Medical Physiology, 3rd Edition

The Electrocardiogram

An ECG generally includes five waves

The electrocardiogram (ECG) is the standard clinical tool used to measure the electrical activity of the heart. It is a recording of the small extracellular signals produced by the movement of action potentials through cardiac myocytes. To obtain a standard 12-lead ECG, one places two electrodes on the upper extremities, two on the lower extremities, and six on standard locations across the chest. In various combinations, the electrodes on the extremities generate the six limb leads (three standard and three augmented), and the chest electrodes produce the six precordial leads. In a lead, one electrode is treated as the positive side of a voltmeter and one or more electrodes as the negative side. Therefore, a lead records the fluctuation in voltage difference between positive and negative electrodes. By variation of which electrodes are positive and which are negative, a standard 12-lead ECG is recorded. Each lead looks at the heart from a unique angle and plane; that is, from what is essentially its own unique point of view.

The fluctuations in extracellular voltage recorded by each lead vary from fractions of a millivolt to several millivolts. These fluctuations are called waves and are named with the letters of the alphabet (Fig. 21-7). imageN21-13 The P wave reflects depolarization of the right and left atrial muscle. The QRS complex represents depolarization of ventricular muscle. The T wave represents repolarization of both ventricles. Finally, the rarely seen U wave may reflect repolarization of the papillary muscle. The shape and magnitude of these waves are different in each lead because each lead views the electrical activity of the heart from a unique position in space. For his discovery of the mechanism of the ECG, Willem Einthoven was awarded the 1924 Nobel Prize in Physiology or Medicine. imageN21-14

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FIGURE 21-7 Components of the ECG recording.

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Nomenclature and Durations of Electrocardiographic Waves

Contributed by Emile Boulpaep, requested by Walter Boron

The various waves of the ECG are named P, Q, R, S, T, and U.

• P wave: a small, usually positive, deflection before the QRS complex

• QRS complex: a group of waves that may include a Q wave, an R wave, and an S wave; note, however, that not every QRS complex consists of all three waves

• Q wave: the initial negative wave of the QRS complex

• R wave: the first positive wave of the QRS complex, or the single wave if the entire complex is positive

• S wave: the negative wave following the R wave

• QS wave: the single wave if the entire complex is negative

• R′ wave: extra positive wave, if the entire complex contains more than two or three deflections

• S′ wave: extra negative wave, if the entire complex contains more than two or three deflections

• T wave: a deflection that occurs after the QRS complex and the following isoelectric segment (i.e., the ST segment that we will define below)

• U wave: a small deflection sometimes seen after the T wave (usually of the same sign as the T wave)

In addition to the totally qualitative wave designations defined above, cardiologists may use uppercase and lowercase letters as a gauge of the amplitude of Q, R, and S waves.

• Capital letters Q, R, S are used for deflections of relatively large amplitude.

• Lower case letters q, r, s are used for deflections of relatively small amplitude. For instance: an rS complex indicates a small R wave followed by a large S wave.

The various intervals are as follows:

• PR interval: measured from the beginning of the P wave to the beginning of the QRS complex; normal duration is between 0.12 and 0.2 second (three to five small boxes on the recording)

• QRS interval: measured from the beginning to the end of the QRS complex, as defined above; normal duration is <0.12 second

• QT interval: measured from the beginning of the QRS complex to the end of the T wave; the QT interval is an index of the length of the overall ventricular action potential; duration depends on heart rate because the action potential shortens with increased heart rate

• R-R interval: the interval between two consecutive QRS complexes; duration is equal to the duration of the cardiac cycle

• ST segment: from the end of the QRS complex to the beginning of the T wave

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Willem Einthoven

For more information about Willem Einthoven and the work that led to his Nobel Prize, visit http://www.nobel.se/medicine/laureates/1924/index.html (accessed March 2015).

Because the ECG machine uses electrodes attached to the skin to measure the sum of the heart's electrical activity, it requires special amplifiers. The ECG machine also has electrical filters that reduce the electrical noise. Moving the limbs, breathing, coughing, shivering, and faulty contact between the skin and an electrode produce artifacts on the recorded ECG.

