Evaluation of Right Ventricular Myocardial Function in Healthy Horses With Recurrent Airway Obstruction Using Tissue Doppler Imaging

Journal of Equine Veterinary Science 34 (2014) 1096–1104

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Original Research

Evaluation of Right Ventricular Myocardial Function in Healthy Horses With Recurrent Airway Obstruction Using Tissue Doppler Imaging Heidrun Gehlen DVM a, *, Anna H. Stahl DVM b a b

Equine Clinic, Veterinary Faculty, Free University of Berlin, Germany Equine Clinic, Veterinary Faculty, Ludwig-Maximilians University of Munich, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 March 2014 Received in revised form 26 May 2014 Accepted 13 June 2014 Available online 21 June 2014

It has been described in humans that chronic obstructive bronchitis leads to pathologic changes, especially in the right ventricular myocardium due to hypoxia, which can be assessed by tissue Doppler imaging (TDI). In our study, different TDI techniques, that is, pulsed-wave TDI and color TDI, were evaluated for applicability in different scan planes (apical long-axis view and short-axis view) for the analysis of right ventricular myocardial function in six healthy horses (control) and six horses affected with recurrent airway obstruction (RAO) (RAO group). Tissue Doppler imaging was applicable to all scan planes described. Myocardial movement directions in general and the absolute values of TDI parameters were assessable. Significantly reduced early diastolic filling velocities (E), elevated late diastolic filling velocities (A), thereby decreased E/A quotient, prolonged electromechanical coupling periods between electrocardiograph Q-wave and maximal velocities, and compensatory elevated systolic strain as well as diminished displacement could be observed in horses with RAO compared with the control group. To conclude, equine right ventricular myocardial function is assessable by TDI. Significant changes of right ventricular myocardial function could be demonstrated by TDI in horses with RAO compared with matched healthy controls. The hypothesis that RAO potentially leads to right ventricular dysfunction detectable by TDI before conventional echocardiography changes are evident is supported by this explorative study. Ó 2014 Elsevier Inc. All rights reserved.

Keywords: Equine Echocardiography Right heart myocardial function Tissue Doppler imaging RAO

1. Introduction Myocardial sarcomeres contract in a three-dimensional way during systole: longitudinal, circumferential, and transverse. Doppler myocardial imaging (tissue Doppler imaging [TDI]) captures these sarcomere contractions by

This study was performed at the Equine Clinic, Ludwig-MaximillianUniversity Munich, Germany, and the Equine Clinic, Free University of Berlin, Germany. * Corresponding author at: Prof Dr Heidrun Gehlen, DVM, Dipl. ECEIM, Equine Clinic, Veterinary Faculty, Free University of Berlin, Oertzenweg 19b, 14163 Berlin, Germany. E-mail address: [email protected] (H. Gehlen). 0737-0806/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jevs.2014.06.012

special filters, highly reflecting, slowly moving myocardium separated from weakly reflecting, quickly moving blood flows [1]. Thereby, right ventricular myocardial contractility and relaxation is measurable by TDI objectively and noninvasively [2]. Different techniques of TDI are used to characterize myocardial function: pulsed-wave TDI (PW-TDI) and/or spectral Doppler myocardial imaging evaluates the peak velocities (m/s) online, whereas offline color TDI defines mean velocities (cm/s) within a defined region of interest (ROI). The velocity parameters are isovolumic velocity (IVV) (reflecting the tensioning phase at the beginning of the cardiac cycle), isovolumic acceleration (IVA) of the IVV, systolic velocity S (reflecting the ventricular contraction and ejection phase), early diastolic filling

