Survival of 4 dogs with persistent atrial standstill treated by pacemaker implantation.

Pacemakers were implanted in 4 client-owned female dogs which had persistent atrial standstill. Three dogs were alive after 14 to 39 months and 1 dog was euthanized after 10.5 years. This report demonstrates that some dogs with persistent atrial standstill can survive for extended time periods.

Survie de 4 chiennes atteintes de paralysie auriculaire persistante traitées à l'aide de l'implantation d'un cardiostimulateur. Des cardiostimulateurs ont été implantés chez 4 chiennes, appartenant à des propriétaires, atteintes de paralysie auriculaire persistante. Trois chiennes étaient vivantes après 14 à 39 mois et 1 chienne a été euthanasiée après 10,5 ans. Ce rapport démontre que certains chiens atteints de paralysie auriculaire persistante peuvent survivre pendant des périodes de temps prolongées.(Traduit par Isabelle Vallières).

Atrioventricular Dissociation and Congestive Heart Failure in a Ring-Necked Pheasant ( Phasianus colchicus ).

The aim of this clinical report was to describe a case of complete atrioventricular dissociation in a 9-month-old, male ring-necked pheasant ( Phasianus colchicus ). The case was incidentally determined during the evaluation of electrocardiograms recorded from pheasants. There were no observed clinical symptoms in the bird before or after the electrocardiogram. The PR interval varied, and there was no association of the P waves and QRS complexes in the electrocardiogram. Although the ventricular rhythm was regular (130 beats/min), atrial rhythm was irregular (approximately 91 beats/min). Pathological examination showed there was left ventricular hypertrophy with degeneration of the mitral valve of the heart and well-marked congestion in the liver and lung.

Clinical presentation, management, and survival in dogs with persistent atrial standstill in the United Kingdom.

OBJECTIVES: To investigate the clinical and echocardiographic presentation of dogs with persistent atrial standstill (PAS), identify variables measured at first presentation that could predict their survival, and document the progression of the disease after pacing. MATERIALS AND METHODS: Retrospective study of medical records of dogs diagnosed with PAS at three referral hospitals of the United Kingdom over seven years. RESULTS: Twenty-six dogs were diagnosed with PAS during the study period. Median age of the population was three years (range: seven months-12.5 years). The most common clinical sign was syncope (14/26). Twenty-four dogs received artificial pacemakers (PM). Major complications after PM implantation were observed in four dogs (four/24). Serial echocardiographic examinations showed that cardiac dimensions of PAS dogs with left atrial or left ventricular dilation at first presentation did not return to reference range after pacing. Further dilation of the cardiac chambers, recurrence of congestive heart failure (CHF), or development of new episodes of CHF were documented in seven, four, and 10 PAS dogs, respectively, despite pacing. Median survival time for cardiac-related deaths after PM implantation was 1512 days (18-3207). Neither CHF nor echocardiographic variables at presentation predicted survival after PM implantation in PAS dogs. CONCLUSIONS: Persistent atrial standstill (PAS) is an uncommon bradyarrhythmia, occurring in young adult dogs. Affected dogs were often presented with syncope. Whilst syncope resolved, cardiac remodeling persisted after PM implantation. Long-term survival was favorable after PM implantation and was not predicted by congestive status or cardiac chamber size at first presentation.

Frequency of cardiac arrhythmias in horses during straight and untethered swimming.

BACKGROUND: Cardiac arrhythmias have not been previously reported in horses while swimming. OBJECTIVES: To describe the type and frequency of encountered arrhythmias during repetitive swimming cycles. STUDY DESIGN: Descriptive observational study. METHODS: Sixteen horses swam five pool lengths (75 m), each separated by an active recovery walk. Continuous electrocardiograms (ECGs) were recorded (n = 80) and analysed during the pre-swim, swim and active-recovery periods. Arrhythmias were categorised as sinus arrhythmia (SA), sinus block, sinus pause (compensatory and non-compensatory), second degree atrioventricular block (2AVB) for physiological arrhythmias, supraventricular premature depolarisation (SVPD) and ventricular premature depolarisation (VPD) for non-physiological arrhythmias. A linear mixed-effects model was used to examine the effects of repetitive swim lengths on arrhythmias and swimming parameters. Data were reported as median [interquartile range]. RESULTS: Fifteen horses (94%) experienced at least one arrhythmia; however, the frequency remained low and 2AVB were only observed during the pre-swim period. The swimming heart rate (HR) was 162 bpm [141;173]. Sinus blocks, sinus pauses, SA, SVPD and VPD were all recorded at least once during swimming. Except for one VPD couplet, all premature depolarisations were isolated. During active-recovery, the HR was 105 bpm [103;106], with SA observed in 13 horses (81%), isolated SVPD in six horses (38%), sinus pause in one horse (6%) but no VPD present. MAIN LIMITATIONS: Limited number of horses precluding population prevalence assessment. CONCLUSION AND CLINICAL IMPORTANCE: High-quality underwater ECGs were acquired in swimming horses for the first time. The frequency of arrhythmias remained low and rare pathological arrhythmias were observed during repetitive swimming and active-recovery cycles. Swimming with active-recovery periods is not a high-risk cardio-arrhythmic exercise.

