This guide demonstrates how to read an ECG in a systematic and effective manner. If you want to put your ECG interpretation knowledge to the test, check out our ECG quiz on the new Geeky Medics quiz platform.
CHAPTER 2 ECG Basics: Waves, Intervals, and Segments The first purpose of this chapter is to present two fundamental electrical properties of heart muscle cells: (1) depolarization (activation), and (2) repolarization (recovery). Second, in this chapter and the next we define and show how to measure the basic waveforms, segments, and intervals essential to ECG interpretation. DEPOLARIZATION AND REPOLARIZATION In Chapter 1, the term electrical activation (stimulation) was applied to the spread of electrical signals through the atria and ventricles. The more technical term for the cardiac activation process is depolarization. The return of heart muscle cells to their resting state following depolarization is called repolarization. These key terms are derived from the fact that normal “resting” myocardial cells are polarized; that is, they carry electrical charges on their surface. Fig. 2.1A shows the resting polarized state of a normal atrial or ventricular heart muscle cell. Notice that the outside of the resting cell is positive and the inside is negative (about −90 mV [millivolt] gradient between them).a When a heart muscle cell (or group of cells) is stimulated, it depolarizes. As a result, the outside of the cell, in the area where the stimulation has occurred, becomes negatively charged and the inside of the cell becomes positive. This produces a difference in electrical voltage on the outside surface of the cell between the stimulated depolarized area and the unstimulated polarized area (Fig. 2.1B). Consequently, a small electrical current is formed Please go to expertconsult.inkling.com for additional online material for this chapter. aMembrane polarization is due to differences in the concentration of ions inside and outside the cell. A brief review of this important topic is presented in the online material and also see the Bibliography for references that present the basic electrophysiology of the resting membrane potential and cellular depolarization and repolarization (the action potential) underlying the ECG waves recorded on the body surface. that spreads along the length of the cell as stimulation and depolarization occur until the entire cell is depolarized (Fig. 2.1C). The path of depolarization can be represented by an arrow, as shown in Fig. 2.1B. Note: For individual myocardial cells (fibers), depolarization and repolarization proceed in the same direction. However, for the entire myocardium, depolarization normally proceeds from innermost layer (endocardium) to outermost layer (epicardium), whereas repolarization proceeds in the opposite direction. The exact mechanisms of this wellestablished asymmetry are not fully understood. The depolarizing electrical current is recorded by the ECG as a P wave (when the atria are stimulated and depolarize) and as a QRS complex (when the ventricles are stimulated and depolarize). Repolarization starts when the fully stimulated and depolarized cell begins to return to the resting state. A small area on the outside of the cell becomes positive again (Fig. 2.1D), and the repolarization spreads along the length of the cell until the entire cell is once again fully repolarized. Ventricular repolarization is recorded by the ECG as the ST segment, T wave, and U wave. In summary, whether the ECG is normal or abnormal, it records just two basic events: (1) depolarization, the spread of a stimulus (stimuli) through the heart muscle, and (2) repolarization, the return of the stimulated heart muscle to the resting state. The basic cellular processes of depolarization and repolarization are responsible for the waveforms, segments, and intervals seen on the body surface (standard) ECG. FIVE BASIC ECG WAVEFORMS: P, QRS, ST, T, AND U The ECG records the electrical activity of a myriad of atrial and ventricular cells, not just that of single f ibers. The sequential and organized spread of stimuli through the atria and ventricles followed by their return to the resting state produces the electrical currents recorded on the ECG. Furthermore, each phase of cardiac electrical activity produces a specific wave or deflection. QRS waveforms are referred to as complexes (Fig. 2.2). The five basic ECG waveforms, labeled alphabetically, are the: P wave – atrial depolarization QRS complex – ventricular depolarization ST segment T wave U wave } ventricular repolarization
