Third-degree atrioventricular (AV) block is a rhythm

Transcription

1 J Vet Intern Med 2006;20: Third-Degree Atrioventricular Block in 21 Cats ( ) H.B. Kellum and R.L. Stepien The effect of 3rd-degree atrioventricular block on long-term outcome in cats is unknown. Clinical findings and long-term outcome of 21 cats with 3rd-degree atrioventricular block were studied retrospectively. Median age of cats studied was 14 years (range 7 19 years). Presenting signs included respiratory distress or collapse, but 6 cats had no clinical signs of disease. Eight cats had congestive heart failure (CHF) at the time that 3rd-degree atrioventricular block was detected. Heart rates ranged from 80 to 140 beats per minute (bpm; median 120 bpm) with no difference in heart rate between cats with and those without CHF. Eleven of 18 cats that had echocardiograms had structural cardiac disease, and 6 cats had cardiac changes consistent with concurrent systemic disease. No atrioventricular nodal lesions were detected by echocardiography. One cat had atrioventricular nodal lesions detected on histologic examination. Median survival of 14 cats that died or were euthanized was 386 days (range 1 2,013 days). Survival did not differ between cats with or without CHF or between cats with or without structural cardiac disease. Thirteen cats with 3rd-degree atrioventricular block survived.1 year after diagnosis, regardless of presenting signs or underlying cardiac disease. Third-degree heart block in cats is often not immediately life threatening. Survival was not affected by the presence of underlying heart disease or congestive heart failure at the time of presentation. Even cats with collapse might survive.1 year without pacemaker implantation. Key words: Conduction disturbance; Escape rhythm; Heart block; Survival; Veterinary. Third-degree atrioventricular (AV) block is a rhythm disturbance that has been identified in many species, including domestic cats. 1 5 The rhythm abnormality is characterized by complete disruption of conduction between the atria and ventricles and is diagnosed on the basis of a characteristic appearance on the surface electrocardiogram (ECG) consisting of rhythmic depolarization of the atria with a simultaneous, unassociated ventricular rhythm originating in the conduction system or ventricular tissue. 6 Third-degree AV block can occur as an idiopathic arrhythmia in cats but has also been associated with cardiac diseases such as hypertrophic cardiomyopathy, intermediate cardiomyopathy, dilated cardiomyopathy, coronary arteriosclerosis with myocardial fibrosis or mitral endocardiosis, infiltrative cardiomyopathies such as lymphoma, dysautonomia or other systemic or metabolic diseases such as hyperthyroidism, as an agonal rhythm, and as a result of aging-associated fibrosis of the nodal tissue. 5,7 16 Degeneration and fibrosis of the AV node has been reported in cats with cardiomyopathy and was suspected to contribute to conduction disturbances. 17 In available case reports of 3rd-degree AV block in cats, 5,7,10,11,14,18,19 the atrial depolarization (P wave) rate is typically within the normal reference range for cats (up to 240 beats per minute [bpm] during ECG recordings 20,21 ), but the ventricular depolarization rate has been substantially lower ( bpm 5,7,11,14,18,19 ). Escape rate and QRS morphology are used to establish the possible origin of the escape rhythm and might reflect the propensity for sudden death. Wide and From the Department of Medical Sciences, University of Wisconsin School of Veterinary Medicine, Madison, WI. Reprint requests: Rebecca L. Stepien, DVM, MS, University of Wisconsin School of Veterinary Medicine, 2015 Linden Drive, Madison, WI 53706; stepienr@svm.vetmed.wisc.edu. Submitted December 22, 2004; Revised March 3, 2004; Accepted July 7, Copyright E 2006 by the American College of Veterinary Internal Medicine /06/ /$3.00/0 bizarre QRS complex morphology, especially when associated with heart rates consistent with idioventricular escape rhythms, usually indicates a ventricular origin for the escape rhythm. Wide escape complexes are also produced when complexes of nonventricular origin are conducted with a bundle branch block, but when supraventricular complexes are conducted with a bundle branch block, the escape rate is typically higher than that associated with idioventricular escape rhythms. Narrow escape complexes are usually thought to originate in the AV node below the level of the block. 