Acquired third-degree (complete) atrioventricular block

INTRODUCTION — 

Atrioventricular (AV) block is defined as a delay or interruption in the transmission of an impulse from the atria to the ventricles due to an anatomical or functional impairment in the conduction system. The level of block may be within the AV node or in the His-Purkinje system. The conduction disturbance can be transient or permanent, with conduction that is delayed, intermittent, or absent. Commonly used terminology includes:

●First-degree AV block – Slowed conduction without missed beats.

●Second-degree AV block – Missed beats, often in a regular pattern (eg, 2:1, 3:2), which is further classified into Mobitz type I (Wenckebach) and Mobitz II AV block.

●Third-degree (complete) AV block – No atrial impulses reach the ventricle.

●High-grade AV block – Intermittent atrial conduction to the ventricle with two or more consecutive blocked P waves but without complete AV block.

The clinical presentation, evaluation, and management of acquired third-degree AV block will be discussed here. Congenital third-degree heart block, the etiology of AV block in general, and the management of other specific types of AV block are discussed separately. (See "Etiology of atrioventricular block" and "Congenital third-degree (complete) atrioventricular block" and "First-degree atrioventricular block" and "Second-degree atrioventricular block: Mobitz type I (Wenckebach block)" and "Second-degree atrioventricular block: Mobitz type II".)

ETIOLOGY — 

There are many causes of acquired third-degree atrioventricular (AV) block. A pathologic or iatrogenic cause can be identified in approximately half of cases, while the remainder are idiopathic. Identifiable causes of third-degree AV block include the following (table 1):

●Pathologic – Pathologic causes of third-degree AV block include myocardial ischemia or infarction (acute or chronic) involving the conduction system, cardiomyopathy (eg, amyloidosis, sarcoidosis), myocarditis (eg, Lyme disease, COVID-19), endocarditis with abscess formation, hyperkalemia, profound hypothyroidism (or, rarely, hyperthyroidism), hypervagotonia, infiltrative malignancies, and neuromuscular diseases.

●Iatrogenic – Iatrogenic causes include medications (ie, AV nodal-blocking drugs) and procedures (cardiac surgery, catheter ablation, transcatheter aortic valve implantation).

Idiopathic third-degree AV block is typically related to progressive cardiac conduction disease with myocardial fibrosis and/or sclerosis that affects the conduction system.

A more extensive discussion of the etiology of AV block is presented separately. (See "Etiology of atrioventricular block".)

CLINICAL PRESENTATION — 

In patients with third-degree (complete) atrioventricular (AV) block, symptoms typically depend upon the rate of the underlying escape rhythm and the presence or absence of comorbid conditions. Nearly all patients with acquired third-degree AV block will present with symptoms of some kind, including fatigue, dyspnea, chest pain, dizziness, syncope, or sudden cardiac arrest.

The type of escape rhythm impacts the severity of the symptoms. Patients who have a junctional escape rhythm (ie, rate of 40 to 60 beats per minute with narrow QRS complex) may be asymptomatic at rest because their cardiac output is adequate for the resting state; however, with exertion, most will experience symptoms because they are unable to augment their cardiac output due to the fixed ventricular rate. By contrast, patients with a ventricular escape rhythm (ie, rate <40 beats per minute with wide QRS complex) usually experience symptoms at rest. In rare cases, a patient may die suddenly due to the absence of an escape.

Patients with underlying coronary artery disease or heart failure often experience symptoms that are more severe than patients without underlying cardiac conditions. These patients may experience abrupt exacerbation of chronic symptoms (chest pain, dyspnea) when they develop third-degree AV block, in addition to new symptoms (dizziness, syncope).

The physical examination of patients with third-degree AV block is often unrevealing, with the exception of bradycardia. However, some patients may appear pale or diaphoretic due to the reduction in cardiac output. Patients with underlying heart failure that is exacerbated by the development of heart block may have crackles on lung examination, elevated jugular venous pulsations, and peripheral edema.

EVALUATE THE ELECTROCARDIOGRAM — 

We use the 12-lead electrocardiogram (ECG) to diagnose third-degree (complete) atrioventricular (AV) block, assess the escape rhythm, and diagnose ST-elevation myocardial infarction (STEMI).

