Electrocardiogram and Its Interpretation of Cardiac Diseases in Cattle

Abstract

Electrocardiogram is a non-invasive method and applied easily to determine the cardiac functions and its abnormalities in cattle. This chapter deals with the anatomy and position of the heart in the thoracic cavity in cattle. Describes the indications, precautions and requirements for ECG in cattle. Positioning of the animals and methods to be followed for ECG examination. Guidelines to be followed to read the ECG graph, normal waves and values of ECG and interpretation of ECG in different diseases conditions. ECG is an aid in the diagnosis of diseases like atrial enlargement, ventricular hypertrophy, anemia, mineral and electrolyte imbalance, myocardial infarction and classification of cardiac arrhythmias.

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].

2.4 Positioning of the animals

The positioning of the animals is important to get an accurate ECG. The cattle should be kept in the standing position inside the trevis for an ECG examination. Animals affected with milk fever, diarrhea, lactic acidosis, Downer cow syndrome, etc. may be sternally or laterally recumbent. In such cases, animals may be subjected to the ECG examination in their current position.

2.5 Methods of ECG and ECG machine

Bipolar lead systems (Base apex I, II, III and X, Y and Z of the orthogonal system) and Unipolar leads (aVF, aVR, aVL and thoracic) have been described in animals. In large animals for the detection of cardiac arrhythmias base lead II and Y lead of the orthogonal system are most commonly used [9].

In cattle, ECG examinations method followed is base apex limb lead II. After adequate rest, the animal is kept in standing position in a trevis. ECG gel is applied on the lead attachment sites. Three sites are selected for application of the clips (Figure 1). Positive lead I is attached between the 3rd-5th intercostal space on the left side behind the elbow. Negative lead II is attached to the caudal 1/3rd of the jugular furrow and neutral lead III is attached away from the other two leads at the wither point [11].

Figure 1.

Positive lead I attached at the 3rd-5th intercostal space. Negative lead II attached at the caudal 1/3rd of the jugular furrow. Neutral lead III attached away from the first two leads at the point of withers.

The ECG machine is simply a voltmeter or galvanometer that records the changes in the electrical potentials occurring during each cardiac cycle. These changes are recorded on an ECG paper with a grid by a stylus. The stylus gets deflected according to the intensity of the electrical activity. The ECG machine is connected to a monitor and has an inbuilt printer. The results are obtained almost immediately. The clip used in ECG examinations is called an alligator clips. These clips have a pointed mouth with serrated ends for firm holding of the thick cattle skin. The serrated ends are very sharp and hence before usage it should be rasped or blunted to prevent injury to the animals. Before application of the alligator clips the sites should be cleaned with surgical spirit and adequate ECG gel should be applied for better contact.

2.6 Guidelines for reading the ECG

The ECG machine is equipped with a printer that generates a graphical representation on gridded paper of the electrical activity of the heart. The speed of the paper is set at 25 mm/second and the stylus present in the ECG records the deflection produced by the electrical impulses. The vertical axis of the grid denotes the voltage and direction of the deflection, either positive or negative, with respect to the baseline. The horizontal axis represents the time taken for each event in the cardiac cycle as well their sequence. The ECG paper is composed of a grid with small and large boxes. Each small box on the horizontal axis corresponds to a 0.04 second interval. Five small boxes form a large box which consequently represents 0.2 seconds. On the vertical axis, each small box corresponds to 0.1 mV.

2.7 ECG graph and its Normal waves

An ECG gives a graphical representation of the electrical changes occurring in the heart as a P-QRS-T complex. The electrical depolarization starting at the SA node travels across the atria creating a brief upward deflection of the stylus forming the P wave, representing atrial depolarization. The P-R interval is formed when the depolarization passes from the SA node to the AV node and into the ventricular tissue. The ventricular septum is the first to get depolarized in a direction away from the positive electrode creating a small negative deflection called the Q wave. A large positive/negative deflection, termed R wave, is created when the bulk of the ventricular myocardium gets depolarized. The S wave is formed when the remaining basilar portion of the ventricles gets depolarized. Thus the ORS complex is representative of ventricular depolarization. Complete ventricular depolarization is followed by repolarization before the next cardiac cycle. There is a difference in repolarization of the ventricular tissue creating a potential difference across the ventricular myocardium which forms the T wave.

