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Long QT Syndrome

Some families have a rare inherited abnormality called congenital long QT syndrome (LQTS). Individuals with LQTS are often discovered because the individual or a family member presents to a physician with episodes of syncope (fainting) or because an otherwise healthy person dies suddenly and an alert physician suggests that their close relatives get an ECG. The ECG of affected individuals reveals either a long, irregular T wave, a prolonged ST segment, or both. Their hearts have delayed repolarization, which prolongs the ventricular action potential. In addition, when repolarization does occur, the freshly repolar-ized myocardium is subject to sudden, early depolarizations, called afterdepolarizations. These occur because the membrane potential in a small region of myocardium begins to depolarize before it has stabilized at the resting value. Afterdepolarizations may disrupt the normal, synchronized pattern of depolarization, and the ventricles may begin to depolarize in a chaotic pattern called ventricular fibrillation. With ventricular fibrillation, there is no synchronized contraction of ventricular muscle and the heart cannot pump the blood. Arterial pressure drops, blood flow to the brain and other parts of the body ceases, and sudden death occurs.

A single mutation of one of at least four genes, each of which codes for a particular cardiac muscle ion channel, causes LQTS. Mutations of three potassium channels have been discovered. The mutations decrease their function, decreasing potassium current and, thereby, increasing the tendency of the membrane to depolarize. A mutation of the sodium channel has also been found in some patients with LQTS. This mutation increases the sodium channel function, increasing sodium current and the tendency of the membrane to depolarize.

Individuals with congenital LQTS may be children or adults when the abnormality is identified. It is now apparent that at least one cause of sudden infant death syndrome (SIDS) involves a form of LQTS.

trodes of the traditional bipolar limb leads are placed on the left arm, right arm, and left leg (Fig. 13.12). The potential differences between each combination of two of these electrodes give leads I, II, and III. By convention, the left arm in lead I is the positive pole, and the left leg is the positive pole in leads II and III. A unipolar lead is the pair of electrical conductors giving the potential difference between an exploring electrode and a reference input, sometimes called the indifferent electrode. The reference input comes from a combination of electrodes at different sites, which is supposed to give roughly zero potential throughout excitation of the heart. Assuming this to be the case, the recorded electrical activity is the result of the influence of cardiac electrical activity on the exploring electrode. By convention, when the exploring electrode is positive relative to the reference input, an upward deflection is recorded.

The exploring electrode for the precordial or chest leads is the single electrode placed on the anterior and left lateral chest wall. For the chest leads, the reference input is obtained by connecting the three limb electrodes (Fig. 13.13). The observed ECGs recorded from the chest leads are each the result of voltage changes at a specified point on the surface of the chest. Unipolar chest leads are designated V1 to V6 and are placed over the areas of the chest

proposing that certain conventions be followed. The heart is considered to be at the center of a triangle, each corner of which serves as the location for an electrode for two leads to the ECG recorder. The three resulting leads are I, II, and III. By convention, one electrode causes an upward deflection on the recorder when it is under the influence of a positive dipole relative to the other electrode.

proposing that certain conventions be followed. The heart is considered to be at the center of a triangle, each corner of which serves as the location for an electrode for two leads to the ECG recorder. The three resulting leads are I, II, and III. By convention, one electrode causes an upward deflection on the recorder when it is under the influence of a positive dipole relative to the other electrode.

Unipolar Chest Lead

jflGUHHPBB^ Unipolar chest leads. V1 is just to the right of the sternum in the fourth intercostal space. V2 is just to the left of the sternum in the fourth interspace. V4 is in the fifth interspace in the midclavicular line. V3 is midway between V2 and V4. V5 is in the fifth interspace in the anterior axillary line. V6 is in the fifth interspace in the midaxillary line. The three limb leads are combined to give the reference voltage (zero) for the unipolar chest lead (V).

jflGUHHPBB^ Unipolar chest leads. V1 is just to the right of the sternum in the fourth intercostal space. V2 is just to the left of the sternum in the fourth interspace. V4 is in the fifth interspace in the midclavicular line. V3 is midway between V2 and V4. V5 is in the fifth interspace in the anterior axillary line. V6 is in the fifth interspace in the midaxillary line. The three limb leads are combined to give the reference voltage (zero) for the unipolar chest lead (V).

shown in Figure 13.13. The generation of the QRS complex in the chest leads can be explained in a way similar to that for lead I.

The exploratory electrode for an augmented limb lead is an electrode on a single limb. The reference input is the two other limb electrodes connected together. Lead aVR gives the potential difference between the right arm (exploring electrode) and the combination of the left arm and the left leg (reference). Lead aVL gives the potential difference between the left arm and the combination of the right arm and left leg. Lead aVF gives the potential difference between the left leg and the combination of the left arm and right arm.

A standard 12-lead ECG, including six limb leads and six chest leads, is shown in Figure 13.14. The ECG is calibrated so that two dark horizontal lines (1 cm) represent 1 mV, and five dark vertical lines represent 1 second. This means that one light vertical line represents 0.04 sec.

The ECG Provides Information About Cardiac Dipoles as Vectors

Cardiac dipoles are vectors with both magnitude and direction. The net vector produced by all cardiac dipoles at a given time can be determined from the ECG. The direction of the vectors can be determined in the frontal and horizontal planes of the body.

Bipolar Leads Ekg

The bipolar limb leads (leads I, II, and III) and the augmented limb leads (aVR, aVL, and aVF) provide information about the electrical activity of the heart as observed in the frontal plane. As we have seen, lead I is the record of potential differences between the left and right arms. It records only the component of the electrical vector that is parallel to its axis. Lead I can be symbolized by a horizontal line (axis) going through the center of the chest (Fig. 13.15A) in the direction of right arm to left arm. Likewise, lead II can be symbolized by a 60° line drawn through the middle of the chest in the direction of right arm to left leg. The same type of representation can be done for lead III and for the augmented limb leads. The positive ends of the leads are shown by the arrowheads (see Fig. 13.15A). The diagram that results (see Fig. 13.15A) is called the hexaxial reference system.

A net cardiac dipole with its positive charge directed to ward the positive end of the axis of a lead results in the recording of an upward deflection. A net cardiac dipole with its positive charge directed toward the negative end of the axis of a lead results in a downward deflection. A net cardiac dipole with its positive charge directed at a right angle to the axis of a lead results in no deflection. The hexaxial reference system can be used to predict the influence of a cardiac dipole on any of the six leads in the frontal plane. As we will see, this system is useful in understanding changes in the leads of the ECG associated with different diseases.

The unipolar chest leads provide information about cardiac dipoles generated in the horizontal plane (Figure 13.15B). Each chest lead can be represented as having an axis coming from the center of the chest to the site of the exploring electrode in the horizontal plane. The deflections recorded in each chest lead can be understood in terms of this axial system.

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