Supra-hisian block

• what happens with conduction signal in type 1 second degree AV block
• how to differentiate type 1 and 2
• what is the clinical relevancy of this findings

• The designation as being either type 1 or type 2 second-degree AV-Block refers solely to surface-ECG patterns and does not tell you the exact anatomical site of the conduction disturbance.
• The diagnosis of a typical type 1 second-degree AV block requires:

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1. The presence of a sinus rhythm with at least 2 consecutive conducted P waves followed by a single blocked atrial impulse.
2. From one P wave to the next, the PR interval has to progressively lengthen prior to failure of an atrial impulse to conduct to the ventricles.
3. This PR interval lengthening has to occur at progressively decreasing increments resulting in progressively shorter R-R intervals.
4. For that reason, the pause between the two QRS complexes encompassing the non-conducted P wave is less than the sum of R-R intervals of any two consecutively conducted beats.
5. The PR interval immediately after the non-conducted P wave returns to its baseline value with the whole sequence then starting all over.

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• The progressively PR interval lengthening is based on an abnormally long relative refractory period of the AV node in conjunction with its decremental behavior. Thus, Wenckebach periodicity can be explained by each consecutive atrial impulse arriving earlier and earlier in the relative refractory period, leading to a longer and longer conduction delay, until one impulse finally falls into the absolute refractory period.
• The vast majority of type 1 second degree AV block is located above the level of HIS – those being benign, rare forms of an infrahisian block do exist, which carry the risk of a complete heart block and warrant pacing even in the absence of symptoms.
• During the EP study, a suprahisian conduction disturbance would show a prolongation in the AH with an unchanged HV interval and a sole A recording on the HIS catheter in case of the non-conducted atrial impulse. Whereas in an infrahisian conduction abnormality the HV interval would be prolonged, and during the non-conducted P wave both an A as well as an H can be expected on the HIS catheter.