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Second-Degree Atrioventricular Block

Practice Essentials

Second-degree atrioventricular (AV) block, or second-degree heart block, is a disease of the cardiac conduction system in which the conduction of atrial impulse through the AV node and/or His bundle is delayed or blocked. Patients with second-degree AV block may be asymptomatic or they may experience variety of symptoms such as lightheadedness and syncope. Mobitz type II AV block may progress to complete heart block, with an associated increased risk of mortality.

Signs and symptoms

In patients with second-degree AV block, symptoms may vary substantially, as follows:

No symptoms (more common in patients with type I, such as well-trained athletes and persons without structural heart disease)

Light-headedness, dizziness, or syncope (more common in type II)

Chest pain, if the heart block is related to myocarditis or ischemia

A regularly irregular heartbeat

Bradycardia may be present

Symptomatic patients may have signs of hypoperfusion, including hypotension

See Clinical Presentation for more detail.

Diagnosis

ECG is employed to identify the presence and type of second-degree AV block. The typical ECG findings in Mobitz I (Wenckebach) AV block—the most common form of second-degree AV block—are as follows:

Gradually progressive PR interval prolongation occurs before the blocked sinus impulse

The greatest PR increment typically occurs between the first and second beats of a cycle, gradually decreasing in subsequent beats

Shortening of the PR interval occurs after the blocked sinus impulse, provided that the P wave is conducted to the ventricle

Junctional escape beats may occur along with nonconducted P waves

A pause occurs after the blocked P wave that is less than the sum of the 2 beats before the block

During very long sequences (typically >6:5), PR-interval prolongation may be inapparent and minimal until the last beat of the cycle, when it abruptly becomes much greater

Post-block PR-interval shortening remains the cornerstone of the diagnosis of Mobitz I block, regardless of whether the periodicity has typical or atypical features

R-R intervals shorten as PR intervals become longer

The typical ECG findings in Mobitz II AV block are as follows:

Consecutively conducted beats with the same PR interval are followed by a blocked sinus P wave

PR interval in the first beat after the block is similar to the PR interval before the AV block

A pause encompassing the blocked P wave is equal to exactly twice the sinus cycle length

The level of the block, AV nodal or infranodal (ie, in the specialized His-Purkinje conduction system), carries prognostic significance, as follows:

AV nodal blocks, which are the vast majority of Mobitz I blocks, carry a favorable prognosis

AV nodal blocks do not carry the risk of direct progression to a Mobitz II block or a complete heart block
; however, if there is an underlying structural heart disease as the cause of  the AV block, a more advanced AV block may manifest in the later stages of the disease 

Infranodal blocks carry significant risk of progression to complete heart block

Evaluating for stability of the sinus rate is important because conditions associated with increases in vagal tone may cause simultaneous sinus slowing and AV block and, therefore, mimic a Mobitz II block. In addition, diagnosing Mobitz II block in the presence of a shortened post-block PR interval is impossible.

An invasive His bundle recording is required to make the diagnosis of an infranodal block; however, ECG indications regarding the site of the block are as follows:

A Mobitz I block with a narrow QRS complex is almost always located in the AV node

A normal PR interval with minuscule increments in AV conduction delay should raise the suggestion of an infranodal Wenckebach block; however, larger increments in AV conduction do not necessarily exclude infranodal Wenckebach block

In the presence of a wide QRS complex, AV block is more often infranodal

An increment in PR interval of longer than 100 msec favors a block site in the AV node

Diagnostic electrophysiologic testing can help determine the level of the block and the potential need for a permanent pacemaker. Such testing is indicated for patients in whom His-Purkinje (infranodal) block is suspected but has not been confirmed, such as those with the following:

Mobitz I second-degree AV block associated with a wide QRS complex in the absence of symptoms

2:1 second-degree AV block with a wide QRS complex in the absence of symptoms

Mobitz I second-degree block with a history of unexplained syncope

Other indications for electrophysiologic testing are as follows:

Patients with pseudo-AV block and those with premature, concealed junctional depolarization, which may be the cause of second- or third-degree AV block

Patients with second- or third-degree AV block in whom another arrhythmia is suspected as the cause of the symptoms (eg, those who remain symptomatic after pacemaker placement)

In most cases, however, further monitoring (either inpatient rhythm monitoring or ambulatory ECG monitoring) provides adequate diagnostic information such that, currently, it is rare to perform an electrophysiology study solely for the evaluation of conduction disease.

