The electrodes placed on the skin are positive terminals attached to lead wires.
When P waves are absent or of abnormal size or shape, it is likely that the impulse originated outside the SA node. The period from the start of the P wave to the beginning of the QRS complex is called the PR interval.
During this interval, which normally does not exceed 0.20 second, the impulse traverses the atria and the AV node. The QRS complex represents depolarization of the ventricular muscle, and consists of an initial downward deflection (Q wave), a large upward deflection (R wave), and a second downward deflection (S wave).
Upward spikes are called “positive” deflections; downward spikes are called “negative” deflections.
As the positive wave of depolarization within the heart cells moves toward a positive (skin) electrode, there is a positive (upward) deflection recorded on EKG.
The advancing wave of positive charge in depolarization creates a positive deflection on EKG when this wave is moving toward a positive skin sensor. The P wave represents the electrical activity associated with the impulse generation at the SA node and its subsequent spread through the atria.
When the P wave is of normal size and shape, it is assumed that the impulse began in the SA node.
The electrical stimulus originating from the SA node proceeds away from the node concentrically in all directions.
This electrical impulse spreads across the atria, the atria contract, and a P wave is recorded on the EKG.
This total QRS complex represents the electrical activity of ventricular contraction. There is a pause after the QRS complex, then a T wave appears.
This pause is the ST segment, a flat piece of baseline between the QRS complex and the T wave.
Repolarization restores the negative charge to the interiors of myocardial cells.