An ECG (electrocardiogram) is a graphic which shows the electrical activity of the heart. The heart muscles create electrical waves by working. These waves can be measured by electrodes that are in contact with the human body.
Accurate ECG interpretation is dependent on recognition and analysis of each of the components of the tracing. In this article, the wave forms which contribute to the ECG are reviewed briefly.
When analyzing any ECG deflection, it is important to point out that the size and configuration of the deflection are affected by the direction from which the electrical events are viewed. Accordingly, points of observation along the electrical axis of a deflection will yield waves which are larger (in either a positive or negative direction) than deflections viewed from observation points more nearly perpendicular to the axis of the wave form.
Some leads are more useful than others for the diagnosis of atrial enlargement and ventricular hypertrophy. It also makes it essential that more than one lead be viewed in the analysis of cardiac arrhythmias, as wave form patterns that are obvious in one lead may be obscure, absent, or uninterpretable in other leads.
The ECG tracing is recorded usually at a rate of 25 mm/sec. Occasionally 50 mm/sec is used to facilitate interval measurement or wave form interpretation.
The P Wave (Atrial Depolarization)
The P wave originates normally from the sinus node, which lies at the junction of the superior vena cava and the right atrium, and proceeds from right to left across the atria. In the coronal plane, right atrial forces are directed anteriorly and left atrial forces both posteriorly and to the left. The P wave is a “composite” wave form, consisting of components originating from both the right and left atria.
The right atrial deflection precedes the left, as one would expect from the discussion above. The P wave, then, is the electrical summation of these two component parts.
The QRS Complex (Ventricular Depolarization)
The QRS complex reflects depolarization of the interventricular septum and both ventricles. Normally, the electrical activation of this portion of the heart proceeds from the AV node to the inferior and left side of the interventricular septum. The septal depolarization from left to right, in a superior direction, is the first (earliest) event in the QRS complex; it represents the Q wave seen in the left precordial (chest) leads of the normal ECG. The remainder of the QRS deflection reflects depolarization of the right and left ventricles.
The nomenclature of this wave form can be confusing:
A Q wave refers to an initial negative deflection.
An R wave is the first positive deflection.
An S wave is a negative deflection following a positive deflection.
The size, axis, and configuration of the QRS complex are useful for the diagnosis of ventricular hypertrophy.
The T Wave (Ventricular Repolarization)
Normally, the T wave axis is similar to the QRS axis because of two cellular electrical factors: cellular repolarization causes a deflection opposite in direction to cellular depolarization, but ventricular repolarization occurs in a sequence opposite to that of depolarization. Therefore, although the direction of repolarization is opposite to that of depolarization, the cellular (and, therefore, ECG) electrical forces are reversed, resulting in a T wave axis similar to that of the QRS complex.
The PR Interval
The interval in time between the onset of the P wave and the onset of the QRS complex is termed the PR interval.
The PR interval varies with both age and heart rate. The higher the heart rate, the shorter the PR interval; for a given rate, the younger the age, the shorter the PR interval. A short PR interval suggests aberrant atrial to ventricular conduction.
The QT Interval
The interval between the onset of the QRS complex and the end of the T wave is referred to as the QT interval.
The QT interval is affected by heart rate, shortening normally at more rapid rates and lengthening at slower rates. For this reason, the QT interval is best “corrected” for rate, by referencing it to a rate of 60 bpm. This is done by dividing the absolute QT interval by the square root of the interval in seconds between R waves (that is, the cycle length).
The QT interval is modulated by the autonomic nervous system, and affected by a variety of electrolyte, mineral, and acid-base disorders.