Оригинал

FIGURE 2-15 Measurement of heart rate (beats per minute) by counting the number of QRS complexes in a 6-second interval and multiplying this number by 10. In this example, 10 QRS complexes occur in 6 seconds. Therefore the heart rate is 10 × 10 = 100 beats/min. The arrows point to 3-second markers.

By definition, a heart rate exceeding 100 beats/min is termed tachycardia, and a heart rate slower than 60 beats/min is called bradycardia. (In Greek, tachys means

“swift,” whereas bradys means “slow.”) Thus during exercise you probably develop a sinus tachycardia, but during sleep or relaxation your pulse rate may drop into the 50s or even lower, indicating a sinus bradycardia.

THE ECG AS A COMBINATION OF ATRIAL AND VENTRICULAR WAVEFORMS

The ECG actually consists of two separate but normally related parts: an atrial ECG, represented by the P wave; and a ventricular ECG, represented by the QRS-T sequence. With completely normal rhythm, when the sinus node is pacing the heart, the P wave (atrial stimulation or depolarization) always precedes the QRS complex (ventricular stimulation or depolarization) because the atria are electrically stimulated first. Therefore the P-QRS-T cycle is usually considered as a unit. In some abnormal conditions, however, the atria and the ventricles can be stimulated by separate pacemakers. For example, suppose that the AV junction is diseased and stimuli cannot pass from the atria to the ventricles. In this situation, a new (subsidiary) pacemaker located below the level of the block in the AV junction may take over the task of pacing the ventricles while the sinus node continues to pace the atria. In this case, stimulation of the atria is independent of stimulation of the ventricles, and the P waves and QRS complexes have no relation to each other. This type of arrhythmia is called complete heart block and is described in detail in Chapter 17. (Figure 17-5 shows an example of this abnormal condition in which the atrial and ventricular ECGs are independent of each other.)

THE ECG IN PERSPECTIVE

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Up to this point, this chapter has considered only the basic components of the ECG. Several general items need to be emphasized before actual ECG patterns are discussed.

1.The ECG is a recording of cardiac electrical activity. It does not directly measure the mechanical function of the heart (i.e., how well the heart is contracting and performing as a pump). Thus a patient with acute pulmonary edema may have a normal ECG. Conversely, a patient with a grossly abnormal ECG may have normal cardiac function.

2.The ECG does not directly depict abnormalities in cardiac structure such as ventricular septal defects and abnormalities of the heart valves. It only records the electrical changes produced by structural defects. In some patients, however, the clinician can infer a specific structural diagnosis such as mitral stenosis, pulmonary embolism, or myocardial infarction from the ECG because typical electrical abnormalities may develop in such patients.

3.The ECG does not record all the heart's electrical activity. The electrodes placed on the surface of the body record only the currents that are transmitted to the area of electrode placement. Therefore there are actually “silent” electrical areas of the heart. For example, the ECG is not sensitive enough to record depolarization of pacemaker cells in the sinus node occurring just before the P wave. As noted earlier, the conventional electrocardiograph does not generally record repolarization of the atria. (For an important exception, see the discussion of pericarditis in Chapter 11.) In addition, the conventional ECG does not detect the spread of stimuli through the AV junction. The electrical activity of the AV junction can be recorded using a special apparatus and a special electrode placed in the heart (His bundle electrogram). Further, the ECG records the summation of electrical potentials produced by innumerable cardiac muscle cells. Therefore the presence of a normal ECG does not necessarily mean that all these heart muscle cells are being depolarized and repolarized in a normal way.

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For these reasons, the ECG must be regarded as any other laboratory test, with proper consideration for both its uses and its limitations (see Chapter 23).

The 12 ECG leads are described in Chapter 3. Normal and abnormal ECG patterns are discussed in subsequent chapters.

CHAPTER 3 – ECG Leads

As discussed in Chapter 1, the heart produces electrical currents similar to those of the familiar dry cell battery. The strength or voltage of these currents and the way they are distributed throughout the body over time can be measured by a suitable recording instrument such as an electrocardiograph.

The body acts as a conductor of electricity. Therefore recording electrodes placed some distance from the heart, such as on the arms, legs, or chest wall, are able to detect the voltages of the cardiac currents conducted to these locations. The usual way of recording these voltages from the heart is with the 12 standard ECG leads. The leads actually show the differences in voltage (potential) between electrodes placed on the surface of the body.

