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Increased Mitral Valve Regurgitation and Myocardial Hypertrophy in Two Dogs With Long-Term Pimobendan Therapy R. Tissier,2,4,* V. Chetboul,1,4 R. Moraillon,3 A. Nicolle,1 C. Carlos,1 B. Enriquez,2,4 and J-L. Pouchelon1,4 1

Unité de Cardiologie, 2Unité Pédagogique de Pharmacie-Toxicologie, Unité Pédagogique de Médecine, Ecole Nationale Vétérinaire d’Alfort, Maisons-Alfort, France; and 4INSERM E00-01, Faculté de Médecine Paris XII, Créteil, France

3

Abstract

*Author to whom all correspondence and reprint requests should be addressed: Renaud Tissier, DVM, PhD, Unité Pédagogique de PharmacieToxicologie, Ecole Nationale Vétérinaire d’Alfort, 7 Avenue du Général de Gaulle, 94704 Maisons-Alfort cedex, France. E-mail: rtissier@ vet-alfort.fr Received: 03/06/04 Revised: 05/10/04 Accepted: 05/24/04 Cardiovascular Toxicology, vol. 5, no. 1, 43–51, 2005

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The aim of this article is to describe original adverse effects in two dogs chronically treated with the inodilator pimobendan. We report a German shepherd (i.e., dog 1) and a poodle (i.e., dog 2) that were referred to our cardiology unit after receiving pimobendan for 10 and 5 mo, respectively. In both dogs, conventional echo-Doppler examination demonstrated mitral valve regurgitation and myocardial hypertrophy. Tissue Doppler imaging (TDI) was performed in the first case and revealed an abnormal relaxation phase. After the first examination, pimobendan administration was stopped in both cases and dogs were re-examined 3 and 1 mo later, respectively. Mitral valve regurgitation assessed by echocardiography decreased in both dogs, and the systolic heart murmur disappeared in dog 1. Importantly, most echocardiographic and TDI parameters tended to normalize in dog 1, suggesting, at least partial reversal of both myocardial hypertrophy and relaxation abnormality produced during inodilator therapy. This is the first report to describe an increase in mitral regurgitation under clinical conditions in dogs treated with pimobendan. We also suggest that pimobendan may induce ventricular hypertrophy. However, prospective studies are needed to confirm this observation. Key Words: Dog; pimobendan; inodilator; inotrope; echocardiography; tissue Doppler imaging; ventricular hypertrophy; mitral valve regurgitation.

Introduction Inodilators are pharmacological compounds producing vasodilation and inotropic effects mediated by phosphodiesterase III inhibition and calcium-sensitizing properties. Despite controversy, the principal inodilators (i.e., levosimendan and pimobendan) were demonstrated to be beneficial in the treatment of left ventricular systolic failure in both human (1–3) and veterinary cardiology (4). However, Schneider et al. (5) demonstrated that repeated pimobendan administration can be cardiotoxic in healthy dogs, for example, leading to mitral jet lesions after 4 wk even at close to therapeutic dosages. These findings raised an important issue, that is, the potential adverse effects of chronic treatment with pimobendan in dogs without systolic myocardial dysfunction (e.g., in myxomatous valvular disease). Indeed, the usefulness

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of inodilator therapy in canine valvular insufficiency has been greatly debated and remains an important issue in the absence of definitive data. A study was recently initiated to obtain more information on the subject (6). During routine clinical work at our veterinary cardiology unit, we observed original adverse effects of pimobendan in two dogs that had been treated chronically with this inodilator and did not show decreased left ventricular systolic function. The aim of this short article is to describe these effects (i.e., increased mitral valve regurgitation, myocardial hypertrophy, and alterations in left ventricular relaxation) and to demonstrate that they were, at least in part, reversed after cessation of pimobendan administration.

Materials and Methods Two dogs were referred to the Cardiology Unit of Alfort. They underwent complete clinical exams followed by conventional echocardiography. Tissue Doppler imaging (TDI) examination was also performed in the first case.

