TYPE I SECOND-DEGREE AV BLOCK

Rhythm strip patterns

The more you look at rhythm strips, the more you’ll notice patterns. The symbols below represent some of the patterns you might see as you study rhythm strips.

Normal, regular (as in normal sinus rhythm)

Slow, regular (as in sinus bradycardia)

Fast, regular (as in sinus tachycardia)

Premature (as in a premature ventricular contraction)

Grouped (as in type I second-degree AV block)

Irregularly irregular (as in atrial fibrillation)

Paroxysm or burst (as in paroxysmal atrial tachycardia)

Lonely Ps, light-headed patients

Usually asymptomatic, a patient with type I second-degree AV block may show signs and symptoms of decreased cardiac output, such as light-headedness or hypotension. Symptoms may be especially pronounced if the ventricular rate is slow.

How you intervene

No treatment is needed if the patient is asymptomatic. For a symptomatic patient, atropine may improve AV node conduction. A temporary pacemaker may be required for long-term relief of symptoms until the rhythm resolves.

When caring for a patient with this block, assess his tolerance for the rhythm and the need for treatment to improve cardiac output. Evaluate the patient for possible causes of the block, including the use of certain medications or the presence of ischemia.

Check the ECG frequently to see if a more severe type of AV block develops. Make sure the patient has a patent I.V. line. Teach him about his temporary pacemaker, if indicated.

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Type II second-degree AV block

Type II second-degree AV block, also known as Mobitz type II block, is less common than type I but more serious. It occurs when occasional impulses

node fail to conduct to the ventricles. On an ECG, you won’t see the PR

interval lengthen before the impulse fails to conduct, as you do with type I second-degree AV block. You’ll see,

instead, consistent AV node conduction and an occasional dropped beat. This block is like a line of people passing through a doorway at the same speed, except that, periodically, one of them just can’t get through.

How it happens

Type II second-degree AV block is usually caused by an anterior wall MI, degenerative changes in the conduction system, or severe coronary artery disease. The arrhythmia indicates a problem at the level of the bundle of His or bundle branches.

Type II block is more serious than type I because the ventricular rate tends to be slower and the cardiac output diminished. It’s also more likely to cause symptoms, particularly if the sinus rhythm is slow and the ratio of conducted beats to dropped beats is low such as 2:1. (See 2:1 AV block.) Usually chronic, type II second-degree AV block may progress to a more serious form of block. (See High-grade AV block.)

2:1 AV block

In 2:1 second-degree atrioventricular (AV) block, every other QRS complex is dropped, so there are always two P waves for every QRS complex. The resulting ventricular rhythm is regular.

Keep in mind that type II block is more likely to impair cardiac output, lead to symptoms such as syncope, and progress to a more severe form of block. Be sure to monitor the patient carefully.

Don’t skip this strip

High-grade AV block

When two or more successive atrial impulses are blocked, the conduction disturbance is called high-grade atrioventricular (AV) block. Expressed as a ratio of atrial-to-ventricular beats, this block will be at least 3:1. With the prolonged refractory period of this block, latent pacemakers can discharge. As a result, you’ll commonly see escape rhythms develop.

Complications

High-grade AV block causes severe complications. For instance, decreased cardiac output and reduced heart rate can combine to cause Stokes-Adams syncopal attacks. In addition, high-grade AV block usually progresses quickly to thirddegree block. Look for these distinguishing characteristics.

Three P waves occur for each QRS complex.

The PR interval remains constant.

What to look for

When monitoring a rhythm strip, look for an atrial rhythm that’s regular and a ventricular rhythm that may be regular or irregular, depending on the block. (See Identifying type II second-degree AV block, page 162.) If the block is intermittent, the rhythm is irregular. If the block is constant, such as 2:1 or 3:1, the rhythm is regular.

Overall, the strip will look as if someone erased some QRS complexes. The PR interval will be constant for all conducted beats but may be prolonged. The QRS complex is usually wide, but normal complexes may occur.

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Don’t skip this strip

Identifying type II second-degree AV block

This rhythm strip illustrates type II second-degree atrioventricular (AV) block. Look for these distinguishing characteristics.

…but the ventricular rhythm is

irregular.

The atrial rhythm is regular…

The PR

interval is

Jumpin’ palpitations!

Most patients who experience a few dropped beats remain asymptomatic as long as cardiac output is maintained. As the number

of dropped beats increases, patients may experience palpitations, fatigue, dyspnea, chest pain, or light-headedness. On physical examination, you may note hypotension, and the pulse may be slow and regular or irregular.

How you intervene

If the dropped beats are infrequent and the patient shows no symptoms of decreased cardiac output, the practitioner may choose only to observe the rhythm, particularly if the cause is thought

to be reversible. If the patient is hypotensive, treatment aims to improve cardiac output by increasing the heart rate. Atropine, dopamine, or epinephrine may be given for symptomatic bradycardia. Discontinue digoxin, if it’s the cause of the arrhythmia.