Left ventricular function quantified by myocardial strain imaging in small-breed dogs with chronic mitral regurgitation

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Abstract

Background

The presence of left ventricular (LV) systolic dysfunction may influence prognosis or therapy in dogs with chronic mitral regurgitation (MR). Assessment of LV function in MR by conventional echocardiography is confounded by altered ventricular loading. Myocardial deformation (strain) imaging might offer more sensitive estimates of LV function in this disease.

Objective

Prospectively measure myocardial strain in dogs with asymptomatic MR compared to a control group.

Animals, materials and methods

Forty healthy dogs (3.5–11.5 kg): 20 Controls; 20 dogs with MR and LV remodeling (Stage B2), were evaluated in this study. LV size and function were assessed in a short-axis plane. Segmental radial strain and strain rate and global circumferential strain were measured using a 2D echocardiographic speckle-tracking algorithm (GE EchoPAC). Groups were compared using Bonferroni t-tests. Influences of heart rate and body weight were explored with linear regression.

Results

The MR group had significantly greater mean values for heart rate, LV size, and LV systolic function. Specifically, LV diastolic diameter, diastole area, shortening fraction, averaged peak systolic and early diastolic radial strain, global circumferential strain, and averaged radial strain rate were significantly greater in the MR group (p < 0.015 to p < 0.001). Strain was unrelated to weight, but weakly correlated with heart rate.

Conclusions

Similar to conventional indices, Stage B2 dogs with MR demonstrate hyperdynamic deformation in the short-axis plane. Short-axis strain variables measured by 2D speckle tracking are greater than for controls of similar age and weight. These results imply either preserved LV systolic function or that LV dysfunction is masked by altered ventricular loading.

Introduction

Mitral regurgitation (MR) caused by degenerative valvular disease (endocardiosis) is the most common cause of left ventricular (LV) remodeling and heart failure in dogs.1 Echocardiographic (Echo) studies of LV function in spontaneous mitral disease have demonstrated hyperdynamic global function when conventional indices such as LV shortening fraction are used.2, 3, 4 Excursion of the ventricular septum in particular is often exuberant with advanced MR. However, in canine models of MR and in spontaneous mitral disease affecting larger-sized dogs, impairment of myocardial contractility has been reported,5, 6, 7 a finding not conclusively identified in small-breed dogs with naturally-occurring endocardiosis. The presence of left ventricular dysfunction could affect prognosis or be important when timing medical, surgical, or interventional therapies. Accordingly, newer and potentially more sensitive methods for identifying LV dysfunction have been advanced, including assessment of myocardial deformation or strain.8, 9, 10

Assessment of global LV systolic and diastolic function in MR is confounded by altered ventricular loading conditions.11 Progressive increases in LV preload and reductions in LV afterload typically create a hyperdynamic LV with increases in ejection fraction and also in the rate of early diastolic ventricular filling.12 Consequently, conventional measures like fractional shortening and ejection fraction are insensitive for measuring early LV dysfunction,13 becoming abnormal only in advanced stages of disease, if at all. While the LV end-systolic volume index has been advanced as a possible indicator of LV dysfunction,2, 3 this measurement has not been validated against a gold standard, and may be limited by the methods used to estimate LV volumes as well as the chamber remodeling that attends chronic MR.4, 14, 15

It has been suggested that measurements of segmental myocardial deformation, namely myocardial strain and strain rate, represent more sensitive methods for identifying LV dysfunction.16 These variables are measured by Echo using either a tissue-Doppler-based method or with 2D Echo speckle tracking.17, 18, 19 Tissue-Doppler-based methods are angle-dependent; whereas, speckle tracking is purportedly less affected by orientation of the ultrasound beam. With either of these methods, percent strain or strain rate per second can be calculated in radial, circumferential, and longitudinal dimensions. Radial strain is positive during myocardial thickening and negative during ventricular relaxation. Circumferential and longitudinal strains are calculated as negative in systole and positive in diastole. Peak strain generally occurs near the end of ventricular ejection; whereas, strain rate, the temporal derivative of strain, typically peaks in early systole and is considered a good correlate of LV global systolic function.16, 17, 18 Importantly, these strain variables are derived from proprietary (“black-box”) algorithms that are not uniform across vendors.20 Additionally, Echo images must be of sufficient quality to permit tissue tracking and a trained observer must verify the cardiac cycle timing, real-time tracking, and resultant strain curves.

Strain measurements derived from speckle-tracking methods relate very closely to sonomicrometry and to magnetic resonance imaging data in anesthetized dogs.21 These variables also correlate significantly to global LV systolic function in healthy anesthetized dogs.22 The technique has been used in clinical veterinary settings including studies of dilated cardiomyopathy23 and muscular dystrophy.24 The method also can be used to evaluate ventricular dyssynchrony in dogs.19, 25, 26

There are limited studies of myocardial strain in dogs with spontaneous MR. Tidholm and colleagues27 reported on tissue-Doppler-derived strain values from dogs affected by various stages of mitral disease. They reported either no difference from controls or increased measures of myocardial deformation with MR. A recent proceedings report by the same laboratory has indicated that speckle-tracking-based strain and strain rate also increase in dogs with spontaneous MR.28 This study also included dogs with varying degrees of MR, including some with congestive heart failure.

The present study was prospectively designed to generate reference data for myocardial strain in healthy, older small-breed dogs and to compare these data to a group of dogs with stage B2 heart disease29 due to chronic MR. Our hypothesis was that myocardial strain indices could identify systolic dysfunction in dogs with MR and LV remodeling but without CHF.

Section snippets

Animals, materials and methods

This study was approved by the institutional Laboratory Animal Use Committee of The Ohio State University and by the Clinical Trials Office of The Ohio State University Veterinary Medical Center.

Results

The Control and MR groups were not significantly different in terms of mean age or body weight (Table 1). As shown in Table 1, dogs with MR demonstrated findings typical of LV volume overload with significantly greater mean values for average heart rate, LV diastolic diameter, LV short-axis area, and LV shortening fraction (all p < 0.005).

Radial myocardial strain and strain rate and circumferential global strain could be obtained from all 40 dogs of this study (Table 2, Table 3). Operator

Discussion

This study demonstrates the limitations of myocardial strain imaging for the identification of LV systolic dysfunction in asymptomatic dogs with Stage B2 mitral regurgitation. Seemingly, there is no obvious advantage in applying this technology to small-breed dogs during this stage of disease. These findings are congruous with those of Tidholm and associates who have used both the tissue-Doppler-based methodology27 and speckle-tracking technology28 for assessing myocardial strain in dogs with

Conclusions

While radial strain, strain rate, and circumferential global strain can be consistently measured in dogs with significant mitral regurgitation, these indices are actually increased with ventricular remodeling, presumably influenced by the same loading conditions that confound other Echo indices. A clear clinical application for this technology in Stage B2 dogs with MR is not evident from the results of this study.

Conflict of interest

There are no conflicts of interest for any of the authors listed.

Acknowledgments

This study was supported in part by the ACVIM Foundation (Cardiology Resident Grants) and by the State of Ohio Canine Research Funds.

The authors appreciate the assistance of Drs. Agnieszka Kent, Brian Scansen, and Richard Cober for acquiring some of the echocardiographic images in the MR group of dogs.

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