The electrocardiogram (ECG or EKG) is a noninvasive test that is used to reflect underlying heart conditions by measuring the electrical activity of the heart. By positioning leads (electrical sensing devices) on the body in standardized locations, information about many heart conditions can be learned by looking for characteristic patterns on the ECG.
Electrodes, or leads, attached to the chest, neck, arms, and legs record the pathway of electrical impulses through the heart muscle.
Drawing of the EKG, with labels of intervals P=P wave, PR=PR segment, QRS=QRS complex, QT=QT interval, ST=ST segment, T=T wave. 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, although it can deviate to the right in very tall people and to the left in obesity. Extreme deviation is abnormal and indicates a bundle branch block, ventricular hypertrophy or (if to the right) pulmonary embolism. It also can diagnose dextrocardia or a reversal of the direction in which the heart faces, but this condition is very rare and often has already been diagnosed by some other investigation (such as a chest x-ray).
The P wave is the electrical signature of the current that causes atrial contraction. Both the left and right atria contract simultaneously. Irregular or absent P waves may indicate arrhythmia. Its relationship to QRS complexes determines the presence of a heart block.
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 Q wave, when present, represents the small horizontal (left to right) current as the action potential travels through the interventricular septum. Very wide and deep Q waves do not have a septal origin, but indicate myocardial infarction.
The R and S waves indicate contraction of the myocardium. Abnormalities in the QRS complex may indicate bundle branch block (when wide), ventricular origin of tachycardia, ventricular hypertrophy or other ventricular abnormalities. The complexes are often small in pericarditis.
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. Electrically, the cardiac muscle cells are like loaded springs. When a small impulse sets them off, they depolarize and contract. Setting the spring up again is repolarization (more at action potential).
In most leads, the T wave is positive. Negative T waves can be signs of disease, although an inverted T wave is normal in V1 (and V2-3 in black people).
The ST segment connects the QRS complex and the T wave. It can be depressed in ischemia and elevated in myocardial infarction, and downslopes in digoxin use.
T wave abnormalities may indicate electrolyte disturbance, such as hyperkalemia.
The QT interval is measured from the beginning of the QRS complex to the end of the T wave. The QT intervals, 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 most commonly used method for correcting the QT interval for rate is the one formulated by Bazett and published in 1920. Bazett's formula is ,vascular where qtc is the QT interval corrected for rate, and RR is the interval from the onset of one QRS complex to the onset of the next QRS complex, measured in seconds. However, this formula tends to not be accurate, and over-corrects at high heart rates and under-corrects at low heart rates.