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Third-degree atrioventricular block
Classification and external resources
ICD-10 I44.2
ICD-9 426.0
DiseasesDB 10477
eMedicine emerg/235



Lead I and II in the same patient, demonstrating complete AV block. Note that the P waves are not related to the QRS complexes (PP interval and QRS interval both constant), demonstrating that the atria are electrically disconnected from the ventricles. The QRS complexes represent an escape rhythm arising from the ventricle.

Third-degree AV block, also known as complete heart block, is a medical condition in which the impulse generated in the SA node in the atrium does not propagate to the ventricles. [1]

Because the impulse is blocked, an accessory pacemaker in the lower chambers will typically activate the ventricles. This is known as an escape rhythm. Since this accessory pacemaker also activates independently of the impulse generated at the SA node, two independent rhythms can be noted on the electrocardiogram (ECG).

  • The P waves with a regular P to P interval represents the first rhythm.
  • The QRS complexes with a regular R to R interval represent the second rhythm. The PR interval will be variable, as the hallmark of complete heart block is no apparent relationship between P waves and QRS complexes.

Note:One of the most important characteristic of this block is the absolute absence of the opportunity for atrial impulses to enter and capture the ventricles(unlike AV dissociation). It means that, in the presence of this block, we never see any fusion or capture beats (Professor.Nader Ahmad Exeer.kabul Medical University).

Patients with third-degree AV block typically experience a lower overall measured heart rate (as low as 28 beats per minute during sleep), low blood pressure, and poor circulation. In some cases, exercising may be difficult, as the heart cannot react quickly enough to sudden changes in demand or sustain the higher heart rates required for sustained activity.


Many conditions can cause third-degree heart block, but the most common cause is coronary ischemia. Progressive degeneration of the electrical conduction system of the heart can lead to third-degree heart block. This may be preceded by first-degree AV block, second-degree AV block, bundle branch block, or bifascicular block. In addition, acute myocardial infarction may present with third-degree AV block.

An inferior wall myocardial infarction may cause damage to the AV node, causing third-degree heart block. In this case, the damage is usually transitory, and the AV node may recover. Studies have shown that third-degree heart block in the setting of an inferior wall myocardial infarction typically resolves within 2 weeks[citation is needed]. The escape rhythm typically originates in the AV junction, producing a narrow complex escape rhythm.

An anterior wall myocardial infarction may damage the distal conduction system of the heart, causing third-degree heart block. This is typically extensive, permanent damage to the conduction system, necessitating a permanent pacemaker to be placed. The escape rhythm typically originates in the ventricles, producing a wide complex escape rhythm.

Third-degree heart block may also be congenital and has been linked to the presence of lupus in the mother. It is thought that maternal antibodies may cross the placenta and attack the heart tissue during gestation. The cause of congenital third-degree heart block in many patients is unknown. Studies suggest that the prevalence of congenital third-degree heart block is between 1 in 15,000 and 1 in 22,000 live births.


Third-degree AV block can be treated by use of a dual-chamber artificial pacemaker. This type of device typically listens for a pulse from the SA node and sends a pulse to the AV node at an appropriate interval, essentially completing the connection between the two nodes. Pacemakers in this role are usually programmed to enforce a minimum heart rate and to record instances of atrial flutter and atrial fibrillation, two common secondary conditions that can accompany third-degree AV block.

Treatment may also include medicines to control blood pressure and atrial fibrillation, as well as lifestyle and dietary changes to reduce risk factors associated with heart attack and stroke.

Treatment in emergency situations ultimately involves electrical pacing. However the American Heart Association states that giving a trial of atropine while waiting for the pacer to be set up is acceptable. Atropine is effective for treating early Heart Blocks (1rst Degree and 2nd Degree type 1) but generally thought to have no effect on 3rd Degree Blocks.

The 2005 Joint European Resuscitation and Resuscitation Council (UK) guidelines state that Atropine is the first line treatment especially if there were any adverse signs, namely: 1) heart rate <40bpm 2) systolic blood pressure < 100mmHg 3) signs of heart failure and 4) ventricular arrhythmias requiring suppression. If these fail to respond to atropine or there is a potential risk of asystole, transvenous pacing is indicated. The risk factors for asystole include 1) previous asystole, 2) complete heart block with wide complexes and 3) ventricular pause for > 3 seconds. Mobitz Type 2 AV block is another indication for pacing.

See also


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