Sinoatrial block
Synonyms and keywords:; SA nodal exit block; exit block; Sino atrial exit block; Sinoatrial nodal block; sino-auricular block; Sino-auricular heart block

# Overview
Sinoatrial block is an uncommon dysrhythmia in which the electrical impulse is delayed or blocked on the way to the atria, thus delaying atrial depolarization. It is characterized by the omission of P waves in the setting of a basic regular rhythm.  It is found incidentally in normal asymptomatic subjects and in some having pre syncope or syncope.  It may occur as an isolated dysrhythmia or in association with sinus bradycardia, tachycardia or sometimes with atrioventricular conduction disorders.

# Classification

## First Degree SA Exit Block
There is a lag between the time that the SA node fires and actual depolarization of the atria.  This rhythm is not recognizable on an ECG strip because a strip does not denote when the SA node fires.  It can only be detected during an electrophysiology study.

## Second Degree SA Exit Block
Second degree SA blocks are broken down into two subcategories just like AV blocks.

### Type I (Wenckebach Phenomenon) Sinoatrial Exit Block
This rhythm is irregular, and the R-R interval gets progressively smaller until a QRS segment is dropped.  Note that this is different from the Wenckebach AV block in which the PR interval gets progressively longer before the dropped QRS segment.

### Type II Second Degree Sinoatrial Exit Block
A second degree type II, or sinus exit block, is a regular rhythm which may be normal or slow.  It is followed by a pause that is a multiple of the R-R interval.  Conduction across the SA node is normal until the time of the pause when it is blocked.

### Third Degree Sinoatrial Exit Block
A third degree sinoatrial block looks very similar to a sinus arrest.  However, a sinus arrest is caused by a failure to form impulses.  A third degree block is caused by failure to conduct them.  The rhythm is irregular and either normal or slow.  It is followed by a long pause that is not a multiple of the R-R interval.  The pause ends with a P wave, instead of a junctional escape beat the way a sinus arrest would.

# Causes

## Life Threatening Causes
Life-threatening causes include conditions which may result in death or permanent disability within 24 hours if left untreated.
- Acute coronary syndrome
- Acute renal failure
- Diabetic ketoacidosis
- Lateral medullary syndrome
- Myocardial rupture
- NSTEMI
- Organophosphate poisoning
- Parathion poisoning
- Poisonous spider bites
- Septic shock
- Severe brain injury
- STEMI

## Common Causes
- Acetylcholine
- Acute coronary syndrome
- Amiodarone
- Beta blockers
- Calcium channel blockers
- Digoxin
- Dilated cardiomyopathy
- Hyperkalemia
- Hypermagnesemia
- Hypertrophic cardiomyopathy
- Myocarditis
- Pericarditis
- Sick sinus syndrome
- Sinus node fibrosis
- STEMI

## Causes by Organ System

## Causes in Alphabetical Order

# Differentiating Sinoatrial block from other Diseases
Other types of SA nodal dysfunction are discussed in detail in other chapters on wikidoc.  Follow the hyperlinks for details and those include:
- Sinus pause or sinus arrest
- Sinus arrhythmia
- Wandering atrial pacemaker
- Sick sinus syndrome

# Diagnosis

### Electrocardiogram
- First degree SA block: Delay between impulse generation and transmission to the atrium
This abnormality is not detectable on the surface ECG.
- This abnormality is not detectable on the surface ECG.
- Second degree SA block, type 1(Wenchebach): Progressive lengthening of the interval between impulse generation and transmission, culminating in failure of transmission.
The P-P interval progressively shortens prior to the dropped P wave.
- The P-P interval progressively shortens prior to the dropped P wave.
- Second Degree SA block, Type II:   Intermittent dropped P waves with a constant interval between impulse generation and atrial depolarisation.
Intermittent dropping of the P wave, while other P waves are normal.
- Intermittent dropping of the P wave, while other P waves are normal.
- Third Degree SA Block: None of the sinus impulses are conducted to the right atrium.
There is a complete absence of P waves.
- There is a complete absence of P waves.

# Treatment
- Sinoatrial block principles of treatment are the same as sinus pause or sick sinus syndrome.
- Usually no treatment is indicated if the patient is asymptomatic.
- Stopping the offending drug is generally reasonable.
- When symptoms occur and become intolerable or life-threatening, then a permanent pacemaker would be indicated.