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Outcome and survival in dogs with sick sinus syndrome | VETgirl Veterinary Continuing Education Podcasts

In this VETgirl online veterinary CE podcast, we review the outcome and survival in dogs with sick sinus syndrome, a life-threatening bradyarrhythmia.

First, let’s review sick sinus syndrome (SSS). SSS is the second most common reason for permanent artificial pacemaker implantation (PAP) intervention in dogs. Definitive diagnosis of SSS technically requires electrophysiologic studies, but in most cases the practical/clinical diagnosis of SSS in dogs is made based on identification of sinoatrial dysfunction on an ECG in conjunction with clinical signs (e.g., syncope, weakness, etc). You can do this in your clinic, yo.

Typical ECG findings of SSS include inappropriate sinus arrhythmia (e.g., during exertion, stress, etc., when tachycardia would be expected), sinus bradycardia, sinus arrest, and in some cases supraventricular tachycardia (SVT) as part of a bradycardia-tachycardia syndrome. Some feel that ECG findings described above in patients with no clinical signs should be termed sinus node dysfunction (SND) as opposed to SSS, while others contend that SSS classification should also not include patients who show a response to anticholinergic administration – drugs like atropine (indicating autonomic (vagal) dysfunction). In reality, SND and SSS likely represent a continuum of conduction systemic disease.

So Ward et al out of the North Carolina State University Veterinary Hospital wanted to evaluate SSS in dogs in a paper called Outcome and survival in canine sick sinus syndrome and sinus node dysfunction: 93 cases (2002-2014), which was published in the Journal of Veterinary Cardiology in 2016. The goal of this study was to evaluate SSS patients from the time of presentation or diagnosis through treatment (including modality), effectiveness of treatment, and overall outcome. Surprisingly, no large-scale studies with this specific aim had been previously reported in the veterinary literature.

So, what’d they find? In this retrospective study, they included a total of 93 dogs defined with SSS or SND. The average age at the time of diagnosis was 11 years, with no difference between the SSS and SND group. Females were overrepresented, with 63% (n=59) being female and 37% (n=34) being male. Certain breeds were overrepresented, including Miniature Schnauzers, West Highland White Terriers, and Cocker Spaniels. 66% of the dogs had clinical signs of their bradyarrythmia and were therefore considered to have SSS; the most common clinical signs was syncope in 95% of these dogs (58/61). 34% (n=32) of the dogs were asymptomatic, and therefore classified as SND. In this study, there was no statistically significant differences in ECG or Holter monitor findings between these two groups of dogs. 74/93 of these dogs had sinus arrest (which is defined as a pause longer than 2 x R-R interval) on ECG. 21% (n=20) of the dogs had bradycardia-tachycardia syndrome. Atropine response testing (when you administer 0.04 mg/kg atropine SQ) was performed in 63% (n=59) of the total dogs (of which 40 had SSS and 19 dogs had SND). 26/59 (44%) of the dogs had an adequate response (defined as a heart rate > 150 bpm), while 20/59 (34%) of dogs had a partial response (defined as a HR < 150 bpm but > 25% improvement from baseline). 22% (13/59) of dogs had no response to atropine. The dogs with SSS were more likely to have no response to atropine as compared to SND dogs.

In this study, the majority of dogs (85%) had some degree of structural heart disease on echocardiography, but virtually all were mitral valve endocardiosis with simply varying degree of left atrial enlargement (defined as mild, moderate, or severe). Also, 43% of dogs (40/93) had concurrent disease of a type that could contribute to elevated autonomic (vagal) tone, including general categories of respiratory, ocular, endocrine (e.g., hypothyroidism), neurologic, gastrointestinal, or neoplastic disease (e.g., adrenal mass).

Medical treatment in these dogs included the use of oral positive chronotropes such as theophylline (which was used most commonly in 41/49 dogs), propantheline, hyoscyamine, and/or terbutaline. These medications were prescribed in 49 dogs (the majority having SSS, n=46) with the goal of decreasing the frequency of syncope (SSS) or increasing the baseline heart rate (SND). Approximately half of these SSS dogs (n=21/46) were treated lifelong with medical therapy only; of these, 10 had adequate control of their syncope until the time of their death. Two dogs were euthanized due to progressive syncope. Nine of the 9 SSS dogs were treated with medical therapy as a bridge until PAP was necessary (which averaged 11 months until PAP was performed). In 6 of the 46 dogs, medical therapy was not effective at reducing syncope. However, medical management often seemed to be successful, with 54% (25/46) of SSS dogs treated achieved > 1 year of successful control of their signs until PAP or death/euthanasia. Statistically significant factors associated with a greater likelihood of success of medical therapy included those dogs that had a higher resting heart rate at baseline, and had a partial or complete response to atropine response testing.

