D.J. Brockman, BVSc, CVR, CSAO, DACVS, DECVS, FHEA, MRCVS
Although many trained surgeons are comfortable performing closed cardiac procedures such as ligation of a patent ductus arteriosus and sub-total pericardectomy, only a few veterinary surgeons the world over 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 prevalence of surgically correctable cardiac disease in veterinary patients, the apparent success of non-surgical treatment (e.g., Balloon dilation for pulmonic stenosis), the risk surgical treatment poses to the patient, the techniques currently available and 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, as a result of experiments with dogs and cats. More recently, open heart surgery programmes have been or are being developed at different centres throughout the world, increasing the availability, safety and success of open heart surgical therapies in the dog.
Venous Inflow Occlusion
Traditionally, open heart surgery was performed under total venous inflow occlusion (TVIO), placing time constraints on the duration of procedures. The trade-off for increasing the 'safe' occlusion time for a patient was a reduction in the likelihood of successfully reviving the patient at the end of the procedure.
Venous inflow occlusion:
Normothermic: 3-5 mins (8 mins--Hunt)
Mild hypothermia (30°C): 9 mins
Moderate hypothermia (25°C): 15 mins
Deep hypothermia (20°C): 45 mins
This time constraint often meant that procedures were designed to be fast rather than accurate and that some procedures needed to be aborted before the surgical goals were achieved. These techniques did, however, allow resection of fibromuscular atrial bands (as in cor triatriatum dexter), resection of pulmonic valve leaflets or placement of a pulmonary outflow patch to treat pulmonic stenosis, and the resection of right ventricular outflow tract tumours. Studies demonstrating the long-term results of, for example, pulmonary outflow patching under TVIO, do not exist. Although short-term survival for patients undergoing such procedures is high at some institutions, the evidence, via personal communication, is that long-term results are either poor or at least very inconsistent.
The ability to open the heart, without the time constraint that TVIO creates, would allow the surgeon to perform a more accurate operation for even the relatively simple congenital cardiac diseases such as pulmonic stenosis. This should result in better long-term results. Theoretically, such techniques would open the way not only for treatment of congenital heart diseases but also offer some hope for all the dogs that develop progressive acquired valvular disease. Longer duration open heart surgery can be achieved in two ways:
The use of a large dog as the 'oxygenator' as in controlled cross circulation is considered unethical in many countries, leaving the heart-lung machine as the best means to perform prolonged open heart surgery in dogs.
In addition to standard surgical and anaesthetic equipment, in order to perform open heart surgery, an artificial pump (roller head pump) and an artificial lung (oxygenator) are required.
A dedicated coordinated team effort is required to perform operations under cardiopulmonary bypass. This team includes: two surgeons, two scrub-nurses, one anaesthetist, and an anaesthesia nurse or technician, a perfusionist to run the heart-lung machine and critical care specialists to perform the post-operative care for these animals. In addition, cardiologists need to be prepared to offer surgical therapy to their patients and need to co-ordinate the medical heart therapy prior to and following surgical therapy.
Re-inventing the Wheel
Unfortunately, over the last few years, those involved in such programmes have to a large extent had to re-learn the lessons learned by early workers in the human field of cardiac surgery many years ago. The author of this presentation has been involved in an open heart surgery programme of over the last eight years and has concluded, as other workers have, that:
1. 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.
2. 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 (>15kg) dogs.
3. At the start of a programme, attempt relatively simple operations in dogs that have not exhausted their myocardial reserve, preferably animals with congenital disease.
4. Dogs with end-stage heart disease are not good candidates for surgical therapy performed by an inexperienced team.
5. Because of problems associated with long-term patient anticoagulation, mechanical valve replacement is fraught with long-term problems; tissue valves, that don't require patient anticoagulation, may be a better valve option for small animals.
6. 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, that is the patients must at least survive.
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 perhaps even horses.
Atrioventricular Valve Replacement
The mitral and tricuspid valve can suffer from both congenital and acquired (endocardiosis) deformity that creates predominantly valvular incompetence and occasionally stenosis. Replacement of such valves in people is commonly performed and typically is done before secondary myocardial changes are advanced.
Valve Replacement in Dogs
The tricuspid valve is approached through a right thoracotomy via a right atriotomy once cardiopulmonary bypass has been initiated and cardioplegia solution delivered. The mitral valve is approached through a left thoracotomy and a left atrial incision. For each AV valve replacement, the native valve is examined and the septal valve leaflet excised. A series of ten to twelve simple interrupted mattress sutures with PTFE 'pledgets' are placed in the valve annulus with the pledgets on the atrial side (mitral) or ventricular side (tricuspid) 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.
The Story So Far
Experience with replacement valves in dogs is in its infancy. Mechanical valves have been placed in the mitral position in dogs. Although dramatic results were obtained in the short-term, difficulty in maintaining life-long anti-coagulation resulted in devastating failure of the valve in many patients. Tissue valves (valves made from bovine pericardium or porcine valves) theoretically obviate the need for life-long anticoagulation in people and should be similar in the dog. Early results of tricuspid valve and mitral 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.