An electrocardiogram or ECG can provide a "snapshot" of your heart’s performance and show irregularities in your heart’s rhythm or rate by recording the heart’s electrical activity. The ECG machine is composed of three main parts: electrodes, oscilloscope and a recorder.
To chart the heart’s electrical impulses, electrodes (receivers of current) on the chest, arms and legs are connected to a cathode ray oscilloscope that records the passage of electrical energy from the heart by sending beams of electrons across a graph. This provides an immediate visible record of electrical activity within the heart. The voltage of these impulses is only a matter of millivolts, yet it is enough to excite heart muscle.
On a normal ECG, each vertical line on the graph represents one-fifth of a second in time, and each horizontal line indicates one-half of a millivolt.
The first part of the ECG, the P-wave, indicates the discharge of electrical energy by the sinoatrial node (natural pacemaker). This discharge of electrical energy swiftly fans out over the right and left atria. This is called depolarization. The P wave thus reflects the activation of the atria. The QRS complex follows the subsequent depolarization of the atrioventricular node and reflects the journey of the electrical current through the conduction system of the heart and into the muscular walls of the right and left ventricles. Measurement of the interval between the P wave and the R wave indicates the length of time between the discharges of the sinoatrial node and the atrioventricular node.
There are several waves within the QRS complex. The first downward or negative deflection is called the Q wave; the initial upward or positive deflection is called the R-wave; and the second negative deflection is called the S wave.
The T-wave indicates how long it takes for the recovery or repolarization of the ventricles. From the Q to T-wave, the physician can observe the duration, excitement, ventricular contraction, and recovery of the heart.
By closely observing each of the segments and waves, the physician can see irregularities in the rhythm of the heart. Sometimes, a patient will be asked to run in place (Exercise Stress
Test) so that the electrocardiogram can show the effect of exercise on the heart.