ECG AND ITS INTERPRETATION
S. Prathaban
Professor and Head, Department of Clinics Madras Veterinary College, Chennai –600 007
Electrocardiography is established as an atraumatic, relatively inexpensive and extremely useful technique for gaining information about the heart and it is accepted as a necessary part of the cardiac examination in a dog or cat. As early as 1912, the electrocardiogram was considered an important method of studying the heart. Augustus D. Waller was the first to demonstrate that the electrical impulses of the heart could be recorded from the surface of the body.
In 1895, Einthoven introduced the terms P, Q, R, S and T for the electrocardiographic deflections; where P correspond to atrial depolarization or contraction, QRS correspond to ventricular depolarization or contraction and the T wave represented ventricular repolarisation or relaxation. Waller was the first to use the term electrocardiogram.
The electrocardiogram is a graph of the variations in voltage produced by a mass of associated cardiac muscle cells or bundle of muscle fibers. The electrocardiograph as a galvanometer has a delicate writing instrument that will indicate a single positive or negative charge. An electrocardiogram should always be obtained without chemical restraint as any drug may influence the heart rate or rhythm: (Dukes McEwan 2000).
The electrocardiograph can be used to look at the electrical activity of the heart from different angles to get a complete picture. Each different angle or pair of electrodes is called a lead. The following lead systems are necessary to view the heart from different directions.
I. Bipolar Standard Limb Leads:
- Lead I – Right arm (-) to the left arm (+).
- Lead II – Right arm (-) to the left leg (+).
- Lead III – Left arm (-) to the left leg (+).
II. Augmented Unipolar Limb Leads:
Lead avR (augmented vector right)-Right arm (+) compared to left arm and left leg (-). Lead avL (augmented vector left) – Left arm (+) compared to the right arm and left leg (-). Lead avF (augmented vector – frontal)- Left leg (+) compared to the right and left arm (-).
III. UNIPOLAR PRECORDIAL LEADS (EXPLORATING LEADS)
Position of explorating electrodes in different leads is as follows:
Lead V 10 – Over the dorsal spinous process of the seventh thoracic vertebra. Lead CV6LL – Sixth left intercostal space near the edge of the sternum.
Lead CV6LU – Sixth left intercostal space at the costochondral junction.
Lead CV5RL – Fifth right intercostal space near the edge of the sternum. The standard leads are especially useful for studying abnormalities in the P, Q, and R, S and T deflections, diagnosing cardiac arrhythmias and determining the mean electrical axis.
Electrocardiography is a useful tool in two major areas:
- Diagnosing most cardiac arrythmias since the electrocardiogram can determine the source of the rhythm and the frequency with which the impulse arises ; and
- Providing information on the electrocardiographic tracing is often altered by either pathologic or physiologic
Indications for an electrocardiogram:
- Cardiac
- Acute onset of dyspnea
- Shock
- Fainting or seizures
- Cardiac monitoring during and after surgery.
- Cardiac murmur.
- Cardiomegaly found on thoracic radiographs.
- Cyanosis.
- Pre operatively in older animals.
- Evaluating the effect of cardiac drugs – especially digitalis, quinidine and propanolol.
- Electrolyte disturbances, especially potassium abnormalities.
- Systemic diseases that affect the heart.
- Serial electrocardiograms as an aid in the prognosis and diagnosis of cardiac disease.
Normal Canine Electrocardiographic Values Rate
70 to 160 beats /min for adult dogs
60 to 140 beats /min for giant breeds
Up to 180 beats /min for toy breeds
Up to 220 beats /min for puppies.
Rhythm
Normal sinus rhythm Sinus arrhythmia Wandering SA pacemaker
Measurements (lead II, 50 mm / sec, 1 cm = 1mv) P wave
Width: maximum, 0.04 sec (2 boxes wide).
maximum, 0.05 sec (2 ½ boxes wide) in giant breeds.
Height: maximum, 0. 4 mV (4 boxes tall).
P-R interval
Width: 0.06 to 0.13 sec (3 to 6 ½ boxes).
QRS complex
Width: maximum, 0.05 sec (2 ½ boxes wide) in small breeds. maximum, 0.06 sec (3 boxes) in large breeds.
Height of R wave*: maximum, 3.0 mV (30 boxes) in large breeds. maximum, 2.5 mV (25 boxes) in small breeds.
S-T segment
No depression: not more than 0.2 mV (2 boxes). No elevation: not more than 0.15 mV (1 ½ boxes). T wave
Can be positive, negative or diphasic
Not greater than one fourth amplitude of R wave; amplitude range + 0.05 – 1.0 mV (1/2 to 10 boxes) in any lead.
Q-T interval
Width: 0.15 to 0.25 sec (7 ½ to 12 ½ boxes) at normal heart rate; varies with heart rate (faster rates have shorter Q- T intervals and vice versa).
Electrical axis (frontal plane) : + 40° to + 100°
Sinus rhythm:
A sequence of beats originating from the sino atrial node forms a rhythm, known as the sinus rhythm. there are four common sinus rhythms.
1. Normal sinus rhythm:
In normal sinus rhythm, the stimulus originates regularly at a constant rate from the SA Node, depolarizing the atria and ventricles normally and producing a co ordinate atrioventricular contraction. The ECG shows a normal P wave followed by normal QRS and T wave. The rhythm is regular (constant) and the rate is normal for age, breed and species.
2. Sinus arrhythmia:
In the case of sinus arrhythmia the stimulus originates from the SA Node, but the rate varies (increases and decreases) regularly. The ECG shows normal P wave followed by normal QRS and T wave. The rhythm varies in rate often associated with respiration. The rate is normal for age, breed and species.