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velocity E (reflecting the ventricular relaxation and filling phase), and late diastolic filling velocity A (reflecting the atrial contraction). Furthermore, color TDI additionally calculates myocardial displacement as well as independent preload and afterload deformation (strain) and deformation rate (strain rate [SR]). Tissue Doppler imaging has been used in human studies to assess right ventricular myocardial dysfunction secondary to chronic obstructive pulmonary disease (COPD) [3,4], to assess right ventricular myocardial adaption to hypoxia [5], and to assess improvement in right ventricular function during reversibility testing in pulmonary arterial hypertension [6]. The diagnostic value of TDI in dogs has been described during pulmonary arterial hypertension [7], with tricuspid regurgitation [8] and during volume loading–related changes derived from the tricuspid valve annulus in anesthetized dogs [9]. Tissue Doppler imaging has been applied to date in equine medicine to the left and right ventricles and atrium from the parasternal short-axis view. In particular, regional differences in ventricular myocardial velocity, left atrial size and mechanical function, left atrial mechanical dysfunction after conversion of atrial fibrillation to sinus rhythm, methods and reliability of TDI for assessment of left ventricular radial wall motion, influence of different pharmacologic drugs, and quantitative analysis of stress echocardiograms have been investigated in horses by TDI

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[10–19]. An influence of recurrent airway obstruction (RAO) on the left ventricular function has also been demonstrated before [20]. Conventional echocardiography combined with invasive right heart catheterization has been performed to evaluate cardiopulmonary interaction in horses, especially of right heart function. Tricuspid valve insufficiency [21], exercise-induced pulmonary hemorrhage [22], and atrial fibrillation combined with valve insufficiencies and RAO [23] have been evaluated by conventional echocardiography and/or right heart catheterization. Horses affected with RAO having clinical symptoms and systemic arterial hypoxia had been diagnosed with pulmonary artery hypertension [24]. However, there was no structural evidence of right heart failure in horses with end-stage RAO in a postmortem evaluation [25]. Based on the literature review displayed previously, we hypothesized that there might be an influence on right ventricular myocardial function by RAO, one of the most common equine airway diseases. Although not leading to permanent changes of heart dimensions, hypoxia and/or elevated pulmonary pressure might lead to intrinsic heart muscle remodeling [26,27] comparable with human COPD pathogenesis [28,29]. This right ventricular myocardial dysfunction [30,31] might be detectable by noninvasive TDI before right-sided heart failure secondary to pulmonary disease is evident to the full extent in conventional equine echocardiography in rare cases [32–34]. Therefore, the purpose of this study was to

Fig. 1. (A) Transducer position for apical long-axis four chamber view from right parasternal, which focus on the right ventricular free wall. (B) B-mode scan of the scan window as described in (A). (C) Color-coded right ventricular free wall with coronary vessel and tricuspid valve (tissue velocity imagingdDMI) developed from B-mode scan of the scan window as described in (A). Depth set at 12 cm and angle set at 45
to achieve necessary frame right for TDI. LV, left ventricle; RA, right atrium; RV, right ventricle; RVW, right ventricular wall.

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healthy horses and horses with RAO (matched to healthy controls by age, weight, height, and athletic condition), postulating that TDI is able to detect subtile changes in equine right ventricular myocardial function secondary to recurrent pulmonary hypertension, limited oxygen saturation, and, thereby, plausible right ventricular myocardial remodeling. 2. Materials and Methods 2.1. Animals

Fig. 2. Pulsed-wave tissue Doppler imaging of the basal right ventricular myocardium (scan as described in Fig. 1A) with maximum myocardial velocities marked. A, atrial contraction/late diastolic filling velocity; E, early diastolic filling velocity; IVV, isovolumic velocity; S, systolic velocity.

examine right ventricle myocardial function in different echocardiographic scan windows and with different TDI techniques and to compare these parameters between

Six cardiopulmonary healthy Warmblood horses (age 6–13 years; mean height: 169 cm, standard deviation (SD): 3.9; mean body weight: 574 kg, SD: 32.6) from the Bavarian stud farm Schwaiganger and six matched horses regarding age, sex, athletic condition (comparable training statusdapproximately 1-hour pleasure riding per day), and body parameter (age, 6–13 years; mean height: 170 cm, SD: 6.1; mean body weight: 559 kg, SD: 51.1) diagnosed with RAO in exacerbation, from the Equine Clinic of the Ludwig Maximilians University in Munich, were included.

