Managing atrioventricular blocks

Editor’s note: This is the author’s second article on atrioventricular (AV) blocks. The first article,
which appeared in the March 2013 issue, addressed how to identify AV blocks and their common causes.

Bill Marcum, a 60-year-old construction worker with hypertension and coronary artery disease, has been having dizzy spells—usually when working and doing a lot of lifting. When the spells grow more frequent and occur even when he’s not working, he decides to see his physician. After several tests and consultation with a cardiologist, Mr. Marcum is diagnosed with transient second-degree atrioventricular (AV) block type II. He wonders why this condition has occurred and what needs to be done about it.

AV block is marked by a disturbance in electrical impulse conduction from the atria to the ventricles. Depending on the type of AV block, the disturbance may be insignificant or it could lead to potentially fatal arrhythmias. AV blocks are classified as first-degree, second-degree, and third-degree. First-degree AV blocks are the least concerning; third-degree blocks are the most dangerous.

Management and treatment of an AV block usually hinge on:

  • cause or suspected cause of the condition
  • whether the cause can be reversed
  • signs and symptoms.

Each type of AV block is managed differently, and treatment is tailored to the individual. Two patients with a similar AV block may receive different treatments.

The first goal of management is to identify the underlying cause of the block and evaluate the patient’s signs and symptoms. Once the cause has been identified and treated, the AV block typically resolves. If the cause isn’t treatable (for example, an irreversible cardiac disease), management aims to return cardiac conduction to as near normal as possible and to treat the patient’s signs and symptoms. Typically, as normal conduction resumes, signs and symptoms resolve. (See Assessment findings by clicking the PDF icon above.)

Pathophysiology and causes

Disturbed impulse conduction from the atria to the ventricles may result from physiologic, pathophysiologic, or iatrogenic impairment of the conduction system. It may be transient or permanent. For instance, many drugs that alter impulse conduction through the AV node (such as those used to treat other arrhythmias and hypertension, including beta blockers, calcium-channel blockers, and digitalis) affect AV node function and can cause a brief or even permanent AV block. The block may resolve when the drug is withheld or the dosage is reduced. If symptoms are severe, additional treatment may be needed until drug
effects wear off. Occasionally, a temporary pacemaker must be used to restore near-normal conduction and relieve symptoms while waiting for drug effects to wear off. In some cases, the clinician determines the AV block is permanent and the patient requires other treatment.

Common causes of AV block are coronary artery disease, myocardial infarction (MI), cardiomyopathies, electrolyte abnormalities, certain cardiac procedures, advanced age, and drugs used to treat other rhythm disturbances (as discussed earlier). For a more complete list of potential causes of AV block, see “How to identify atrioventricular blocks” in the March 2013 issue of American Nurse Today.

First-degree AV block

First-degree AV block is a sinus rhythm with a PR interval greater than 0.20 seconds. (Normally, duration of the PR interval ranges from 0.12 to 0.20 seconds.) In this block, no impulses are blocked completely and no heartbeats are missed. First-degree AV block rarely causes significant symptoms and generally requires only monitoring. Clinicians may consider treating it if the PR interval exceeds 0.30 seconds, a bundle branch block is present, the block occurs just after an MI, or a known cause of the block exists.

When a PR interval greater than 0.30 seconds occurs in conjunction with a bundle branch block, the blockade mechanism is probably in both the AV node and one of the bundle branches. This situation increases the risk of a higher-degree block and possible loss of AV communication. In this case, a permanent pacemaker may be considered.

When first-degree AV block follows an MI, the clinician must rule out suspected ischemia or infarction in the conduction system. The patient should be monitored closely during recovery from MI to determine if the block is transient or permanent. If it’s permanent, signs and symptoms determine treatment. An MI also can affect the bundle branches, so the patient should be monitored for a new first-degree AV block and a bundle branch block. This combination may signal development of a higher-grade block.

If a specific cause of first-degree AV block is found, treatment focuses on eliminating it. The patient’s medication use should be monitored closely. Because advanced age can cause AV block, monitoring for changes in patient status is the only option until symptoms occur or the block progresses.

Second-degree AV block

This block has two forms—Mobitz type I and Mobitz type II. Both types can progress to an advanced state.

Second-degree Mobitz type I

Also called Wenckebach, Mobitz type I block is marked by a sinus rhythm with progressively lengthening PR intervals until an impulse is completely blocked, resulting in a missed beat. Many patients with this block have preexisting first-
degree AV block. Signs and symptoms may occur with a slower rate of impulse formation from the atria, coupled with missed beats. However, uncomplicated Mobitz type I block (in which atrial impulses occur at a normal rate of 60 to 100 beats/minute [bpm]) rarely causes symptoms. Patients typically are monitored for changes, progression, or resolution of the block.

