Surgical Correction of a Partial Atrioventricular Septal Defect With a Ventricular Septal Defect in a Dog

Introduction

An atrioventricular (AV) septal defect is a complex congenital heart defect resulting from an embryological malformation of the endocardial cushions.^1–3 Atrioventricular septal defect has also been called AV canal defect, endocardial cushion defect, and persistent AV ostium.⁴ An AV defect is a rare congenital heart disease in dogs and is more commonly reported in cats.⁵ During embryological development, the dorsal endocardial cushion fuses with the septum primum to close the ostium primum in the atrial septum. The dorsal and ventral endocardial cushions fuse to divide the common AV canal into the left and right AV canals. If the dorsal and ventral endocardial cushions do not close, they cannot fuse with the septum primum to close the ostium primum, resulting in abnormal development of the inflow portion of the interventricular septum and abnormal development of AV valves.

Normal AV valves have two separate rings that lie at different levels in the heart. By contrast, in AV defects, a common AV orifice with a common fibrous ring and a five-leaflet valve are present.⁶ This abnormal development also results in the mitral valve being displaced ventrally in the ventricle and the aortic valve being displaced dorsally. With AV septal defects, the longer left ventricular outflow tract is often observed as a “goose-neck sign” during angiography.⁷

A partial AV septal defect is defined as a combination of an ostium primum atrial septal defect (ASD) and AV valvular malformation, in which the tricuspid valve and the mitral valve are located at the same level as the ventricular septum.¹⁸⁹ The purpose of this paper is to report on the diagnosis and management of this rare congenital heart disease.

Case Report

A 2-month-old, 2.4-kg, male Shiba Inu was referred for evaluation of a heart murmur and suspicion of congenital heart disease. The heart murmur was identified during physical examination prior to routine vaccination. The dog was otherwise normal and appeared to be healthy. Upon auscultation, a grade III/VI holosystolic murmur was found and was loudest over the left cardiac apex. The sinus heart rate was 180 beats per minute. The physical examination was otherwise normal. Complete blood count and serum biochemical tests were within reference ranges.

Electrocardiography revealed a sinus rhythm, deep S waves (1.2 mV in lead II), increased P wave width (0.06 seconds in lead II), and wide QRS complexes (0.04 seconds in lead II). The mean electrical axis was -30°. Phonocardiography demonstrated both systolic and diastolic regurgitant murmurs. The systolic murmur was loudest on the left side at the apex, and the diastolic murmur was loudest on the right side at the apex. Thoracic radiography showed moderate cardiomegaly involving both the right and left sides. The vertebral scale heart size was 13.7 (reference range 9.7±0.5).¹⁰¹¹ The presence of prominent pulmonary vessels suggested pulmonary overcirculation.

Two-dimensional echocardiography revealed left atrial and left ventricle enlargement (end-diastolic diameter was 19.5 mm), an ASD at the lower portion of the atrial septum, and a ventricular septal defect (VSD) at the upper portion of the ventricular septum [Figure 1]. Left and right ventricular valves were separated and located at the same level as the crest of the ventricular septum [Figure 2]. Color-flow Doppler echocardiography indicated left-to-right blood flow across both the ASD and VSD as well as mitral valve regurgitation [Figure 3]. Further echocardiographic evaluation revealed a cleft in the septal cusp of the mitral valve [Figure 4], and that the length of the mitral septal cusp was shorter than normal.

Based on echocardiography, the dog had three lesions: a ventral ASD, a dorsally located VSD, and mitral valve displacement. These findings were atypical for a partial AV defect, because the ASD was complicated by a VSD.⁸ Although the dog was clinically normal, there were echocardiographic findings of volume overload in the left heart, which were presumably caused by mitral regurgitation, the VSD, and the ASD. Treatment with digoxin was initiated at a low dose to improve hemodynamics by suppression of sympathetic stimulation and to dilate peripheral blood vessels, rather than for its inotropic effects.²¹²¹³ Digoxin^a was given orally at a dose of 0.01 mg/kg daily.

Four months after the first examination and when the dog was 6 months old (7.1 kg), the vertebral scale heart size on the dorsoventral thoracic radiograph was 14.5, which was markedly increased compared to the initial vertebral scale heart size of 13.7. An echocardiogram showed that the left ventricular fractional shortening was 34.1% (reference range 35% to 57%) and the left ventricular end-diastolic diameter was 51.9 mm (reference range 16 to 28 mm). Cardiac catheterization and angiography were subsequently performed.

