Single Coronary Type R2A

Single right coronary artery type R2A is a congenital coronary anomaly that causes pulmonic stenosis predominantly in English bulldogs. It was described in 3 bulldogs and one Boxer in 1990 and since then has been recognized in these breeds worldwide (1).

1. Normal coronary anatomy
2. Single coronary pathology
3. Embryology
4. Etiology of SCA
5. Epidemiology
6. Diagnosis
7. Significance
8. Treatment
9. References

1. Normal coronary anatomy

Coronary artery (CA) patterns in dogs and humans are similar (Fig 1). The left and right main CA originate from the left and right sinuses of Valsalva behind the respective leaflets of the aortic valve. The right CA extends around the right atrioventricular groove and supplies the posterior descending (subsinuosal) CA in over half of human patients. In dogs, the left main CA bifurcates 2-10 mm beyond its origin into the left circumflex artery (LCx) and the paraconal (left anterior descending) CA. Dogs almost always have a dominant left CA pattern; i.e. the LCx supplies the subsinuosal (posterior descending) artery (Fig 2). In addition, many dogs have a large ventricular septal artery that originates near the bifurcation of the left main CA and supplies a major portion of the interventricular septum.

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Fig. 1. Cranial perspective drawing of the cardiac chambers and major coronary arteries (Modified from Ciba Medical illustrations: Vol 5 The heart; pg 16,1969.

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Fig. 2. Aortic injection lateral angiocardiogram in a normal mongrel dog two seconds after the onset of brief asystole (induced with acetylcholine) demonstrating the right (RCA), left circumflex (LCx), paraconal (Pc), and subsinuosal (S ) coronaries.

2. Single coronary pathology

Various abnormal coronary distribution patterns have been described in humans (Fig 3). In one report of 134 human patients, 70 patients had single left CA and 64 had single right CA (2). The R2A pattern shown in figure 3 is the only one described in dogs and it causes pulmonic stenosis. The association between these two anomalies has not been reported in humans. (3)

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Fig. 3. Diagram of normal CA and common patterns of single right CA distributions. In the R1 pattern, the right coronary artery (RCA) continues as a single vessel and crosses the caudal crux of the atrioventricular sulcus (open arrow), then continues as the left circumflex (LCx) and the paraconal (Pc) arteries. In type 2 patterns, the single RCA divides shortly after leaving the aorta. Subclassifications are made depending on whether the crossing vessel (left main CA, solid arrow) passes in front of the pulmonary trunk (R2A), between the aorta and pulmonary trunk (R2B) or behind the aorta (R2C). The paraconal usually originates from the circumpulmonary left main CA but it may arise directly from the RCA and cross the right ventricular outflow tract independent of the LCx.

The circumpulmonary segment of the left main CA in the R2A pattern in dogs causes external compression of the pulmonary outflow tract and clinical signs of pulmonic stenosis at the valvular or immediate subvalvular level. The diagram below (Fig 4) indicates the size and positional relationships more accurately in dogs with single R2A and pulmonic stenosis.

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Fig. 4.Diagram of type R2A single right CA in 4 dogs. The anomalous circumpulmonary left main CA (arrow) compressed the base of the pulmonary valve (PV). The right pulmonic leaflets and sinuses were hypoplastic. The left aortic sinus (LAS) was small (23% of aortic circumference) and lacked a coronary ostium. The right aortic leaflet and sinus (RAS) were almost twice as big as the left sinus and leaflet. Mean portions of the aortic circumference associated with each aortic sinus in 3 dogs are indicated in percentages. The paraconal (Pc) originated from the circumpulmonary left main CA. The subsinuosal (S) was a terminal branch of the left circumflex artery (LCx). In a recent case the subsinuosal artery was the terminal branch of the right CA.

3. Embryology

Myocardial perfusion is accomplished initially by outward flow through a capillary network from the endocardium to the epicardium where blood is collected in larger vessels located at the atrioventricular groove and drains into the right atrium. At the same time, right and left CA precursors (anlagen) begin to bud from the aorta (Fig 5) (4).

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Fig. 5. Frontal view of the developing human heart. Note the endothelial buds from the truncus arteriosus (truncus) which have hollowed out and are about to join the anlagen of the paraconal artery (LAD) which lies in the groove between the left (LV) and right ventricles (RV) and the anlagen of the RCA lying in the sulcus between the RV and the right atrium (RA). The left atrium (LA) is also identified.

The coronary anlagen join the capillary plexus and increase epimyocardial perfusion. Vessels in the atrioventricular groove enlarge and form an arterial ring at the atrioventricular junction passing external to the pulmonary trunk and right ventricular outflow tract (Fig 6) (4).

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Fig. 6. Diagram showing anastomosis between the RCA and LCA creating an arterial ring encircling the right ventricular outflow tract (circle of Vieussens).

