Pashu Sandesh, 29th June 2018
Dr. M. Areshkumar
INTRODUCTION:
Electrocardiography is a noninvasive, inexpensive technique that yields useful information in classification of arrhythmias, diagnosing conduction abnormalities and it is a valuable aid in prognostic and therapeutic considerations.
VALUE OF ECG:
INDICATIONS OF THE ECG:
ü Tachycardia, Bradycardia or arrhythmia heard on auscultation
ü Acute dyspnoea
ü Shock
ü Preoperatively in older animals.
ü Cardiac monitoring during and after surgery
ü Fainting or seizures
ü All cardiac murmurs
ü Cardiomegaly found on thoracic radiographs
ü Cyanosis
ü Evaluating the effect of cardiac drugs
ü Electrolyte disturbances, especially potassium abnormalities
LIMITATIONS OF ELECTROCARDIOGRAPHY:
ECG IN LARGE ANIMALS:
No single electrocardiographic lead system has been universally accepted for use in large animals. Bipolar leads (I, II, III, base-apex X, Y, and Z of the orthogonal lead system) and unipolar leads (aVF aVR, aVL, thoracic) have been described, but the amplitude, duration and configuration of the different waveforms vary widely depending on an animal’s breed, size, body type, and sex. In addition, there is liability of certain waveforms within each animal depending on the level of exercise, excitement, or organic heart disease. Large animals have a deeply penetrating Purkinje system, and depolarization from the ventricular endocardium to the epicardium occur explosively and in many directions at once. This period of ventricular activation is responsible for the electrocardiographic criteria that indicate ventricular enlargement in small animals but contribute little to generating the QRS complex of large animals. Establishing specific diagnostic criteria for chamber enlargement in large animal species has been difficult because changes in the QRS complex are not sensitive or specific for ventricular enlargement.
The electrocardiogram (ECG) is used primarily to detect cardiac arrhythmias. For this purpose a single-channel machine can be used, and the lead system chosen can be any that generates distinctive P, QRS, and T complexes. If an arrhythmia is detected, another lead should then be obtained to further characterize the QRS and T complexes and confirm their origin. The lead system should be easy to apply, and the tracing free of artifacts created by muscle tremors, skin movement, shifting of weight, and changes in limb position. Two such leads commonly used to obtain a rhythm strip for the diagnosis of cardiac arrhythmias are the base-apex lead1 and the Y lead of the orthogonal lead system.
The ECGs recorded using the limb leads in cattle have low polarities and great variability. This could be due to the position of electrodes relative to position of the heart in the thoracic cavity, and also to distribution of conduction system in the myocardium which causes the cancellation of wave fronts.
To overcome these problems different lead systems have been tried by different researchers and found base apex lead was working good.
ECG RECORDING SITE:
A normal consultation area of a large animal clinic can be used without any special installations.
NOISE REDUCTION:
A troublesome problem which may disturb recording of ECG is electrical noise caused mainly by alternate current. In order to prevent such disturbances, a rubber sheet can be used in between the trevises.
ELECTRODES:
Electrodes for skin applications were devised by using alligator clips in order to obtain a constant contact condition on applied regions.
APPLICATION OF SKIN ELECTRODES:
For setting electrodes on the skin surface, isopropyl alcohol or surgical spirit can be applied and then have to apply ECG gel.
Calibration of most electrocardiographs is such that an input of 1 mV produces a 1 cm deflection of the recording pen. Recording speeds are generally 25 or 50 mm/s. In recording an ECG, certain standard electrode positions are used for recording.
The positive electrode of the bipolar lead on the left thorax in the fifth intercostal space at the level of elbow (Green), the Negative electrode attached on the right jugular furrow 2/3 of the way from the ramus of the mandible (Red) and the ground electrode (neutral) can be attached to any site remote from the heart, can be on the withers.
OBSERVATION
The Morphologies of the P wave and the QRS complex was observed. The amplitude and duration of P, QRS, and T waves and intervals of P-R and Q-T were also recorded
PROCEDURE FOR RECORDING THE ELECTROCARDIOGRAM:
POINTS TO BE NOTED WHILE RECORDING:
ANALYSIS OF ELECTROCARDIOGRAM:
CALCULATION OF HEART RATE:
MEASUREMENT OF THE COMPLEXES:
P wave:
P-R interval
QRS complex
S-T segment
T- Wave
Q-T interval
Dr. M. Areshkumar, M.V.Sc., M.Sc., PGDORT
AK Pet Clinic, Chennai -41