Theresa W. Fossum, DVM, MS, PhD, DACVS
Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station, TX, USA
Patent Ductus Arteriosus
The ductus arteriosus is a fetal vessel that connects the main pulmonary artery and descending aorta. During development it shunts blood away from the collapsed fetal lungs. Normally it closes shortly after birth during the transition from fetal to extrauterine life. Continued patency of the ductus arteriosus for more than a few days after birth is termed "patent ductus arteriosus." PDA is the most common congenital heart defect of dogs; it also occurs in cats. PDA causes a left-to-right shunt that results in volume overload of the left ventricle and produces left ventricular dilation and hypertrophy. Progressive left ventricular dilation distends the mitral valve annulus causing secondary regurgitation and additional ventricular overload. This severe volume overload leads to left-sided congestive heart failure and pulmonary edema, usually within the first year of life. Atrial fibrillation may occur as a late sequela due to marked left atrial dilation.
Rarely, dogs with PDA develop suprasystemic pulmonary hypertension that reverses the direction of flow through the shunt causing severe hypoxemia and cyanosis (Eisenmenger's physiology). Right-to-left PDA can occur as a late sequela to untreated PDA. When right-to-left PDA is noted in very young animals it may be due to persistent pulmonary hypertension after birth. Reversal of PDA lessens the risk for developing progressive left-sided heart failure, but causes severe debilitating systemic hypoxemia, exercise intolerance, and progressive polycythemia.
PDA is seen more commonly in purebred, female dogs. Maltese, Pomeranians, Shetland sheepdogs, English springer Spaniels, Keeshonds, Bichon frise, Miniature and Toy poodles, and Yorkshire terriers are at increased risk to develop PDA. A genetic basis has been established in poodles.
Most young animals with PDA are asymptomatic or have only mild exercise intolerance. The most common complaint in symptomatic animals with left-to-right shunts are cough or shortness of breath (or both) due to pulmonary edema. Animals with right-to-left or reverse PDA may be asymptomatic or have exercise intolerance and hind limb collapse on exercise.
Physical Examination Findings
The most prominent physical finding associated with PDA is a characteristic continuous (machinery) murmur heard best at the left heart base. The left apical cardiac impulse is prominent and displaced caudally and a palpable cardiac a "thrill" often is present. Femoral pulses are strong or hyperkinetic (water hammer pulse) due to a wide pulse pressure caused by diastolic runoff of blood through the ductus. Tall R waves (> 2.5 mV) or wide P waves on a lead II electrocardiogram are supportive of the diagnosis, but not always present. Atrial fibrillation or ventricular ectopy may be present in advanced cases.
The physical examination findings in animals with right-to-left or reverse PDA differ from those with left-to-right shunts. "Differential" cyanosis is typically present (i.e., cyanosis is most apparent in the caudal mucous membranes), but cyanosis may also be noted in the cranial half of the body in some animals.
Surgical correction of PDA is accomplished by ligation of the ductus arteriosus. Ligation of PDA is considered curative and should be performed as soon as possible after diagnosis. Secondary mitral regurgitation usually regresses after surgery due to reduction in left ventricular dilation. Inadvertent ductal rupture during dissection is the most serious complication associated with PDA repair. The risk of this complication decreases as the surgeon's experience increases.
Small ruptures, especially those on the back side of the ductus, often respond to gentle tamponade, but will enlarge and worsen if dissection is continued. Large ruptures must be controlled immediately with vascular clamps and then repaired with pledget-buttressed mattress sutures. Once bleeding is controlled, a decision must be made whether to continue surgery, or to abandon surgery in favor of repair at a later time. Second surgeries are more difficult due to adhesions at the surgical site, so complete occlusion should be attempted during the initial procedure, if possible. Often, simple ductal ligation is not possible after a rupture has occurred. In such instances, surgical alternatives include ductal closure with pledget-buttressed mattress sutures or ductal division and closure between vascular clamps. The divided ductal ends are closed with a continuous mattress suture oversewn with a simple continuous pattern. Ductal closure without division is safer than surgical division, but re-cannulation of the ductus may occur. Because ductal division requires added technical expertise, it should be undertaken only by experienced surgeons.
The ductus arteriosus in dogs and cats is usually wide (- 1 cm), but relatively short (< 1 cm). It is located between the aorta and main pulmonary arteries, caudal to the origin of the brachycephalic and left subclavian arteries. As a result, most mixing of oxygenated and non-oxygenated blood occurs in the descending aorta in dogs with reverse PDA. Thus, normally oxygenated blood is supplied to the head and neck, while desaturated blood is presented to the caudal half of the body. The left vagus nerve always passes over the ducts arteriosus and must be identified and retracted during dissection. The left recurrent laryngeal nerve can often be identified as it loops around the ductus.
