Electrocardiogram is a visual record of the electrical activity of the heart recorded at the surface of the body of humans and animals during cardiac systole and diastole. It is one of the simplest and fasted technique employed for evaluating the rhythm, rate, and electrical activity (impulse formation and its conduction) of the heart. Though, Waller initially considered electrocardiography as an experimental method useful to physiologists rather than physicians, it has now become the most useful, handy, non-invasive and cost effective technique with clinicians for evaluating electrical activity of the heart in routine clinical practice.

Electrocardiograph, electrocardiography and electrocardiogram are the three terms commonly associated with ECG or EKG. Though, these terms are interdependent, there is clear differentiation and demarcation between these terms and should not be used as synonym. Electrocardiograph The term “Electrocardiograph” refers to an instrument or machine that records surface potentials associated with the electrical activity of the heart to generate a graphic representation, the electrocardiogram (Fig.1).

An electrocardiographic unit has a compact electrocardiograph machine, power cord, patient cable, thermal recording ECG paper roll, and conducting gel. Electrocardiographic Unit The Electrocardiograph is a sensitive galvanometric device that is capable of recording difference in electric potential up to 1 mV (1/1000 of a volt). The machine has a voltage meter and amplifier combined with a strip recording device (Fig.5) to provide a permanent record. Various basic features viz. running (paper) speed of the machine , sensitivity, lead selector, off/on switch, f ilter etc are displayed on the screen (Fig. 5). Electrocardiographic machine can record electrocardiogram at the paper speed of 25 or 50 mm per second as per our selection. The sensitivity switch controls amplitude of the waves. The amplitude or sensitivity can be selected as 0.5,1. or 2.

The process of recording an electrocardiogram involves preparation of sites, application of conduction gel, positioning and restraining the patient, attaching the electrodes and then actual recording. Errors in placement of electrodes and artifacts during electrocardiography may generate erroneous electrocardiogram. Sites for Placement of Electrodes in Dogs and Cats There are definite sites (Fig. 15 and Fig.16) for placement of electrodes both in dogs and cats .Improper placement of electrode may lead to erroneous electrocardiogram.

Electrocardiogram (ECG) Electrocardiogram is a graphic record of electrical events of the heart recorded by the electrocardiograph. It represents the sum up surface voltage and direction of electrical activity produced during cardiac muscle depolarization and repolarization plotted against time (Fig. 33). The time is represented on the X- axis (horizontal line) and amplitude of voltage is represented on the Y-axis (vertical line) of the graph. The potential difference occurring over the surface of the body (at different lead placement locations) is associated with depolarization and repolarization of heart muscle (myocardium). The amplitude of the potential difference is measured as milli volt (mV) and their duration is measured in second or milliseconds.

After successful recording of the electrocardiogram, the next step is its reading, analysis, interpretation and report writing. It is the most important part of electrocardiography. Patients details ( name, species of animal, breed, age ,sex, owners’ name and address with E-mail and mobile number) and date of tracing are to be mentioned in the electrocardiograph prior to actual recording of the electrocardiogram or can be jotted down on the ECG strip after completion of recording as per available facility in the electrocardiographic machine. Clinical complaints and observations should be kept in mind while interpreting the electrocardiogram.

An understanding of normal values of electrocardiographic indices of healthy dogs and cats is utmost necessary for interpreting an abnormal electrocardiogram. The criteria for normal configuration and measurements of the waves, segments and intervals in an electrocardiogram of healthy subjects provides basis for detecting abnormalities in the electrocardiogram of the ailing animals. The electrocardiogram, like other diagnostic aids, is not an absolute indicator of health and disease and therefore it is required to be interpreted in conjunction with patient’s history, clinical manifestations, and the results of other investigations.

