Guyton and Hall Physiology Chapter 11 Fundamentals of Electrocardiography PDF

CHAPTER 11 UNIT III Fundamentals of Electrocardiography When a cardiac impulse passes through the heart, elec- In F...

CHAPTER 11 UNIT III Fundamentals of Electrocardiography When a cardiac impulse passes through the heart, elec- In Figure 11-2A, depolarization, demonstrated by red trical current also spreads from the heart into the adja- positive charges inside and red negative charges outside, cent tissues surrounding the heart. A small portion of the is traveling from left to right. The first half of the fiber has current spreads all the way to the surface of the body. If already depolarized while the remaining half is still polar- electrodes are placed on the skin on opposite sides of the ized. Therefore, the left electrode on the outside of the fiber heart, electrical potentials generated by the current can is in an area of negativity, and the right electrode is in an area be recorded; the recording is known as an electrocardio- of positivity, which causes the meter to record positively. To gram (ECG). A normal ECG for two beats of the heart is the right of the muscle fiber is shown a record of changes in shown in Figure 11-1. potential between the two electrodes, as recorded by a high- speed recording meter. Note that when depolarization has reached the halfway mark in Figure 11-2A, the recording on WAVEFORMS OF THE NORMAL the right has risen to a maximum positive value. ELECTROCARDIOGRAM In Figure 11-2B, depolarization has extended over The normal ECG (see Figure 11-1) is composed of a P the entire muscle fiber, and the recording to the right has wave, a QRS complex, and a T wave. The QRS complex is returned to the zero baseline because both electrodes are often, but not always, three separate waves: the Q wave, now in areas of equal negativity. The completed wave is a the R wave, and the S wave. depolarization wave because it results from the spread of The P wave is caused by electrical potentials generated depolarization along the muscle fiber membrane. when the atria depolarize before atrial contraction begins. Figure 11-2C shows halfway repolarization of the The QRS complex is caused by potentials generated when same muscle fiber, with positivity returning to the outside the ventricles depolarize before contraction—that is, as the of the fiber. At this point, the left electrode is in an area of depolarization wave spreads through the ventricles. There- positivity, and the right electrode is in an area of negativ- fore, both the P wave and the components of the QRS com- ity. This polarity is opposite to the polarity in Figure 11- plex are depolarization waves. 2A. Consequently, the recording, as shown to the right, The T wave is caused by potentials generated as the ven- becomes negative. tricles recover from depolarization. This process normally In Figure 11-2D, the muscle fiber has completely occurs in ventricular muscle 0.25 to 0.35 second after depo- repolarized, and both electrodes are now in areas of posi- larization. The T wave is known as a repolarization wave. tivity so that no potential difference is recorded between Thus, the ECG is composed of both depolarization and them. Thus, in the recording on the right, the potential repolarization waves. The principles of depolarization returns once more to zero. This completed negative wave and repolarization are discussed in Chapter 5. The dis- is a repolarization wave because it results from the spread tinction between depolarization waves and repolarization of repolarization along the muscle fiber membrane. waves is so important in electrocardiography that further clarification is necessary. Relation of the Monophasic Action Potential of Ventricular Muscle to the QRS and T Waves in the Standard Electrocardiogram. The monophasic action CARDIAC DEPOLARIZATION WAVES potential of ventricular muscle, discussed in Chapter 10, VERSUS REPOLARIZATION WAVES normally lasts between 0.25 and 0.35 second. The top part Figure 11-2 shows a single cardiac muscle fiber in four of Figure 11-3 shows a monophasic action potential re- stages of depolarization and repolarization, with the color corded from a microelectrode inserted into the inside of a red designating depolarization. During depolarization, single ventricular muscle fiber. The upsweep of this action the normal negative potential inside the fiber reverses and potential is caused by depolarization, and the return of becomes slightly positive inside and negative outside. the potential to the baseline is caused by repolarization. 