An AV block is a delay or block in the transmission of an impulse from the atria to the ventricles.
AV blocks occur in 12% to 25% of patients with acute MI (Issa, et al., 2012). They are classified into (1) degree AV block, (2) second-degree AV block, and (3) third-degree AV block. With first-degree AV block, impulses from the SA node to the ventricles are delayed; they are not blocked. With second-degree AV blocks, there is an intermittent disturbance in the conduction of impulses between the atria and the ventricles. With third-degree AV block, there is a complete block in the conduction of impulses between the atria and the ventricles.
First-Degree Atrioventricular Block [Objectives 1, 2]
A first-degree AV block is associated with a delay in impulse conduction that results in a constant PR interval of more than 0.20 second in duration ( Table 6.7, Fig. 6.4). First-degree AV block may be per-manent or transient (Latcu & Nadir, 2010). When the QRS complex associated with a first-degree AV block is narrow, the conduction abnormality is most commonly in the AV node (Hamdan, 2010). When the QRS complex associated with a first-degree AV block is wide, the conduction abnormality may be located in the AV node, the bundle of His, or the bundle branches.
Fig. 6.3 Ventricular escape rhythm. (From Aehlert B: ECGs made easy, ed 3, St. Louis, 2006, Mosby.) TABLE 6.6 Characteristics of Ventricular Escape Rhythm
Regularity Ventricular rhythm is essentially regular Rate Ventricular rate is 20 to 40 beats/min
P waves Usually absent or with retrograde conduction to the atria; may appear after the QRS (usually upright in the ST segment or T wave)
PR interval None
QRS duration 0.12 sec or greater; the T wave is frequently in the opposite direction of the QRS complex
172 CHAPTER 6 Bradycardias
The patient with a first-degree AV block is often asymptomatic; however, marked first-degree AV block can lead to symptoms even in the absence of higher degrees of AV block ( Barold, 1996). First-degree AV block that occurs with acute MI should be monitored closely to detect progression to higher-degree AV block (Blank, et al., 2014). If first-degree AV block accompanies a symptomatic bra-dycardia, treat the bradycardia.
Second-Degree Atrioventricular Blocks
The term second-degree AV block is used when one or more, but not all, sinus impulses are blocked from reaching the ventricles. Intermittent AV conduction is reflected on the ECG as more P waves than QRS complexes.
Second-degree AV block is classified as type I or type II, depending on the behavior of the PR inter- vals associated with the dysrhythmia. The type I or type II designation is used to describe the ECG
pat-tern of the PR intervals and should not be used to describe the anatomic site (ie, location) of the AV block (Issa, et al., 2012). At least two consecutively conducted PR intervals must be observed to determine their pattern.
Second-Degree Atrioventricular Block Type I [Objectives 1, 2]
Blockage of the right coronary artery resulting in an inferior MI or right ventricular infarction can result in conduction delays such as first-degree AV block and second-degree AV block type I. Second-degree AV block type I is also known as type I block, Mobitz I, or Wenckebach. The term Wenckebach phenomenon is used to describe a progressive lengthening of conduction time in any cardiac conducting tissue that even-tually results in the dropping of a beat or a reversion to the initial conduction time. It is generally rec-ognized that all of the classic Wenckebach features are found in less than 50% of cases (Latcu &
Nadir, 2010).
Second-degree AV block type I is associated with a cyclic pattern that consists of conducted P waves (ie, each P wave is followed by a QRS) and then a P wave that is not conducted (ie, the P wave is not followed by a QRS) ( Table 6.8, Fig. 6.5). The P wave that is not conducted ends a group of beats. The cycle then begins again. The repetition of this cyclic pattern is called grouped beating.
The patient with this type of AV block is usually asymptomatic because the ventricular rate often remains nearly normal, and cardiac output is not significantly affected. If the patient is symptomatic and the dysrhythmia is a result of medications (eg, digoxin, beta-blockers), these substances should be withheld. When it is associated with an acute inferior MI, this dysrhythmia is usually transient and resolves within 48 to 72 hours as the effects of parasympathetic stimulation disappear.
TABLE 6.7 Characteristics of First-Degree Atrioventricular Block
Regularity Regular
Rate Usually within normal range, but depends on underlying rhythm P waves Every positive (ie, upright) P wave is followed by a QRS complex PR interval Fixed duration of more than 0.20 sec
QRS duration Usually 0.11 sec or less unless abnormally conducted
Fig. 6.4 Sinus rhythm with a first-degree AV block, ST-segment depression. (From Aehlert B: ECG study cards, St. Louis, 2004, Mosby.)
If the heart rate is slow and serious signs and symptoms occur because of the slow rate, treatment should include applying a pulse oximeter and administering oxygen (if indicated), obtaining the patient ’s vital signs, and establishing IV access. A 12-lead ECG should be obtained. Atropine, administered IV, is the drug of choice. Reassess the patient ’s response and continue monitoring the patient. When this rhythm occurs in conjunction with acute MI, the patient should be observed closely for increasing AV block and expert consultation should be sought with regard to patient management decisions.
Second-Degree Atrioventricular Block Type II [Objectives 1, 2]
Second-degree AV block type II is also called type II block or Mobitz II AV block ( Table 6.9, Fig. 6.6).