Conventional Echocardiography Two-dimensional (2D) and M-mode echocardiography, color flow imaging, and spectral Doppler examinations were performed by the same trained observer with continuous ECG monitoring using a Vingmed system 5 (General Electric Medical System, Waukesha, WI) equipped with a 2.5- to 3.5-MHz phased-array transducer. Ventricular measurements were taken from the right parasternal location (shortaxis view) using the 2D-guided M-mode, according to the recommendations of the American Society of Echocardiography (7). Measurements of the aorta and the left atrial diameter were performed with a 2D method (8), using a short-axis right-sided parasternal view obtained at the level of the aortic valve, where the commissures of the cusps are visualized during diastole. For all ultrasound examinations, dogs were awake, gently restrained in the standing position. This method has already been proven in our group to have good repeatability and reproducibility (9). A left parasternal apical four-chamber view was used to record mitral inflow by pulsed wave Doppler. Peak diastolic velocities were measured in early (Em) and late (Am) diastole, and the Em/Am ratio was then calculated. Finally, mitral regurgitation was assessed Cardiovascular Toxicology

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semiquantitatively by measuring the size of the systolic color-flow jet originating from the mitral valve and spreading into the left atrium using the left apical four-chamber view. As previously described in dogs (10), images were carefully analyzed frame by frame to determine the maximum area of the regurgitant jet signal.

2D Color TDI Examination The same materials and procedures described in “Conventional Echocardiography” were used for TDI examinations. Real-time color Doppler was superimposed on the gray scale with a frame rate ³100 frames per second. The Doppler receive gain was adjusted to maintain optimal coloring of the myocardium, and Doppler velocity range was set as low as possible to avoid occurrence of aliasing. Left ventricular free wall (LVFW) velocities resulting from the radial left ventricular motion were measured using the right parasternal ventricular short-axis view between the two papillary muscles, as previously described (11). The angle of interrogation of the beam was carefully aligned to be perpendicular to the LVFW. Measurements were made on an endocardial and an epicardial segment (2 ´ 2 mm) of the LVFW. Simultaneous endocardial and epicardial velocity profiles were obtained using a stand-alone off-line measuring system (Echo Pac for Vingmed System 5, General Electric Medical System). TDI parameters included maximal systolic (S), early (E), and late (A) diastolic LVFW velocities. This method has also been proved in our group to have good repeatability and reproducibility (11).

Results Case Report: Dog 1 First Visit A 6-yr-old 30-kg female German shepherd was referred to the Cardiology Unit for exercise intolerance and depression that had been increasing for several weeks. The owner reported that the animal was anxious for no specific reason. The dog had been treated with pimobendan for 10 mo (0.33 mg/kg PO q12h) without any prior echocardiographic examination. Biochemical parameters, blood cell count, and systemic blood pressure (145 mmHg, 75 mmHg) were within normal ranges. Physical examination

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Table 1 Conventional M-Mode and Two-Dimensional Echocardiographic Parameters of Dogs 1 and 2 During Two Exams, Before and After Stopping Pimobendan a Second exam after stopping pimobendan therapy

Normal rangesa for corresponding weight

Care report 1 Day 0 Left ventricular end-diastolic diameter (mm) 34.6 Left ventricular end-systolic diameter (mm) 15.4 Interventricular septal diastolic thickness (mm) 12.9 Interventricular septal systolic thickness (mm) 21.1 Left ventricular free-wall diastolic thickness (mm) 15.4 Left ventricular free-wall systolic thickness (mm) 18.6 Shortening fraction (%) 56 Left atrium size (mm)/aorta diameter (mm) 0.89 Maximal mitral regurgitation jet area (mm2) 232 Mitral E wave (m/s) 0.21 Mitral A wave (m/s) 0.60 Mitral E wave/mitral A wave 0.35 Time of mitral regurgitation Whole systole

Day 0 + 3 mo 40.0 25.0 9.6 15.7 10.5 16.2 37.5 0.83 22 0.74 0.43 1.72 Early systole

German shepherd 30 kg 40.3–43.6 25.0–27.4 10.2–11.3 15.4–16.7 8.2–9.2 13.2–14.4 33.5–45.9 0.83–1.13 — 0.59–1.18 0.33–0.93 1.04–2.42 —

Case report 2 Day 0 Left ventricular end-diastolic diameter (mm) 26.3 Left ventricular end-systolic diameter (mm) 12.0 Interventricular septal diastolic thickness (mm) 7.8 Interventricular septal systolic thickness (mm) 11.4 Left ventricular free-wall diastolic thickness (mm) 7.5 Left ventricular free-wall systolic thickness (mm) 13.6 Shortening fraction (%) 54 Left atrium size (mm)/aorta diameter (mm) 0.84 95 Maximal mitral regurgitation jet area (mm2) Mitral E wave (m/s) 0.64 Mitral A wave (m/s) 0.87 Mitral E wave/mitral A wave 0.74 Time of mitral regurgitation Whole systole

