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Atrial Fibrillation and Sudden Cardiac Death in Dogs | VETgirl Veterinary Continuing Education Podcasts

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In this VETgirl online veterinary continuing education podcast, we discuss atrial fibrillation (AF) in dogs. AF is a common arrhythmia in our canine patients, characterized by that classic “tennis shoes in a dryer” sound on auscultation. This arrhythmia is caused by disorganized, rapid electrical activity of the atria (Pedro). After identification of this abnormality on physical examination, it is important to confirm the arrhythmia with an electrocardiogram (ECG) and consider a full cardiac assessment to better understand the cause and severity of this finding. AF can itself lead to structural remodeling, and in humans with AF, an increased risk of sudden cardiac death (SCD) is observed (Waldmann). However, it is unclear whether this risk for SCD is also true in dogs.

So, Borgeat et al wanted to evaluate this in a study entitled Prevalence of sudden cardiac death in dogs with atrial fibrillation. In this study, the authors hypothesize that a higher proportion of dogs with AF suffer from SCD than those with sinus rhythm and that SCD would be associated with complex ventricular arrhythmias as identified on a Holter monitor. This was a retrospective, case-control, multicenter study that reviewed medical records of dogs with AF from 7 veterinary clinics. To be included, all dogs must have had an ECG and 24-hour Holter ECG performed within 2 weeks of diagnosis and with AF occurring at the time of the Holter study. Information about the patient’s outcome was also required to be included, and the investigators extracted thorough details about the patient’s clinical status, diagnostic results, therapeutic regimen, and outcome. A second control group of dogs in sinus rhythm was also included, but this group was recruited from a single clinic rather than from multiple. It was required that these dogs had both an echocardiogram performed and outcome data available. The authors selected dogs for the control group that had a similar cardiac diagnoses to the AF group, but the control group was not matched to the AF group by sex, age, or body weight. In other words, if a dog with AF was diagnosed with dilated cardiomyopathy (DCM), the authors recruited a control dog that was diagnosed with DCM but that demonstrated sinus rhythm. The investigators classified the diagnoses as DCM, myxomatous mitral valve degeneration (MMVD), arrhythmogenic cardiomyopathy, congenital heart disease, and normal, meaning no pathology detected via echocardiography. Finally, the outcome of interest in this study was SCD, so it is important to understand how the authors defined this. Here, SCD indicated that a patient was found deceased without an apparent cause. The patient must have been seemingly well in the preceding 24 hours, and without obvious or apparent clinical symptoms in the hour prior to death.

Let’s hear about some results! First, 142 dogs with AF fit the study’s enrollment criteria and were included in the study, and an additional 127 dogs were enrolled as controls. Dogs were recruited from a time period ranging from January 2013 to March 2020. There were no statistically significant differences between the AF and control group dogs in terms of age, sex, syncopal history, ventricular arrhythmias while in the clinic, disease diagnosis, normal versus abnormal echocardiogram, cardiac euthanasia, surgery or procedure-related death, noncardiac deaths, or ejection fraction. However, dogs in the AF group had a significantly higher body weight, median left atrial size, and current or previous diagnosis of heart failure. The left ventricular internal diameter was also reported to be higher in the control group when in diastole, but there was no significant difference while in systole.

In the AF group, more than 20% of dogs (30/142) had comorbidities. Neoplasia (n = 8), endocrinopathies (n = 6), and renal disease (n = 6) were the most commonly reported. German shepherd and Labrador retriever dogs were the most common breeds in the AF group (n = 13 each), followed by Newfoundlands (n = 11), Dogue de Bordeaux (n = 10), Boxers (n = 9), Irish wolfhounds (n = 7), Rottweilers (n = 7), and Golden retrievers (n = 6). The average 24-hour heart rate for dogs with AF on Holter monitoring was 132 bpm, with highly variable ventricular premature complexes (VPC) reported, ranging from no VPCs to over 42,000 in a 24-hour period! More than half of the AF dogs had ventricular couplets or triplets recorded, and 39% had ventricular bigeminy or trigeminy observed. Finally, 13% of dogs had ventricular tachycardia documented, and 2 dogs had paroxysmal third-degree atrioventricular block. Importantly, no dogs in the control group were receiving any form of antiarrhythmic therapy in this study, whereas more than 80% of dogs in the AF group were receiving some type of antiarrhythmic drug. The most common therapy was a combination of digoxin and diltiazem (n = 53), followed by diltiazem alone (n = 24) and digoxin alone (n = 11). However, several other medications, such as amiodarone, atenolol, mexiletine, and sotalol were also represented.