135 UNIT III The Heart Atria Ventricles RR interval +1.0 R +0.5 S-T Millivolts T segment P 0 Q P-R interval = 0.16 sec S Q-T interval –0.5 Figure 11-1. Normal electrocardio- 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 gram. Time (sec) 0 The lower half of Figure 11-3 shows a simultaneous recording of the ECG from this same ventricle. Note that − + the QRS waves appear at the beginning of the monopha- − + sic action potential, and the T wave appears at the end. Note especially that no potential is recorded in the ECG when the ventricular muscle is either completely polarized −−−−−−− +++++++++ + or completely depolarized. Only when the muscle is partly +++++++ −−−−−−−−− polarized and partly depolarized does current flow from +++++++ −−−−−−−−− −−−−−−− +++++++++ one part of the ventricles to another part, and therefore A − Depolarization current also flows to the surface of the body to produce wave + the ECG. −−−−−−−−−−−−−−−− ++++++++++++++++ ++++++++++++++++ −−−−−−−−−−−−−−−− − B RELATIONSHIP OF ATRIAL AND VENTRICULAR CONTRACTION TO THE +++++++++−−−−−−− + WAVES OF THE ELECTROCARDIOGRAM −−−−−−−−−+++++++ −−−−−−−−−+++++++ Before contraction of muscle can occur, depolarization +++++++++−−−−−−− − must spread through the muscle to initiate the chemical C Repolarization processes of contraction. Refer again to Figure 11-1; the ++++++++++++++++ + wave P wave occurs at the beginning of contraction of the atria, −−−−−−−−−−−−−−−− and the QRS complex of waves occurs at the beginning of −−−−−−−−−−−−−−−− contraction of the ventricles. The ventricles remain con- ++++++++++++++++ − D 0.30 second tracted until after repolarization has occurred—that is, Figure 11-2. Recording the depolarization wave (A and B) and the until after the end of the T wave. repolarization wave (C and D) from a cardiac muscle fiber. The atria repolarize about 0.15 to 0.20 second after ter- mination of the P wave, which is also approximately when the QRS complex is being recorded in the ECG. There- Depolarization fore, the atrial repolarization wave, known as the atrial T Repolarization wave, is usually obscured by the much larger QRS com- plex. For this reason, an atrial T wave is seldom observed on the ECG. The ventricular repolarization wave is the T wave of the normal ECG. Ordinarily, ventricular muscle begins R to repolarize in some fibers about 0.20 second after the P T beginning of the depolarization wave (the QRS complex), Q but in many other fibers, it takes as long as 0.35 second. S Thus, the process of ventricular repolarization extends over a long period, about 0.15 second. For this reason, the T wave in the normal ECG is a prolonged wave, but the Figure 11-3. Top, Monophasic action potential from a ventricular mus- cle fiber during normal cardiac function showing rapid depolarization and voltage of the T wave is considerably less than the volt- then repolarization occurring slowly during the plateau stage but rapidly age of the QRS complex, partly because of its prolonged toward the end. Bottom, Electrocardiogram recorded simultaneously. length. 136  Chapter 11 Fundamentals of Electrocardiography ELECTROCARDIOGRAPHIC CALIBRATION 0 0 0 AND DISPLAY − + − + − + All recordings of ECGs are made with appropriate cali- − + − + − + bration lines on the display grid. Historically, ECGs were UNIT III recorded electronically and printed onto paper; ECGs are +++++ now usually displayed digitally. As shown in Figure 11-1, + + ++++++++ + +++++ +−+−+−+−+−+ +++++ + the horizontal calibration lines are arranged so that 10 of the ++++++ −−−−−−−−−−−−−−− +−+++++ + ++++++ −− −− −− −− −− −− −− −− −− − +++++ + small line divisions upward or downward in the standard +++++ + − − − − − − − − − − +++++ + − − − − − − − − +++++ − − − − − − − − − +++++ − − ECG represent 1 millivolt, with positivity in the upward +++++ +−−−−−−−−−−−−−−−−− +++++ + direction and negativity in the downward direction. ++++++ − − − −− −− −− −− ++++++ +++++ + + + − − + ++++++ + The vertical lines on the ECG are time calibration lines. +++++++++++ +++ ++++++++++ + A typical ECG is run at a speed of 25 millimeters per sec- ond, although faster speeds are sometimes used. Therefore, Figure 11-4. Instantaneous potentials develop on the surface of a cardiac muscle mass that has been depolarized in its center. each 25 millimeters in the horizontal direction is 1 second, and each 5-millimeter segment, indicated by the dark ver- tical lines, represents 0.20 second. The 0.20-second inter- Heart Rate as Determined from the Electrocardio- vals are then broken into five smaller intervals by thin lines, gram. The rate of the heartbeat can be determined easily each of which represents 0.04 second. from an ECG because