Laboratory studies to identify possible underlying causes are as follows:

Serum electrolytes, calcium, and magnesium levels

A digoxin level in patients on digoxin

Cardiac biomarker testing in patients with suspected myocardial ischemia

Myocarditis-related laboratory studies (eg, Lyme titers, HIV serologies, enterovirus polymerase chain reaction [PCR], adenovirus PCR, Chagas titers), if clinically relevant

Infection-related studies, apropos a valve ring abscess

Thyroid function studies if appropriate

See Workup for more detail.

Management

Acute treatment of Mobitz type I second-degree AV block is as follows:

In patients who have symptoms or who have concomitant acute myocardial ischemia or myocardial infarction (MI), admission is indicated to a unit with telemetry monitoring and transcutaneous pacing capabilities

Symptomatic patients should be treated with atropine and transcutaneous pacing immediately, followed by transvenous temporary pacing until further workup detemines the etiology of the disease

Atropine should be administered with caution in patients with suspected myocardial ischemia, as ventricular dysrhythmias can occur. Atropine increases the conduction in the AV node. If the conduction block is infranodal (eg if the block is Mobitz II), an increase in AV nodal conduction by atropine only worsens the infranodal conduction delay and increases the AV block. 

Acute treatment of Mobitz type II second-degree AV block is as follows:

Admit all patients to a unit with monitored beds, where transcutaneous and transvenous pacing capabilities are available

Apply transcutaneous pacing pads to all patients with Mobitz II second-degree AV block, including those who are asymptomatic, because of the risk of progression to complete heart block. Test the transcutaneous pacemaker to ensure capture; if capture cannot able be achieved, then insertion of a transvenous pacemaker is indicated, even in asymptomatic patients

Urgent cardiology consultation is indicated for patients who are symptomatic or are asymptomatic but unable to achieve capture with transcutaneous pacing

It is reasonable to insert a transvenous pacemaker for all new Mobitz type II blocks

Hemodynamically unstable patients for whom an emergency cardiology consult is not available should undergo placement of a temporary transvenous pacing wire in the emergency department, with confirmation of correct positioning by chest radiography

Guidelines recommend the following as indications for permanent pacing in second-degree AV block
:

Second-degree AV block associated with signs such as bradycardia, heart failure, and asystole for 3 seconds or longer while the patient is awake

Second-degree AV block with neuromuscular diseases, such as myotonic muscular dystrophy, Erb dystrophy, and peroneal muscular atrophy, even in asymptomatic patients (progression of the block is unpredictable in these patients); in some of these patients, an implantable cardioverter defibrillator (ICD) may be appropriate

Mobitz II second-degree AV block with wide QRS complexes

Asymptomatic Mobitz I second-degree AV block with the block at intra- or infra-His level found on electrophysiologic testing. Some of the electrophysiologic findings of an intra-His block include an HV interval longer than 100 ms, doublng of the HV interval after administration of procainamide, and the presence of split double potentials on the His recording catheter.

In some cases, the following may also be indications for permanent pacemaker insertion:

Persistent, symptomatic second-degree AV block after MI, especially if it is associated with bundle-branch block; AV block resulting from right coronary artery occlusion tends to resolve over a few days after revascularization versus left anterior descending artery MI, which results in permanent AV block

High-grade AV block after anterior MI, even if transient

Persistent second-degree AV block after cardiac surgery

Permanent pacing may not be required in the following situations:

Transient or asymptomatic second-degree AV block after MI, particularly after right coronary artery occlusion

Second-degree AV block in patients with drug toxicity, Lyme disease, or hypoxia in sleep

Whenever correction of the underlying pathology is expected to resolve second-degree AV block

AV block after transcatheter aortic valve implantation may occur. This is a relatively new technology, and there is not enough adequte evidence to guide the patient’s therapies in this situation. In some cases, depending on the type of the implanted valve, baseline ECG features, degree and location of the aortic valve calcification, and the patient’s comorbidities, implanting a permanent pacemaker outside of conventional criteria may be a reasonable and safe approach. 

See Treatment and Medication for more detail.

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