Nancy Laste, DVM, DACVIM (Cardiology)
This is the third article in a series of three. You can find the first article here and the second article here.
The ductus arteriosus is a normal fetal blood vessel running between the aorta and the main pulmonary artery. Oxygenated blood flows through the pulmonary artery (high pulmonary vasculature resistance in the fetus and is directed through the ductus arteriosus into the ascending aorta and out to the body (bypassing the lungs, which are non-functional in the fetus). “Right to left” flow is the normal flow pattern in the fetus. At birth, when the neonate takes their first breathe and expand the lungs, the tissue of the ductus arteriosus should respond to the increase in the oxygen tension with a vasoconstrictive response, resulting in closure of the ductus arteriosus in the minutes to hours after birth. If the ductus arteriosus fails to close (remains “patent”) than a patent ductus arteriosus (PDA) is present with blood now flowing from the high pressure arterial side to the pulmonary artery: “left to right” flow. The continuous flow of blood through the ductus arteriosus through all phases of the cardiac cycle give the PDA its typical “machinery”, continuous heart murmur that is best ausculted cranial and high on the left hemithorax (in the left axillary region). A palpable thrill is often appreciated. The hemodynamic consequences of the PDA depend on the overall size of the PDA and how much blood is being shunted left to right through the PDA (shunt fraction, Qp/Qs). A PDA is the most common cause of left-sided congestive heart failure in a neonate. Pets with a PDA generally have strong, hyperkinetic femoral pulses. The majority of patients with an uncorrected PDA will develop congestive heart failure (pulmonary edema) within the first few years of life. Occasionally, a PDA is diagnosed in a much older patient. In these cases, the shunt is usually relatively small and the condition may be incidentally diagnosed when the patient develops another cardiac condition, most typically chronic degenerative valvular disease.
Figure 1: Arteriography: Patient in right lateral recumbency. Pigtail measuring catheter in place in the ascending aorta, contrast noted in the ascending aorta (AA), PDA, dilated main pulmonary artery (MPA) and descending aorta. The minimal ductal diameter has been measured on the (labeled) image to the right at approximately 2.3 mm.
PDA is one of the more common causes of congenital heart disease in the dog (ranking in the top three causes, alongside pulmonic or Subaortic stenosis). In feline patients, PDA is approximated as the sixth most common congenital heart disease.
Figure 2: ACDO in place at the pulmonic ostium of the ductus arteriosus.
A small subset of patients may have (or develop) a “reverse PDA” which is the term for a right to left shunting PDA which develops in response to severe pulmonary hypertension. It is generally thought that some patients with PDA are genetically predisposed to develop significant PH related to the PDA flow. Corgi dogs are the most commonly described breed with right to left PDA, suggesting a genetic predisposition in this breed. Reverse PDA is one type of Eisenmenger syndrome where blood flows right to left secondary to the development of pulmonary hypertension related to a cardiac shunt. Patients with reverse PDA (that do not have a history of a left to right PDA) can be difficult to diagnose without maintaining an index of suspicion for a shunt. These patients most typically present for syncope, exercise intolerance and/or high leg weakness. On physical examination there is either no cardiac murmur or only a mild cardiac murmur or split second heart sound (typical with PH). The cranial mucous membrane (oral) are pink while the caudal (vulvar, penile) mucous membranes are cyanotic (although this can be difficult to appreciate). Echocardiography will reveal changes consistent with significant pulmonary hypertension: right ventricular hypertrophy, pulmonary artery enlargement and often a small left ventricular chamber. Definitive diagnosis can be made with an echocardiographic contrast study (Bubble study). Agitated saline is administered through a cephalic catheter while visualizing the four chambers of the heart on echocardiography. The microbubbles in the saline will opacify the heart but no bubbles will be seen crossing at the atrial or ventricular levels. If the study is negative at the level of the heart, an additional injection is given while visualizing the abdominal aorta (usually imaged adjacent to the kidneys). If a right to left shunting PDA is present, microbubbles will be seen in the aorta. This bubble study result: negative at the cardiac level, positive at the abdominal level is diagnostic for a right to left PDA. Unfortunately other than managing the pulmonary hypertension (with Viagra) and managing any associated polycythemia (with phlebotomy and/or hydroxyurea) there is no surgical or interventional procedure that will help a patient with reverse PDA. Luckily, most PDA will be identified and corrected long before they develop a reverse shunt.
