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Organ Systems I: Heart and ECGs (Part 3)


P wave:
--depolarization of atria
--terminates when atria are completely depoalarized
--no deflection corresponds to repolarlization of the atria, too unsynchronized to produce ECG wave

QRS wave:
--depolarization of the ventricles
--wave terminates when all ventricle cells depolarized

T wave:
--repolarization of ventricles
--positive wave because repolarization progresses from epicardium to endocardium

Three leads forming a triangle and detect components of a mean electrical axis.
Projection of polarities gives graphical representation of mean vector. Each lead records a different deflection of same event. Alignment of electrodes influences reading of charge, parallel to flow gives greatest deflection, ie highest reading of polarity. Each of 3 leads records a different deflection of same event.

Leads: convention for illus: left is negative, right is positive

"forget AVF"

Progression of Depolarization (two slides)
--initially all cells are positive on outside
--wave of negative depolarization starts in atria building P WAVE, all three leads record this event
--the more parallel a pathway to a lead pair, the greater the deflection
--conduction travels from av node to ventricles, starts on interventricular septum, diagonal arrow indicates polarity across septum, negative deflection creating Q WAVE.
--R wave indicated by arrow thru bottom of heart
--negative depolarization spreads around entire intereior of ventricle and heart is at maximum polarity, still R wave
--next page, arrow to right because still some + on right side of heart, R WAVE coming down, less polarity
--both ventricles are negative, depolarized, so ECG is at zero, no polarity
--left ventricular RE (not DE as it says on slide) shows - on inside and + on outside, + cells have stopped producing action potentials, - cells are still doing it, arrow is down and right, - cells have different membrane properties and inactivate sooner

Mean Electrical Vector
= the average direction in which the dipole of the heart is oriented during the R peak, maximal peak, direction and magnitude (vector quantity) is different from person to person
--typcial mean EV lies btw 0 and 90 degrees on clockwise grid (down & right), a short person's heart is tilted up more, others down, lots of variation
--deviation can indicate hypertrophy/atrophy, MI's, etc:
--left axis deviation is due to left ventricular hypertrophy (less than 0 degrees) or right atrophy
--right axis deviation (greater than 90, down and left) is due to right ventricular hypertrophy

ABNORMAL ECGs (from mild to severe)
--atrial flutter = AV node cannot follow rapid SA activity
--atrial fibrillation = irregular heartbeat, atrial depolarization arises from ectopic sites, not SA node... no P waves.
--blocks = cannot conduct thru AV node = 1st degree AV conduction block increases space between P wave and QRS. AV is pacemaker, so heart rate is slow
--premature complex = ectopic pacemaker sites create extra QRS waves, sometimes negative deflecting peaks
--v tachycardia = reentry paths of ectopic sites in ventricles lead to dissociation of atria and ventricles, repeated large peaks, can become v-fib
--ventricular fibrillation = ventricles flutter but don't pump

REENTRY PATHS = signal loops back
Wolff-Parkinson-White syndrome
--A: very strange ECG, P wave OK, QRS is rounded & diminished. Two APs come in, one ahead of the other, slight desynchronization. Opposite polarity from normal. Delta wave.
--B: P wave occurs AFTER QRS waves. Signal cycles back into AV node after short loop thru ventricle
The accessory pathway into the heart

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