AV Blocks

AV BlocksDawn B. Altman, R.N., EMT-P

WHAT ARE AV BLOCKS?When conduction between the atria and ventricles is slowed or impeded, even though there is no physiologic reason for the lack of conduction, the patient is said to have an AV block.

Physiologic blocks occur for good reasons, such as when atrial flutter is conducted partially, resulting in a better heart rate. Another example of physiologic block is the non-conducted PAC, when the premature P wave occurs very early after the preceding beat, finding the ventricles refractory and unable to produce a QRS. AV blocks occur when there is no underlying physiological reason for the disturbance in conduction.

Physiologic block: Atrial flutter with variable conduction.

LOCATION OF A-V BLOCK

AV blocks occur at different locations in the condution system. The AV node can have first degree AV block; second degree AV block, Type I; or third degree AV block. The bundle of His can have third degree AV block. The bundle branches can have second degree AV block, Type II or third degree AV block.

In general, the AV blocks which occur at the level of the AV node are transient and linked to another condition the patient has. When that condition is resolved, the AV nodal block resolves, too.

AV blocks that occur below the AV node tend to me much more ominous and life-threatening. They are more likely to result from permanent lesions, and to progress to more dangerous blocks.

FIRST DEGREE AV BLOCK

First degree AV block usually occurs at the AV node. It causes a prolonged P-R interval, greater than .20 seconds (200 milliseconds). The P-R interval will be constant.

First degree AV block can be caused by medication, especially digitalis. It is also caused by many other conditions, one of which is acute myocardial infarction.It is especially common in inferior wall M.I. (Up to 13% of IWMI patients develop first degree AV block).

First degree AV block can progress to second degree AVB type I or third degree AV block with junctional escape.

First-degree AV block. P-R interval .32 seconds.

SECOND DEGREE AV BLOCK

In second degree AV block, some of the P waves are not conducted, and some are. The underlying rhythm is usually sinus, and gives no reason for a physiologic block. That is, the rate is below 140. Second degree AV block can occur in the AV node (type I) or below the AV node (type II).

SECOND DEGREE AV BLOCK, TYPE I (Wenckebach)Type I is also called Wenckebach. In this rhythm, the P-R interval progressively prolongs. This prolongation moves the QRS complex progressively toward the right. Eventually, the next sinus P wave will land in the refractory period from the previous beat. At this point, the P will fail to conduct and produce a QRS. After this happens, the shortest P-R interval will reoccur, and the cycle starts again.

Second degree AV block, type IType I second degree AV block results in grouped beating. That is, the QRS complexes will appear in groups, and the pulse will be irregular. The length of the QRS groups will be determined by how often a sinus P wave is non-conducted. If the change in the P-R interval length is very subtle with each beat, it may take quite some time before the QRS is dropped. To recognize the type I AV block in this situation, find the non-conducted P wave. Then, look at the last P-R interval before the dropped beat, and compare it to the first P-R interval after the dropped beat. The last interval will be the longest in the cycle, and the first will be the shortest.

Second degree AV block, Type I with long Wenckebach cycle.Type I second degree AV block is, like most AV blocks that occur in the AV node, usually transient and harmless. It is seen with digitalis intoxication, inferior wall M.I., acute myocarditis, and post open-heart surgery. The prognosis is linked to the condition causing the AV block.EKG signs of type I second degree AV block are

Some P waves conducted, some are not.

Progressive prolongation of the P-R intervals.

Grouped beating

Usually normal (narrow) QRS

Only one consecutive P wave fails to conduct then cycle starts again.

The EKG diagnosis of second degree AV block, type I can be made more difficult by the additional presence of premature beats, escape beats, or bundle branch block.

Second degree AV block, type I, with a PVC.SECOND DEGREE AV BLOCK, TYPE IIType II second degree AV block, like type I, has some P waves that are conducted and some that are not. Also, like type I, there will be an underlying rhythm which is usually a sinus rhythm, with a reasonable rate. Unlike type I, the P-R intervals will remain constant.Type II, however, is caused by a lesion or lesions in the bundle branches. There are three primary pathways into the ventricles: the right bundle branch, the anterior-superior fascicle of the left bundle branch, and the posterior-inferior fascicle of the LBB.

Often, the patient with second degree type II has a pre-existing bundle branch block, usually of the right bundle branch. They sometimes have a hemiblock on the left, as well. (Bi-fascicular block). When the remaining fascicle develops an intermittent block, second degree type II is produced. You could say that second degree AV block type II is an intermittent tri-fascicular block, or an intermittent complete heart block.

