Table 1: The waves and the intervals to interpret the electrical activity of the hearts.

FeatureDescriptionDuration

RR intervalThe interval between an R waveand the next R wave. Normal resting heart rate is between 60 and 100 bpm0.6 to 1.2 s
P waveDuring normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node and spreads from the rightatriumto the left atrium. This turns into the P wave on the ECG 80 ms
PR intervalThe 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 entering the ventricles. The PR interval is therefore a good estimate of AV node function 120 to 200 ms
PR segmentThe PR segment connects the P wave and the QRS complex. The impulse vector is from the AV node to the bundle of His to the bundle branches and then to the Purkinje Fibers. This electrical activity does not produce a contraction directly and is merely traveling down towards the ventricles and this shows up flat on the ECG. The PR interval is more clinically relevant50 to 120 ms
QRS complexThe QRS complex reflects the rapid depolarization of the right and left ventricles. They have a large muscle mass compared to the atria and so the QRS complex usually has much larger amplitude than the P wave80 to 120 ms
J-pointThe point at which the QRS complex finishes and the ST segment begins. Used to measure the degree of ST elevation or depression present N/A
ST segmentThe ST segment connects the QRS complex and the T wave. The ST segment represents the period when the ventricles are depolarized. It is isoelectric 80 to 120 ms
T waveThe T wave represents the repolarization (or recovery) of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as theabsolute refractory period. The last half of the T wave is referred to as therelative refractory period(or vulnerable period) 160 ms
ST intervalThe ST interval is measured from the J point to the end of the T wave 320 ms
QT intervalTheQT intervalis measured from the beginning of the QRS complex to the end of the T wave (<0.4 sec = 400 ms). A prolonged QT interval is a risk factor for ventricular tachyarrhythmias and sudden death. It varies with heart rate and for clinical relevance requires a correction for this, giving the QTc 300 to 430 ms
U waveThe U wave is hypothesized to be caused by the repolarization of the interventricular septum. They normally have a low amplitude, and even more often completely absent. They always follow the T wave and also follow the same direction in amplitude. If they are too prominent we suspect hypokalemia, hypercalcemia, or hyperthyroidism usually
J waveThe J wave, elevated J-Point or Osborn Wave, appears as a late delta wave following the QRS or as a small secondary R wave. It is considered pathognomonicof hypothermiaorhypocalcemia