HISTORIAL: Arritmias cardiacas no han sido previamente descritas en caballos nadando. OBJETIVOS: Describir el tipo y frecuencia de arritmias encontradas durante ciclos de natación repetitivos. DISEÑO DEL ESTUDIO: Estudio descriptivo observacional. MÉTODOS: Diez y seis caballos nadaron cinco largos de piscina (75 m), cada uno separado por una caminata de recuperación. Electrocardiogramas continuos (ECGs) fueron grabados (n = 80) y analizados durante el pre-natación, natación y periodos de recuperación activa. Las arritmias fueron categorizadas en arritmia sinusal (SA), bloqueo sinusal, pausa sinusal (compensatoria y no compensatoria), bloqueo atrio ventricular de segundo grado (2AVB) para las arritmias fisiológicas, y en despolarización supraventricular prematura (SVPD) y despolarización ventricular prematura (VPD) para las arritmias no fisiológicas. Se utilizo un modelo linear de efecto mixto para examinar los efectos de nadar largos de piscina en forma repetitiva sobre las arritmias y parámetros de natación. Los datos fueron reportados como mediana [rango intercuartil]. RESULTADOS: Quince caballos (94%) sufrieron de al menos una arritmia, sin embargo la frecuencia permaneció baja. 2AVB fueron observados solo durante el periodo de pre-natación. La frecuencia cardiaca durante la natación (HR) fue de 162 bpm [141;173]. Bloqueos sinusales, pausas sinusales, SA, SVPD y VPD fueron todos registrados por lo menos una vez durante la natación. Excepto por un par de VPD, todas las despolarizaciones fueron aisladas. Durante la recuperación activa, la HR fue de 105 bpm [103;106], con SA observado en 13 caballos (81%), SVPD aislados en 6 caballos (38%), pausa sinusal en 1 caballos (6%) pero ningún VPD presente. LIMITACIONES PRINCIPALES: Número limitado de caballos lo que imposibilita hacer un asesoramiento de la prevalencia en la población. CONCLUSIÓN E IMPORTANCIA CLÍNICA: Por primera vez, se adquirieron ECGs debajo del agua de alta calidad en caballos nadando. La frecuencia de las arritmias permaneció baja y se observaron arritmias patológicas raramente durante la natación repetitiva y los ciclos de recuperación activa. Nadar con periodos de recuperación activa no es un ejercicio de alto riesgo cardio-arrítmico.

Atrioventricular dissociation in a cat with persistent truncus arteriosus.

A 10-month-old female domestic shorthaired (DSH) cat was presented with peracute respiratory problems. Physical examination revealed dyspnoea, tachypnoea, cyanosis, weak pulse and bradycardia. Auscultation showed pulmonary crepitation and attenuated heart sounds and a pansystolic grade V/VI murmur. The electrocardiogram showed atrioventricular dissociation identified as third-degree sinoatrial block. X-rays showed increased density in the ventral and middle zones of the thorax and loss of definition of the cardiac silhouette and increased diffuse radiographic density of the entire abdomen. Echocardiography revealed dilatation of the right atrium and concentric biventricular hypertrophy. A type 1 persistent truncus arteriosus was diagnosed at necropsy. This is the first case report of this type of arrhythmia in a cat with persistent truncus arteriosus, and its relationship with the described congenital cardiac anomaly is discussed.

Toceranib phosphate resolves right heart obstruction secondary to a heart base neoplasm in a dog.