Second degree AV block, type II. This patient has a pre-existing right bundle branch block and an anterior-superior fascicular block (hemiblock). The remaining fascicle, the left posterior-inferior, is being intermittently blocked. The underlying rhythm is sinus tachycardia at 120/min. The ventricular rate is 30/min. (4:1 conduction).Second degree AV block, type II usually represents a necrotic, progressive lesion. It is very likely to progress to complete heart block (third-degree). If it does progress to third degree AV block, the escape rhythm, if there is one, will be idioventricular. The bradycardia caused by the AV block may cause syncopal episodes. Patients with second degree AV block, type II usually are treated emergently with a temporary pacemaker until they can receive a permanent, implanted pacemaker.

When there is a high grade second degree type II, several consecutive P waves will be non-conducted. This can result in a severe symptomatic bradycardia.

To review, EKG signs of second degree AV block, type II are:

Some P waves are conducted, some are not.

P-R intervals remain constant.

Usually signs of pre- or co-existing bundle branch disease (wide QRS).

P-R interval may be normal or prolonged.

Sometimes, more than one consecutive P wave fails to conduct.2:1 SECOND DEGREE AV BLOCK

When P waves are conducted with a ratio of 2:1 with the QRS complexes, it can be difficult to determine which type of AV block is present. It is necessary to see two P waves conduct consecutively to see the progressive prolongation of type I.

One way to definitively diagnose the AV block is to see two consecutive P waves conduct during an otherwise 2:1 AV block. If the lesion is in the bundle branches, a type II conduction will occur, and the P-R intervals will stay the same. If the AV node is blocked, the P-R intervals will prolong.

Second degree AV block, 2:1 conduction. The last three cycles prove block to be Type I, with progressively prolonging P-R intervals. Additional clues: QRS is narrow, non-conducted P wave is on or near T wave.

Second degree AV block, 2:1 conduction. Last four cycles prove block to be Type II, with constant P-R intervals. Additional clue: QRS is wide, indicating bundle branch block.REMEMBER: Type I is seldom dangerous, but Type II can rapidly progress to a life-threatening condition.

One of the biggest worries with 2:1 conduction, regardless of type, is that the conduction ratio automatically cuts the patients rate in half. An underlying sinus rate of 70 bpm with 2:1 AV conduction will produce a pulse rate of 35 bpm!

Second degree AV block, 2:1 conduction. Atrial rate is 66 / min. Ventricular rate is 33 / min.

In this case, the patient will be treated according to the symptoms produced by the bradycardia. THIRD DEGREE AV BLOCK (Complete Heart Block)Third degree AV block is diagnosed when no atrial impulses are conducted to the ventricles. This is one form of AV dissociation.Third degree AV block can occur in the AV node, bundle of His, or bundle branches. If the block is in the AV node, it will usually have a reliable junctional escape rhythm. This is a type I block, and it is usually transient. When the condition causing the block is successfully resolved, the block will be, too.

Third degree AV block at the AV node level. Atrial rate 95 (sinus rhythm), ventricular rate 42 (junctional).EKG criteria for third degree AV block with junctional escape are:

Atria and ventricles are controlled by separate pacemakers. (AV dissociation).

Atrial rate is usually within normal limits.

QRS complexes are narrow, rate is about 40-60 (meets criteria for junctional rhythm).`Third degree AV block occurring in the His bundle or below will result in an idioventricular escape rhythm. This type of AV block represents a trifascicular block. It may occur suddenly, with a block of the His bundle, or the right and left bundle branches simultaneously. Or, third degree AV block may be a result of progressive disease of the fascicles, resulting after a period of second degree AVB, type II.

Third degree AV block at the fascicular level. Atrial rate is 47 / min. (sinus bradycardia). Ventricular rate is 34 / min. with wide QRS complexes (idioventricular rhythm). There is complete A-V dissociation.EKG criteria for third degree AV block with ventricular escape are: Atria and ventricles are controlled by separate pacemakers. (AV dissociation.)

Atrial rate is usually within normal limits.

QRS complexes are wide, and the rate is below 40 / min. (meets criteria for idioventricular rhythm.)

Third degree AV block can also occur without the benefit of an escape rhythm. This results in ventricular standstill. Sometimes there is a warning, with a progressive AV block occurring. At other times, sudden ventricular standstill can occur!

Second degree AV block to third degree with ventricular standstill. Underlying rhythm is NSR at 80 / min.

Third degree AV block with ventricular standstill, except for one junctional escape beat. AV BLOCKS TREATMENTTreatment of AV blocks depends upon 1) the likelihood of progression to a more dangerous rhythm and 2) the patients tolerance of the rate (usually bradycardia).

Various medications can be used to speed up the rate. Some of these work better for the Type I blocks and some for the Type II blocks. The superior method of treating AV blocks remains the electronic pacemaker. A temporary transcutaneous or transvenous pacemaker can be used in an emergency situation. Implanted transvenous pacemakers remain the gold standard for treatment of AV blocks. D.Bean Services, 2005

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