2. The Normal Electrocardiogram PDF

2. THE NORMAL ELECTROCARDIOGRAM Belén Merck MD PhD  THE NORMAL ELECTROCARDIOGRAM When the cardiac impulse passes through the heart, electrical current also spreads from the heart into the adjacent tissues surrounding the heart. A small portion of...

2. THE NORMAL ELECTROCARDIOGRAM Belén Merck MD PhD  THE NORMAL ELECTROCARDIOGRAM When the cardiac impulse passes through the heart, electrical current also spreads from the heart into the adjacent tissues surrounding the heart. A small portion of the current spreads all the way to the surface of the body. If electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the current can be recorded; the recording is known as an electrocardiogram.  THE NORMAL ELECTROCARDIOGRAM The normal electrocardiogram is composed of a P wave, a QRS complex, and a T wave. The QRS complex is often, but not always, three separate waves: the Q wave, the R wave, and the S wave. The P wave is caused by electrical potentials generated when the atria depolarize before atrial contraction begins.  THE NORMAL ELECTROCARDIOGRAM The QRS complex is caused by potentials generated when the ventricles depolarize before contraction, that is, as the depolarization wave spreads through the ventricles. Therefore, both the P wave and the components of the QRS complex are depolarization waves. The T wave is caused by potentials generated as the ventricles recover from the state of depolarization.  THE NORMAL ELECTROCARDIOGRAM This process normally occurs in ventricular muscle 0.25 to 0.35 second after depolarization, and the T wave is known as a repolarization wave. Thus, the electrocardiogram is composed of both depolarization and repolarization waves.  STAGES OF DEPOLARIZATION AND REPOLARIZATION During depolarization, the normal negative potential inside the fiber reverses and becomes slightly positive inside and negative outside. Depolarization is demonstrated by red positive charges inside and red negative charges outside, is traveling from left to right. The first half of the fiber has already depolarized, while the remaining half is still polarized.  STAGES OF DEPOLARIZATION AND REPOLARIZATION Therefore, the left electrode on the outside of the fiber is in an area of negativity, and the right electrode is in an area of positivity; this causes the meter to record positively. To the right of the muscle fiber is shown a record of changes in potential between the two electrodes. Note that when depolarization has reached the halfway mark, the record has risen to a maximum positive value.  STAGES OF DEPOLARIZATION AND REPOLARIZATION Now depolarization has extended over the entire muscle fiber, and the recording to the right has returned to the zero baseline because both electrodes are now in areas of equal negativity. The completed wave is a depolarization wave because it results from spread of depolarization along the muscle fiber membrane.  STAGES OF DEPOLARIZATION AND REPOLARIZATION This figure shows halfway repolarization of the same muscle fiber, with positivity returning to the outside of the fiber. At this point, the left electrode is in an area of positivity, and the right electrode is in an area of negativity. Consequently, the recording becomes negative.  STAGES OF DEPOLARIZATION AND REPOLARIZATION The muscle fiber has completely repolarized, and both electrodes are now in areas of positivity so that no potential difference is recorded between them. Thus, in the recording to the right, the potential returns once more to zero. This completed negative wave is a repolarization wave because it results from spread of repolarization along the muscle fiber membrane.  DEPOLARIZATION AND REPOLARIZATION No potential is recorded in the electrocardiogram 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 current also flows to the surface of the body to produce the electrocardiogram.  RELATIONSHIP OF CONTRACTION TO EKG WAVES Before contraction of muscle can occur, depolarization must spread through the muscle to initiate the chemical processes of contraction. The 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 contracted until after repolarization has occurred, that is, until after the end of the T wave.  RELATIONSHIP OF CONTRACTION TO EKG WAVES The atria repolarize about 0.15 to 0.20 second after termination of the P wave. This is also approximately when the QRS complex is being recorded in the electrocardiogram. Therefore, the atrial repolarization wave, known as the atrial T wave, is usually obscured by the much larger QRS complex. For this reason, an atrial T wave seldom is observed in the electrocardiogram.  