the heart rate is the reciprocal of the time interval between two successive heartbeats (the Normal Voltages in the Electrocardiogram. The re- R-R interval). If the interval between two beats as deter- corded voltages of the waves in the normal ECG depend mined from the time calibration lines is 1 second, the on the manner in which the electrodes are applied to the heart rate is 60 beats/min. The normal interval between surface of the body and how close the electrodes are to the two successive QRS complexes in an adult is about 0.83 heart. When one electrode is placed directly over the ven- second, which is a heart rate of 60/0.83 times/min, or 72 tricles, and a second electrode is placed elsewhere on the beats/min. body remote from the heart, the voltage of the QRS com- plex may be as high as 3 to 4 millivolts. Even this voltage is FLOW OF CURRENT AROUND THE small in comparison with the monophasic action poten- HEART DURING THE CARDIAC CYCLE tial of 110 millivolts recorded directly at the heart muscle membrane. When ECGs are recorded from electrodes on the two arms or on one arm and one leg, the voltage of the Recording Electrical Potentials from a QRS complex usually is 1.0 to 1.5 millivolts from the top Partially Depolarized Mass of Syncytial of the R wave to the bottom of the S wave, the voltage of Cardiac Muscle the P wave is between 0.1 and 0.3 millivolts, and the volt- Figure 11-4 shows a syncytial mass of cardiac muscle that age of the T wave is between 0.2 and 0.3 millivolts. has been stimulated at its most central point. Before stimu- lation, all the exteriors of the muscle cells had been positive, P-Q or P-R Interval. The time between the beginning of and the interiors had been negative. For reasons presented the P wave and the beginning of the QRS complex is the in Chapter 5 in the discussion of membrane potentials, as interval between the beginning of electrical excitation of soon as an area of cardiac syncytium becomes depolarized, the atria and the beginning of excitation of the ventricles. negative charges leak to the outsides of the depolarized This period is called the P-Q interval. The normal P-Q muscle fibers, making this part of the surface electronega- interval is about 0.16 second. (Often, this interval is called tive, as represented by the minus signs in Figure 11-4. The the P-R interval because the Q wave is likely to be ab- remaining surface of the heart, which is still polarized, is sent.) The P-R interval shortens at faster heart rates due represented by the plus signs. Therefore, a meter connected to increased sympathetic or decreased parasympathetic with its negative terminal on the area of depolarization and activity, which increase atrioventricular node conduction its positive terminal on one of the still polarized areas, as speed. Conversely, the P-R interval lengthens with slower shown to the right in the figure, records positively. heart rates as a consequence of slower atrioventricular Two other electrode placements and meter readings nodal conduction caused by increased parasympathetic are also demonstrated in Figure 11-4. These placements tone or withdrawal of sympathetic activity. and readings should be studied carefully, and the reader should be able to explain the causes of the respective meter Q-T Interval. Contraction of the ventricle lasts almost readings. Because the depolarization spreads in all direc- from the beginning of the Q wave (or R wave, if the Q tions through the heart, the potential differences shown wave is absent) to the end of the T wave. This interval in the figure persist for only a few thousandths of a sec- is called the Q-T interval and ordinarily is about 0.35 ond, and the actual voltage measurements can be accom- second. plished only with a high-speed recording apparatus. 137 UNIT III The Heart + 0.5 mV 0 – + – + Lead I 0 – + – + – – – + + + B A – 0.2 mV + + ++ + 0.3 mV – – –– + + – –+ + + – –+ + ++ –– –+ ++ + + +–+ + + + ++ + + + + ++ + 1.2 mV + 0.7 mV 0 0 – + – + Figure 11-5. Flow of current in the chest around partially depolar- – + – + ized ventricles. A and B are electrodes. Lead II Lead III + 1.0 mV Flow of Electrical Currents in the Chest Around the Heart Figure 11-5 shows the ventricular muscle lying within the Figure 11-6. Conventional arrangement of electrodes for recording chest. Even the lungs, although mostly filled with air, con- the standard electrocardiographic leads. Einthoven’s triangle is super- duct electricity to a surprising extent, and fluids in other imposed on the chest. tissues surrounding the heart conduct electricity even more easily. Therefore, the heart is actually suspended