5 When a ventricular escape rhythm is relied on to support ventricular function, the animal might be predisposed to sudden death if the escape focus fails and sudden death has occurred anecdotally in dogs with 3rd-degree AV block. Pacemaker implantation is the accepted method of therapy in dogs with 3rd-degree AV block and has been recommended for cats with clinical signs associated with 2nd-degree or 3rd-degree AV block. 5,19,22,23 Although pacemaker implantation for therapy of 3rddegree AV block in cats has been reported, it is not available or chosen for some cats, 9,19 and published information regarding clinical outcome of cats with untreated 3rd-degree AV block is lacking. In the reported cases of feline 3rd-degree AV block, presenting clinical signs have included syncope, 7 lethargy or episodic weakness, 13 congestive heart failure (CHF) or sudden death, 8 or have been absent, 15 but it is unclear whether the presence of structural heart disease or severe clinical signs affects long-term clinical outcome. In addition, even though both narrow 5,11 and wide complex 12,14,19 escape rhythms have been reported in cats with 3rd-degree AV block, it is unclear whether the apparent difference in escape focus location (AV node or bundle of His for narrow complexes or ventricular tissue for wide complexes) of some cats has an effect on outcome or whether the morphology of the escape complexes is related to the rate of depolarization. The purpose of this retrospective study was to investigate presentation, clinical findings, and clinical outcome of 21 cats with 3rd-degree AV block.

2 98 Kellum and Stepien Materials and Methods Records from the University of Wisconsin Veterinary Medical Teaching Hospital Cardiology Service from 1997 to 2004 were reviewed. Cases of 2nd- and 3rd-degree AV block in cats were identified (n 5 29). The ECGs from these cases were reviewed. On review, nineteen cases with an original diagnosis of 3rd-degree AV block were confirmed, and 2 of 10 cases with an original diagnosis of 2nd-degree AV block were diagnosed as 3rd-degree AV block. Third-degree AV block in 21 cats was studied further. The following information was extracted from the medical records: signalment; presenting complaint or reason for referral; physical examination findings on presentation; and diagnostic testing results at the time of original diagnosis, including thoracic radiographs, ECGs, echocardiograms, blood pressure, serum thyroxine concentration, and complete blood cell and serum chemistry evaluations, when available. Cats with left ventricular (LV) hypertrophy were diagnosed with idiopathic hypertrophic cardiomyopathy only if systemic hypertension was excluded (all cats), the cats had normal serum thyroxine concentrations (if age appropriate), and if the hypertrophy was considered to be noninfiltrative by the examining echocardiographer. Mild rightsided chamber dilation in cats receiving fluid therapy or inconsistent minor chamber size abnormalities considered nondiagnostic or clinically insignificant by the examining echocardiographer were recorded as of uncertain clinical significance. Mild or moderate symmetrical LV hypertrophy without outflow obstruction in cats with documented systemic hypertension or hyperthyroidism was considered to be consistent with the systemic disease diagnosis. Cats were categorized as having CHF if evidence of fluid retention (ie, radiographic pulmonary infiltrates consistent with edema, confirmed ascites, or pleural effusion) was accompanied by venous congestion (ie, pulmonary venous distention or jugular distention) on physical examination or diagnostic testing. Cause and date of death was ascertained by medical record review. When the date of death was not recorded, the information was obtained by phone contact with the owner or referring veterinarian. Survival time was calculated from the date of original diagnosis of 3rddegree AV block until death or euthanasia. Clinical outcome and length of follow-up was calculated from the date of original diagnosis of 3rd-degree AV block to the date of last contact for patients lost to follow-up, or to October 4, 2004, for patients still alive at the end of the period under review. Data Analysis Data analysis was performed with standard statistical software. a Length of follow-up was calculated as the time from diagnosis of 3rd-degree AV block until death, euthanasia, loss to follow-up, or the end of the period under