An electrocardiogram records the electrical activity of the heart each time it contracts. In this procedure, skin patches with wires (electrodes) are placed the chest. The set of electrodes measure electrical activity, which is recorded on a special ECG paper. The ECG helps determine the direction of the electrical waves inside the heart as well as any evidence of heart chamber enlargement or if the muscular wall of the right or left ventricle are thickened (right or left ventricular hypertrophy).
In a normal heart, the electrical waves originate in the sinus node, which is the main natural pacemaker of the heart. It then travels through the atrium into a middle electrical station called the AV node. The electrical wave then moves into the right and left bundle branches and then into the Purkinje cells found in the ventricular heart muscle.
As the electrical wave moves from the upper portion of the heart into the lower portion, it first produces an atrial contraction (P wave in the ECG) and this is followed by a delay in the AV node (PR segment in the ECG).
As the electrical wave keeps moving down the heart, it will produce a ventricular contraction that consists of ventricular systole (R wave in the ECG). Later on, the heart will relax and repolarize (T wave in the ECG).
Conduction disorders, such as heart block, can be easily detected on the ECG. Arrhythmias can also be identified and classified into atrial or ventricular dysrhythmias by the ECG.
The ECG will also help determine how slow or fast the heart is going.