RELATIONSHIP OF CONTRACTION TO EKG WAVES The ventricular repolarization wave is the T wave of the normal EKG. Ventricular muscle begins to repolarize in some fibers about 0.20 s after the beginning of the depolarization wave, but in many other fibers, it takes as long as 0.35 s. The process of ventricular repolarization extends over 0.15 second. For this reason, the T wave in the normal electrocardiogram is a prolonged wave, but the voltage of the T wave is considerably less than the voltage of the QRS complex, partly because of its prolonged length.  VOLTAGE AND TIME CALIBRATION All recordings of electrocardiograms are made with appropriate calibration lines on the recording paper. The horizontal calibration lines are arranged so that 10 of the small line divisions upward or downward in the standard electrocardiogram represent 1 millivolt, with positivity in the upward direction and negativity in the downward direction.  VOLTAGE AND TIME CALIBRATION The vertical lines on the electrocardiogram are time calibration lines. A typical EKG is run at a paper speed of 25 mm/s. Each 25 mm in the horizontal direction is 1 s, and each 5-mm segment (dark vertical lines) represents 0.20 s. The 0.20-s intervals are then broken into five smaller intervals by thin lines, each of which represents 0.04 s.  VOLTAGE AND TIME CALIBRATION The recorded voltages of the waves in the normal EKG depend on the manner in which the electrodes are applied to the surface of the body and how close the electrodes are to the heart. When one electrode is placed directly over the ventricles and a second electrode is placed elsewhere on the body remote from the heart, the voltage of the QRS complex may be as great as 3 to 4 millivolts.  VOLTAGE AND TIME CALIBRATION The time between the beginning of the P wave and the beginning of the QRS complex is the interval between the beginning of electrical excitation of the atria and the beginning of excitation of the ventricles. This period is called the P-Q interval. The normal P-Q interval is about 0.16 second. Often this interval is called the P-R interval because the Q wave is likely to be absent.  VOLTAGE AND TIME CALIBRATION Contraction of the ventricle lasts almost from the beginning of the Q wave to the end of the T wave. This interval is called the Q-T interval and is about 0.35 s. The rate of heartbeat can be determined easily from an electrocardiogram because the heart rate is the reciprocal of the time interval between two successive heartbeats.  VOLTAGE AND TIME CALIBRATION If the interval between two beats as determined from the time calibration lines is 1 second, the heart rate is 60 beats per minute. The normal interval between two successive QRS complexes in the adult person is about 0.83 second. This is a heart rate of 60/0.83 times per minute, or 72 beats per minute.  FLOW OF CURRENT AROUND THE HEART Before stimulation, all the exteriors of the muscle cells had been positive and the interiors negative. As soon as an area of cardiac syncytium becomes depolarized, negative charges leak to the outsides of the depolarized muscle fibers, making this part of the surface electronegative. The remaining surface of the heart, which is still polarized, is represented by the positive signs.  FLOW OF CURRENT AROUND THE HEART Therefore, a meter connected with its negative terminal on the area of depolarization and its positive terminal on one of the still-polarized areas records positively. Because the depolarization spreads in all directions through the heart, the potential differences shown in the figure persist for only a few thousandths of a second, and the actual voltage measurements can be accomplished only with a high-speed recording apparatus.  FLOW OF CURRENT AROUND THE HEART Even the lungs, although mostly filled with air, conduct electricity to a surprising extent, and fluids in other tissues surrounding the heart conduct electricity even more easily. Therefore, the heart is actually suspended in a conductive medium. When one portion of the ventricles depolarizes and therefore becomes electronegative with respect to the remainder, electrical current flows from the depolarized area to the polarized area in large circuitous routes.  FLOW OF CURRENT AROUND THE HEART The cardiac impulse first arrives in the ventricles in the septum and shortly thereafter spreads to the inside surfaces of the remainder of the ventricles. This provides electronegativity on the insides of the ventricles and electropositivity on the outer walls of the ventricles, with electrical current flowing through the fluids surrounding the ventricles along elliptical paths. If one algebraically averages all the lines of current flow (the elliptical lines), one finds that the average current flow occurs with negativity toward the base of the heart and with positivity toward the apex.  