review for 21 cats. Survival analysis was performed on the 14 cats that died during the period under review. Median heart rate on presentation and median atrial and ventricular rate on ECG of cats with radiographic evidence of CHF were compared with those of cats without CHF by the Mann- Whitney U-test (P,.05). Prevalence of heart murmurs in cat with and without structural heart disease were compared by Fisher s exact test (P,.05). Median length of follow-up for cats with CHF at the time of 3rd-degree AV block diagnosis (n 5 8) was compared with cats presenting without CHF (n 5 13), and length of followup for cats with structural cardiac disease as a final primary diagnosis (n 5 12) was compared with cats with cardiac findings considered likely to represent secondary cardiac changes (n 5 9). Lastly, survival of cats with CHF at presentation (n 5 5) was compared with those without CHF (n 5 9), and survival of cats with structural heart disease (n 5 8) was compared with survival of cats without primary structural heart disease (n 5 6). All group comparisons were performed by log-rank analysis (P,.05). Fig 1. Distribution by age at diagnosis of cats with 3rd-degree atrioventricular block (n 5 21). Median age of affected cats at presentation was 14 years. Survival data are presented as Kaplan-Meier curves with patient data censored at the time of death or euthanasia. Results Of the 21 cats included in the study, 12 were neutered males, 8 were spayed females, and 1 was a sexually intact female. Fifteen cats were of mixed breed. The remaining cats consisted of 2 Burmese, 2 Siamese, 1 Russian Blue, and 1 Persian. Age of the cats at the time of diagnosis ranged from 7 to 19 years (median 14 years; Fig 1). Many cats were receiving medications for concurrent diseases, but only 1 cat was receiving an antiarrhythmic medication (diltiazem b ) at the time of diagnosis. History Many cats had more than 1 presenting complaint, but the primary reason for presentation or referral was frequently related to clinical signs suggestive of cardiac disease. Presenting complaints included respiratory distress or dyspnea (n 5 10, 6 cats with CHF and 4 cats without CHF) and collapse (n 5 3). Two cats were referred for evaluation of known pleural effusion without respiratory signs. Two cats without CHF had respiratory distress only when stressed. Other clinical signs (eg, neurologic signs, anorexia, dehydration, lethargy, hypothermia) were the reason for evaluation in 2 cats. Six cats were referred for evaluation when cardiovascular abnormalities were found on routine physical examination by the referring veterinarian. Of these 6 cats, 2 cats were referred for evaluation of a heart murmur and 4 were referred for evaluation of bradycardia or auscultated arrhythmias. Some cats had more than 1 reason for evaluation stated. Physical Examination Heart rate obtained by auscultation was recorded in 20 cats. All 20 cats had recorded heart rates between 80 and 140 bpm (median 120 bpm). Thirteen cats had abnormal heart rates noted as a problem. Ten of 13 cats

3 Heart Block in Cats 99 Fig 2. Lead II ECG recordings from 4 cats with 3rd-degree atrioventricular block. Paper speeds and calibrations are indicated. (a) Atrial rate 180 beats per minute (bpm), ventricular rate 100 bpm, with narrow QRS complexes (#0.04 second). P waves are indicated by open arrows. (b) Atrial rate 140 bpm, ventricular rate 100 bpm, with wide QRS complexes. P waves are indicated by open arrows. (c) Atrial rate 180 bpm, ventricular rate 100 bpm, with variable escape morphology. (d) Atrial rate 260 bpm, ventricular rate 140 bpm, with splintered QRS complexes and 1 ventricular ectopic complex (black arrow). with heart rates of 120 bpm or less had bradycardia listed as an abnormality, whereas 3 of 6 cats with heart rates of 140 had bradycardia listed as a problem. There was no difference in median heart rate between cats with CHF (107 bpm) and those without CHF (120 bpm, P 5.16). Heart murmurs were noted in 13 cats. Eight of these 13 cats with murmurs had structural heart disease, and 4 of the 5 cats with murmurs but without structural heart disease had concurrent hypertension, hyperthyroidism, or diabetes mellitus. There was no difference in murmur prevalence between cats with structural heart disease and those without (P 5.43). Other physical abnormalities included increased respiratory effort or respiratory distress noted in 6 cats, with pulmonary crackles detected in