The ductus arteriosus is a foetal blood vessel derived from the sixth aortic arch on the left and connects the main pulmonary artery to the descending aorta. During foetal development it deviates blood so that it does not pass through the collapsed lungs. Within the first hours of extrauterine life, with the increase in oxygen tension, there is a functional closure of the duct, followed by its anatomical obliteration. Persistent patency of the ductus arteriosus for more than a few days after birth is a congenital cardiovascular disorder called “patent ductus arteriosus” (PDA).
PDA is the most common congenital heart defect in dogs (prevalence, 25-32%); there is gender-related predilection and a predisposition in some breeds. PDA is also found in cats, but with a lower prevalence (11%).
PATHOLOGY
The lack of closure of the ductus arteriosus in dogs is characterized by distinctive histological changes in the wall of the vessel itself. In puppies with PDA various parts of the wall of the ductus arteriosus are composed of elastic fibres instead of contractile smooth muscle fibres. According to Patterson et al., PDA is characterized by an “extension of the non-contractile parietal structure of the aorta to a larger segment of the ductus arteriosus, with progressive reduction of the capacity of this latter to undergo physiological closure”.
PATHOPHYSIOLOGY
In this condition blood is deviated from the descending aorta to the pulmonary artery, in both systole and diastole, because the pressure in the aorta is higher than that in the pulmonary artery throughout the whole cardiac cycle.
This shunting of blood in a left to right direction causes a volume overload in the pulmonary circulation, in the left atrium and in the left ventricle, inducing eccentric hypertrophy of the ventricle. The volume of the shunt is directly proportional to the difference in pressure between the two circulations and to the diameter of the ductus arteriosus. The progressive dilatation of the left ventricle distends the mitral annulus causing secondary regurgitation and a further volume overload of the ventricle. This severe volume overload can lead to left-sided congestive heart failure with pulmonary oedema, often within the first year of life.
DIAGNOSIS
Clinical characteristics
Signalment: PDA is more often seen in bitches, with frequently affected breeds including the Maltese, Pomeranian, Collie, Shetland Sheepdog, German Shepherd dog, English Springer Spaniel, Keeshund, Bichon Frisé, Miniature and Toy Poodle, Yorkshire Terrier, Chihuahua and Cavalier King Charles Spaniel. PDA seems to have a polygenic hereditary background.
History: Most young animals with PDA are asymptomatic or have only mild intolerance to exercise. The problems most commonly reported by owners of symptomatic animals with left to right shunting are effort-related fatigue, cough and tachypnoea due to the pulmonary oedema.
Clinical examination: The most obvious clinical finding associated with PDA is a characteristic, continuous murmur, which is most intense over the cranial part of the left hemithorax. The murmur is often accompanied by a continuous, precordial thrill and a wide, fast pulse, described as being like a “pneumatic hammer”.
The characteristic clinical signs make it easy to reach the diagnosis in most animals simply from the clinical examination. It should, however, be remembered that a combination of aortic stenosis /aortic regurgitation or of a defect of the interventricular septum/aortic regurgitation can give rise to a hyperkinetic pulse and a “to-and-fro” murmur which can be difficult to differentiate from the continuous murmurs of PDA.
Electrocardiography: Electrocardiography is not a useful examination for the purpose of diagnosing PDA. Delays in left intraventricular conduction and, sometimes in advanced cases, persistent atrial fibrillation and ventricular arrhythmias of various degrees may be seen.
Radiography: Chest X-rays in a dorso-ventral view typically show enlargement of the left atrium and ventricle, overload of the pulmonary circulation and an evident convexity of the descending aorta, pulmonary trunk and left auricle (between 12 o’clock and 3 o’clock).
Echocardiography: Echocardiography is the diagnostic examination that confirms the presence of PDA and can contribute to excluding concomitant cardiac defects. The characteristic echocardiographic findings in PDA are enlargement of the left atrium and ventricle, eccentric hypertrophy of the left ventricle, dilatation of the pulmonary trunk and the presence of the ductus arteriosus itself in the parasternal, short axis view. The characteristic picture of continuous turbulent blood flow in the pulmonary artery, demonstrated by Doppler imaging, is extremely useful for identifying the shunt (Figs. 1 and 2).
