Abstract: The present invention relates to an If blocker or a pharmaceutically acceptable salt thereof for the treatment and/or prevention of a canine patient suffering from heart diseases, preferably heart diseases such as dilated cardiomyopathy (DCM), mitral valve insufficiency (MI), arrthymias, preferably tachyarrthymias, preferably arterial arrhythmias, atrioventricular nodal arrhythmias and/or tachycardia. Each of these diseases may or may not result in heart failure (HF) in canine patients. The invention also relates to improving the quality of life, improving the general health condition as well as a prolonging the life expectancy in canine patients suffering from heart diseases and/or heart failure due to one or more of the following etiologies dilated cardiomyopathy (DCM), mitral valve insufficiency (MI), arrthymias, preferably tachyarrthymias, preferably arterial arrhythmias, atrioventricular nodal arrhythmias and/or tachycardia.