Major coronary vessels evolve on the surface of the heart and subsequent myocardial perfusion is almost entirely from the epicardium to subendocardium via right and left CA (Fig 7). The arterial ring over the right ventricular outflow tract atrophies and only vestiges remain as arterioles in that region.

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Fig. 7. Diagram of postnatal arteries showing the relatively independent right and left coronary blood flow. Flow through anastomotic channels between the two coronary circulations is minimal.

4. Etiology of SCA

Failure of the right or left coronary anlagen to connect to the atrioventricular capillary plexus is thought to result in enlargement and continued circulation through the connecting vessels between the right and left sides of the myocardium. A study of 134 human patients with SCA found almost equal numbers of single right and single left coronary arteries (2). In some instances an intimal dimple was found in the aortic sinus of Valsalva and it was thought to be an anlagen remnant of the missing CA. Serial section histology in a stillborn bulldog with single CA revealed abnormal inward growth of elastic tissue at the site where the left coronary artery should have originated (Fig 8) (5). This provides support for the theory of failed contralateral connection as the cause of single CA. In this dog, the abnormal circumpulmonary left main CA coursed in the wall of the right ventricular outflow tract causing subvalvular pulmonic stenosis (Fig 9).

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Fig. 8. Histologic cross-section at the level of the aortic sinuses of Valsalva in a stillborn dog with single right CA and subvalvular pulmonic stenosis. The right coronary sinus (R) and noncoronary sinus (N) are slightly larger than the left sinus and appear normal except for increased thickness of the wall of the right sinus and increased diameter of the right CA. In the left sinus, notice the inward protrusion of elastic media (arrows) at a point where the left CA should have extended outward from the aorta (A). Pulmonary valves (P). Right Atrium (RA). Left Atrium (LA). Van Gieson stain 20X. Bar = 1mm.

Fig. 9 (A) Histologic cross-section in the same dog at the level of the right ventricular (RV) outflow tract just beyond the branch point of the circumpulmonary left main artery (C) from the single right CA (R). The proximal segment of the left circumflex CA (X) is also evident. (B) Notice the circumpulmonary left main CA (C) coursing in the myocardium around the RV outflow tract and compressing it. The artery becomes epicardial when it approaches the great cardiac vein (V) just before branching into the left circumflex, and paraconal CA (not seen in this section).
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5. Epidemiology

A predisposition for pulmonic stenosis in English bulldogs was reported in the 1968 when 6 bulldogs with PS were observed and only 2 were expected in a study of breed frequencies in 233 dogs with congenital heart disease (6). The predisposition was confirmed in a later study that found PS was 19 times more frequent in bulldogs than in other breeds among 1320 dogs with congenital heart disease (Table 1) (7). The association of PS and single CA type R2A in bulldogs was reported in 1990 when 3 bulldogs and a Boxer were found to have both anomalies. Subsequent studies found R2A in 3 of 4 more bulldogs with PS. The associated anomalies have also been observed in French Bulldogs. Two of the dogs with R2A also had fibrous ring subvalvular PS.

Table 1. Pulmonic Stenosis in 1320 dogs

6. Diagnosis

Dogs with R2A have typical signs of PS ranging from asymptomatic left basal systolic murmur to congestive heart failure with ECG evidence of right ventricular hypertrophy and radiographic and echocardiographic signs of right heart enlargement and RV hypertrophy. These signs may be progressive as noted in one dog with R2A examined sequentially (Table 2). Breed awareness is a legitimate consideration in recommending follow up studies and coronary angiography.

Table 2 "Binnabik"

The most reliable way to diagnose R2A is by coronary angiography. An aortic root injection of contrast material in a lateral angiocardiogram reveals an enlarged right CA giving off a circumpulmonary left main (CLM) artery that crosses the aortic sinus of Valsalva (Fig 10A). The CLM may be superimposed on the right CA or it may appear as a separate knob adjacent to the right CA. The knob is the "aortic root sign" reported in humans (Fig 10B).

Fig. 10. Angiocardiograms illustrating single right CA. (A) LV contrast injection shows a single right CA (R) that gives rise to a circumpulmonary left main artery (C) which curves caudally before branching into the paraconal (P) and left circumflex (LCx) arteries. The LCx characteristically crosses over the aortic sinuses of Valsalva. (B) When the CLM is not superimposed on the right CA, it may appear as a separate knob (arrow) on the cranial aspect of the aorta and is called the aortic root sign. The LCx still crosses the sinuses of Valsalva.
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A right ventricular injection angiocardiogram usually shows eccentric valvular or subvalvular pulmonic stenosis with a caudally positioned outflow orifice and a cranial filling defect in the outflow tract (Fig 11A). Superimposition of the pulmonic stenosis image and the coronary angiogram (Fig 11B) shows the CLM coursing through the filling defect (Fig 11C). This finding also may be demonstrated by digital subtraction angiography (Fig 12).

