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Bectrocardiogram: I

CARDIOYASCULAR PHYSIOLOGY

Normal $cqucncc ot Cardiac Depolarization and Repolari/ation and l)erivation of ECG

A. Impulse origin and atrial depolarization

Impulse nriginates at SA norie, and wave ol depolarization spreads over atria. resulting in electrical vector directed downward and to left. This causes upward (po$itive) deflection in ECG tracing in leads I and aVF (P wave)

ReCOfding axis uf lead I {horizontal. right to leń)

Recording axis of lead I {horizontal, nght to leń)

Recording axis ol lead aVF {vertical downward)

Common bundle    Right and left

of His    bundle brancbes

Resultant of electrical activity

B. Septal depolari/atinn

After brief delay at AV node, impulse traverses common bundle of His and right and left bundle branches and then enters interventricular septum, < ausing myocardial depolarization with electrical vector directed to right and downward. This results in smali negathre {downward) deflection in lead I (Q wave) and positive (upward) deflection in lead aVF (R vvave)

Figurę 4.6 Cardiac Depoi arization and Repolarization Part 1

axis of lead aVF Wertical, downward)

^r«M

The elements of the electrocardiogram (ECG) are shown in the next three figures. Beginning with the SA imhIh (pacemaker), depolarization spreads over the atria, causing an upward deflection of the ECG tracing (P wave) (panel A). The delay at the AV node ensures that the ventricle$ will have ample time to fili, and then the impulse passes

through the bundle of His and the bundle branches of the interven tricular septum. The resulting depolarization of the myocardium of the septum yields the Q wave of the ECG tracing (panel B).

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