First-Degree AV Block or Prolonged PR Interval

The PR interval is a reflection of intra-atrial and AV conduction time and is measured from the onset of the P wave to the beginning of the QRS complex. Conduction delay in the atrium, AV node, bundle of His, or bundle branches all may result in a prolonged PR interval. The longer the PR interval becomes, the more likely the delay resides in the AV node.

Second-Degree Type I AV Block (AV Wenckebach)

In second-degree AV block, type I (AV Wenckebach), there is progressive lengthening of the PR interval culminating in a blocked P wave (Fig. 98.3). The QRS duration may be narrow or wide, depending on the presence of bundle branch block. When the QRS is narrow, it suggests the block is at the level of the AV node and will improve with atropine or exercise and worsen with carotid sinus massage. It can be seen in young healthy individuals with increased vagal tone, especially nocturnally. It is commonly due to reversible causes such as AV nodal slowing drugs.

Second-Degree Type II AV Block

In second-degree AV block, type II the PR intervals are normal or slightly prolonged and are exactly the same length before and after the nonconducted P waves (Fig. 98.4). As opposed to second-degree, type I block, AV conduction may worsen with sinus acceleration due to atropine or exercise and may improve with carotid massage. Type II second-degree AV block is most likely located below the AV node, residing in the His bundle if there is a narrow QRS duration (rare) or in the bundle branches if there is wide QRS duration (common). These patients are often symptomatic with dyspnea, fatigue, and syncope, and can progress to complete AV block.

Two-to-One AV Block

In a two-to-one AV block, every other P wave conducts to the ventricle, and the conducted PR interval may be either normal or prolonged (Fig. 98.5). The level of block—AV nodal versus infranodal—cannot be determined with certainty without an electrophysiology study. A narrow QRS duration suggests that the block is in the AV node or rarely the His bundle, whereas a wide QRS suggests block in the bundle branches.

Third-Degree AV Block or Complete AV Block

In complete AV block there is no P wave to QRS relationship. The sinus rate (P-to-P intervals) is faster than the escape rate. The ventricular escape rate is usually less than 50 beats per minute (bpm), with the exception of a congenital AV block (Fig. 98.6). If the escape rhythm has a narrow QRS complex, then it originates in the AV junction and the site of block is either AV nodal or, less likely, the His bundle. If the QRS is wide, the site of block is likely within the bundle branches. If occasional P waves are observed to capture the ventricle but most do not, then the term high-grade or advanced AV block may be used.

TABLE 98.1 Drugs Affecting Sinus Node Function

TABLE 98.2 Noninvasive Interventions to Determine Site of Atrioventricular (AV) Block

General Pacing Considerations for AV Conduction Abnormalities

1. Record a 12-lead ECG and try to determine site of block by noninvasive methods.
   
2. In the setting of an acute inferior wall MI, the site of block is usually the AV node and is usually transient with successful reperfusion.
   
   
   
   1. Insert a temporary pacemaker if there is hemodynamic compromise secondary to heart block.
      
   2. Proceed to permanent pacing if there is persistent hemodynamic compromise, continued symptoms due to ongoing AV nodal block, or persistent high-grade or complete AV block.
   
   
3. In the setting of an acute anterior MI, the site of block is considered likely to be infranodal especially in the context of a wide QRS.
   
   
   
   1. Insert a temporary pacemaker if there is hemodynamic compromise secondary to heart block.
      
   2. Proceed to permanent pacing if there is persistent hemodynamic compromise, continued symptoms due to ongoing AV nodal block, or persistent high-grade or complete AV block.
      
   3. Proceed to permanent pacing, if the transient AV block is suspected to be infranodal and associated with a persistent bundle branch block.
   
   
4. Second-degree AV block, Type I
   
   
   
   1. Progressive PR prolongation is present prior to AV conduction block.
      
   2. Proceed to temporary pacing if there is hemodynamic compromise secondary to AV nodal block (rare).
      
   3. Proceed to permanent pacing if there is persistent hemodynamic compromise or continued symptoms due to ongoing AV nodal block.
   
   
5. Second-degree AV block, Type II
   
   
   
   1. If there is no PR progressive prolongation prior to block, then the site of block is most likely in the infranodal tissue.
      