one of these cats. An irregular heart rhythm was present in 3 cats, and a gallop rhythm was identified in 2 cats. Three cats had palpable thyroid gland enlargement. Diagnostic Test Results Electrocardiographic findings were available in 21 cats. Atrial depolarization rates ranged from 140 to 260 bpm (median 200 bpm) and ventricular rates ranged from 100 to 140 bpm (median 120 bpm). There was no difference in atrial rate (CHF, 210 bpm; no CHF, 200 bpm; P 5.73) or ventricular rate (CHF, 110 bpm; no CHF, 140 bpm; P 5.29) on the basis of presence of CHF. In 19 cats, the escape rhythm had a narrow (#0.04 second) QRS morphology (Fig 2a); 12 of these 19 cats had escape rates of $120 bpm. Two cats had a wide escape complex morphology (Fig 2b), and 1 cat had both morphologies with narrow and wide escape complexes dominating at different times (Fig 2c). Neither cat with wide QRS complexes had sustained 3rd-degree AV block; 1 cat was hypothermic (atrial rate 140 bpm, ventricular rate 100 bpm) and converted to a sinus rhythm with 2nd-degree AV block when normothermic, and 1 cat with wide complexes of right bundle branch block morphology (atrial rate 240 bpm, ventricular rate 140 bpm) converted to sinus rhythm with a similar right bundle branch block morphology within 24 hours of admission. Seven cats had an intraventricular conduction disturbance diagnosed (on the basis of an irregular or splintered-appearing QRS complex with width # 0.04 second) and 1 cat had a left

4 100 Kellum and Stepien anterior fascicular block pattern noted. Ten cats had premature ectopic ventricular complexes different in appearance from their primary escape rhythm noted (Fig 2d). Escape rates of bpm were present in cats with both narrow and wide QRS complexes. Thoracic radiographic results were available for 18 cats, with multiple abnormalities noted in some cats. Cardiomegaly was noted in 15 cats, pleural effusion in 6 cats, pulmonary alveolar infiltrates consistent with edema in 1 cat, a bronchial pattern in 3 cats, pulmonary masses in 2 cats, lung lobe rounding and retraction in 1 cat, and pneumothorax (after thoracocentesis) in 1 cat. Thoracic radiographic findings were normal in 1 cat. Echocardiographic examinations were recorded in 18 cats. Seven of 18 cats had primary myocardial diseases, including hypertrophic, dilated, restrictive, or unclassified cardiomyopathy at the time of presentation. In 1 cat diagnosed with an unclassified cardiomyopathy, infiltrative myocardial disease was suspected. Six cats had mild, moderate, or severe mitral insufficiency, recorded as an echocardiographic abnormality. Of these, 4 cats had thickening of the mitral leaflets noted as the primary abnormality, and 2 cats had mitral insufficiency diagnosed as part of a myocardial disease diagnosis. Of the remaining 7 cats that underwent echocardiographic examination, 1 cat was recorded as having no echocardiographic abnormalities. The remaining 6 cats had echocardiographic findings consisting of varying combinations of right- or left-sided chamber dilation or wall thickening, considered to be of uncertain significance or typical of concurrent systemic disease diagnoses, including systemic hypertension or hyperthyroidism. Most of these middle-aged to elderly cats had a least 1 concurrent systemic disease at the time of diagnosis. Serum thyroxine concentration was within reference range in 11 of 15 cats tested, high in 2 cats, and below reference range values in 2 cats. Both hypothyroid cats had been treated previously for hyperthyroidism with surgery or radioiodine therapy. Overall, 5 cats had an ongoing diagnosis of hyperthyroidism. Six cats had evidence of renal insufficiency (chronic or acute renal failure based on medical evaluation or unexplained azotemia), 2 cats had neurologic disease or neurologic signs thought to be unrelated to the arrhythmia, and 2 cats had diabetes mellitus. Other noted abnormalities thought to be unrelated to the arrhythmia included dental disease, urinary tract infection, primary lung masses, and an elbow abscess. One cat was tested for antibodies to Borrelia burgdorferi and had a positive titer at 1 : 512. Systolic blood pressure (Doppler ultrasonic method) was measured in 8 cats and ranged from 96 to 230 mm Hg. Three cats were hypertensive (systolic blood pressure. 