?url_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Adc&rft.title=Changing+QRS+Morphology%3A+What+is+the+mechanism%3F+&rft.creator=Haghjoo%2C+Majid&rft.creator=Arya%2C+Arash&rft.creator=Dehghani%2C+Mohammad+Reza&rft.creator=Sadr-Ameli%2C+Mohammad+Ali&rft.subject=Indian+Pacing+and+Electrophysiology+Journal&rft.description=ECG+in+sinus+rhythm+with+ventricular+preexcitation+and+changing+QRS+morphology+was+seen+that+was+initially+interpreted++as+the+multiple+accessory+pathway+from+elsewhere.+(Figure+1A).+%0A++++++++++++The+following+mechanisms+are+potentially+involved+in+the+electrogenesis+of+changing+QRS+morphology+in+WPW+syndrome%3A+1)+multiple+accessory+pathways1%3B+2)+simultaneous+occurrence+of+aberrant+atrioventricular+conduction+with+accessory+pathway+conduction+2%3B+3)+ventricular+fusion+of+preexcited+sinus+impulse+with+ectopic+impulse.+%0A++++++++++++Electrophysiologic+study+showed+short+PR+(75+ms)+interval+with+wide+QRS+(152+ms)+and+negative+HV+(-12+ms)+interval.+No+change+in+delta+wave+polarity+was+observed+during+HRA+and+CS+pacing.+In+full+preexcitation%2C+no+breakthrough+was+seen+in+the+CS.+During+incremental+ventricular+pacing%2C+atrial+breakthrough+site+is+initially+recorded+on+the+HRA+catheter+and+then+changed+to+distal+pole+of+CS+catheter+with+progressive+decrease+in+pacing+cycle+length.+During+ventricular+pacing+at+cycle+length+of+500+ms+(S1)%2C+earliest+atrial+activity+is+recorded+on+HRA+catheter.+%0A++++++++Changing+QRS+could+not+be+explained+by+presence+of+multiple+APs+because+only+right-sided+AP+had+bidirectional+conduction+and+no+distal+CS+breakthrough+was+seen+simultaneous+with+changing+QRS+morphology.+The+possibility+of+aberrant+conduction+is+excluded+by+presence+of+negative+HV+interval+in+the+beats+with+differing+QRS+morphology.+No+sinus+cycle+length+variation+before+and+after+the+beats+with+different+morphologies+are+against+the+occurrence+of+functional+LBBB.+The+prematurity+of+ventricular+electrogram+in+His+recording+catheter+with+variable+HV+(H-electrogram+is+recorded+after+V-electrogram+in+second+beat+and+before+V-electrogram+in+third+beat)+and+fixed+V-RB+intervals+(interval+from+ventricular+electrogram+in+His+to+the+RB+potential)+are+compatible+with+ventricular+fusion+of+preexcited+sinus+impulse+with+ectopic+ventricular+impulse+originating+from+parahissian+area+(explaining+LBBB+and+inferior+axis+morphology+of+the+beats+with+changing+QRS)+but+not+from+the+His+bundle+or+RBB+itself++(because+H-electrogram+and+RB+potential+is+recorded+after+V-electrogram+in+the+second+beat+with+greater+degree+of+ventricular+fusion)(Figure+1B).+&rft.publisher=Indian+Pacing+and+Electrophysiology+Group&rft.contributor=Singh%2C+Balbir&rft.contributor=Lokhandwala%2C+Yash&rft.contributor=Francis%2C+Johnson&rft.contributor=Gupta%2C+Anup&rft.contributor=Ghosh%2C+Joydeep&rft.date=2006-01&rft.type=Journal+(Paginated)&rft.type=PeerReviewed&rft.format=text%2Fhtml&rft.identifier=http%3A%2F%2Fcogprints.org%2F4688%2F1%2Fhaghjoo4.htm&rft.format=application%2Fpdf&rft.identifier=http%3A%2F%2Fcogprints.org%2F4688%2F2%2Fhaghjoo4.pdf&rft.identifier=++Haghjoo%2C+Majid+and+Arya%2C+Arash+and+Dehghani%2C+Mohammad+Reza+and+Sadr-Ameli%2C+Mohammad+Ali++(2006)+Changing+QRS+Morphology%3A+What+is+the+mechanism%3F++%5BJournal+(Paginated)%5D+++++&rft.relation=http%3A%2F%2Fcogprints.org%2F4688%2F