[PLEASE PLACE BRACKETS OVER THE 3 GROUPS OF 3 AND 1 GROUP OF 4]
The inability of some of the atrial impulses to propagate to the ventricles is referred to as 2nd degree AV block. When this occurs, all of the QRS complexes are preceded by a P wave, but some of the P waves are blocked within the AV conducting system and are therefore not followed by a QRS complex. The site of block may be within the AV node or in the more distal portions of the conducting system. When this becomes repetitive, it results in regularly spaced pauses, a phenomenon referred to as group beating.
The phenomenon of group beating was originally described in 1899 by Carl Frederick Wenckebach, a Dutch born physician working in Vienna. He recognized the group beating from arterial pulse recordings, such as that reproduced here from his original description. We have added the brackets to enclose the 3 groups of three and the single group of four.