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CARDIOvASCuLAR ``CARdIOvASCulAR—PHYSIOlOGY CARDIOvASCuLAR ``CARdIOvASCulAR—PHYSIOlOGY SECTION III 293

Electrocardiogram Conduction pathway: SA node Ž atria P wave—atrial depolarization.
Ž AV node Ž bundle of His Ž right and PR interval—time from start of atrial
left bundle branches Ž Purkinje fibers depolarization to start of ventricular
Ž ventricles; left bundle branch divides into depolarization (normally 120-200 msec).
left anterior and posterior fascicles. QRS complex—ventricular depolarization
SA node—located at junction of RA and SVC; (normally < 100 msec).
“pacemaker” inherent dominance with slow QT interval—ventricular depolarization,
phase of upstroke. mechanical contraction of the ventricles,
AV node—located in posteroinferior part of ventricular repolarization.
interatrial septum. Blood supply usually T wave—ventricular repolarization. T-wave
from RCA. 100-msec delay allows time for inversion may indicate ischemia or recent MI.
ventricular filling. J point—junction between end of QRS complex
Pacemaker rates: SA > AV > bundle of His/ and start of ST segment.
Purkinje/ventricles. ST segment—isoelectric, ventricles depolarized.
Speed of conduction: His-Purkinje > Atria > U wave—prominent in hypokalemia (think
Ventricles > AV node. He Parks At Ventura hyp“U”kalemia), bradycardia.
Avenue.

5 mm +1.0
0.2 seconds +1.0
5 mm
Aorta R 0.2 seconds
Aorta R
Superior
vena cava Superior P-R S-T
vena cava Bachmann segment P-R segment S-T +0.5
+0.5
segment
segment
bundle Bachmann
SA node SA node bundle
P P T T
AV node AV node J point J point U U
Left bundle Left bundle 0 mV
0 mV
branch branch Q
Bundle of His Bundle of His Q
S S
QRS
P-R
P-R interval interval
QRS
interval interval Q-T interval –0.5
Right bundle Left anterior Q-T interval
Right bundle branch Left anterior –0.5
fascicle
branch fascicle
Purkinje fibers Atrial Ventricular Ventricular
Purkinje fibers Left posterior Atrial depolarization depolarization Ventricular
repolarization
Ventricular
Left posterior depolarization depolarization repolarization
fascicle
fascicle

FAS1_2019_07-Cardio.indd 293 11/7/19 4:24 PM