Perform a left 4th space intercostal thoracotomy. Identify the left vagus nerve as it courses over the ductus arteriosus and isolate it using sharp dissection at the level of the ductus. Place a suture around the nerve and gently retract it. Isolate the ductus arteriosus by bluntly dissecting around it without opening the pericardial sac. Pass a right-angle forceps behind the ductus, parallel to its transverse plane, to isolate the caudal aspect of the ductus. Then, dissect the cranial aspect of the ductus by angling the forceps caudally approximately 45 degrees. Complete dissection of the ductus by passing forceps from medial to the ductus in a caudal to cranial direction. Grasp the suture with right-angle forceps. Slowly pull the suture beneath the ductus. If the suture does not slide easily around the ductus, do not force it. Regrasp the suture and repeat the process, being careful not to include surrounding soft tissues in the forceps. Pass a second suture using the same maneuver. Alternatively, the suture may be passed as a double loop and the suture cut so that you have 2 strands. Slowly tighten the suture closest to the aorta first. Then, tighten the remaining suture.
Vascular Ring Anomalies
Vascular ring anomalies are congenital malformations of the great vessels and their branches that cause constriction of the esophagus and signs of esophageal obstruction.
The most common type of vascular ring anomaly is a persistent fourth right aortic arch, right dorsal aortic root, and rudimentary left ligamentum arteriosum (left sixth arch). The left pulmonary artery and the descending aorta are connected by the ligamentum arteriosum. The esophagus is encircled by the ligamentum arteriosum (or patent ductus arteriosus) on the left, the base of the heart and pulmonary artery ventrally, and the aortic arch on the right. The esophagus is constricted by this vascular "ring" and begins to dilate cranially as food accumulates. Food not passing beyond the constriction is intermittently regurgitated. Chronic regurgitation predisposes to aspiration pneumonia. Approximately 95% of those diagnosed with vascular ring anomalies will have a persistent right aortic arch (PRAA). Persistent left vena cava occurs in conjunction with PRAA in about 40% of the cases.
Thoracic radiographs may reveal a dilated esophagus cranial to the heart containing air, water, or food. The trachea may be displaced ventrally and the esophagus may overlap it. Signs of pneumonia may be identified. Positive contrast radiography using a barium suspension or barium with food will demonstrate esophageal constriction at the base of the heart with varying degrees of esophageal dilatation extending cranially. The caudal esophagus is usually a normal size, although sometimes it is dilated. Fluoroscopy is beneficial in evaluating esophageal motility. The dilated esophagus does not usually demonstrate normal peristaltic contractions. Although not routinely performed, angiography is beneficial in preoperatively identifying the type of vascular ring anomaly and other cardiac anomalies. Echocardiography may also be beneficial. Endoscopic examination of the esophagus helps rule out other causes of esophageal stricture or obstruction and may reveal esophageal ulceration.
Surgical treatment of PRAA is described below. Other types of vascular ring anomalies can be managed in a similar fashion. A persistent left vena cava often covers the left ventral area of the vascular ring. A persistent right ligamentum arteriosum and some aberrant right subclavians should be approached from the right side. Angiograms are helpful in patients with double aortic arches to determine which arch is dominant and if adequate circulation can be maintained after transection of the other arch.
Surgical transection of the constricting structure(s) is recommended before esophageal dilatation becomes severe. Transection is feasible with most vascular ring anomalies with the exception of some double aortic arches. Perform a lateral thoracotomy at the left fourth (fifth) intercostal space for patients with PRAA. Pack the cranial lung caudally to expose the mediastinum dorsal to the heart. Identify the aorta, pulmonary artery, ligamentum arteriosum, vagus, and phrenic nerves. Identify the anomalous structure(s). If a persistent left cranial cava is present, dissect and retract the vena cava to improve visualization. If a prominent hemiazygous vein is also present, dissect, ligate, and divide it. If a constricting subclavian artery is identified, isolate, ligate, and transect it. Incise the mediastinum, dissect, and elevate the ligamentum arteriosum. Double ligate the ligamentum arteriosum and then transect it. Pass a ballooned catheter or large orogastric tube through the constricted esophagus to aid identification of constricting fibrous bands and to dilate the site. Dissect and transect these fibrous bands from the esophageal wall. Lavage the area, reposition the lung lobes, place a thoracostomy tube if necessary, and close the thorax routinely.