Patent ductus arteriosus

What it is

In the fetus the ductus arteriosus connects the pulmonary trunk to the descending aorta and carries blood past the non-ventilated lungs. It normally closes within the first days after birth and becomes the ligamentum arteriosum. When the ductal wall lacks enough smooth muscle, it stays open. Because aortic pressure exceeds pulmonary pressure, blood flows continuously from the aorta into the pulmonary artery (left-to-right shunt), recirculates through the lungs and returns to the left atrium and ventricle, which dilate from volume overload. A small minority develop severe pulmonary hypertension and the flow reverses (right-to-left or "reversed" PDA).

Who it affects

It is present from birth and is usually detected at the first puppy examinations. Predisposed breeds include the miniature and toy poodle, Pomeranian, German shepherd, collie, Shetland sheepdog, Maltese, bichon frise, Yorkshire terrier and cocker spaniel; females are affected more often. A hereditary basis has been shown in poodles.

Anatomy involved

• Ductus arteriosus: runs from the descending aorta, just caudal to the origin of the left subclavian artery, to the pulmonary trunk near its bifurcation, on the left side of the base of the heart. The left vagus nerve crosses over it, and the left recurrent laryngeal nerve loops around it; both are landmarks during ligation.
• Aorta and pulmonary trunk: the descending aorta and the main pulmonary artery are enlarged at the ductus.
• Left atrium and ventricle: dilation from volume overload; secondary mitral regurgitation is common.
• Lungs: pulmonary overcirculation and, when the heart fails, pulmonary oedema.

Clinical signs

A continuous ("machinery") murmur, loudest at the left heart base, often with a palpable thrill and hyperkinetic (bounding) femoral pulses. Many puppies show no signs at first; without treatment, left-sided congestive heart failure develops in a large proportion of dogs, often within the first year. Reversed PDA instead causes differential cyanosis (caudal mucous membranes cyanotic, cranial ones pink), pelvic limb weakness on exercise and polycythaemia, without the continuous murmur.

Diagnosis

• Auscultation: continuous left basal murmur in a young dog is highly suggestive.
• Thoracic radiographs: on the dorsoventral view, three bulges between 1 and 3 o'clock (descending aorta, pulmonary trunk, left auricle) — the "three knuckles" sign — with left-sided cardiomegaly and pulmonary overcirculation.
• Echocardiography with Doppler: shows the ductus and continuous turbulent flow into the pulmonary artery, measures the ductus for occluder sizing and detects reversal.
• Angiography: during catheter-based closure.

Treatment

In left-to-right PDA, closure is recommended as soon as possible: surgical ligation through a left 4th intercostal thoracotomy, or minimally invasive transarterial occlusion with a device (canine ductal occluder) or coils. After closure most dogs have a normal life expectancy. Reversed PDA must not be closed, because the ductus acts as a relief valve for the pulmonary circulation; it is managed with periodic phlebotomy or hydroxyurea for polycythaemia and with sildenafil for pulmonary hypertension.

In the cat

In the cat patent ductus arteriosus is less common than in the dog; ventricular septal defect and atrioventricular valve dysplasias predominate among feline congenital heart defects, and PDA can coexist with other defects. The clinical picture is similar but often less typical: many cats have no signs when the defect is found, and the murmur at the left heart base is continuous in only about half of cats (systolic in the rest). There may be bounding femoral pulses and, without treatment, left-sided heart failure, which in the cat shows as tachypnoea, dyspnoea from pulmonary oedema or pleural effusion and stunted growth. Doppler echocardiography confirms the diagnosis. Closure is done mainly by surgical ligation through a left thoracotomy, because the small size of feline vessels limits the use of occlusion devices designed for dogs; coils have been used in some cats. The prognosis after closure is good. Pulmonary hypertension and other cardiac defects are common in cats with PDA, and reversed PDA, although less frequent, is not exceptional; it must not be closed either.

Key studies in the cat

• Schrope (2015), records of 57,025 mixed-breed cats: the prevalence of congenital heart disease was 0.14%. Ventricular septal defect was the most common, followed by aortic stenosis and obstructive hypertrophic cardiomyopathy.
• Tidholm et al. (2015), 162 cats with congenital heart defects at two referral centres in Sweden. Patent ductus arteriosus accounted for 3% of cats, far behind ventricular septal defect (50%).
• Bascuñán et al. (2017), retrospective study of 50 cats from five referral hospitals, the largest series published. Median age at presentation was 6 months, with no sex difference, and 70.2% had no signs. The murmur was continuous in only 55% and systolic in the rest; 17.5% had right-to-left shunting, 54.5% had other cardiac anomalies and 45.7% had pulmonary hypertension. Closure was performed in 68%, with complications in 14.7% (one intraoperative death), and the long-term outcome after closure was favourable. Because it is retrospective and from referral centres, it may overrepresent complex cases.