- In Sinus tachycardia, the SA Node generates an impulse and depolarization occurs faster than The ECG shows a normal sinus rhythm but at a faster rate than normal.
- In Sinus bradycardia, the SA Node generates an impulse and depolarization occurs more slowly than This can be a normal feature in some giant breed dogs and in athletically fit animals. The ECG shows a normal sinus rhythm but at a slower rate than normal.
Ventricular premature complexes (VPC) are a common finding in dogs and cats and arise from an ectopic focus or foci within the ventricular myocardium.
The QRS complex morphology is abnormal. The complex is usually a) abnormal in shape, (bizarre) and slightly widened (prolonged). The T wave of VPC is often large and opposite in direction to the QRS wave. A run of three or more VPC is known as Ventricular tachycardia.
ECG interpretation essentially involves four main steps:
- Calculation of the heart rate;
- Determination of the heart rhythm;
- Measurement of the complex amplitudes and intervals;
- Measurement of the mean electrical
Calculation of the heart rate
The simplest way to calculate the rate from an ECG is to mark a second strip on a representative part of the tracing, count the number of complexes which appear within this time frame and multiply this figure by 10. If the P wave rate and QRS-T complex rate differ, these should be recorded separately.
Electrical alternans refers to an alternation in the size of the QRS amplitude that occurs nearly every other beat.
Artifacts include electrical interference, muscle tremor artifacts and movement artifacts.
ECG ABNORMALITIES
A good understanding of the electrical activity of the heart is key to the accurate interpretations of ECG (Martin, 2002).
1. Right atrial enlargement
P wave grater than 0.4 mV (4 boxes) .Tall, slender and peaked P wave. Eg. Bronchitis/ Pneumonia/ Congenital defects.
2. Left atrial enlargement
P wave more than 0.04 sec (2 boxes).
3. Right ventricular enlargement
S wave in L.I. greater than 0.05 sec (1/2 box).
4. Left ventricular enlargement
Tall ‘R’ waves in L II.
QRS duration- Wide more than (2 ½ box) 0.05 sec. ST – coving.
5. T wave abnormalities
Should not be greater than 1/4 of the R wave.
Sharply pointed (or) Notched – Electrolyte imbalances
Associated conditions
Myocardial hypoxia (O2 deficiency). Myocardial infarction.
Right or Left Bundle branch block. Ventricular enlargement.
Hyperkalemia – Large and Spiked. Hypokalemia – Small and diphasic. Digitalis Quinidine – Toxicity.
Myocardial infarction
Notched R wave
Sudden deviation of ST segment Tall peaked T wave.
Wandering pacemaker
It is normal in dogs Wandering pace maker is a shift of the pace maker from the S A node to AV node or from within the SA node .Change in the configuration of the P wave which becomes positive / negative or diphasic. It is an irregular, multiform, supra ventricular rhythm with changing P wave morphology.
Q – T interval
Normal: 0.15 – 0.25 sec (7 ½ – 12 ½ Boxes)
Prolonged QT interval
- Hypocalcemia due to hypothyroidism renal failure
- Hypokalemia – Metabolic and respiratory alkolosis
Shortened QT interval
- Hypercalcemia, Degitilisation, Hyperkalemia
ST segment abnormalities
ST segment depression 0.2 mV (2 Boxes) ST segment elevation 0.15 mV – in Lead I
S-T elevation is seen in animals with: Pericarditis, Severe ischaemia/infarction (eg, full wall thickness).
ST segment depression
Myocardial ischemia Myocardial infarction Hyper and Hypokalemia Trauma to the heart.
Right bundle branch block
Wide S waves – in Rt. Bundle Branch block QRS complex greater than 0.08 sec.
Left bundle branch block
QRS complex in greater than 0.08 sec. duration QRS complex in wide and +ve.
Atrial premature complexes
‘P’ represents the premature complex
‘P’ wave is superimposed on the T wave of the preceding complex seen in atrial diseases. Congenital cardiac defects (Mitral insufficiency) or PDA.
Atrial flutter (F waves)
Rapid, regular atrial rhythm at a rate varying usually from 300 to 500 beats / min ‘ P ’ waves – replaced by saw tooth waves (Called F waves).
Atrial fibrillation
Fibrillation means rapid, irregular small movements of fibres. In atrial fibrillation, one of the most common arrythmias seen in small animals, depolarization waves occur randomly throughout the atria.
Normal QRS morphology, the R-R interval is irregular and chaotic and the QRS complexes often vary in amplitude. There are no recognizable P waves preceding the QRS complex. Fine irregular movements of the base line – known as F waves are seen as a result of the atrial fibrillation waves. Associated conditions include Sinus arrhythmia and wandering pacemaker – frequent in the dog.
Ventricular fibrillation is nearly always a terminal event associated with cardiac arrest. The depolarization waves occur randomly throughout the ventricles. The ECG shows coarse (larger) or fine (smaller) rapid, irregular and bizarre movement with no normal waves or complexes. (Martin, 2002).
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REFERENCE
- Abbott, 1998. Diagnosing congestive heart failure in dogs and cats. Vet.Med. 811 -829.
- Dukes McEwan, Canine dilated cardiomyopathy 1. Breed Manifestations and diagnosis. In Practice. 522-530.
- Ettinger, J.1987.Cardiac arrythmias. In Text book of Veterinary Internal Medicine. Edited by S.J.Ettinger.2ndedition. W.B.Saunders, Philadelphia.
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- M. 2002 . ECG interpretations in small animals . 1. Understanding the electricity of the heart. In Practice . 114 – 123.
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- Tilley , P.1992. Essentials of canine and Feline Electrocardiography . 3rdedn. Lea & Febiger, Philadelphia.