Fig. 3. (A) Tissue Doppler imaging of the basal right ventricular myocardium (scan as described in Fig. 1A), mean myocardial velocities calculated within the region of interest (ROI) set at the basal free wall next to the coronary vessel; ROI tracked offline throughout the cardiac cycle. (B) Tissue Doppler imaging of the basal right ventricular myocardium (scan as described in Fig. 1A), mean myocardial strain (percentage of myocardial deformation) calculated within the ROI set at the basal free wall next to the coronary vessel; ROI tracked offline throughout the cardiac cycle. (C) Tissue Doppler imaging of the basal right ventricular myocardium (scan as described in Fig. 1A), mean myocardial strain rate (maximum systolic velocity of myocardial deformation) calculated within the ROI set at the basal free wall next to the coronary vessel; ROI tracked offline throughout the cardiac cycle. (D) Tissue Doppler imaging of the basal right ventricular myocardium (scan as described in Fig. 1A), mean myocardial displacement in mm at the end of systole calculated within the ROI set at the basal free wall next to the coronary vessel; ROI tracked offline throughout the cardiac cycle. A, atrial contraction/late diastolic filling velocity; AVC, aortic valve closure; AVO, aortic valve opening; E, early diastolic filling velocity; IVV, isovolumic velocity; S, systolic velocity; SImax, maximum strain; SRImax, strain rate, systolic maximum; TTmax, endsystolic maximum displacement.

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2.2. Preparticipation Examination After detailed clinical examination and questioning of the owners regarding the history of each horse, a specific examination of the cardiovascular system and a clinical examination of the respiratory apparatus, including auscultation and percussion, were performed. Horses were eligible for participation in the study provided that the owner had given informed consent. Horses were excluded from the study if they had clinical signs of additional heart disease or any kind of medication up to 4 weeks before. 2.2.1. Lung Examination The lung examination included history, clinical lung auscultation, arterial blood gas analysis (ABG), endoscopic examination, cytologic evaluation of tracheobronchial secretions (TBS), and radiographic examination.

2.3. Echocardiographic Examination and Offline Data Analysis Echocardiographic examinations and offline analyses were performed by one experienced observer throughout the entire study period, using a Vivid i echocardiograph (General Electric [GE] Healthcare, Medical Systems [Application Software 6.1.110; System software 1.36.18]) with continuous base–apex electrocardiograph (ECG). A 3.5MHz annular phased-array probe (Vivid i, 3s-RS probe; GE Medical Systems) with a maximum depth of 30 cm was used. Standardized right and left parasternal B-mode and color-flow Doppler images were performed and recorded for the evaluation of cardiac dimensions and valvular functions. A right parasternal short-axis M-mode image of the left ventricle was acquired at the chordal level for the calculation of fractional shortening (FS%). Analysis of different myocardial velocities and time parameters with different TDI techniques (PW-TDI and color TDI) were performed using EchoPAC software (Versio7.0 EchoPAC Software Only; Firma GE Healthcare, Horton, Norway). All horses remained unsedated in their stalls restrained by experienced grooms. Electrocardiograph was obtained simultaneously. All horses had a physiological heart rate during echocardiography. Data of three consecutive artefact-free cardiac cycles were stored for offline analyses. 2.3.1. Apical Long-Axis View of the Right Heart Evaluation of the right ventricular basal longitudinal myocardial function comparable with human medicine was achieved by first placing the transducer in the conventional right cardiac window to assess the long-axis four-chamber view. Second, the transducer was lowered to the level of the olecranon until the right ventricular free wall was visible in total and the coronary vessel and tricuspid valve were clearly definable (Figs. 1A and 1B). Scan plane presets were adjusted at a depth of 12–14 cm and an angle of 45
with an average of 88.3 fps (Fig. 1C). Pulsed-wave TDI and color TDI were performed in this scan plane.