The site of block in type II block is most often in the bundle branches (Issa, et al., 2012). Although second-degree AV block type II is less common than type I, type II is more serious and it is associated
TABLE 6.8 Characteristics of Second-Degree Atrioventricular Block Type I
Regularity Ventricular irregular; atrial regular; grouped beating may be present Rate Atrial rate is greater than the ventricular rate
P waves Normal in size and shape; some P waves are not followed by a QRS complex
PR interval Progressive prolongation of the PR interval (although lengthening may be very slight) until a P wave appears without a QRS complex; the PR interval after a nonconducted P wave is shorter than the interval preceding the nonconducted beat
QRS duration Usually 0.11 sec or less; complexes are periodically dropped
Fig. 6.5 Second-degree AV block type I. (From Aehlert B: ECG study cards, St. Louis, 2004, Mosby.)
TABLE 6.9 Characteristics of Second-Degree Atrioventricular Block Type II
Regularity Ventricular irregular; atrial regular
Rate Atrial rate is greater than the ventricular rate; ventricular rate is often slow P waves Normal in size and shape; some P waves are not followed by a QRS complex
PR interval Within normal limits or prolonged, but constant for the conducted beats; the PR intervals before and after a blocked P wave are constant
QRS duration Within normal limits if the block occurs above or within the bundle of His; greater than 0.11 sec if the block occurs below the bundle of His; complexes are periodically absent after P waves
Lead II
Fig. 6.6 Second-degree AV block type II. (From Aehlert B: ECG study cards, St. Louis, 2004, Mosby.)
174 CHAPTER 6 Bradycardias
with an increased risk of mortality because it has a relatively high risk of progression to advanced or third-degree AV block (Blank, et al., 2014).
Because second-degree AV block type II may abruptly progress to third -degree AV block, the patient should be closely monitored for increasing AV block. If the heart rate is slow and serious signs and symp-toms occur because of the slow rate, treatment should include obtaining the patient ’s vital signs, applying a pulse oximeter and administering oxygen (if indicated), and establishing IV access. Although atropine is the first-line drug for acute symptomatic bradycardia, it is unlikely to be effective when the site of an AV block is below the AV node. In situations such as this, pacing or the administration of
beta-adrenergic medications is preferable (Link, et al., 2015). The choice of transcutaneous versus temporary transvenous pacing varies by institution and equipment availability. If TCP is available, it should be read-ied for immediate use should the patient ’s condition deteriorate and become unstable. A 12-lead ECG should be obtained and a cardiology consult should be sought.
2:1 Atrioventricular Block
With second-degree AV block in the form of 2:1 AV block, there is one conducted P wave followed by a blocked P wave; thus two P waves occur for every one QRS complex (ie, 2:1 conduction) ( Table 6.10).
Because there are no two PQRST cycles in a row from which to compare PR intervals, 2:1 AV block cannot be conclusively classified as type I or type II. To determine the type of block with certainty, it is necessary to continue close ECG monitoring of the patient until the conduction ratio of P waves to QRS complexes changes to 3:2, 4:3, and so on, which would enable PR interval comparison. If the QRS com-plex measures 0.11 second or less, the block is likely to be a form of second-degree AV block type I. A 2:1 AV block associated with a wide QRS complex (ie, more than 0.11 second) is usually a type II block. The causes and emergency management for 2:1 AV block are those of type I or type II block previously described. A comparison of the types of second-degree AV blocks is shown in Fig. 6.7.
TABLE 6.10 Characteristics of Second-Degree 2:1 Atrioventricular Block
Regularity Ventricular regular; atrial regular Rate Atrial rate is twice the ventricular rate
P waves Normal in size and shape; every other P wave is not followed by a QRS complex PR interval Constant
QRS duration May be narrow or wide; complexes are absent after every other P wave
Lead II
Lead II
Lead II
A
B
C
Fig. 6.7 Types of second-degree AV block. A, Second-degree AV block type I. B, Second-degree AV block type II. C, 2:1 AV block. (From Grauer K: A practical guide to ECG interpretation, ed 2, St Louis, 1998, Mosby.)
Third-Degree Atrioventricular Block [Objectives 1, 2]
With third-degree AV block, there is a complete block in conduction of impulses between the atria and the ventricles ( Table 6.11, Fig. 6.8). The site ofblock may occur atthe level ofthe AVnode,the bundle of His, or distal to the bundle of His. A secondary pacemaker (either junctional or ventricular) stimulates the ventricles; therefore the QRS may be narrow or wide, depending on the location of the escape pace-maker and the condition of the intraventricular conduction system.
If the patient is symptomatic because of the slow rate, treatment should include obtaining the patient ’s vital signs, applying a pulse oximeter, administering oxygen (if indicated), establishing IV access, and obtaining a 12-lead ECG. Because atropine is unlikely to be effective in the management of a third-degree AV block, TCP may be used as a temporizing measure to provide immediate stabilization while preparations are made for transvenous pacing. Other interventions that may be used in the treatment of third-degree AV block include epinephrine, dopamine, or isoproterenol IV infusions (Link, et al., 2015).
Frequent patient reassessment is essential. Most patients with third-degree AV block have an indication for permanent pacemaker placement. The bradycardia algorithm is shown in Fig. 6.9.
ACLS Pearl
Although calcium administration is not part of the symptomatic bradycardia algorithm, IV calcium is useful in the treatment of many types of bradydysrhythmias, especially those that occur because of an overdose of a calcium channel blocker (eg, verapamil, diltiazem) or because of hyperkalemia.