FLOW OF CURRENT AROUND THE HEART During most of the remainder of the depolarization process, current also continues to flow in this same direction, while depolarization spreads from the endocardial surface outward through the ventricular muscle mass. Then, immediately before depolarization has completed its course through the ventricles, the average direction of current flow reverses for about 0.01 s, flowing from the ventricular apex toward the base, because the last part of the heart to become depolarized is the outer walls of the ventricles near the base of the heart.  FLOW OF CURRENT AROUND THE HEART Thus, in normal heart ventricles, current flows from negative to positive primarily in the direction from the base of the heart toward the apex during almost the entire cycle of depolarization, except at the very end. And if a meter is connected to electrodes on the surface of the body, the electrode nearer the base will be negative, whereas the electrode nearer the apex will be positive, and the recording meter will show positive recording in the electrocardiogram.  THREE BIPOLAR LIMB LEADS The term “bipolar” means that the EKG 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 connecting from the body but a combination of two wires and their electrodes to make a complete circuit between the body and the electrocardiograph. The electrocardiograph in each instance is represented by an electrical meter in the diagram, although the actual electrocardiograph is a high-speed recording meter with a moving paper.  THREE BIPOLAR LIMB LEADS In recording limb lead I, the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal to the left arm. 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 EKG. When the opposite is true, the electrocardiograph records below the line.  THREE BIPOLAR LIMB LEADS To record limb lead II, the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal to the left leg. When the right arm is negative with respect to the left leg, the EKG records positively. To record limb lead III, the negative terminal of the electrocardiograph is connected to the left arm and the positive terminal to the left leg. This means that the electrocardiograph records positively when the left arm is negative with respect to the left leg.  CHEST LEADS (PRECORDIAL LEADS) Often EKG are recorded with one electrode placed on the anterior surface of the chest directly over the heart. This electrode is connected to the positive terminal of the electrocardiograph, and the negative electrode, called the indifferent electrode, is connected through equal electrical resistances to the right arm, left arm, and left leg all at the same time.  CHEST LEADS (PRECORDIAL LEADS) Usually six standard chest leads are recorded, one at a time, from the anterior chest wall, the chest electrode being placed sequentially at the six points shown in the diagram. The different recordings are known as leads V1, V2, V3, V4, V5, and V6. Because the heart surfaces are close to the chest wall, each chest lead records mainly the electrical potential of the cardiac musculature immediately beneath the electrode.  CHEST LEADS (PRECORDIAL LEADS) Therefore, relatively minute abnormalities in the ventricles, particularly in the anterior ventricular wall, can cause marked changes in the EKG recorded from individual chest leads. In leads V1 and V2, the QRS recordings of the normal heart are mainly negative because, the chest electrode in these leads is nearer to the base of the heart than to the apex, and the base of the heart is the direction of electronegativity during most of the ventricular depolarization process.  CHEST LEADS (PRECORDIAL LEADS) Conversely, the QRS complexes in leads V4, V5, and V6 are mainly positive because the chest electrode in these leads is nearer the heart apex, which is the direction of electropositivity during most of depolarization.  AUGMENTED UNIPOLAR LIMB LEADS Another system of leads in wide use is the augmented unipolar limb lead. In this type of recording, two of the limbs are connected through electrical resistances to the negative terminal of the electrocardiograph, and the third limb is connected to the positive terminal. When the positive terminal is on the right arm, the lead is known as the aVR lead; when on the left arm, the aVL lead; and when on the left leg, the aVF lead.  AUGMENTED UNIPOLAR LIMB LEADS Normal recordings of the augmented unipolar limb leads are all similar to the standard limb lead recordings, except that the recording from the aVR lead is inverted.