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2.3.1.1. Pulsed-Wave TDI. The sample volume was placed in the middle of the right ventricular basal segment enddiastolic at a width of 12 mm. This defined region and sampling width was chosen, as these settings allowed measurement of the entire longitudinal right ventricular movement throughout the cardiac cycle (there is myocardium in the fixed sample volume throughout the cardiac cycle). Different myocardial peak velocities, that is, IVV, systolic velocity (S), early diastolic filling velocity (E), and atrial contraction and/or late diastolic filling velocity (A), and different timing intervals, that is, isovolumic contraction time (IVCT), isovolumic relaxation time (IVRT), IVA, and electromechanical coupling periods (time between beginning of ECG Q-wave and peak velocities) (Fig. 2), were measured. The E/A quotient as a parameter for diastolic function and the Tei index (IVCT þ IVRT/ET) for global myocardial function were also calculated [35]. 2.3.1.2. Color TDI. The B-mode scan was color coded by color TDI. The ROI was defined by offline software analysis with a length of 24 mm, width of 12 mm, and deviation of 9
to the left. The ROI was placed in the right ventricular free wall including endo- and epicard. Landmarks for the ROI setting were the base of the tricuspid valve and the coronary vessel. The ROI was tracked offline throughout the cardiac cycle. The ROI defined was chosen to be as large as possible. Mean velocities, the myocardial deformation parameters strain (%) and SR (1/s), and the displacement (mm) were evaluated (Figs. 3A–D). 2.3.2. Short-Axis View of the Right Heart The short-axis view was obtained from the standard right parasternal cardiac window at the level of the papillary muscles. First, this scan plane was aimed to compare myocardial contraction onsets of all ventricular walls simultaneously. Second, the maxima of systolic velocities by each ventricular wall were to be compared regarding time with the onset of ECG. Only color TDI was performed in this scan plane. Therefore, the depth was adjusted at 21–22 cm and the angle by 60
. Thus, the frame rate was 63.3 fps. Three regions of interest of 10  10 mm were placed one each in the middle between the papillary muscles in the left and right ventricular myocardium and interventricular septum (IVS) (Figs. 4A–C). The ROIs were tracked manually throughout three consecutive cardiac cycles. The maxima of velocities, deformation and displacement parameters, and electromechanical coupling periods were measured.

2.4. Statistical Analysis Statistical analysis was performed using PASW Statistics 17.0 (SPSS Inc, Chicago, IL). The mean, median, SD, range, and interquartile range (25th and 75th percentile) were reported for descriptive purposes as quantitative data. Furthermore, the individually based coefficient of variation (CV) was calculated and summarized using the root mean square CV to obtain a comparable statistic of sample variability within repeated measurements. The Mann–Whitney U test was used to compare the central tendency of

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Fig. 4. (A) Tissue Doppler imaging of the short-axis view obtained from standard right parasternal cardiac window at the level of the papillary muscles. Region of interest set simultaneously in the left ventricledred velocity curve, interventricular septumdyellow velocity cure, and right ventricledblue velocity curve.

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quantitative data between two independent samples. All statistical tests were performed two sided with a P-value  0.05. 3. Results 3.1. History and Lung Examination Only horses free of cardiac disease were included. Six horses were pulmonary healthy (control). The other six horses (RAO group) had a history of RAO diagnosed previously. They showed dyspnea and/or tachypnea and chronic and/or recurrent cough at rest during clinical examinations. Percussion revealed an enlarged lung field. The alveolar– arterial oxygen difference was between 12 and 25 mm Hg in the ABG analysis. Endoscopy revealed an increased tracheal secretion and thickened bifurcation tracheae, and microscopic examination of the TBS showed a dominating picture of neutrophils and Curshmann spirals in the cell count. Additionally, peribronchial infiltrations were visible in the radiographic examination. Thus, the diagnosis of moderate-to-severe RAO in exacerbation could be confirmed in all six horses. 3.2. Standard Echocardiographic Examination Measurements of the end-diastolic diameters of the left and right ventricles in B-mode were within normal ranges in all horses. There were no heart valve insufficiencies detectable with color-flow Doppler. B- and M-mode measurements (FS%) did not show any significant changes. 3.3. Echocardiographic TDI Examination 3.3.1. Apical Long-Axis View of the Right Heart 3.3.1.1. Pulsed-Wave TDI. Pulsed-wave TDI at the basal segment of the right ventricular free wall was applicable at a good quality for all study subjects. All peak velocities and timing intervals were clearly definable at an adjusted maximum velocity range. Systolic peak velocities in all horses were of positive values and early diastolic and late diastolic atrial filling velocities were of negative values, reflecting the movement of the myocardium toward and, respectively, away from the transducer. Double peaks were often evident (biphasic contraction pattern) in the systolic velocity profiles of healthy horses and those with RAO. The highest peak was measured every time double peaks occurred. The mean of peak E, resembling diastolic filling velocity, was significantly reduced in horses with RAO (Table 1). The E/A quotient, a parameter of diastolic function, was significantly higher in control horses (Table 1).

