Page 12 - VetCPD Jnl Volume 7, Issue 4

VETcpd - Cardiology
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
"The gold standard for the diagnosis of AF remains an ECG. The hallmark of AF is based on the absence of P waves, irregular R-R intervals and generally fast heart rates"
SubOscnrilbye
Box 1:
ECG Refresher
Single lead of a normal dog ECG with labelling of the P-QRS-T components
The normal heart rhythms in dogs are sinus rhythm (a regular rhythm with the rate appropriate for the level of activity; see above) and also sinus arrhythmia where the heart rate is generally <140bpm and there are regular rhythmic fluctuations in the R-R interval associated with physiological fluctuation in vagal tone, e.g. associated with breathing.
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).
iagnosis. Even in cases
e atr Diagnosis initiate atrial contraction. Normal atrial
surface ECG), irregular R-to-R intervals, 5b): most dogs are affected by permanent QRS complexes with normal morphol- AF therefore a simple ECG is sufficient ogy (except if a conduction disturbance is for diagnosis, however there are reports of present (Figure 1) and a fast HR (Figure paroxysmal AF in dogs (PorteiroVázquez
2) (although this may be within normal et al. 2016) where, because of its intermit- limits in cases of lone AF) (Figure 3) .The tent nature, a 24-hour Holter is more
rate of the atrial impulses during AF is
likely to pro
much higher than the reported ventricular of permanent AF, a 24-hour Holter may
rate (around 600 bpm), however the AV be beneficial, not only for heart rate node will block the conduction of all assessment but also to detect other
those impulses to the ventricles (which concurrent arrhythmias (such as ventricu-
avoids an even faster HR). lar arrhythmias, a common finding in
The lack of typical P waves is a result of a patients with myocardial failure) which lack of a uniform, synchronized electrical may affect treatment decision making.
impulse travellin
The presence of AF should be easily contraction contributes to approximately Clinical signs
suspected based on physical examination: cardiac auscultation usually reveals a
fast heart rate (HR) with an irregularly irregular (chaotic) rhythm, associated with variable pulse quality and pulse deficits. In many cases, a heart murmur may also be heard, which would raise further suspicion of an underlying cardiac condition. In some cases, however, whilst the rhythm may be irregular, the HR may be within normal limits (rather than fast) – a common finding in dogs with lone AF. In these cases, AF can be an incidental finding during an otherwise normal physical examination.
The gold standard for the diagnosis of AF remains an ECG.The hallmark of AF is based on the absence of visible P waves (fibrillatory f waves may be identified
in some cases as an undulation of the baseline, but are not always evident on a
Page 12 - VETcpd - Vol 7 - Issue 4
25% of the ventricular filling. Loss of atrial contraction in combination with a fast HR (which will reduce further the time for ventricular filling) will have a signifi- cant impact on cardiac output (Figure 4) and trigger the maladaptive mechanisms that will ultimately lead to CHF (Meisner et al. 1991). If clinical signs are suggestive of CHF, thoracic radiographs or at least
a T-FAST scan should be performed for confirmation.
Echocardiography should also be part of the work-up of an AF patient, in order to investigate a possible underlying cardiac disease (Video 1 and 2).AF associated with extra-cardiac conditions is rare, but if suspected, further investigations should be performed (complete blood work, abdominal imaging, etc.).
A 24-hour Holter ECG monitor is not required to diagnose AF (Figure 5a and
Dogs with “lone” or primary AF tend to have a normal HR with a structurally normal heart and therefore may be completely asymptomatic. In those cases, the arrhythmia is an incidental finding (Menaut et al. 2005; Gelzer et al. 2015; Pedro et al. 2018). Occasionally, dogs with lone AF may develop poor exercise tolerance due to reduced cardiac output. When an underlying cardiac disease is present, clinical signs may differ depending on the severity of the primary disease and potential presence of concurrent CHF: these could vary from lethargy, weakness, exercise intolerance and inappetence, to dyspnoea, coughing and occasionally even syncope (Menaut et al. 2005). However, right sided CHF is also commonly associated with AF, therefore ascites, pleural and pericardial effusions may be detected (Ward et al. 2019).
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