While you can be fairly confident of your diagnosis of a PDA on physical examination, it is important to refer these patients to a cardiologist to further delineate the morphology and size of the PDA to determine the possible treatment plans. Dogs who are at least 4 kg and have a ductal morphology that is tapering and with an appropriate minimal ductal diameter will be candidates for ACDO closure in the cardiac catheterization lab. The ACDO sizes run from 3-14 mm with an appropriate sizing ratios of 1.5 -2.5 times the minimum ductal diameter as measured on angiography (and estimated on echocardiography) allows for closure of minimal ductal diameters of 1.5 to 9.0 mm. It is also important that the ductus have a tapering shape (not a straight tube) so as to retain the device. With smaller patients it is important to consider the relative size of the ductus related to the size of the dog (and the dog’s femoral arteries) as in small dogs with a relatively large ductal diameter, the femoral arterial size may be the limiting factor. Dogs with a small ductal diameter (under 3.0 mm) may also have the PDA closed using embolization coils. These tend to remain in place and work well for successful embolization in small ducts and required a much smaller device for coil placement. Em
Figure 3: Post ACDO placement angiography. Contrast fills the PDA but no longer crosses the ductus into the pulmonary artery.
bolization coils are more likely to embolize through the PDA into the pulmonary arteries. Luckily, coils embolized to the pulmonary arteries seem to be reasonably well tolerated as they can be very difficult to retrieve. ACDO are much less likely to embolize, provided they have been sized and placed appropriately. While the ACDO devices (Inifiniti medical) are not inexpensive ($875 with placement sheaths running in the $200 range), the shorter procedural time means less time under anesthesia (less expensive) as well as reduced intensity of post-operative care (with significant decrease in patient mortality) and short hospitalization (patients are discharged the day after the procedure).
PDA closure in the cardiac catheterization lab usually results in a total bill of approximately $2,800-3,500, while a surgical PDA closure ends up closer to $4,000-5,000 depending on the intensity of post-operative care. Closure of PDA via thoracotomy is generally reserved for very small patients, patients with a non-tapering ductus, patients with a very large ductus or small patients with a proportionately large ductus arteriosus. Although most times surgery goes very well, acute hemorrhage related to a tear in the ductus arteriosus of the junction with the main pulmonary artery may be fatal. Most reports suggest an incidence of 5-10% of fatal hemorrhage with the procedure. Although device closure is less invasive, it is not without potential serious complication such as vascular perforation (with hemorrhage), embolization of the ACDO or coils (may necessitate subsequent surgical closure in some cases), device dislodgement or fatal arrhythmia.
Patients who have the PDA closed without any residual PDA flow are thought to have an excellent prognosis. However, the typical left ventricular remodeling: dilation and sphericity of the LV with some decrease in wall motion, tends to persist. When some of these patients develop degenerative valvular disease later in life, they may not tolerate the subsequent volume load and develop CHF at what seems to be an earlier point in the disease process. Patients who had congestive heart failure at the time of surgery, can often be weaned off some or all of the medications (diuretics, ace-inhibitors, pimobendan, sometimes digitalis glycosides).
Figure 4: Smaller canine patient with two embolization coils in the ductus arteriosus.
Cats with PDA have been closed both by thoracotomy as well as coil embolization, depending on the size of the cat and the size and morphology of the ductus arteriosus. The ACDO devices require larger femoral artery access than is typically available in the feline patient and has not been successfully done, to the author’s knowledge.
For more information, please contact Angell’s Cardiology Service at 617-541-5038 or email@example.com.
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