UNDERSTANDING ELECTROCARDIOGRAM (ECG) IN CATTLE

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:

  • In diagnosing most cardiac arrhythmias.
  • Providing information on the status of the myocardium

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:

  • No information about the mechanical status of the heart.
  • The ECG cannot always indicate the prognosis.
  • The ECG does not record the pathologic features of valve, coronary arteries, endocardium or pericardium except myocardium.
  • Body conformation of small animals may alter the accepted standard measurements.

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.

  • A lead is the recording or circuit between two recording points. Depending upon the wiring within the  electrocardiograph the same potential difference across a lead could result in an upward or downward deflection of the recording pen
  • In order to allow standard recording and comparison between recordings the polarity of the  electrodes for standard leads has been  established by convention and the leads are always  recorded at  these polarities
  • The electrodes of a lead  are commonly called positive or negative
  • A positive electrode in a lead is one that, when electrically positive relative to the other, due to a potential difference between them, yields an upward or positive deflection of the recording pen.

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:

  • Turn on the power switch.
  • Position the stylus to the centre of the chart.
  • Check that the sensitivity switch is at position 1.
  • Turn on the record switch to a paper speed of 25 mm / Sec.
  • Push the standardization button to check sensitivity.
  • Record the leads of the electrocardiogram:Remove the electrode clips from the animal.
    • Set the lead selector to I and record three to four good complexes.
    • Without switching off, change to lead II and record three to four good complexes. Repeat this procedure for leads III, aVR, aVL, and aVF.
    • Again set to lead II and record at least 15 to 20 inches of rhythm strip.
    • If the precordial chest leads are used, stop the recording and prepare the electrodes position and move the lead selector to V and turn on the recorder. Repeat this procedure for other V positions.
    • Move the lead selector to ‘O’ and then turn off the record and then turn off the machine
  • Write the animal’s case number and date on the tracing.
  • File the electrocardiogram.

POINTS TO BE NOTED WHILE RECORDING:

  • The recording should always be near the centre of the paper.
  • When the QRS complexes go off the paper, the sensitivity switch must be set at position ½ to reduce the amplitude. If the P-QRS complexes are too small, the sensitivity switch can be set to position 2 to increase in amplitude.
  • If arrhythmia is recognized, a longer lead rhythm strip should be taken. 
  • The R waves should normally be positive in lead I.

ANALYSIS OF ELECTROCARDIOGRAM:

  1. 1.      Calculation of heart rate
  2. 2.      Evaluation the heart rhythm
  3. 3.      Measure the complexes and intervals4.      Determine the mean electrical axis
    1. a.      P-wave
    2. b.      P-R interval
    3. c.       QRS complex
    4. d.      S-T segment
    5. e.      T wave
    6. f.        Q-T interval

CALCULATION OF HEART RATE: 

  • One small box is 0.02 sec in width
  • The no. of small boxes equalling 1 min. is pided by 0.02 (3000)
  • Therefore the no. of small boxes in one R-R interval pided into 3000 gives the heart rate.
  • For a paper speed of 25mm/sec, the no. of small boxes in an R-R interval is pided into 1500.

MEASUREMENT OF THE COMPLEXES:

P wave:

  • The P wave represents depolarization of atria, and its duration indicates the time required for an impulse to pass from SA node to AV node.  The P wave may be positive, notched, diphasic, or negative depending on the particular lead recorded.  The normal P wave in lead II is usually rounded.

P-R interval

  • The normal P-R interval indicates the time required for an impulse to travel from SA node to the ventricle.  It is measured from the beginning of P wave to the beginning of Q wave.  The P-R interval should be approximately the same from the complex to complex.

QRS complex

  • It denotes the depolarization of ventricles.

S-T segment

  • This segment represents the time interval from the end of QRS interval to the onset of the T wave and indicates the early phase of ventricular repolarization.

T- Wave

  • It indicates the repolarization of the ventricles.  It may be positive, negative, notched or biphasic.

Q-T interval

  • It is measured from the onset of Q wave to the end of the T wave. It represents the ventricular systole.

Dr. M. Areshkumar, M.V.Sc., M.Sc., PGDORT

AK Pet Clinic, Chennai -41