RT Book, Section
A1 Ferry, David R.
SR Print(0)
ID 1128942536
T1 SA and AV Nodal Conduction Abnormalities
T2 ECG in 10 Days, 2e
YR 2007
FD 2007
PB McGraw-Hill Education
PP New York, NY
SN 9780071465625
LK accesscardiology.mhmedical.com/content.aspx?aid=1128942536
RD 2024/04/19
AB Intracardiac  electrogramsConduction  disturbances  in  the  surface  ECG  have  their  genesis  in  specific  locations  in  the  conduction  system.  Surface  ECG  disturbances  are  more  clearly  appreciated  by  concomitant  analysis  of  the  intracardiac  electrogram.Components  of  the  intracardiac  electrogramSinoartrial  (SA)  nodeThere  is  no  surface  ECG  representation  of  SA  nodal  depolarization;  a  recurrent,  normal  axis  P  wave  implies  that  the  SA  node  is  responsible.Careful  recordings  from  a  tiny  area  in  the  upper  right  portion  of  right  atrium  have  demonstrated  SA  nodal  activity  preceding  atrial  depolarization.AtriaAtrial  depolarization  produces  the  P  wave  on  the  surface  ECG.The  P  wave  axis  is  demonstrative  of  the  direction  of  atrial  depolarization.Atrioventricular  (AV)  nodeThe  AV  node  is  responsible  for  most  of  the  delay  between  the  P  wave  and  the  QRS  complex.On  the  intracardiac  electrogram,  the  delay  in  the  AV  node  is  represented  by  the  P  wave  to  His  bundle  spike  interval  (the  A—H  interval).Disturbances  of  AV  nodal  conduction  result  in  prolongation  of  the  A—H  interval.  His  bundleThere  is  no  surface  representation  of  His  bundle  activation;  it  is  implied  by  a  succeeding  QRS  complex.On  the  intracardiac  electrogram,  careful  positioning  of  an  electrode  cam  demonstrate  a  small  deflection  coincident  with  the  activation  of  the  His  bundle.The  time  between  the  His  bundle  spike  and  the  QRS  complex  is  the  H—V  interval.The  sum  of  A—H  and  H—V  intervals  equals  the  PR  interval.Bundle  branchesDepolarization  of  the  right  and  left  bundles  produce  the  QRS  complex  on  the  surface  ECG.Defects  of  bundle  branch  conduction  were  discussed  on  Day  2.AV  conduction  abnormalitiesFirst  degree  AV  block  (Day  3-01)  (Day  3-02)In  first  degree  AV  block,  the  PR  interval  >  200  msec.The  PR  interval  is  dependent  on  heart  rate,  so  that  at  very  slow  rates,  a  PR  interval  >  200  may  be  normal.First  degree  AV  block  is  almost  always  due  to  a  prolongation  of  the  A—H  interval.Second  degree  AV  blockType  I  (Wenckebach)  (Day  3-03)  (Day  3-04)In  second  degree  AV  block  type  I,  there  is  progressive  prolongation  of  the  PR  interval  until  there  is  a  dropped  QRS  complex.The  Wenckebach  phenomenon  usually  produces  group  beating  of  the  QRS  complexes.In  the  His  bundle  electrogram,  there  is  progressive  prolongation  of  the  A—H  interval  until  there  is  no  His  spike  produced.The  H—V  interval  is  usually  normal.Second  degree  AV  block  type  II  (Day  3-05)  (Day  3-06)In  second  degree  AV  block  type  II,  there  are  regular  P  waves  with  an  occasional  loss  of  the  QRS  complex.The  PR  interval  does  not  change  before  the  conducted  beats.On  the  His  bundle  electrogram,  this  type  of  block  is  usually  associated  with  an  intermittent  failure  of  H—V  conduction.Third  degree  AV  block  (Day  3-07)  (Day  3-8)In  third  degree  AV  block,  there  is  complete  failure  of  conduction  from  the  atria  to  the  ventricles.The  atrial  rate  is  always  faster  than  the  ventricular  rate.The  escape  rhythm  may  arise  from  the  junctional  area,  in  which  case  its  rate  will  typically  be  40–60,  or  it  may  arise  from  a  ventricular  focus  with  a  rate  of  20–40.  DAY  3-01  DAY  3-02  DAY  3-03  DAY  3-04  DAY  3-05  DAY  3-06  DAY  3-07  DAY  3-08  Junctional  escape  rhythms  have  a  narrow  QRS  complex  (unless  there  is  ...