial exit block

• 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

• 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).

Image Credit: Dr Steve Smith’s ECG Blog

Related Topics

• Clinical Case: “Wenckebach Squared!” A case of simultaneous SA exit block and AV Mobitz I block
• AV block: 1st degree
• AV block: 2nd degree, Mobitz I (Wenckebach)
• AV block: 2nd degree, Mobitz II
• AV block: 3rd degree (complete heart block)

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Mastery: Yellow Belt online course. Understand ECG basics. Medmastery
• Wiesbauer F, Kühn P. ECG Mastery: Blue Belt online course: Become an ECG expert. Medmastery
• Kühn P, Houghton A. ECG Mastery: Black Belt Workshop. Advanced ECG interpretation. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Zimmerman FH. ECG Core Curriculum. 2023
• Mattu A, Berberian J, Brady WJ. Emergency ECGs: Case-Based Review and Interpretations, 2022
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Hampton J, Adlam D. The ECG Made Practical 7e, 2019
• Kühn P, Lang C, Wiesbauer F. ECG Mastery: The Simplest Way to Learn the ECG. 2015
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest