Fatigue, concealed conduction and supernormality as determined of paroxismal AV block

pp 415-419

Abstract

The paroxysmal atrioventricular block (PAVB) in phase 3 has been attributed to concealed conduction, On the other hand, "fatigue", may prolong the refractory period of the His-Purkinje system and therefore create the necessary conditions of concealed conduction to induce PAVB. These observation is about a patient whose P AVB was induced by fatigue in distal His bundle; before PAVB, supernormal phase ot conduction was present as concealed conduction. A 55 year old man, who had dizziness, His ECG showed sinus rhythm with 2:1, 3:2 and 4:2 second degree (Mobitz II) AV block and left anterior hemiblock, An electrophysiological study with His bundle electrogram, atrial and ventricular stimulation was performed. Control records revealed 1:1 AV conduction with HV interval of 120 msec. The refractory period was 550 msec, measured with single atrial stimulus delivered during sinus rhythm. Concealed conduction, as block of the next P wave (P2) and HV prolongation of the third P wave (P3) was observed. PAVB up to 6 consecutive P waves was induced with atrial stimulation at rates of 110 bpm or more. After atrial stimulation was stopped, PAVB was sustained with 4:2 and 4:1 periods, in spite of a reduced heart rate. In these periods the conduction of the first beat had a HV interval of 200 msec and the second beat one of 120 msec. Ventricular stimulation also induced PAVB by fatigue of the bundle, although it had no conduction to the atria. When ventricular stimulation was interrupted after a short period of PAVB, a second degree AV block (Mobitz II) with supernormal conduction was present. We analyze the supernormal conduction as concealed conduction in distal His bundle. Increments in the refractory period could be determined by fatigue and be responsible for the PAVB. Although during 1:1 AV conduction the refractory period was almost normal, fatigue-induced prolongation might reveal supernormal conduction.