A 9-year-old 6.3-kg (13.9-lb) sexually intact male Maltese was referred to the Cardiology Unit of the Department of Veterinary Sciences of the University of Parma because of episodes of transient loss of consciousness (T-LOC) with vocalization, limb stiffness, opisthotonus, and urine incontinence. During the preceding few months, the T-LOC episodes became more frequent and occurred during both periods of rest and exercise. Physical examination findings were unremarkable except for a grade 1/6 midsystolic cardiac murmur, which was audible over the left apex, and an irregular cardiac rhythm. Echocardiography revealed mild left ventricular systolic dysfunction and trivial mitral valve regurgitation. During the echocardiographic examination, ECG monitoring revealed respiratory sinus arrhythmia with a mean heart rate of 85 beats/min and frequent episodes of isolated second-degree atrioventricular block (AVB). On the basis of the dog's history and echocardiographic findings, Holter monitoring for a 24-hour period and clinicopathologic analyses were performed.
ECG Interpretation
During the 24-hour period of Holter monitoring, the dog did not have any episodes of T-LOC. The heart rate ranged from 44 to 140 beats/min (mean heart rate, 66 beats/min). The dominant rhythm was respiratory sinus arrhythmia with sinus bradycardia during periods of rest and sleeping. Frequent episodes of atypical Mobitz type I second-degree AVB (Figure 1) and episodes of ventricular asystole (range of duration, 1.7 to 5.7 seconds) due to sinoatrial arrest with no escape rhythm (Figure 2) were evident regardless of the activity of the dog (as reported in a patient diary completed by the owner).
Three-lead tracings obtained during a 24-hour period of Holter monitoring of a Maltese that was evaluated because of episodes of transient loss of consciousness with vocalization, limb stiffness, opisthotonus, and urinary incontinence. The dominant rhythm is sinus rhythm with a mean heart rate of 90 beats/min. Notice the P wave that is not followed by a QRS complex. The PQ interval following the blocked P wave is shorter than the PQ interval preceding the blocked P wave. Other typical features of the Mobitz type I atrioventricular block are lacking (eg, progressive lengthening of the PQ interval before the blocked P wave, maximum lengthening of the PQ interval associated with the second QRS complex after the blocked P wave, and progressive shortening of the R-R interval before the blocked P wave). Hence, this is an atypical Mobitz type I (Wenckebach) second-degree atrioventricular block, which is characterized by variable PQ and R-R intervals before the blocked P wave and a shorter PQ interval after the blocked P wave. Paper speed = 25 mm/s; 1 cm = 1 mV.
Citation: Journal of the American Veterinary Medical Association 250, 8; 10.2460/javma.250.8.851
Continuous tracings (duration of each segment is 10 seconds) obtained from the 24-hour Holter monitoring of the dog in Figure 1. A—The dominant rhythm is respiratory sinus arrhythmia with a mean heart rate of 54 beats/min. Notice a period of sinoatrial arrest of 5.7 seconds with no escape beats. The arrest is not followed by an increased heart rate as expected, suggesting autonomic dysfunction. Paper speed = 16 mm/s; 1 cm = 1 mV. B—Focused view of the sinoatrial arrest in panel A. Sinus arrest is characterized by a P-P interval greater than twice the normal P-P interval and a pause P-P interval that is not a multiple of the preceding P-P interval. Paper speed = 25 mm/s; 1 cm = 1 mV.
