This was a case of ventricular extrasystoles superimposed on sinus rhythm with P waves blocked by concealed and incomplete penetration of extrasystoles in the bundle of His1,2 (correct answer, response 2). Figure shows the surface and bundle of His electrocardiograms (A, atrium; H, His; V, ventricle). The patient is in sinus rhythm and the P waves have a relatively constant cycle duration (924ms) and typical and stable morphology (response 3 and 4, incorrect). Capture of the ventricular extrasystole by the bundle of His leads to suprahisian block of the following atrial pulse (incomplete penetration). The suprahisian block then enables atrioventricular node (AVN) recovery and improved suprahisian conduction in the following pulse (shortening of the A-H interval from 171 to 126ms). There is no intrinsic defect in conduction, but rather a physiological response of the AVN to incomplete penetration of a ventricular extrasystole (response 1, incorrect).
.