Waveform and Waves

Suppose we have our patient hooked up to our cardiac monitor and the machine is set to record lead II. Here is what we hope to see on the screen:

The waveform is the overall electrical activity of the heart during one cardiac cycle. What we want to see is a single repeating waveform consistent in shape and size. If you isolate one of those waveforms, you'll notice that each waveform is comprised of a series of waves and phases that comprise the different parts of the cardiac cycle.

Isoelectric Line

The isoelectric line is the baseline of the ECG waveform and represents no electrical activity in the heart at that time. It typically occurs after the cardiac muscles depolarize or repolarize.

The isoelectric line for a waveform is based on the TP Interval, which is the segment before the P wave and after the QRS complex - in other words, between two consecutive waveforms.

P Wave

The P wave represents atrial depolarization (and contraction). It's typically small, upright, and rounded.

PR Interval

The PR interval is the time from the start of the P wave to the start of the QRS complex. It's normally around 0.12 to 0.20 seconds.

The PR interval represents the time for the electrical impulse to depolarize the atria and travel to the ventricles. Physiologically, this represents blood getting pumped from the atria and filling the ventricles.

QRS Complex

The QRS complex represents ventricular depolarization (and contraction). It's typically larger than the P or T wave, but very narrow - typically less than 0.12 seconds. This is because of the large muscle mass of the ventricles compared to the atria, and the coordinated electrical pathways of the Bundle of His, Bundle Braches, and Purkinje fibers that allow for rapid depolarization of the ventricles.

ST Segment

The ST segment is the time from the end of the QRS complex to the start of the T wave, and represents the delay from depolarization to repolarization. Because of this delay, there's no electrical activity occuring during this time and the ST segment is "flat" or at the isoelectric line. An ST segment above the isoelectric line is elevated, while an ST segment below the isoelectric line is depressed. There's usually a 1 mm variance allowed for the ST segment before it's considered elevated or depressed.

T Wave

The T wave represents ventricular repolarization (and relaxation). It's generally upright and rounded, and larger than the P wave. In lead II, it's usually no larger than 0.5 mV in height.

QT Interval

The QT interval is the time from the start of the QRS complex to the end of the T wave, and represents the time for the ventricles to depolarize and repolarize. The "normal" QT interval depends on the standard, but the usual stated range is 0.36 to 0.44 seconds. As the QT interval increases, the risk of ventricular arrhythmias increases, especially if the QT interval is longer than 0.50 seconds.