Electrocardiography (EKG)

Examines how Depolarization occurs in the Heart

• If a wavefront of depolarization travels towards the electrode attached to the + input terminal of the ECG amplifier and away from the electrode attached to the - terminal, a positive deflection will result.

• If the waveform travels away from the + terminal lead towards the - terminal, a negative going deflection will be seen.

• If the waveform is travelling in a direction perpendicular to the line joining the sites where the two leads are placed, no deflection or a biphasic deflection will be produced.

ECG examines how depolarization events occur in the heart

•The electrical activity of the heart originates in the sino-atrial node.  The impulse then rapidly spreads through the right atrium to the atrioventricular node.  (It also spreads through the atrial muscle directly from the right atrium to the left atrium.)  This

generates the P-wave

•The first area of the ventricular muscle to be activated is the interventricular septum, which activates from left to right.  This generates the Q-wave

•Next the bulk of the muscle of both ventricles gets activated, with the endocardial surface being activated before the epicardial surface.   This generates the R-wave

•A few small areas of the ventricles are activated at a rather late stage.  This generates the S-wave

•Finally, the ventricular muscle repolarizes. This generates the T-wave

•Since the direction of atrial depolarization is almost exactly parallel to the axis of lead II (which is from RA to LL), a positive deflection (P wave) would result in that lead.

•Since the ventricular muscle is much thicker in the left than in the right ventricle, the summated depolarization of the two ventricles is downwards and toward the left leg: this produces again a positive deflection (R-wave) in lead II, since the depolarization vector is in the same direction as the lead II axis.

•Septal depolarization moves from left to right, the depolarization vector is directed towards the - electrode of lead II (RA), and therefore a negative deflection (Q-wave) is produced.

Electrocardiography