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ACE-inhibitors are often used in horses, although the effect has been shown only in a few studies in horses suffering from heart disease. This study aimed to evaluate the efficacy of enalapril for treatment of mitral or aortic valve insufficiency. Diagnostics included clinical examination of the cardiovascular system, standard echocardiography, colour and pulsed-wave tissue Doppler imaging as well as two-dimensional speckle tracking. The study included 29 horses with aortic and/or mitral valve insufficiency. The horses were treated with enalapril (Enalatab®, CP-Pharma, 0.5mg/kg p.o.) over a period of three months. All horses were examined immediately prior to the course of medication and after the three month therapy period. The Horses were divided into two groups. Horses in group 2 were examined a third time, three months before the start of medication. Thereby, the progress of the disease could be assessed over a longer period of time. Clinical examination and echocardiography were performed at every examination time-point. Standardized right and left parasternal B-mode and color flow Doppler images were recorded for evaluation of cardiac dimensions, cardiac contraction (fractional shortening) and valvular function. For tissue Doppler imaging and two-dimensional speckle tracking, a right parasternal short axis image was acquired at the chordal level immediately below the mitral valve. Tissue Doppler imaging was used to measure myocardial velocities in the left ventricular free wall and the interventricular septum. Myocardial deformation was measured by two-dimensional speckle tracking. None of the horses in the present study showed any signs of adverse effects in response to treatment with ACE-inhibitors. Furthermore, the owners of three horses reported a generally improved condition. A statistically significant reduction in the dimension of the aorta was demonstrated after enalapril treatment in horses with aortic valve insufficiency and dilatation. The average dimension of the aorta was D=0.39cm (P=0.047) larger before starting the therapy. Horses with mitral valve insufficiency showed a significant increase in the dimension of the left atrium. No significant changes were detected with the PW-tissue Doppler imaging. The color tissue Doppler technique suggested a significant decrease in the early diastolic myocardial velocity in the interventricular septum. Within group 1, the average values before starting the medication wereD=0.03cm/s (P=0.023) greater than after the treatment with enalapril over the three months. Conversely, no effect was found in the period without medication. These results indicate a positive effect and an improvement of myocardial relaxation by enalapril. The change in the early diastolic velocity could not be confirmed in group 2. The myocardial movement measured using the speckle tracking method also showed significant changes. However, this findings must be interpreted with caution due to high variance within these parameters. This study showed an effect of the ACE-inhibitor enalapril in horses with left sided heart valve insufficiency on the myocardium and the renin-angiotensin-aldosterone-system. However, a beneficial effect was not detected in every case, therefore additional studies are advised to investigate this further.