Atrial fibrillation (AF) is the most common cardiac arrhythmia seen in dogs. It can occur in any condition where there is significant atrial stretch, with mitral valve disease and dilated cardiomyopathy being the top two causes. Atrial fibrillation typically indicates that there is advanced and marked underlying cardiac chamber dilation but it is occasionally seen in the absence of cardiomegaly, the so called “lone” AF. Atrial fibrillation is a rapid and disorganised discharge of electrical activity from the atria with a variable ventricular response rate, which results in the erratic and irregular rhythm. Where the AF heart rate is fast and/or where there is a low cardiac output, dogs will be symptomatic and AF management is indicated. Rate control is the initial treatment of choice where there is underlying heart disease; based on recent retrospective studies, good control of AF with an improved survival and prognosis may require rate control to <125bpm. In addition to medical management, electrical DC cardioversion may be an option for some dogs which can revert them back to a sinus rhythm; this requires no or minimal structural heart disease for the greatest chance of success. AF can be managed readily in general practice and good quality of life can be expected.
Introduction
Atrial fibrillation (AF) is the most common non-physiological arrhythmia in dogs. It has a significant impact on morbidity and mortality, therefore it is important for the clinician to be able to identify and treat this arrhythmia appropriately.
Epidemiology
In dogs, AF is more common in males; adult to old dogs are most commonly affected, mainly because of the higher prevalence of underlying cardiac diseases in that population, which is frequently seen in association with this arrhythmia. Any breed can be affected by AF, however large and giant breed dogs are overrepresented, particularly Dogue de Bordeaux, Great Dane, Newfoundland, Mastiff, Irish Wolfhound, German Shepherd, Rottweiler, Labrador Retriever and Australian Shepherd. Considering these breeds, it is not surprising to see an association between AF and dilated cardiomyopathy (DCM). In fact, up to 50 % of dogs with DCM have been reported to also be affected by AF. However, dogs with myxomatous degenerative valvular disease (MDVD) can also develop this arrhythmia: a recent study showed that AF was detected in 52% of dogs with MDVD and congestive heart failure (CHF) weighing over 15 kg, while dogs weighing over 20 kg have a 5.8 times higher chance than smaller breed dogs of developing the arrhythmia.
Pathophysiology
Frequently AF presents in two different scenarios:
Lone or Primary AF
In some cases, AF is detected in animals without an obvious underlying structural heart disease. This is the so called “lone”or primary AF. In these cases, the atria appear within normal dimensions (as no obvious remodelling is noted on echocardiography), although histopathological analysis has revealed subclinical cardiomyopathies and atrial fibrosis in Irish Wolfhounds (Vollmar and Aupperle 2016), a breed where a genetic component also appears to play an importantrole (Fousse et al. 2019). Furthermore, it has been reported that Irish Wolfhounds diagnosed with primary AF had higher chances of developing DCM than those in sinus rhythm (Vollmar et al. 2019a). It is therefore important for the clinician to closely monitor these dogs as progressive remodelling may be expected.
AF Secondary to Structural Heart Disease
In the vast majority of cases where AF is diagnosed, there is an underlying structural heart disease (with marked atrial dilation) and frequently concurrent CHF (Li et al. 1999; Iwasaki et al. 2011; Kirchhof et al. 2016). In these cases, AF is considered secondary to the cardiac disease. In fact, atrial size was shown to be the main risk factor for the development of AF in dogs and humans: the larger the atrial surface area, the more re-entry wavelets can be accommodated on the atrial surface without colliding, allowing the arrhythmia to persist (Allessie et al. 1977). Large breed dogs will have larger atria, which increases the chances of developing AF (Guglielmini et al. 2000), whether or not the dog has underlying structural heart disease. Independently of the breed and underlying cardiac disease, virtually any dog whose atrial surface increases enough to accommodate multiple re-entry wavelets may develop AF. However, atrial mass is not the only determinant of AF in dogs. Changes in conduction velocity and refractory periods are also important, as recently confirmed by a study where dogs that spontaneously develop AF within 6 months had longer conduction times than controls (Neves et al. 2018).
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