Third-degree atrioventricular block

• The P waves with a regular P-to-P interval (in other words, a sinus rhythm) represent 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 the lack of any apparent relationship between P waves and QRS complexes.

Presentation

People with third-degree AV block typically experience severe

Many conditions can cause third-degree heart block, but the most common cause is

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.[4] The escape rhythm typically originates in the AV junction, producing a narrow complex escape rhythm.^{[citation needed]}

An anterior wall myocardial infarction may damage the distal conduction system of the heart, causing third-degree heart block. Initially demonstrated by animal studies, this is due to a stark reduction in the Kv β-subunit of the voltage-gated K+ channels in the pacemaker cells of the atrioventricular junction, causing significantly decreased propagation of ions across gap junctions between cardiac cells and thus prolonging the PR interval.^[5] This is typically extensive, permanent damage to the conduction system, eliciting a necessity for a permanent pacemaker to be placed. ^[6] The escape rhythm typically originates in the ventricles, producing a wide complex escape rhythm.

Third-degree heart block may also be

Hyperkalemia in those with previous cardiac disease^[8] and Lyme disease can also result in third-degree heart block.^[9]

Hypermagnesemia

AV block may be observed in patients with

Atropine is often used as a first line treatment of a third-degree heart block in the presence of a narrow QRS which indicates a nodal block, but, may have little to no effect in an infra-nodal block.^[11] Atropine works by reducing vagal stimulation through the AV node but will not be effective in those who have had a previous heart transplant.^[12] Other drugs may be utilized such as epinephrine or dopamine which have positive chronotropic effects and may increase the heart rate.^[13] Treatment in emergency situations can involve electrical transcutaneous pacing in those who are acutely hemodynamically unstable and can be used regardless of the persons level of consciousness.^[14] Sedative agents such as a benzodiazepine or opiate may be used in conjunction with transcutaneous pacing to reduce the pain caused by the intervention.^[13][14]

In cases of suspected

Once the patient is hemodynamically stable, a potentially reversible cause should be evaluated and treated. If no reversible cause is identified, a permanent pacemaker is inserted.^[citation needed] Most stable patients have persistent bradycardia-related symptoms and require identification and treatment of any reversible cause or permanent implantable pacemaker.

Reversible causes of complete AV block should be ruled out before the insertion of a permanent pacemaker, such as drugs that slow heart rate and which induce hyperkalemia.

Complete atrioventricular block in acute myocardial infarction should be treated with temporary pacing and revascularization.^{[18][citation needed]}

Complete atrioventricular block caused by hyperkalemia should be treated to lower serum potassium levels and patients with hypothyroidism should also receive thyroid hormone.^[18]

If there is no reversible cause, the clear treatment of complete atrioventricular block is mostly permanent pacemaker placement.^{[citation needed]}

Prognosis

The prognosis of patients with complete heart block is generally poor without therapy. Patients with 1st and 2nd-degree heart block are usually asymptomatic.^[19]

See also

• Cardiac pacemaker
• Electrical conduction system of the heart
• Electrocardiogram
  (ECG)
• Atrioventricular block
• First-degree AV block
• Second-degree AV block

References