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Electricity Of The Heart
ABBAS SHAREEF
Introduction – Cardiac Conduction
 The heart has a specialized cardiac conduction
system which spreads to all parts of the myocardium
 Depolarization – Change in membrane potential
from a more negative state to a more positive state
 Repolarization – change in membrane potential
from a positive state to a more negative state
Introduction – Sequence Of Heart Contraction
 Atrial Systole, contraction of the atria
 Atrial Systole is followed by ventricular systole,
contraction of the ventricles
 During diastole all four chambers are relaxed
 Structures that make up conduction system:
sinoatrial node(SA node), internodal atrial pathways,
the atrioventricular node, the bundle of His, its
branches, and the Purkinje system.
Introduction
 Due to its rapid discharge, the SA Node is the normal
cardiac pacemaker. Its rate of discharge determines
the rate at which the heart beats.
 Impulse from SA node pass through atrial pathways
to and through the AV node, to the bundle of His,
through the branches of the bundle of His via
Purkinje system to the ventricular muscle.
Locations
 The SA Node is located at the junction of superior
vena cava with the right atrium
 The AV node is located in the right posterior portion
of the interatrial septum
 Bundle Of His runs from the AV Node to fascicular
branches which leads to Purkinje Fibers
 Purkinje Fibers are located in the inner ventricular
walls of the heart
Path Of Conduction
Spread Of Cardiac Excitation
 Depolarization initiated in SA node and converges
onto AV Node (Atrial Depolarization).
Time Length – completed in .1 s
 Conduction in AV Node is slow, which causes a delay
of .1 s (AV nodal delay), before excitation spreads to
the ventricles.
 From the top of the septum, a wave of depolarization
spreads to the purkinje fibers from .08 - .1 s.
Electrocardiogram
 Electrocardiogram
Electro – electricity
Cardio – heart
Gram – Record
Electrocardiogram is a record of electricity of the
heart.
Waves Of Electrocardiogram
 The normal ECG shows 5 waves named as P,Q,R,S,T
 The P wave represents the impulse from the SA node
sweeping over the atria (atrial depolarization)
 The QRS complex represents the spread of impulse
from the AV node to Av bundle (bundle of His) and
purkinje fibers(ventricular depolarization)
 The T wave represents relaxation of the muscles
(ventricular repolarization)
Clinical Relevance
 Complete Third Degree Heartblock – occurs when
conduction from atria to the ventricles is completely
interrupted and ventricles beat at a low rate
(idioventricular rhythm)
 Causes of Third Degree Heartblock – disease of the
AV node or in conducting system below the
node(infranodal block)
 In patients with AV nodal block the nodal tissue
becomes the pacemaker and the rate of
idioventricular rhythm is approximately 45
beats/min.
Clincal Revelance
 Increased Automaticity – present when His-Purkinje
fibers or the myocardial fibers discharge
spontaneously
 If an ectopic focus discharges once, this results in a
beat that occurs before the expected next normal
beat which interupts the normal cardiac rhythm
(extrasystole or premature beat). If the focus
discharges repetitively at a higher rate than that of
SA node, it produces rapid, regular tachycardia.
Clinical Revelance
 Atrial Arrhytmias - Excitation spreading from an
independent focus in atria stimulates AV node
prematurely and is conducted to the ventricles.
The P waves of atrial extrasystoles are abnormal,
which may depolarize the SA node, which must
repolarize and then depolarize to the next level
before it can initiate the next normal beat.
 Results in a pause between normal beats preceding
the extrasystole
 Results in low cardiac output and symptoms are of
heart failure appear
Clinical Revelance – Long QT Syndrome
 Patients in whom QT interval is prolonged, cardiac
repolarization is irregular
increases incidence of ventricular arrhythmias and
sudden death increases
 Caused by number of different drugs, electrolyte
abnormalities, and by myocardial ischemia
PHSYIOLOGY IS FUN!!!!!