Third-Degree Atrioventricular Block (Complete Heart Block) Treatment & Management

New-onset third-degree atrioventricular (AV) block (complete heart block) is a medical emergency. Treatment of third-degree AV block is based on the level of the block. A common misconception of an inexperienced clinician is to gauge a patient’s stability according to the heart rate and blood pressure rather than according to the symptoms and level of the block.

An asymptomatic patient with inferior wall myocardial infarction (MI) causing complete heart block at the AV node (AVN) level and a heart rate of 35 beats/min is at very little immediate risk. A patient in the acute phase of an anterior wall MI with intermittent distal high-grade block is at immediate danger of impending asystole and requires immediate preparation for pacing of some kind, even though the heart rate between asystolic episodes may be 90 beats/min.

The first, and sometimes most important, medical treatment for heart block is the withdrawal of any potentially aggravating or causative medications. Many antihypertensive, antianginal, antiarrhythmic, and heart failure medications cause AV block that resolves after withdrawal of the offending agent.

Review patient medication lists upon presentation to help rule out medication-induced or medication-aggravated heart block. Common drugs that induce AV block include beta-blockers, calcium channel blockers, antiarrhythmics, and digoxin. Withdrawal of the offending drugs is the first treatment for heart block.

Cases in which complete heart block results from a calcium channel blocker should be managed in much the same fashion as cases involving other causes of third-degree block (eg, pacemaker), but affected patients should also receive appropriate treatment for toxicity from calcium channel blockers. This therapy includes the administration of intravenous (IV) fluids, calcium, glucagons, vasopressors, and high-dose insulin (hyperinsulinemic euglycemia [HIE] therapy). (See Calcium Channel Blocker Toxicity.)

Overdoses of beta-blockers are managed similarly to overdoses of calcium channel blockers, although HIE therapy for beta-blocker overdoses is less well established. (See Beta-Blocker Toxicity.)

Medical treatment of complete heart block is limited to patients with conduction disease in the AVN. Patients with block at the AVN level, in the absence of ischemia, can benefit from sympathomimetic agents or vagolytic agents.

Initial efforts should focus on assessing the need for temporary pacing and initiating the pacing. Except in the case of AV block caused by medications that can be withdrawn or infections that can be treated, most patients with acquired complete heart block will require a permanent pacemaker or an implantable cardioverter defibrillator (ICD).

A study by Zhao et al in 38 patients who underwent dual-chamber pacemaker implantation for third-degree AV block found that compared with those who underwent implantation in the right ventricular apex, patients whose pacemaker was implanted in the right ventricular outflow tract exhibited better results with regard to systolic function and systolic dyssynchronization, at 12-month follow-up. ^[23]

All patients with suspected third-degree atrioventricular (AV) block (complete heart block) should be rapidly transported to the nearest available facility, receiving advanced life support (ACLS) with continuous cardiac monitoring, as per local protocols. In all patients, oxygen should be administered and intravenous (IV) access established. Avoid maneuvers likely to increase vagal tone (eg, Valsalva maneuvers, painful stimuli). Atropine can be administered but should be given cautiously.

Treatment in the emergency department (ED) should continue that already established in the prehospital setting, which includes administering oxygen, maintenance of an IV line, frequent monitoring of blood pressure, and continuous cardiac monitoring. Transcutaneous pacing pads should be applied and tested, if this has not already been done.

All patients with third-degree heart block need to be admitted to either a telemetry floor (if hemodynamically stable and transcutaneous pacing achieves capture) or an intensive care unit (ICU). The decision between the two locations should be made in conjunction with the cardiologist. Any patient who is hemodynamically unstable, has persistent complete heart block, has electrolyte abnormalities, or who is in complete heart block as a result of an overdose or myocardial infarction should be admitted to the ICU.

Patients may be transferred to a higher level of care if the hospital does not have intensive care capabilities or if appropriate consultation services (eg, cardiology) are not available.

Transcutaneous pacing is the treatment of choice for any symptomatic patient. All patients who have third-degree atrioventricular (AV) block (complete heart block) associated with repeated pauses, an inadequate escape rhythm, or a block below the AV node (AVN) should be stabilized with temporary pacing. Transcutaneous pacing is demonstrated in the video below.

When assessing capture with transcutaneous pacing, it is important to avoid the common mistake of looking for electrical capture on the monitor. The pacing artifact is usually large and that QRS complex can rarely be seen reliably. Instead, palpation for the pulse is the best indication of capture.

Although the transcutaneous pacer should be placed on all patients, this mode of pacing is not highly reliable and is extremely uncomfortable for the patient. Symptomatic patients in whom capture cannot be obtained with a transcutaneous pacemaker need urgent placement of a transvenous pacemaker. Placement of a transvenous pacemaker is also indicated for asymptomatic patients in whom capture cannot be obtained; the timing of this should be discussed with the consulting cardiologist.

The decision to place a transvenous pacing wire depends on the availability of fully trained personnel and equipment for placing a transvenous wire. All patients with persistent block below the AVN should be prepared for temporary wire placement.

