An electrocardiogram (ECG or EKG) is a graphic produced by an electrocardiograph, which records the electrical voltage in the heart in the form of a continuous strip graph. It is the prime tool in cardiac electrophysiology, and has a prime function in screening and diagnosis of cardiovascular diseases.
An ECG is constructed by measuring electrical potential between various points of the body using a galvanometer. There are twelve leads in total. Each, by their nature, record information from particular parts of the heart:
The inferior leads (leads II, III and aVF) look at electrical activity from the vantage point of the inferior region (wall) of the heart. This is the apex of the left ventricle.
The lateral leads (I, aVL, V5 and V6) look at the electrical activity from the vantage point of the lateral wall of the heart, which is the lateral wall of the left ventricle.
The anterior leads, V1 through V6, and represents the anterior wall of the heart, or the frontal wall of the left ventricle.
aVR is rarely used for diagnostic information, but indicates if the ECG leads were placed correctly on the patient.
A typical ECG tracing of a normal heartbeat consists of a P wave, a QRS complex and a T wave. A small U wave is not normally visible.
The axis is the general direction of the electrical impulse through the heart. It is usually directed to the bottom left (normal axis: -30o to +90o), although it can deviate to the right in very tall people and to the left in obesity.
The P wave is the electrical signature of the current that causes atrial contraction.
The QRS complex corresponds to the current that causes contraction of the left and right ventricles, which is much more forceful than that of the atria and involves more muscle mass, thus resulting in a greater ECG deflection.
The T wave represents the repolarization of the ventricles. The QRS complex usually obscures the atrial repolarization wave so that it is not usually seen. T wave abnormalities may indicate electrolyte disturbance, such as hyperkalemia and hypokalemia.
The ST segment connects the QRS complex and the T wave.
The QT interval is measured from the beginning of the QRS complex to the end of the T wave. The QT interval as well as the corrected QT interval are important in the diagnosis of long QT syndrome and short QT syndrome. The QT interval varies based on the heart rate, and various correction factors have been developed to correct the QT interval for the heart rate.
The PR interval is measured from the P wave to the QRS complex. It is usually 0.12 to 0.20 seconds. A prolonged PR indicates a first-degree heart block, while a shorting may indicate an accessory bundle, such as seen in Wolff-Parkinson-White syndrome