Strut fracture mechanisms of the Björk-Shiley convexo-concave heart valve.

Investigations of convexo-concave (C/C) valve outlet strut fractures (OSFs) were initially confounded by knowledge that the strut was subject to bending forces in arresting the opening disc. Pulse duplicator studies subsequently showed that closing loads were all born by the inlet strut, along with an understandable focus on the nature of the welds, where most fractures occurred. As observations of explanted valves accumulated, certain features pointed to unusual closing loads that might be contributory factors, but these hypothetical forces could not be verified. Epidemiological extrapolations and case-matched control studies have shown that certain valve and patient characteristics were each associated independently with increased OSF risk, leading to clinically valuable risk stratification, but little additional understanding of why OSFs continued to occur. Detection of the causative, highly transient (< 0.5 ms), outlet-strut-tip impacts due to closing disc over-rotation that have almost ten times the force of disc opening, and the capability of inducing leg-base bending stresses beyond the strut wire's fatigue endurance limit had to await the development of computer-controlled pulse duplicators and strut-leg strain gaging. Exercised young animals easily achieved such strut loading, but most human patients would probably have more difficulty. The actual OSF mechanism is a long-term, valve-patient interaction that requires the concurrence of susceptible valve geometry and sufficient ventricular contractility potential to develop the isovolumic, high dP/dt needed for forceful disc over-rotation. Critical strut tip loading must then occur often enough to fatigue fracture both strut legs within the patient's lifetime with the valve.

Rapid intraoperative construction of autologous small caliber blood vessels.

Autologous pericardium, briefly tanned in glutaraldehyde, is an excellent biomaterial when used in various cardiac procedures, including repair and replacement of heart valves. A generalized lack of calcification and an absence of inflammatory response in these combined experiences has led the authors to consider the use of treated autologous pericardium for the construction of substitute small caliber blood vessels. Rapid, intraoperative construction of a vascular graft from autologous pericardium, briefly treated in 0.62% glutaraldehyde, is accomplished through the use of two concentric, mating helical stents that support a rectangular pattern of tissue into a cylindrical, nonkinking, compliant conduit. A disposable automated assembly tool provides for precise assembly of the tissue and stents. The tools and methods for construction of vascular grafts from nonvascular tissue are known as the Rapidgraft (Ramus Medical Technologies, Santa Barbara, CA). The technique can be used with any suitable tissue. Accelerated fatigue test studies have confirmed that stent supported pericardial grafts are capable of withstanding physiologic pressures and flows beyond 7 equivalent years. Six autologous pericardial grafts measuring 5 mm in diameter by 5.5 cm in length showed 100% patency beyond 5 months in a growing calf carotid artery model. Pathological examination of explanted grafts confirm the presence of a continuous endothelial lined surface, infiltration of the tissue by fibrous connective tissue such that the individual layers of the pericardial vessel wall could not be identified, and there was no inflammatory response. Based on the encouraging results in animal studies, a small multicenter clinical trial has been initiated to evaluate the performance of the Rapidgraft as a replacement conduit for the radial artery in cases in which the radial artery has been harvested as a coronary artery bypass conduit. Results from the radial artery trial will be used as an indication to evaluate the Rapidgraft as a coronary artery substitute. We conclude that the material properties, including the biological origin of a vessel wall, may be significant determinants of graft patency, and that the Rapidgraft may be an answer to the need for small caliber arterial graft with the potential for long-term patency and durability.

Removal of a cholesteatoma in a dog, using a caudal auricular approach.

A new surgical method for treating cholesteatoma in dogs is described. Although rarely reported in dogs, cholesteatomas may be more common than previously believed. Complete excision of a cholesteatoma is associated with low recurrence and good long-term prognosis. Surgical intervention, with total ear canal ablation and lateral bulla osteotomy, has been recommended in dogs with tumors of the middle ear; however, this technique often results in conductive hearing loss. Through a caudal auricular approach to the tympanic bulla, we were able to preserve the external ear canal, reconstruct the auditory ossicles, and graft the tympanic membrane. Results of brain stem auditory-evoked response tests in the dog revealed intact conduction potentials. A caudal auricular approach to the tympanic bulla is technically possible, preserves normal appearance, and may maintain, or even improve, hearing conduction of affected ears in dogs.