TREATMENT
The treatment of choice for PDA is surgery. Medical therapy is limited to those cases addressed to surgery late, already with signs of congestive heart failure. In these cases the treatment used is that for the heart failure (furosemide, ACE inhibitors, digoxin, positive inotropes). If haemodynamically significant arrhythmias, such as atrial fibrillation, are present, these must be controlled pharmacologically with digoxin. It should be realised that complete resolution of the clinical signs of congestive heart failure with medical therapy alone can be difficult.
SURGICAL TREATMENT
Surgical closure is the treatment of choice for PDA and should be performed as soon as possible after the diagnosis. There are two different surgical approaches: thoracotomy with ligature of the ductus arteriosus or mini-invasive closure of the vessel through the placement of an appropriate device. The treatment of PDA with left to right shunting in dogs younger than 2 years is associated with an excellent post-operative prognosis.
Ligation of a patent ductus arteriosus
Ligation of the ductus arteriosus requires a thoracotomy at the level of the fourth intercostal space on the left; the vessel can then be ligated with a suture thread. Surgical ligation is necessary in small animals with a large PDA in which it is not possible to introduce a sufficiently large catheter. The technique is relatively safe, with the most serious complication being iatrogenic rupture of the ductus arteriosus itself during the dissection. The risk of this complication decreases as the experience of the surgeon increases.
Mini-invasive closure of a patent ductus arteriosus
Closure of a PDA through a transcutaneous venous access is being ever increasingly used also in veterinary practice because of the minimal invasiveness of the procedure, its excellent outcome and the short time that must be spent in hospital after the intervention.
A new device for veterinary use has recently been developed: the Amplatzer Canine Ductal Occluder® (ACDO). The ACDO has the shape of a champagne cork and is conveyed to the ductus arteriosus through the femoral artery and ascending aorta by a catheter and positioned across the minimum diameter of the ductus by expansion (Figs. 3 and 4). For most cases of PDA closure, the ACDO has replaced coils, spiral-shaped devices with a thrombogenic effect. Given the shape, size and method of deployment, coils carry a high risk of pulmonary embolisation. Nowadays they are only indicated for small PDA with a diameter no larger than 4 mm.
The devices used to close a PDA are always deployed after angiography; the angiographic studies are used to classify the PDA on the basis of its morphology and to measure the minimal ductal diameter. Miller described a classification made on the basis of angiographic examination of a large population of dogs according to which four types of ductus arteriosus were identified (I, IIA, IIB and III). The type IIA anomaly was the most common, being found in 54% of cases. The correlation between angiographic and transoesophageal echocardiographic (TEE) determinations of the dimensions of the ductus arteriosus has been studied. In most cases, the size of the ductus arteriosus measured by TEE closely reflects that determined by angiography and TEE seems to be an excellent method for assisting the placement of the device, reducing the risks related to exposure to X-rays.
The choice of the most appropriate device to use is related to the size and morphology of the ductus arteriosus. For straight vessels with a tunnel morphology (type III), surgical closure via a thoracotomy is advised. Coils can be used for small ducts (< 4 mm). There was a recent report of the closure of 21 PDA with coils in toy dogs weighing less than 3 kg. In these cases, too, the minimum ductal diameter was less than 2.4 mm. In contrast, the ACDO is preferable for large ducts, even if the dimensions of the release catheter restrict the use of this occluder in small animals with limited arterial accesses.
PROGNOSIS
Dogs with untreated PDA usually develop progressive, left-sided congestive heart failure and pulmonary oedema. From data reported in the literature, it appears that 70% of dogs with untreated PDA die before 1 year of age.
REVERSE PATENT DUCTUS ARTERIOSUS
Reverse PDA is a very serious condition characterized by hypoxaemia and cyanosis. PDA with a right-to-left shunt can develop as a late sequel of an untreated, left-to-right PDA or, more commonly, is found in very young animals following pulmonary hypertension persisting after birth. The reversal of flow through PDA reduces the risk of left-sided heart failure but causes a debilitating, severe, systemic hypoxaemia, exercise intolerance and progressive compensatory polycythemia which gives rise to a hyperviscosity syndrome.