Diagnosis of third-degree AV block — Most patients who present with third-degree AV block are in sinus rhythm, but some are in atrial fibrillation (AF).

●Patients in sinus rhythm – For patients in sinus rhythm, we assess the ECG for evidence of atrial (P waves) and ventricular (QRS complexes) activity that are independent of each other (waveform 1). In nearly all cases, the atrial rate will be faster than the ventricular rate, and there will be dissociation of the P waves and QRS complexes. In rare cases, third-degree AV block may be mistaken for second-degree (2:1) AV block if the sinus rate is exactly twice the rate of the ventricular escape rhythm. However, within a few seconds or minutes, a slight variation in the atrial or ventricular rate should allow AV dissociation to be appreciated.

●Patients in atrial fibrillation – For patients in AF, the diagnosis is more challenging than for patients in sinus rhythm. Since ventricular activity normally appears irregular when patients are in AF, we suspect third-degree AV block if the QRS complexes appear regular because the regularity suggests the presence of an escape rhythm. However, a regular rhythm does not always indicate that third-degree AV block is present; patients with bradycardia of any etiology may develop a junctional rhythm in the absence of AV block. If the diagnosis of third-degree AV block is unclear, consultation with a cardiac electrophysiologist is appropriate. In rare cases, an electrophysiology study may be necessary to make or exclude the diagnosis.

Once we have diagnosed third-degree AV block, we assess the appearance of the escape rhythm, which provides clues about the location of the block in the conduction system (eg, within the AV node, in the bundle of His, infrahisian) [1-3].

●Narrow QRS complex – A narrow QRS complex escape rhythm (waveform 1) is known as a junctional escape rhythm. The presence of a junctional escape rhythm indicates that block is occurring within the AV node or at the level of the bundle of His [4]. Junctional escape rhythms typically have a rate between 40 and 60 beats per minute. Patients with a junctional escape rhythm may have a reversible cause of third-degree AV block (eg, Lyme disease, medications, acute inferior MI) or an irreversible cause (eg, idiopathic, cardiomyopathy, endocarditis with abscess).

●Wide QRS complex – A wide QRS complex escape rhythm is usually (ie, 80 percent of the time) due to infrahisian block, in which case it is referred to as a ventricular escape rhythm. Ventricular escape rhythms typically have a rate of <40 beats per minute (waveform 2) and are associated with more severe symptoms than junctional escape rhythms. A ventricular escape rhythm almost always suggests an irreversible etiology of third-degree AV block, with hyperkalemia being a notable exception.

In 20 percent of patients with wide QRS complexes, the escape rhythm is junctional, not ventricular [4]; in these patients, the QRS is wider than normal because of underlying disease elsewhere in the conduction system (eg, bundle branch block) [4-6]. An escape rate ≥40 beats per minute with a wide QRS complex suggests that the escape rhythm is junctional. These patients may have a reversible or irreversible etiology of AV block.

Evaluation for acute myocardial infarction — For all patients with third-degree AV block, it is important to evaluate the ECG for evidence of STEMI, which is a common cause of reversible AV block.

●For patients with a narrow QRS complex (ie, junctional) escape rhythm, we use standard criteria to diagnose STEMI and refer for emergency coronary angiography if indicated. Diagnosis of STEMI is discussed elsewhere. (See "Diagnosis of acute myocardial infarction", section on 'ECG'.)

●For patients with a wide QRS complex escape rhythm, STEMI may be difficult to diagnose on ECG. For this reason, we refer most patients with a wide complex escape rhythm and ischemic symptoms for emergent coronary angiography.

If a patient with suspected STEMI is unstable, we administer therapies (eg, atropine, transcutaneous pacing) to improve stability (see 'Ensure hemodynamic stability' below). While we are doing so, we ask another team member to contact the interventional cardiologist or cardiac catheterization team to avoid delaying the procedure.

ENSURE HEMODYNAMIC STABILITY — 

The initial management of third-degree (complete) atrioventricular (AV) block depends on the patient’s stability (algorithm 1). To determine stability, we assess the patient’s vital signs, mental status, and symptoms. Patients with third-degree AV block are unstable if they have one or more of the following features: hypotension, altered mental status, signs of shock, ischemic chest discomfort, or acute heart failure.

