Second-degree AV block type

Distinguishing second-degree AV block type II from nonconducted PACs... 

Type I block and type II second-degree AV block are electrocardiographic patterns and as such should not be automatically equated with the anatomical site of block. 

Fig. 10.3 Diagrammatic representation of various forms of second-degree AV block with the same format as in Fig. 10.2. (A) Relatively long and atypical type I sequence with several constant PR intervals before a dropped beat. Note the shorter PR interval after the blocked P-wave. This pattern should not be called type 11 AV block. It is essential to examine all the PR intervals in long rhythm strips and not merely several PR intervals preceding a blocked impulse. (B) True type II AV block. Every atrial impulse successfully traverses the AV node which is not afforded a long recovery time as occurring in type I AV block. Note that the PR interval after the blocked beat is unchanged. Reproduced with permission from (11).

Increments in AV conduction (AV nodal - AH interval) in Type I AV nodal block are typically large. Type I infranodal block typically exhibits small increments in AV conduction (confined to the HV interval) and large increments in AV condnction (confined to the HV interval) occur uncommonly. The increments in AV nodal block may occasionally be so tiny that they superficially mimic type H second-degree AV block. 

Type I second-degree AV block with bundle branch block (which is far less common than narrow QRS type I block) must not be automatically labeled as AV nodal. Outside of acute myocardial infarction, type I block and bundle branch block (QRS > 0.12 s) occur in the His-Purkinje system in 60-70% of the cases (10) (Fig. 10.5). In such cases exercise is likely to aggravate the degree of AV block. Yet, many still believe that type I blocks are all AV nodal and therefore basically benign. It is believed that the prognosis of infranodal type I block is as serious as that of type II block and a permanent pacemaker... 

Fig. 10.5 Sinus rhythm with second-degree Type 13 2 iniranodal AV block, and RBBB. Note that the AH interval remains constant. TTie HV interval increases from 80 (following first P-wave) to 150ms (following second P-wave). The third P-wave is followed by an H deflection but no QRS complex. AV block occurs in the His-Purkinje system below the site of recording of the His bundle potential (arrow). Note the shorter PR interval after the nonconducted P-wave, a feature typical of Type I second-degree AV block. HBE = His bundle electrogram, A = atrial deflection, H = His bundle deflection, V = ventricular deflection, P = P-wave. TL = time lines 50ms. (Barold SS. Pacemaker treatment of bradycardias and selection of optimal pacing modes. In Zipes DP (Ed.). Contemporary Treatments in Cardiovascular Disease, 1997 1 123, with pamission.)...

Type II according to the strict definition occurs in the His-Purkinje system and rarely above the site of recording of the His bundle potential in the proximal His bundle or nodo-Hisian junction. Type II block has not yet been convincingly demonstrated in the N zone of the AV node (3). Most if not all the purported exceptions involve reports where type I blocks (shorter PR interval after the blocked beat) are claimed to be type II blocks by using loopholes in the definitions of second-degree AV block. Because type II invariably occurs in the His-Purkinje system, it should be a class I indication for pacing. 

Type II Second-Degree AV Block True or False ... 

AV block. 2 1 AV block can be AV nodal or in the His-Puikinje system. It cannot be classified as type I or type II block because there is only one PR interval to examine before the blocked P-wave (Fig. 10.9). 2 1 AV block is best labeled simply as 2 1 block (3,15). For the purpose of classification according to the World Health Organization and the ACC, it is considered as advanced block as are 3 1, 4 1 etc. AV block. Confusion arises when the term advanced AV block (defined in the ACC/AHA guidelines as a form of second-degree AV block of two or more P-waves) is used to describe both second- and third-degree AV blocks (1). 

The site of the lesion in 2 1 AV block can often be determined by seeking the company 2 1 AV block keeps. An association with either type 1 or type II second-degree AV block helps localization of the lesion according to the correlations already discussed. Outside of acute myocardial infarction, sustained 2 1 and 3 1 AV block with a wide QRS complex occurs in the His-Purkinje system in 80% of cases and 20% in the AV node (3). It is inappropriate to label 2 1 or 3 1 AV nodal block as type I block and infranodal 2 1 or 3 1 AV block as type II block because the diagnosis of type I and type II blocks is based on electrocardiographic patterns and not on the anatomical site of block. 

Permanent pacing is almost never needed in inferior MI and narrow QRS AV block. Pacemaker implantation should be considered only if second- or third-degree AV block persists for 14-16 days (26,27). The use of permanent pacing is required in only 1-2% of all the patients who develop acute second or third-degree AV block regardless of thrombolytic therapy. Narrow QRS type II second-degree AV block has not yet been reported in acute inferior MI (28-31). 

Barold SS, Jais P, Shah DC, Takahashi A, Haissaguerre M, Clementy J. Exercise-induced second-degree AV block is it type I or type n J Cardiovasc Electrophysiol 1997 8 1084-1086. 

Prophylactic Temporary Pacemaker Insertion. Approximately 1% of patients with acute myocardial infarction develop a Type n second-degree AV block. Although this rhythm is often tolerated hemodynamically, because there can be sudden progression to complete AV block, temporary pacing should be considered. New bundle-branch block (BBB) has been associated with an 18% risk of transient complete AV block (9-11). The development of BBB usually signifies an extensive infarction, typically involving the anterior wall. Death in these patients usually results from left ventricular pump failure, although 9% of deaths have been attributable to complete AV block (9). 

Atropine Symptomatic SB AV block Asystole Bradycardic PEA Monitor heart rate and rhythm. Use the drug cautiously in patients with myocardial ischemia. Atropine isn t recommended for third-degree AV block or infranodal type II second-degree AV block. In adults, avoid doses less than 0.5 mg because of the risk of paradoxical slowing of the HR. 

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