ECG of the Month

Elisa Martinelli Department of Veterinary Medical Sciences, University of Parma, 43126 Parma, Italy.

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Ilaria Spalla Department of Veterinary Science and Public Health (DIVET), University of Milan, 20133 Milan, Italy.

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Cecilia Quintavalla Department of Veterinary Medical Sciences, University of Parma, 43126 Parma, Italy.

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Paola Brambilla Department of Veterinary Science and Public Health (DIVET), University of Milan, 20133 Milan, Italy.

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Giulia Riscazzi Department of Veterinary Science and Public Health (DIVET), University of Milan, 20133 Milan, Italy.

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Chiara Locatelli Department of Veterinary Science and Public Health (DIVET), University of Milan, 20133 Milan, Italy.

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A 14-year-old spayed female domestic shorthair cat was referred to the Cardiology Service of the Department of Veterinary Science and Public Health, University of Milan, for evaluation of an arrhythmia. No clinical signs were reported by the owner. On physical examination, the cat was bright, alert, and responsive and had a good body condition score (3/5; weight, 2.85 kg [6.27 lb]). Mucous membranes were pink and moist, and capillary refill time was < 2 seconds. Respiratory rate was 28 breaths/min, and femoral pulse rate was 140 beats/min. Rectal temperature was considered normal (38.0°C [100.4°F]). A prominent left apical precordial beat and a hyperkinetic femoral pulse were detected. On cardiac auscultation, bradycardia (< 160 beats/min)1 was evident, despite the cat's signs of anxiety. Heart sounds appeared to be arrhythmic, but no distinct heart murmur could be detected. The remainder of the physical examination findings were unremarkable.

Diagnostic testing included a CBC, serum biochemical analyses (including assessment of serum concentrations of thyroxine and free thyroxine), and cardiac evaluation (ECG and echocardiography). A CBC revealed no abnormalities. Results of serum biochemical analyses were within reference ranges except for mildly high alanine aminotransferase activity (164 U/L; reference range, 12 to 130 U/L). High serum concentrations of thyroxine (5.8 μg/dL; reference range, 0.8 to 4.7 μg/dL) and free thyroxine (64.1 pmol/L; reference range, 9.0 to 33.5 pmol/L) were found.

Echocardiography revealed mild left ventricular enlargement, mild left ventricular eccentric hypertrophy, and normal left atrial dimension; trivial mitral valve regurgitation was detected with color Doppler ultrasonography.2 These findings were in accordance with reported echocardiographic patterns of high-output states.3 Systolic arterial blood pressure, measured by a Doppler method, was 140 mm Hg. An ECG examination was performed with the cat in right lateral recumbency; tracings from leads I, II, III, aVR, aVL, and aVF were recorded for approximately 5 minutes.

ECG Interpretation

The initial ECG examination (6 lead tracings) revealed that the ventricular rate was 140 beats/min and the atrial rate was 200 beats/min (Figure 1). P waves were seen at a regular (P-P interval, 240 milliseconds) and fast rate; QRS complexes had a slower fairly irregular rate (R-R intervals were somewhat variable; range, 340 to 420 milliseconds). P waves were of normal appearance; amplitude was 0.2 mV, and duration ranged from 20 to 30 milliseconds. Two consecutive P waves were blocked and did not conduct QRS complexes (atrioventricular [AV] block) indicative of a fixed relationship between the atria and the ventricles (3:1 AV block). There was an alternation between QRS complexes of apparently normal morphology (amplitude, 0.8 mV; duration, 40 milliseconds) and QRS complexes of abnormal morphology (range of amplitude, 1.1 to 1.6 mV; range of duration, 50 to 90 milliseconds) with a 1:1 ratio. The morphology of the abnormal QRS complexes was variable (polymorphism), and those complexes occurring during bradycardia were considered ventricular escape beats (VEBs). A single episode (4 consecutive ventricular ectopic beats) of ventricular tachycardia (VT) was observed during the 5-minute period of ECG monitoring. The final ECG diagnosis was an advanced second-degree AV block with polymorphic VEBs and a single run of VT. Treatment of the cat with methimazole at a standard dosage (2.5 mg/cat, PO, q 12 h) was initiated.

Figure 1—
Figure 1—

Surface 6-lead ECG tracings obtained at the initial evaluation of a 14-year-old cat with hyperthyroidism. The atrial rate is 200 beats/min, and the ventricular rate is 140 beats/min. P waves appear at a regular (P-P interval, 240 milliseconds) and fast rate (arrow). The QRS complexes appear at a slower fairly irregular rate (R-R intervals were variable; range, 340 to 420 milliseconds). The appearance of the P waves is considered normal (P-wave amplitude, 0.2 mV; duration, 20 to 30 milliseconds). Two consecutive P waves are blocked (3:1 atrioventricular block). There is an alternation between QRS complexes of apparently normal morphology (amplitude, 0.8 mV; duration, 40 milliseconds) and QRS complexes of abnormal morphology (amplitude range, 1.1 to 1. 6 mV; duration range, 50 to 90 milliseconds [arrowheads]) with a 1:1 conduction ratio. Notice the sequence of 4 abnormal and wide QRS complexes (episode of VT). Paper speed = 50 mm/s; 1 cm = 1 mV.

Citation: Journal of the American Veterinary Medical Association 243, 6; 10.2460/javma.243.6.787

Figure 2—
Figure 2—

Surface 6-lead ECG tracing obtained from the cat in Figure 1 after 1 month of treatment with methimazole. The abnormal QRS complexes (arrowheads) have all the same morphology (monomorphic), and there are no episodes of VT. Paper speed = 50 mm/s; 1 cm = 1 mV.

