Wednesday, March 13, 2013

Cardiac electrophysiology


Cardiac electrophysiology



2 billion heart cells
100,000 beats per day
<50 cells can produce arrythmias

gap junctions = connexins

  • 2 um gaps
  • modulated by Ca, pH, neurotransmitters, etc
anisotropy = longitudinal conduction/transverse conduction = nl = 3

av node slows conduction (giving time for ventricles to fill) - slows even more at higher hr (decremental conduction)  e.g., atrial flutter with atria rate of 300 bpm has ventricular rate of 150 bpm, not 300.

sa node 
  • connected to left atrium by Bachmann's bundle
  • sick sinus syndrome - too slow hr  (may need pacemaker)
bundle branchs
  • left - more robust - essential
  • right - less essential
pace making
  • sa>av>HIS>ventricular escape
  • overdrive suppression = faster pacemaker dominates
EKG
  • bipolar - I,II,III
  • unipolar
    • aVR, aVF, aVL
    • precordial
      • V1, V2  RV
      • V3 - V6 LV
  • mean electrical axis (nl = -30 to +90 degees)
    • determined by vector analysis
    • right axis deviation 
    • left axis deviation

Conduction Intervals

  • PR - from P to Q (since Q may not br present in some leads)
  • P
  • QT - changes with heart rate  - longer with slow hr
    • corrected for hr by Bazett formula  QTc = QT/sq.rt. RR
    • quick method = if QT is > .5 RR, QT is abnormally long
    • long QT interval
      • polymorpic ventricular tachycardia (Torsade de Pointe)
        • hr too high for filling - sudden death
        • may be drug induced (e.g., tricyclic antidepressants)
        • may be channelopathy (genetic)


heart rate


5 big boxes = 1 sec

1 big box = 1/5 sec

instead of memorizing, go from 1 box to next:
if R-R interval = 1 small box, hr is 5 beats/sec = 300 bpm 
if R-R interval = 2 small boxes, hr is 1/2 as fast = 150 bpm
if R-R interval = 3 small boxes, hr is 1/3 as fast = 100 bpm
if R-R interval = 4 small boxes, hr is 1/4 as fast = 75 bpm
if R-R interval = 5 small boxes, hr is 1/5 as fast = 60 bpm
if R-R interval = 6 small boxes, hr is 1/6 as fast = 50 bpm
if R-R interval = 7 small boxes, hr is 1/7 as fast = 43 bpm
if R-R interval = 8 small boxes, hr is 1/8 as fast = 37 bpm

mean electrical axis nl range = -30 to +90 degrees

if aVf and I have positive R wave axis is 0 to + 90 degrees (within nl range)

if aVf is negative and I is positive, could still be normal.  Use lead II to decide.

if I and II are positive, axis is between -30 to 90 degrees = normal.






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