Citation: Journal of the American Veterinary Medical Association 250, 8; 10.2460/javma.250.8.851
Results of a CBC and plasma biochemical analysis were within reference intervals except for high cholesterol and triglycerides concentrations. The findings of a serum thyroid panel were as follows: free thyroxine concentration, 1.2 ng/dL (reference interval, 0.93 to 3.5 ng/dL); thyroxine concentration, 1.95 ng/dL (reference interval, 1.47 to 4.50 ng/dL); and thyroid-stimulating hormone concentration, 1.16 ng/dL (reference interval, 0.04 to 0.35 ng/dL). A diagnosis of hypothyroidism was made, and treatment of the dog with L-thyroxinea (20 μg/kg [9.1 μg/lb], PO, q 24 h) was initiated. The episodes of T-LOC progressively resolved during the first month of treatment, and a cardiac recheck examination was performed 4 months after the start of treatment. At that time, echocardiography revealed normalization of systolic function and Holter monitoring revealed a sinus rhythm, respiratory sinus arrhythmia, and complete resolution of the atypical Mobitz type I second-degree AVB and sinoatrial arrest.
Discussion
The dog of the present report was initially examined because of episodes of T-LOC. Cardiologic examination of the dog revealed sinoatrial arrest and intermittent Mobitz type I second-degree AVB. A diagnosis of hypothyroidism was also made. Hypothyroidism generally develops in middle-aged or older dogs, and both sexes can be affected. Certain breeds of dog are more commonly affected than others, and the Maltese breed is reportedly predisposed for the primary form of hypothyroidism.1
Thyroid hormones control many systems, and hypothyroidism can cause a great variety of clinical signs, the most common of which are dermatologic abnormalities, gastrointestinal signs, and infertility.2 Neurologic signs, alteration in mental status (dullness, lethargy, or aggressive behavior), low body temperature, exercise intolerance, and increased body weight in the face of normal or decreased appetite have also been reported for dogs with hypothyroidism.3,4 Cardiovascular abnormalities are not common in dogs with hypothyroidism; however, weak apex beats and peripheral pulses; low heart rate; low QRS complex voltage; prolongation of P waves, QRS complexes, and QT intervals; inverted T waves; atrial fibrillation; and bradyarrhythmia have been described.4–7 In hypothyroid dogs, systolic dysfunction may develop, although it rarely progresses to heart failure and can be reversible after hormonal treatment.2 There are several proposed causes of these cardiovascular alterations, including reduction of myocardial cytoplasmic concentration of calcium and potassium, reduction in the number of β1-adrenergic receptors, low circulating concentration of norepinephrine, decreased myocardial response to catecholamines, and alterations in myosin metabolism.5
First- and second-degree AVBs are the most common bradyarrhythmias associated with hypothyroidism in both humans and dogs.1,5–7 Sinoatrial arrest has been described in humans with hypothyroidism as part of the so-called sick sinus syndrome.8 Sick sinus syndrome is an electric disorder characterized by alterations of sinoatrial automaticity and sinoatrial conduction. Electrocardiographic signs are sinus arrest (pauses of 2 to 8 seconds’ duration) or sinoatrial block, with or without an escape rhythm.7 Recently, a retrospective study9 of 93 dogs identified an association between hypothyroidism and sick sinus syndrome or sinus node dysfunction. The direct association between hypothyroidism and bradyarrhythmias has not been demonstrated, to our knowledge; it has been hypothesized that hypothyroidism might affect the autonomic nervous system, induce cardiovascular signs (eg, syncope), and unmask underlying sinus node disease. However, the report of the retrospective study9 did not include the clinical outcome of hypothyroid dogs after hormonal treatment.
For the dog of the present report, the cause of the T-LOC episodes could not be determined from the Holter monitoring findings. We speculate that the T-LOC episodes might have been related to periods of ventricular asystole (arrhythmic syncope) or orthostatism (dysautonomic syncope) because of the sinus node dysfunction. The second hypothesis seems more likely. However, even if bradyarrhythmias were not the cause of the T-LOC episodes, the hormonal treatment given to the dog contributed to the resolution of both syncope and arrhythmias. To our knowledge, this is the first case report of hypothyroidism-associated sinus node dysfunction and T-LOC episodes that completely resolved with L-thyroxine treatment in a dog.
Footnotes
Leventa, MSD Animal Health S.r.l. Italia, Via Fratelli Cervi, Italy.
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