   2. Temporary pacemaker is required when associated with significant hemodynamic compromise or unrelenting symptoms.
      
   3. A permanent pacemaker should be considered in most cases of infranodal AV block.
   
   
6. Two-to-one AV block
   
   
   
   1. A relatively short PR interval (≤160 msec) with a wide QRS suggests infranodal block.
      
   2. A temporary pacemaker is indicated in patients with significant hemodynamic compromise, unrelenting symptoms, or with a wide QRS following an acute anterior MI.
      
   3. Observe other periods on telemetry or rhythm strips for evidence of 3:2 (Wenckebach) second-degree AV block, type I or second-degree AV block, type II to help define the level of block.
   
   
7. Third-degree AV block
   
   
   
   1. A temporary pacemaker is indicated in patients with hemodynamic compromise, unrelenting symptoms, or in acute anterior MI with a wide QRS escape rhythm.
      
   2. A permanent pacemaker should be considered in older patients with acquired, unresolved third-degree AV block and in most cases of acquired, transient complete AV block.
      
   3. Some young patients with congenital complete AV block and a narrow QRS escape rhythm under autonomic influence (i.e., appropriate increases in rate with exercise) may not require permanent pacing.

NARROW QRS TACHYCARDIA

Narrow QRS tachycardia is defined as an arrhythmia with a rate faster than 100 bpm and QRS duration of <120 msec. Patients are often symptomatic complaining of palpitations, lightheadedness, shortness of breath, or anxiety. ECG documentation of the tachycardia is extremely important to help determine the mechanism of the tachycardia. The differential diagnosis for narrow QRS tachycardia includes the following: sinus tachycardia, atrial tachycardia, atrial flutter, atrial fibrillation, junctional tachycardia, AV nodal reentry tachycardia (AVNRT), and AV reentry tachycardia (AVRT) using an accessory pathway.

A 12-lead ECG can be helpful in distinguishing sinus tachycardia from other narrow QRS tachycardias by allowing the evaluation of the morphology of the P wave. A P-wave morphology that is distinctly different than sinus or a P wave that is notably absent suggests the mechanism is unlikely to be originating from the sinus node. Atrial fibrillation is an irregular tachycardia due to a rapid, irregular atrial rhythm with variable AV conduction. The ventricular response to the more organized atrial flutter rhythm can be regular or irregular depending on conduction through the AV node. The three most common causes of regular paroxysmal narrow QRS tachycardia are AVNRT, AVRT, and atrial tachycardia, respectively (3,4).

Atrial Fibrillation

Atrial fibrillation is the most common supraventricular tachyarrhythmia in the United States (3,4). The atrial rhythm is irregular with an atrial rate of 350 to 500 bpm (Fig. 98.8). It is a major cause of cardiovascular morbidity and mortality, with an increased risk of death, congestive heart failure, and stroke. The incidence of atrial fibrillation increases with age, with a lifetime risk of one in four men and women older than the age of 40 years (4). Similarly, the risk of embolic stroke from atrial fibrillation increases with age (>65 years), as well as other risk factors including hypertension, prior history of stroke, congestive heart failure, coronary or peripheral vascular disease, female gender, and diabetes. Stroke prevention is the key focus in the management of atrial fibrillation. The above stroke risk factors are compiled in a risk stratification scheme called the CHA₂DS₂-VASc score (5). Anticoagulation with warfarin, with an international normalized ratio (INR) between 2.0 and 3.0, a direct thrombin inhibitor or a factor Xa inhibitor is recommended in patients with atrial fibrillation and a CHA₂DS₂-VASc score of 2 or greater and can be considered in patients with a CHA₂DS₂-VASc score of 1. These recommendations, along with other updated practice guidelines for the management of atrial fibrillation, have recently been jointly published by the AHA, ACC, and HRS (5).

Atrial fibrillation can be divided into five categories: (i) new onset; (ii) paroxysmal (lasting <7 days); (iii) persistent (lasting >7 days); (iv) longstanding persistent (lasting >12 months); and (v) permanent (a clinical decision to forgo restoration of sinus rhythm). These categories are important considerations when discussing whether to try to restore and maintain sinus rhythm as the longer atrial fibrillation persists, the harder it becomes to achieve that goal.

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