160 mm Hg) at time of admission (1 cat with hyperthyroidism, 1 cat with chronic renal failure, and 1 cat with idiopathic hypertension). Two cats were being treated for hypertension at the time of diagnosis. Fig 3. Duration of survival after diagnosis (days) in 14 cats with 3rd-degree atrioventricular block. Data censored at time of death or euthanasia. Final Diagnosis and Outcome Twelve cats had a final diagnosis of structural cardiac disease as a cause for their presenting signs or physical abnormalities based on previous diagnostic testing or testing performed at the time of arrhythmia diagnosis. Third-degree AV block continued throughout the length of follow-up in 17 cats, converted to 2nd-degree AV block in 2 cats and converted to sinus rhythm the day after presentation in 1 cat. A pacemaker was implanted successfully in 1 cat and was recommended and declined in 3 additional cats. Median length of follow-up for all cats included in the study was 540 days (range 1 2,013 days). Fourteen cats died in the period under review; median survival of these cats was 386 days (range 1 2,013 days; Fig 3). Six cats were alive at the end of the study period, and 1 cat was lost to follow-up at 30 days. Although numerically quite different, there was no statistical difference in median length of follow-up between cats presenting with evidence of CHF (n 5 8, 1,825 days) and those presenting without signs of CHF (n 5 13, 540 days). Median survival time was not different between cats with CHF (n 5 5, 268 days) and cats presenting without CHF (n 5 9, 407 days; Fig 4a). Similarly, there was no difference in median length of follow-up between cats with structural cardiac disease as a primary problem (n 5 12, 540 days) and those with no cardiac changes or changes thought to be secondary to systemic disease (n 5 9, 590 days) or of median survival time between cats with (n 5 8, 338 days) or without (n 5 6, 399 days Fig 4b) structural cardiac disease. Of the 14 cats that did not survive, 6 died or were euthanized for unknown causes. Five cats died or were euthanized because of known noncardiac disease processes (renal failure n 5 2, neoplasia n 5 2) or trauma (n 5 1). One cat was euthanized as a result of aortic thromboembolism, and 2 cats were euthanized because of poor quality of life. No cats were known to have died suddenly. One cat had an epicardial pacemaker implanted after pleural effusion and pulmonary edema

5 Heart Block in Cats 101 Fig 4. (a) Comparison of survival after diagnosis in cats with congestive heart failure at time of presentation (n 5 5, solid line) and those presenting without signs of congestive heart failure (n 5 9, dotted line). There is no difference in median survival time. (b) Comparison of survival after diagnosis in cats with structural cardiac disease at time of presentation (n 5 8, solid line) and those with cardiac changes considered to be consistent with systemic disease or clinically insignificant (n 5 6, dotted line). There is no difference in median survival time. developed 7 days after presentation for collapse. This cat was alive and receiving no diuretics 566 days after presentation. Pacemaker implantation was recommended for 3 additional cats because of persistent clinical signs. None of these cats underwent pacemaker implantation, and all survived to the end of the study period (length of follow-up 472, 486, and 851 days). One cat was euthanized 2 days postpresentation because of acute renal failure and was submitted for postmortem examination. Histopathologic findings in the AV node and bundle of His included a segmental zone of replacement of myocardial tissue by fibroelastic connective tissue with remnants of Purkinje fibers and fibroblastic tissue. Scattered lymphoplascytic cells were noted in these areas. Discussion Third-degree AV block was diagnosed in 21 cats presented for a variety of clinical signs. Although some cats had severe clinical signs at presentation, approximately one third of the cats with 3rd-degree AV block had their arrhythmia diagnosed as an incidental finding. Cats in this study were usually diagnosed in middle to old age and had concurrent systemic diseases that might have accounted for many of their clinical signs. The presence of CHF at diagnosis or the presence of identifiable structural heart disease did not affect median survival. There is conflicting information in the veterinary literature about the importance of 3rd-degree AV block in cats; some authors report that most cats with this condition show no clinical signs, 15 some report dramatic clinical signs, 8,13 or the relationship or the arrhythmia to clinical signs has been unclear. 