in surface outward through the ventricular muscle mass. a conductive medium. When one portion of the ventricles Then, immediately before depolarization has completed depolarizes and therefore becomes electronegative with its course through the ventricles, the average direction of respect to the remainder, electrical current flows from the current flow reverses for about 0.01 second, flowing from depolarized area to the polarized area in large circuitous the ventricular apex toward the base, because the last part routes, as noted in the figure. of the heart to become depolarized is the outer walls of It should be recalled from the discussion of the Pur- the ventricles near the base of the heart. kinje system in Chapter 10 that the cardiac impulse first Thus, in normal heart ventricles, current flows from arrives in the ventricles in the septum and shortly there- negative to positive primarily in the direction from the after spreads to the inside surfaces of the remainder of base of the heart toward the apex during almost the entire the ventricles, as shown by the red areas and the negative cycle of depolarization, except at the very end. If a meter signs in Figure 11-5. This process provides electronega- is connected to electrodes on the surface of the body, as tivity on the insides of the ventricles and electropositivity shown in Figure 11-5, the electrode nearer the base will on the outer walls of the ventricles, with electrical cur- be negative, whereas the electrode nearer the apex will rent flowing through the fluids surrounding the ventricles be positive, and the recording meter will show a positive along elliptical paths, as demonstrated by the curving recording in the ECG. arrows in the figure. If one algebraically averages all the lines of current flow (the elliptical lines), the average cur- ELECTROCARDIOGRAPHIC LEADS rent flow occurs with negativity toward the base of the heart and with positivity toward the apex. During most of the remainder of the depolarization Three Standard Bipolar Limb Leads process, current also continues to flow in this same direc- Figure 11-6 shows electrical connections between the tion, whereas depolarization spreads from the endocardial patient’s limbs and the electrocardiograph for recording 138  Chapter 11 Fundamentals of Electrocardiography ECGs from the so-called standard bipolar limb leads. The term bipolar means that the ECG is recorded from two electrodes located on different sides of the heart—in this case, on the limbs. Thus, a lead is not a single wire con- I necting from the body but a combination of two wires and UNIT III their electrodes to make a complete circuit between the body and the electrocardiograph. The electrocardiograph in each case is represented by an electrical meter in the diagram, although the actual electrocardiograph is a high- speed, computer-based system with an electronic display. II Lead I. In recording limb lead I, the negative terminal of the electrocardiograph is connected to the right arm, and the positive terminal is connected to the left arm. There- fore, when the point where the right arm connects to the chest is electronegative with respect to the point where the left arm connects, the electrocardiograph records positively—that is, above the zero-voltage line in the III ECG. When the opposite is true, the electrocardiograph records below the line. Lead II. To record limb lead II, the negative terminal of Figure 11-7. Normal electrocardiograms recorded from the three standard electrocardiographic leads (I–III). the electrocardiograph is connected to the right arm and the positive terminal is connected to the left leg. There- fore, when the right arm is negative with respect to the with respect to the average potential in the body, the left left leg, the electrocardiograph records positively. arm is +0.3 millivolts (positive), and the left leg is +1.0 millivolts (positive). Observing the meters in the figure, Lead III. To record limb lead III, the negative terminal of one can see that lead I records a positive potential of +0.5 the electrocardiograph is connected to the left arm, and millivolts because this is the difference between the −0.2 the positive terminal is connected to the left leg. This millivolts on the right arm and the +0.3 millivolts on the configuration means that the electrocardiograph records left arm. Similarly, lead III records a positive potential of positively when the left arm is negative with respect to +0.7 millivolts, and lead II records a positive potential of the left leg. +1.2 millivolts because these are the instantaneous poten- tial differences between the