An eight-year-old castrated male, 45 kg labrador retriever presented for evaluation of a two-week history of cough and tachypnoea. Echocardiography revealed an approximately 10 cm heart base mass, which extended to the right atrioventricular junction, causing compression of both the right atrium and ventricle resulting in right-sided congestive heart failure (abdominal effusion). Cytology of the mass was consistent with a neuroendocrine carcinoma. Given the location and description, a chemodectoma or ectopic thyroid carcinoma was suspected. The patient was treated with toceranib phosphate and famotidine. At the follow-up appointment four weeks later, the right heart compression had resolved due to a clinically significant reduction in the size of the mass. The patient was prescribed furosemide and enalapril to treat right-sided congestive heart failure. When considering treatment options for haemodynamically significant heart base masses, treatment with toceranib phosphate may result in rapid clinical benefit.

Optimizing single-chamber pacing in dogs Part 1: Rate determinations, rate interventions and hysteresis.

Determining ideal pacing rates to meet physiological needs and optimizing programming to prevent unnecessary right ventricular pacing in dogs requires an understanding of heart rate profiles and applicable pacing technology. The heart rate and rhythm of the dog is complex necessitating investigation of rate requirements of activity and circadian influences. Overlaying this information are a multiplicity of other factors such as age, breed, temperament, cardiovascular disease and underlining rhythm disorders that contribute to the difficulty in making general conclusions. However, all such information permits better implementation of programming options with the goal of better outcomes. In this review (Part 1 of a two-part review) instantaneous heart rate, rolling average heart rate, simple average heart rate, heart rate tachograms, RR interval tachograms (2D, 3D and dynamic), and Poincaré plots (2D, 3D and dynamic) are discussed as they apply to decisions in the determination and examination of pacing rates for dogs programmed in the VVI pacing mode (Ventricular paced, Ventricular sensed, Inhibited pacing). The applicable pacing operations available for three pacemaker companies are reviewed (Abbott, Biotronik/Dextronix, and Medtronic). The programmable options considered include: slowest pacing rate without additional features to extend the pacing interval, sleep/rest rate preferences, hysteresis to lengthen pacing interval following intrinsic beats, and intermittent increases in pacing following abrupt loss of intrinsic rhythm. Recommendations are suggested for follow-up of individual dogs with examination of pacing statistics and Holter monitoring.

Insights into sinus arrhythmia of the dog: Acetylcholine perfusion of canine right atrium results in beat-to-beat patterns that mimic sinus arrhythmia supporting exit block in the sinoatrial conduction pathways.

Sinus arrhythmia of the dog is unique because of the pronounced alternating beat-to-beat intervals. The clustering of these short (faster rates) and long (slower rates) intervals is not just influenced by autonomic input from breathing; sinus arrhythmia can persist in the panting or apneic dog. The multiplicity of central and peripheral influences on the sinus node complicates the unraveling of the mechanisms of sinus arrhythmia. Studies of the sinus node suggest that acetylcholine can slow cellular depolarization and block sinoatrial conduction. Electrocardiographic monitoring of the dog supports this notion in that abrupt bifurcation into short and long intervals develop at lower heart rates. We sought to determine whether this phenomenon could be recapitulated in canine atrial preparations perfused with acetylcholine and whether selective pharmacologic blockade of the voltage and calcium clocks could provide insight into its mechanism. Spontaneous beat to beat (A-A) intervals were obtained from monophasic action potential recordings of perfused canine right atrial preparations before and during perfusion with acetylcholine (2-5 µM). The calcium clock was blocked with ryanodine (2-3 µM). The membrane clock was blocked with diltiazem hydrochloride (ICa,L blocker; 0.25 µM) and ZD7288 (If blocker; 3 µM). Hyperpolarization was hindered by blockade of IK,Ado/IK,Ach with tertiapin Q (100 nM) before and during acetylcholine perfusion. Acetylcholine resulted in beat clusters similar to those seen in sinus arrhythmia of the dog. Beat clusters were consistent with intermittent 2:1 and 3:1 sinoatrial conduction block. Tertiapin Q abolished this patterning suggesting a role of IK,Ado/IK,ACh in the mechanism of these acetylcholine-induced beat-to-beat patterns.

Pathological Features of Persistent Atrial Standstill Syndrome in Three Dogs.