Because the movement of charge (i.e., the spreading wave of electrical activity in the heart) has both a three-dimensional direction and a magnitude, the signal measured on an ECG is a vector. The system that clinicians use to measure the heart's three-dimensional, time-dependent electrical vector is simple to understand and easy to implement, but it can be challenging to interpret.

A pair of ECG electrodes defines a lead

To record the complicated time-dependent electrical vector of the heart, the physician or ECG technician constructs a system of leads in two planes that are perpendicular to each other. One plane, the frontal plane, is defined by the six limb leads (Fig. 21-8A). A perpendicular transverse plane is defined by the six precordial leads (see Fig. 21-8B). Each lead is an axis in one of the two planes, onto which the heart projects its electrical activity. The ECG recording from a single lead shows how that lead views the time-dependent changes in voltage of the heart.

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FIGURE 21-8 The ECG leads.

Older ECG machines recorded data from the 12 leads one at a time, sequentially. Thus, relatively rare events captured by the recording in one lead might not be reflected in any of the others, which were obtained at different times. Modern ECG machines record from leads synchronously in groups of 3 or 12. Because the real electrical vector of the heart consists of just one time-dependent vector signal, you might think that a three-lead recording would suffice to localize the vector signal in space. In principle, this is true: only two leads in one plane and one lead in another plane are needed to fully define the original electrical vector of the heart at all moments. However, recording from all 12 leads is extremely useful because a signal of interest may be easier to see in one lead than in another. For example, an acute myocardial infarction involving the inferior (diaphragmatic) portion of the heart might be easily visualized in leads II, III, and aVF but go completely undetected (or produce so-called reciprocal changes) in the other leads.

The Limb Leads

One obtains a 12-lead ECG by having the patient relax in a supine position and connecting four electrodes to the limbs (see Fig. 21-8A). Electrically, the torso and limbs are viewed as an equilateral triangle (Einthoven's triangle) with one vertex on the groin and the other two on the shoulder joints (Fig. 21-9A). Because the body is an electrical “volume conductor,” an electrical attachment to an arm is electrically equivalent to a connection at the shoulder joint, and an attachment to either leg is equivalent to a connection at the groin. By convention, the left leg represents the groin. The fourth electrode, connected to the right leg, is used for electrical grounding. The three initial limb leads represent the difference between two of the limb electrodes:

• I (positive connection to left arm, negative connection to right arm). This lead defines an axis in the frontal plane at 0 degrees (see Fig. 21-9A, B).

• II (positive to left leg, negative to right arm). This lead defines an axis in the frontal plane at 60 degrees.

• III (positive to left leg, negative to left arm). This lead defines an axis in the frontal plane at 120 degrees.

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FIGURE 21-9 Axes of the limb leads. A, The frontal plane limb leads behave as if they are located at the shoulders (RA [right arm] and LA [left arm]) and groin (LL [left leg]). Leads I, II, and III are separated from one another by 60 degrees. The augmented leads, referenced to the center of the heart, bisect each of the 60-degree angles formed by leads I, II, and III. B, Translating each of the six frontal leads so that they pass through a common point defines a polar coordinate system, providing views of the heart at 30-degree intervals.

An electronic reconstruction of the three-limb connection defines an electrical reference point in the middle of the heart (see Fig. 21-9A) that constitutes the negative connection for the augmented “unipolar” limb leads and for the chest leads. The three augmented unipolar limb leads compare one limb electrode to the average of the other two:

• aVR (positive connection to right arm, negative connection is electronically defined in the middle of the heart): The axis defined by this limb lead in the frontal plane is −150 degrees (see Fig. 21-9B). The a stands for augmented, and the V represents unipolar.

• aVL (positive to left arm, negative is middle of the heart): The axis defined by this limb lead in the frontal plane is −30 degrees.

