Electrocardiogram (ECG) is the recording of the electrical activity of the heart on a graph paper. The electrical activity is generated by the activation (depolarization) and inactivation (repolarization) of the atrial and the ventricular muscle (myocardium). The myocardium is activated by the conduction system of the heart. The electrical activity of the conduction system is itself too small to be recorded by the ECG! ECG machine is a modified galvanometer that outputs the electrical activity of the heart in the form of tracings. The machine is normally standardized to output one millivolt of electrical activity as 10 millimeter deflection along the vertical axis of the graph paper (Figure 1.1). The standard speed of the paper is 25 mm/sec.
CONDUCTION SYSTEM OF THE HEART
The conduction system of the heart consists of the sinoatrial node (SA node), preferential interatrial conduction fibers, atrioventricular node (AV node), bundle of His, bundle branches (right and left), and the Purkinje network (Figure 1.2). The left bundle divides into two fascicles (anterosuperior and posteroinferior) before ramification into the Purkinje network. Any of the cells in the conduction system is capable of spontaneous firing and initiating the heartbeat. The SA node is the normal pacemaker of the heart and impulse originating from it activates the rest of the heart. Under normal conditions, the frequency of depolarization of the SA node is faster, and so it overrides the other foci in the conduction system. However, when the SA node is diseased, other subsidiary pacemakers can take over the rhythm of the heart.
Impulses originating from the SA node activate the atria, with the right atrium being activated first, followed by the left atrium. The impulse also travels towards the AV node by preferential conduction pathways formed by the atrial musculature. Once the impulse reaches the AV node, there is a delay due to slowing of conduction in the AV node. From the AV node, the impulse spreads to the bundle of His, the bundle branches and the fascicles. Finally, the impulse reaches the Purkinje fibers in the endocardium of the heart.
The impulse reaches the endocardium of all regions of the heart near simultaneously and results in synchronized depolarization of the entire ventricular myocardium.
Leads
For recording the electrical activity, electrode pairs called leads are used. Each of these electrode pairs (leads) will record the electrical activity of the heart as seen by them. It is true that the more the number of leads we use, the more complete picture can be obtained about the electrical activity of the heart. However, using too many leads will be cumbersome and using less will lead to insufficient data. The standard practice is to use 12 leads to record the heart's electrical activity. Six of these leads record the electrical activity in the frontal plane (limb leads) and six of them record the electrical activity in the horizontal plane (chest leads, Figure 1.3 and Table 1.1).
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The leads can be bipolar or unipolar. Bipolar leads record the electrical activity between two active electrodes (positive and negative), while the unipolar leads record the electrical activity between an active and an inactive electrode. For the unipolar recordings, reference electrode is formed by connecting the right arm, left arm and left leg electrodes together to form a Wilson Central Terminal (WCT).
The limb leads include lead I, II, III, aVR, aVL and aVF. Leads I, II and III are bipolar leads and leads aVR, aVL and aVF are unipolar leads. For lead I, the positive pole is the left arm and the negative pole is the right arm. For lead II, the positive pole is the left leg and the negative pole is the right arm. For lead III, the left leg is the positive pole and the left arm is the negative pole. Misplacement of the electrodes can result in abnormal looking ECGs! Leads aVR, aVL and aVF are augmented unipolar leads recorded with the active pole on the right arm, left arm and left leg respectively. The indifferent electrode for these leads is formed by removing the corresponding active electrode from the WCT, which amplifies the signal by 50%.5
The six chest leads include V1 to V6 and all are unipolar leads. It is important to place the chest leads in standard positions to make a meaningful interpretation of the ECG. V1 is positioned in the right fourth intercostal space adjacent to the sternum and V2 is placed in the left fourth intercostal space adjacent to the sternal border. Next V4 is placed in the fifth intercostal space in the midclavicular line. V3 is placed midway between V2 and V4. V5 is placed in the anterior axillary line along the same line as V4 and V6 is place in the mid- axillary line along the same line as V4 and V5.
In addition to these 12 leads, special leads are sometimes used for recording activity from the atria, right ventricle and posterior side of the heart. Normal ECG tracing consists of waves, intervals and segments. The waves include P, QRS, T and U. The intervals include PR and QT intervals. The segments include PR and ST segments. Each of them will be addressed in subsequent chapters.
Uses
The ECG can be used to diagnose coronary artery disease, chamber enlargements, arrhythmias, inherited electrical disorders, drug toxicities and electrolyte imbalances. Each of these conditions affects the activation (depolarization) or inactivation (repolarization) of the heart and thus gives an indirect clue to their presence. Since ECG is not a direct measure of any of these abnormalities except for some arrhythmias, their use in these conditions is not fool proof. Therefore, there can be false negatives and false positives. Despite this, ECG is immensely helpful as an initial investigation in these conditions. As with any investigation, the significance of an abnormal finding has to be considered along with the total clinical picture. ECG being inexpensive, noninvasive and widely available, basic knowledge of the same will benefit the practicing primary care physician.