ECG Waveforms And Components

The electrocardiogram (ECG) is a graphic recording ofelectric potentials generated by the heart.The signals are detected by means of metal electrodes attached to the extremities and chest wall and are then amplified and recorded by the electrocardiograph. ECG leads actually display the instantaneous differences in potential between these electrodes.

The clinical utility of the ECG derives from its immediate availability as a noninvasive, inexpensive, and highly versatile test. In addition to its use in detecting arrhythmias, conduction disturbances, and myocardial ischemia, electrocardiography may reveal other findings related to life-threatening metabolic disturbances (e.g., hyperkalemia) or increased susceptibility to sudden cardiac death (e.g., QT prolongation syndromes). The widespread use of coronary fibrinolysis and acute percutaneous coronary interventions in the early therapy of acute myocardial infarction has refocused attention on the sensitivity and specificity of ECG signs of myocardial ischemia.

An electrocardiogram (ECG) waveform has three basic components: the P wave, QRS complex, and T wave. These elements can be further divided into the PR interval, J point, ST segment, U wave, and QT interval.

P wave and PR interval

The P wave represents atrial depolarization. The PR interval represents the time it takes an impulse to travel from the atria through the atrioventricular nodes and bundle of His. The PR interval measures from the beginning of the P wave to the beginning of the QRS complex.

QRS complex

The QRS complex represents ventricular depolarization (the time it takes for the impulse to travel through the bundle branches to the Purkinje fibers).

The Q wave appears as the first negative deflection in the QRS complex; the R wave, as the first positive deflection. The S wave appears as the second negative deflection or the first negative deflection after the R wave.

J point and ST segment

Marking the end of the QRS complex, the J point also indicates the beginning of the ST segment. The ST segment represents part of ventricular repolarization.

T wave and U wave

Usually following the same deflection pattern as the P wave, the T wave represents ventricular repolarization. The U wave follows the T wave, but isn't always seen.

QT interval

The QT interval represents ventricular depolarization and repolarization. It extends from the beginning of the QRS complex to the end of the T wave.