An electrocardiogram, or ECG or EKG, is a recording of the heart's electrical activity as a graph over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body. The graph can show the heart's rate and rhythm. It can also detect enlargement of the heart, decreased blood flow, or the presence of current or past heart attacks. ECG's are inexpensive, non-invasive, quick, and painless.
How it Works
The ECG device detects and amplifies the tiny electrical changes on the skin that are caused when the heart muscle depolarizes during each heartbeat. At rest, each heart muscle cell has a negative charge across its outer wall. Decreasing this negative charge towards zero via the influx of the positive cations, Na+ and Ca++, is called depolarization, which activates the mechanisms in the cell that cause it to contract. During each heartbeat, a healthy heart will have an orderly progression of a wave of depolarization that is triggered by the cells in the sinoatrial node, spreads out through the atrium, passes through the atrioventracular node, and then spreads all over the ventricles. This is detected as tiny rises and falls in the voltage between electrodes placed on either side of the hear. It is displayed as a wavy line either on a screen or on paper. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle.
ECGs detect the electrical charges from the heart using electrodes, also known as leads. The electrodes are attached to the skin and to the ECG device , which translates the electrical pulses of the heart into a linear representation.
Waves and Intervals
A typical ECG tracing of the cardiac cycle (heartbeat) consists of a P wave, a QRS complex, and a T wave . An additional wave, the U wave, is visible in 50 to 75% of ECGs.
The P wave represents the movement of the electrical signal from the sinoatrial node to the atrioventricular node, spreading from the right atrium to the left atrium during atrial depolarization .
The QRS complex represents the rapid depolarization of the left and right ventricles. The QRS complex has a larger amplitude than the P wave because the ventricles have more muscle mass than the atria.
The T wave represents the repolarization of the ventricles. The atria repolarize while the ventricles are depolarizing, so the large QRS complex blocks the reading.
The U wave is hypothesized to be caused by the repolarization of the interventricular septum.
Intervals between the waves on an ECG reading can be used for diagnosing heart issues. The space between successive R peaks is known as the RR interval. This interval represents the heart rate. The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The PR interval reflects the time the electrical impulse takes to travel from the sinus node through the AV node and enter the ventricles. The PR interval is, therefore, a good estimate of AV node function. The ST segment is the space between the end of the S wave and the beginning of the T wave, which represents the amount of time taken for the ventricles to repolarize. Changes in the ST segment can be diagnostic of a myocardial infarction. The QT interval is measured from the beginning of the QRS complex to the end of the T wave. A prolonged QT interval is a risk factor for ventricular tachyarrhythmias and sudden death.