Veterinary Medicine, Shri Surat Panjarapole Prerit Nandini Veterinary Hospital, Surat, Gujarat, India
3.1 What Is an Electrocardiogram (ECG)?
An electrocardiogram is a graphic representation of the final total surface voltage and direction of electrical activity produced during depolarization and repolarization of cardiac cells plotted against time (Fig. 3.1). The potential difference occurs over the surface of the body owing to electrical activity in the heart. The electrical activity in the heart is associated with depolarization and repolarization of the myocardium. The amplitude of these potential differences between various points on the body surface is measured as millivolt (mV), and their duration is measured in seconds. ECG provides an information about heart rate; its rhythm; size of chambers in dogs and humans; conduction defects; myocardial diseases or ischemia; and electrolyte imbalances such as hypocalcemia, hypokalemia, hypercalcemia, or hyperkalemia. Information about some drug toxicities can also be obtained. Electrocardiographic parameters in different breeds of dog differ slightly.
3.2 Electrical Activity in Cardiac Cell
The inner and outer surfaces of cardiac muscle cell have different electrical charge. The inside of the cell is negatively charged, and outer cell surface is positively charged (Fig. 3.2). The contraction of K+ inside the cell is higher, while concentration of sodium and calcium is higher outside the cell. Cardiac muscle cells can differentiate between sodium and potassium ions. The sodium ions are kept out, while the potassium ions are retained within the cell. By this process (sodium and potassium pump), resting membrane potential in cardiac cell is maintained. On stimulation membrane potential of resting myocardial cell is reduced suddenly with a change in cell membrane permeability facilitating the entry of Na+ ions into the cell and exit of K+ ions from the cell. As a result the surface charge of the cardiac muscle cell starts becoming negative, and inside of the cell starts becoming positive leading to the event of depolarization. Electrocardiogram records this event as an upward deflection if depolarization flows toward positive electrode. On complete depolarization of the cardiac cell, potential difference between various portions of the cell surface is equalized, hence recording returns to baseline. Now repolarization begins with the exit of Na+ from the cell and entry of K+ into the cell, and outer surface starts getting positive charge leading to deflection of stylet as per current flow from positive to negative or vice versa.
3.3 Current Generation and Conduction System in the Heart
Canine and feline heart has four chambers, but from electrical point of view, it is considered as having only two units. The right and left atria are synchronized as one unit, and right and left ventricles are synchronized as other unit. The contraction of one unit (atrium) is followed by the contraction of the other unit (ventricles). The seat of generation of the current for each cardiac cycle is “sinoatrial” node (SA node) that is situated in the right atrium. Cardiac conduction system (Fig. 3.3) consists of sinoatrial node, the interatrial bundle, the atrioventricular node (AV node), the common bundle of His, the right and left bundle branches, and the Purkinje fibers. The SA node is a powerhouse from where depolarization is initiated and spread across the right and left atrium and to the AV node through internodal fibers. The right atrium depolarizes first followed by the depolarization of the left atrium. There is a difference of approximately 0.01 s between depolarization of both atrium. The speed of depolarization wave from SA node to the right to left atrium, to the bundle of His, to the bundle branches, and to the Purkinje fibers is variable. Depolarization wave is stopped for a fraction at all points except atrioventricular node (AV node). In ECG this delay is represented by a straight line, i.e., P-R segment. From the AV node current passes to the bundle of His to bundle branches to Purkinje fibers to ventricular myocardium. The bundle of His is divided into the right and left bundle branches. The right bundle branch passes down the right ventricle side. The left bundle branch passes down the left ventricular side of the septum. The left bundle branch further divides into anterior and posterior fascicles. These fascicles divide into the Purkinje fibers, and these are distributed to the ventricular myocardium. The entire ventricular muscle mass contracts synchronously. Finally when depolarization is complete, repolarization of the cells of the ventricular myocardium starts. Its earliest stages are normally not detectable at the body surface, but later stages are represented as T wave.
3.4 Conduction and Electrocardiogram
Depolarization impulse spreads from the SA node through the right atrium toward the left atrium and the AV node and produces “P” wave. The “P” wave represents the total electrical force generated during depolarization of the right and left atrium. First half part of “P” wave (ascending arm) indicates depolarization of the right atrium, and second half part of the “P” wave (descending arm) indicates depolarization of the left atrium. The delay at the AV node is reflected in ECG as a straight line following “P” wave and is called P-R segment. The P-R interval represents the total time taken in depolarization of the right and left atrium and delay in AV node. Depolarization impulse passes into the ventricle through the bundle of His. Three phases of electrical activity produce “Q,” “R,” and “S” deflections on ECG. After ventricular depolarization, ECG complex returns to baseline producing S-T segment. “T” wave represents repolarization of ventricles. “T” wave in dogs is quite variable; hence there is a need to establish normal values of “T” wave. Thus, each normal heartbeat is represented in the electrocardiographic tracing by three successive complexes, namely, the P wave, the QRS-complex, and the T wave, corresponding to atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively.
3.5 The Shape of the ECG
Electrical changes accompanying the depolarization of the atrium is small owing to small mass. In electrocardiogram, atrium depolarization is represented by “P” wave. The ventricular mass is large, so there is a large deflection during ventricular contraction leading to formation of “QRS” complex. Repolarization of ventricular mass leads to formation of “T” wave. The letters P, Q, R, S, and T are arbitrary and were selected during early days of ECG history.
Different waves in electrocardiogram are shown in Fig. 3.4. The first positive deflection is upward and is called as “P.” Then the first downward deflection is termed as “Q.” A second upward deflection is called “R” wave. Many times it is not preceded by “Q.” Any deflection below the baseline following “R” is called as “S” wave. After this any positive or negative deflection is called as “T” wave. Many normal dogs may have T wave in the same direction as of QRS, in the opposite direction as of QRS, no T wave at all, or biphasic (plus/minus or minus/plus) T waves. T wave is extremely labile. Configuration and duration of “T” wave is influenced by many factors. Alterations in either “J” point or ST-T contour or in both can be produced by myocardial ischemia, digitalis toxicity, electrolyte imbalance, or pericarditis.