The hearts of three dogs, clinically diagnosed as having persistent atrial standstill syndrome (PAS), were studied post mortem. The most significant gross findings in the hearts of all three dogs were dilatation and marked reduction in the thickness of both atrial walls. Histopathologically, all three had widespread progressive loss of the atrial myocardium with replacement by fatty or fibrofatty tissue, consistent with atrial myopathy. The lesion mainly affected the upper half of both atria and was more severe in the epimyocardium and midmyocardium than in the endomyocardium. On the basis of these observations, it is proposed that the atrial myopathy commences in the upper regions of both atria and progresses downwards, as has been demonstrated electrophysiologically in PAS in humans, and extends from the epicardium towards the endocardium.

Mediastinal lymphoma with complete atrioventricular block in a horse.

A 15-year-old, neutered-male pony presented with a history of weight loss during 4 months. Clinical evaluation revealed severe bradycardia and complete atrioventricular block. At necropsy, a lobulated mass in the anterior mediastinum and moderate enlargement of the superficial cervical lymph nodes were observed. The vagus nerve and the brachiocephalic trunk were embedded in this anterior mediastinal tumor. Histologically, the mass was composed of sheets of neoplastic lymphoid cells expressing CD3, with a low mitotic rate. To the best of our knowledge, this is the first reported case of mediastinal lymphoma associated with complete atrioventricular block in horses.

A histological study of the cardiac conduction system in canine cases of mitral valve endocardiosis with complete atrioventricular block.

The cardiac conduction system was examined histologically in four canine cases of endocardiosis of the mitral valve (MV) with complete atrioventricular (AV) block. In all cases, moderate to severe reduction of the conduction fibres due to fibrous or fibro-fatty replacement was observed in the penetrating and branching portions of the AV bundle. In addition, degenerative and fibrotic lesions were commonly seen at the upper portions of the left and right bundle branches. These changes in the AV conduction system were associated with marked degeneration and fibrosis of the base of the central fibrous body and the upper part of the ventricular septum. The degenerative and sclerotic changes of the AV junctional region, affecting the AV bundle and bundle branches, were qualitatively similar to those in age-matched control dogs, but were more severe. It is possible that the pathological process occurred as a result of ageing and may have been exaggerated or accelerated by the abnormal mechanical forces created by excessive motion of the prolapsed MV and the long-term haemodynamic stresses of mitral regurgitation, resulting in interruption of the AV conduction system to produce complete AV block. Conduction abnormalities represent a possible complication in some canine cases of MV endocardiosis.

Results of pacemaker implantation in 104 dogs.

OBJECTIVES: To document the outcome, survival and complications involved in pacemaker implantation in dogs in a retrospective study. METHODS: Case records for all dogs in which pacemaker implantation was performed were reviewed. RESULTS: A total of 104 dogs underwent pacemaker implantation. Dogs were presented with atrioventricular (AV) block (71), sick sinus syndrome (25) or vasovagal syncope (eight). Age at presentation varied from six months to 13 years with a median age of seven years and two months. The Labrador was the most commonly represented breed (17 cases). All but one dog survived pacemaker implantation, with 93 showing resolution of their clinical signs while 10 dogs showed intermittent residual signs. One-, three- and five-year survival estimates were 86, 65 and 39 per cent, respectively. Major complications after implantation were documented in 15 dogs and three of these led to fatalities. Minor complications were noted in 23 dogs. Sudden death occurred in six dogs three to 55 months following successful pacemaker implantation. CLINICAL SIGNIFICANCE: Transvenous pacemaker implantation was successful in reducing or eliminating clinical signs in over 90 per cent of dogs with third-degree atrioventricular (AV) block or sick sinus syndrome. In dogs with vasovagal syncope, six of eight dogs had greatly reduced frequency of collapse and two became asymptomatic. Although the procedure was associated with complications, these were rarely life threatening and good survival was documented in the majority of cases.

Influence of 2nd-degree AV blocks, ECG recording length, and recording time on heart rate variability analyses in horses.