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Table 1 Different pulsed-wave tissue Doppler imaging parameters of horses affected by recurrent airway obstruction (group 1) compared with age, sex, weight, and athletic condition matched healthy control horses (group 0)dapial long-axis four chamber view of the right ventricular free wall Variable E (cm/s)

Group Mean  SD

0 1 A (cm/s) 0 1 E/A 0 1 IVV (cm/s) 0 1 S (cm/s) 0 1 QS (ms) 0 1

27.71 23.9 15.53 18.31 1.83 1.32 10.81 11.42 17.13 16.93 27 52

           

2.91 2.52 3.01 2.14 0.30 0.20 2.23 1.52 1.42 1.54 17 13

Median Minimum– Maximum

P

27.72 23.34 15.33 17.74 1.96 1.27 11.34 12.42 17.23 16.74 29 55

.037

6.6

.119

7.5

.009

6.0

.732

7.6

.79

5.2

23.31–30.72 21.72–28.03 11.74–19.02 16.32–21.34 1.37–2.09 1.11–1.71 7.71–13.32 9.01–12.72 15.34–18.74 15.11–19.02 0–51 30–65

CV (%)

.026 24.5

A (cm/s), late diastolic filling velocity, equivalent to atrial contraction; CV, coefficient of variation; E (cm/s), early diastolic filling velocity (cm/s); E/A, quotient of early and late diastolic filling velocity, a parameter of diastolic function; IVV (cm/s), isovolumic velocity; QS (ms), electromechanical coupling time from the onset of Q-wave in the ECG to the maximum of PW Doppler-derived systolic velocity; S, systolic velocity (cm/s); SD, standard deviation.

Comparing the groups, electromechanical coupling times were not altered, except time from the onset of Q-wave in the ECG to maximum of systolic wave in the PW-TDI (Table 1). There was no IVRT measurable in either group. The calculated Tei index, a marker for global right ventricular function, was not altered between the study groups. PW-derived parameters in three consecutive measurements at the right ventricular basal free wall were of very low to low intraindividual variability (1%–15%), except time from the onset of Q-wave in the ECG to maximum of systolic wave in the PW (up to 25% intraindividual variability) (Table 1). 3.3.1.2. Color TDI. Average longitudinal myocardial velocities within a defined ROI were measured by offline ROI setting and tracking throughout the cardiac cycle. Mean peak velocities and timing intervals were clearly definable. In contrast to the PW, afterload independent deformation parameters (strain, shortening of muscle fibers in percentage; SR, tissue tracking and/or displacement) could be measured by this technique. Similar to that in the PW, late diastolic filling velocity A was significantly higher and E/A quotient was, thereby, significantly decreased in the RAO compared with matched control subjects (Table 2). There was a tendency to higher IVV in the RAO patients (Table 2). Evaluating the deformation parameters, systolic SR (myocardial deformation per second) in RAO subjects was significantly lower than in control horses (Table 2).

= Negative systolic mean velocities for the right ventricle and interventricular septum (IVS_RV_S) and positive systolic mean velocity for the left ventricle (LV_S). (B)

Tissue Doppler imaging of the short-axis view obtained from standard right parasternal cardiac window at the level of the papillary muscles. Region of interest set simultaneously in the left ventricledred strain curve, interventricular septumdyellow strain curve, and right ventricledblue strain curve. Highest mean strain for the left ventricle (LV), followed by the right ventricle (RV) and interventricular septum (IVS), maximum strain was achieved for all ventricular walls almost simultaneously. (C) Tissue Doppler imaging of the short-axis view obtained from standard right parasternal cardiac window at the level of the papillary muscles. Region of interest set simultaneously in the left ventricledred strain rate curve, interventricular septumdyellow strain rate curve, and right ventricledblue strain rate curve. Highest mean systolic strain rate (S) for all ventricular walls was achieved almost simultaneously. AVC, aortic valve closure; AVO, aortic valve opening; MVC, mitral valve closure; MVO, mitral valve opening.

