**1. Introduction**

Electrocardiography is the simple non- invasive technique that records the changing electrical activity/potential difference in the heart by positive and negative electrodes. It is a valuable aid in assessing the heart rate, rhythm, chamber size, cardiac conduction system and cardiac functions. It is considered one of the more sensitive tool for diseases of the heart [1, 2]. ECG in ruminants is primarily used in the detection of cardiac arrhythmias, disturbances in conduction and electrolyte imbalances. This will help to determine the prognosis of cardiac disease and for therapeutic considerations [3]. Whereas mild enlargement of chambers cannot be detected in cattle due to deeply penetrating Purkinje fibers and the depolarization and repolarization occurring concurrently over multiple minor fronts [4]. Devadevi *et al*. [5] reported that electrocardiography is a useful aid in diagnosis of anemia in cattle affected with benign bovine theileriosis. Fetal electrocardiograms in dairy cattle were found to be useful in detection of multiple pregnancies [6]. ECG was successfully used in the detection of cardiac arrhythmias and conduction disturbances in buffaloes affected with traumatic reticuloperitonitis [7].

### **2. ECG examination in cattle**

#### **2.1 Anatomy and position of heat in the thoracic cavity of cattle**

The bovine heart lies in the middle mediastinum between the 3rd to 5th intercostal space predominantly to the left side of the median plane with the base lying dorsally and the apex present ventrally, close to the sternum. The heart is rotated to the left side about its vertical axis such that the right auricle is present on the left side.

The heart consists of four chambers – two atria and two ventricles and has a base and an apex with the apex being formed solely by the left ventricle. The heart is surrounded by the fibrous and serous pericardium with a small amount of serous fluid present in the pericardial cavity. Each electrical discharge begins at the sinoatrial (SA) node present on the right atrium and the depolarization spreads through the atrial muscles to the atrioventricular (AV) node. The conduction passes from the atria to the ventricles through the bundle of His which divides into the left and right bundle branches supplying the left and right ventricles respectively. The left bundle branch is further divided into the anterior and posterior fascicles. The conduction is then passed through the Purkinje fibers to the myocardium [8].

#### **2.2 Precautions and requirements**

Cattle are docile in nature but they are excited and restless in a new environment. The cattle should be kept in a pleasant environment and be allowed to take rest. Animals should not be subjected to an ECG immediately after arrival because of the increase in the heart rate and respiration rate due to stress during transport. This will in turn give a wrong diagnosis. The animal is allowed to take rest for 15 minutes before performing the ECG examination. This will reduce the error in the ECG. Cattle should be kept in the trevis for ECG examination and calves should be kept on a wooden table, floor or nonconductive materials. Animals should not be leaning on the trevis as it will interfere with the ECG examination. A thick rubber sheet is placed over the trevis to prevent electrical noise produced by alternate currents when the animal comes in direct contact with the trevis [9].

#### **2.3 Indications for ECG**

ECG is the best method to read the different kinds of arrhythmias in cattle. If the heart rate exceeds 90 beats/minute, it is called as sinus tachycardia and if it is less than 50 beats/minute it is called as sinus bradycardia. ECG is an ideal additional tool to find out the electrolyte and mineral deficiency in cattle. These imbalances commonly occur during the first 3 months of production in animals suffering from milk fever, hypomagnesemia, post parturient hemoglobinuria and ketosis. Animals suffering from severe diarrhea may be deficient in certain minerals like sodium, potassium, chloride, iron and magnesium which may result in poor prognosis and should be replaced with ideal intravenous or oral electrolyte therapy. ECG is one of the methods to find out the enlargement of the atrium and ventricles. Although in cattle, mild enlargement of the heart is difficult to detect because of the deep penetration of the Purkinje fibers. ECG can be used to check the heart rate in cattle which may be cross verified with auscultation methods. The average of the last six R-R intervals of the trace is calculated to obtain the heart rate as the animals are more relaxed toward the end of the recording [10].
