Use of tissue Doppler imaging and two-dimensional speckle tracking for quantification of left ventricular function in horses

In horses with cardiac disease or poor performance, echocardiography is an essential tool for non-invasive evaluation of myocardial function. Left ventricular (LV) function is usually quantified by calculating fractional shortening (FS). Tissue Doppler imaging (TDI) and two-dimensional speckle tracking (2DST) are two echocardiographic techniques that might provide a more extensive quantification of left ventricular function. TDI measures myocardial velocities based on the Doppler principle. Myocardial deformation can be calculated from the velocity gradient between the endo- and epicardium. The total amount of wall deformation is described as strain (%), the rate of deformation as strain rate (s-1). Using 2DST, acoustic speckle patterns which are unique for each myocardial region are tracked in the two dimensions of the ultrasound image. As a result, longitudinal and circumferential shortening can be measured in addition to radial wall thickening. The aim of this study was to evaluate the use of TDI and 2DST for quantification of LV function in horses. First, the feasibility and reproducibility of TDI and 2DST measurements were described in ten healthy trotter horses. Using a modified four-chamber view, 2DST allowed quantification of global and segmental longitudinal LV velocity, displacement, strain rate and strain. Using short-axis images acquired at chordal and papillary muscle level, circumferential and radial LV function could be assessed by 2DST. Using TDI, radial velocity, strain rate and strain measurements were feasible from left and right parasternal short-axis views at chordal and papillary muscle level. The high frame rate allowed accurate measurements of systolic and diastolic time intervals. Next, the influence of the atrioventricular (AV) delay on mitral valve closure and LV peak velocity and acceleration during isovolumic contraction was investigated. This study demonstrated that AV interaction should be kept in mind when using peak pre-ejectional velocity and acceleration as parameters of LV contractility. The clinical use of TDI and 2DST was evaluated in horses with myocardial damage caused by an accidental ionophore intoxication on a farm in Belgium with eighty-one horses. Twenty horses were included in the echocardiographic study as they showed signs of myocardial damage. These horses showed abnormal TDI, 2DST and FS measurements, indicating impaired LV function. In a horse with myocardial fibrosis, TDI and 2DST could detect regional alterations of myocardial function caused by the predominant presence of fibrosis in the interventricular septum. Over all horses, TDI measurements, SL, SR at chordal level and FS correlated significantly with maximal cTnI. Over all examinations, TDI and 2DST measurements correlated well with FS. The ability of TDI and 2DST to detect myocardial dysfunction in horses with normal FS was investigated in twelve horses with atypical myopathy (AM). AM is an acute, highly fatal rhabdomyolysis in grazing horses. Eleven horses with AM had elevated cTnI concentrations and ten horses ventricular premature depolarizations (VPDs). All horses presented a prolonged corrected QT (QTcf) interval and an abnormal LV wall motion pattern. FS was normal in all horses. TDI measurements revealed abnormal LV relaxation with prolonged contraction duration, increased isovolumic relaxation time and decreased ratio of early to late diastolic (E/A) LV radial velocities. Decreased LV global longitudinal strain and increased mechanical dispersion between myocardial segments could be detected by 2DST. One of four surviving horses still showed VPDs and a mildly prolonged QTcf after ten weeks follow-up, although the wall motion abnormality had disappeared. In conclusion, TDI and 2DST can be considered complimentary for the comprehensive quantification of LV function. In the future, TDI and 2DST might be applied for detection of subclinical myocardial dysfunction in horses with valvular disease, for evaluating the effect of training or for quantification of right ventricular and atrial function.

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