But a few exceptions exist. This block may warrant aggressive treatment in patients who’ve had inferior-wall MIs. In most individuals, the right coronary artery supplies blood to the inferior wall of the heart and the AV node; therefore, an AV block can occur as a complication of an inferior-wall MI. This situation increases mortality risk; the initial block quickly can progress to a third-degree (complete) AV block. In this case, the clinician may consider using a permanent pacemaker.

Clinicians should check for reversible causes of second-degree Mobitz type I block (such as drugs linked to AV blocks or increased vagal tone) and treat the patient accordingly. Irreversible causes warrant treatment only in symptomatic patients. Asymptomatic patients don’t require specific treatment but should have regular clinician follow-up. A permanent pacemaker should be considered for patients with symptomatic bradycardia caused by this type of block.

Second-degree Mobitz type II

In Mobitz type II block, the PR interval remains unchanged but P waves fail to conduct, causing missed beats. The mechanism for the block may be located below the AV node or in the bundle of His (about 20% of cases), or it may be in the bundle branches (about 80%). The PR interval may be normal or slightly prolonged but remains constant, in contrast to the progressive PR-interval lengthening seen in Mobitz type I.

In patients with a normal sinus rhythm and a few dropped beats, significant symptoms are rare. But signs and symptoms may arise if the rhythm slows to bradycardia or the number of blocked impulses increases significantly. This rhythm may worsen with exercise, stress, atrial pacing, and certain drugs. Because the conduction system needs time to repolarize, any activity, drug (such as atropine), or artificial formation of impulses that increases the heart rate reduces repolarization time and may lead to more dropped impulses and more missed beats. On the other hand, vagal maneuvers that slow conduction may improve impulse conduction.

Treatment of Mobitz type II block focuses on the underlying cause and the patient’s signs and symptoms. This block rarely is transient and has a high potential for progressing to third-degree AV block. Treatable causes should be addressed and medications that prolong AV node conduction time should be avoided whenever possible. A permanent pacemaker is almost always considered.

Advanced second-degree AV block (high-grade block)

Advanced second-degree AV block occurs when either type of second-degree AV block (Mobitz type I or type II) progresses to cause blockade of multiple impulses in a row, or when more than 50% of impulses fail to conduct. These blocks almost always cause symptoms and necessitate a permanent pacemaker to help restore conduction to as near normal as possible.

Third-degree AV block

In third-degree AV block or complete heart block, no atrial impulses reach the ventricles and an escape pacemaker takes over to keep the heart beating. If the block is situated high in the conduction system (the AV node or bundle of His), a nodal or junctional escape pacemaker may take over, causing a heart rate between 40 and 60 bpm with or without symptoms. If the block is lower in the bundle of His or the bundle branches, a lower escape pacemaker initiates impulses at a rate of 20 to 40 bpm and symptoms may arise based on the heart rate and condition of the heart.

Reversible and treatable causes should be addressed. Patients with high junctional pacemakers may respond temporarily to atropine, exercise, or catecholamines (such as epinephrine), which can restore AV communication. Vagal maneuvers, which slow impulse conduction, may temporarily improve communication between the heart chambers. Patients with lower escape pacemakers are less likely to respond to these treatments.

Third-degree AV block can be transient or permanent, depending on the cause. Transient third-degree blocks usually resolve as the cause is treated. With a permanent block, nearly all patients require a permanent pacemaker to restore near-normal conduction.

Nursing considerations

Nursing care for patients with AV blocks depend on how the block affects the patient. Lower-degree AV blocks are less likely to cause hemo¬dynamic alterations and usually require only monitoring for progression. But as the AV block progresses, hemodynamic instability may lead to signs and symptoms. Nursing diagnoses that may be appropriate include:

  • decreased cardiac output
  • increased fluid volume
  • acute pain related to ischemia
  • ineffective breathing patterns
  • ineffective tissue perfusion
  • activity intolerance, fatigue, or both
  • impaired gas exchange.

Focus your care in these areas while assisting the management team to treat underlying causes and restore impulse conduction
to as near normal as possible. Once near-normal conduction returns, many signs and symptoms resolve and treatment is no longer necessary.

Although Mr. Marcum’s AV block is transient, it’s causing symptoms. The physician decides to implant a permanent pacemaker to help restore near-normal conduction and avoid potentially life-threatening conduction problems in the future. At his 3-month checkup, Bill is happy to report that he has experienced no dizzy spells since he has had the pacemaker.

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Ira Gene Reynolds is a staff nurse on the medical/oncology unit at Utah Valley Regional Medical Center, Intermountain Healthcare, in Provo.

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