Citation: Journal of the American Veterinary Medical Association 243, 6; 10.2460/javma.243.6.787

After 1 month of treatment, the cat's thyroid hormone concentrations had normalized and ECG examination was repeated (Figure 2). Six-lead tracings of 5 minutes' duration were recorded and revealed persistence of advanced second-degree AV block with a ventricular rate of 140 beats/min characterized by monomorphic VEBs without episodes of VT.

Discussion

The cat of the present report was a geriatric patient with hyperthyroidism, but without signs of cardiomyopathy or other diseases. Although a connection between advanced age or hyperthyroidism and arrhythmias (AV block, VEBs, and VT) has been reported,4–12 it is difficult to establish a clear cause-and-effect relationship in this patient.

Atrioventricular block occurs when conduction through the AV junction is delayed or interrupted. Intermittent failure of AV conduction characterizes second-degree AV block.1,3 The frequency of the blocks is usually constant. For example, in second-degree AV block with a 2:1 conduction ratio, for each P wave conducted through the AV node, there is 1 P wave that is blocked (not followed by a QRS complex). Advanced second-degree AV block is characterized by 2 or more blocked P waves (ie, a 3:1 [or greater] conduction ratio).13 In cats, reported causes of AV block include age-associated fibrosis of the nodal tissue, inflammatory or degenerative myocardial lesions (involving the conduction system) associated with cardiomyopathy, toxoplasmosis, feline infectious peritonitis, and hyperthyroidism.4,5,14–16 To the authors' knowledge, 8 studies4–11 involving ECG examination of hyperthyroid cats were reported between 1982 and 2012; in those studies, 275 hyperthyroid cats were examined and 11 (4%) cats with AV block were identified. Four of those cats had first-degree AV block, 3 cats had second-degree AV block (one of which had advanced second-degree AV block), and 4 cats had third-degree AV block. Because no ECG recordings obtained prior to the onset of hyperthyroidism in these cats were available, it is also possible that AV block was present before the onset of hyperthyroidism and that the 2 conditions were unrelated. In humans, AV blocks associated with thyrotoxic heart disease have been reported and are explained by inflammatory changes in the AV node.17 Case reports of AV block in healthy geriatric cats are less common in the veterinary medical literature, probably because of underdiagnosis of the arrhythmia. First- and second-degree AV block in cats generally are occasional findings because they are not associated with overt clinical signs such as weakness or syncope; however, advanced second-degree AV block and third-degree AV block generally cause clinical signs over time.3,18

Ventricular escape beats are spontaneous depolarizations originating in the specialized ventricular conduction system and, rarely, in the ventricular myocardium during bradycardia because of a lack of an overriding and more rapid beat.1,3 Ventricular escape beats of identical shape are called unifocal and suggest a single ectopic origin; when ectopic QRS complex shapes are variable, they are termed multiform or multifocal and are related to multiple sites of depolarization.1,3 For the cat of this report, the variable shape of the QRS complexes in the ECG tracings at the initial evaluation suggested multiple ventricular origins, whereas 1 month later, the shape and width of the QRS complexes suggested a single site of origin.

The effects of thyroid hormones are considered similar to the effects of sympathomimetic agents.19 It has been proposed that some of the effects of thyroid hormones are mediated by increased activity of the sympathoadrenal system or, more likely, that thyroid hormones increase cardiac responsiveness to circulating catecholamines by increasing the number or affinity of cardiac β-adrenergic receptors.6,19 The activation of the sympathetic nervous system has been implicated in the onset of thyroid storm.20 Within the 5-minute-long first ECG recording obtained from the cat of this report, 1 brief episode of paroxysmal multiform VT occurred. Polymorphic VEBs and an episode of VT were not detected in the second ECG recording obtained from the cat after treatment with methimazole.

It is not clear whether AV block secondary to hyperthyroidism resolves after appropriate treatment for hyperthyroidism.21 In 1 study,6 16 of 17 cats that were examined at the time of diagnosis with hyperthyroidism and after successful treatment had a complete resolution of the arrhythmias. However, ECG recordings obtained from these cats before the diagnosis of hyperthyroidism were not available. In another report,8 third-degree AV block persisted throughout treatment for hyperthyroidism in only 1 cat.

It has also been reported that resolution of arrhythmias other than sinus tachycardia is less consistent.19 The prevalences of atrial fibrillation and atrial tachycardia increase in people with hyperthyroidism, compared with age-matched controls, and decrease with treatment for hyperthyroidism. However, the prevalence of ventricular arrhythmias and conduction disturbances (in combination or separately) does not differ between people with or without hyperthyroidism and is not altered with treatment for hyperthyroidism.19 No similar studies on cats are available, to the authors' knowledge.

For the cat of the present report, both hyperthyroidism and advanced age could be considered causes of the detected arrhythmias. One month after initiation of treatment with methimazole, no change in severity of the second-degree AV block (ie, 3:1 conduction ratio) was evident, but polymorphic VEBs and episodes of VT were not detected. Considering the reduction in the malignant characteristics of ventricular arrhythmia after treatment, we hypothesized a link between the endocrinopathy and the presence of polymorphism and VT. Increased cardiac responsiveness to circulating catecholamines induced by hyperthyroidism19 may be responsible for the more malignant appearance of the ventricular arrhythmia at the first examination.

For the cat of the present report, Holter monitoring was not performed because of the owner's financial constraints. Consequently, we cannot rule out the possibility that the apparent improvements in the arrhythmia (absence of polymorphism and VT) evident during the recheck ECG examination were a reflection of random variation in the heart rhythm throughout the day. Furthermore, although toxoplasmosis and feline infectious peritonitis were ruled out on the basis of clinical examination and general laboratory findings, no specific tests were performed to confirm the cat's negative status for those conditions.

References

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