Taking an ECG is like recording an event such as a baseball game with a video camera. Multiple camera angles are necessary to capture the event completely. One view is not enough. Similarly, multiple ECG leads must be recorded to describe the dynamic electrical activity of the heart adequately. Figure 3-1 shows the ECG patterns that are obtained when electrodes are placed at various points on the chest. Notice that each lead presents a different pattern.

Figure 3-2 is an ECG illustrating the 12 leads. The leads can be subdivided into two groups: the six limb (extremity) leads (shown in the left two columns) and the six chest (precordial) leads (shown in the right two columns).

The six limb leads—I, II, III, aVR, aVL, and aVF—record voltage differences by means of electrodes placed on the extremities. They can be further divided into two subgroups based on their historical development: three standard “bipolar” limb leads (I, II, and III), and three augmented “unipolar” limb leads (aVR, aVL, and aVF).

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The six chest leads—V1, V2, V3, V4, V5, and V6—record voltage differences by means of electrodes placed at various positions on the chest wall.

The 12 ECG leads can also be viewed as 12 “channels.” In contrast to television channels (which can be tuned to different events), however, the 12 ECG channels (leads) are all tuned to the same event (the P-QRS-T cycle), with each lead viewing the event from a different angle.

FIGURE 3-1 Multiple chest leads give a three-dimensional view of cardiac electrical activity.

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FIGURE 3-2 A, Sample ECG showing the 12 standard leads. B, A lead II rhythm strip recorded simultaneously.

LIMB (EXTREMITY) LEADS

STANDARD LIMB LEADS: I, II, AND III

The limb (extremity) leads are recorded first. In connecting a patient to an electrocardiograph, first place metal electrodes on the arms and legs. The right leg electrode functions solely as an electrical ground, so you need concern yourself with it no further. As shown in Figure 3-3, the arm electrodes are attached just above the wrist and the leg electrodes are attached above the ankles.

FIGURE 3-3 Metal electrodes are used to take an ECG. The right leg (RL) electrode functions solely as a ground to prevent alternating-current interference.

LA, left arm; LL, left leg; RA, right arm.

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The electrical voltages of the heart are conducted through the torso to the extremities. Therefore an electrode placed on the right wrist detects electrical voltages equivalent to those recorded below the right shoulder. Similarly, the voltages detected at the left wrist or anywhere else on the left arm are equivalent to those recorded below the left shoulder. Finally, voltages detected by the left leg electrode are comparable to those at the left thigh or near the groin. In clinical practice, the electrodes are attached to the wrists and ankles simply for convenience.

As mentioned, the limb leads consist of standard “bipolar” (I, II, and III) and augmented (aVR, aVL, and aVF) leads. The bipolar leads were so named historically because they record the differences in electrical voltage between two extremities.

Lead I, for example, records the difference in voltage between the left arm (LA) and right arm (RA) electrodes:

Lead I = LA – RA

Lead II records the difference between the left leg (LL) and right arm (RA) electrodes:

Lead II = LL – RA

Lead III records the difference between the left leg (LL) and left arm (LA) electrodes:

Lead III = LL – LA

Consider what happens when you turn on the electrocardiograph to lead I. The LA electrode detects the electrical voltages of the heart that are transmitted to the left arm. The RA electrode detects the voltages transmitted to the right arm. Inside the electrocardiograph, the RA voltages are subtracted from the LA voltages, and the difference appears at lead I. When lead II is recorded, a similar situation occurs between the voltages of LL and RA. When lead III is recorded, the same situation occurs between the voltages of LL and LA.

Leads I, II, and III can be represented schematically in terms of a triangle, called

Einthoven's triangle after the Dutch physiologist who invented the electrocardiograph in the early 1900s. At first the ECG consisted only of recordings from leads I, II, and III. Einthoven's triangle (Fig. 3-4) shows the spatial orientation of the three standard 26

limb leads (I, II, and III). As you can see, lead I points horizontally. Its left pole (LA) is positive and its right pole (RA) is negative. Therefore lead I = LA – RA. Lead II points diagonally downward. Its lower pole (LL) is positive and its upper pole (RA) is negative. Therefore lead II = LL – RA. Lead III also points diagonally downward. Its lower pole (LL) is positive and its upper pole (LA) is negative. Therefore lead III = LL – LA.