Fig. 11.

Fig. 12. (A) Dextrophase digital angiocardiogram in a dog with R2A and pulmonic stenosis showing eccentric narrowing at the immediate subvalvular level, moderate right ventricular hypertrophy and post stenotic dilation of the main pulmonary artery. (B) Levophase showing bright, end-on contrast in the CLM (arrow). (C) Subtraction image showing the CLM is superimposed on the subpulmonic filling defect (arrow). Courtesy of Christophe Lombard, University of Berne, Switzerland.

Echocardiography sometimes reveals the presence of an R2A in dogs with pulmonic stenosis (Fig 13). But coronary angiography usually is necessary to confirm the diagnosis. If R2A is evident by echocardiography, balloon valvuloplasty for PS is contra-indicated and the expense of coronary angiography can be avoided.

Fig. 13. Right parasternal short axis 2D echocardiogram and color Doppler demonstrating the circumpulmonary left main artery in type R2A single right CA in a bulldog with pulmonic stenosis. The artery appears as two parallel echogenic bands at right angles to the right ventricular outflow tract at the level of the pulmonic valve.
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7. Significance

The major significance of R2A is that it causes pulmonic stenosis and limits treatment options. Closed valvulotomy, patchgraft surgery (Fig 14) or balloon valvuloplasty to relieve pulmonic stenosis will tear or transect the circumpulmonary left main CA which supplies the entire left ventricle and interventricular septum. Fatal surgeries of this type led to discovery of the coronary anomaly in bulldogs.

Fig. 14. Diagrams (A and B) and necropsy photograph (C ) of patchgraft surgery. A cutting wire is passed into the right ventricle, through the pulmonic valve and out the pulmonary artery wall. An elliptical patch of Dacron, Gortex, or pericardium is sutured over the cutting wire and the suture is tied after sawing action of the wire cuts the outflow tract under the patch.
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Non-iatrogenic myocardial infarction has not been recognized in dogs with R2A except in the stillborn bulldog shown in figure 9. That puppy had an acute myocardial infarct at the apex of the left ventricle (Fig 15) which may have been caused by the intramyocardial course of the circumpulmonary left main CA. Sudden death has been reported in humans with type R2B anomalies but R2B has not been recognized in dogs.

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Fig. 15. Necropsy photograph of the heart shown in figure 9. Histology confirmed an acute hemorrhagic infarct (I) at the LV apex. The paraconal (Pc) and left circumflex (LCx) arteries came out of the RV myocardium instead of originating from the aorta in the normal location dorsal to the interventricular septum and behind the pulmonary trunk.

8. Treatment

Surgical options to treat pulmonic stenosis in dogs with R2A theoretically include coronary artery bypass from the aorta to the left circumflex and paraconal arteries followed by double ligation and division of the circumpulmonary left main CA and a patchgraft in the RV outflow tract. Submaximal balloon valvuloplasty has been performed in a bulldog with R2A without tearing the CLM but the dog developed congestive heart failure 2 years later and died 30 minutes after patchgraft surgery severed the CLM. The other surgical option is an extracardiac conduit from the right ventricle to the pulmonary artery (Fig 16) (8). Techniques used in this operation are shown in figures 17-21.

Fig. 16. (A) Diagram illustrating a cross-clamped conduit in position after anastomosis to the pulmonary artery. (B) Myocardial core removed with a cork borer rotated against a Foley urinary balloon catheter during inflow venous occlusion. (C) Median sternotomy view after insertion of a conduit from the right ventricle (RV) to the pulmonary artery (PA) in a bulldog with pulmonic stenosis and R2A. Left ventricle (LV).
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Surgery to insert a conduit can be performed through a median sternotomy. Place loose tourniquets around the vena cavae in preparation for inflow venous occlusion. Cut the conduit to a desired length and anastomose the distal end to the main pulmonary artery after isolating a segment of the PA with a double angled vascular clamp (Fig 17).

Fig. 17. (A) Place stay sutures at the ends of the pulmonary artery incision (PA) and the beveled conduit (C). (B) Complete the anastomosis with continuous sutures, first medially then laterally. Right atrium (RA). Right ventricle (RV).
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Cross clamp the conduit near the pulmonary artery and test the hemostatic integrity of the anastomosis. Place three equidistant, buttressed mattress sutures in the right ventricular outflow tract separated by distances greater than the diameter of the conduit flange and pass the sutures through the flange (Fig 18).

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Fig. 18. Three untied buttressed mattress sutures are placed in the right ventricular outflow tract and conduit flange in preparation for conduit insertion.

Make a side hole in the Foley catheter and insert a rigid rod (Fig 19) to stiffen the catheter for right ventricular puncture in the center of the triangulated area defined by the mattress sutures.