The normal P wave amplitude and duration in Holstein cattle is 0.05–0.32 mV and 0.05–0.12 seconds respectively (Figure 2). In cattle, the QR and QS amplitudes are recorded separately and the normal values are 0.05–0.95 mV and 0.9–1-1 mV respectively. The normal duration of the QRS complex is 0.04–1.0 seconds. The P-R interval and Q-T interval in normal Holstein cattle was found to be 0.12–0.26 seconds and 0.22–0.48 seconds respectively. In large animals, the T wave is more variable than in small animals and is generally not relevant in the detection of cardiac problems. The normal T wave amplitude and duration are 0.05–0.8 mV and 0.05–0.16 seconds respectively [10]. In Table 1 the normal ECG values of cattle is given.

Figure 2.

Normal ECG pattern in cattle.

S. No	Parameters	Normal values
1	P wave amplitude (mV)	0.05–0.32
2	P wave duration (sec)	0.05–0.12
3	QR amplitude (mV)	0.05–0.95
4	QS amplitude (mV)	0.9–1.1
5	QRS duration (sec)	0.04–1.0
6	P-R duration (sec)	0.12–0.26
7	Q-T interval (sec)	0.22–0.48
8	T wave amplitude (mV)	0.05–0.8
9	T wave duration (sec)	0.05–0.16

Table 1.

Normal ECG values in cattle.

2.8 ECG graph and its interpretation

Variations in an ECG from the normal values of amplitude and duration in the P-QRS-T complex is indicative of cardiac dysfunction. In general, an increase in the amplitude of the P wave is suggestive of right atrial enlargement and increase in duration of the P wave is seen in left atrial enlargement (Figure 3). The QRS complex in bovines is negative and an increase in amplitude of QRS complex and duration of Q-T interval is characteristic of ventricular hypertrophy (Figure 3). A positive S wave is indicative of myocardial infarction. Variations in T was may be helpful in the diagnosis of electrolyte and mineral imbalances. ECG alterations in some of the common cardiac diseases and electrolyte imbalances are described below.

Figure 3.

ECG shows notching of P wave indicative of atrial enlargement and increase in amplitude of QRS complex and QT interval indicative of ventricular enlargement in cattle.

Atrial fibrillation (AF) is occurs due to focal ectopic firing arising from within the pulmonary veins and spreading to the atria [12]. AF is the most common cardiac arrhythmia present in milking cows. 50% of the cows with AF were previously diagnosed with ketosis suggestive of a relationship between the two conditions [13]. ECG recording of AF reveals absence/decreased amplitude of the P wave with a series of rapid and irregular waves (F wave) on the base line. The QRS complex is normal in amplitude and duration but irregular [12]. In cattle, the heart rate exceeds 90 beats/minute, it is called as sinus tachycardia (Figure 4) and if it is less than 47 beats/minute it is called as sinus bradycardia (Figure 5).

Figure 4.

ECG shows sinus tachycardia in cattle.

Figure 5.

ECG shows sinus bradycardia in cattle.

Endocarditis is an inflammatory disease affecting the layers of the endocardium in cattle due to bacterial infection. Bacterial endocarditis is most commonly of vegetative form (Healy, 1996). ECG examination in bovine vegetative endocarditis revealed deep QRS complex (1.7–2.5 mV) and tall T waves (0.8–1.5 mV) shown in the Figure 6 [14].

Figure 6.

ECG shows deep QRS complex and tall T wave in cattle with endocarditis.

Pericarditis is the inflammation of the pericardium with accumulation of serous or fibrinous inflammatory debris [15]. The most common cause of pericarditis in cattle is traumatic pericarditis [16] due to penetration of the pericardium by a hard foreign body. ECG taken in cattle affected with traumatic pericarditis shows decreased amplitude of QRS complex (<1.5 mV), regular alteration of P, QRS or T complexes and slurring/elevation in ST interval [17, 18, 19] shown in Figure 7.

Figure 7.

ECG shows regular alteration in P, QRS or T wave and elevation of ST interval in cattle with pericarditis.

