Jacobus ECG Tips

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ECG Primer  Christian Jacobus, MD  Ross University  Introduction to Clinical Medicine    When I dictate ECGs for a patient’s medical record, I use the following format: rate, rhythm, axis,  intervals, hypertrophy, ischemia. For example, “The rate is 60, the rhythm is sinus, axis is minus 45  degrees, intervals are normal, there is no evidence of hypertrophy, and no signs of ischemia.” So that's  the order in which I'm going to go over these today.      Rate    There are two easy ways to figure o
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  ECG   Primer   Christian   Jacobus,   MD   Ross   University   Introduction   to   Clinical   Medicine   When   I   dictate   ECGs   for   a   patient’s   medical   record,   I   use   the   following   format:   rate,   rhythm,   axis,   intervals,   hypertrophy,   ischemia.   For   example,   “The   rate   is   60,   the   rhythm   is   sinus,   axis   is   minus   45   degrees,   intervals   are   normal,   there   is   no   evidence   of    hypertrophy,   and   no   signs   of    ischemia.”   So   that's   the   order   in   which   I'm   going   to   go   over   these   today.   Rate   There   are   two   easy   ways   to   figure   out   the   heart   rate   by   looking   at   the   ECG.   1)   Standard   ECGs   record   for   10   seconds   so   an   easy   way   to   calculate   the   rate   is   to   count   the   number   of    QRS   complexes   on   one   page   and   multiply   by   six   to   get   the   number   in   one   full   minute.   (#   QRS   complexes   on   one   page)   *(6)   =   BPM   2)   The   300   rule:   count   the   number   of    big   boxes   between   two   adjacent   QRS   complexes   and   divide   300   by   that   number.   So   one   big   box   between   complexes   will   mean   a   heart   rate   of    300;   two   big   boxes   would   be   150;   three   is   100;   four   is   75;   five   is   60;   six   is   50.   This   is   most   easily   done   by   finding   one   complex   that   falls   right   onto   a   darker   line   and   counting   the   number   of    big   boxes   to   the   next   QRS   complex.   If    the   next   complex   falls   between   the   lines   then   you   can   estimate   based   on   the   line   before   and   after.   For   example,   if    I   find   an   R   wave   that   falls   right   on   a   dark   line,   and   the   next   R   wave   falls   between   four   and   five   big   boxes   later,   then   I   know   the   rate   is   between   75   and   60.   If    it’s   closer   to   four   big   boxes   then   I   might   estimate   70,   closer   to   five   big   boxes   I   might   estimate   65.   (300)   /   (#   big   boxes   between   2   adjacent   QRS   complexes)   =   BPM   Rhythm   First,   some   basics.   The   P   wave   is   atrial   de polarization;   the   QRS   complex   is   ventricular   de polarization;   the   T   wave   is   ventricular   re polarization   (atrial   re polarization   occurs   at   the   same   time   as   ventricular   de polarization   and   so   is   lost   in   the   QRS   complex).   When   the   ventricles   depolarize   normally,   current   moves   through   the   His ‐ Purkinje   system,   specialized   cells   that   conduct   electrical   charge   quickly,   like   electrical   wiring   for   the   heart.   Because   they   propagate   that   wave   of    depolarization   quickly,   and   because   time   equals   horizontal   distance   on   ECG,   a   normal   QRS   complex   is   narrow.   In   contrast,   slow   depolarization   causes   a   widened   QRS   complex   because   time   is   on   the   x ‐ axis   of    an   ECG,   as   stated   above.   For   instance,   when   depolarization   starts   in   the   ventricular   myocardium   and    spreads   from   myocyte   to   myocyte   it   is   not   using   the   His ‐ Purkinje   system   and   so   discharge   propagates   more   slowly.   The   other   cause   for   a   widened   QRS   is   a   block   in   the   His ‐ Purkinje   system,   delaying   depolarization   and   widening   the   complex.   Sinus   rhythm   is   a   regular    rhythm   with   a   P    wave   occurring   before   every    QRS   complex  .   These   are   heartbeats   that   srcinate   in   the   SA   node.   Junctional   rhythm   is   an   impulse   that   begins   in   the   AV   node   rather   than   the   SA   node.   If    the   SA   node   is   diseased   or   ischemic   it   will   not   function   properly,   causing   the   AV   node   to   have   to   take   over   pacemaking   duties.   The   AV   node   has   an   intrinsic   rate   that   is   slower   than   the   SA   node:   40    –   60   bpm   (vs.   the   SA   node   which   is   60    –   100).   These   complexes   don’t   usually   have   a   P   wave   because   the   charge   starts   in   the   AV   node   and   goes   down   to   the   ventricles.   At   the   same   time,   the   current   does   go   back   up   to   the   atria,   but   because   this   occurs   at   the   same   time   as   the   ventricular   depolarization,   the   P   wave   is   lost   in   the   QRS   complex.   If    the   current   gets   to   the   atria   slightly   before   the   ventricles,   you   may   see   inverted   (because   the   wave   of    depolarization   is   going   up,   rather   than   down)   P   waves   immediately   before   the   QRS.   So   on   the   ECG   you’ll   see   slow,   regular,   narrow    QRS   complexes   with   no   P    waves   before   them   or    inverted    P    waves .   An   idioventricular   rhythm   srcinates   in   the   myocardium   of    the   ventricles   themselves.   