The dog was premedicated with atropine and acepromazine. Anesthesia was induced with ketamine hydrochloride and maintained with isoflurane in 100% oxygen. Manometric^b and oximetry^c recordings were made, and results are shown in the Table. Higher oxygen saturation in the pulmonary artery than in the right ventricle suggested a left-to-right shunt through the VSD. Lower oxygen saturation in the right atrium than in the vena cava and higher oxygen saturation in the right atrium than in the right ventricle suggested a slight left-to-right shunt through the ASD. Pressure measurements were always higher in the left chambers when compared to the right chamber, further suggesting that a right-to-left shunt did not exist.

Following the pressure recordings, selective right and left angiocardiography was performed. On right ventricular angiocardiography, the contrast medium did not enter the right atrium, which meant there was no evidence of tricuspid regurgitation. On left ventricular angiocardiography, contrast medium injected into the left ventricle entered the left atrium secondary to mitral regurgitation and then entered the right atrium via the ASD. The contrast medium also entered the right ventricle from the left ventricle across the VSD (as opposed to entering from the right atrium via the ASD or via both routes) [Figure 5]. Left ventricular angiocardiography also demonstrated a “goose-neck sign,” representing elongation of the left ventricular outflow tract and an enlarged left ventricle.⁷

Results of the cardiac catheterization corresponded well with the echocardiographic findings, and they provided a definitive diagnosis of partial AV septal defect with a VSD. Even though the dog exhibited no abnormal clinical signs, radiographic and echocardiographic findings indicated worsening of the dog’s condition, so surgical intervention was considered to correct the disorder.

When the dog was 7 months old (7.4 kg), open-heart surgery was conducted under cardiopulmonary bypass using an artificial heart-lung machine.^d,14–16 The dog was premedicated with atropine sulfate, butorphanol tartrate, and ace-promazine maleate. Anesthesia was induced with ketamine hydrochloride and maintained with isoflurane in 100% oxygen. Intraoperative pain control was achieved through periodic administration of butorphanol tartrate. Intermittent, positive-pressure ventilation was maintained throughout the procedure except during the cardiopulmonary bypass. Electrocardiography, arterial blood pressure, arterial blood gases, central venous pressure (CVP),^e end-tidal carbon dioxide (CO₂), arterial oxygen saturation (SaO₂),^f esophageal and rectal temperatures, urinary output, serum electrolyte concentrations, packed cell volume (PCV), and total protein concentrations were monitored continuously during surgery. Ampicillin^g (50 mg/kg intravenously [IV]) and dexamethasone sodium phosphate^h (1 mg/kg IV) were administered preoperatively. Heparinⁱ was given (100 U/kg IV) and added to the priming solution for the cardiopulmonary bypass machine in order to achieve an activated clotting time of >480 seconds.¹⁴¹⁷

The cardiopulmonary bypass circuit consisted of a reservoir, roller pump, membrane oxygenator, and circulating heater/cooler water bath. The bypass circuit was primed with a balanced crystalloid solution. Additives to the crystalloid priming solution included mannitol^j (0.5 g/kg), sodium bicarbonate^k (1.4 g/L of priming solution), and heparin (1,400 U/L of priming solution).¹⁷

A catheter^l was placed into the jugular vein to provide central venous access and to monitor CVP. An arterial catheter^m was placed into the right femoral artery to monitor direct arterial pressure and arterial blood gases, and an additional arterial cannulaⁿ was inserted into the left femoral artery. A right fourth intercostal thoracotomy was performed. The pericardium was opened dorsal to the phrenic nerve and sutured to the skin, making a pericardial basket. Two venous cannulas^o were inserted through pre-placed purse-string sutures of 4-0 nylon into the cranial and caudal vena cavae, and they were connected to venous lines of the cardiopulmonary bypass machine. A cannula^p was inserted in the same fashion into the coronary sinus as a retrograde cardioplegic root.¹⁸

Blood in the operative field was recovered using a vent pump and was returned into the reservoir via a suction line. Perfusion flow was 1.6 to 2.1 L per minute per m² [38 to 48 mL per minute per lb]. The core body temperature of the dog was cooled to 26°C during bypass. The aorta was cross-clamped with vascular forceps, and total perfusion was achieved. The heart was arrested with a 40-mL injection of Young’s solution^q at 4°C delivered into the coronary sinus root. Thereafter, 100 mL of cardioplegic solution (i.e., GIK solution^r) at 4°C was administered at 10-minute intervals. Additional protection was accomplished by surface cooling of the heart with an iced saline slush.

