Introduction

Although many trained surgeons are comfortable performing closed cardiac procedures such as ligation of a patent ductus arteriosus and subtotal pericardectomy, only a few veterinary surgeons perform open heart surgery regularly and perhaps even fewer can boast reliable long-term results for the therapy they perform. The reasons for this include:

 The low prevalence of surgically correctable cardiac disease in veterinary patients

 The apparent success of non-surgical treatment (e.g., balloon dilation for pulmonic stenosis and endovascular occlusion for patent ductus arteriosus)

 The risk associated with surgical treatment

 The techniques currently available

 The cost of such treatments

For these reasons, cardiac surgery in animals has fallen massively behind when compared to the repertoire of procedures currently offered to human patients with heart disease. Ironically, the techniques used in human patients to facilitate open heart surgery today are largely the same as the techniques developed by pioneers such as Gibbon, Lillehei, Taussig and others, during experiments with dogs and cats. More recently, open heart surgery programmes have been or are being developed at different veterinary centres throughout the world, increasing the availability, safety and success of open heart surgical therapies in the dog.

What Does It Take?

The author of this presentation has been involved in an open heart surgery programme for over 15 years. Progress has been slow and occasionally stuttering. Like others working in this field we have concluded that:

 All members of the team (cardiologists, surgeons, anaesthetist, perfusionist, critical care clinicians) have to be dedicated to the programme, understand their roles and be coordinated by a single leader.

 Size matters: the insult associated with being on a heart-lung machine using current protocols is magnified in small dogs this means that small dogs often die of bypass-related complications using protocols that are well tolerated by larger (> 15 kg) dogs.

 Initially, it is important to select patients requiring relatively simple operations that have not exhausted their myocardial reserve, preferably animals with congenital disease.

 Dogs with end-stage heart disease associated with mitral valve incompetence are not good candidates for surgical therapy performed by an inexperienced team.

 This is an expensive, high-risk endeavour that will be a drain on hospital resources; in order to survive within a hospital, it must be successful early on in its development.

What Can Be Done?

As centres around the world gain experience and expertise in the field of open heart surgery, the repertoire of procedures will be extended to include congenital and acquired valve defects, along with other congenital heart disease. In addition, as skill levels increase, it will become possible for workers in this field to offer therapies to dogs of all sizes, along with cats and possibly even horses.

Total Valve Replacement (TVR) in Dogs

The mitral and tricuspid valve can both suffer from congenital and acquired (endocardiosis) deformity that creates predominantly valvular incompetence and occasionally stenosis. Repair or replacement of such valves is commonly performed in people, and typically is done before secondary myocardial changes are advanced.

The tricuspid valve is approached through a right thoracotomy. The cannulation strategy is: bicaval cannulation via the right atrium for venous drainage, right carotid artery for arterial return and aortic root for cardioplegia delivery and venting. The valve annulus is approached through a right atriotomy once cardiopulmonary bypass has been initiated and cardioplegia solution delivered. The native valve is examined and the septal valve leaflet excised. A series of 10–12 interrupted mattress sutures with PTFE 'pledgets' are placed in the valve annulus with the pledgets on the ventricular side of the annulus. The parietal valve leaflet is typically 'reefed' into these sutures to preserve chordae attachments to the papillary muscles and therefore preserve ventricular function. The valve annulus is measured and a valve of appropriate size selected. The sutures are placed evenly around the artificial valve through the 'sewing ring' and the valve is gently parachuted into position. The mattress sutures are tied, the heart is de-aired and the atriotomy closed. The dog is weaned from bypass and recovered from anaesthesia.

Right Ventricular Outflow Tract (RVOT) Reconstruction in Dogs

Surgical access to the RVOT in dogs is via a left fourth or fifth intercostal incision. The cannulation strategy is either: a single atriocaval cannula via the right auricular appendage or bicaval cannulation (cranial cava direct and caudal cava via right auricular appendage), for venous drainage, left carotid artery for arterial return and aortic root for cardioplegia delivery and venting. The RVOT is opened equidistant between the right coronary sulcus and the paraconal interventricular coronary artery, once cardiopulmonary bypass has been initiated and cardioplegia solution delivered. The outflow tract is cleared of any fibrous valve or fibromuscular obstruction and reconstructed using a sheet of PTFE vascular graft cut to the appropriate size, sutured to the right ventricular myocardium and pulmonary artery using a simple continuous pattern. Prior to placement of the final suture, the heart is de-aired. The dog is weaned from bypass and recovered from anaesthesia.

