The ductus arteriosis is a normal structure that is present in the canine fetal heart.
Its purpose is to divert blood from the pulmonary artery (the vessel that brings blood from the heart to the lungs for oxygenation) directly to the aorta (the vessel that delivers blood from the heart to the rest of the body). Therefore, the ductus arteriosis is present in the fetal heart to allow the majority of canine fetal blood to flow around rather than through the lungs. This is necessary because the canine fetus lives in the fluid environment of the mother’s uterus, receiving oxygen from its mother’s bloodstream. The ductus arteriosus normally closes at the time of birth when the young animal begins to breathe, oxygen is obtained by the lungs and normal circulation is established. The problem occurs when the ductus arteriosus does not close.
With the condition patent ductus arteriosus (PDA), the ductus arteriosus does not close, causing abnormal blood flow through the heart and lungs. The result is a connecting vessel that allows blood to travel in a circular fashion from the left side of heart through the lungs and immediately back to the left side of the heart. The heart must work much harder to maintain a normal amount of blood flow to the rest of the body. This extra workload eventually causes the heart to fail. The degree to which a patient is affected depends on the magnitude of the defect. This can range anywhere from a small blind pocket off the aorta which does not cause any problems, to varying degrees of abnormal blood flow through the ductus arteriosis between the aorta and the pulmonary artery. Most commonly, there is a shunt from the left to the right side of the heart, with blood from the higher pressure aorta continuously shunted to the pulmonary artery. This means an increased volume of blood is sent to the lungs which results in fluid build-up (pulmonary edema) and volume overload of the left side of the heart. In some cases, however, the increased flow of blood into the lungs injures the pulmonary blood vessels. This can reverse the path of blood flow from right to left. In this case, un-oxygenated blood flows into the aorta. PDA is the second most commonly diagnosed congenital heart defect of dogs. It affects about 7 out of every 1000 puppies. The condition is usually inherited as a genetic trait. This condition most commonly affects the Miniature Poodle, Collie, Maltese, Shetland sheepdog, German Shepherd, Cocker Spaniel, Pomeranian, Yorkshire Terrier and Labrador Retriever. Female dogs are predisposed.
Clinical symptoms of the disease include: coughing, reduced tolerance of exercise, loss of weight, and eventually, congestive heart failure. Affected puppies initially appear normal, although they are usually smaller and play less vigorously than their littermates. Typically, there are no clinical signs until congestive heart failure develops. This leads to fluid accumulation in the lungs that causes the previously described clinical symptoms. In most cases, clinical signs develop within a year. About 60% of affected dogs will die without surgical treatment.
Patent ductus arteriosus is diagnosed by history, auscultation of a “machinary” heart murmur on physical examination, cardiac enlargement and pulmonary edema on chest x-ray, and visualization of the defect with cardiac ultrasound (echocardiagram). The majority of cases are first diagnosed upon the initial visit to a veterinarian when the characteristic heart murmur is detected on routine physical examination.
When caught early and following treatment with successful closure of the PDA, most dogs live a normal life. Unless there are complications from other heart defects or heart failure has already developed, there is rarely any future need for medication. While special circumstances can influence the prognosis, most cases are straightforward.
The conventional treatment is surgery, which should be performed shortly after the diagnosis is confirmed. Dogs as young as 8 weeks are considered surgical candidates, and it is recommended to carry out the procedure when the dog is between 8 and 16 weeks of age. There is no benefit to delaying surgery. In fact, the chances of a dog developing heart failure or suffering irreversible damage to the heart muscle only increase with time. Anesthetic and surgical risks become greater as the heart fails and the heart and lungs become irreversibly damaged. Medical therapy may be necessary prior to and immediately after surgery if significant clinical symptoms are present. Dogs with a right to left shunting PDA are to be treated medically as surgery cannot be successfully performed in these pets. The polycythemia caused by right to left shunting is treated periodically by phlebotomy, which is removing blood to control the red blood count and viscosity of the blood. When surgery is not an option, and heart failure has occurred, drug therapy with furosemide, enalapril or benazepril, and digoxin is often prescribed. A salt-restricted diet is enforced. Aspirin, indomethacin, and other prostaglandin inhibitors sometimes used to close the PDA in premature human babies do not work in dogs and should not be given to close the ductus because the canine ductus lacks the smooth muscle capable of responding to these drug therapies.