FIGURE 3-4 Orientation of leads I, II, and III. Lead I records the difference in electrical potentials between the left arm and right arm. Lead II records it between the left leg and right arm. Lead III records it between the left leg and left arm.

Einthoven, of course, could have hooked the leads up differently. Yet because of the way he arranged them, the bipolar leads are related by the following simple equation:

Lead I + Lead III = Lead II

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In other words, add the voltage in lead I to that in lead III and you get the voltage in lead II.[*] You can test this equation by looking at Figure 3-2. Add the voltage of the R wave in lead I (+9 mm) to the voltage of the R wave in lead III (+4 mm) and you get +13 mm, the voltage of the R wave in lead II. You can do the same with the voltages of the P waves and T waves.

It is a good practice to scan leads I, II, and III rapidly when you first look at a mounted ECG. If the R wave in lead II does not seem to be the sum of the R waves in leads I and II, this may be a clue that the leads have been recorded incorrectly or mounted improperly.

Einthoven's equation is simply the result of the way the bipolar leads are recorded; that is, the LA is positive in lead I and negative in lead III and thus cancels out when the two leads are added:

Thus, in electrocardiography, one plus three equals two.

In summary, leads I, II, and III are the standard (bipolar) limb leads, which, historically, were the first invented. These leads record the differences in electrical voltage between selected extremities.

FIGURE 3-5 A, Einthoven's triangle. B, The triangle is converted to a triaxial diagram by shifting leads I, II, and III so that they intersect at a common point.

In Figure 3-5, Einthoven's triangle has been redrawn so that leads I, II, and III intersect at a common central point. This was done simply by sliding lead I

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downward, lead II rightward, and lead III leftward. The result is the triaxial diagram in Figure 3-5B. This diagram, a useful way of representing the three bipolar leads, is employed in Chapter 5.

AUGMENTED LIMB (EXTREMITY) LEADS: aVR, aVL, AND aVF

Nine leads have been added to the original three “bipolar” extremity leads. In the 1930s, Dr. Frank N. Wilson and his colleagues at the University of Michigan invented the “unipolar” limb leads and also introduced the six “unipolar” chest leads,

V1 through V6. A short time later, Dr. Emanuel Goldberger invented the three augmented unipolar limb leads: aVR, aVL, and aVF. The abbreviation a refers to augmented; V tovoltage; R, L, and F to right arm, left arm, and left foot (leg), respectively. Today 12 leads are routinely employed, consisting of the six limb leads (I, II, III, aVR, aVL, and aVF) and the six precordial leads (V1 to V6).

A so-called unipolar lead records the electrical voltages at one location relative to an electrode with close to zero potential rather than relative to the voltages at another single extremity, as in the case of the bipolar limb leads.[*] The zero potential is obtained inside the electrocardiograph by joining the three extremity leads to a central terminal. Because the sum of the voltages of RA, LA, and LL equals zero, the central terminal has a zero voltage. The aVL and aVF leads are derived in a slightly different way because the voltages recorded by the electrocardiograph have been augmented 50% over the actual voltages detected at each extremity. This augmentation is also done electronically inside the electrocardiograph.[†]

Just as Einthoven's triangle represents the spatial orientation of the three standard limb leads, the diagram in Figure 3-6 represents the spatial orientation of the three augmented limb leads. Notice that each of these “unipolar” leads can also be represented by a line (axis) with a positive and negative pole. Because the diagram has three axes, it is also called a triaxial diagram.

As would be expected, the positive pole of lead aVR, the right arm lead, points upward and to the patient's right arm. The positive pole of lead aVL points upward and to the patient's left arm. The positive pole of lead aVF points downward toward the patient's left foot.

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Furthermore, just as leads I, II, and III are related by Einthoven's equation, so leads aVR, aVL, and aVF are related:

aVR + aVL + aVF = 0

FIGURE 3-6 Triaxial lead diagram showing the relationship of the three augmented (“unipolar”) leads (aVR, aVL, and aVF). Notice that each lead is represented by an axis with a positive and negative pole. The term unipolar was used to mean that the leads record the voltage in one location relative to about zero potential, instead of relative to the voltage in one other extremity.

In other words, when the three augmented limb leads are recorded, their voltages should total zero. Thus the sum of the P wave voltages is zero, the sum of the QRS voltages is zero, and the sum of the T wave voltages is zero. Using Figure 3-2, test this equation by adding the QRS voltages in the three unipolar limb leads (aVR, aVL, and aVF).

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