As just noted, the electrocardiograph must be properly standardized so a 1 mV signal produces a 10 mm deflection. Therefore every electrocardiograph has a special standardization button that produces a 1 mV wave. The standardization mark (St) produce when the machine is correctly calibrated is a square wave 10 mm tall. If the machine is not standardized correctly, the 1 mV signal will produce a deflection either more or less than 10 mm, and the amplitudes of the P, QRS, and T deflection will be larger or smaller than they should. The standardization deflection is also important because the standardization can be varied in the newer electrocardiographs. When very large deflections are present (as occurs, for example, in some patients who have an electronic pacemaker that produces very large spikes), it may be advisable to take the ECG at half standardization to avoid damaging the stylus and to get the entire tracing on the paper. If the ECG complexes are very small, it may be advisable to double the standardization (for example, to study a small Q wave more thoroughly). The standardization need be set only once on an ECG—just before the first lead is recorded.
Because the ECG is standardized, we can describe any part of the P, QRS, and T deflections in two ways. We can measure the amplitude (voltage) of any of the deflections, and we can also measure the width (duration) of any of the deflections. We can therefore measure the amplitude and width of the QRS complex, the amplitude of the ST segment deviation (if present), and the amplitude of the T wave. For clinical purposes, if the standardization is set at 1 mV = 10 mm, the height of a wave is usually recorded in millimeters and not in millivolts. For example, the P wave is 1 mm in amplitude, the QRS complex is 8 mm, and the T wave is about 3.5 mm.
In describing the amplitude of any wave or deflection, it is also necessary to specify if it is positive or negative, by convention, an upward deflection or wave is called positive. A downward deflection or wave is called negative. A deflection or wave that rests on the baseline is said to be isoelectric. A deflection that is partly positive and partly negative is called biphasic. For example, the P wave is positive, the QRS complex is biphasic (initially positive, then negative), the ST segment is isoelectric (flat on the baseline), and the T wave is negative.
In this chapter we shall describe the P, QRS, ST, T, and U waves in a general way and the measurement of the heart rate, the PR interval, the QRS width, the QT interval, and their normal values in detail.
The P wave, which represents atrial depolarization, is a small deflection before the QRS complex. The normal values for P wave amplitude and width are described in
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The PR interval may vary slightly in different leads, and the shortest PR interval should be noted. The PR interval represents the time it takes for the stimulus to spread through the atria and to pass through the AV junction. (This physiologic delay allows the ventricles to fill fully with blood before ventricular depolarization occurs.) in adults the normal PR interval is between 0.12 and 0.2 second (three to five small boxes). When conduction through the AV junction is impaired, the PR interval may become prolonged. Prolongation of the PR interval above 0.2 second is called first-degree heart block.
One of the most confusing aspects of electrocardiography for the beginning student is the nomenclature of the QRS complex. The QRS complex, as noted previously, represents the spread of a stimulus through the ventricles. However, not every QRS complex contains a Q wave, an R wave, and an S wave; hence the confusion. This bothersome but unavoidable nomenclature becomes understandable if you remember the following: if the initial deflection of the QRS complex is negative (below the baseline), it is called a Q wave. The first positive deflection in the QS complex is called an R wave. A negative deflection following the R wave is called an S wave. Thus this QRS complex contains a Q wave, an R wave, and an S wave. If the entire QRS complex is positive, it is simply called an R wave. However, if the entire complex is negative, it is termed a QS wave (not just a Q wave as you might expect). Occasionally the QRS complex will contain more than two or three deflections, and in such cases the extra waves are called R’ (R prime) waves if they are negative. Shows the various possible QRS complexes and the nomenclature of the respective waves. Note that the capital letters (QRS) are used to designate waves of relatively large amplitude while small letters (qrs) are used to label relatively small waves.
This nomenclature is confusing at first, but it allows you to describe any QRS complex over the phone and to evoke in the mind of the trained listener an exact mental picture of the complex named. For example, in describing an ECG you might say that lead V1 showed an rS complex (“small r, capital S”) while lead aVF showed a QS wave.
QRS Width (Interval)
The QRS width represents the time required for a stimulus to spread through the ventricles (ventricular depolarization) and is normally 0.1 second or less. If the spread of stimulus through the ventricles is slowed, for example, by a block in one of the bundle branches, the QRS width will be prolonged.
The ST segment is the portion of the ECG cycle from the end of the QRS complex to the beginning of the T wave. It represents the beginning of ventricular repolarization. The normal ST segment is usually isoelectric (that is, flat on the baseline, neither positive nor negative), but it may be slightly elevated or depressed normally (usually by less than 1 mm). Some pathologic conditions, such as myocardial infarction, produce characteristic abnormal deviations of the ST segment. The very beginning of the ST segment (actually the junction between the end of the QRS complex and the beginning of the ST segment) is sometimes called the J point.