Just the facts

In this chapter, you’ll learn:

the proper way to identify the various forms of atrioventricular (AV) block and interpret their rhythms

the reason that AV block is a significant arrhythmia

patients who at risk for developing AV block

signs and symptoms of AV block

nursing care for patients with AV block.

A look at AV block

Atrioventricular (AV) heart block results from an interruption in the conduction of impulses between the atria and ventricles. AV block can be total or partial or it may delay conduction. The block can occur at the AV node, the bundle of His, or the bundle branches.

The heart’s electrical impulses normally originate in the sinoatrial (SA) node, so when those impulses are blocked at the AV node, atrial rates are commonly normal (60 to 100 beats/minute). The clinical effect of the block depends on how many impulses are completely blocked, how slow the ventricular rate is as a result, and how the block ultimately affects the heart. A slow ventricular rate can decrease cardiac output, possibly causing lightheadedness, hypotension, and confusion.

The cause before the block

Various factors can lead to AV block, including underlying heart conditions, use of certain drugs, congenital anomalies, and conditions that disrupt the cardiac conduction system. (See Causes of AV block, page 154.)

In AV block, the impulses between the atria and ventricles are totally stopped, partially stopped, or delayed.

Causes of AV block

Atrioventricular (AV) blocks can be temporary or permanent. Here’s a look at causes of each kind of AV block.

Causes of temporary block

•Myocardial infarction (MI), usually inferior wall MI

•Digoxin (Lanoxin) toxicity

•Acute myocarditis

•Calcium channel blockers

•Beta-adrenergic blockers

•Cardiac surgery

Causes of permanent block

•Changes associated with aging

•Congenital abnormalities

•MI, usually anteroseptal MI

•Cardiomyopathy

•Cardiac surgery

Typical examples include:

•myocardial ischemia, which impairs cellular function so cells repolarize more slowly or incompletely. The injured cells, in turn, may conduct impulses slowly or inconsistently. Relief of the ischemia can restore normal function to the AV node.

•myocardial infarction (MI), in which cell death occurs. If the necrotic cells are part of the conduction system, they no longer conduct impulses and a permanent AV block occurs.

•excessive dosage of, or an exaggerated response to, a drug which can cause AV block or increase the likelihood that a block will develop. Although many antiarrhythmic medications can have this effect, the drugs more commonly known to cause or exacerbate AV blocks include digoxin, beta-adrenergic blockers, and calcium channel blockers.

•congenital anomalies such as congenital ventricular septal defect that involve cardiac structures and affect the conduction system. Anomalies of the conduction system, such as an AV node that doesn’t conduct impulses, can also occur in the absence of structural defects. (See AV block in elderly patients.)

Under the knife

AV block may also be caused by inadvertent damage to the heart’s conduction system during cardiac surgery. Damage is most likely to occur in surgery involving the mitral or tricuspid valve or in the closure of a ventricular septal defect. If the injury involves tissues

Ages

and stages

AV block in elderly patients

In elderly patients, atrioventricular (AV) block may be due to fibrosis of the conduction system. Other causes include use of digoxin and the presence of aortic valve calcification.