Resection of subvalvular aortic stenosis. Surgical and perioperative management in seven dogs

Subvalvular Aortic Stenosis: Learning From Human and Canine Clinical Research

Subvalvular aortic stenosis (SAS) is the most common congenital heart disease (CHD) in dogs and is also prevalent in human children. A fibrous ridge below the aortic valve narrows the left ventricular outflow tract (LVOT) and increases blood flow velocity, leading to devastating side effects in diseased patients. Due to the similarities in presentation, anatomy, pathophysiology, cardiac development, genomics, and environment between humans and dogs, canine SAS patients represent a critical translational model of human SAS. Potential adverse outcomes of SAS include arrhythmias, left-sided congestive heart failure, endocarditis, exercise intolerance, syncope, and sudden cardiac death. The greatest divergence between canine and human SAS clinical research has been the standard of care regarding treatment of these outcomes, with pharmacological intervention dominating best practices in veterinary medicine and surgical intervention comprising the standard practice for human SAS patients. Regardless of the species, the field has yet to identify a treatment option to prevent disease progression or permanently remove the fibrous ridge, but historical leaps in SAS research support a continued translational approach as the most promising method for achieving this goal.

Open-heart techniques and mitral valve plasty for mitral regurgitation in toy- and small-breed dogs: A review

In human medicine, in the past, open-heart techniques for low-bodyweight children and newborn babies with congenital heart disease were more difficult than high-bodyweight adults. In toy- and small-breed dogs with mitral regurgitation (MR), an acquired heart disease, these techniques are more difficult to perform than for congenital heart diseases in young medium-sized or large dogs because of old age and low body weight. Therefore, improved open-heart techniques and mitral valve surgery for severe MR in older toy- and small-breed dogs are essential. Through our surface-cooling hypothermia (sHT) studies, we designed a new, improved open-heart method, namely, “the low-flow cardiopulmonary bypass (CPB) combined with deep sHT in toy- and small-breed dogs (Japan method)”; sHT was later replaced by blood-cooling hypothermia (bHT). At the same time, we devised a new, improved mitral valve plasty (MVP) applicable to severe MR, instead of mitral valve replacement, in toy- and small-breed dogs. This MVP technique was combined with artificial chordal reconstruction, semi-circular suture annuloplasty (AP), and direct scallop-suture valvuloplasty. These MVP techniques are simple, durable, and lead to good long-term quality of life in toy- and small-breed dogs. This review highlights the benefits of our improved CPB and MVP techniques (Japan method) for severe MR in toy-and small-breed dogs, which have led to a high success rate for MVP in severe clinical MR cases in Japan. It may further contribute to the development of more robust techniques for MR in toy- and small-breed dogs. This also represents the first comprehensive review of the history of open-heart surgery, CPB techniques, and MVP methods for MR in toy- and small-breed dogs.

The history of veterinary cardiology

Throughout civilization, animals have played a pivotal role in the advancement of science and medicine. From as early as 400 BC when Hippocrates recognized that diseases had natural causes, the steadfast advances made by biologists, scientists, physicians and scholars were fueled by timely and important facts and information- much of it gained through animal observations that contributed importantly to understanding anatomy, physiology, and pathology. There have been many breakthroughs and historic developments. For example, William Harvey in the 16th and 17th centuries clarified the importance of the circulatory system, aided by observations in dogs and pigs, which helped to clarify and confirm his concepts. The nineteenth century witnessed advances in physical examination techniques including auscultation and percussion. These helped create the basis for enhanced proficiency in clinical cardiology. An explosion of technologic advances that followed in the 20th century have made possible sophisticated, accurate, and non-invasive diagnostics. This permitted rapid patient assessment, effective monitoring, the development of new cardiotonic drugs, clinical trials to assess efficacy, and multi-therapy strategies. The latter 20th century has marshaled a dizzying array of advances in medical genetics and molecular science, expanding the frontiers of etiologies and disease mechanisms in man, with important implications for animal health. Veterinary medicine has evolved during the last half century, from a trade designed to serve agrarian cultures, to a diverse profession supporting an array of career opportunities ranging from private, specialty practice, to highly organized, specialized medicine and subspecialty academic training programs in cardiology and allied disciplines.

