Patent Ductus Arteriosus

Matthew W. Miller, College Station, Texas

Ashley B. Saunders, College Station, Texas

Patent ductus arteriosus (PDA) is the persistence of a fetal communication between the descending aorta and the main pulmonary artery. Depending on the study, PDA is considered the most or second most common congenital cardiac malformation diagnosed in dogs (see Chapter 174). There is a higher incidence in females, with an odds ratio of approximately 3 : 1, although this is not evident in all breeds. Numerous breeds appear predisposed, including the bichon frise, Chihuahua, cocker spaniel, collie, English springer spaniel, German shepherd, Labrador retriever, Maltese, Pomeranian, poodle, Shetland sheepdog, Welsh corgi, and Yorkshire terrier. Without correction, the estimated 1-year mortality for PDA approaches 60%. Although subaortic stenosis is the other congenital malformation reported as the most common in many case series, many diagnosed cases of subaortic stenosis are of trivial hemodynamic significance, requiring no intervention and having little effect on an individual patient’s quality or duration of life. Therefore PDA is likely the most commonly diagnosed defect that routinely requires intervention.

Diagnosis

The initial diagnosis of PDA still depends on careful physical examination and detection of the classic continuous murmur. In animals with very rapid heart rates or elevations in pulmonary artery pressure, the diastolic component of the murmur may not be appreciated if careful auscultation is not performed. Ensuring that the stethoscope is placed high in the left axillary region over the main pulmonary artery increases the likelihood of an accurate initial diagnosis. Large volumetric left-to-right shunts frequently result in femoral pulses that are hyperdynamic or bounding.

Practicing veterinarians must be able to recognize the murmur of PDA so that appropriate diagnostic tests and treatments can be offered. In addition to concentrating on the dorsal left axillary region during auscultation, slowly moving the stethoscope chest piece between the point of maximal murmur intensity and the left apex often allows the examiner to appreciate the continuous nature of the murmur at the base. In some puppies and kittens the continuous murmur may be equally loud or even more prominent immediately cranial to the left edge of the manubrium. In very young puppies or kittens and in cats with developing pulmonary hypertension the murmur may be very faint to absent at end diastole.

In a very small percentage of cases, shunt reversal (right-to-left PDA) may occur secondary to severe pulmonary hypertension, a condition that is not amenable to closure in any manner. Dogs tend to develop pulmonary hypertension early in life and relatively quickly from a large PDA, but the development of pulmonary hypertension in cats typically is more gradual. This allows some cats to benefit from closure of the ductus, despite the presence of pulmonary vascular disease. It may be possible to close a PDA successfully when there is left-to-right shunting even when pulmonary hypertension is present (Seibert et al, 2010), and this judgment is best made by considering not only pulmonary artery pressure but calculated pulmonary to systemic vascular resistance. This generally requires cardiac catheterization.

Diagnostic imaging allows for characterization of the type and severity of the defect. Classic radiographic findings include left ventricular and atrial dilation with pulmonary overcirculation, with venous prominence often evident. The cardiac silhouette may be especially elongated on the ventrodorsal or dorsoventral projection. It is also common to appreciate dilation of both the main pulmonary artery and descending aorta. Pulmonary edema is present with the onset of congestive heart failure.

A complete Doppler echocardiographic study is the test of choice for definitive diagnosis. Echocardiographic features of PDA include dilation of the left atrium, left ventricle, ascending aorta, and pulmonary artery. A portion of the PDA itself frequently can be visualized from several imaging planes, but the left cranial short-axis view usually provides the optimal image. Saunders and colleagues (2007) described the transthoracic and transesophageal echocardiographic (TEE) appearance of PDA as it relates to angiographic anatomy, demonstrating the superiority of TEE for critical visualization of ductal anatomy in the dog. Doppler echocardiography confirms continuous flow in the main pulmonary artery with reversal of flow in diastole. Mild to moderate mitral regurgitation secondary to annular dilation is a frequent finding. Depending on its severity, mitral regurgitation usually resolves following ductal ligation or occlusion. Evidence of systolic dysfunction (increased left ventricular internal diameter during systole, reduced percent fractional shortening) is common in dogs with large volumetric shunts and typically persists or appears worse following successful ligation or occlusion. Nevertheless, this finding rarely translates into long-term morbidity or mortality unless ventricular function is severely depressed or atrial fibrillation or congestive heart failure is evident.

Transaortic flow velocities commonly are accelerated and may be as high as 3.75 m/sec with large shunts (normal velocity, <2.5 m/sec). This may lead to an erroneous diagnosis of concurrent subaortic stenosis. Therefore caution should be used when interpreting transaortic flows in animals with large shunts. Aortic velocities normalize after ductal occlusion or ligation.

Most animals are in sinus rhythm when the diagnosis is established. Although the electrocardiogram (ECG) often demonstrates criteria indicating left-sided heart enlargement (widened P waves, increased R-wave amplitude), when the rhythm is normal an ECG contributes little to the overall diagnosis. An arrhythmia is the principal indication for recording an ECG in this disease. Atrial fibrillation may develop with long-standing PDA, especially in larger-breed dogs; is the most common arrhythmia seen with PDA; and generally is associated with a more unfavorable prognosis.

Concurrent cardiac defects are relatively uncommon but may be more likely in large-breed dogs. In the authors’ canine population, approximately 9% of dogs with PDA have an additional congenital malformation, the most common of which is pulmonic stenosis.

Treatment

Most dogs and virtually all cats with a PDA with left-to-right shunting will benefit from closure of the ductus. The outcomes with PDA closure are likely to be best when experienced operators perform the procedure, and this should be discussed with clients. Furthermore, closure of a ductus never should be delayed until a dog is “mature” or “develops symptoms” since irreversible myocardial damage, congestive heart failure, atrial fibrillation, and death are likely outcomes. Currently there are two options: surgical ligation and catheter-based closure. The latter approach represents the most important advance in PDA management over the past 15 years. Surgical ligation of the PDA long has been considered the standard of care for PDAs with left-to-right shunting and is a highly successful procedure with acceptable mortality (<5%) in experienced hands. However, in 1995 Snaps and colleagues published the first clinical report describing the use of vascular occlusion coils for treatment of PDA in a dog. Since that time, several devices have been evaluated for ductal occlusion in dogs.

Device Occlusion of Patent Ductus Arteriosus

Following the initial report by Snaps and colleagues, numerous reports have detailed the use of either free-release or detachable occlusion coils. In addition, there have been two reports of the use of the human Amplatzer ductal occluder (Sisson, 2003). The high cost of this device and the requirement for transvenous delivery precludes its routine use in veterinary medicine. The Amplatzer vascular plug and now the Amplatz canine duct occluder have been evaluated in dogs (Hogan et al, 2006; Nguyenba and Tobias, 2007). Generally these are deployed using femoral artery catheterization. Most dogs can be treated successfully with one of these two devices (or with coils) provided the operator is experienced and the catheterization laboratory carries a sufficient inventory of devices. Inappropriate ductal morphology or small size most commonly precludes use of catheter-based techniques. Lower-profile devices have been developed, but the majority of centers tend to recommend surgical correction in dogs weighing less than approximately 3 kg.

All catheter-delivered devices are deployed under general anesthesia using fluoroscopic guidance following angiographic evaluation of ductal morphology. Web Figure 64-1 provides lateral radiographs showing the three most commonly used occlusion devices deployed in canine clinical patients.