• aVF (positive to left leg [foot], negative is middle of the heart): The axis defined by this limb lead in the frontal plane is +90 degrees.

Thus, the positive and negative ends of these six leads define axes every 30 degrees in the frontal plane (see Fig. 21-9B).

The Precordial Leads

The precordial leads lie in the transverse plane, perpendicular to the plane of the frontal leads. The positive connection is one of six different locations on the chest wall (see Fig. 21-8B), and the negative connection is electronically defined in the middle of the heart by averaging of the three limb electrodes. The resultant leads are named V1 to V6, where the V stands for unipolar:

• V1: fourth intercostal space to the right of the sternum

• V2: fourth intercostal space to the left of the sternum

• V4: fifth intercostal space at the midclavicular line

• V3: halfway between V2 and V4

• V6: fifth intercostal space at the midaxillary line

• V5: halfway between V4 and V6

It is also possible, on rare occasions, to generate special leads by employing the same negative connection used for the unipolar limb and precordial leads and a positive “probe” connection. Special leads that are used include esophageal leads and an intracardiac lead (e.g., that used to obtain a recording from the His bundle).

A simple two-cell model can explain how a simple ECG can arise

We can illustrate how the ECG arises from the propagation of action potentials through the functional syncytium of myocytes by examining the electrical activity in two neighboring cardiac cells, A and B, connected by gap junctions (Fig. 21-10A). The depolarization and action potential begin first in cell A (VA in Fig. 21-10A, green record). The current from cell A then depolarizes cell B through the gap junctions and a brief time later triggers an action potential in cell B (VB). If we subtract the VB record from the VA record, we obtain a record of the intracellular voltage difference VA − VB (see Fig. 21-10B).

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FIGURE 21-10 Two-cell model of the ECG.

We have already seen that according to Ohm's law (see Equation 21-1), the intracellular current from cell A to cell B (IAB) is proportional to (VA − VB). The extracellular current flowing from the region of cell B to the region of cell A is equal but opposite in direction to the intracellular current flowing from cell A to cell B. Imagine that an extracellular voltmeter has its negative electrode placed to the left of cell A and its positive electrode to the right of cell B (forming a lead with an axis of 0 degrees). During the upswing in the action potential of cell A, while cell B is still at rest, (VA − VB) and IAB are both positive, and the voltmeter detects a positive difference in voltage (see Fig. 21-10C)—analogous to the QRS complex in a real ECG. Later, during the recovery from the action potential in cell A, while cell B is still depolarized, (VA − VB) and IAB are both negative, and the voltmeter would detect a negative difference in voltage. From the extracellular voltage difference in Figure 21-10C, we can conclude that when the wave of depolarization moves toward the positive lead, there is a positive deflection in the extracellular voltage difference.

If we place the two electrodes at the junction between the two cells, with the positive connection on the bottom and the negative connection on the top, we create a lead with an axis of 90 degrees to the direction of current flow (see Fig. 21-10D). Under these conditions, we observe no voltage difference because both extracellular electrodes sense the same voltage at each instant in time. Thus, when a lead is perpendicular to the wave of depolarization, the measured deflection on that lead is isoelectric.

If we put our extracellular electrodes in yet a third configuration—with the positive electrode on the left and the negative electrode on the right—we observe a negative deflection during the depolarization of cell A because the wave of depolarization is moving away from the positive electrode (see Fig. 21-10E).

This simple two-cell model demonstrates that the wave of depolarization behaves like a vector, with both magnitude and direction. Two practical methods to determine the direction (or axis) of the vector are presented in Box 21-2.

Box 21-2

Basic Interpretation of the Electrocardiogram

An ECG provides a direct measurement of the rate, rhythm, and time-dependent electrical vector of the heart. It also provides fundamental information about the origin and conduction of the cardiac action potential within the heart. Because the different parts of the heart activate sequentially, we can attribute the time-dependent changes in the electrical vector of the heart to different regions of the heart. The P wave reflects the atrial depolarization. The QRS complex corresponds to ventricular depolarization. The T wave reflects ventricular repolarization.