Challenges of cardiopulmonary bypass-a review of the veterinary literature

Cardiopulmonary bypass (CPB) has been used in veterinary medicine in experimental surgery and to address congenital and acquired diseases. We review the veterinary literature and expose common challenges of CPB in dogs and cats. Specifically, we describe the most specific elements of this technique in veterinary patients. The variety in animal size has made it difficult to standardize cannulation techniques, oxygenators, and priming volumes and solutions. The fact that one of the most common cardiovascular disorders, mitral valve disease, occurs predominantly in small dogs has limited the use of bypass in these patients because of the need for small, low prime oxygenators and pumps that have been unavailable until recently. Coagulation, hemostasis, and blood product availability have also represented important factors in the way CPB has developed over the years. The cost and the challenges in operating the bypass machine have represented substantial limitations in its broader use.

Open heart surgery with deep hypothermia and cardiopulmonary bypass in small and toy dogs

OBJECTIVE

To evaluate open heart surgery with deep surface-induced hypothermia (sHT) and low-flow cardiopulmonary bypass (CPB) in small and toy-breed dogs.

STUDY DESIGN

Case series.

ANIMALS

Small breed dogs (n=8) weighing <5.5 kg with naturally occurring cardiac disease.

METHODS

Deep sHT under isoflurane anesthesia and low-flow rate CPB with a small-volume prime circuit were used. Ventricular septal defect was closed directly in 2 dogs and severe mitral regurgitation was corrected with mitral valvuloplasty (MVP) in 5 dogs and mitral valve replacement in 1 dog.

RESULTS

All dogs survived surgery; 1 dog died 6 days and 1 died 2 months after MVP. The other 6 dogs lived (mean follow-up, 32.8 months; range, 12-65 months). Mean body weight at surgery was 3.6 kg (range, 2-5.3 kg). Mean lowest esophageal temperature was 21.4 degrees C (range, 19.8-23.8 degrees C). Mean lowest pump flow volume was 29.2 mL/kg/min (range, 9.4-57.7 mL/kg/min) during aortic cross-clamping (mean, 53.5 minutes; range, 25-79 minutes). Mean hematocrit before CPB was 38.6% (range, 33-47%) and 20.3% (range, 13-24%) during CPB with a small circuit priming volume of 225-260 mL.

CONCLUSION

Deep sHT with low-flow rate CPB may be used for open heart surgery in small dogs weighing <5.5 kg.

CLINICAL RELEVANCE

Open heart surgery for selected congenital defects and acquired defects in small and toy-breed dogs may be successfully performed using deep sHT and CPB.

Anesthetic management of an off-pump open-heart surgery in a dog

OBSERVATIONS

A 9 year-old, 40 kg, female spayed Bouvier des Flandres was anesthetized for surgical removal of an intra-cardiac mass. Pre-anesthetic work-up included thoracic radiographs, which revealed moderate pleural effusion, and cardiac ultrasound, which identified a mass attached to the wall of the right ventricular outflow tract (RVOT). The mass caused dynamic obstruction of the RVOT during systole. The dog was pre-medicated with intravenous (IV) hydromorphone (0.05 mg kg(-1)). Following pre-oxygenation, anesthesia was induced with ketamine (3.75 mg kg(-1), IV) and diazepam (0.18 mg kg(-1), IV). Anesthesia was maintained with isoflurane in oxygen, an intravenous constant rate infusion (CRI) of fentanyl (10-30 microg kg(-1) hour(-1)) and a CRI of lidocaine (50-200 microg kg(-1) minute(-1)). A right lateral thoracotomy was performed. The heart was stopped transiently with a cold cardioplegic solution for 7.83 minutes to allow the removal of the mass through an open-heart procedure. No cardiopulmonary bypass was used. The heart was successfully restarted after cardiopulmonary resuscitation with internal cardiac massage and internal defibrillation. The dog recovered uneventfully from anesthesia without any apparent neurological sequelae. Post-operative analgesia consisted of intercostal nerve blocks with bupivacaine, CRIs of fentanyl (2-5 microg kg(-1) hour(-1)) and lidocaine (40 microg kg(-1) minute(-1)) and with oral meloxicam (0.1 mg kg(-1)). Five days following surgery, the dog was discharged from the hospital. Histopathology and immunohistochemistry of the mass identified an ectopic thyroid carcinoma.

CONCLUSIONS

This case showed the feasibility of whole body hypothermia and using a cold cardioplegic solution to induce cardiac arrest for a short open-heart procedure.