An incision was made in the right atrium, and an ASD (25 × 15 mm in diameter) was identified in the lower portion of the atrial septum immediately above the ventricular septum [Figure 6A]. The mitral valve was seen through the ASD, and there was a cleft in the septal cusp [Figure 6A]. The cleft separated the septal cusps into two portions, both of which had thick edges. The cleft was repaired with mattress sutures of 5-0 polypropylene^s [Figures 6B, 7]. The ASD was then closed with sutures of 5-0 polypropylene using pledgets^t [Figures 6C, 6D, 8]. A small VSD (5 mm in diameter) was observed behind the septal cusp of the tricuspid valve [Figure 6E]. The VSD was closed with simple mattress sutures of 5-0 polypropylene [Figure 6F]. The right atrium was sutured closed with a simple continuous pattern of 5-0 polypropylene.

Before the cross-clamp was removed from the aorta, the air was removed from the left ventricular apex using a needle. At the same time, the dog was warmed to a temperature of 30°C. Cardiac arrest lasted for 42 minutes. Electrical defibrillation (20 J)^u was used to restore sinus rhythm, and the dog was gradually weaned from the cardiopulmonary bypass machine. Dopamine^v (10 μg/kg per minute IV) and lidocaine^w (1 mg/kg IV) were administered to maintain the sinus rhythm. A blood transfusion of fresh whole blood was administered for anemia (PCV of 22%). The two venous cannulas were removed, and residual blood volume in the reservoir was returned from the arterial line. Total perfusion time was 174 minutes.

The pericardium was closed with simple interrupted sutures. Two drainage tubes were placed on each side of the thorax, and the thoracotomy incision was closed routinely. The femoral arteries were sutured with 6-0 polypropylene. Recovery from anesthesia was uncomplicated.

About 4 hours after surgery, the dog developed ventricular premature contractions that were treated with lidocaine (1 to 2 mg/kg IV), given as needed for 2 days. Postoperative pain management was accomplished by administration of butorphanol tartrate^x (0.05 to 0.1 mg/kg q 2 to 3 hours IV). Ampicillin (50 mg/kg q 8 hours IV) was administered for 8 days after surgery. Thoracic drainage tubes were removed 4 days after surgery. The dog began to eat 2 days postoperatively, recovered without serious complications, and was discharged 9 days after surgery. At the time of discharge, enalapril^y (0.35 mg/kg per day per os) was initiated instead of digoxin.

Thoracic radiography and echocardiography were repeated 1 month after surgery. The heart size was reduced, and the pulmonary vessels were less remarkable than before surgery. The vertebral scale heart size was reduced to 11.3 (from 14.5). Doppler echocardiography revealed no blood flow through the prior sites of the ASD and VSD, a reduction in the volume of mitral regurgitation, and no evidence of tricuspid regurgitation. Two months after surgery, a grade II/VI systolic murmur was heard over the left apex. On two-dimensional echocardiography, the ASD and VSD remained closed, and the cleft of the mitral valve was not detected. On Doppler echocardiography, there was no blood flow through either the atrial or ventricular septum, but mild mitral insufficiency persisted. On two-dimensional echocardiography, left ventricular fractional shortening and left ventricular end-diastolic diameter were reduced to 27.8% (from 34.1%) and 46 mm (from 51.9 mm), respectively. The dog was still alive 1 year after surgery and was clinically normal.

Discussion

Atrioventricular septal defects are classified as either complete or partial. In both types of AV defects, a five-leaflet valve is present and made up of a left mitral leaflet, a right cranial-ventral leaflet, and two bridging leaflets. With complete AV defects, the bridging leaflets are not connected to each other, and the common AV annulus is wide open. In partial AV defects, the common AV annulus is separated by a connecting strand of bridging leaflets into right and left orifices. With partial AV defects, the mitral valve has the cleft in the septal cusp so that it looks like a tricuspid valve, and the edges of the cusps are usually thick and curled.¹⁹ Generally, with a complete AV defect, there is a common orifice of the right and left AV valves, with ostium primum ASD and VSD just under the AV valves.¹ With a partial AV defect, on the other hand, an ostium primum ASD and AV valve malformation separate the common annulus into right and left orifices.¹ In a partial AV defect, AV valves are present at the same level as the ventricular septum and cause valvular regurgitation.