The P wave represents the spread of a stimulus through the atria (atrial depolarization). The QRS waveform, or complex, represents stimulus spread through the ventricles (ventricular depolarization). As the name implies, the QRS set of deflections (complex) includes one or more specific waves, labeled as Q, R, and S. The ST (considered both a waveform and a segment) and T wave (or grouped as the “ST-T” waveform) represent the return of stimulated ventricular muscle to the resting state (ventricular repolarization). Furthermore, the very beginning of the ST segment (where it meets the QRS complex) is called the J point. The U wave is a small deflection sometimes seen just after the T wave. It represents the final phase of ventricular repolarization, although its exact mechanism is not known. You may be wondering why none of the listed waves or complexes represents the return of the stimulated (depolarized) atria to their resting state. The answer is that the atrial ST segment (STa) and atrial T wave (Ta) are generally not observed on the routine ECG because of their low amplitudes. An important exception is described in Chapter 12 with reference to acute pericarditis, which often causes subtle, but important deviations of the PR segment. Similarly, the routine body surface ECG is not sensitive enough to record any electrical activity during the spread of stimuli through the atrioventricular
Depolarization and repolarization. (A) The resting heart muscle cell is polarized; that is, it carries an electrical charge, with the outside of the cell positively charged and the inside negatively charged. (B) When the cell is stimulated (S), it begins to depolarize (stippled area). (C) The fully depolarized cell is positively charged on the inside and negatively charged on the outside. (D) Repolarization occurs when the stimulated cell returns to the resting state. The directions of depolarization and repolarization are represented by arrows. Depolarization (stimulation) of the atria produces the P wave on the ECG, whereas depolarization of the ventricles produces the QRS complex. Repolarization of the ventricles produces the ST-T complex. QRS P ST T U Fig. 2.2 The P wave represents atrial depolarization. The PR interval is the time from initial stimulation of the atria to initial stimulation of the ventricles. The QRS complex represents ventricular depolarization. The ST segment, T wave, and U wave are produced by ventricular repolarization.
Fig. 2.3 Summary of major components of the ECG graph. These can be grouped into 5 waveforms (P, QRS, ST, T, and U), 4 intervals (RR, PR, QRS, and QT) and 3 segments (PR, ST, and TP). Note that the ST can be considered as both a waveform and a segment. The RR interval is the same as the QRS–QRS interval. The TP segment is used as the isoelectric baseline, against which deviations in the PR segment (e.g., in acute pericarditis) and ST segment (e.g., in ischemia) are measured.
(AV) junction (AV node and bundle of His) en route to the ventricular myocardium. This key series of events, which appears on the surface ECG as a straight line, is actually not electrically “silent,” but reflects the spread of electrical stimuli through the AV junction and the His–Purkinje system, just preceding the QRS complex. In summary, the P/QRS/ST-T/U sequence represents the cycle of the electrical activity of the normal heartbeat. This physiologic signaling process begins with the spread of a stimulus through the atria (P wave), initiated by sinus node depolarization, and ends with the return of stimulated ventricular muscle to its resting state (ST-T and U waves). As shown in Fig. 2.3, the basic cardiac cycle repeats itself again and again, maintaining the rhythmic pulse of life. ECG SEGMENTS VS. ECG INTERVALS ECG interpretation also requires careful assessment of the time within and between various waveforms. Segments are defined as the portions of the ECG bracketed by the end of one waveform and the beginning of another. Intervals are the portions of the ECG that include at least one entire waveform. There are three basic segments: 1. PR segment: end of the P wave to beginning of the QRS complex. Atrial repolarization begins in this segment. (Atrial repolarization continues during the QRS and ends during the ST segment.) 2. ST segment: end of the QRS complex to beginning of the following T wave. As noted above, the ST-T complex represents ventricular repolarization. The segment is also considered as a separate waveform, as noted above. ST elevation and/or depression are major signs of ischemia, as discussed in Chapters 9 and 10. 3. TP segment: end of the T wave to beginning of the P wave. This interval, which represents the electrical resting state, is important because it is traditionally used as the baseline reference from which to assess PR and ST deviations in conditions such as acute pericarditis and acute myocardial ischemia, respectively. In addition to these segments, four sets of intervals are routinely measured: PR, QRS, QT/QTc, and PP/ RR.b The latter set (PP/RR) represents the inverse of the ventricular/atrial heart rate(s), as discussed in Chapter 3. 1. The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. bThe peak of the R wave is often selected. But students should be aware that any consistent points on sequential QRS complexes may be used to obtain the “RR” interval, even S waves or QS waves. Similarly, the PP interval is also measured from the same location on one P wave to that on the next. This interval gives the atrial rate. Normally, the PP interval is the same as the RR interval (see below), especially in “normal sinus rhythm.” Strictly speaking, the PP interval is actually the atrial–atrial (AA) interval, since in two major arrhythmias—atrial flutter and atrial fibrillation (Chapter 15)—continuous atrial activity, rather than discrete P waves, are seen.