Historically, surgical ligation has been the standard method of correction. Surgery consists of performing a thoracotomy on the left side of the chest through the third or fourth intercostal space to gain access to the surgical site. The standard technique has been to dissect immediately cranial and caudal to the ductus and then carefully create a passage on the medial aspect of the ductus by blind dissection with right-angle forceps and tying off the patent ductus. Operative success should be greater than 90 percent, even in the smallest dogs, and the prognosis is excellent for a normal life if surgery is completed early. Although highly successful, surgical ligation is associated with some operative morbidity and mortality.
The least invasive treatment available is to feed a coil via a catheterized large vessel into the patent ductus arteriosis to block the flow of blood through it. However, depending on the size of the patient, anatomical and other factors, not every case is a candidate for this procedure. In these cases, open heart surgery to ligate the patent ductus ateriosus closed is necessary. If the patient is reasonably stable immediately prior to treatment, prognosis tends to be fair to good with treatment.
While traditional surgery to close the PDA has a very high success rate, more recently, thorascopic PDA occlusion using Titanium ligating clips or a custom-designed thoracoscopy clip applicator has been described. Although technically demanding, minimally invasive PDA occlusion via thorascopy is a safe and reliable technique in dogs; however, preoperative measurement of the diameter of the PDA is crucial to determine if complete closure with metal clips can be achieved. Assuming the diameter of the PDA is amenable to clip ligation, minimally invasive thorascopic PDA occlusion can be considered as an alternative to occlusion via conventional thoracotomy.
More recently, minimally invasive transcatheter techniques have been employed for PDA occlusion. Transarterial PDA coil embolization is a safe, less invasive alternative for PDA occlusion. This procedure involves catheterization of the femoral artery under general anesthesia. An angiogram is then performed to delineate PDA morphology and facilitate coil selection. Coils are commercially available and composed of surgical stainless steel with prothrombotic poly-Dacron fibers. Coil occlusion has been widely accepted as a relatively safe and effective treatment for PDA in dogs, although careful patient selection is helpful in achieving a high success rate. Important patient factors that affect successful coil occlusion include the size and morphology of the ductus and the patient’s body weight. The standard arterial approach for PDA coil embolization requires placement of a 4-French sheath introducer into the femoral artery. The estimated minimal patient body weight needed to safely introduce this device is ∼2.5 kg, although there is variability associated with operator experience as well as the breed and age of the patient. Coils are advanced through a catheter into the PDA under fluoroscopic guidance until satisfactory angiographic occlusion is documented. Patients are then recovered and released the following day. This procedure requires substantial technical expertise and specialized equipment.
Many devices are available to close a PDA, and in each case, the device that is used is determined by the size of the PDA. For narrow PDAs, a vascular coil, as described above, can be used to close the vessel. Larger PDAs can be closed by canine ductal occluders (CDO) or vascular plugs. To place any of these devices, a small incision is made allowing entry into the femoral artery and then the device is threaded through the artery to close the PDA. The Amplatz canine duct occluder (ACDO) is a nitinol mesh device with a short waist that separates a flat distal disc from a cupped proximal disc. The device is designed to conform to the morphology of the canine patent ductus arteriosus. PDA dimensions are determined by angiography, and a guiding catheter is advanced into the main pulmonary artery via the aorta and PDA. An ACDO with a waist diameter approximately twice the angiographic minimal ductal diameter (MDD) is advanced via the catheter using an attached delivery cable until the flat distal disc deploys within the main pulmonary artery. The partially deployed ACDO, guiding catheter, and delivery cable are retracted until the distal disc engages the pulmonic ostium of the PDA. With the delivery cable stabilized, the catheter is retracted to deploy the waist across the pulmonic ostium and cupped proximal disc within the ductal ampulla. Tension on the delivery cable is released, and correct ACDO positioning and stability are confirmed by observing that the device assumes its native shape, back-and-forth maneuvering of the delivery cable, and a small contrast injection made through the guiding catheter. The delivery cable is detached and removed with the guiding catheter. To assess for any residual ductal flow, an angiogram is performed at the conclusion of the procedure, followed by Doppler echocardiography. PDA occlusion in dogs with the ACDO is straightforward and extremely effective across a wide range of body weights, MDDs, and ductal morphologies.
In conclusion, 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 be utilized at the surgeon’s discretion to achieve success.