OBJECTIVES: To assess the influence of 2nd-degree AV blocks (AVB) on RR interval-based heart rate variability (HRV) variables; to investigate the effect of using PP interval time series and of artifact filtering on HRV analyses; to investigate the influence of electrocardiogram (ECG) recording length and time of recording; and to calculate day-to-day variability and reference intervals of HRV variables. ANIMALS: Thirty healthy adult horses. METHODS: RR and PP interval time series were extracted from 10-h Holter ECGs and an automated filter was applied to the RR time series (RRf). Time-domain HRV variables were calculated based on RR, PP, and RRf time series and their relation to the number of AVBs was assessed. Hourly 10- and 60-min segments were extracted to investigate the influence of segment length and recording time on HRV variables. Day-to-day variability and reference intervals of HRV variables were calculated. RESULTS: Variables of short-term HRV were significantly influenced by the number of AVBs when based on RR, but not when based on PP- and RRf time series. PP- and RRf-based HRV variables were in good agreement. The majority of HRV variables were influenced by recording time and ECG segment length. Day-to-day variability of HRV variables was low when based on 10-h ECG recordings but moderate to high when based on 60-min and 10-min recordings. CONCLUSIONS: Second-degree AVBs significantly influence conventional RR-based, but not PP- and RRf-based time-domain HRV variables. However, PP and RRf analyses have limitations and recording length and time of recording must be considered.

Survival time with pacemaker implantation for dogs diagnosed with persistent atrial standstill.

OBJECTIVES: To evaluate survival time in dogs with persistent atrial standstill after pacemaker implantation and to compare the survival times for cardiac-related vs. non-cardiac deaths. Secondary objectives were to evaluate the effects of breed and the presence of congestive heart failure (CHF) at the time of diagnosis on survival time. ANIMALS: Twenty dogs with persistent atrial standstill and pacemaker implantation. METHODS: Medical records were searched to identify dogs diagnosed with persistent atrial standstill based on electrocardiogram that underwent pacemaker implantation. Survival after pacemaker implantation was analyzed using the Kaplan-Meier method. RESULTS: The median survival time after pacemaker implantation for all-cause mortality was 866 days. There was no significant difference (p=0.573) in median survival time for cardiac (506 days) vs. non-cardiac deaths (400 days). The presence of CHF at the time of diagnosis did not affect the survival time (P=0.854). No difference in median survival time was noted between breeds (P=0.126). CONCLUSIONS: Dogs with persistent atrial standstill have a median survival time of 866 days with pacemaker implantation, though a wide range of survival times was observed. There was no difference in the median survival time for dogs with cardiac-related deaths and those without. Patient breed and the presence of CHF before pacemaker implantation did not affect median survival time.

Presumptive partial atrial standstill secondary to atrial cardiomyopathy in a Greyhound.

Persistent atrial standstill is a rare arrhythmia in both human and veterinary patients. In recent decades, cases of partial atrial standstill have been recognized in humans. We describe a case of presumptive partial atrial standstill in a Greyhound, in which there was disparate left and right atrial electromechanical function and rapid progression to congestive heart failure over the span of fourteen weeks. An atrial cardiomyopathy characterized by severe, diffuse, fibrofatty replacement of the atrial myocardium was identified histologically.

The anatomical basis of complete atrioventricular block in cats with hypertrophic cardiomyopathy.

The cardiac conduction system was examined histologically in 13 feline cases of hypertrophic cardiomyopathy (HCM) with complete atrioventricular (AV) block. Marked degeneration and fibrous replacement of the AV conduction system were consistently observed in the combined regions of the branching portion of the AV bundle and the upper portion of the left bundle branch. These changes were associated with extensive fibrosis of the central fibrous body and endocardial and myocardial fibrosis in the upper border of the ventricular septum. Chondrometaplastic lesions or osseous lesions, or both, present in the base of the central fibrous body, compressed the underlying penetrating or branching (or both) portions of the AV bundle, causing apparent reduction of the conduction fibres. The pathological process and the nature and predilection sites of the lesions resembled those associated with ageing in human patients with complete AV block. It is possible that the pathological process in the cats was fundamentally related to the normal ageing phenomenon and may have been exacerbated by mechanical forces created by the cardiac hypertrophy associated with HCM.

Physiologic VDD versus nonphysiologic VVI pacing in canine 3rd-degree atrioventricular block.

