The electrocardiograph (ECG) complex consists of five waveforms labeled with the letters P, Q, R, S, and T. In addition, sometimes a U wave appears.
P wave: Represents atrial depolarization (conduction of the electrical impulse through the atria); the first component of ECG waveform.
PR interval: Tracks the atrial impulse from the atria through the AV node, from the SA node to the AV node. Measures from the beginning of the P wave to the beginning of the QRS complex. Normal PR is 0.12 to 0.2 seconds.
QRS complex: Follows the PR interval and represents depolarization of the ventricles (the time it takes for the impulse to travel through the bundle branches to the Purkinje fibers) or impulse conduction and contraction of the myocardial cells (ventricular systole). 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. Normal QRS is 0.06 to 0.1 seconds.
ST segment: Represents the end of ventricular conduction or depolarization and the beginning of ventricular recovery or repolarization; the J point marks the end of the QRS complex and the beginning of the ST segment.
T wave: Represents ventricular recovery or repolarization.
QT interval: Measures ventricular depolarization and repolarization; varies with the heart rate (i.e., the faster the heart rate, the shorter the QT interval); extends from the beginning of the QRS complex to the end of the T wave. Normal QT is ;0.4 seconds but can vary with heart rate.
U wave: Represents the recovery period of the Purkinje fibers or ventricular conduction fibers; not present on every rhythm strip.
Normal ECG waveforms, intervals, and correlation with events of the cardiac cycle.
Normal ECG waveforms, intervals, and correlation with events of the cardiac cycle.