5 Reportedly, the fixed, subnormal heart rate of cats with AV block might contribute to the development of heart failure if structural heart disease is present and worsens prognosis in these patients. 5 Approximately one third of the cats in this study (n 5 8) presented with evidence of heart failure as the primary problem, but median group survival did not differ in these cats from those presenting with noncardiac or no clinical signs. This finding suggests that 3rd-degree AV block might not contribute to the development or persistence of CHF consistently and that the arrhythmia might not need to be addressed directly to resolve signs of heart failure. Eight of the 11 cats presented for reasons other than respiratory distress were without clinical signs and were diagnosed after cardiac abnormalities were found on routine physical examination. The majority of cats without clinical signs lived.1 year postdiagnosis and died or were euthanized for noncardiac problems even when detectable cardiac disease was present. The ausculted heart rates of cats in this study ranged from 80 to 140 bpm and were well below what is typical of cats in a hospital setting. 16 Despite the prevalence of clinical signs typical of cardiac disease in these cats, these low heart rates were occasionally not recognized as abnormal during the physical examination. Ausculted bradycardia might have been attributed to the apparent severity of illness of clinical presentation, but even in the 6 cats without clinical signs, 2 were not noted as having abnormal heart rates. The lack of recognition of subnormal or inappropriate heart rate as an abnormality in clinical patients can contribute to the apparent low clinical prevalence of this condition in cats. Sixty-five percent of patients had heart murmurs, but the presence or absence of a murmur did not add information as to the presence of structural heart disease in most cats. Approximately equal numbers of cats with murmurs in this study had and did not have structural cardiac disease, and approximately equal numbers of cats without murmurs did and did not have structural cardiac disease. Detection of heart murmurs remains an important indicator of cardiovascular abnormality in cats, but detection of a heart murmur does not reliably signify structural cardiac disease in these patients. In contrast to the wide escape complexes frequently seen in dogs with 3rd-degree AV block, 5,15 90% of cats in this study had narrow QRS complexes, implying that the

6 102 Kellum and Stepien escape rhythm had its origin in the AV node or in the bundle of His. 6,23 The ventricular escape depolarization rates varied between 100 and 140 bpm and were not consistently related to the width of the complexes, but the ventricular rates in most cats in this study were generally higher than ventricular escape rates previously described in cats with 3rd-degree AV block ( bpm). 13 Sudden death, reported to be a common outcome in cats with 3rd-degree AV block, 8 was not reported in the cats in this study, but 6 cats had unknown causes of death. Clinical signs were often absent or less dramatic in cats in this study than those reported previously. These findings suggest that the presence of severe clinical signs, rather than heart rate, might be an important indicator of the need for immediate intervention. The AV nodal or bundle of His origin implied by the narrow QRS complex morphology might be a reason that sudden death does not appear to occur in cats with the frequency with which it occurs in dogs, a species in which escape rhythms originating in the bundle branches or distal Purkinje system appear to be more prevalent. Escape rhythms originating in the bundle of His tend be more stable and more rapid than escape rhythms originating in the bundle branches or the Purkinje fibers. 