Now let’s focus on the survival analysis of these dogs. First of all, dogs that survived less than 7 days were not included in this study. The authors note that at the time of analysis, 54% of dogs in the AF group were deceased, compared to 32% of control dogs. These numbers represent dogs that died from any cause, and recall that since this was retrospective, the age of the dog and interval between this analysis and diagnosis varied. Therefore, looking at median survival time (MST) will be more informative. In the AF group, MST was 492 days (16.4 months), as compared to a median of 593 days (19.8 months) in the dogs without AF and with sinus rhythm (P = .02). These MSTs represent death from any cause, so now let’s look more closely at cardiac deaths. In the AF group, 59 dogs (77%) died from cardiac-related causes, leaving 23% of deaths from noncardiac causes. Peri-anesthetic mortality was classified as a cardiac-related cause. In the control group, only 17% of deaths were classified as having a noncardiac cause. Importantly, SCD was significantly higher in the AF group compared to the control group. The prevalence of SCD was nearly 15% in AF dogs, but only 5.5% in controls (P = .01). In other words, AF was associated with a higher prevalence of SCD in dogs, which is the primary question this study was attempting to answer.

Next, the authors wanted to determine if patient factors were associated with SCD in the dogs with AF. The investigators first looked at several factors with a univariate analysis. These included age, heart failure, previous therapies, syncope, and echocardiographic or Holter findings. In this initial analysis, they found that dogs that experienced SCD were significantly younger at the time of diagnosis (6.6 years versus 8.1 years). Interestingly, there was no significant difference between the time between diagnosis of AF and sudden versus non-sudden cardiac deaths. The authors then included variables with a P value < .1 on univariate analysis to be included in a multivariable analysis. Variables that met this criteria were age, history of syncope, and left atrium diameter to aortic root diameter ratio (LA:Ao) on echocardiogram. They found that this logistic regression model correctly classified 82% of cases. Younger age, syncopal history, and increased LA:Ao ratio were all associated with an increased risk of SCD. In fact, they discovered that dogs with a history of syncope were 4.3 times more likely to experience SCD than dogs without syncopal episodes.

The authors discuss several limitations to this study, which were largely related to its retrospective nature. For instance, dogs were managed by different clinicians, at different clinics, and without standardization in management protocols. Necropsies were not routinely performed in patients that passed away. The authors also note differences between case and control groups. For example, both weight and left atrial size were higher in dogs with AF. Plus Holter monitoring was not required in control dogs, and dogs with AF were recruited from a larger number of clinics. So with those limitations in mind, what can we take away from this VETgirl podcast? Well first of all, SCD was significantly more prevalent in dogs with AF when compared to dogs with matched echocardiographic diagnoses but with sinus rhythm. Recall that SCD occurred in nearly 15% of dogs with AF, compared to only 5.5% of dogs without AF. In the dogs with AF, a few variables were associated with a higher risk of SCD, which included younger age at diagnosis, larger left atrial size, and syncopal history. The authors originally hypothesized that ventricular arrhythmias, as detected on Holter monitoring, would be associated with a higher risk of SCD, but this study did not support that hypothesis. This VETgirl will be aware of this higher risk of SCD in dogs with AF moving forward, and remember that quick intervention and referral to a cardiologist are imperative. While this study was not designed to investigate therapeutic interventions and the effect of different management strategies on outcome, hopefully prospective studies to follow up on this study’s excellent foundational information will be available in the future.

References:
Borgeat K, Pack M, Harris J, et al. Prevalence of sudden cardiac death in dogs with atrial fibrillation. J Vet Intern Med. 2021; 35(6): 2588-2595.

Pedro B, Fontes-Sousa AP, Gelzer AR. Canine atrial fibrillation: Pathophysiology, epidemiology, and classification. Vet J. 2020; 265:105548.

Waldmann V, Jouven X, Narayanan K, et al. Association between atrial fibrillation and sudden cardiac death: pathophysiological and epidemiological insights. Circ Res. 2020;127:301-309. 18.

Abbreviations:
AF – Atrial fibrillation
DCM – Dilated cardiomyopathy
ECG – Electrocardiogram
LA:Ao – Left atrium diameter to aortic root diameter ratio
MMVD – Myxomatous mitral valve degeneration
MST – Median survival time
SCD – Sudden cardiac death
VPC – Ventricular premature complex

Today’s VETgirl podcast is sponsored by Royal Canin. Recommend multifunction Royal Canin Urinary SO® + Calm complete and balanced adult cat food to help support a healthy urinary tract while providing calming nutrients for cats facing stress. Learn more at my.royalcanin.com.

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