Electrical Conduction in the Heart

8/14/2019 Electrical Conduction in the Heart

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Electrical conduction in the

Heart The Sinoatrial node (SA node), is a group of autorhythmic cells (main

pacemaker of the heart) in the right atrium near the entry of the superior

vena cava.

An internodal pathway connects the SA node to the atrioventricular

node (AV node), a group of autorhythmic cells found near the floor of

the right atrium. From the AV node action potentials move into fiber known as the

bundles of his or atrioventricular bundle. The bundle passes from the

AV node into the wall of the septum between the ventricles.

A short way down the septum the bundle divides into left and right

bundle branches.

These fibers continue downward to the apex where they divide into many

small purkinje fibers that spread outward among the contractile cells.


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If the electrical signals from the atria were conducted directly

into the ventricles, the ventricles would start to contraction at

the top. Then the blood would be squeezed downward andtrapped at the bottom of the ventricle.

The apex to base contraction squeezes blood toward the

arterial opening at the base of the heart.

The AV node also delays the transmission of action

potentials slightly, allowing the atria to complete their

contraction before the ventricles begin their contraction.

This AV node delay is accomplished by slowing conductionthrough the AV node cells.


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Electrocardiogram (ECG) Composite of all action potentials of nodal and

myocardial cells detected, amplified and recordedby electrodes on arms, legs and chest


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Normal Electrocardiogram (ECG)


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1)atria begin todepolarize

2) atria depolarize

3)ventricles begin to

depolarize at apex;

atria repolarize

4)ventricles depolarize

5) ventricles begin torepolarize at apex

6) ventricles repolarize

Electrical Activity of Myocardium


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Diagnostic Value of ECG

Invaluable for diagnosing abnormalities in

conduction pathways, MI, heart

enlargement and electrolyte and hormone

imbalances


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ECGs, Normal & Abnormal

No P waves


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ECGs, Abnormal

Arrhythmia: conduction failure at AV node

No pumping action occurs


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Cardiac Cycle

One complete contraction and relaxation

of heart

Atrial systole

Atrial diastole

Ventricle systole

Ventricle diastole Quiescent period


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Opposing pressures

always positive blood

pressure in aorta, holds

aortic valve closed

ventricular pressure must

rise above aortic pressure

forcing open the valve

Change in volume creates a pressure

gradient

Principles of Pressure and Flow

Measurement: compared to force

generated by column of mercury (mmHg)- sphygmomanometer


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Heart Sounds

Auscultation - listening to sounds made by

body

First heart sound (S1), louder and longer

lubb, occurs with closure of AV valves

Second heart sound (S2), softer and

sharper dupp occurs with closure of

semilunar valves

S3 - rarely heard in people > 30


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Phases of Cardiac Cycle

Quiescent periodQuiescent period

all chambers relaxedall chambers relaxed

AV valves openAV valves open

blood flowing into ventriclesblood flowing into ventricles

Atrial systoleAtrial systole

SA node fires, atria depolarizeSA node fires, atria depolarize

P wave appears on ECGP wave appears on ECG

atria contract, force additional blood intoatria contract, force additional blood into

ventriclesventricles

ventricles now contain end-diastolicventricles now contain end-diastolic

volume (EDV) of about 130 ml of bloodvolume (EDV) of about 130 ml of blood


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Isovolumetric Contraction of

Ventricles

Atria repolarize and relax

Ventricles depolarize

QRS complex appears in ECG Ventricles contract

Rising pressure closes AV valves

Heart sound S1 occurs No ejection of blood yet (no change in

volume)


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Ventricular Ejection

Rising pressure opens semilunar valves

Rapid ejection of blood

Reduced ejection of blood (less pressure)

Stroke volume: amount ejected, about 70 ml SV/EDV= ejection fraction, at rest ~ 54%, during

vigorous exercise as high as 90%, diseased heart

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Electrical Conduction in the Heart