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Table 2 Different color–tissue Doppler imaging parameters of horses affected by recurrent airway obstruction (group 1) compared with age, sex, weight, and athletic condition matched healthy control horses (group 0) derived from apical long-axis four chamber view of the right ventricular free wall Variable

Group

Mean  SD

Median

Minimum–Maximum

P

E (cm/s)

0 1 0 1 0 1 0 1 0 1 0 1 0 1

15.16 15.35 7.03 11.34 2.23 1.39 4.98 6.76 2.8 1.7 10.05 11.14 40 51

14.71 15.08 7.34 10.69 1.94 1.34 4.77 6.85 2.8 1.7 10.24 11.72 43 53

21.31 to 11.79 19.28 to 11.63 8.83 to 4.37 13.74 to 9.31 1.69–3.03 0.85–1.87 2.34–8.00 5.16–7.92 4.0 to 1.8 3.1 to 0.8 8.55–11.54 8.11–12.8 0–103 30–70

1.00

A (cm/s) E/A IVV (cm/s) SR (1/s) S (cm/s) QS (ms)

             

3.4 3.47 1.62 1.76 0.57 0.39 1.99 1.14 0.7 0.8 1.26 1.69 39 15

CV (%) 7.2

.002

10.9

.009

12.4

.180

14.5

.041

15.1

.223

6.4

.489

31.1

A (cm/s), late diastolic filling velocity, equivalent to atrial contraction; CV, coefficient of variation; E (cm/s), early diastolic filling velocity (cm/s); E/A, quotient of early and late diastolic filling velocity, a parameter of diastolic function; IVV (cm/s), isovolumic velocity; QS (ms), electromechanical coupling time from the onset of Q-wave in the ECG to the maximum of PW Doppler-derived systolic velocity; S, systolic velocity (cm/s); SR (1/s), strain rate systolic; SD, standard deviation.

4. Discussion

Three consecutive measurements of TDI parameters at the tricuspid annulus were mainly of very low to low intraindividual variability (15%, Table 2).

Right ventricular function has been incompletely studied by echocardiography in equine medicine because of anatomic and technical difficulties. The entire right heart could not be imaged in total because of the massive sternum and muscles, as well as the heart’s right cranial-to-left caudal orientation [36–38]. Anatomic and technical limitations of right ventricular geometry are overcome in this study by defining a new right ventricular scan window to which TDI is applicable. Longitudinal muscle fibers in human medicine are known to be affected, first, by acute or chronic hypoxia and pulmonary pressure elevation secondary to COPD [2,39]. Therefore, the aim was to define a scan window adopted from human medicine where longitudinal right ventricular myocardial movement at the basal right ventricular segment could be evaluated. A newly defined “lowered” long-axis four-chamber view assessed by PW-TDI and color TDI was applied to the horses, abstracted from tricuspid annulus settings and alterations in human medicine [2]. The circumstance of convex right ventricular free wall architecture and movement deviation to the “left” was accounted for calculating that the overall quantitative longitudinal myocardial movement was measured with the “same error” in all horses.

3.3.2. Short-Axis View of the Right Heart 3.3.2.1. Color TDI. Color TDI of the short-axis view was applicable to all horses. Tracking of all three ventricular walls simultaneously was possible throughout the cardiac cycle. During the evaluation of the entire myocardial movement, the IVS and the right ventricular wall were moving away from the transducer. Therefore, the direction of systolic radial movement was negative for the RV and IVS. The highest systolic velocities were achieved in the right ventricle followed by the IVS. The highest strain was measured for the right ventricular myocardium, followed by the IVS, revealing a slightly lower contractility. Strain was lowest in the IVS. Strain rate (“velocity” of strain) was also highest in the right ventricular myocardium, followed by the IVS (Table 3). No significant differences between horses affected with RAO and healthy controls regarding systolic velocities, strain, and SR, as well as corresponding electromechanical coupling times, were evident. The systolic velocities measured were of very low to low intraindividual variability (5%–15%, Table 3), whereas strain and SR were of high intraindividual variability of up to 25%.