Day 0 + 1 mo 25.4 14.5 8.0 11.9 8.1 12.3 43 0.84 34 1.18 0.87 1.36 Early systole

Poodle 8.5 kg 16–28 8–16 4–6 6–10 4–6 6–10 35–57 0.83–1.13 — 0.59–1.18 0.33–0.93 1.04–2.42 —

Echocardiographic parameters

aSee

First exam before stopping pimobendan therapy

ref. 12–15. —, not applicable.

revealed a weak and a tachypneic animal with a left apical systolic heart murmur (grade III/VI). Heart rate averaged 95 beats per minute (BPM). Conventional M-mode and 2D echocardiographic parameters are shown in Table 1. Right ventricular and atrial dimensions were normal. 2D echocardiography revealed irregular and thickened mitral valve leaflets on the right and left parasternal fourchamber views (maximal thickness = 4.0 mm). A systolic anterior motion of the mitral valve was not observed on M-mode images. Echo-Doppler examination showed a severe symmetric myocardial hyperCardiovascular Toxicology

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trophy in which the ventricular septum and the LVFW were both affected (Fig. 1). This hypertrophy was associated with a marked elevation of the shortening fraction, reduced systolic and diastolic left ventricular cavity, and significant systolic mitral regurgitation. However, no left atrial dilation was observed. The reversed Em/Am ratio (0.35) on Doppler examination of mitral inflow suggested impaired left ventricular relaxation. As shown in Table 2, TDI examination revealed high systolic radial myocardial velocities and an impaired relaxation phase with a characteristic decrease

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Tissier et al. complete exam was performed. The systolic murmur had totally disappeared and heart rate averaged 97 BPM. Color flow Doppler examination showed a nearly total disappearance of the mitral regurgitation (Table 1). On Doppler examination of mitral inflow, Em/ Am ratio (1.72) returned to normal. Shortening fraction and interventricular septal systolic thickness returned to normal ranges. Interventricular septal diastolic thickness strongly decreased under normal ranges. Both LVFW diastolic and systolic thickness remains slightly elevated. As shown in Table 2, systolic and early diastolic myocardial velocities also returned to normal ranges, despite a persistent slight increase in A wave. Moreover, and as illustrated in Fig. 2B, normalized E/A ratio was observed in both epicardial and endocardial layers (1.18 and 1.21, respectively).

Case Report: Dog 2 First Visit

Fig. 1. (A, B) Echocardiography of dog 1 at the first visit. (A) Two-dimensional echocardiogram showing the marked left ventricular hypertrophy (end-systolic frame of the left ventricle obtained from the right parasternal shortaxis view at the level of the papillary muscles). (B) M-mode echocardiogram showing the symmetric myocardial hypertrophy and the reduction of the left ventricular diameter. LV, left ventricle; RV, right ventricle; IVS, interventricular septal wall; LVFW, left ventricular free wall.

in the E-to-A ratio (E/A < 1) in the endocardial as well as in the epicardial layers (0.46 and 0.48, respectively). Figure 2A illustrates an example of the TDI velocity profile. Second Visit: Follow-Up After the first visit, pimobendan therapy was stopped and changed to benazepril (0.33 mg/kg/d). The owner reported that the dog was less depressed and less anxious during the week following the new treatment. Three months later, the dog was alert and a second Cardiovascular Toxicology

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A 10-yr-old 8.5-kg male poodle was referred to the Cardiology Unit for exercise intolerance, lethargy, and cough with tachypnea that had been increasing for several weeks. Like the owner of dog 1, the owner of dog 2 reported that the animal was getting more and more anxious and nervous, particularly at night. The dog had a prior history of tracheal collapse and chronic bronchitis for several years. The dog underwent pimobendan treatment for 5 mo (0.29 mg/kg PO q12h) without any prior echocardiographic examination. On physical examination, a nonproductive cough was easily elicited by palpation of the trachea, but no heart murmur was detected. Heart rate averaged 120 BPM during this first visit. 2D echocardiography revealed irregular and thickened mitral valve leaflets on the right and left parasternal four-chamber views (maximal thickness = 5.2 mm). A systolic anterior motion of the mitral valve was not observed on M-mode images. Right ventricular and atrial dimensions were normal. As shown in Table 1, conventional echo-Doppler examination demonstrated slight myocardial hypertrophy associated with an elevated shortening fraction and mitral regurgitation during the whole systole. However, no left atrial dilation was observed. Similar to the results for dog 1, the reversed Em/Am ratio (0.73) on Doppler examination of mitral inflow suggested impaired left ventricular relaxation.

