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4. A Systematic Reading of an Electrocardiogram
After completion of the tracing, the electrocardiogram (ECG) should be filled in properly and stored. Basic information such as name, breed, age, and sex of the dog, owner’s name and address with mobile number, date of tracing, clinical complaints, and observations should be filled in. Leads I, II, III, aVR, aVL, aVF, CV5RL, CV6LL, CV6LU, and V10 should be marked on the tracings itself if ECG is taken using a manual machine. Speed (25 or 50 mm/s) and sensitivity (0.5, 1 or 2) should be specified for calculating heart rate and measurements of different wave forms in case of manual machines. Modern automatic machines automatically record these details as per their setting. Taking an ECG tracing is an easy task, but its interpretation is an important and difficult task requiring fundamental knowledge of cardiac anatomy, physiology, and cardiology. Casual approach in interpretation may be unrewarding. An electrocardiogram should be interpreted keeping in view the history and clinical manifestations of the patient. Definite interpretations from ECG regarding anatomic defects or change in physiological states may not be always predictable. Therefore clinician is the best person to interpret an ECG findings in the background of his patient’s history, clinical status, and other investigations. A veterinarian should also be conversant with wide variations in the normal and overlapping ECG patterns between various conditions.
4.1 Systematic Approach to ECG
Calculation of heart rate: Average heart rate is generally calculated in lead II. The numbers of predominant complex (QRS) are counted in 3 s (15 large squares, i.e., 75 small squares if machine is running at the speed of 25 mm/s or 30 large squares, i.e., 150 small squares if machine is running at the speed of 50 mm/s) and is multiplied by 20 to get heart rate per minute. This is the simplest way of calculating heart rate.
Determining cardiac rhythm: In normal ECG, each complex occurs with a “P” wave followed by a short “P-R” segment followed by “QRS” followed by, S-T segment and finally T wave. If there is “P” wave for every “QRS” complex and “R-R” or “P-R” intervals are of normal duration, then rhythm is said, to be sinus rhythm. For evaluating heart rhythm, lead II strip is examined. In case of frank arrhythmia, gross irregularities are obvious and can be detected even at first sight, while small irregularities need proper attention warranting measurement of “P-R” and “R-R” intervals. In normal sinus rhythms, these intervals (P-R interval and R-R interval) do not vary much from complex to complex.
Measuring the amplitude and duration: Amplitude and duration of waves (“P,” “QRS,” “T”), segments (P-R segment, S-T segment) and intervals (“P-R” interval, “R-R” interval, “Q-T” interval) are to be measured in each electrocardiogram. Generally these measurements are recorded in lead II. But other leads (I, III, aVR, aVL, and aVF) are also analyzed for confirming the changes in lead II in confusing situations and determining mean electrical axis on frontal plane, etc. Some of this information is available on the ECG strip obtained through automatic ECG machines. Measurement procedure of amplitude, duration of waves, segments, and intervals is given in Chap. 3.
“P” wave—Amplitude and duration of “P” is measured. In dogs “P” wave may be positive, notched, biphasic, or negative depending on the lead (Fig. 4.1). In lead II, normal “P” wave is small, rounded, and positive, or it may be M-shaped. It is negative (−ve) in lead aVR and avL. Sometimes biphasic “P” is seen possibly owing to shift of pace maker site.
P-R interval—It is measured from the beginning of the “P” to the beginning of the “Q” (Fig. 4.2). Generally P-R interval is approximately same from complex to complex. Variation in P-R interval from beat to beat indicates arrhythmia.
“Q” wave—It is a first negative deflection after “P” wave, and it reflects right ventricle depolarization (Fig. 4.3).
“R” wave—In QRS complex it is the first positive deflection indicating depolarization of left ventricle (Fig. 4.4).
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