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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