In Table 2, ECG pattern of cattle with electrolyte and mineral imbalance was given. Hyperkalemia in an increase in potassium ion concentration in the blood caused by renal failure, severe dehydration and acidosis in calves with diarrhea [20]. The ECG in such animals shows wider QRS complex, tall T waves and deeper and higher S-T segment. A decrease in potassium ion concentration or hypokalemia occurs in case of inappetance, metabolic acidosis and increased excretion due to corticosteroid therapy combined with treatments used for ketosis [20]. The ECG recordings have tall and tent shaped T wave with an absence of P wave.

S. No	Condition	P wave	QRS complex	T wave	S wave	others
1	Hyperkalimia; (Increased K^+)	*	Longer	Taller	Deeper; Higher ST segment	Conduction Abnormalities
2	Hypokalimia; (decreased K^+)	No; P wave	*	Tall and Tent shape	*	Recorded in diarrhea calves
3	Hypercalcimia; (Increased Ca^2+) / Hypothermia	*	Short QT interval	Widened /flattented	Short ST segment	Cause Acute Myocardial Infaction
4	Hypocalcimia; (decreased Ca^2+)	*	Prolong QT interval	*	Lengthened ST segment	Recorded in Milk fever, Downers cow syndrome/ Creepers cow; Arrythmias and complete heart block
5	Hypomagnesimia; (Decreased Mg^2+)	*	Short PQ interval and QRS complex; Lesser degree shortening of QT	Negative	*	Causes Tachycardia, Arrythmias and sudden death.
6	Severe Hypomagnesimia	*	*	High Peak	ST depression	Causes Sinus Tachycardia
7	HypoPhosphatemia; (decreased P)	*	*	*	*	No change in ECG

Table 2.

ECG patterns in cattle with electrolyte and mineral imbalance.

Note: * indicate No change in waves.

Hypocalcemia or parturient paresis is a condition common in periparturiant cattle due to deficiency of calcium characterized by progressive neuromuscular degeneration, circulatory collapse and depressed consciousness [21]. Hypocalcemia is a common finding in case of Milk fever, Downer cow syndrome, Creeper cows, etc. The ECG is has a characteristic prolonged Q-T interval and lengthened S-T segment (Figure 8). Cows with hypocalcemia may have arrhythmias and sometimes complete heart block. Hypercalcemia is an increase in the levels of serum calcium most commonly due to primary hyperparathyroidism and malignancy [22, 23] as well as over supplementation of calcium in treatment of milk fever. Hypercalcemia decreases the atrial activity and increases ventricular activity [24]. ECG reveals short Q-T interval, flattened/widened T wave and short S-T segment. Hypercalcemia may cause acute myocardial infarction.

Figure 8.

ECG shows prolong QT interval and lengthened ST segment in cattle with hypocalcemia.

Hypomagnesemia tetany or grass tetany is caused by a deficiency of magnesium characterized by ataxia, recumbency, neuromuscular irritability, convulsions and tetanic spasms [25]. Cattle with hypomagnesemia (Figure 9) subjected to ECG shows short P-Q interval and QRS complex. The T wave is negative and there is a lesser degree of shortening of the Q-T interval.

Figure 9.

ECG shows short PQ interval and QRS complex with negative T wave in cattle with hypomagnesemia.

3. Conclusions

Electrocardiography is currently the best method available for the detection of cardiac arrhythmias and conduction abnormalities [26]. Since Einthoven’s invention of the first electrograph in 1895, there have been major advances in the study of cardiac diseases in both humans and animals. In cattle the base apex lead system is the most suitable for monitoring of cardiac function in cattle [9, 10]. Today, ECG is widely used in the field of veterinary medicine for the detection of cardiac arrhythmias and conduction abnormalities as well as to identify electrolyte and mineral imbalances which is valuable in the diagnosis of various disease conditions.

Acknowledgments

The technical assistance of Mr. Dias Nolan, veterinary student, is greatly appreciated. Thanks are extended to Ms. Carmel Prins, veterinary student, for typing the manuscript, to Dr. V. Vijayalakshmi, veterinary post-graduate student, for assisting in all the procedures involved in obtaining the required ECG readings. The authors extend their sincere gratitude to the Department of Veterinary Medicine and the Dean, Rajiv Gandhi Institute of Veterinary Education and Research, Puducherry.

Conflict of interest

The authors declare no conflict of interest.