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|Third-degree atrioventricular block|
|Classification and external resources|
12 lead ECG showing complete heart block
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).
One of the pathognomonic characteristic of this block is the absolute absence of the opportunity for atrial impulses to enter and capture the ventricles (unlike AV dissociation with functional block) due to the organic nature of block (e.g., fibrosis, calcification, or infiltration of the node). In the presence of complete heart block, fusion or capture beats will never be seen. Patients with third-degree AV block typically experience bradycardia (an abnormally low measured heart rate), hypotension, and at times, hemodynamic instability. 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 prolonged 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. Studies have shown that third-degree heart block in the setting of an inferior wall myocardial infarction typically resolves within 2 weeks. 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.
Lastly, Lyme disease can also result in third-degree heart block.
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 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 (1st 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 < 40 bpm, 2) systolic blood pressure < 100 mm Hg, 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.
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