2. The QRS interval (duration) is measured from the beginning to the end of the same QRS. 3. The QT interval is measured from the beginning of the QRS to the end of the T wave. When this interval is corrected (adjusted for the heart rate), the designation QTc is used, as described in Chapter 3. 4. The RR (QRS–QRS) interval is measured from one point (sometimes called the R-point) on a given QRS complex to the corresponding point on the next. The instantaneous heart rate (beats per min) = 60/RR interval when the RR is measured in seconds (sec). Normally, the PP interval is the same as the RR interval, especially in “normal sinus rhythm.” We will discuss major arrhythmias where the PP is different from the RR, e.g., sinus rhythm with complete heart block (Chapter 17).c cYou may be wondering why the QRS–QRS interval is not measured from the very beginning of one QRS complex to the beginning of the next. For convenience, the peak of the R wave (or nadir of an S or QS wave) is usually used. The results are equivalent and the term RR interval is most widely used to designate this interval.
Fig. 2.5 The ECG is recorded on graph paper divided into millimeter squares, with darker lines marking 5-mm squares. Time is measured on the horizontal (X) axis. With a paper speed of 25 mm/sec, each small (1-mm) box side equals 0.04 sec and each larger (5-mm) box side equals 0.2 sec. A 3-sec interval is denoted. The amplitude of a deflection or wave is measured in millimeters on the vertical (Y) axis.
5–4–3 Rule for ECG Components To summarize, the clinical ECG graph comprises waveforms, intervals, and segments designated as follows: 5 waveforms (P, QRS, ST, T, and U) 4 sets of intervals (PR, QRS, QT/QTc, and RR/PP) 3 segments (PR, ST, and TP) Two brief notes to avoid possible semantic confusion: (1) The ST is considered both a waveform and a segment. (2) Technically, the duration of the P wave is also an interval. However, to avoid confusion with the PR, the interval subtending the P wave is usually referred to as the P wave width or duration, rather than the P wave interval. The P duration (interval) is also measured in units of msec or sec and is most important in the diagnosis of left atrial abnormality (Chapter 7). The major components of the ECG are summarized in Fig. 2.3. ECG GRAPH PAPER The P–QRS–T sequence is recorded on special ECG graph paper that is divided into grid-like boxes
(Figs. 2.4 and 2.5). Each of the small boxes is 1 millimeter square (1 mm2). The standard recording rate is equivalent to 25 mm/sec (unless otherwise specified). Therefore, horizontally, each unit represents 0.04 sec (25 mm/sec × 0.04 sec = 1 mm). Notice that the lines between every five boxes are thicker, so that each 5-mm unit horizontally corresponds to 0.2 sec (5 × 0.04 sec = 0.2 sec). All of the ECGs in this book have been calibrated using these specif ications, unless otherwise indicated. A remarkable (and sometimes taken for granted) aspect of ECG analysis is that these recordings allow you to measure events occurring over time spans as short as 40 msec or less in order to make decisions critical to patients’ care. A good example is an ECG showing a QRS interval of 100 msec, which is normal, versus one with a QRS interval of 140 msec, which is markedly prolonged and might be a major clue to bundle branch block (Chapter 8), hyperkalemia (Chapter 11) or ventricular tachycardia (Chapter 16). We continue our discussion of ECG basics in the following chapter, focusing on how to make key measurements based on ECG intervals and what their normal ranges are in adults.