How to Make Basic ECG Measurements

ECG as a Dynamic Heart Graph

As shown in Fig. 3.1 , the standardization mark produced when the machine is routinely calibrated is a square (or rectangular) wave 10 mm tall, usually displayed at the left side of each row of the ECG. If the machine is not standardized in the expected way, the 1-mV signal produces a deflection either more or less than 10 mm, and the amplitudes of the P, QRS, and T deflections will be larger or smaller than they should be.

The standardization deflection is also important because it can be varied in most electrocardiographs (see Fig. 3.1 ). When very large deflections are present (e.g., as occurs in some patients who have an electronic pacemaker that produces very large stimuli [“spikes”] or who have high QRS voltage caused by hypertrophy), there may be considerable overlap between the deflections on one lead with those one above or below it. When this occurs, it may be advisable to repeat the ECG at one-half standardization to get the entire tracing on the paper. If the ECG complexes are very small, it may be advisable to double the standardization (e.g., to study a small Q wave more thoroughly or augment a subtle pacing stimulus). Some electronic electrocardiographs do not display the calibration pulse. Instead, they print the effective paper (“sweep”) speed and standardization at the bottom of the ECG paper (“25 mm/sec, 10 mm/mV”).

Because the ECG is calibrated, any part of the P, QRS, and T deflections can be precisely described in two ways; that is, both the amplitude (voltage) and the width (duration) of a deflection can be measured. For clinical purposes, if the standardization is set at 1 mV = 10 mm, the height of a wave is usually recorded in millimeters, not millivolts. In Fig. 3.2 , 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.

The ECG recoding also includes other nonphysiologic deflections. Notably, as described next, electronic 12-lead recorders inscribe vertical lines to separate leads on typical 12-lead displays. Artifacts, for example because of electrical interference, poor electrode contact, and tremor, are described in Chapter 23 .

P Wave and PR Interval

The P wave, which represents atrial depolarization, is a small positive (or negative) deflection before the QRS complex. The normal values for P wave axis, amplitude, and width are described in Chapter 7 . The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex ( Fig. 3.3 ). The PR interval may vary slightly in different leads, and the shortest PR interval should be noted when measured by hand. The PR interval represents the time it takes for the stimulus to spread through the atria and pass through the atrioventricular (AV) junction. (This physiologic delay allows the ventricles to fill fully with blood before ventricular depolarization occurs, to optimize cardiac output.) In adults the normal PR interval is between 0.12 and 0.2 sec (three to five small box sides). When conduction through the AV junction is impaired, the PR interval may become prolonged. As noted, prolongation of the PR interval above 0.2 sec (200 msec) is called first-degree heart block (delay) (see Chapter 17 ). With sinus tachycardia, AV conduction may be facilitated by increased sympathetic and decreased vagal tone modulation. Accordingly, the PR may be relatively short (e.g., about 0.10-0.12 sec [100-120 msec]), as a physiologic finding, in the absence of Wolff–Parkinson–White (WPW) preexcitation (see Chapter 18 ).

QRS Complex

The QRS complex 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 possibility of confusion. The slightly awkward (and arbitrary) nomenclature becomes understandable if you remember three basic naming rules for the components of the QRS complex in any lead ( Fig. 3.4 ):

• 1.

  When the initial deflection of the QRS complex is negative (below the baseline), it is called a Q wave.

• 2.

  The first positive deflection in the QRS complex is called an R wave.

• 3.

  A negative deflection after the R wave is called an S wave.

Thus the following QRS complex contains a Q wave, an R wave, and an S wave. In contrast, the following complex does not contain three waves:

If, as shown earlier, 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 contains more than two or three deflections. In such cases the extra waves are called R ′ (R prime) waves if they are positive and S ′ (S prime) waves if they are negative.

Fig. 3.4 shows the major possible QRS complexes and the nomenclature of the respective waves. Notice that capital letters ( QRS ) are used to designate waves of relatively large amplitude and small letters ( qrs ) label relatively small waves. However, no exact thresholds have been developed to say when an s wave qualifies as an S wave, for example.

