A 12-year-old 23.0-kg (50.6-lb) neutered male mixed-breed dog was referred to the Veterinary Teaching Hospital at Colorado State University for evaluation of a chronic cough. For approximately 2 months prior to the referral evaluation, the dog had had daily episodes of nonproductive coughing, which had recently increased in frequency. During this period, the dog had been taken on 2-hour-long walks daily without any problems, and the owner reported no additional concerns.
At the evaluation, the dog was quiet and alert. Respiratory abnormalities were not discernible during thoracic auscultation, and a cough could not be elicited via tracheal palpation. The dog had pink mucous membranes, and capillary refill time was 2 seconds. A grade 2/6 left apical, systolic heart murmur was present. Heart rate was 70 beats/min, and a regular rhythm was detected. Femoral pulses were synchronous with the heart beat and strong bilaterally. Throughout the examination, the dog was perceptibly (and progressively) apprehensive.
Diagnostic tests performed initially included thoracic radiography, serum biochemical analysis, whole blood hematologic analysis, urinalysis, fecal flotation analysis, heartworm antigen testing, and ELISA testing for antibodies against Anaplasma phagocytophilum, Borrelia burgdorferi, and Ehrlichia canis. All of these test results revealed no clinically important abnormalities. A bronchoscopic examination was subsequently scheduled for this patient, and prior to induction of anesthesia, ECG and echocardiography were performed. Echocardiography revealed mild to moderate left atrial enlargement and left ventricular enlargement. However, left ventricular systolic function was considered to be normal. Peak left and right ventricular outflow velocities were abnormally high (2.6 m/s and 2.1 m/s, respectively [reference range, < 2 m/s]), and this was believed to account for the murmur. Electrocardiography was performed.
ECG Interpretation
Multiple ECG tracings were acquired during the evaluation. The initial ECG examination revealed an underlying sinus rhythm and high-grade second-degree atrioventricular (AV) block (Figure 1). Ventricular rate was 68 beats/min, atrial rate was 215 beats/min, and the atrial-to-ventricular conduction rate was 3:1. As the dog became more anxious during the ECG examination, changes in the ECG tracings became evident and revealed an alternation between the aforementioned second-degree AV block and third-degree, or complete, AV block (Figure 2). During periods of third-degree AV block, ventricular escape beats had a wide QRS complex duration (80 milliseconds), occurred at a rate of 63 beats/min, and were of left-bundle branch block (LBBB) morphology; atrial rate was approximately 190 beats/min. Ventricular escape complexes had marked ST segment depression (0.6 to 1.0 mV), whereas marked ST segment depression was not observed in the sinus-conducted QRS complexes. Occasional conduction of a sinus beat was observed, and these conducted beats were characterized by a consistent PR interval (100 milliseconds) and normal QRS complex duration (40 milliseconds). Toward completion of the examination, when only the ECG leads of the echocardiographic recorder were attached to the dog, the patient suddenly became extremely nervous. The ECG recording obtained at that time (Figure 3) revealed a progression from third-degree AV block with ventricular escape complexes of LBBB morphology to a brief period of ventricular asystole (2.8 seconds' duration), followed by complexes of a right-bundle branch morphology. These complexes were most likely the result of a second focus of escape activity. A single conducted sinus complex was identified preceding resumption of third-degree AV block with the initial ventricular escape focus (LBBB morphology).
Initial lead II ECG rhythm strip obtained from a mixed-breed dog that had a chronic cough during evaluation prior to anesthesia for a bronchoscopic examination. An underlying sinus rhythm is present with high-grade second-degree atrioventricular (AV) block. P waves (arrowheads) are conducted in a 3:1 ratio. The ventricular rate is approximately 68 beats/min, and the atrial rate is approximately 215 beats/min. Paper speed = 25 mm/s; 5 mm = 1 mV.
Citation: Journal of the American Veterinary Medical Association 239, 8; 10.2460/javma.239.8.1060
Initial lead II ECG rhythm strip obtained from a mixed-breed dog that had a chronic cough during evaluation prior to anesthesia for a bronchoscopic examination. An underlying sinus rhythm is present with high-grade second-degree atrioventricular (AV) block. P waves (arrowheads) are conducted in a 3:1 ratio. The ventricular rate is approximately 68 beats/min, and the atrial rate is approximately 215 beats/min. Paper speed = 25 mm/s; 5 mm = 1 mV.
Citation: Journal of the American Veterinary Medical Association 239, 8; 10.2460/javma.239.8.1060
Initial lead II ECG rhythm strip obtained from a mixed-breed dog that had a chronic cough during evaluation prior to anesthesia for a bronchoscopic examination. An underlying sinus rhythm is present with high-grade second-degree atrioventricular (AV) block. P waves (arrowheads) are conducted in a 3:1 ratio. The ventricular rate is approximately 68 beats/min, and the atrial rate is approximately 215 beats/min. Paper speed = 25 mm/s; 5 mm = 1 mV.