15 Because only 2 cats in this study had wide QRS morphology and their escape rates (100 and 140 bpm) overlap the escape rates of those cats with narrow QRS morphology, no conclusions can be drawn regarding the relationship of QRS width with depolarization rate. Premature ectopic complexes different in morphology from the primary escape complexes were evident in 10 cats. These complexes appeared to be ventricular in origin on the basis of wide morphology and lack of associated P waves, but no cat had ectopy considered frequent enough to warrant therapy. Seven of the 10 cats with ectopy had structural cardiac disease as the final diagnosis, suggesting that the presence of such premature ectopic complexes might identify the presence of structural cardiac disease in some cats. Eight cats had a history of CHF or signs of CHF present on presentation. Five of 7 cats with pleural effusion had structural heart disease, but 2 had systemic disease as their primary diagnosis. The presence of bradycardia could potentiate the accumulation of pleural effusion in an impaired heart by preventing the necessary increase in heart rate that supports cardiac output in the face of decreased stoke volume. Despite this concern, 4 of 7 cats presenting with pleural effusion in this study lived.1 year postdiagnosis. On the basis of this small number of cats, the presence of CHF at presentation of cats with 3rd-degree AV block might not have a negative effect on survival, and CHF might represent an underlying cardiac condition of many of these cats separate from their rhythm status. However, cats presenting without CHF might have had their survival limited by severe systemic disease and therefore might not provide a reasonable comparison group. Although cats with CHF in this study were treated with angiotensin-converting enzyme inhibitors, diuretics, and other cardiovascular medications at the discretion of the attending clinician, therapy was not standardized or analyzed in this study. Echocardiographic findings were highly variable in this group of cats, and many of the findings might have been reflective of cardiac changes in response to systemic disease rather than structural heart disease. No anatomic abnormalities were noted in the area of the AV node by echocardiography in these cats, but microscopic fibrosis or replacement of myocardial tissue with fibroelastic connective tissue (such as was noted on postmortem examination of 1 cat) might be difficult to identify by echocardiography. Identifiable structural cardiac disease did not affect median survival times in these cats. Although 3rd-degree AV block in cats often is reported to indicate the presence of serious myocardial disease, 5 that finding is not supported by the results of this study. Echocardiographic results in this study were obtained from reports in the records, and echocardiograms were not reviewed. Conclusions regarding echocardiographic findings might be obscured by variability in record keeping and diagnostic criteria for myocardial disease among echocardiographers. In addition, therapy before echocardiography was not recorded; administration of fluids and other medical manipulations might have affected echocardiographic findings in some cats. Although 3rd-degree AV block is often considered an irreversible arrhythmia in dogs, 2 cats in this study converted to 2nd-degree AV block within days, and 1cat converted to sinus rhythm 24 hours after presentation. No specific clinical abnormality presented in these cats that would appear to predict this phenomenon. Pacemaker implantation is the preferred therapy for patients with refractory heart failure thought to be heart rate related or for patients with overt clinical signs affecting quality of life (eg, syncope). 5,22,23 The continued survival to.1 year postpresentation of 3 cats considered to have overt clinical signs of arrhythmia suggests that the clinical signs were tolerated despite persistence of the arrhythmia. Two cats in this study were known to have died or were euthanized because of cardiovascular problems, but the exact cause of death or euthanasia was not known or was vague (eg, poor quality of life) for 8 cats (4 of 8 cats with CHF and 4 of 13 cats without CHF). Cats might be partially protected from sudden death if their escape foci are situated in the more stable bundle of His and other subsidiary ventricular pacemakers are available more distal to the AV node. It has previously been suggested that 3rd-degree AV block in cats tends to cause clinical signs over time, 13 but that was not apparent in this study. The survival times of the cats in our study with an unknown cause of death ranged from 25 to 2,013 days, with 5 cats living.1 year after diagnosis and 3 cats surviving.4.5 years. Limited conclusions can be drawn from this study regarding the ultimate circumstances of death or euthanasia in cats with 3rd-degree AV block, but the risk of sudden death soon after diagnosis appears to be small. This study was subject to a number of limitations that are typical of retrospective studies. A small number of