A computer-controlled, continuously circulating, hot irrigating system for endometrial ablation.

OBJECTIVE: This study evaluated the efficacy of a computer-controlled thermal catheter system to ablate the endometrium by use of 5 to 15 ml of hot saline solution. STUDY DESIGN: Five in vivo sheep uteri and 32 freshly excised human uteri were studied by inserting a self-contained heater catheter equipped with thermocouples and circulating hot saline solution for 15 minutes within the uteri at various temperature settings ranging from 60 degrees C to 80 degrees C. The uteri were grossly examined and microscopically evaluated by hematoxylin and eosin and reduced nicotinamide adenine dinucleotide-diaphorase stains for degree of acute destruction, depth of thermal injury, and predicted viability. RESULTS: All 80 degrees C sheep treatments resulted in complete loss of viability at mucosal, submucosal, and myometrial levels. In human uteri temperatures > or = 80 degrees C produced grossly recognizable thermal damage to the endometrium. Full-thickness irreversible damage was seen in the endometrium. Loss of viability was also demonstrated 1 to 3 mm deep in the superficial myometrium. Intrauterine pressure was demonstrated to range from 20 to 40 mm Hg, with a mean of 26 mm Hg. No leakage of fluid from the oviducts was observed. CONCLUSION: The data presented here demonstrate that a simple technique coupled with a sophisticated monitoring control system could successfully ablate the endometrium without the vagaries associated with individual operator skill levels.

Long-term clinical outcome after partial ligation of single extrahepatic vascular anomalies in 20 dogs.

This study reports the long-term clinical outcome after partial ligation of single extrahepatic vascular anomalies in 20 dogs. Outcome was classified as successful or complicated based on review of follow-up evaluations that consisted of telephone contacts with owners and veterinarians (n = 20), in hospital reevaluation (n = 15), and per-rectal portal nuclear scintigraphy (n = 12). Partial ligation was initially effective in resolving clinical signs in all 20 dogs. Long-term outcome was completely satisfactory in 50% of the cases. The mean time interval between surgery and onset of signs, or diagnosis of complications in the 10 dogs with a complicated outcome, was approximately 3 years. Central nervous system, gastrointestinal, and complications related to the urinary tract were most common. Acquired multiple extrahepatic venous communications occurred in at least 3 of 10 dogs (30%) with a complicated outcome. Long-term survival occurred in 13 of 20 dogs (65%) with partial ligation. The mean survival time for the seven dogs that died or were euthanatized was 3.6 years and the mean survival time for the 13 dogs still alive at the last follow-up was 5.0 years.

The effect of hypovolemic shock and reperfusion on the hepatic oxygen supply-uptake relationship in the dog.

The hepatic oxygen supply-uptake relationship was investigated during hypovolemic shock using a right heart bypass technique. The results were dissimilar to those previously reported in that the ratio of liver oxygen delivery to systemic oxygen delivery was significantly decreased during shock. The decreased ratio was due to a significant decrease in the portal venous oxygen delivery when compared to the decrease in the systemic oxygen delivery. The decrease in portal venous oxygen delivery was caused not only by the decrease in portal venous blood flow, but also by the decrease in oxygen content of portal blood. The ratio of hepatic arterial oxygen delivery, on the other hand, was significantly increased during shock. Hypovolemic shock increased the liver oxygen extraction ratio to nearly 100% of the pre-shock value. These findings suggest a hepatic protective mechanism for matching oxygen uptake to rising hepatic oxygen requirements. Liver oxygen delivery returned to pre-shock value after correction of hypovolemia primarily due to a significant increase in hepatic arterial oxygen delivery. A significant negative correlation between the liver oxygen extraction ratio and the oxygen content of hepatic venous blood was observed. The hepatic venous oxygen content appears to be a simple and appropriate index of liver oxygenation in clinical medicine because it is difficult to evaluate the liver oxygen extraction ratio directly.