ECG paper has a grid of small 1-mm square boxes and larger 5-mm square boxes. The vertical axis is calibrated at 0.1 mV/mm; the horizontal (time) axis, at 0.04 s/mm (small box) or 0.2 s/5 mm (large box). Thus, five large boxes correspond to 1.0 second (Fig. 21-11). Table 21-5 summarizes the steps for interpretation of an ECG.

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FIGURE 21-11 Normal 12-lead ECG recording. The recordings were obtained synchronously, three leads at a time (I, II, and III simultaneously; aVR, aVL, and aVF simultaneously; V1, V2, and V3 simultaneously; and V4, V5, and V6 simultaneously). A 1-mV, 200-ms calibration pulse is visible on the left of each of the three recordings in the top left panel. The leads are marked on the traces. (We thank the Division of Cardiology, University of Maryland School of Medicine, for obtaining this ECG recording from the author.)

TABLE 21-5

Eightfold Approach for Reading an Electrocardiogram

1

Search for P waves.

2

Determine the relationship of P waves and QRS complexes.

3

Identify the pacemaker.

4

Measure the heart rates from different waves (e.g., P-P interval, R-R interval).

5

Characterize the QRS shape (i.e., narrow versus wide).

6

Examine features of the ST segment.

7

Estimate the mean QRS axis (and the axes of the other waves of interest).

8

Examine the cardiac rhythm (e.g., look at a 20- to 30-s ECG record from lead II).

Rate

We can measure rate in two ways. The direct method is to measure the number of seconds between waves of the same type, for example, the R-R interval. The quotient of 60 divided by the interval in seconds is the heart rate in beats per minute.

A quick alternative method is quite popular (Table 21-6). Measure the number of large boxes that form the R-R interval and remember the series 300, 150, 100, 75, 60, 50—which corresponds to an interval of one, two, three, four, five, or six large boxes. Thus, rate = 300/(number of large boxes). For example, if four large boxes separate the R waves, the heart rate is 75 beats/min.

TABLE 21-6

Determination of Heart Rate from the Electrocardiogram

R-R INTERVAL (in number of large boxes of 0.2 s)

CALCULATION

HEART RATE (beats/min)

1

300/1

300

2

300/2

150

3

300/3

100

4

300/4

75

5

300/5

60

6

300/6

50

Rhythm

The determination of rhythm is more complex. One must answer the following questions: Where is the heart's pacemaker? What is the conduction path from the pacemaker to the last cell in the ventricles? Is the pacemaker functioning regularly and at the correct speed? The normal pacemaker is the SA node; the signal then propagates through the AV node and activates the ventricles. When the heart follows this pathway at a normal rate and in this sequence, the rhythm is called a normal sinus rhythm. imageN21-18

Careful examination of the intervals, durations, and segments in the ECG tracing can reveal a great deal about the conducted action potential (see Fig. 21-7). The P-wave duration indicates how long atrial depolarization takes. The PR interval indicates how long it takes the action potential to conduct through the AV node before activating the ventricles. The QRS duration reveals how long it takes for the wave of depolarization to spread throughout the ventricles. The QT interval indicates how long the ventricles remain depolarized and is thus a rough measure of the duration of the overall “ventricular” action potential. The QT segment gets shorter as the heart rate increases, which reflects the shorter action potentials that are observed at high rates. In addition, many other alterations in these waves—and the segments separating them—reflect important physiological and pathophysiological changes in the heart.

Vector (or Axis) of a Wave in the Frontal Plane

Determination of the vector of current flow through the heart is not just an intellectual exercise but can be of great clinical importance. The normal axis of ventricular depolarization in the frontal plane lies between −30 and +90 degrees. However, this axis can change in a number of pathological situations, including hypertrophy of one or both ventricular walls (a common sequela of severe or prolonged hypertension) and conduction blocks in one or several of the ventricular conducting pathways.

We can use two approaches to measure the axis of a wave within the frontal plane (i.e., with use of limb leads). The first is more accurate but the second is quicker and easier and is usually sufficient for clinical purposes.

