Basics of electrocardiogram (ECG)

The electrocardiogram (ECG or EKG) is primarily a tool for evaluating the electrical events within the heart. The action potentials of cardiac muscle cells can be viewed as batteries that cause charge to move throughout the body fluids. These moving charges currents, in other words – are caused by all the action potentials occurring simultaneously in many individual myocardial cells and can be detected by recording electrodes at the surface of the skin.

In a typical ECG , the first deflection is called the P wave. It corresponds to current flows during atrial depolarization. It generates about 0.2 mv and lasts for 0.1s

The second deflection, the QRS complex, occurs approximately 0.15s later. It is the result of ventricular depolarization. It is a complex deflection because the paths taken by the wave of depolarization through the thick ventricular walls differ from instant to instant, and the current generated in the body fluids change direction accordingly.

The S-T interval represents the time during which the entire ventricular muscle is depolarized.

The final deflection, the T wave , is the result of ventricular repolarization. Atrial repolarization is usually not evident on the ECG because it occurs at the same time as the QRS complex.A typical ECG makes use of multiple combinations of recording locations on the limbs and the chest (called ECG leads).

It is not a direct record of the changes in membrane potential across individual cardiac muscle cells. Instead it is a measure of the currents generated in the extracellular fluid