What is the ductus arteriosus?
The ductus arteriosus is an arterial shunt between the aorta and the pulmonary artery, the two main blood vessels leading from the heart. It is normally present during fetal development.
What is its purpose?
Before birth, the developing fetus receives all of its oxygen needs through the placenta, and the developing lungs are deflated. As part of the fetal circulation, the ductus arteriosus allows the majority of circulating blood to bypass the lungs. Therefore, while the fetus is in the uterus, the ductus arteriosus is normally open, or “patent”.
When the newborn kitten takes its first breath, the ductus is stimulated to close down. Closure of the ductus arteriosus ensures that blood will circulate through the inflated lungs and become oxygenated.
What is patent ductus arteriosus?
Patent ductus arteriosus (PDA) is a heart defect that occurs when the ductus arteriosus fails to close down at birth.
What happens if the ductus arteriosus does not close at birth?
In the normal heart blood flows from the body into the right side of the heart, and then is pumped through the pulmonary artery into the lungs to be oxygenated. From the lungs it flows through the pulmonary vein into the left side of the heart. The left side of the heart then pumps the blood through the aorta, distributing oxygen-rich blood to the body. The muscles in the left side of the heart are normally thicker and stronger than those in the right side of the heart. The pressure within the left side of the heart and the aorta is higher than the pressure in the right side and the pulmonary artery and veins. This is because it is harder to pump blood into the body than it is to pump blood through the lungs.
If the ductus arteriosus fails to close properly after birth, the difference in pressure between the pulmonary artery and the aorta means that the blood will take the path of least resistance and flow from the aorta through the patent ductus arteriosus into the pulmonary artery. The pulmonary artery then needlessly recirculates this oxygenated blood back to the lungs. Since less blood is now being pumped into the main circulation through the aorta, the left side of the heart is forced to work harder to meet the demands of the body.
The size of the patent ductus arteriosus defect will determine how much harder the heart must work. If the defect is small, the left side of the heart becomes mildly enlarged (mild left ventricular hypertrophy). If the defect is medium-sized, the left side of the heart becomes moderately enlarged. As the heart enlarges, it is forced to work harder and harder and ultimately this increased work can lead to congestive heart failure.
In some cases when the PDA is large, a significant amount of blood will shunt into the lungs, forcing the pulmonary artery, the pulmonary vein, and the right side of the heart to become thicker (called right ventricular hypertrophy), in order to deal with the excess load of blood. This leads to pulmonary hypertension (elevated blood pressure in the lungs) and may ultimately cause the direction of the shunt to reverse.
What are the signs of a PDA?
In its early stages, a cat that has a small PDA defect may not show any obvious signs of disease at all. The larger the PDA is, the more blood will be shunted through it, causing more significant signs. These may include difficulty breathing, a loud heart murmur, abnormal pulses, and exercise intolerance. A kitten with a PDA may sleep more than usual, may be stunted in growth, or may be thinner than average.
"The larger the PDA is, the more blood will be shunted through it,
causing more significant signs."
As blood continues to shunt through the PDA, it will cause irreversible changes in the heart muscle and eventually may lead to congestive heart failure. It is reported that symptoms will develop in most cats with a PDA before the cat reaches one year of age. If the PDA is significant enough to cause congestive heart failure, symptoms may also include a heart arrhythmia (abnormal heart beats), increased lung sounds when listened to with a stethoscope, and accumulation of fluid in the lungs and/or abdomen.
When the shunt is very large, the pressure in the pulmonary circulation may continue to increase until it exceeds the pressure in the aorta. If this happens, the shunt may reverse, so that blood returning from the body shortcuts through the aorta before going through the lungs to receive oxygen. If the cat develops this “reverse PDA”, the heart murmur will disappear, and the signs will become severe and debilitating, including abnormal heartbeats, cyanosis (blueness) of the foot pads on the hind legs, collapse of the hind legs when the cat exercises, and weakness or lethargy.
How common is this heart defect in cats?
Although PDA is not that common in cats (approximately 1 in 5000 cats), it is reported to be the most common shunt in this species. Affected cats should not be bred, even if the condition is successfully treated.
How is PDA diagnosed?
A PDA will usually be diagnosed when your veterinarian hears a continuous heart murmur during a routine physical examination of your kitten. A continuous murmur means that the murmur is present continuously through the entire heart cycle. It is often described as a ‘machinery murmur’ or a ‘washing machine murmur’ because it sounds like water being agitated through the wash cycle. The murmur will be graded on a scale of either 1 to 4, or 1 to 6. If the murmur is pronounced, you may be able to feel a buzzing when you touch the kitten’s chest – this buzzing is called a thrill.
Your veterinarian will recommend radiographs (X-rays) of the chest to assess the heart and lungs and an electrocardiogram (ECG) to assess the heart rhythm. Blood tests will show whether other organs are being affected by the abnormal blood flow, or if there is evidence of abnormal red blood cell values.
Echocardiography, or cardiac ultrasound, will be needed to definitively diagnose a PDA. The ultrasonographer will examine a moving image of the heart to assess the degree of enlargement of its walls and the efficiency of its pumping ability. Doppler echocardiography, which evaluates the direction and speed of blood flow, will be used to help pinpoint the location and size of the shunt, and to determine the amount of turbulence associated with the shunt.
Could the signs be caused by something else?
Other complex heart defects can also cause a continuous heart murmur. Echocardiography will usually differentiate the type of defect, helping to determine the appropriate treatment options.
What is the treatment for a PDA?
The goal of treatment for a forward flowing PDA is to stop the blood flowing through the shunt. Your veterinarian will refer you to a veterinary cardiovascular surgeon, who will determine the optimal treatment for your cat. In some cases, the ductus will be tied off during heart surgery.
"The goal of treatment for a forward flowing PDA is to stop the
blood flowing through the shunt."
In other cases, a less invasive surgical treatment may be an option. The less invasive procedure involves occlusion (blocking) of the ductus arteriosus with either a transarterial coil or a ductal occluder device, which is placed by means of cardiac catheterization. These procedures, however, are more difficult to perform in cats, usually because of the cat’s small size.
Surgical repair should be performed as soon as possible. The longer the surgery is delayed, the more likely that irreversible heart damage will occur. If the cat is showing signs of heart failure, pre-surgical stabilization will be necessary.
There is no surgical treatment for a reverse PDA. Medical treatment will help manage the symptoms, but a cure is not possible for cats with a reverse PDA.
What is the success rate for surgical treatment of PDA?
Provided that the condition is treated before heart failure develops, the success rate associated with surgical closure is very high and the prognosis for a normal life after surgery is excellent.
If irreversible heart damage was already present before surgery, the cat may require heart medication in the future.
What is the success rate for medical management of a reverse PDA?
If signs of disease are able to be controlled with medications, the cat may live comfortably for several years.
Prompt and appropriate diagnosis and treatment of this congenital, hereditary cardiac disorder is associated with an excellent long-term prognosis. Both standard surgical and minimally invasive transcatheter techniques can successfully treat the condition.