Type I second-degree

First-degree AV nodal blockade occurs due to inhibition of conduction within the upper portion of the node.15 Mobitz type I second-degree AV nodal blockade occurs as a result of inhibition of conduction further down within the node.12,15 Mobitz type II second-degree AV nodal blockade is caused by inhibition of conduction within or below the level of the bundle of His.12,15 Third-degree AV nodal blockade maybe a result of inhibition of conduction either within the AV node or within the bundle of His or the His-Purkinje system.12,15 AV block may occur as a result of age-related AV node degeneration. 

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.)...

However, it is unknown whether underlying RBBB (unifascicular block) is prognostically different from underlying LBBB (bifascicular block) in the setting of asymptomatic type I second-degree infranodal block. 

Barold SS. Lingering misconceptions about type I second-degree atrioventricular block. Am J Cardiol 2001 88 1018-1020. 

Cardiovascular Chronic aluminium exposure associated with cardiotoxic manifestations is rather rarely reported. A recent case study reported Mobitz type I second-degree atrioventricular block and nonsustained ventricular tachycardia (VT) following chronic occupational aluminium exposure [4 ]. 

First degree, second degree (Mobitz type I), third degree AV junctional escape rhythms, junctional tachycardia Atrial arrhythmias with slowed AV conduction or AV block Particularly paroxysmal atrial tachycardia with AV block Sinus bradycardia... 

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. 

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. 

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. 

Couper recognised two valencies of carbon, one in carbon monoxide and one in carbon dioxide, and says the highest known power of combination of carbon is that of the second degree, i.e., 4. His formulae are really nearer the modern ones than Kekule s, since the latter long afterwards continued to use either his own peculiar formulae (see p. 540) or the notation of types he represented acetic acid (I) as derived from a type (II) derived from the mixed type 4H2 + 2H2O, which in turn could be derived from a type 8H2 such a formula, since it is derived from all the metamorphoses of acetic acid and expresses all these, has not the advantage which some require from a rational formula, since it does not emphasise one or other group of metamorphoses ... 

The presence of a new feature may be qualitatively explained in terms of the three types of clusters mentioned above. At low frequency only clusters of type (i) and (ii) are active, while, at high frequency, the double layer capacitance reduces the magnitude of the contact impedance and type (iii) clusters progressively switch in , producing a second arc. The degree of overlap of the arcs depends on the volume fraction of the electrode phase, an effect for which Sunde provides a tentative explanation at low volume fractions, where the overall impedance is dominated by the electrolyte, type (iii) clusters have a significant effect and generate a well-... 

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).

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. 

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). 

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. 

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