Hemodynamically stable patients in whom transcutaneous pacing can be successfully performed can go to a telemetry unit or ICU at the discretion of the treating cardiologist. Hemodynamically unstable patients for whom timely cardiologic consultation is unavailable should undergo temporary transvenous pacemaker insertion in the emergency department (ED).

Hemodynamically unstable patients may be treated with atropine. This should be done with a degree of caution. The goal of atropine therapy is to improve conduction through the AVN by reducing vagal tone via receptor blockade. Atropine often improves the ventricular rate if the site of block is in the AVN. The peak increase in heart rate occurs in 2-4 minutes after IV administration; the half-life is 2-3 hours.

However, if the block is in the His bundle, atropine may lead to an increased atrial rate, and a greater degree of block can occur with a slower ventricular rate. Atropine is unlikely to be successful in wide-complex bradyarrhythmias where the level of the block is below the level of the AVN.

In addition, care should be taken in administering atropine to a patient with a suspected acute myocardial infarction (MI), in that the resulting vagolysis leads to unopposed sympathetic stimulation, which can cause increased ventricular irritability and potentially dangerous ventricular arrhythmias. Furthermore, atropine is ineffective in patients with a denervated heart (eg, those patients who have undergone a cardiac transplant procedure).

Similarly, use of isoproterenol may be attempted to accelerate a ventricular escape rhythm with a low probability for efficacy and the same concerns in patients with suspected acute MI. Isoproterenol is more likely to facilitate conduction with a distal level of block, but patients with a block at the distal level are more likely to have a contraindication, such as active ischemic heart disease. Isoproterenol should only be used as a temporary measure until more definitive and less risky treatments (eg, transvenous pacing) can be arranged.

Once the patient has been stabilized, a decision must be made regarding permanent pacemaker implantation. ^{[24, 25, 26, 27, 28]} The admitting cardiologist will determine the need for and timing of permanent pacemaker implantation.

Unless the heart block is due to a medication that can be discontinued or an infectious process that can be effectively treated, most patients with acquired third-degree atrioventricular (AV) block (complete heart block) should receive a permanent pacemaker or an implantable cardioverter-defibrillator (ICD) (if a high risk of sudden cardiac death exists on the basis of severe left ventricular dysfunction or other criteria).

The ultimate decision whether to place a permanent pacemaker in patients with persistent heart block without a reversible cause depends on many factors. A clinical statement from the American College of Cardiology (ACC), the American Heart Association (AHA), and the Heart Rhythm Society (HRS) was published in 2008; this paper outlined the indications for permanent pacing. ^[24] A focused update of these guidelines was published in 2012 and is presented below. ^[25]

2012 ACC/AHA/HRS focused update recommendations

Class I recommendations

Permanent pacemaker implantation is indicated for third-degree and advanced second-degree AV block at any anatomic level ^[25] :

• Associated with bradycardia with symptoms (including heart failure) or ventricular arrhythmias presumed to be due to AV block
• Associated with arrhythmias and other medical conditions that require drug therapy that results in symptomatic bradycardia
• In awake, symptom-free patients in sinus rhythm with documented periods of asystole 3 seconds or longer or any escape rate below 40 beats/min, or with an escape rhythm that is below the AVN
• In awake, symptom-free patients with atrial fibrillation and bradycardia with one or more pauses of at least 5 seconds or longer
• After catheter ablation of the AV junction
• Associated with postoperative AV block that is not expected to resolve after cardiac surgery
• Associated with neuromuscular diseases with AV block (eg, myotonic muscular dystrophy, Kearns-Sayre syndrome, Erb dystrophy [limb-girdle muscular dystrophy], and peroneal muscular atrophy), with or without symptoms

Permanent pacemaker implantation is indicated for second-degree AV block with associated symptomatic bradycardia, regardless of type or site of the block.

Permanent pacemaker implantation is indicated for asymptomatic persistent third-degree AV block at any anatomic site with average awake ventricular rates of 40 beats/min or faster if cardiomegaly or left ventricular dysfunction is present or if the site of the block is below the AVN.

Permanent pacemaker implantation is indicated for second- or third-degree AV block during exercise in the absence of myocardial ischemia.

 

Class IIa recommendations

Permanent pacemaker implantation is reasonable for the following ^[25] :

• Persistent third-degree AV block with an escape rate faster than 40 beats/min in asymptomatic adult patients without cardiomegaly
• Asymptomatic second-degree AV block at intra- or infra-His levels found at electrophysiologic study
• First- or second-degree AV block with symptoms similar to those of pacemaker syndrome or hemodynamic compromise
• Asymptomatic Mobitz II second-degree AV block with a narrow QRS; when Mobitz II second-degree AV block occurs with a wide QRS, including isolated right bundle-branch block, pacing becomes a class I recommendation

 

Class IIb recommendations

Permanent pacemaker implantation may be considered for the following ^[25] :

• Neuromuscular diseases such as myotonic muscular dystrophy, Erb dystrophy (limb-girdle muscular dystrophy), and peroneal muscular atrophy with any degree of AV block (including first-degree AV block), with or without symptoms, because there may be unpredictable progression of AV conduction disease
• AV block in the setting of drug use and/or drug toxicity when the block is expected to recur even after the drug is withdrawn.