An Experimental Study of Apico-Aortic Valved Conduit (AAVC) for Surgical Treatment of Aortic Stenosis in Dogs

A new valved conduit was developed using a canine aortic valve. The bioprosthetic valve was fixed with glutaraldehyde and epoxy compound (Denacol-EX313/810). A vascular graft composed of ultra-fine polyester fiber (10 mm in diameter, 200 mm in length) was used. Four dogs underwent apico-aortic valved conduit (AAVC) implantation and aortic banding (bypass group, BG), while another 4 dogs underwent aortic banding without AAVC implantation (control group, CG). Cardiac catheterization and angiocardiography were performed for assessment of hemodynamics 2 weeks and 6 months after surgery. Left ventricular systolic pressure, left ventricular end-diastolic pressure and the left ventricular-aortic pressure gradient differed significantly (P<0.01) between the BG and CG dogs. Left ventricular angiocardiography showed patency of the valved conduit in all the BG dogs. Echocardiography was performed before and 2, 4 and 6 months after surgery, and showed that while pressure overload caused concentric myocardial hypertrophy in the CG dogs, the left ventricle dilated eccentrically in the BG dogs. Furthermore, relief of left ventricular pressure overload by AAVC was maintained.

Surgical correction of subaortic stenosis via right ventriculotomy and septal resection in a dog

After 2 years of medical management with a beta-adrenoreceptor blocking agent, a 30-month-old castrated male Golden Retriever with subaortic stenosis was treated surgically because of progression of its condition. In an attempt to achieve complete relief of the left ventricular outflow obstruction, a modified Konno procedure consisting of right ventriculotomy and septal myectomy from the infundibular portion of the right ventricle was performed; this combination of procedures allowed wide resection of the septal portion of the left ventricular outflow obstruction. Two years after surgery, the mass of the dog's left ventricle had decreased and the peak calculated pressure gradient across the aortic valve had decreased to 40 mm Hg, compared with a preoperative value of 240 mm Hg; at that evaluation, the dog had gained weight and was able to play normally. It is suggested that use of this modified approach to the outflow tract may have a positive effect on long-term survival time in dogs with subaortic stenosis.

Surgical Correction of Subvalvular Aortic Stenosis Using Cardiopulmonary Bypass in a Dog

A three-month-old male Golden Retriever had symptoms including exercise intolerance, dyspnea and syncope and was diagnosed with subvalvular aortic stenosis. Cardiac catheterization revealed a left ventricular-aortic systolic pressure gradient of 90 mm Hg. Surgical correction of the condition was achieved using cardiopulmonary bypass. The subvalvular fibrous lesion was resected through an aortotomy. The stenosis was dilated from 8.5 mm to 12.0 mm in diameter. Postoperatively the dog was asymptomatic. Seven months after surgery, the pressure gradient decreased to 44 mm Hg. However, after another three months, the dog died suddenly without any premonitory signs. Postmortem examination revealed that pathologic changes caused by increased left ventricular pressure overload were not severe.

Cardiopulmonary bypass in the cat

OBJECTIVE

To assess the physiologic response to, and acute survival of, cats undergoing cardiopulmonary bypass (CPB) and to evaluate the efficacy of a commercial human pediatric oxygenator system on cats weighing less than 6 kg.

STUDY DESIGN

Experimental study.

ANIMALS

Six intact male cats

METHODS

Cats were placed on cardiopulmonary bypass by cannulating the cranial and caudal vena cavae and the carotid artery. The pediatric CPB circuit was primed with 150 mL of a balanced crystalloid solution. Venous drainage was enhanced by a controlled, vacuum-assist system. A cross-clamp was placed on the ascending aorta and cardiac arrest was induced by antegrade infusion of a cold cardioplegia solution. After 45 minutes of arrest time, the cross-clamp was removed and the cats were weaned off bypass and decannulated. No blood products were administered. Heart rate, mean arterial pressure (MAP), central venous pressure, arterial blood gas, hematocrit (HCT), total plasma protein concentration (TP), serum electrolyte concentrations, and activated clotting time (ACT) were measured at baseline period (BL), during CPB, 60 minutes after CPB (CPB 60) and 90 minutes after CPB (CPB 90). A complete blood count (CBC), blood chemistry profile, and urinalysis were performed at BL, during CPB, and CPB 90. Cats were euthanatized after CPB 90.

