?url_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Adc&rft.title=Sinoatrial+Reentry+Tachycardia%3A+A+Review&rft.creator=Simmers%2C+TA&rft.creator=Sreeram%2C+N&rft.subject=Indian+Pacing+and+Electrophysiology+Journal&rft.description=The+concept+of+reentry+within+the+sinus+node+is+by+no+means+new.+In+their+1943+report%2C+Barker+and+co-workers+postulated+that+%E2%80%9C%E2%80%A6a+circus+rhythm+could+be+accommodated+in+auricular+muscle+and+in+one+of+the+specialized+nodes+at+known+rates+of+conduction+and+with+cycle+lengths+such+as+occur+in+paroxysmal+tachycardia%E2%80%9D+1.+Lack+of+invasive+electrophysiology+at+that+time+and+subsequent+failure+to+appreciate+the+heterogeneity+of+supraventricular+arrhythmias%2C+left+their+astute+observation+in+the+realm+of+conjecture.+It+was+not+until+1968+that+Han%2C+Malozzi+and+Moe+finally+demonstrated+the+existence+of+sinoatrial+echoes+2.+In+a+superfused+isolated+rabbit+right+atrial+preparation%2C+they+examined+the+response+to+premature+extrastimuli+with+an+18+electrode+grid+at+the+sinus+node+and+surrounding+atrial+tissue.+They+found+that+critically+timed+extrastimuli+led+to+early+re-excitation+of+the+atrium%2C+supposedly+due+to+sinoatrial+reentry.+Sequential+microelectrode+measurements+of+sinus+node+and+atrial+transmembrane+potentials+were+performed+in+an+attempt+to+provide+a+temporal+and+anatomical+map+of+the+observed+phenomenon.+Based+on+findings%2C+the+authors+concluded+that+sinoatrial+reciprocation+was+caused+by+entrance+block+at+one+site%2C+slow+conduction+within+the+node+with+disparate+refractoriness%2C+and+re-excitation+of+the+atrium+at+the+original+area+of+entrance+block.+They+went+on+to+suggest+that+repetition+of+this+phenomenon+may+conceivably+form+the+basis+for+clinically+relevant+tachycardia.+In+vivo+confirmation+of+sinoatrial+echoes+was+inferred+from+work+by+Childers+3+and+Paulay+4.+In+the+former+study%2C+programmed+electrical+stimulation+was+performed+in+dogs.+Three+responses+to+premature+atrial+stimuli+were+noted%3A+complete+interpolation+(i.e.+the+subsequent+sinus+complex+was+on+time)%2C+incomplete+interpolation+(i.e.+the+subsequent+beat+was+delayed)%2C+and+sinus+echo.+In+the+latter%2C+the+first+post-extrastimulus+complex+was+electrocardiographically+the+same+as+sinus%2C+but+earlier+than+expected.+The+authors+attributed+this+to+sinoatrial+reentry%2C+and+went+on+to+describe+each+of+the+three+responses+to+premature+atrial+beats+in+a+70-year-old+man+in+an+elegant+electrocardiographic+deduction+of+underlying+mechanisms.+An+in+vivo+canine+model+was+also+employed+in+the+report+by+Paulay+et+al.+While+confirming+Childers%E2%80%99+findings%2C+Paulay+offered+two+additional+pieces+of+evidence+implicating+the+sinus+node%2Fperinodal+area%3A+firstly%2C+timing+of+the+echo+was+independent+of+stimulation+site+(they+performed+pacing+at+the+sinus+node%2C+Bachmann%E2%80%99s+bundle%2C+right+and+left+atrial+appendages+and+low+atrial+septum)%2C+and+secondly+echoes+were+eliminated+by+crushing+the+sinus+node.+As+has+long+been+the+case+for+AV+nodal+reentry+tachycardia%2C+it+is+apparent+that+considerable+discussion+also+revolves+around+the+issue+of+whether+sinoatrial+echoes+do+or+do+not+involve+perinodal+atrial+tissue+as+well+as+the+sinus+node+itself.+In+1979%2C+Allessie+and+Bonke+published+work+suggesting+confinement+of+the+re-entrant+circuit+to+the+node+per+se+5.+The+experimental+set-up+was+similar+to+earlier+work+by+Han+2%2C+with+the+significant+difference+that+resolution+of+the+measuring+electrodes+was+superior%3A+32+unipolar+surface+atrial+electrograms+were+recorded+during+initiation+of+sinoatrial+echoes+as+opposed+to+the+18+used+by+Han.+In+addition%2C+up+to+130+transmembrane+sinus+node+potentials+were+recorded+sequentially+in+cases+of+sustained+reciprocation.+These+detailed+observations+led+to+the+conclusion+that+sinus+node+reentry+was+confined+to+a+circuitous+pathway+in+an+extremely+small+area+(1-2mm)+with+low+conduction+velocities+(2.5cm%2Fs)+within+the+node+itself.+It+is+of+note+that+the+same+authors+pointed+out+the+inability+to+induce+sustained+arrhythmia+in+this+model%2C+and+later+concluded+that+sinoatrial+reentry+tachycardia+(SART)+may+not+be+feasible+without+border+zone+atrial+tissue.+Given+the+limitations+inherent+to+in+vivo+study+in+man%2C+it+may+be+fair+to+continue+to+use+%E2%80%9Csinus+node+reentry%E2%80%9D+and+%E2%80%9Csinoatrial+reentry%E2%80%9D+synonymously+for+the+time+being.+In+the+wake+of+in+vivo+studies+in+dogs%2C+and+reports+of+sinus+node+reentry+in+man+by+Paulay+6+and+Childers+3%2C+Narula+first+described+sustained+SART+in+two+patients+in+1974+7.+Out+of+300+patients+undergoing+electrophysiologic+study%2C+he+observed+sinus+node+reentry+beats+in+20%2C+and+sustained+tachycardia+in+two.+The+criteria+for+the+diagnosis+of+SART+proposed+by+Narula+are+still+valid%3A+1.+atrial+activation+and+P-wave+morphology+are+the+same+or+highly+similar+to+sinus+rhythm%2C+with+activation+from+high+to+low+right+atrium%2C+2.+the+arrhythmia+is+inducible+with+atrial+extrastimuli+at+specific+coupling+intervals%2C+independent+of+AV+nodal+conduction+intervals+and+site+of+stimulation%2C+and+3.+the+arrhythmia+can+be+terminated+by+atrial+stimuli.+SART+is+thus%2C+by+definition%2C+a+paroxysmal+arrhythmia.%0A&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.date=2003-07&rft.type=Journal+(Paginated)&rft.type=PeerReviewed&rft.format=text%2Fhtml&rft.identifier=http%3A%2F%2Fcogprints.org%2F4259%2F1%2Fsimmers.htm&rft.identifier=++Simmers%2C+TA+and+Sreeram%2C+N++(2003)+Sinoatrial+Reentry+Tachycardia%3A+A+Review.++%5BJournal+(Paginated)%5D+++++&rft.relation=http%3A%2F%2Fcogprints.org%2F4259%2F