respective pairs of limbs. Einthoven’s Triangle. In Figure 11-6, the triangle, called Now, note that the sum of the voltages in leads I and Einthoven’s triangle, is drawn around the area of the heart. III equals the voltage in lead II; that is, 0.5 plus 0.7 equals This triangle illustrates that the two arms and left leg form 1.2. Mathematically, this principle, called Einthoven’s law, apices of a triangle surrounding the heart. The two apices holds true at any given instant while the three “standard” at the upper part of the triangle represent the points at bipolar ECGs are being recorded. which the two arms connect electrically with the fluids around the heart, and the lower apex is the point at which Normal Electrocardiograms Recorded from the the left leg connects with the fluids. Three Standard Bipolar Limb Leads. Figure 11-7 shows recordings of the ECGs in leads I, II, and III. It is Einthoven’s Law. Einthoven’s law states that if the ECGs obvious that the ECGs in these three leads are similar are recorded simultaneously with the three limb leads, the to one another because they all record positive P waves sum of the potentials recorded in leads I and III will equal and positive T waves, and the major portion of the QRS the potential in lead II: complex is also positive in each ECG. On analysis of the Lead I potential + Lead III potential = Lead II potential three ECGs, it can be shown, with careful measurements and proper observance of polarities, that at any given In other words, if the electrical potentials of any two instant, the sum of the potentials in leads I and III equals of the three bipolar limb electrocardiographic leads are the potential in lead II, thus illustrating the validity of known at any given instant, the third one can be deter- Einthoven’s law. mined by simply summing the first two. Note, however, Because the recordings from all the bipolar limb that the positive and negative signs of the different leads leads are similar to one another, it does not mat- must be observed when making this summation. ter greatly which lead is recorded when one wants For instance, let us assume that momentarily, as noted to diagnose different cardiac arrhythmias, because in Figure 11-6, the right arm is −0.2 millivolts (negative) diagnosis of arrhythmias depends mainly on the time 139 UNIT III The Heart 6 5 1 2 4 34 5 6 1 2 3 V1 V2 V3 V4 V5 V6 Figure 11-9. Normal electrocardiograms recorded from the six standard chest leads. RA 5000 LA ohms 5000 ohms 0 – + – + aVR aVL aVF Figure 11-10. Normal electrocardiograms recorded from the three 5000 augmented unipolar limb leads. ohms Figure 11-9 illustrates the ECGs of the healthy heart as recorded from these six standard chest leads. Because the heart surfaces are close to the chest wall, each chest Figure 11-8. Connections of the body with the electrocardiograph lead records mainly the electrical potential of the cardiac for recording chest leads. LA, Left arm; RA, right arm. musculature immediately beneath the electrode. There- fore, relatively minute abnormalities in the ventricles, particularly in the anterior ventricular wall, can cause relationships between the different waves of the car- marked changes in the ECGs recorded from individual diac cycle. However, when one wants to diagnose dam- chest leads. age in the ventricular or atrial muscle or in the Purkinje In leads V1 and V2, the QRS recordings of the normal conducting system, it matters greatly which leads are heart are mainly negative because, as shown in Figure recorded, because abnormalities of cardiac muscle 11-8, the chest electrode in these leads is closer to the contraction or cardiac impulse conduction change the base of the heart than to the apex, and the base of the patterns of the ECGs markedly in some leads yet may heart is the direction of electronegativity during most of not affect other leads. Electrocardiographic interpreta- the ventricular depolarization process. Conversely, the tion of these two types of conditions—cardiac myopa- QRS complexes in leads V4, V5, and V6 are mainly positive thies and cardiac arrhythmias—is discussed separately because the chest electrode in these leads is closer to the in Chapters 12 and 13. heart apex, which is the direction of electropositivity dur- ing most of depolarization. Precordial Leads Often ECGs are recorded with one electrode placed on the anterior surface of the chest directly over the Augmented Limb Leads heart at one of the points shown in Figure 11-8. This Another system of leads in wide use is the augmented electrode is connected to the positive terminal of the limb leads. In this type of