●Unstable patients – Patients who are pulseless should be managed according to the advanced cardiac life support (ACLS) cardiac arrest algorithm (algorithm 2).

For unstable patients who have a pulse, our management (algorithm 1) is in accord with the ACLS bradycardia algorithm:

•We give atropine 1 mg intravenously (IV). We repeat this dose every three to five minutes while the patient remains unstable, not exceeding a total of three doses. A favorable response to atropine suggests that AV block is occurring within the AV node. Atropine is preferred over transcutaneous pacing as the first-line treatment because of its rapid onset of action.

•For unstable patients who do not respond to atropine, we initiate transcutaneous pacing. We do not necessarily wait until three doses of atropine have been administered to initiate transcutaneous pacing, especially if the patient has a ventricular escape rhythm (ie, wide QRS complex rhythm <40 beats per minute), because atropine is less likely to be effective on distal conduction system block than block within the AV node. If possible, we administer IV sedation before initiating transcutaneous pacing, which is uncomfortable for the patient. Details about performing transcutaneous pacing are found elsewhere. (See "Temporary cardiac pacing", section on 'Transcutaneous'.)

While transcutaneous pacing is occurring, we prepare the patient for transvenous pacing (ie, obtain central venous access, place the pacing wire). Transcutaneous pacing is unreliable (ie, ventricular capture cannot be assured) and is meant to be a temporizing measure until transvenous pacing can occur. If the patient has hypotension and/or heart failure while undergoing transcutaneous pacing, we manage them in the following manner until transvenous pacing can be performed:

-For patients with hypotension, we start a dopamine IV infusion at 5 mcg/kg/minute and titrate as needed to achieve an optimal hemodynamic response, to a maximum dose of 20 mcg/kg/minute [7]. Dopamine is preferred over dobutamine due to its vasopressor effects.

-For patients with heart failure and a normal blood pressure, we start a dobutamine IV infusion at 2 to 5 mcg/kg/minute and titrate as needed to achieve an optimal clinical response. The usual maintenance dose is 2 to 10 mcg/kg/minute. Dobutamine is preferred over dopamine because it has chronotropic and inotropic effects and does not act as a vasopressor.

-Our contributors do not use epinephrine IV infusion for unstable third-degree AV block, but some experts do (algorithm 3).

After an unstable patient has been stabilized, we perform further evaluation as discussed below. (See 'Exclude reversible causes' below.)

●Stable patients – Patients who are stable do not require the acute interventions (ie, atropine, transcutaneous or transvenous pacing, dopamine, dobutamine) that are appropriate for unstable patients. However, it is important to keep in mind that patients who are initially stable may become unstable and require acute intervention. We perform further evaluation as discussed below. (See 'Exclude reversible causes' below.)

EXCLUDE REVERSIBLE CAUSES

Evaluation — After patients with third-degree (complete) atrioventricular (AV) block have been stabilized, we evaluate them for underlying conditions that may cause AV block. (See 'Etiology' above.)

History and physical — We perform a history and physical examination to determine whether any precipitating conditions may be present. We ask about the following:

●Recent onset of chest, jaw, or neck discomfort, which may indicate acute myocardial infarction (MI) or unstable angina. (See "Conduction abnormalities after myocardial infarction".)

●History of heart disease (coronary artery disease, cardiomyopathy, endocarditis) or sarcoidosis.

●History of chronic kidney disease, which may cause hyperkalemia.

●Signs and symptoms of severe hypothyroidism (eg, fatigue, cold intolerance, weight gain, myxedema) or hyperthyroidism (eg, proptosis, sweating, hyperpigmentation). (See "Clinical manifestations of hypothyroidism" and "Overview of the clinical manifestations of hyperthyroidism in adults".)

●Recent history of tick exposure, tick bite, or erythema migrans, which may suggest Lyme disease. (See "Clinical manifestations of Lyme disease in adults".)

●Family history of AV block, which may suggest familial AV block.

●Ingestion of medications that block the AV node (eg, beta blockers, nondihydropyridine calcium channel blockers, digoxin, certain antiarrhythmic drugs).

Laboratory and imaging tests — We perform the following laboratory tests on all patients with third-degree AV block:

●Serum potassium and creatinine to assess for hyperkalemia and acute kidney injury (which can occur secondary to hypoperfusion from reduced cardiac output).

