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Atrial Fibrillation as a Prognostic Indicator in Dogs | VETgirl Veterinary Continuing Education Podcasts

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In this VETgirl online veterinary continuing education podcast, we review atrial fibrillation in medium- to large-sized dogs. Are there any other prognostic factors can we look at when it comes to heart disease? Is the presence of atrial fibrillation a prognostic indicator in certain dogs with Myxomatous Mitral Valvular Degeneration (MMVD) and congestive heart failure?

Now, don’t worry – we haven’t forgotten about our smaller dogs with MMVD. If you treat a lot of smaller dogs with heart murmurs, you should read up on the EPIC study, by the way. Your pet owners want their dogs to live longer, and knowing what medications can help with that are important.

Atrial fibrillation is more common in medium- and large-sized dogs with congestive heart failure secondary to MMVD than in small dogs with congestive heart failure from MMVD. Atrial fibrillation occurs in CHF/MMVD due to electrical remodeling associated with structural changes within the myocardium. Negative effects of atrial fibrillation include irregular tachycardia, loss of atrial contribution to diastolic cardiac filling, decreased cardiac output, and increased left atrial pressure, all of which can result in progressive cardiomegaly and decompensation to CHF. The prognosis for combined CHF and atrial fibrillation in humans is worse than for either condition occurring in isolation. Up to now, there was no reported data on the effect of atrial fibrillation on survival in medium- and large-sized dogs with CHF.

So, Jung et al out of Auburn and UC-Davis wanted to investigate the presence of atrial fibrillation as a prognostic indicator in larger dogs (e.g., > 15 kgs) with MMVD in their publication called Atrial fibrillation as a prognostic indicator in medium to large-sized dogs with myxomatous mitral valvular degeneration and congestive heart failure. This study aimed to investigate the effect of atrial fibrillation on survival in medium- and large-sized dogs with CHF, as well as to investigate the prevalence of AF in this population, and whether pharmacologic heart rate control of AF affects survival.

In a retrospective study from 2005-2010, they evaluated 64 dogs > 15 kg that met the inclusion criteria that had MMVD and had been treated for congestive failure failure. Of the 64 dogs, 33 (52%) had atrial fibrillation. In order to eliminate dogs that had dilated cardiomyopathy, dogs with a shortening fraction (FS%, echocardiographic parameter) < 22% were excluded. In this study, exclusion criteria included all other forms of heart disease not classified as MMVD, dogs with lone atrial fibrillation (e.g., where there was absence of structural heart disease), concurrent ventricular arrhythmias, and concurrent systemic disease. Medications that were used to control the heart rate in these dogs included: diltiazem (regular or extended release), digoxin, and/or atenolol (alone or in combination). “Adequate heart rate control” was defined as a heart rate < 160 bpm on average on examination. The primary endpoint was defined as cardiac-related death or euthanasia.

What’d they find in this study? Overall, the median age at the time of the diagnosis of congestive heart failure was 11 years (range: 6-15), with a median body weight of 24 kg. 50 dogs had left-sided congestive heart failure only, while 14 dogs had bilateral congestive heart failure. The most common medications used for treatment of CHF included furosemide (n = 64), enalapril (n =59), and pimobendan (n = 32). Other medications also included hydrochlorothiazide (3), and spironolactone (3). 21 dogs had atrial fibrillation at the time of diagnosis of CHF, while 12 dogs developed atrial fibrillation sometime after diagnosis of CHF.

Two statistically significant outcome variables between the atrial fibrillation group versus the non-atrial fibrillation group included differences in the median survival times and body weight. In the atrial fibrillation group, the median survival was 142 days as compared to 234 days in the non-atrial fibrillation group. As for body weight, the atrial fibrillation group had a median body weight of 28 kg as compared to the non-atrial fibrillation group of 22 kgs.

What was also notable was the effect of heart rate control treatment on survival time. The median survival time for dogs with poorly controlled HR (> 160 bpm) was significantly shorter as compared to dogs that had control of tachycardiac; in the 13 dogs that had their heart rate controlled (< 160 bpm), survival was 171 days as compared to only 61 days (n=20 dogs) in uncontrolled dogs (> 160 bpm). There was no difference in systolic function at baseline between these two groups. The median heart rate prior to anti-arrhythmic treatment was 220 bpm (range: 160-270). This study also found that heart rate control was better with diltiazem and digoxin combination therapy (median HR: 144 bpm) versus with diltiazem alone (median HR: 180 bpm). The median survival time for dogs treated with a combination of both diltiazem and digoxin therapy was significantly longer (130 days) versus diltiazem alone (35 days). Only 3 dogs in this study received atenolol for heart rate control, which was inadequate in all 3 dogs.

So, what do we take away from this VETgirl podcast?

The authors found that the prognosis for medium- and large-sized dogs with concurrent atrial fibrillation and congestive heart failure from MMVD appears to be worse than for dogs with congestive heart failure from MMVD alone (without the presence of atrial fibrillation). The median survival time of dogs with congestive heart failure from MMVD (without atrial fibrillation) in this study was 234 days, comparable to prior studies. The findings of a significantly shorter survival time (142 days) for dogs with atrial fibrillation plus congestive heart failure from MMVD had not previously been reported in this study population. This data supports the physiologic basis for worsened cardiac function with atrial fibrillation based on the effects of chronic tachycardia. However, the lack of a detectable difference in systolic function between the atrial fibrillation and non-atrial fibrillation groups in this study suggest that the worsened prognosis with atrial fibrillation may specifically be secondary to other factors (e.g., lack of atrial contribution to filling, etc.) associated with the arrhythmia. The determination that dogs with adequate pharmacologic heart rate control have improved survival (versus those that do not) is an important one, as is the finding that combination diltiazem/digoxin therapy in atrial fibrillation is superior in both heart rate control and survival to diltiazem therapy alone (the effect on rate control confirms a previous report). The question remains as to whether prognosis could be further improved with even stricter heart rate control (e.g. < 140 bpm), as well as to whether similar improvement in survival could be achieved with diltiazem monotherapy at higher doses (median dose in this study was 1 mg/kg q 8h) provided a similar degree of heart rate control could be demonstrated. In other words, control that heart rate, folks!

References
Jung SW, Sun W, Griffiths LG, et al. Atrial fibrillation as a prognostic indicator in medium to large-sized dogs with myxomatous mitral valvular degeneration and congestive heart failure. J Vet Intern Med 2016;30:51-57.

Abbreviations:
AF: Atrial fibrillation
CHF: Congestive Heart Failure
MMVD: Myxomatous valvular degeneration
Kg: Kilograms
HR: Heart rate
BW: Body weight
Mg: Milligram

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