Principles of ECG
The heart consists of conductive tissue, shown here in blue, and muscle or myocardial tissue shown here in red and pink.
Anatomical and physiological components behind ECG
Electrical impulses are transmitted through the heart through specialized conductive cells. Electrical impulses begin in the sinoatrial (SA) node, a group of cells located on the wall of the right atrium at the junction of the SVC and the right atrium. The SA node is sometimes called the "heart's natural pacemaker."
The electrical impulse travels from the SA node to the atrioventricular (AV) node. In response to the impulse, the atrium contracts pushing blood into the ventricles. At the AV node, the electrical impulse is delayed just long enough to allow blood to flow from the atrium and fill the ventricle. The electrical impulse is then transmitted through the bundle of His and down the bundle branches terminating in the Purkinje fibers within the myocardium. Working cells respond to stimulation by contracting.
- SA node is not a single location, but a specialized bundle of cells
- Can differ in size and shape in every patient
- If SA node does not function, there is no P-Wave
On the electrocardigram, the passage of electrical impulses through the heart are traced on a curve.
- The P-Wave signifies the activation of the atria as current flows from the SA node to the AV node and through the atria
- The QRS complex signifies the depolarization and activation of the ventricles
- The T-Wave signifies the recovery or repolarization of the ventricles