Patent ductus arteriosus (PDA) is a common birth defect. The pressure in the aorta is higher than the pressure in the pulmonary artery, therefore, blood crosses the ductus from left (aorta) to right (pulmonary artery). Very infrequently, the increased blood flowing into the lungs injures the pulmonary blood vessels. This can reverse the path of blood flow from right to left. In this case, unoxygenated blood flows into the aorta to the rear limbs, causing weakness and complications such as an elevated red blood cell count (polycythemia), which makes the blood thick. Symptoms are usually precipitated by exercise and include weakness and even seizures. Anemia and fever can lead to soft to moderate intensity murmurs.
There are a number of reasons for heart murmurs in kittens. These conditions require different management and include:
Ventricular septal defect, a "hole in the heart" between the two ventricles, is common.
Aortic stenosis (or subaortic stenosis) is the most common heart defect in cats today. This is a narrowing of the outlet from the left ventricle and obstructs blood flowing into the aorta.
Pulmonic stenosis is similar to aortic stenosis but affects the other side of the heart.
Innocent murmurs (not pathologic) are common in growing puppies. These sound different from PDA to the experienced examiner. Furthermore, these murmurs become softer and most always go away by the time of the rabies vaccine, which is usually given between four and 6 months of age.
There are also many causes of stunted growth and of shortness of breath. Your veterinarian should investigate these symptoms.