"New Techniques"--Cardiopulmonary Bypass in Small Animals
World Small Animal Veterinary Association World Congress Proceedings, 2007
D.J. Brockman, BVSc, CVR, CSAO, DACVS, DECVS, ILTM, MRCVS
Department of Small Animal Medicine & Surgery, Royal Veterinary College
North Mymms, Hatfield, Herts, UK

Introduction

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.

Techniques

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 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:

Cardiopulmonary Bypass

 Controlled cross circulation: Unlimited time

 Heart-lung machine: Unlimited time

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.

Equipment

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.


 

Personnel

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.

Experience at the RVC

Since January 2005, we have performed twelve open-heart procedures under cardiopulmonary bypass at The Royal Veterinary College. These have included two dogs with pulmonic stenosis that had failed balloon dilation attempts, two dogs with double-chambered right ventricle and seven dogs with tricuspid valve dysplasia.

Mortality

All dogs were successfully weaned off bypass and transferred to ICU. Overall, seven dogs have survived the attempt to treat their disease in the long term. Two dogs died within 24 hours of surgery. One dog with post-bypass coagulopathy had blood loss that was so great, that a second chest exploration was performed. No site of bleeding was seen and the dog went into ventricular fibrillation at the end of the procedure. A second dog developed post bypass systemic inflammatory response syndrome and developed refractory hypotension and anuria the morning after her surgery, and was euthanatized.

One dog died of acute thromboembolic disease six days after an uncomplicated recovery from heart valve replacement. This dog was not managed with anticoagulant therapy. Two dogs developed a multi resistant bacterial septicaemia resulting in death.

Morbidity

Two dogs have developed acute transient respiratory distress, associated with hypoxia, both responded to supportive care (oxygen rich environment, nebulisation and coupage). Both of these dogs had had RVOT patches.

One dog developed pressure-related sciatic neuropathy that resolved over a six month period. One dog developed a post-operative pneumothorax that was managed by placement of a chest drain.

Anticoagulant Therapy

Morbidity associated with anticoagulant therapy has included large-bowel haemorrhage in one dog. Variation in the success of anticoagulant therapy (as determined by the international normalised ratio) has been associated with valve thickening in all dogs with artificial (tissue) valves. None have had fatal thromboembolic events or durable compromise to the valve function. One dog developed an enormous right atrial incisional thrombus. All dogs have been taken off warfarin therapy three months after valve replacement.

Post Operative Arrhythmia

Atrial tachyarrhythmia (presumed incisional re-entrant pathway) has been seen in two dogs, one dog required electroversion and was stabilized on sotalol, the other dog is stable in normal sinus rhythm, on sotalol.

Follow-up

Four dogs that have had tricuspid valve replacement all enjoy a better quality of life and are on less or no heart medication. One of these dogs died of osteosarcoma 8months after surgery. Two dogs that have had PS patches are clinically normal 20 months and ten months post op. One dog that had a DCRV patch is normal five months after surgery.

Comments

The smallest dog we have successfully treated was 12kg, the rest have been between 17kg and 60kg. Our progress over the first eleven patients has been better than we expected but could be, and will get better. We would be happy to treat left AV valve disease in an appropriately sized patient.

The Future

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.

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Daniel J. Brockman, BVSc, CVR, CSAO, DACVS, DECVS, ILTM, MRCVS
Royal Veterinary College
Hertfordshire, UK


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