Ultimately, 30% (28/93) of dogs required pacemaker placement; 10 of these were performed on the initial visit to the cardiologist. 20 dogs received a transvenous/endocardial pacemaker, while 8 dogs had an epicardial pacemaker placed. 4/28 of the PAP dogs experienced major complications, of which 3 were fatal. Statistically significant factors associated with an increase in likelihood of receiving a pacemaker included those patients that had no response to atropine, that had clinical signs on presentation (SSS), that had longer periods of sinus arrest, had more frequent clinical signs, had bradycardia-tachycardia syndrome, or that lacked disease believed to increase vagal tone. In this study, a total of 17 dogs developed CHF, but all of these dogs were documented to have mitral valve endocardiosis with severe left atrial enlargement. Patients with bradycardia-tachycardia syndrome were statistically more likely to develop CHF. There was no difference in survival for CHF dogs with or without PAP placement.

Overall, 63/93 of the dogs died at time of publication of this study, with 43/63 being euthanized for non-cardiac reasons, and approximately 1/3 (20/63) being euthanized for cardiac reasons. 13 of the 20 euthanized for cardiac reasons did so because of CHF; 3/20 euthanized due to PAP complications, 2/20 euthanized due to progressive clinical signs of SSS (where the owner declined PAP intervention), 1/20 due to arterial embolism, and one due to unexpected death (at a boarding facility). There was no statistically significant difference in survival between dogs with SSS and SND in study. The median survival in dogs with SSS was 480 days, and for dogs with SND was 754 days. Survival did not differ based on success of medical therapy, atropine response, or PAP placement. Keep in mind that the majority of dogs with SSS received medical management with or without PAP therapy, as compared to only 3 dogs with SND. Dogs with SSS were statistically at a higher risk of death associated with heart disease vs SND dogs.

Syncope was clearly the most common clinical sign in dogs with SSS. However, a significant number of dogs presented with SND, defined as asymptomatic sinoatrial node dysfunction. Medical therapy improved patient clinical signs of SSS in a significant number of dogs, often for a significant period of time (> 1 year) such that PAP placement was delayed or not necessary altogether. That said, improvement or ‘success’ was defined purely by owner/client subjective interpretation rather than objective measurement of frequency of syncope. However, a significant number of dogs with SSS did ultimately require PAP placement eventually. Dogs with SSS were less likely to respond to atropine as compared to dogs with SND, and dogs with no response to atropine were less likely to have success with medical therapy. This supports that a significant number of dogs with sinoatrial dysfunction have a vagally-mediated contribution. Atropine response testing is advised in all dogs with SSS and SND. PAP implantation was highly effective at controlling clinical signs and relatively safe, although 3 fatal complications were encountered. PAP implantation had no effect on survival, possibly because of the impact of these 3 fatal complications. Sudden cardiac death (SCD) was also rare (1 case) in this study population, which was hypothesized by the owners to be the cause. The lack of statistically significant difference in survival between SSS and SND dogs should not be interpreted to suggest that prognosis is good regardless of treatment or lack thereof, but rather that prognosis can be good provided the appropriate therapy (none, medical, or PAP implantation) is chosen to resolve/improve clinical signs and prevent euthanasia. Congestive heart failure was relatively common in the population, with the predominant factor likely unrelated to the conduction disease, namely primary mitral valve endocardiosis. However, it is well documented that PAP implantation can affect ventricular function and what effect this might have on advancing primary valvular heart disease is unclear, but a topic for further study in the future.

So, what did we learn from this VETgirl podcast?
Syncope is the most common presenting clinical sign with SSS. SND terminology can be used to describe dogs with sinoatrial dysfunction but lack of clinical signs, if you want to get into the semantics of it all. A significant number of dogs with SSS or SND will display partial or complete response to atropine, suggesting a significant vagal contribution to their conduction disorder. This is good. The response to atropine was associated with likelihood of successful (transient or long-term) treatment with medical therapy in dogs with SSS, allowing delay or lack of need for pacemaker therapy (which is less expensive and invasive for the pet owner). Those SSS patients with successful treatment with medical therapy often displayed success for > 1 year. However, many patients with SSS will still ultimately require PAP implantation, which is highly successful at resolving clinical syncope. Pet owners should be warned of this impending possibility. Most importantly, know that congestive heart failure occurs relatively frequently (18%) on dogs with SSS/SND, but in most of these dogs concurrent mitral valve endocardiosis is believed to be the primary contributor.

References:
Ward JL, DeFrancesco TC, Tou SP, et al. Outcome and survival in canine sick sinus syndrome and sinus node dysfunction: 93 cases (2002-2014). J Vet Cardiol (2016)18:199-212.

Abbreviations:
SSS: Sick sinus syndrome
PAP: Permanent artificial pacemaker implantation
ECG: Electrocardiography
SND: Sinus node dysfunction
CHF: Congestive heart failure
SCD: Sudden cardiac death

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