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Table 2 Radial Tissue Doppler Imaging Parameters Measured in Endocardial and Epicardial Layers of the Left Ventricular Free Wall of Dog 1 During Two Exams, Before and After Stopping Pimobendana Tissue Doppler imaging parameters S wave (cm/s) Endocardial Epicardial E wave (cm/s) Endocardial Epicardial A wave (cm/s) Endocardial Epicardial

First exam before stopping pimobendan therapy

Second exam after stopping pimobendan therapy

Day 0

Day 0 + 3 mo

9.7 7.4

5.9 3.8

4.7–9.0 1.9–4.7

4.1 2.5

6.7 4.5

5.2–12.0 1.5–5.2

9.0 5.2

5.7 3.7

1.9–5.8 0.5–2.9

Normal rangesa

aSee

ref. 16. A, peak velocity of the left ventricular free wall during late diastole: E, peak velocity of the left ventricular free wall during early diastole: S, peak velocity of the left ventricular free wall during systole.

Second Visit: Follow-Up Pimobendan treatment was stopped after the first visit and no other treatment was given. Like the owner of dog 1, the owner of dog 2 reported that the dog was more alert and less anxious and nervous 1 wk later and that cough and tachypnea had markedly decreased. One month later, the dog was in good condition and a second echo-Doppler exam was performed. As illustrated in Table 1, myocardial wall thicknesses were still slightly elevated, but the shortening fraction had diminished. Mitral inflow profile returned to normal (Em/Am ratio = 1.35). Finally, mitral regurgitation markedly diminished. Heart rate averaged 107 beats/min during this second visit.

Discussion Our reports demonstrate original adverse effects associated with chronic treatment with pimobendan in two dogs. In both cases, mitral valve regurgitation strongly decreased when pimobendan therapy was stopped. Ventricular hypertrophy was also demonstrated and appeared to be at least partially reversible in one dog after pimobendan treatment was replaced with the angiotensin-converting enzyme inhibitor benazepril. Relaxation abnormalities were characterized by TDI and pulsed wave Doppler (mitral inCardiovascular Toxicology

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flow) in dog 1 and by pulsed wave Doppler only in dog 2. These diastolic alterations were reversed after cessation of pimobendan administration. Because heart rate was similar between the two visits for dog 1, and decreased slightly (-13 BPM) for dog 2, a variation of heart rate could not explain the abnormal relaxation phase during pimobendan therapy or its normalization after cessation of the treatment. To our knowledge, this is the first study to report reversible mitral valve regurgitation, myocardial hypertrophy, and diastolic dysfunction in dogs under pimobendan treatment and in clinical conditions. Importantly, both dogs were treated with dosages (0.33 and 0.29 mg/kg PO q12h, respectively) close to those recommended in canine systolic heart failure (0.3–0.6 mg/ kg/d) (4). The described potential adverse effects could not therefore be related to high dosage and might be observed at the therapeutic level. In our study, the first important finding is pimobendan-induced increase in mitral valve regurgitation. The imputability of this phenomenon to pimobendan is highly probable because regurgitation was strongly reduced after stopping this therapy. Obviously, increased valve regurgitation might be explained by ventricular hypercontractility, which was characterized by greater shortening fraction during pimobendan therapy (Table 1). In dog 1, increased magnitude

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Fig. 2. (A, B) Analysis of left ventricular free wall (LVFW) radial motion (right panels) of dog 1 at the first visit (A) and second visit (B) (after stopping pimobendan treatment). Two-dimensional color tissue Doppler imaging mode recording from the right parasternal short-axis view was used (left panels). The yellow and green curves correspond to the endocardial and epicardial velocity profiles, respectively. LV, left ventricle; A, peak velocity of the LVFW during late diastole; E, peak velocity of the LVFW during early diastole; IVCT, isovolumic contraction phase; IVRT, isovolumic relaxation phase; S, peak velocity of the LVFW during systole.

of S waves (assessed by TDI) also reflects this hypercontractile state. These findings further support the conclusion that pimobendan’s vasodilating properCardiovascular Toxicology

