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Echocardiographic predictors of first onset of atrial fibrillation in dogs with CVHD using Speckle Tracking | VETgirl Veterinary Continuing Education Podcasts

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In this VETgirl online veterinary continuing education podcast, we review echocardiographic predictors of first onset of atrial fibrillation in dogs with chronic valvular heart disease (CVHD) AKA myxomatous mitral valve disease (MMVD). The onset of atrial fibrillation (AF) is a negative prognostic indicator in dogs with underlying structural heart disease, and the likelihood of onset of atrial fibrillation is higher with increases in left atrial size and body weight. Furthermore, the greater the patient’s ventricular response rate, or ultimately heart rate, in atrial fibrillation, the worse the prognosis is known to be. As a result, pharmacologic heart rate control is a major goal of treatment in dogs with atrial fibrillation secondary to structural heart disease. Early detection of onset of atrial fibrillation, if possible, would reduce the amount of time during which a dog may be experiencing as-yet undetected rapid atrial fibrillation. Data in humans have determined that certain markers on surface electrocardiogram (ECG) and echocardiogram (ECHO) can be useful in predicting the onset of atrial fibrillation. Specifically, tissue Doppler and speckle tracking (STE) of the left atrium via echocardiogram have proven to be most useful in humans, and these methods have been studied in dogs as well, both healthy and those with myxomatous mitral valve disease (MMVD).

A recent study in dogs determined that tissue Doppler assessment of electromechanical atrial delay, or the time from onset of electrical activity in the left atrium to onset of mechanical movement of the left atrium, to have utility in predicting onset of AF in dogs with cardiac disease. Speckle-tracking is a complex parameter obtained by advanced image analysis software, but is actually quite simple to conceptualize. This technology uses the natural speckled pattern of the myocardium on two-dimensional echocardiogram to track the simultaneous motion of multiple regions of myocardium, both indepedently, and in their relation to nearby “speckle” regions. This can provide insight into the distribution and severity of myocardial dysfunction in cardiac disease. Left atrial strain, a speckle-tracking derived marker, has been shown to decrease as MMVD worsens.

So, Toaldo et al wanted to evaluate this in a study entitled Echocardiographic predictors of first onset of atrial fibrillation in dogs with myxomatous mitral valve disease. The goal of this study was to evaluate the utility of several echocardiographically-derived indicators of left atrial and ventricular size and function in predicting onset of atrial fibrillation in dogs with MMVD. The study was retrospective in nature. Dogs with MMVD that developed AF within 12 months of diagnosis and had at least one echocardiogram with confirmation of normal sinus rhythm prior to onset of AF were included. Onset of atrial fibrillation was confirmed via 6 or 12 lead echocardiography. Baseline clinical variables were collated including congestive heart failure status, drugs received, and time elapsed between echocardiogram and onset of AF. Digital echocardiographic images were retrospectively retrieved and analyzed with software specifically designed for advanced image analysis, including speckle-tracking. A control group was created from dogs with MMVD with matching baseline characteristics (including echocardiographic data) that did not develop atrial fibrillation. Two-dimensional echocardiographic data was obtained, including a variety of left atrial volume parameters that assessed not only minimal and maximal left atrial volumes, but the relative distributions of early and late filling phases to changes in volume of the left atrium during diastole. Speckle-tracking was then performed to assess left atrial strain, including the peak strain on the left atrial myocardium during left ventricular contraction, known as peak atrial longitudinal strain (or PALS), and the strain value immediately prior to left atrial contraction, known as the peak atrial contraction strain (or PACS). A strain index was then calculated as [PACS/PALS] x 100.

Phew… all that echo-techno is a mouthful. Ready for some real-world results? Me too! And let’s translate them into layman’s terms while we are at it. Forty-four dogs were ultimately included in the study, with 22 in the AF group and 22 controls. No baseline differences in signalment, body weight, or heart failure class were identified. The majority of dogs (77%) were considered to be in American College of Veterinary Internal Medicine (ACVIM) class C, or congestive heart failure. No differences were detected in drugs administered between groups. Some combination of variety of classes of diuretics in addition to angiotensin converting enzyme inhibitors (ACEI), pimobendan, amlodipine, sotalol and mexiletine were in use as treatments for many dogs at the time of inclusion in the study.

The time elapsed between the most recent echocardiogram and the onset of AF (in the AF group) was significantly shorter than the time to final follow-up in the non-AF control group. Median time to onset of atrial fibrillation from the most recent echocardiographic exam was 142 days in the AF group.

Let’s look at the echocardiographic variables that differed between groups in a way that doesn’t require echocardiographic training to understand. Dogs in the AF group had larger two dimensional left atrial and end-diastolic left ventricular size than control dogs. Interestingly, none of the multitude of Doppler (aka blood flow) or left atrial volume parameters differed between groups. Remember those complicated speckle-tracking variables? Well only the PALS, or peak atrial longitudinal strain, which assesses the peak strain on the left atrium during contraction of the neighboring left ventricle, exhibited a difference. PALS was lower in the dogs with AF. Remember, strain decreases as the myocardium becomes more diseased. This may seem counterintuitive – strain sounds like a bad thing to have, so one might initially be inclined to think that decreased strain is a good thing. But the opposite is true. A baseline level of strain is normal and appropriate for healthy muscle, cardiac or otherwise, to experience. It indicates that the muscle fibers are contracting. When strain decreases, either globally or within certain regions of the myocardium relative to others, it indicates that those regions of myocardium are becoming LESS functional. So, a reduced level of strain is a bad thing. That muscle is scarring, dying, or both.

As it turned out in the statistical analysis, PALS was the only variable shown to have a significant, independent predictive impact on the future development of AF in this population, with a cutoff of PALS < 28% displaying the optimal predictive value. The authors did not include a frame of reference for what a “normal” PALS value is, either in dogs or humans. After consulting with Dr. Google myself, it appears that a normal PALS value in humans is about 42%, with a range of 36-48%.

Larger left atrial size, in general (e.g. with larger body size), and in relative terms associated with progressive disease-induced dilation, is associated with increased risk of development of atrial fibrillation. As the left atrium in any individual increases in size secondary to disease, significant remodeling at the cellular level occurs, most notably via not only hypertrophy of the myofibrils, but remodeling fibrosis. Fibrosis, or scarring, disrupts both the electrical and the contractile apparatus and creates what is commonly referred to as “substrate for arrhythmia.” Assessment of left atrial function on echocardiogram in humans has proven to be valuable in assessing the degree of remodeling that has occurred, and thus predicting the relative risk for onset of atrial fibrillation. This study has taken a first step in investigating if this will prove to have a similar utility in dogs with cardiac disease, and appears to show promise in doing so.

Perhaps the greatest challenge to utilizing this technology on a more widespread basis in veterinary medicine at this time is the need for specialized software, which requires specialized training to use. There is, therefore, an additional cost and time investment required to evaluate speckle tracking parameters using such software. In the clinical setting, particularly in private specialty practice, this is not necessarily a very practical option, or one where the benefit-to-cost ratio can be justified – at least not yet. But we do need to continue to explore these technologies – because as we need look no further than our pockets and purses to know, as technology advances, it can become less expensive and easier to use.

References:
1. Toaldo MB, Mazzoldi C, Romito G. Echocardiographic predictors of first onset of atrial fibrillation in dogs with myxomatous mitral valve disease. J Vet Intern Med 2020;34:1787–1793.

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