Effects of hypovolemic shock and reperfusion on liver blood flow in the dog.

Liver blood flow was investigated in hypovolemic shock using a modified right heart bypass technique which can obtain accurate portal blood flow. Findings were similar to those previously reported: hepatic blood flow accounted for 34% of cardiac output in this study; 76% of hepatic blood flow was delivered from the portal vein and 24% from the hepatic artery. Hypovolemic shock markedly decreased total liver blood flow by a reduction in portal venous blood flow. The findings of this study provide evidence that mesenteric blood flow is a peripheral circulation circuit where blood flow is restricted during reduced circulatory volume. Development of a hepatic arterial buffer response during hypovolemic shock was confirmed by an increased ratio of hepatic arterial flow to cardiac output. Reduced total hepatic blood flow during hypovolemic shock returned to control flow by an increase in hepatic arterial flow after reperfusion. The results of this study demonstrate that compensated reactions for maintaining liver blood flow mainly due to the hepatic arterial buffer response were functioned both during hypovolemic shock and after elimination of shock.

Dynamics of Björk-Shiley Convexo-Concave mitral valves in sheep.

The outlet strut of Björk-Shiley (BSCC) Convexo-Concave heart valves has fractured in some implanted valves resulting in disc escape and emergency reoperation or death. The closing dynamics of BSCC heart valves was studied in situ to determine the forces acting on the outlet strut during valve closure. BSCC valves with strain gages attached to the outlet strut were implanted in the mitral position in sheep. Impact forces were measured under various circumstances including anesthetized, sedentary, and exercise conditions. Left ventricular (LV) pressures were recorded simultaneously via catheter-tip pressure transducers. Impact forces increased with the 1/3 power of left ventricular pressure rise and the square root of LV pressure at valve closure. Good agreement between the in vivo measurements and theoretical models was observed. Based on this study, it is concluded that sheep provides a good in situ model for the testing of mechanical mitral valves.

Tricuspid and mitral valvular disease: valve replacement.

Using the methods described, the author has performed AV-valve replacement surgery in seven clinical dogs with AV valve dysfunction. Two dogs received autologous tissue tricuspid valves, three dogs received autologous tissue mitral valves, one dog received a mechanical mitral prosthesis, and one dog received a mechanical mitral and autologous tricuspid prosthesis. The two dogs that received a tricuspid prosthesis recovered from surgery, one dog is normal nearly 3 years after valve replacement surgery, and the second dog was euthanized 3 weeks after replacement surgery because of complications related to other congenital cardiovascular defects (ie, pulmonary artery coartations). The dog that received the mechanical mitral valve prosthesis made a complete immediate postoperative recovery but died from hemorrhage complications related to a femoral artery catheter removal 3 days after surgery. Hemorrhage from the left atrium resulted in the death of one dog after implantation of a bioprosthetic mitral valve. Two dogs with mitral valve bioprostheses and one dog with a mitral and tricuspid valve replacement could not be weaned from the cardiopulmonary bypass pump, even with pharmacological support. Although the initial results in this small clinical trial have not been spectacular, it is apparent from the few clinical cases operated on that the equipment, supplies, and techniques have advanced sufficiently in veterinary surgical practice to perform AV-valve replacement surgery in companion animals. Prosthetic valves are available presently to perform such surgery even in small animal patients (5 kg). However, a significant obstacle that must be overcome before such techniques become routine is the selection of animal patients that have sufficient myocardial reserve to undergo valve replacement surgery.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Open heart surgery was performed during cardiopulmonary bypass (CPB) to surgically correct subvalvular aortic stenosis in seven dogs. After initiation of total CPB, cardiac arrest was induced by antegrade and retrograde administration of blood cardioplegia. The subvalvular fibrous stenosis was resected through a transverse aortotomy. Intraoperatively and postoperatively, dobutamine, nitroprusside, lidocaine, blood(-products), and crystalloid solutions were used to manage hypotension and optimize cardiac index. Aortic cross-clamp time varied from 73 to 166 minutes, and duration of CPB varied from 130 to 210 minutes. Iatrogenic incision into the mitral valve in two dogs was the most significant intraoperative complication. Postoperative complications included: hypoproteinemia (n = 7), premature ventricular depolarization (n = 6), increased systemic vascular resistance index (n = 5), increased O2 extraction (n = 3), pulmonary edema (n = 2), and decreased cardiac index (n = 1). All seven dogs were discharged alive and in stable condition. Six dogs are alive and in stable condition after a mean follow up of 15.8 months. This is the first detailed report of CPB in a series of clinical veterinary patients. Using the techniques described in this paper, open heart surgery of considerable duration can be performed successfully in dogs with significant myocardial hypertrophy and endomyocardial fibrosis secondary to subvalvular aortic stenosis.