Other Procedures

Mitral valve replacement, mitral valve repair, complete repair of tetralogy of Fallot, repair of septal defects, resection of the fibromuscular band in animals with cor triatriatum dexter and resection of subaortic stenosis lesions have all been reported in dogs. Open heart therapies in cats under venous inflow occlusion and, more recently, under cardiopulmonary bypass have also been reported.

RVC Experience

Experience with replacement valves in dogs is still in its infancy and numbers of animals treated this way are still small. Tissue valves (valves made from bovine pericardium or porcine aortic valves) have been used at the RVC because they, theoretically, obviate the need for life-long anticoagulation. Early results of tricuspid valve replacement using both bovine pericardial and porcine aortic valves, suggest that inability to manage anticoagulation even in the short term can prove devastating in the long term for dogs undergoing valve replacement.

We have operated on seven dogs with pulmonic stenosis in which balloon valvuloplasty had failed. These dogs all had either severe pressure gradients, were in right heart failure, or both. One dog could not be weaned from bypass and was euthanased in the operating theatre. All the other dogs survived and have had a normal quality of life with either no or minimal residual pressure gradient evident echocardiographically. One dog was euthanased 5 years after his heart operation because of metastatic osteosarcoma and one dog was euthanased because of refractory right heart failure secondary to tricuspid valve dysplasia. We have operated on two dogs with double-chambered right ventricle (DCRV). One dog died of blood loss (from the patch) the day of surgery, the other dog is living a normal life with no demonstrable pressure gradient.

Clinical Decision Making

To summarise what we have learned so far: the main concern with valve replacement in dogs is no longer getting them through bypass surgery but is with managing the anticoagulant therapy after surgery. Mitral valve repair, which has only been done successfully with any consistency by workers in Japan, is technically demanding. The repair is rarely perfect, which means the valve remains incompetent after surgery (unlike the situation in people) so this is only palliative. Cardiopulmonary bypass facilitates accurate and effective right ventricular outflow tract reconstruction using ePTFE in dogs. Good long-term success is anticipated with this approach. Once the bypass techniques are established in a centre, the possibility of attempting more rare conditions such as tetralogy of Fallot, septal defects and triatriatum dexter exists. Open patch grafting of pulmonic stenosis lesions would seem to be a reliable and successful treatment for larger dogs at least and, with increased experience, smaller dogs will benefit from these operations too. This approach should be reserved for dogs that have failed balloon dilation. Other one-off procedures such as tetralogy repair, septal defect repairs, where there is no good non-surgical therapy, should be considered for open repair. Currently, until new valve materials or new anticoagulant therapies are developed, valve replacement, whether it is at open surgery or using a minimally invasive delivery device, remains a high-risk option.

References

1.  Behr L, Chetboul V, et al. Beating heart mitral valve replacement with a bovine pericardial bioprosthesis for treatment of mitral valve dysplasia in a Bull Terrier. Veterinary Surgery 2007;36(3):190–198.

2.  Gordon SG, Nelson DA, et al. Open heart closure of an atrial septal defect by use of an atrial septal occluder in a dog. Journal of the American Veterinary Medical Association 2010;236(4):434–439.

3.  Kanemoto I, Taguchi D, et al. Open heart surgery with deep hypothermia and cardiopulmonary bypass in small and toy dogs. Veterinary Surgery (e-publish) 2010 Apr 29.

4.  Orton EC, Mama K, et al. Open surgical repair of tetralogy of Fallot in dogs. Journal of the American Veterinary Medical Association 2001;219(8):1089–1093.

5.  White RN, Boswood A, et al. Surgical management of subvalvular aortic stenosis and mitral dysplasia in a golden retriever. Journal of Small Animal Practice 1997;38(6):251–255.