Citation: Journal of the American Veterinary Medical Association 239, 8; 10.2460/javma.239.8.1060
Lead II ECG rhythm strip obtained from the dog in Figure 1 several minutes after the initial ECG strip was recorded. An underlying sinus rhythm is present, and an alternating pattern of high-grade second- and third-degree AV block is evident. During periods of third-degree AV block, ventricular escape complexes with left-bundle branch block morphology (LBBB) and marked ST segment depression (arrows) are present. During periods of high-grade second-degree AV block, the PR interval remains constant (100 milliseconds) for sinus-conducted QRS complexes (arrowheads). Paper speed = 25 mm/s; 5 mm = 1 mV.
Citation: Journal of the American Veterinary Medical Association 239, 8; 10.2460/javma.239.8.1060
Lead II ECG rhythm strip obtained from the dog in Figure 1 several minutes after the initial ECG strip was recorded. An underlying sinus rhythm is present, and an alternating pattern of high-grade second- and third-degree AV block is evident. During periods of third-degree AV block, ventricular escape complexes with left-bundle branch block morphology (LBBB) and marked ST segment depression (arrows) are present. During periods of high-grade second-degree AV block, the PR interval remains constant (100 milliseconds) for sinus-conducted QRS complexes (arrowheads). Paper speed = 25 mm/s; 5 mm = 1 mV.
Citation: Journal of the American Veterinary Medical Association 239, 8; 10.2460/javma.239.8.1060
Lead II ECG rhythm strip obtained from the dog in Figure 1 several minutes after the initial ECG strip was recorded. An underlying sinus rhythm is present, and an alternating pattern of high-grade second- and third-degree AV block is evident. During periods of third-degree AV block, ventricular escape complexes with left-bundle branch block morphology (LBBB) and marked ST segment depression (arrows) are present. During periods of high-grade second-degree AV block, the PR interval remains constant (100 milliseconds) for sinus-conducted QRS complexes (arrowheads). Paper speed = 25 mm/s; 5 mm = 1 mV.
Citation: Journal of the American Veterinary Medical Association 239, 8; 10.2460/javma.239.8.1060
Lead II ECG rhythm strip obtained by use of an echocardiographic recorder from the dog in Figures 1 and 2 after a period of extreme stress. An underlying sinus rhythm with third-degree AV block is initially present with the same LBBB morphology (A) as seen in Figure 2. P waves (arrowheads) are identified during a period of ventricular asystole (2.8 seconds' duration). Development of an alternate ventricular escape focus with right-bundle branch block morphology is indicated (B). One sinus beat is conducted (C) before the third-degree AV block with ventricular escape complexes of LBBB morphology resumes. Because the recording was obtained by use of an echocardiographic recorder, paper speed and voltage information are unavailable.
Citation: Journal of the American Veterinary Medical Association 239, 8; 10.2460/javma.239.8.1060
Lead II ECG rhythm strip obtained by use of an echocardiographic recorder from the dog in Figures 1 and 2 after a period of extreme stress. An underlying sinus rhythm with third-degree AV block is initially present with the same LBBB morphology (A) as seen in Figure 2. P waves (arrowheads) are identified during a period of ventricular asystole (2.8 seconds' duration). Development of an alternate ventricular escape focus with right-bundle branch block morphology is indicated (B). One sinus beat is conducted (C) before the third-degree AV block with ventricular escape complexes of LBBB morphology resumes. Because the recording was obtained by use of an echocardiographic recorder, paper speed and voltage information are unavailable.
Citation: Journal of the American Veterinary Medical Association 239, 8; 10.2460/javma.239.8.1060
Lead II ECG rhythm strip obtained by use of an echocardiographic recorder from the dog in Figures 1 and 2 after a period of extreme stress. An underlying sinus rhythm with third-degree AV block is initially present with the same LBBB morphology (A) as seen in Figure 2. P waves (arrowheads) are identified during a period of ventricular asystole (2.8 seconds' duration). Development of an alternate ventricular escape focus with right-bundle branch block morphology is indicated (B). One sinus beat is conducted (C) before the third-degree AV block with ventricular escape complexes of LBBB morphology resumes. Because the recording was obtained by use of an echocardiographic recorder, paper speed and voltage information are unavailable.
Citation: Journal of the American Veterinary Medical Association 239, 8; 10.2460/javma.239.8.1060
Considering the dog's marked cardiac arrhythmia, the bronchoscopic examination was cancelled and empiric treatment for presumed chronic bronchitis was initiated. An atropine-response test was performed, and there was no effect on atrial rate, ventricular rate, or AV conduction. A few days later, a permanent transvenous cardiac pacemaker was implanted. The pacemaker placement and recovery from anesthesia occurred without incident. One month after pacemaker placement, the owner reported no problems with the dog at home.
