Second-Degree Atrioventricular Block

Etiology

Mobitz type I (Wenckebach) is often a normal variant and seen in individuals with a high vagal tone without evidence of structural heart disease. However, this rhythm can result from inferior myocardial ischemia, medication toxicity (AV nodal blocking agents), hyperkalemia, cardiomyopathy (Lyme disease), or following cardiac surgery.

Common drugs that are known to cause AV block include beta-blockers, digoxin, calcium channel blockers, and many antiarrhythmic agents. Recently the immunosuppressant agent, fingolimod, used to treat relapsing multiple sclerosis was also found to cause AV block.

Mobitz type II is rarely seen in patients without structural heart disease. It is often associated with myocardial ischemia and fibrosis or sclerosis of the myocardium. This rhythm often progresses to third-degree atrioventricular block.

Other causes of AV block include the following:

• Infiltrative heart disease- amyloidosis, hemochromatosis
• Acute rheumatic fever
• Malignancies- lymphoma
• Hyperthyroidism, myxedema
• Collagen vascular disorders
• Cardiac tumors
• Following transcatheter placement of valves
• Ethanol injection for hypertrophic cardiomyopathy

Genes

In some patients, the AV conduction block may have a genetic trait. Several mutations in the SCN5A gene have been linked to familial AV conduction block.

Epidemiology

There have not been large population-based studies on the prevalence of Mobitz type I or II atrioventricular blocks. At this time, there is no associated age, racial, or gender correlation. AV block is sometimes seen in athletes and in patients with congenital heart disorders.

Pathophysiology

In Mobitz type I (Wenckebach) there is a progressive prolongation of the PR interval (AV conduction) until eventually an atrial impulse is completely blocked. When an atrial impulse is completely blocked there will be a P wave without a QRS complex. This pattern is often referred to as a “dropped beat.” Mobitz type I occurs because each depolarization results in the prolongation of the refractory period of the atrioventricular (AV) node. When an atrial impulse comes through the AV node during the relative refractory period, the impulse will be conducted more slowly, resulting in a prolongation of the PR interval. Eventually, an impulse comes when the AV node is in its absolute refractory period and will not be conducted. This will manifest on the ECG as a P wave that is not followed by a QRS complex. This non-conducted impulse allows time for the AV node to reset, and the cycle continues. This phenomenon leads to a grouped beating.

 In Mobitz type II there is a constant PR interval across the rhythm strip both before and after the non-conducted atrial beat. Each P wave is associated with a QRS complex until there is one atrial conduction or P wave that is not followed by a QRS. Mobitz type II is often a problem in the infra-nodal conduction system, and therefore, is associated with a widened QRS complex, bundle-branch block, or fascicular block. Be aware that if more than one P wave is not conducted this is no longer a Mobitz type II and is considered a high degree AV block.

History and Physical

Questions should involve any history of cardiac disease both current and congenital, recent cardiac procedures and current medications. Patients with Mobitz type II often present with fatigue, dyspnea, chest pain, presyncope or syncope, and even sudden cardiac arrest.

Physical examination will often reveal bradycardia. Patients who have severe bradycardia resulting in hypotension and decreased cardiac output may be diaphoretic and pale. Patients who have a second-degree atrioventricular block secondary to myocardial ischemia may present with chest pain.

Evaluation

ECG is used to determine if a second-degree AV block is present. If ischemia is suspected based on the clinical picture and/or ECG findings, then cardiac biomarkers and chest radiography may be indicated. Serum electrolytes, calcium, and magnesium should be checked. If a patient is on digoxin, a level should be obtained to evaluate for toxicity. Routine imaging is not required for initial evaluation of second-degree atrioventricular block.[6][7][8][9]

Electrophysiologic testing is sometimes done to determine the level of the block and the need for a pacemaker.

Treatment / Management

Treatment for a Mobitz type I (Wenckebach) is often not necessary. Occasionally type I blocks may result in bradycardia leading to hypotension. If hypotension and bradycardia occur, type I blocks respond well to atropine. If unresponsive to atropine, pacing (transcutaneous or transvenous) should be initiated for stabilization. 

If the patient is on any beta blockers, calcium channel blockers or digoxin, the dose of these medications should be reduced or the medication discontinued. All patients with Mobitz 1 block should be admitted and monitored.

Treatment for a Mobitz type II involves initiating pacing as soon as this rhythm is identified. Type II blocks imply structural damage to the AV conduction system. This rhythm often deteriorates into complete heart block. These patients require transvenous pacing until a permanent pacemaker is placed. Unlike Mobitz type I (Wenckebach), patients that are bradycardic and hypotensive with a Mobitz type II rhythm often do not respond to atropine.

Differential Diagnosis

Differential diagnosis includes congenital heart block, sinoatrial exit block, non-conducted premature atrial complexes, first-degree atrioventricular block, myocardial infarction, medication toxicity, and third-degree atrioventricular block.

Prognosis

The prognosis for Mobitz type I (Wenckebach) is excellent. This is usually asymptomatic and requires no intervention in asymptomatic patients.

The prognosis for Mobitz type II varies based on the cause of the rhythm. Patients require placement of implanted pacemaker devices to avoid complications of Mobitz type II AV blocks such as decreased cardiac output, symptomatic bradycardia, and sudden cardiac arrest.

Complications

• Complete heart block
• Syncope
• Dizziness
• Chest pain
• Death