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ties might not permit avoidance of ventricular overload and a secondary increase in mitral regurgitation. Interestingly, previous experimental data in

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healthy dogs demonstrated that mitral jet lesions could be induced, even with the lack of valvular disease, by pimobendan administration (5). One might, however, argue that previous studies (17,18) demonstrated that inotropic therapies (i.e., dobutamine) were able to decrease mitral regurgitation in humans. However, the pathophysiology of such mitral insufficiency was completely different because it occurred in dilated cardiomyopathies that induced mitral regurgitation by an initial increase in mitral orifice area. In such a situation, some inotropes have been demonstrated to be beneficial by a reduction of mitral orifice area (19). In dogs with myxomatous valvular disease, the regurgitation is triggered by a primitive valvular degeneration, and the effect of inotropes might be completely different because a worsening of regurgitation could be induced by a higher pressure gradient between atria and ventricles. Indeed, a previous clinical report in dogs demonstrated that the acute administration of another inotrope (i.e., digoxin) increased mitral regurgitation in four of five cases (20). These findings suggest that ventricular function should always be assessed before initiating inodilator therapy, especially in dogs with myxomatous valvular disease. One can hypothesize that our conclusions could be explained by inter-day variability of our echocardiographically assessed mitral regurgitation jet size. However, we observed a total disappearance of systolic left apical murmur after stopping pimobendan therapy in dog 1. It also supports a strong decrease in mitral regurgitation because murmur intensity and Doppler-assessed jet size were demonstrated to be well-correlated (21). Finally, it should be noted that exercise tolerance was rapidly better in both dogs (several days) after stopping administration of pimobendan, even though this parameter was assessed subjectively. The second major finding of this short article is the observation of myocardial hypertrophy in these two pimobendan-treated dogs. Obviously, our first hypothesis was that these morphological alterations reflected hypertrophic cardiomyopathy. This disease remains rare, but is well described in dogs (22). However, ventricular hypertrophy was strongly reversed 3 mo after stopping pimobendan therapy in dog 1. This reversal could not be explained by interday variability in echocardiographic procedures. For example, the inter-day variation coefficient for the Cardiovascular Toxicology

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LVFW diastolic thickness averaged 9% in our clinic (9), whereas this parameter evolved from 15.4 to 10.5 mm (i.e., -32%) after cessation of pimobendan administration. These findings strongly suggest that pimobendan triggered concentric hypertrophy in this dog, especially because reversal myocardial diastolic dysfunction was also observed (Fig. 2). Again, this reverse could not be explained by inter-day variability of TDI measurements (inter-day variation coefficient of endocardial E wave = 25% vs a 63% observed variation between the two exams) (11). In dog 2, ventricular hypertrophy decreased but was not reversed 1 mo after stopping pimobendan administration. It is probable that a longer time is needed for reversal of this change. Moreover, it is obvious that diastolic left ventricular myocardial hypertrophy cannot be considered as a usual alteration in canine myxomatous valvular disease and that our observations could therefore not reflect the natural history of this disease. Indeed, such a diastolic myocardial hypertrophy was never reported, to our knowledge, in the numerous studies describing canine valvular insufficiency (23). Nevertheless, a large prospective study is needed to confirm our hypothesis of possible pimobendan-induced hypertrophy and to evaluate its incidence (6). Some mechanistic theories could be hypothesized in order to explain the genesis of such a hypertrophy. Indeed, although the potential hypertrophic effect of longterm administration of inodilators has not yet been described, the ability of other inotropes (e.g., isoproterenol or dobutamine) to induce ventricular hypertrophy is well known (24,25). Whether such a morphological alteration might be caused by hemodynamic factors or by b-adrenergic receptor stimulation is debatable (26–28). Chronic pimobendan administration entered the picture because it mimics hemodynamic changes mediated by b-receptor agonists and is characterized by a similar molecular pathway, that is, activation of the cyclic adenosine monophosphate pathway (by inhibition of phosphodiesterase III). In conclusion, we report increased mitral valve regurgitation and reversal myocardial hypertrophy in two dogs after long-term pimobendan therapy. Reversal diastolic dysfunction was also observed by TDI in dog 1. These results suggest that echocardiography may be useful both before and during pimobendan therapy in order to rule out prior or induced diastolic myocardial dysfunction and myocardial hypertro-

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phy. Further prospective studies are needed to confirm the potential cardiovascular adverse effects of pimobendan. 12.

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