This defect is basically a hole in the heart. Depending on how big the hole is, the difficulties caused by the defect may be mild to life-threatening. Kittens with extreme cases often die before birth or shortly after. An atrial septal defect can lead to damage of the heart or heart failure. If a large hole is present, large volumes of blood will pass through to the left side of the heart, which strains the right side of the heart until it becomes irreversibly damaged. 

The heart of a cat is a small but muscular organ. It is hollow and made up of two sides and four compartments. The heart moves blood to the lungs for and oxygenation and then pumps the blood throughout the body. The upper chambers on each side of the heart are referred to as atria and the lower compartments are called ventricles. Blood collects in the atrial compartments and pumps out of the ventricles. The membranes that separate these four chambers are called septas. Before birth, an opening exists to allow blood to pass from the right atrium to the left, bypassing the lungs, as in utero kittens do not breathe air. After birth, this opening closes over. In cats with an atrial septal defect, a constant opening exists between the two upper sections of the heart, creating added pressure in the right side of the heart.

Kittens with very mild defects may not exhibit noticeable symptoms. In over 30 percent of atrial septal defect cases, other birth defects are also present. If symptoms are recognized, take the cat to a veterinary clinic or animal hospital immediately. Signs to watch for include:

Often, congenital heart defects are passed down genetically from the parents. Some instances occur due to events that happen to the mother while pregnant. All known causes are listed as follows:

An atrial septal defect may be found during the first routine examination of a kitten. While listening to the heart with a stethoscope, the veterinarian may notice a murmur or whooshing sound. This does not always confirm that a heart defect exists, however, the louder the murmur, the more likely it is that a defect is present. The earlier that this defect is identified, the better, as early treatment can prevent heart damage. If the cat is exhibiting symptoms, a complete physical examination will be performed, often with the same outcome. 

An electrocardiogram may be used to record the electrical activity within the heart. This can be useful in finding arrhythmias of the heart. A general x-ray of the chest can help reveal the overall condition of the heart and lungs. An echocardiogram (ultrasound of the heart) can help determine the size, function and blood flow of the heart. The cat will need to go through pre-anesthetic testing to ensure it can handle surgery if necessary. Full blood work may be run before a medication prescription is started to assess the health of the kidneys and liver. 

Small atrial septal defects may not need treatment at all, especially if they are asymptomatic. Appropriate treatment will depend on the extent of the defect present.

If the surgeon has adequate experience, they may suggest implanting a synthetic device to seal off the hole. Open heart surgery carries many risks and is not offered at the majority of veterinary clinics. 

Certain medications may be given to help with the secondary issues caused by atrial septal defects. Diuretics may be prescribed to reduce fluid accumulation in the abdomen or lungs. Angiotensin-converting enzyme (ACE) inhibitors may be prescribed to relax the blood vessels and lower pressure on the heart.

If the defect is mild, the cat is likely to live a long and normal life, however, regular veterinary assessments of the heart condition should be performed throughout the cat's life. If the defect is severe, the prognosis may be poor. The goal in this case will be to minimize all symptoms present and lengthen and enrich the cat's life as much as possible. If symptoms start to worsen, the cat should be brought back to the vet for further testing. 

In some cases, cats may recover well from surgery. Following all at-home care guidelines during recovery is essential for the success of the healing process. All activity may need to be limited while the cat recuperates from this major surgery. If the cat has been prescribed medications, it will need regular blood tests to ensure that the dosage is correct and that no adverse side effects have developed. In cats that have suffered heart damage, the outcome may be much worse.