●Troponin-I or troponin-T, which can be elevated with MI.

●Thyroid-stimulating hormone.

Laboratory and imaging tests that may be indicated based on the history and physical examination include the following:

●Lyme serologies if Lyme disease is suspected. (See "Diagnosis of Lyme disease", section on 'Serologic testing algorithms'.)

●Echocardiography if cardiomyopathy is suspected.

●Cardiac magnetic resonance imaging or ¹⁸F-fluorodeoxyglucose-positron emission tomography is appropriate for patients under age 60 who have unexplained heart block, to rule out cardiac sarcoidosis. Previously undetected cardiac sarcoidosis has been identified in up to 25 to 35 percent of such patients [8,9]. (See "Clinical manifestations and diagnosis of cardiac sarcoidosis", section on 'How to diagnose cardiac sarcoidosis'.)

●Coronary angiography if there is evidence of acute MI. (See 'Evaluation for acute myocardial infarction' above.)

Treatment — After the evaluation described above, we manage patients for any underlying conditions that may be contributing to third-degree (complete) AV block. Many of the conditions that are known to cause third-degree AV block cause irreversible conduction system disease; patients with these conditions will require permanent pacemaker (PPM) implantation.

However, for patients with certain conditions (eg, Lyme disease, inferior MI, hyperkalemia, hypothyroidism, drugs, high vagal tone), treatment may lead to resolution of the AV block.

●Lyme carditis – Patients with Lyme carditis should be treated with appropriate antibiotics (table 2). Third-degree AV block usually resolves over one week, although some degree of AV block (first- or second-degree) may persist for weeks. (See "Lyme carditis", section on 'Prognosis'.)

●Hyperkalemia – Patients with hyperkalemia should be treated according to standard protocols for hyperkalemic emergency (see "Treatment and prevention of hyperkalemia in adults", section on 'Patients with a hyperkalemic emergency'). Third-degree AV block is expected to resolve within hours.

●Acute ST-elevation myocardial infarction – Patients with acute inferior ST-elevation myocardial infarction (STEMI) are treated with percutaneous coronary intervention. A temporary transvenous pacemaker is typically placed in the cardiac catheterization laboratory. Third-degree AV block usually resolves within 24 hours. By contrast, patients with anterior STEMI usually have infrahisian block, which is unlikely to resolve. (See "Conduction abnormalities after myocardial infarction", section on 'Management of patients with AV block'.)

●AV nodal-blocking agents – Patients on AV nodal-blocking drugs (eg, beta blockers, nondihydropyridine calcium channel blockers, digoxin, certain antiarrhythmics) should have those drugs withheld. For patients who present with an overdose, third-degree AV block should resolve as the drug effects wear off. However, for those who were taking the drugs as prescribed, it is unusual to develop third-degree AV block from these drugs without having underlying conduction disease; for these patients, withholding the drug may not lead to resolution of AV block.

●Hypothyroidism – Patients with hypothyroidism should receive thyroid replacement therapy. Third-degree AV block should resolve over a few weeks; if it does not, then the hypothyroidism is not the cause of the AV block. (See "Treatment of primary hypothyroidism in adults".)

●Hypervagotonia – Patients with transient vagally mediated heart block (ie, hypervagotonia), which is often seen on hospital telemetry or event monitors during sleep, do not require any specific treatment. While this type of third-degree AV block is expected to recur, these patients are typically asymptomatic and do not require a permanent pacemaker.

●Cardiac sarcoidosis – For patients with cardiac sarcoidosis, immunosuppressive therapy typically reverses heart block. However, in most cases, a permanent pacemaker is indicated due to the risk of recurrence. Furthermore, many patients qualify for an implantable cardioverter-defibrillator due to an increased risk of sudden cardiac death due to ventricular arrhythmia. (See "Management and prognosis of cardiac sarcoidosis", section on 'Arrhythmias and conduction system disease'.)

If patients with a potentially reversible cause of third-degree AV block are symptomatic at rest, we sometimes use temporary transvenous pacing while waiting for the AV block to resolve. (See "Temporary cardiac pacing".)

