If the QRS complex is wide (>0.12 s/>3 small squares), the ventricular activity may be arising from the ventricle conducting system. caused by a ‘bundle branch block’.
WiLLiaM MaRRoW (classic mnemonic)
Wi – V1
LL – LBBB
M – V6
M – V1
RR – RBBB
W – V6
ViLLheiM MaRRooN (more accurate)
V – V1
LL – LBBB
M – V6
M – V1
RR – RBBB
N – V6
With a Left bundle branch block (LBBB), you get W or more accurately V shaped complex (rS) in V1 and M shaped complex (R) in V6.
With a Right bundle branch block (RBBB), you get M shaped complex (rSR’) in V1 and W or more accurately N shaped complex (qRs) in V6.
If LBBB is present, no comment can be made on the rest of the ST segment, and thus it may be a sign of acute MI.