Recently, there have been a few published reports of a complete AV defect without ASD and a partial AV defect with VSD in humans.⁸ There has been marked variation in the morphology of the septal defect itself. This variation depends upon the arrangement of the bridging leaflets and their relationship to the atrial and ventricular septal structures. The present case is a rare form of a partial AV defect with VSD in a dog. In people, either with complete or partial forms, the response to pharmacotherapy is not favorable, and surgical correction is considered to be the only effective treatment.²⁰

In the dog of this report, a partial AV defect with VSD was diagnosed by echocardiography and cardiac catheterization. On echocardiography, morphological abnormalities were observed, namely ostium primum ASD, inlet portion of VSD, and AV valvular malformation. On cardiac catheterization, not only were the morphological abnormalities identified, but also detailed pathophysiological evaluations were acquired from local pressure measurements and blood gas analyses. In systole, mitral regurgitation resulted in regurgitation from the left ventricle into the left atrium and partially into the right atrium through the ASD. At the same time, the ASD allowed blood flow from the left atrium into the right atrium, and the VSD allowed blood flow from the left ventricle into the right ventricle. If this hemodynamic condition were left untreated, the left-to-right shunting through the ASD and VSD would have resulted in atrial and ventricular volume overload on both sides of the heart. Typically, continuous volume overload on the left side of the heart causes pulmonary hypertension and/or results in heart failure. Surgery was performed to prevent this from occurring. The dog was not a high risk for surgery, because it had not yet developed Eisenmenger’s syndrome (i.e., pulmonary hypertension causing right-to-left shunting of blood through the ASD and/or VSD).

Open-heart correction of AV defects in dogs has been described previously.²¹²² Reports on the surgical treatment of AV defects in dogs are much rarer than reports in humans.⁷²³ In one canine report, the dog died 32 hours after surgery from encephalopathy resulting from hypotension during cross-circulation cardiopulmonary bypass.²¹ In another report, two dogs lived long-term after cardiopulmonary bypass surgery, in which a pericardial patch was placed over the ASD and a commissuroplasty was used to repair the mitral valve.²² One of the two dogs was still clinically normal 42 months after surgery, and the other had mild exercise intolerance 15 months after surgery.²² Blood flow through the septal defect disappeared 5 and 3 months after surgery, respectively.²¹ Moderate to severe mitral insufficiency was detected in both dogs. Based on these results, management of mitral valve defects was considered quite difficult.

Several techniques of mitral valve repair have been reported in people.⁷ Usually patch suturing (prosthetic patch or pericardial patch) of the cleft and mitral valve replacement are performed.²⁰²⁴ Patch suturing is relatively easy to perform, but the septal cusp is usually so thickened and rolled that coaptation is not always achieved.²⁰ In comparison, mitral valve replacement provides a more efficient valve, although it involves various complications, including prosthetic valvular incompetence, thromboembolism, hemolysis, infectious endocarditis, and adverse effects from the anticoagulants used postoperatively.²⁵ In comparing these two techniques, mitral valve replacement is considered to be more advantageous in humans with AV defects.²⁰ Very few mitral valve replacements have been reported in dogs, however.²⁶²⁷ In the present case, a commissuroplasty of the mitral valve was performed with mattress sutures and without a patch. Although mitral regurgitation persisted, the intensity of the cardiac murmur became milder (from a grade III/VI to II/VI), and a subsequent Doppler echocardiogram showed reduction in the mitral regurgitation.

Conclusion

A partial AV septal defect was diagnosed in a young Shiba Inu on echocardiography and angiography. Surgical repair was conducted under extracorporeal circulation. The ASD and VSD were repaired completely, and hemodynamic improvement was achieved, although the mitral valve malformation could not be completely repaired. The dog was clinically normal 1 year after surgery. Although the technique for repair of a partial AV septal defect was effective in this dog, the methods and techniques for complete restoration of normal heart function in dogs with AV septal defects require further investigation.