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Fig. 19. Pass a urinary balloon catheter through a 10 mm cork borer then make a side hole in the catheter and insert a rigid orthopedic rod to stiffen the catheter.

Make a small, shallow incision in the selected area of the RV outflow tract, push the catheter through the RV wall and inflate the balloon (Fig 20). Occlude the vena cavae with the preplaced tourniquets. Maintain outward traction on the catheter and rotate the cork borer until it cuts through the RV wall and meets the balloon. (A cork borer with no edge irregularities did not cut the balloon in 73 experimental surgeries of this type. The Foley catheters were re-sterilized and used multiple times). Deflate the balloon. Withdraw the catheter with its myocardial core. Insert the conduit and tie the pre-placed mattress sutures (Fig 21). Release the vena caval tourniquets. This can be accomplished in less than 3 minutes.

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Fig. 20. Diagrams illustrating use of a balloon catheter and cork borer to cut and remove a core of myocardium from the right ventricle. (A) Insert and inflate a balloon catheter in the RV outflow tract in a purse string or mattress controlled area. (B) Rotate the cork borer to partially cut through the myocardium. (C) After tightening tourniquets on the vena cavae, finish cutting the myocardium and remove the myocardial core and the balloon. (D) Myocardium removed from the heart shown in figure 19.

Fig. 21. Post-implant photograph showing the conduit secured by three buttressed mattress sutures. Additional hemostatic sutures were not required. The non-valved Dacron conduit had a rigid right angle stent and contacted the sternum with chest closure.

Two days after surgery, the dog became dyspneic when standing, developed low cardiac output and died. Necropsy revealed marked right ventricular hypertrophy that probably occluded the conduit orifice in the outflow tract (Fig 22). No thrombus was present in the conduit or at the pulmonary artery anastomosis.

Fig. 22. Necropsy photographs of ventricles cut in cross section. (A) The opened RV outflow tract shows the entrance of the conduit. (B) The conduit appeared to be occluded when the RV outflow tract was not held open.
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Dissection at the base of the heart confirmed the R2A artery distribution and bridging of the circumpulmonary left main CA by the conduit (Fig 23).

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Fig. 23. Necropsy photograph of the conduit (C) between the right ventricle (RV) and the pulmonary artery (PA). The coronary arteries have type R2A single right CA distribution. The right coronary (R) arises from the aorta (A) and continues in the right atrioventricular sulcus after giving off the circumpulmonary left main (CLM) that constricts the pulmonic valves (P) and divides into the paraconal (Pc) and left circumflex (LCx) arteries.

Fig. 24. Gross photograph of the pulmonary outflow tract cut longitudinally in another dog showing the circumpulmonary left main CA (arrow) immediately adjacent to the pulmonary artery and small thick pulmonary valve leaflet (P-P). The associated sinus of Valsalva (S) is only two thirds the depth of the other two sinuses. The dark area in the myocardium surrounding the region is attributable to hemorrhage from surgical dissection.

A non-stented conduit may have worked better in this dog because it would not have transferred sternal pressure to the right ventricular outflow tract. However, non-stented conduits have greater tendency to develop obstructive neointima (8). Insertion of the stent nearer the apex and away from the sternum may have improved the outcome of surgery in this dog. Resection of muscle in the RV outflow tract also would be of benefit but cardiopulmonary bypass would be required.

9. References

1.  Buchanan JW: Pulmonic stenosis caused by single coronary artery in dogs: 4 cases (1965-1984). JAVMA 196:115-120, 1990.

2.  Roberts W: Major anomalies of coronary arterial origin seen in adulthood. Am Heart J 111:941-963,1986.

3.  Roberts W: Personal Communication 1990

4.  Boucek RJ, Morales AR, Romanelli R, et al: Embryology and congenital anomalies of the coronary arteries. In: Sangston JL, ed Coronary Artery Disease. Baltimore : Williams and Wilkins 1984:49,57.

5.  Buchanan JW: Pathogenesis of single right coronary artery and pulmonic stenosis in English Bulldogs. J Vet Intern Med 15:101-104,2001.

6.  Patterson DF: Epidemiologic and genetic studies of congenital heart disease in the dog. Circ Res 32:171-202,1968.

7.  Buchanan JW: Causes and Prevalence of Heart disease. In: Current Veterinary Therapy XI, Kirk RW and Bonagura JD (eds), Philadelphia, W.B. Saunders Co. 1992; p 647-655.

8.  Flore AC, Peigh PS, Sears NJ, et al. The prevention of extracardiac conduit obstruction. J Surg Res 34:463-472,1983

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James W. Buchanan, DVM, M Med Sci
Professor Emeritus of Cardiology
Univ of Penn, Philadelphia