recording, two of the limbs electrocardiograph, and the negative electrode, called are connected through electrical resistances to the neg- the indifferent electrode or Wilson central terminal, is ative terminal of the electrocardiograph, and the third connected through equal electrical resistances to the limb is connected to the positive terminal. When the right arm, left arm, and left leg all at the same time, positive terminal is on the right arm, the lead is known as also shown in the figure. Usually, six standard chest as the aVR lead; when on the left arm, it is known as leads are recorded, one at a time, from the anterior the aVL lead; and when on the left leg, it is known as chest wall, with the chest electrode being placed the aVF lead. sequentially at the six points shown in the diagram. Normal recordings of the augmented limb leads are The different recordings are known as leads V1, V2, V3, shown in Figure 11-10. They are all similar to the stan- V4, V5, and V6. dard limb lead recordings, except that the recording from 140  Chapter 11 Fundamentals of Electrocardiography Standard Limb Leads Augmented Leads Precordial Leads I aVR V1 V4 10mm/mv UNIT III II aVL V2 V5 III aVF V3 V6 Figure 11-11. Normal 12-lead electrocardiogram. the aVR lead is inverted. (Why does this inversion occur? cause strokes. Because the daily variability in the frequency Study the polarity connections to the electrocardiograph of arrhythmias is substantial, detection often requires am- to determine the answer to this question.) bulatory electrocardiographic monitoring throughout the day. Electrocardiographic Display There are several categories of ambulatory electrocar- diographic recorders. Continuous recorders (Holter moni- Leads are typically displayed into three groupings as in Fig- tors), are typically used for 24 to 48 hours to investigate the ure 11-11: the standard bipolar limb leads (I, II, III) fol- relationship of symptoms and electrocardiographic events lowed by the augmented leads (aVR, aVL, and aVF) and that are likely to occur within that time frame. Intermittent then the precordial leads (V1–V6). recorders are used for longer periods (weeks to months) to provide brief intermittent recordings for detection of Ambulatory Electrocardiography events that occur infrequently; these recordings are usually Standard ECGs provide an assessment of cardiac electri- initiated by the patient when experiencing symptoms. In cal events over a brief duration, usually while the patient some cases, a small device, about the size of a large paper is resting. In conditions associated with infrequent but im- clip and called an implantable loop recorder, is implanted portant abnormalities of cardiac rhythms, it may be useful just under the skin in the chest to monitor the heart’s elec- to examine the ECG over a longer period, thereby permit- trical activity continuously for as long as 2 to 3 years. The ting evaluation of changing cardiac electrical phenomena device can be programmed to initiate a recording when the that are transient and may be missed with a standard rest- heart rate falls below, or rises above, a predetermined level, ing ECG. Extending the ECG to allow assessment of car- or it can be activated manually by the patient when a symp- diac electrical events while the patient is ambulating during tom such as dizziness occurs. Improvements in solid-state normal daily activities is called ambulatory electrocardiog- digital technology and recorders equipped with micropro- raphy. cessors now permit continuous or intermittent transmis- Ambulatory electrocardiographic monitoring is typi- sion of digital electrocardiographic data over telephone cally used when a patient demonstrates symptoms that lines, and sophisticated software systems provide rapid on- are thought to be caused by transient arrhythmias or other line computerized analysis of the data as they are acquired. transient cardiac abnormalities. These symptoms may in- Newer wearable devices, including watches or hand-held clude chest pain, syncope (fainting) or near syncope, diz- electrocardiographic monitoring devices, are also being de- ziness, and irregular heartbeats (palpitations). The crucial veloped for home-based heart rhythm monitoring. information needed to diagnose serious transient arrhyth- mias or other cardiac conditions is a recording of an ECG during the precise time that the symptom is occurring. Bibliography These devices can also be used to detect asymptomatic cardiac arrhythmias such as atrial fibrillation that may in- See the bibliography for Chapter 13. crease the risk of embolus formation, which can, in turn, 141