Discussion
In dogs, AV block represents approximately 10% of all arrhythmias.1 Cardiac conduction system abnormalities can develop as a result of either functional or structural lesions.2 Functional causes include high vagal tone and drug- or anesthetic-induced AV conduction abnormalities, and structural causes include inflammatory processes (eg, endocarditis or myocarditis) and degenerative lesions (eg, myocardial fibrosis). Dogs with high-grade second- and third-degree AV block commonly have structural lesions.2 Fibrosis is an important and common cause of advanced conduction system disease, particularly in older dogs.2,3
The AV node possesses several characteristics that allow disruption of conduction to occur. Electrical impulses have decremental conduction through the AV node, meaning that the impulse becomes diminished in intensity as it progresses through the AV nodal tissue.3 The AV node is comprised of a small, sparsely connected system of tissue.3 Cell-to-cell contact becomes impaired as patchy fibrosis develops with increasing age, and this contributes to impaired propagation of the electrical impulse through the AV node.4 All of these attributes predispose the AV node to impulse block when fibrosis is present. In a postmortem study5 of the conduction system of large-breed dogs without known clinical or gross cardiac disease, marked degenerative changes were observed in the hearts of older dogs. Such changes in the conduction system included an increase in fibrous connective tissue, infiltration of fat, and focal fibrosis of the conduction system; loss of conduction fibers; and a drastic reduction in size of the small arterioles supplying the AV conduction system, with some being nearly occluded.5 Considering that these findings in dogs without clinically apparent conduction abnormalities are similar to those in dogs with overt AV conduction block, degeneration of the AV conduction system may thus account for the observation that high-grade second-degree AV block often progresses to third-degree AV block in dogs.6
For routine health screening, humans often undergo nuclear perfusion imaging of the heart during exercise stress testing. There have been many reports of AV block being induced in apparently healthy individuals during such exercise stress testing.7,8 In those persons, a 1:1 AV conduction rate with normal sinus rhythm was present prior to the exercise stress testing. During the exercise stress testing, high-grade AV block developed and the nuclear perfusion images revealed reversible, transient ischemia as the cause for the AV block.7,8 The sinus and AV nodes are well innervated by the autonomic nervous system, whereas the His-Purkinje system is comparatively lacking in this innervation; thus, a mismatch between the shortening of cardiac cycle length and the effective refractory period of the His-Purkinje system could explain this transient AV block.7,8 During periods of increased sympathetic tone, AV nodal conduction is improved. Because His-Purkinje system conduction is unable to similarly improve, the His-Purkinje system becomes relatively stressed by this discrepancy.7 When any subclinical His-Purkinje system disease is present, exercise-induced ischemia exacerbates this His-Purkinje stress and could lead to marked AV block.7
The dog of this report had major ST-segment depression of ventricular escape complexes. Although ST-segment depression may indicate myocardial ischemia in sinus-conducted complexes, this finding during ventricular escape complexes rarely indicates myocardial ischemia as the cause of the ST-segment depression. However, it is possible that the progression to transient third-degree AV block was caused by reversible, transient, stress-induced myocardial ischemia detectable only via nuclear perfusion testing. Transient exercise-induced complete AV block in an 8-year-old Rottweiler that was evaluated for exercise-associated syncope has been previously reported.9
In humans, there is also evidence that individuals with preexisting, overt conduction system disease that develop a progression of their AV block during stress or exercise may be at risk for sudden death.10 In 1 report,10 a person with chronic bifascicular block underwent an exercise stress test for evaluation of progressive pulmonary disease. During the stress testing, third-degree AV block developed, but this was then followed by 30 seconds of ventricular asystole before the escape complexes returned. The progression of AV block during exercise stress testing represents a risk for sudden death because of the unstable nature of escape complex generation, and the recommendation is for an immediate pacemaker implantation in such patients.10 In the dog of this report, a similar instability was observed in the ventricular escape complex focus. Although the period of ventricular asystole in the dog was short, compared with that reported for the aforementioned human with chronic bifascicular block, the instability of the ventricular escape complex generation was nonetheless demonstrated.
In a retrospective review of dogs with high-grade second- or third-degree AV block, several important aspects of high-grade second-degree AV block were highlighted.1 The dogs in that study did not receive pacemakers, and dogs with high-grade second- or third-degree AV block had similar survival times. Both groups of dogs with AV block were at high risk of sudden death during the first 6 months after diagnosis.1 In fact, 12 of those 63 (19%) dogs died suddenly within the first 30 days after diagnosis.1 Negative prognostic indicators for survival time included relatively narrow ventricular escape complexes and relatively fast ventricular escape rates. Both of these ECG characteristics were evident for the dog of this report. In the retrospective study,1 the absence of clinical signs and the atropine response test result had no effect on survival time. The unstable nature of the ventricular escape rhythm in the dog of this report may highlight why dogs with high-grade second- or third-degree AV block are at risk for sudden death. It is possible that the dog of this report had a progression from high-grade second- to third-degree AV block and then to ventricular asystole as a result of stress-induced myocardial ischemia affecting the cardiac conduction system. Dogs with high-grade second-degree AV block are at risk for progression of conduction system abnormalities and at risk for sudden death despite the absence of clinical signs, and should have a permanent pacemaker implanted.
References
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