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       Heart Blocks
AV Heart Blocks:
  These are a result of conduction
   problems at the AV node/Junction or the
   Bundle of HIS.
  The impulses originate in the SA node,
   but have trouble getting to the
  There are four different types,
   categorized by the severity of
   obstruction at the AV node.
First Degree Heart Block:
 All impulses are conducted, but there is a
  delay before being transmitted to the
  ventricles (therefore it’s not really a true “block”).
 Rhythm: usually regular
 Rate: variable
 P waves: normal; one P for each QRS
 PRI: greater than .20 sec. for each beat
 QRS: all are less than .12 sec.
Clinical significance of 1st Degree AVB:

   Usually no clinical change
   Can occur in young athletes and
    individuals with no underlying heart
   Can also be seen in those taking certain
    meds, such as beta blockers, CA
    channel blockers, and digoxin.
   Observe closely if it occurs post MI
Second Degree Heart Block/
Wenckebach (or Mobitz I):
  Each impulse is delayed progressively
   longer at the AV node, until one impulse
   is blocked completely.
  Rhythm: irregular
  Rate: variable
  P waves: some P waves are not followed
   by a QRS
  PRI: gets progressively longer, then a
   beat is blocked, then it repeats
Clinical Significance of 2nd Degree
AVB (Wenckebach)

    Causes:
     • AV node ischemia, medications (digoxin),
       increased vagal tone, MI.
  Can be transient….will come and go,
   then come back again.
  Treatment is based on symptoms (if
   any) and underlying cause.
Second Degree Heart Block/
Classic (or Mobitz II):
  The AV node selectively chooses to
   either conduct or block individual
   impulses sent from the SA node.
  It usually does this in a pattern, called a
   “conduction ratio”.
  There will always be more P waves than
   QRS complexes (P waves that have no
   QRS to follow them)
2nd Degree Heart Block (type II)

  Rhythm: if the conduction ratio is
   consistent, the rhythm is regular. If not,
   the rhythm is irregular.
  Rate: the atrial rate is usually normal,
   the ventricular rate is often slow.
  P waves: there are always more P
   waves than QRS complexes
  PRI: consistent for those beats
   conducted (beats with a QRS)
2nd degree AV Block (typeII)
  QRS complex: all look alike and will
   either be normal (<.12) or wide.
  Clinical significance:
     • Acute MI, Septal wall necrosis, acute
       myocarditis, CAD
     • Patient may or may not be symptomatic. It
       will usually depend on ventricular rate.
     • Treatment depends on symptoms and
       underlying cause
Complete or Third Degree Heart
  All of the impulses generated by the
   SA node are blocked at the AV node.
  Either a junctional focus or a
   ventricular focus takes over to pace
   the ventricles.
  The SA node is functioning normally,
   but none of those impulses can get
   through to the ventricles, so some
   other pacemaker site (either junctional
   or ventricular) takes over.
Third Degree Heart Block:
  Rhythm: regular
  Rate: atrial rate (P to P) is usually
   normal. Ventricular rate (R to R) is 40-
   60 if junctional paced, 20-40 if
   ventricular paced.
  P waves: normal, but more P waves
   than QRS complexes.
  PRI: variable, but no pattern. No
   relationship between P’s to QRS
Third Degree Heart Block:
  QRS: less than .12 sec. if junctional
   paced, greater than .12 sec. if
   ventricular paced.
  The P waves can occasionally be
   hidden within the QRS complex, or seen
   as part of the T waves, or as part of the
   QRS complex.
Clinical significance of 3rd degree AVB:

   Is the most serious type of heart block,
    because it can progress to asytole.
   Patient will most likely show signs and
    symptoms of low cardiac output or
    hypoperfusion….it depends on the
    ventricular rate and if it is junctional or
    ventricular paced.
   Pacemaker is usually indicated.
Bundle Branch Blocks (Ventricular

  Normally, an impulse travels
   simultaneously through the right bundle
   branch and the left bundle branch,
   causing depolarization of both the right
   and left ventricles.
  A bundle branch block represents an
   abnormal conduction of the impulse
   through either the right or left bundle
Bundle Branch Blocks:
  One ventricle depolarizes before the
   other, which will result in a widening of
   the QRS complex.
  All QRS complexes are >.12 seconds
  The cause of the block (underlying
   disease) is what is important, not the
   block itself.
Bundle Branch Blocks
    The impulse origin in a bundle branch block is
     either sinus (atrial) or junctional, therefore,
     each beat will still have a P wave!
    Remember…all that is happening is that each
     impulse is blocked, therefore is delayed
     through one of the ventricles.
    You can determine a BBB with a single lead,
     but to determine a Left or Right BBB, you
     need a 12-Lead ECG
Rules for a Bundle Branch Block
    Rhythm: regular or irregular
    Rate: rate is that of underlying rhythm
     (usually sinus, but it can be anything)
    P waves: usually positive (and each beat will
     have one!)
    PR: usually normal (.12-.20 seconds)
    QRS: Wide, or .12 seconds or greater
    Bundle Branch blocks can (and do) occur in
     the presence of other arrhythmias
Right Bundle Branch Block:
  Right ventricular depolarization occurs
   AFTER the left.
  QRS complexes are > .12 sec.
  Small R waves in V1 and V2
  rSR pattern (“rabbit ears”) in Leads V1
   and V2
  Small Q wave and large S wave in V6
Left Bundle Branch Block:
  May indicate significant myocardial
  All QRS complexes are > .12 seconds
  Deep, wide S waves in Leads V1-3
  rSR patter (“rabbit ears”) in V4-6, no Q
   or S waves in V6
  LBBB will make it difficult to diagnose
   myocardial infarction
When to worry about BBB:
  If it develops as a result of a myocardial
  If it is combined with a 2nd or 3rd degree
   heart block
  If it is a left BBB combined with 1st
   degree heart block

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