In   cases   where   the   SA   node   and   the   AV   node   are   both   diseased   or   ischemic   and   not   generating   impulses,   the   ventricular   myocytes   have   to   take   over.   Their   intrinsic   rate   is   20    –   40   bpm.   Because   the   charges   are   not   traveling   through   the   His ‐ Purkinje   system   these   complexes   will   usually   be   wide   and    bizarre   looking   and    the   rate   will    be   slow  .   Atrial   fibrillation:   in   this   rhythm   the   atria   have   uncoordinated   electrical   activity,   kind   of    like   they're   having   a   seizure.   The   AV   node   is   getting   bombarded   with   electrical   signals   because   of    all   this   electrical   activity.   When   the   AV   node   resets   from   the   previous   firing,   it   is   ready   to   fire   again   and   will   let   the   next   charge   it   receives   through,   which   is   then   conducted   down   to   the   ventricles.   Because   the   charge   goes   through   the   AV   node,   the   rest   of    the   charge   is   conducted   normally,   propagating   down   the   His ‐ Purkinje   system.   As   a   result   the   QRS   complexes   are   narrow   (i.e.   normal).   Every   time   the   AV   node   resets   it   waits   for   the   next   current,   which   starts   the   cycle   over   again.   Because   the   electrical   activity   in   the   atria    is   uncoordinated   and   unpredictable   the   next   charge   could   occur   in   one   millisecond   or   1000.   Thus,   the   frequency   with   which   a   charge   is   sent   down   to   the   ventricles   to   actually   generate   a   heartbeat   is   irregular.   So   what   you   see   is   a   wavy    baseline,   caused    by    the    fibrillating   atria,   and    irregular,   narrow    QRS   complexes .   SVT:   this   is   a   pretty   nondescriptive   name.   SVT   stands   for   supraventricular   tachycardia,   which    just   means   any   tachycardia   (rate   greater   than   100)   caused   by   discharges   above   the   ventricles.   So   anything   fast   and   coming   from   above   the   ventricles   would   qualify:   sinus   tachycardia,   rapid   a ‐ fib,   etc.   What   we   usually   mean   when   we   say   SVT   is   actually   AVNRT:   AV ‐ nodal   reentry   tachycardia.   The   idea   is   that   you   have   a   pathway   in   or   around   the   AV   node,   which,   in   addition   to   conducting   the   charge   down   the   length   of    the   ventricles,   also   loops   back   up   towards   the   AV   node.   So   when   the   AV   node   fires   most   of    the   charge   continues   down   to   the   ventricles   and   make   them   fire   normally,   but   some   charge   will   ride   that   looping   pathway   back   around.   If    the   AV   node   is   ready   to   fire   again   that   charge   will   set   it   off    earlier   than   usual   and   you   get   a   faster   cycle:   the   AV   node   fires,   the   charge   loops   around   and   makes   it   fire   again.   This   can   result   in   heart   rates   of    over   200.   So   think   about   what   we'd   see   on   ECG.   P   waves?   Probably   not   since   the   charge   is   coming   from   the   AV   node   so   this   is   like   a    junctional   rhythm.   Since   the   charge   starts   in   the   AV   node   it   depolarizes   the   ventricles   the   normal   way   but   it   depolarizes   the   atria   going   up .   So   you   might   see   an   inverted   P   wave   right   before   the   QRS   (closer   than   usual),   you   might   see   no   P   waves   (because   it   is   lost   in   the   QRS   complex),   or   you   might   see   a   little   P   wave   after   the   QRS.   The   QRS   will   be   narrow   (normal)   since   depolarization   is   occurring   down   the   His ‐ Purkinje   system.   Classically    this   will    be   a   very     fast    rate   with   narrow    QRS   complexes   and    no   P    waves .   In   the   example   below   the   small   waves   between   the   QRS   complexes   are   T   waves,   not   P   waves.    Ventricular   tachycardia:   this   occurs   when   an   irritated   area   of    myocardium   somewhere   in   the   ventricles   starts   to   fire   on   its   own,   without   waiting   for   a   charge   from   the   His ‐ Purkinje   system.   Irritation   can   be   caused   by   ischemia,   electrolyte   imbalance,   trauma,   or   other   reasons.   The   myocardium   starts   firing   and    just   keeps   going.   Because   the   current   starts   in   the   ventricular   myocardium,   the   charge   is   going   to   spread   myocyte   to   myocyte   rather   than   down   the   His ‐ Purkinje   system.   So   on   ECG   you'll   see   wide   QRS   complexes   occurring   at    a   very     fast    rate .   I   think   that   it   often   looks   like   a   bunch   of    McDonalds   signs   in   a   row.   Ventricular   fibrillation:   this   is   uncoordinated   electrical   activity   in   the   ventricles,    just   like   a   seizure.   There   is   no   coordinated   electrical   activity   at   all   and,   hence,   no   contraction.   On   ECG   this    just   looks   like   a   wavy    line .   Asystole:   this   is   nothing,   no   electrical   activity   at   all.   On   ECG   this   looks   like   a   simple   straight    line .    Axis   Mean   electrical   axis   (MEA)   is   probably   the   most   difficult   part   of    ECG   interpretation   to   teach.   So   I’ll   give   you   my   quick   and   dirty   method   as   well   as   my   more   formal   way.  
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