PLACE PERMANENT PACEMAKER IF AV BLOCK IS NOT REVERSIBLE — 

Patients with third-degree (complete) atrioventricular (AV) block who are not found to have a reversible cause of the conduction abnormality require permanent pacemaker (PPM) implantation [10,11]. Third-degree AV block without an identifiable cause may be persistent or intermittent. The purpose of PPM implantation is twofold: to reduce symptoms (eg, fatigue, dyspnea, dizziness) and to decrease the risk of a serious complication (eg, syncope causing a fall or motor vehicle accident). Dual-chamber (ie, AV) pacing is preferable to single-chamber right ventricular pacing for patients with AV block due to the favorable hemodynamic benefits of AV synchrony [10]. Device selection is discussed in detail elsewhere. (See "Permanent cardiac pacing: Overview of devices and indications" and "Modes of cardiac pacing: Nomenclature and selection".)

For most patients whose third-degree AV block resolves after management of a precipitating condition, PPM implantation is not indicated.

SOCIETY GUIDELINE LINKS — 

Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Arrhythmias in adults" and "Society guideline links: Cardiac implantable electronic devices".)

INFORMATION FOR PATIENTS — 

UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topics (see "Patient education: Bradycardia (The Basics)" and "Patient education: Heart block in adults (The Basics)")

SUMMARY AND RECOMMENDATIONS

●Introduction – In third-degree (complete) atrioventricular (AV) block, all electrical impulses from the atrium fail to reach the ventricle. (See 'Introduction' above.)

●Etiology – A pathologic or iatrogenic cause of third-degree AV block can be identified in approximately half of patients (table 1). The most common causes include acute myocardial infarction (MI), Lyme disease, and cardiac procedures. Idiopathic third-degree AV block is due to myocardial fibrosis of the conduction system. (See 'Etiology' above.)

●Clinical presentation – Typical symptoms include fatigue, dyspnea, chest pain, dizziness, and syncope. The severity of symptoms depends on the rate of the underlying escape rhythm and the presence or absence of comorbid conditions. (See 'Clinical presentation' above.)

●12-lead electrocardiogram (ECG)

•Diagnosis – For patients in sinus rhythm, third-degree AV block can be diagnosed when the atrial rate is faster than the ventricular rate and there is dissociation of the P waves and QRS complexes. For patients in atrial fibrillation, regular QRS complexes are a clue to third-degree AV block. (See 'Diagnosis of third-degree AV block' above.)

•Escape rhythm – A narrow QRS complex escape rhythm, known as a junctional escape rhythm, typically has a rate of 40 to 60 bpm. A wide QRS complex rhythm is usually due to infrahisian block, in which case it is called a ventricular escape rhythm. Most ventricular escape rhythms have a rate <40 bpm. A wide complex escape rhythm with a rate ≥40 bpm suggests a junctional rhythm with underlying bundle branch block.

•Evaluation for acute MI – ST elevations are an indication for emergent coronary revascularization. For patients with a wide escape rhythm and ischemic symptoms, we refer for emergent coronary revascularization even if ST elevations are not present. (See 'Evaluation for acute myocardial infarction' above.)

●Ensure hemodynamic stability

•Unstable patients – For unstable patients, we administer atropine 1 mg intravenously (IV) and repeat the dose every three to five minutes, not exceeding three doses (algorithm 1). For those who do not respond, we initiate transcutaneous pacing and prepare the patient for transvenous pacing. Dopamine or dobutamine infusions may be used for patients with hypotension or heart failure symptoms while awaiting transvenous pacing. (See 'Ensure hemodynamic stability' above.)

•Stable patients – Patients who are stable do not require the acute interventions (ie, atropine, transcutaneous or transvenous pacing, dopamine, dobutamine) that are appropriate for unstable patients. However, it is important to keep in mind that patients who are initially stable may become unstable and require acute intervention.

●Exclude reversible causes – We perform a history, physical examination, and laboratory studies to rule out reversible causes of third-degree AV block. We treat any conditions that are present. (See 'Exclude reversible causes' above.)

●Permanent pacemaker if AV block is not reversible – Patients with third-degree AV block who do not have a reversible cause require permanent pacemaker implantation to reduce symptoms and decrease the risk of a serious complication. (See 'Place permanent pacemaker if AV block is not reversible' above.)

ACKNOWLEDGMENT — 

The UpToDate editorial staff acknowledges Leonard Ganz, MD, FHRS, FACC, who contributed to earlier versions of this topic review.