Electrocardiogram is a visual record of electrical activities going on in the heart during depolarization and repolarization (cardiac cycle). It is one of the simplest and fasted non-invasive diagnostic test available to evaluate cardiac function. Though the heart of large mammals have many similarities, difference in shape, size, positioning in thoracic cavity and innervation of Purkinje f ibers exists in different species of the animals making the electrocardiogram peculiar for that species of animal. Peculiar characteristics that distinguish the electrocardiograms of different species of animals are given in this chapter.

Departure of waves ( P,Q,R,S, T), segments (P-R, S-T and T-P) and /or intervals (P-R, Q-T, R-R) from their normal values (configuration, morphology, amplitude and duration of wave; prolonging /shortening of intervals : elevation /depression of segments from their isoelectric line) is said to be abnormal feature of the electrocardiogram. These abnormalities are suggestive of specific cardiac abnormalities such as a defect in the current generation, its conduction or both; or change in the size of the chambers (atria / ventricles/both.).

Apart from the heart rate and rhythm, the electrocardiogram also provides information pertaining to the chamber size, i.e. the presence of chamber enlargement in dogs. It can more precisely be evaluated if the P QRS and T complexes are clearly visible without any artifacts. Further, the measurement of the chamber enlargements is only possible when all the segments of the ECG are clearly visible. When the impulse is conducted uninterruptedly from sino-atrial node (SA node) to AV node to the ventricles, the measurement of the amplitude of the complexes (P, QRS, T) is more precise in providing information regarding the presence of chamber enlargement.

The word “arrhythmia” has been derived from ancient Greek ?ρρυθμος (arruthmos) via new Latin “arrhythmia”. Arruthmos is made up of ‘a ‘(without) and ‘rruthmos’ (rhythm) means without rhythm. By definition as per dictionary, arrhythmia is defined as any disturbance in the rhythm of the heartbeat. In fact in medical sense arrhythmias are the disturbances not only in cardiac rhythm but also in cardiac rate or both. The seat of generating impulse in the heart is the pacemaker, of highest automaticity, in the SA node. If any impulse is generated outside the SA node pacemaker it leads to arrhythmias.

Though the treatment for arrhythmias has been recommended for even a single premature beat, in the current scenario the treatment is indicated when there are more than 17 VPCs per minute. In certain situations the side effects or proarrhythmic effects of drugs can lead to morbidity or mortality. As a result, new guidelines are proposed when evaluating the need for treatment of arrhythmias. Keeping the adverse effects of antiarrhythmic agents in mind, one should be selective about which arrhythmias are to be treated. The goal of antiarrhythmic therapy includes the treatment of an existing arrhythmia or prophylaxis for anticipated arrhythmias.

This chapter on electrocardiograms for self-exercise is designed to provide opportunities to the readers to assess their understanding of reading and interpreting electrocardiograms and to increase their confidence. Each electrocardiogram has its detail and a question to answer. On the reverse side of each electrocardiographic tracing, detailed reading and interpretation is provided to match your assessment and to improve your interpretation skill. It is to be remembered that each electrocardiogram should be evaluated in conjunction with complete data base (history, clinical manifestations, laboratory investigations, radiological observations, and echocardiographic observations).

A  APC 156, 157, 253 Arrhythmias 8, 65, 76, 78, 93, 95, 99, 100, 122, 131, 143, 144, 145, 146, 147, 148, 149, 150, 164, 165, 175, 185, 186, 187, 188, 189, 191, 192, 192, 193, 196, 197, 198, 200, 201, 202, 203, 227 Atrial fibrillation 6, 131, 145, 146, 147, 148, 152, 158, 159, 160, 187, 189, 191, 192, 193, 196, 197, 198, 199, 201, 203, 241 Atrial flutter 145, 148, 152, 160, 193, 196, 198, 225, 239  Atrial tachycardia 145, 146, 147, 148, 152, 157, 158, 160, 162, 193, 219, 271  Atrium 7, 62, 66, 70, 108, 109, 130, 131, 133, 135, 138, 140, 156, 161, 173, 192  AV junction