Animals with reverse PDA may be asymptomatic or show intolerance to exercise and collapse or incoordination of the hind limbs during physical activity. At clinical examination, the femoral pulse is normal and the murmur may no longer be audible because, following inversion of the pressures, the right-to-left systolic and diastolic pressure gradients are limited.
Typically there is a “differentiated” cyanosis, more evident in the caudal mucosae (penile, vaginal). The cyanosis is due to the mixture of non-oxygenated blood from the pulmonary artery and oxygenated, aortic blood. Chronic hypoxaemia is a stimulation to the overproduction of erythropoietin, which causes a compensatory polycythemia.
In the presence of a PDA with a right-to-left shunt, chest X-rays show signs of enlargement of both ventricles and marked distension of the pulmonary artery. The pulmonary arteries may also appear tortuous, which is an indicator of pulmonary hypertension.
It is not always possible to visualise the ductus arteriosus by echocardiography and in the absence of a large, high velocity shunt, as occurs in left-to-right PDA, Doppler echocardiography may not be easy. An air contrast study (“bubble study”) may be used to document the existence of a PDA. The bubble study is an echocardiogram with a contrast agent – air bubbles dispersed in physiological solution. Given their physical properties, after having been introduced into a peripheral vein, the air bubbles reach the right side of the heart and are not able to bypass the pulmonary circulation because they are trapped in the capillary bed and eliminated by the lungs. Thus, the observation of bubbles in the descending aorta and not in the left chambers of the heart is diagnostic because it implies that the bubbles have passed into the systemic circulation through a shunt.
The treatment of reverse PDA is symptomatic. Ligation of a completely reversed PDA is contraindicated because in this case the ductus arteriosus is acting as a one-way safety valve because of the high right-sided pressures and its closure would cause a further increase in pulmonary pressures.
Suggested readings
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- Buchanan JW. Patent ductus arteriosus. Semin Vet Med Surg (Small Anim) 1994;9:168–176.
- Buchanan JW. Radiographic aspects of patent ductus arteriosus before and after surgery. Acta Radiol Suppl 1972;19:271–278.
- Henrich E, Hildebrand N, Schneider C, Hassdenteufel E, Schneider M. Transvenous coil embolization of patent ductus arteriosus in small (<3 kg) dogs. J Vet Intern Med 2011; 25(1): 65-70.
- Saunders AB, Achen SE, Gordon SG, Miller MW. Utility of trasesofageal echocardiography for trascatheter occlusion of patent ductus arteriosus in dogs: influence on the decision-making process. J Vet Intern Med 2010; 24(6): 1407-13.
- SaundersAB, Miller MW, Gordon SG, Bahr A Echocardiographic and angiographic comparison of ductal dimensions in dogs with patent ductus arteriosus. J Vet Intern Med 2007; 21(1): 68-75.
- Glaus TM, Martin M, Boller M, Stafford Johnson M, Kutter A, Fluckinger M, Tofeig M. Catheter closure of patent ductus arteriosus in dogs: variation in ductal size requires different technique. J Vet Cardiol 2003; 5(1): 7-12.
- Miller MW, Gordon SG, Saunders AB, Arsenault WG, Meurs KM, Lehnkuhl LB, Bonagura JD, Fox PR. Angiographic classification of patent ductus arteriosus morphology in the dog. J Vet Cardiol 2006; 8(2): 109-14.
- Nguyenba TP, Tobias AH. Minimally invasive per-catheter patent ductus arteriosus occlusion in dogs using a prototype duct colluder. J Vet Intern Med 2008; 22(1): 129-34.
- Oyama M, Sisson DD, Thomas WP, Bonagura JD. Congenital heart disease. In: Ettinger SJ, Feldman EC, Textbook of veterinari internal medicine. 6th ed.Elsevier Saunders, St Luois Missouri 2000, pp: 978-87.