7 Heart Block in Cats 103 cases were available for review, especially when subgroups were studied. This might obscure what could be real differences in the survival of cats with CHF versus no CHF, or with structural cardiac disease as a primary diagnosis versus cats with other primary diagnoses. The nature and interpretation of the diagnostic testing or therapy was not consistent, but all ECGs were reviewed for diagnostic accuracy and all radiographs were interpreted by a board-certified radiologist. With regard to echocardiographic findings, variable echocardiographic technique and record keeping as well as changes in diagnostic criteria over time made echocardiographic results less consistent. Incomplete data were available regarding the cause of death for animals with long survival times, and there was a conspicuous lack of portmortem examinations. Footnotes a Prism 4.0a for the Macintosh, GraphPad Software Inc, San Diego, CA b Diltiazem, Teva Pharmaceutical Corporation USA, Sellersville, PA References 1. Pibarot P, Vrins A, Salmon Y, et al. Implantation of a programmable atrioventricular pacemaker in a donkey with complete atrioventricular block and syncope. Eq Vet J 1993;25: Reef VB, Clark ES, Oliver JA, et al. Implantation of a permanent transvenous pacing catheter in a horse with complete heart block and syncope. J Am Vet Med Assoc 1986;189: Rosen KM, Dhingra RC, Loeb HS, et al. Chronic heart block in adults. Clinical and electrophysiological observations. Arch Intern Med 1973;131: Tilley LP. Canine third-degree AV block (complete block). In: Tilley LP, ed. Essentials of Canine and Feline Electrocardiography. Interpretation and Treatment, 3rd ed. Philadelphia, PA: Lea & Febiger; 1992; Johnson L, Sisson DD. Atrioventricular block in cats. Comp Con Educ Sm Anim 1993;15: Schamroth L. Third degree Complete A-V block. In: The Disorders of Cardiac Rhythm. Oxford: Blackwell Scientific; 1980: Harpster NK. Feline cardiomyopathy. Vet Clin N Am Sm Anim Pract 1977;7: Tilley LP. Feline third degree AV block (complete block). In: Tilley LP, ed. Essentials of Canine and Feline Electrocardiography. Interpretation and Treatment, 3rd ed. Philadelphia, PA: Lea & Febiger; 1992; Darke PGG, McAreavey D, Been M. Transvenous cardiac pacing in 19 dogs and one cat. J Sm Anim Pract 1989;30: Moise NS, Dietze AE. Echocardiographic, electrocardiographic, and radiographic detection of cardiomegaly in hyperthyroid cats. Am J Vet Res 1986;47: Jacobs GJ, Otto C. ECG of the month: Third degree atrioventricular block in a cat. J Am Vet Med Assoc 1988;193: Fox PR, Kaplan P. Feline arrhythmias. In: Bonagura JD, ed. Contemporary Issues in Small Animal Practice. New York, NY: Churchill Livingstone; 1987: Fox PR, Harpster NK. Diagnosis and management of feline arrhythmias. In: Fox PR, Sisson D, Moise NS, eds. Textbook of Canine and Feline Cardiology. Principles and Clinical Practice, 2nd ed. Philadelphia, PA: WB Saunders; 1999: Buss DD, Pyle RL, Chacko SK. Pulmonary torsion and complete heart block in a cat. Clinico-Pathologic Conference. J Am Vet Med Assoc 1972;161: Kittleson MD, Kienle RD. Diagnosis and treatment of arrhythmias (dysrhythmias). In: Kittleson MD, Kienle RD, eds. Small Animal Cardiovascular Medicine. St. Louis, MO: Mosby; 1998: Bonagura JD. Cardiovascular Diseases. In: Sherding RG, ed. The Cat: Diseases and Clinical Management. New York: Churchill Livingstone; 1994: Liu S-K, Tilley LP, Tashjian RJ. Lesions of the conduction system in the cat with cardiomyopathy. In: Roy P-E, Rona G, eds. The Metabolism of Contraction. Baltimore, MD: University Park Press; 1975: Orsini D, Buss DD. Complete atrioventricular block in a cat. J Am Vet Med Assoc 1972;172: Fox PR, Moise NS, Woodfield JA, et al. Techniques and complications of pacemaker implantation in four cats. J Am Vet Med Assoc 1991;199: Tilley LP. Analysis of feline P-QRS-T deflections. In: Tilley LP, ed. Essentials of Canine and Feline Electrocardiography. Interpretation and Treatment, 3rd ed. Philadelphia, PA: Lea & Febiger; 1992; Gompf RE, Tilley LP. Comparison of lateral and sternal recumbent positions for electrocardiography of the cat. Am J Vet Res 1979;40: Ferasin L, Van de Stadt M, Rudorf H, et al. Syncope associated with paroxysmal atrioventricular block and ventricular standstill in a cat. J Sm Anim Pract 2002;43: Harpster NK. The cardiovascular system. In: Holzworth J, ed. Diseases of the Cat. Medicine & Surgery. Phildelphia, PA: WB Saunders; 1987: