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17. Electrocardiography in Cats
In India, information on cardiac diseases in cats is scanty as number of feline patients in veterinary hospitals is less than that of canines. Therefore, the prevalence rate of cardiac abnormalities in cats in India is unknown. Studies done abroad have revealed that cardiac diseases (hypertrophic cardiomyopathy, mitral valve disease, unclassified cardiomyopathy, thyrotoxic heart disease and congenital heart disease) are commonly diagnosed in cats. Arrhythmias are quite common in cats. It seems that the diseases of heart are not uncommon in feline population in India also. Diagnosis of cardiovascular diseases in cats is not as an easy task because of small size of the body and heart, rapid heart rate, and uncooperative nature. Further abrupt onset of clinical signs makes the situation complex. Some risk factors such as taurine deficiency and hyperthyroidism are associated with feline heart diseases. Breeds such as Maine Coons, Ragdolls, Persians, and Siamese are said to be more predisposed to cardiomyopathies. There are few differences in canine and felines as related to cardiac diseases.
17.1 Differences Between Dogs and Cats with Regard to Cardiac Diseases
17.1.1 Clinical Manifestations
The signs and lesions in cats with cardiac diseases remain asymptomatic until very advanced stage and manifested suddenly in acute form. Whereas in dogs clinical signs of heart ailments are insidious and slowly progressive, cats with congestive heart failure suddenly develop severe dyspnea. In some cases per acute hind limb paralysis is developed due to arterial thromboembolism. Sometimes cats die suddenly due to hypertrophic cardiomyopathy without manifesting any clinical signs. Coughing (nocturnal cough and gagging) and lung crackles or wheezing on chest auscultation, as seen in dogs with left heart failure, is not common in cats despite having severe lung edema. Vasoconstriction in extremities leads to slightly cool extremities. But it is not a reliable sign of congestive heart failure in cats. Similarly ascites, seen in dogs with right heart failure, is also not very dominant sign in cats. In cats functional murmurs (due to fever, anemia, volume overload, tranquilizing drugs) are very common. There is hardly any change in pulse quality in feline heart disease. Even capillary refill time and mucus membrane color changes are also not very conspicuous in cats with heart diseases. In ventral septal defects and tricuspid valve defects, murmur localization can be of some help.
17.1.2 Cardiac Murmurs
Areas of location of cardiac murmurs in cats are different than dogs. Murmurs are loud in cat and are located along the left or right sternum or cranial/caudal thorax.
17.1.3 Radiographic Features
Cardiac silhouette in normal young cats is slightly elongated (Fig. 17.1), and in geriatric cats, it is more horizontal in lateral radiographs. Dilation of main pulmonary artery (MPA) in cats does not show a bulge at 1–2° clock position, as seen in dogs, in ventro-dorsal chest radiographs. Cardiomegaly in cats results in elongation and widening of cardiac silhouette in general, and detection of specific chamber enlargement in radiographs is rather difficult. Tracheal elevation is not always seen in cats with cardiomegaly. Pulmonary edema in cardiac diseases in cats may be patchy and ventral (Fig. 17.2), while in dogs it is dorsal in caudal perihilar region. Vertebral heart score (VHS) is calculated on the lateral thoracic radiographs to assess cardiac size. Long axis of cardiac silhouette from the carina of the main bronchus to the apex of the heart and short axis at the widest part of the heart are measured. These axes (long and short) are transferred to the vertebrae starting from cranial edge of T4 and count the number of vertebrae fall under each axis. Sum up the number of vertebrae falling under the both axes (Fig. 17.3). Normal vertebral heart score in cats remains within 7.5. Sometimes results of VHS are very illusive as there is no radiographically detectable heart enlargement in hypertrophic cardiomyopathy in cats.
Electrocardiography is a noninvasive diagnostic tool routinely employed for monitoring cardiac rhythm, rate, and conduction disturbances in cats also similar to dogs. Electrocardiography in felines is not an effective tool for screening of occult heart diseases. It has been observed that electrocardiograph is not a very sensitive and precise tool for detection of chamber enlargement, though it can detect MEA. Therefore major utility of electrocardiography in cats is limited to the diagnosis of arrhythmias and conduction disturbances. Early detection of arrhythmias of severe nature, electrolyte imbalance, and conduction disturbances facilitates an exact diagnosis of cardiac ailments so that appropriate remedial measures can be undertaken at the earliest to increase the quality of life of cats with heart disease. Holter 24 hour’s continuous ambulatory electrocardiography is not very successful in felines.
The generation of electrocardiogram in cats is because of depolarization and repolarization of cardiac muscle fibers as seen in canines. The wave of depolarization begins in sinoatrial node (SA node) and passes through atrium producing “P” wave and reaches to AV node. The delay in conduction at AV node results into P-R segment. Then depolarization of ventricles occurs producing “QRS” complex followed by isoelectric period, i.e., S-T segment. This is followed by repolarization of the heart represented by “T wave. The measurements of amplitude and duration of electrocardiographic complexes and intervals in cats differ from that of dogs.
17.2.1 Positioning of Cats for Electrocardiography
The cat is positioned on a table covered with a foam mattress and nonconductive rubber sheet in right lateral recumbency and is restrained by an attendant putting his right arm over the neck and left over hind quarter. Both limbs (left and right) are kept apart. The forelimbs are kept perpendicular to the long axis of the body. If the left thoracic limb is pulled caudally or back, the form of QRS in lead I appears like that of aVF, whereas if that thoracic limb is pulled cranially, QRS in lead I appears like that recorded in aVR. Some cats are uncooperative in right lateral recumbency. In such cases sternal recumbency may be adopted. In sternal recumbency the size of “P” and “R” is slightly tall than in lateral recumbency.
17.2.2 Placement of the Electrodes
Before putting the electrodes, both electrode and the skin are moistened with electrocardiographic gel, paste, or alcohol. Alcohol works well. Electrodes are attached directly to the skin. In case nothing is available, water can be used to increase contact between electrode and skin in the unfavorable condition. Placement of electrode is shown in Fig. 17.4.
Right forelimb clip or needle electrode is attached proximal to the olecranon on the caudal aspect of the right forelimb.
Left forelimb clip or needle electrode is attached proximal to the olecranon on the caudal aspect of the left forelimb.
Right hind limb clip or needle electrode is attached over patellar ligament on the anterior aspect of the right hind limb.
Left hind limb clip or needle is attached over patellar ligament on the anterior aspect of the left hind leg.
17.2.3 Electrocardiogram and Electrocardiographic Indices
Electrocardiogram of feline is having “P,” “QRS,” and “T” complexes; P-R interval, S-T segment, Q-T interval, and R-R interval (Fig. 17.5). In lead I “R” is conspicuous, and “P” and “T” are very small (Fig. 17.6). In lead II, III, and aVF (Fig. 17.6), “P,” “R,” and “T” complexes are conspicuous. In lead aVR complexes are reversed in polarity, and very small “s” is recognizable. In lead aVL complexes are recognizable and reversed in polarity as compared to lead II. This pattern is similar to the pattern seen in dogs. Heart rate varies from 140 to 220 beats per minute. Figs. 17.7 and 17.8 show electrocardiograms of healthy cat. Range values of electrocardiographic parameters are given in the table (Table 17.1). “P” amplitude varies from 0.1–0.2 mV with a duration from 0.03 to 0.04 s. P-R interval ranges from 0.05 to 0.07 s. Amplitude of “R” is small than that of dogs and is variable from 0.1 to 0.8 mV. QRS duration is of 0.04 s. S-T segment is short (0.08 s). T wave is sometimes not appreciable and is very, very small. Its amplitude varies from 0.0 to 0.1 mV and duration 0.04 s. Q-T interval is of 0.16 s, and R-R interval is of 0.4 s and varies as per heart rate. Mean electrical axis on frontal plane in normal cats lies between 0 and + 160° (Fig. 17.9).
17.3 Electrocardiographic Parameters of Healthy Cat
The range values of different electrocardiographic parameters observed at the hospital are given in the Table 17.1
Electrocardiographic parameters of healthy cat
Not appreciable −0.2 mV, +ve, −ve, biphasic
17.4 Interpretations of Normal Cardiac Wave Forms in Cats
Wave forms segments and intervals
Represents atrial muscle depolarization. Normally the wave is positive in lead II and avF
Represents time from the onset of atrial depolarization through conduction over AV node, bundle of His, and Purkinje fibers
Represents depolarization of ventricular muscle. Q is first negative deflection, “R” the first positive deflection, and “S” the second negative deflection
Represents period of phase 2 of action potential
Represents repolarization of ventricular muscle
Represents total time of ventricular depolarization and repolarization
17.5 Characteristics of Feline Electrocardiogram
Complexes are small as compared to dogs.
Intervals are short than that of dogs.
QRS may be of low voltage in some normal cats.
Heart rate is fast as compared to dogs.
17.6 Abnormal Waves, Intervals and Segments with Their Indications
Abnormal waves, intervals and segments
Broad (more than 0.04 s)
Left atrium enlargement (P mitrale)
Tall (more than 0.2 mV)
Right atrium enlargement (P pulmonale)
Tall and broad (more than0.2 mV and more than 0.04 s)
Variable amplitude of P wave
Wandering pace maker
Absence of P wave
Atrial stand still or silent atrium
Ta wave (increased height of descending arm of “P”)
Right atrium enlargement
Tall “R” (more than 0.9 mV) in lead II
Left ventricular enlargement (LVE) or left bundle branch block (LBBB)
Wide “QRS” (more than 0.04 s)
LVE or LBBB
Increased amplitude of “S” (more than 0.5 mV in lead I, II, III, aVF)
Right ventricular enlargement (RVE)
Right bundle branch block (RBBB)
S-T segment/J point
Elevation (more than 0.10 mV)
Depression (more than 0.10 mV)
Slurring or coving
Left ventricular hypertrophy
P-R interval is inversely proportional to heart rate
Increase (more than 0.09 s)
First-degree heart block
Prolong (more than 0.25 s)
Ethylene glycol poisoning
Secondary to prolonged QRS
Short (<0.15 s)
“T” wave changes may indicate myocardial disease
Sharply pointed and notched
No specific abnormality if QRS complex is normal
Large T wave
Intraventricular conduction abnormalities
Pause between R-R < twice of normal R-R interval
Pause between R-R > twice of normal R-R interval