The QRS naming system does seem confusing at first, but it allows you to describe any QRS complex and 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 V ₁ showed an rS complex (“small r, capital S”):

or a QS (“capital Q, capital S”):

QRS Interval (Width or Duration)

The QRS interval represents the time required for a stimulus to spread through the ventricles (ventricular depolarization). Normally, in adults this interval is ≤0.10 sec (100 msec) as measured by the eye, or ≤0.11 sec (110 msec) when electronically measured by computer algorithms ( Fig. 3.5 ). ^a

a You may have already noted that the QRS amplitude (height or depth) often varies slightly from one beat to the next. This variation may be caused by a number of factors. One is related to breathing mechanics: as you inspire, your heart rate speeds up because of decreased cardiac vagal tone ( Chapter 13 ), and it decreases with expiration because of increased vagal tone. Breathing may also change the QRS axis because changes in heart position and chest impedance change QRS amplitude slightly. If the rhythm strip is long enough, you may even be able to estimate the patient’s breathing rate. QRS changes may also occur to slight alterations in ventricular activation, as with atrial flutter and fibrillation with a rapid ventricular response ( Chapter 15 ). Beat-to-beat QRS alternans with sinus tachycardia is a specific but not sensitive marker of pericardial effusion with tamponade pathophysiology because of the swinging heart phenomenon (see Chapter 12 ). Beat-to-beat alternation of the QRS is also seen with certain types of paroxysmal supraventricular tachycardias (PSVTs; see Chapter 14 ) and occasionally with monomorphic ventricular tachycardia ( Chapter 16 ).

If the spread of a stimulus through the ventricles is slowed, for example by a block in one of the bundle branches, the QRS width will be prolonged. The differential diagnosis of a wide QRS complex is discussed in 18, 19, and 25 . ^b

b A subinterval of the QRS, termed the intrinsicoid deflection, is defined as the time between the onset of the QRS (usually measured in a left lateral chest lead) to the peak of the R wave in that lead. A preferred term is R wave peak time . This interval is interpreted as an estimate of the time for the impulse to travel from the endocardium of the left ventricle to the epicardium. The upper limit of normal is usually given as 0.04 sec (40 msec), with increased values seen with left ventricular hypertrophy (>0.05 sec or 50 msec) and left bundle branch block (>0.06 sec or 60 msec). However, this microinterval is hard to measure reliably (especially with notched QRS complexes) and reproducibly at conventional paper speeds used in clinical electrocardiography. Therefore, the R wave peak time has very limited utility in contemporary practice.

ST Segment

The ST segment is that portion of the ECG cycle from the end of the QRS complex to the beginning of the T wave ( Fig. 3.6 ). It represents the earliest phase of ventricular repolarization. The normal ST segment is usually isoelectric (i.e., flat on the baseline, neither positive nor negative), but it may be slightly elevated or depressed normally (usually by less than 1 mm). Pathologic conditions, such as myocardial infarction (MI), that produce characteristic abnormal deviations of the ST segment (see 9, 10 ) are a major focus of clinical ECG diagnosis.

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 called the J point. Fig. 3.6 shows the J point and the normal shapes of the ST segment. Fig. 3.7 compares a normal isoelectric ST segment with abnormal ST segment elevation and depression.

The terms J point elevation and J point depression are descriptive. They do not denote specific conditions (e.g., pericarditis, ischemia, etc.). For example, isolated J point elevation may occur as a normal variant with the early repolarization pattern (see Chapter 10 ) or as a marker of systemic hypothermia (where they are termed Osborn or J waves; see Chapter 11 ). J point elevation may also be part of ST elevations with acute pericarditis, acute myocardial ischemia, left bundle branch block or left ventricular hypertrophy (leads V ₁ to V ₃ usually), and so forth. Similarly, J point depression may occur in a variety of contexts, both physiologic and pathologic, as discussed in subsequent chapters and summarized in Chapter 25 .