A look at atrial arrhythmias

ATRIAL ARRHYTHMIAS

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•Reentry—In reentry, an impulse is delayed along a slow conduction pathway. Despite the delay, the impulse remains active enough to produce another impulse during myocardial repolarization. Reentry may occur with coronary artery disease, cardiomyopathy, or myocardial infarction (MI).

•Triggered activity—An injured cell sometimes only partly repolarizes. Partial repolarization can lead to a repetitive ectopic firing called triggered activity. The depolarization produced by triggered activity is known as afterdepolarization and can lead to atrial or ventricular tachycardia. Afterdepolarization can occur with cell injury, digoxin toxicity, and other conditions. Now let’s examine each atrial arrhythmia in detail.

Premature atrial contractions

Premature atrial contractions (PACs) originate outside the SA node and usually result from an irritable spot, or focus, in the atria that takes over as pacemaker for one or more beats. The SA node fires an impulse, but then an irritable focus jumps in, firing its own impulse before the SA node can fire again.

PACs may be conducted through the atrioventricular (AV) node and the rest of the heart, depending on their prematurity and the status of the AV and intraventricular conduction system. Nonconducted or blocked PACs don’t trigger a QRS complex.

How it happens

PACs, which commonly occur in a normal heart, can be triggered by alcohol, nicotine, anxiety, fatigue, fever, and infectious diseases. A patient who eliminates or controls those factors can correct the arrhythmias.

PACs may also be associated with coronary or valvular heart disease, acute respiratory failure, hypoxia, pulmonary disease, digoxin toxicity, and certain electrolyte imbalances.

PACs are rarely dangerous in a patient who doesn’t have heart disease. In fact, they commonly cause no symptoms and can go unrecognized for years. The patient may perceive PACs as normal palpitations or skipped beats.

The nicotine in cigarattes can cause PACs. Yuk!

Early warning sign

However, in patients with heart disease, PACs may lead to more serious arrhythmias, such as atrial fibrillation and atrial flutter. In a patient with an acute MI, PACs can serve as an early sign of heart failure or an electrolyte imbalance. PACs can also result from the release of the neurohormone catecholamine during episodes of pain or anxiety.

What to look for

The hallmark ECG characteristic of a PAC is a premature P wave with an abnormal configuration when compared with a sinus P wave. (See Nonconducted PACs and second-degree AV block.)

When the PAC is conducted, the QRS complex appears similar to the underlying QRS complex. PACs are commonly followed by a pause.

The PAC depolarizes the SA node early, causing it to reset itself and disrupt the normal cycle. The next sinus beat occurs sooner than it normally would, causing the P-P interval between two normal beats that have been interrupted by a PAC to be shorter than three consecutive sinus beats, an occurrence referred to as noncompensatory. (See Identifying premature atrial contractions, page 90.)

Lost in the T

When examining a PAC on an ECG, look for irregular atrial and ventricular rates. The underlying rhythm is usually regular. An irregular rhythm results from the PAC and its corresponding pause. The P wave is premature and abnormally shaped and may be lost in the previous T wave, distorting that wave’s configuration. (The T wave might be bigger or have an extra bump.) Varying configurations of the P wave indicate more than one ectopic site.

The PR interval can be normal, shortened, or slightly prolonged, depending on the origin of the ectopic focus. If no QRS complex follows the premature P wave, a nonconducted PAC has occurred.

PACs may occur in bigeminy (every other beat is a PAC), trigeminy (every third beat is a PAC), or couplets (two PACs in a row).

The patient may have an irregular peripheral or apical pulse rhythm when the PACs occur. He may complain of palpitations, skipped beats, or a fluttering sensation. In a patient with heart

Mixed signals

Nonconducted PACs and second-degree AV block

Don’t confuse nonconducted premature atrial contractions (PACs) with type II second-degree atrioventricular (AV) block. In type II seconddegree AV block, the P-P interval is regular. A nonconducted PAC, however, is an atrial impulse that arrives early to the AV node, when the node isn’t yet repolarized.

As a result, the premature P wave fails to be conducted to the ventricle. The rhythm strip below shows a

P wave embedded in the preceding T wave.

Nonconducted PAC

disease, signs and symptoms of decreased cardiac output—such as hypotension and syncope—may occur.

How you intervene

Most patients who are asymptomatic don’t need treatment. In symptomatic patients, however, treatment may focus on eliminating the cause, such as caffeine, alcohol, and nicotine.

When caring for a patient with PACs, assess him to help determine what’s triggering the ectopic beats. Tailor your patient teaching to help the patient correct or avoid the underlying cause. For example, if applicable, the patient should eliminate caffeine or nicotine or learn stress reduction techniques to lessen his anxiety.

If the patient has ischemic or valvular heart disease, monitor him for signs and symptoms of heart failure, electrolyte imbalances, and the development of more severe atrial arrhythmias.

PACs may be caused by too much caffeine or alcohol.