Table 3 Color–tissue Doppler imaging parameter of horses affected by recurrent airway obstruction (group 1) compared with age, sex, weight, and athletic condition matched healthy control horses (group 0) derived from right parasternal short-axis view Variable

Group

Mean  SD

TpeakSRV (s)

0 1 0 1 0 1 0 1

0.32 0.298 0.363 0.343 4.7 4.48 2.4 2.28

TpeakSIVS (s) SRV (s) SIVS (s)

       

0.038 0.065 0.021 0.038 1.03 1.07 1.14 0.7

Median

Minimum–Maximum

P

0.315 0.288 0.368 0.35 4.48 4.82 2.36 2.41

0.26–0.367 0.227–0.370 0.327–0.383 0.28–0.375 5.97 to 3.36 5.43 to 2.85 3.74 to 0.76 3.08 to 1.35

.777

CV (%) 7.8

.355

4.8

.818

5.7

.818

17.9

CV, coefficient of variation; SIVS, systolic velocity interventricular septum wall; SRV, systolic velocity right ventricle; SD, standard deviation; TpeakSIVS, time interval until systolic maximum interventricular septum; TpeakSRV, time interval until systolic maximum right ventricle.

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Supporting this hypothesis, these myocardial measurements were of lowest variability and revealed significant differences between horses affected with RAO and healthy horses comparable with results in human medicine [6,40–42]. Reduced early diastolic filling velocities (E), elevated late diastolic filling velocities (A), thereby decreased E/A quotient, prolonged electromechanical coupling periods between ECG Q-wave and maximal velocities, compensatory elevated systolic strain, and diminished displacement in horses affected by RAO compared with matched control horses were the most significant findings of this study. These differences between the two groups may be explainable by an increased afterload because of higher pulmonary pressures in horses affected by RAO. To confirm the hypothesis, further examinations with a larger number of horses affected by RAO is necessary. Additional measurements of pulmonary pressures may also help to support this thesis in further examinations. Furthermore, asynchronism of ventricular walls was detectable in humans with pulmonary hypertension [43]. Therefore, TDI was also applied to the short-axis view to compare myocardial contraction onsets and contractility of all ventricular walls simultaneously. No significant differences were measurable between the controls and horses affected by RAO. However, in contrast to the short-axis view described herein, longitudinal function in human medicine was assessed by an apical scan window not realizable in equine cardiology because of the massive sternum and muscles. Regarding different locally obtained patterns of myocardial movement, more net circumferential shortening of the right ventricular myocardium and IVS (toward the transducer) compared with left ventricular net contraction (away from the transducer) is measured in equine TDI (short-axis view). Therefore, we concluded that the equine short-axis view might not be appropriate to evaluate quantitative myocardial displacement for each ventricular wall but could be useful to determine asynchronism of ventricular walls according to the time of onset of myocardial contraction in general. Overall, three consecutive measurements of TDI parameters were of negligible intraindividual variability when assessed by one experienced observer following predefined image acquisition guidelines. Further studies will be needed to investigate the usefulness of TDI techniques in detail. One should keep in mind the necessity of experienced observers and image acquisition guidelines, as measurement reproducibility was shown to be optimized for color TDI in human medicine by tracking the ROI throughout the cardiac cycle, similar to this study [44]. To conclude, right heart function can be evaluated by TDI techniques applied to defined scan window settings described in this explorative study. Further studies are needed to validate these methods and support our preliminary results, that is, to verify interobserver and intraobserver results for repeatability of TDI parameters, to evaluate the impact of different diseases or medications on right heart function, to determine the usefulness of TDI as a clinical diagnostic method, and, most of all, to address correlations with pulmonary artery pressure measurements.

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Acknowledgments The authors disclose no conflicts of interest.

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