The first approach is a geometric method. It uses our knowledge of the axes of the different leads and the measured magnitude of the wave projected onto at least two leads in the frontal plane. It involves five steps:

Step 1: Measure the height of the wave on the ECG records in two leads, using any arbitrary unit (e.g., number of boxes). A positive deflection is one that rises above the baseline, and a negative deflection is one that falls below the baseline. In the example in Figure 21-12A, we are estimating the axis of the R wave of the QRS complex. The R wave is +2 units in lead II and −1 unit in lead aVR.

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FIGURE 21-12 Estimation of the ECG axis in the frontal plane.

Step 2: Mark the height of the measured deflections on the corresponding lead lines on a circle of axes. Any unit of measure will suffice, as long as you use the same unit for both markings. Starting at the center of the circle, mark a positive deflection toward the arrowhead and a negative deflection toward the tail of the arrow.

Step 3: Draw lines perpendicular to the lead axes through each of your two marks.

Step 4: Connect the center of the circle of axes (tail of the vector) to the intersection of the two perpendicular lines (head of the vector). In our example, the intersection is close to the aVF axis.

Step 5: Estimate the axis of the vector that corresponds to the R wave, using the “angle” scale of the circle of axes. In this case, the vector is at about 95 degrees, just clockwise to the aVF lead (i.e., 90 degrees).

The second approach is a qualitative inspection method. It exploits the varying magnitudes of the wave of interest in recordings from different leads. When the wave is isoelectric (i.e., no deflection, or equal positive and negative deflections), then the electrical vector responsible for that projection must be perpendicular to the isoelectric lead, as we already saw for the two-cell model in Figure 21-10D. The inspection approach requires two steps:

Step 1: Identify a lead in which the wave of interest is isoelectric (or nearly isoelectric). In the example in Figure 21-12B, the QRS complex is isoelectric in aVL (−30 degrees). The vector must be perpendicular (or nearly perpendicular) to that lead (i.e., aVL). In our example, the vector must point 90 degrees from −30 degrees and therefore is at either +60 degrees or −120 degrees. Because the leads in the frontal plane define axes every 30 degrees, every lead has another lead to which it is perpendicular.

Step 2: Identify a lead in which the wave is largely positive. In Figure 21-12B, this would be lead II. The vector must lie roughly in the same direction as the orientation of that lead. Because lead II is at +60 degrees, the axis of the vector of the QRS wave must be about +60 degrees and not −120 degrees.

If the wave of interest is not isoelectric in any lead, then find two leads onto which the projections are of similar magnitude and sign. The vector has an axis halfway between those two leads.

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Electrocardiographic Rhythm Strip

Contributed by W. Jonathan Lederer

Determining the cardiac “rhythm,” as discussed in Box 21-2, requires observation over a longer time interval than that used to obtain the ECG traces shown in Figure 21-11. The rhythm strip in eFigure 21-2 shows leads V1, II, and V5 obtained simultaneously for an extended period.

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EFIGURE 21-2 ECG strip for assessment of heart rhythm. (We thank the Division of Cardiology, University of Maryland School of Medicine, for obtaining this ECG recording from the author.)

The QRS equivalent in the extracellular voltage record of our simplistic two-cell analysis is due to a spreading wave of depolarization. The T-wave equivalent is negative compared with the QRS equivalent, and it reflects the wave of repolarization. If cell A has an action potential that is much longer than that of cell B (so that positive current again propagates from A to B after the action potential in B is completed), then the T-wave equivalent will be upright, as it is in most ECGs. Thus, on average, the ventricular myocytes that depolarize last are the first to repolarize. In other words, the B cells have shorter action potentials than the A cells.

What happened to the P wave that we see in a real ECG? The P wave reflects the depolarization of the atrial myocytes. In our model, we could represent the P wave by introducing a second pair of myocytes (i.e., the atrial cells) and allowing them to fire their action potentials much earlier than the two ventricular myocytes.