Historically, ventricular demand, nonphysiologic (VVI) pacing has been the most commonly used modality to treat 3rd-degree atrioventricular (AV) block. The goal of this study was to determine the feasibility of using a commercial, single-lead, physiologic (VDD) pacemaker in dogs with 3rd-degree AV block. Furthermore, we hoped to characterize and identify differences in the radiographic, echocardiographic, neurohormonal, and quality of life consequences of physiologic versus nonphysiologic pacing. We evaluated 10 dogs during a 12-week crossover study. Acutely, rate-matched physiologic pacing reduced pulmonary capillary wedge pressure by 19% compared with nonphysiologic pacing. VDD pacing significantly reduced left atrial size normalized to body weight, left atrial-to-aortic root ratio, and left ventricular end-systolic dimension and increased fractional shortening, aortic Doppler velocity, cardiac output, and stroke volume compared with VVI pacing. Variable rate VDD pacing resulted in a significantly slower heart rate (HR) during echocardiography than fixed-rate (100 bpm) VVI pacing. AV synchronous pacing reduced circulating N-terminal proatrial natriuretic peptide (ANP), norepinephrine (NOR), and epinephrine (EPI) concentrations compared with asynchronous pacing. There were no significant differences in systemic blood pressure, thoracic radiographs, or owner-perceived quality of life. The median percentage of AV synchronous pacing during the VDD modality was 99.8% (range, 1.2 to 99.9%). This study confirms the potential to achieve physiologic pacing with a commercial, single-lead system in dogs. VDD pacing improved hemodynamics and neurohormonal profiles over asynchronous pacing although the long-term clinical benefits of these changes remain to be determined.

Third-degree atrioventricular block in 21 cats (1997-2004).

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.

Applications, complications, and outcomes of transvenous pacemaker implantation in 105 dogs (1997-2002).

We reviewed the indications for age and breeds of dogs who received transvenous endocardial artificial pacemaker (AP) implantation (n = 105) and complications and survival thereafter at a single institution over a 6-year period. A third-degree atrioventricular (AV) block (59%) and sick sinus syndrome (SSS; 27%) were the most common indications, along with a high-grade second-degree AV block (9%) and atrial standstill (5%). The most common breeds identified were Labrador Retriever (n = 16; 11 with a third-degree AV block), American Cocker Spaniel (n = 14; 10 with SSS), and Miniature Schnauzer (n = 13; all with SSS). Common presenting complaints were syncope (n = 66) and exercise intolerance or lethargy (n = 25). Half of the dogs (n = 52) had a history of acute onset of clinical signs (<2 weeks). Mean survival time for the 60 dogs who died during the study period was 2.2 years (range, 0.1-5.8 years). Major complications occurred in 13% of dogs and included lead displacement (n = 7), sensing problems that led to syncope (n = 3), infection at the pacemaker site (n = 1), bleeding (n = 1), and ventricular fibrillation during implantation (n = 1; successfully defibrillated). Minor complications occurred in 11 dogs (11%). The success rate of transvenous AP implantation was comparatively high (all dogs survived the first 48 hours), and the complication rate was comparatively low when compared with a previous multicenter study, most likely because of how commonly the procedure was performed and supervisory experience.

Signalment, clinical signs, and prognostic indicators associated with high-grade second- or third-degree atrioventricular block in dogs: 124 cases (January 1, 1997-December 31, 1997).

OBJECTIVE: To evaluate signalment, clinical signs, and prognosis associated with high-grade second- or third-degree atrioventricular block (AVB) in dogs. DESIGN: Retrospective case series. ANIMALS: 124 dogs. PROCEDURES: Data were gathered from ECGs, veterinarian questionnaires, echocardiograms, and radiographs submitted for review; compared with data from a large control group; and examined for association between variables and duration of survival. A new classification system for AVB was evaluated. RESULTS: Afghan, Catahoula Leopard Dog, Chow Chow, Cocker Spaniel, German Wirehaired Pointer, and Labrador Retriever breeds were predisposed to high-grade second- or third-degree AVB. Heavier, older, and sexually intact female dogs were overrepresented in the study group. Weakness, lethargy, exercise intolerance, and syncope were the most common clinical signs. The presence of clinical signs was not associated with duration of survival. Dogs with high-grade second-degree AVB had a duration of survival similar to that of dogs with third-degree AVB. Dogs with high-grade second- or third-degree AVB were at high risk for sudden death in the first 6 months after diagnosis. High ventricular escape rhythm rate and narrow escape-complex QRS width were negatively associated with duration of survival. Pacemaker implantation had a significant positive association with survival. CONCLUSIONS AND CLINICAL RELEVANCE: Pacemaker implantation should be strongly considered in all dogs with high-grade second- or third-degree AVB regardless of whether clinical signs are evident. If medical treatment is warranted, vagolytic medications may be the best choice. A new classification system for AVB may merit further investigation.