Aortic stenosis is a congenital heart defect. Interestingly, the lesion tends to develop within the first 3 months of life after the puppy is born, as opposed to being present at the time of birth. Aortic stenosis occurs when a constricting ring or ridge of tissue develops directly below the aortic valve, causing an obstruction to blood flow out of the heart. This means that insufficient blood flow may occur, particularly during exercise or times of stress. It is more commonly seen in certain breeds of dog (Golden Retriever, Boxer, German Shepherd Dog, Newfoundland, Rottweiler).
Since the heart has to work harder to pump blood through a narrowed exit the muscles of the heart get larger (hypertrophy). The hypertrophied heart muscles can become deprived of normal blood flow and oxygen, and this can result in abnormal heart beats and dysrhythmias. This may further reduce the ability of the heart to provide the body with an adequate blood supply. Fainting is caused by an insufficient blood supply to the brain.