RESULTS

Cardiopulmonary bypass resulted in a significant (P <.05) decrease in mean HCT (18.0%) and TP (2.3 gm/dL) at CPB 90 when compared to BL (30.5% and 6.0 gm/dL, respectively). The MAP at CPB 90 (54 mm Hg) was decreased from BL (94 mm Hg). The ACT increased from a mean of 124 seconds to > 400 seconds with heparinization and was reversed to 300 seconds with protamine. Mean platelet counts decreased from BL (369,000 /microL) to CPB 90 (94,500 /microL). Mean white blood cell counts decreased from 13,200 /microL at BL to 2,200 /microL at CPB 90. Upon reperfusion, 1 cat fibrillated but was successfully defibrillated.

CONCLUSIONS

Cardiopulmonary bypass was performed successfully in 6 cats weighing less than 6 kg. Acute survival to 90 minutes after CPB was achieved in all 6 cats

CLINICAL RELEVANCE

The ability to perform CPB in the cat may allow intracardiac repair of various heart defects in this species.

Deep hypothermic low flow cardiopulmonary bypass in small dogs

OBJECTIVE

To evaluate the feasibility of and morbidity and mortality associated with cardiopulmonary bypass (CPB) using deep hypothermia and low flow perfusion in adult dogs weighing less than 10 kg.

STUDY DESIGN

Prospective, descriptive study.

ANIMALS

Two groups of three dogs underwent CPB. Group 1 dogs underwent deep hypothermia (15 to 18 degrees C), 45 minutes of low perfusion flow (20 mL/kg/min) and 1 hour of aortic cross clamp time. In group 2, ultrafiltration of perfusate before discontinuation of bypass was added to the standard treatment. Complete blood counts, serum biochemistry, urine output, ejection fraction, and cardiac output were monitored before and for 7 days after surgery.

RESULTS

All dogs were successfully weaned from bypass. Four of six dogs survived, three without major complications. One dog developed and recovered from septic pleuritis. Two dogs died or were euthanatized after surgery because of respiratory or gastrointestinal complications. Minor complications included anemia, hypoproteinemia, and electrolyte disturbances. Transfusion requirements and edema formation were reduced by ultrafiltration.

CONCLUSIONS

The observations in this study support the feasibility of low flow hypothermic CPB. Meticulous tissue handling, precise equipment, ultrafiltration, and aggressive postoperative potassium supplementation are recommended for smaller patients.

CLINICAL RELEVANCE

Increased sensitivity to adverse sequelae of CPB may be associated with small patient size. Further evaluation is necessary before routine clinical application of low flow hypothermic CPB in this patient population.

Surgical management of subvalvular aortic stenosis and mitral dysplasia in a golden retriever

A 12-month-old neutered male golden retriever was presented with a history of lethargy and exercise intolerance. Clinical examination, electrocardiography, radiography and echocardiography supported a diagnosis of fixed subvalvular aortic stenosis with a Doppler pressure gradient of 77.5 mmHg. Surgical inspection also revealed gross structural abnormalities of the mitral valve consistent with mitral dysplasia. Intervention consisted of resection of the dysplastic mitral valve and the subvalvular aortic stenosis. The mitral valve was replaced with a bioprosthetic valve. Total cardiopulmonary bypass time was 65 minutes and aortic cross-clamp time was 55 minutes. A full recovery was made and 11 months postoperatively the aortic transvalvular gradient was 30 mmHg. At the time of writing, 12 months after surgery, the dog was clinically normal and requires no medication.

Controlled cross circulation in dogs: effects on donor hemodynamics

Controlled cross circulation (CCC) was performed in six pairs of dogs for 45 minutes with aortic cross clamping and cardioplegia. Data were collected in donor dogs at 10 minute intervals three times before, three times during, and three times after CCC and included arterial blood pressure, pulmonary capillary wedge pressure (PCWP), central venous pressure (CVP), cardiac index (CI), heart rate (HR), blood gas analysis, temperature, maximum rate of rise of left ventricular pressure dP/dt max/End diastolic volume (EDV), blood volume (BV), complete blood count (CBC) and activated clotting times (ACT). Pulse pressure (PP), systemic vascular resistance (SVR), oxygen delivery (DO2), and left ventricular cardiac work (LVCW) were calculated. Arterial blood pressure, CVP, blood gas analysis, temperature, BV, CBC, and ACT were measured in recipient dogs. During CCC, donor hemodynamic changes resembled those observed in models of acute onset arteriovenous fistulas. Insidious BV shifts can occur despite the use of occlusive roller pumps. After CCC, donor hemodynamics resembled acute blood loss, characterized by decreases in mean arterial pressure (MAP), CVP, PCWP, and CI, and increases in SVR and dP/dt max/EDV. These changes were probably caused by pump imbalance and BV shift to the recipient dog.