 

Class III recommendations

Permanent pacemaker implantation is not indicated for the following ^[25] :

• Asymptomatic first-degree AV block
• Asymptomatic Mobitz I second-degree AV block at the supra-His (AVN) level or that which is not known to be intra- or infra-Hisian
• AV block that is expected to resolve and is unlikely to recur (eg, drug toxicity, Lyme disease, transient increases in vagal tone or during hypoxia in sleep apnea syndrome in the absence of symptoms)

The American College of Cardiology (ACC), the American Heart Association (AHA), and the Heart Rhythm Society (HRS) have also formulated recommendations for permanent pacing in patients with chronic bifascicular block. ^{[24, 25]}

Class I recommendations

Permanent pacemaker implantation is indicated for the following ^[25] :

• Advanced second-degree AV block or intermittent third-degree AV block
• Mobitz II second-degree AV block
• Alternating bundle-branch block

Class IIa recommendations

Permanent pacemaker implantation is reasonable for the following ^[25] :

• Syncope not demonstrated to be due to AV block when other likely causes have been excluded, specifically ventricular tachycardia (VT)
• An incidental finding at electrophysiologic study of a markedly prolonged HV interval (≥100 ms) in asymptomatic patients
• An incidental finding at electrophysiologic study of pacing-induced infra-His block that is not physiologic

Class IIb recommendations

Permanent pacemaker implantation may be considered in the setting of neuromuscular diseases such as myotonic muscular dystrophy, Erb dystrophy (limb-girdle muscular dystrophy), and peroneal muscular atrophy with bifascicular block or any fascicular block, with or without symptoms.

Class III recommendations

Permanent pacemaker implantation is not indicated for the following ^[25] :

• Fascicular block without AV block or symptoms
• Fascicular block with first-degree AV block without symptoms

The American College of Cardiology (ACC), the American Heart Association (AHA), and the Heart Rhythm Society (HRS) have also provided recommendations for permanent pacing in patients with atrioventricular (AV) block associated with acute myocardial infarction (MI). ^{[24, 25]}

Class I recommendations

Permanent ventricular pacing is indicated for the following ^[25] :

• Persistent second-degree AV block in the His-Purkinje system with alternating bundle-branch block or third-degree AV block within or below the His-Purkinje system after ST-segment elevation MI
• Transient advanced second- or third-degree infranodal AV block and associated bundle-branch block; if the site of block is uncertain, an electrophysiologic study may be necessary
• Persistent and symptomatic second- or third-degree AV block.

Class IIb recommendations

Permanent ventricular pacing may be considered for persistent second- or third-degree AV block at the AV node level, even in the absence of symptoms.

Class III recommendations

Permanent ventricular pacing is not indicated for the following ^[25] :

• Transient AV block in the absence of intraventricular conduction defects
• Transient AV block in the presence of isolated left anterior fascicular block
• New bundle-branch block or fascicular block in the absence of AV block
• Persistent asymptomatic first-degree AV block in the presence of bundle-branch or fascicular block

Bed rest is advisable for patients with new third-degree atrioventricular (AV) block (complete heart block). Bed rest minimizes the chance of significant injury if syncope occurs as a result of ventricular asystole and decreases hemodynamic demand. A temporary pacemaker may be required.

Because these patients have decreased cardiac output, assuming an upright posture after prolonged periods in the supine position can lead to orthostatic hypotension with syncope or near-syncope.

Patients with renal insufficiency or failure, dehydration, and certain electrolyte disturbances are predisposed to develop digoxin toxicity. Careful monitoring of electrolytes, drug levels, and renal function is essential in patients on chronic digoxin therapy.

Patients on multiple nodal agents (eg, beta-blockers and calcium channel blockers) are at an increased risk for the development of third-degree atrioventricular (AV) block (complete heart block); the more nodal blockade that occurs, the higher the chance of developing complete heart block.

Cardiologic consultation is indicated for all patients with third-degree atrioventricular (AV) block (complete heart block). The consultation is emergent in patients with concomitant acute myocardial infarction (MI), active myocardial ischemia, congestive heart failure, wide-complex escape rhythm, or symptoms of hypoperfusion. Patients in this group may require early placement of a permanent pacemaker, or assistance may be needed if difficulty is encountered obtaining capture from an external or transvenous pacer.

The involvement of an electrophysiologist should also be considered when appropriate.

Patients can be discharged from the hospital the following day after pacemaker implantation. Routinely, chest radiography is required before discharge.

Routine postpacemaker care is necessary. This includes transtelephonic checks every 2 months and office visits for pacemaker interrogation every 6-12 months. In the initial postimplantation period, these visits are more frequent.