Experimental evaluation of an autologous tissue heart valve.

The goal of the autologous tissue heart valve (ATHV) prosthesis project has been the development of a non-antigenic, non-calcifying bioprosthesis of greater durability than heterograft or homograft bioprostheses. It is made in the operating room, at a sterile work bench, during surgery for heart valve replacement. Autologous pericardium is used for valve construction after a five minute immersion in 0.6 per cent glutaraldehyde buffered to pH 7.4 with phosphate. The stent-mounted trileaflet prosthesis can be made in five minutes with a semi-automated method that uses two concentric mating stents that substitute clamping for sewing of the tissue. In vitro testing, to include pulse duplicator, accelerated wear tester, static testing for leakage and tensile strength testing, has been performed with ATHVs made with glutaraldehyde-tanned bovine and ovine pericardium. Transvalvular pressure gradients were measured at 3.3-7.3 mmHg at flow rates of 4-5 l/min. Six valves have been tested beyond 800,000,000 cycles with full opening and closing at differential closing pressures of 125 mmHg. One of the valves developed a 2mm leaflet tear after 26,000,000 cycles but the remaining five survived intact. No fractures were seen in the Dacron covered Delrin stents. Six ATHVs were implanted in juvenile sheep for five months. One animal died after five months of infective endocarditis secondary to an unrecognized deep wound infection and the other five were electively sacrificed at the same time interval. Four valves were fully competent at terminal elective cardiac catheterization and one showed minimal insufficiency attributed to a paravalvular leak. The leaflet tissue was free of generalized calcification in all instances. There was no evidence of leaflet tissue thickening or shrinkage. The mean calcium content at necropsy of the 15 leaflets from the five valves was 8.357 mg/g of tissue. There is experimental evidence that an ATHV made of pericardium treated briefly with glutaraldehyde may achieve the goal of a non-calcifying, more durable bioprosthesis.

Splanchnic surface oximetry during experimental portal hypertension and surgical manipulation of portosystemic shunts in dogs.

Surface oxygen tension (PSO2) was measured in dogs during experimental manipulation of the portal vein and hepatic artery, and during surgery to correct portosystemic shunting. There was no alteration in PSO2 of liver, pancreas, duodenum, or jejunum during partial (50%) or complete reduction of hepatic artery flow. After 100% reduction in portal vein blood flow, PSO2 was lower in jejunum, duodenum, and liver. With 50% reduction in portal flow, PSO2 was significantly decreased only in jejunum. In six dogs with single extrahepatic shunts, there was a significant correlation between portal pressure and jejunal PSO2. It was concluded that measurement of visceral organ PSO2 represents an accurate noninvasive means of obtaining objective data on the effect of reduction in hepatic blood flow on perfusion of other select splanchnic organs.

Endotoxin concentrations measured by a chromogenic assay in portal and peripheral venous blood in ten dogs with portosystemic shunts.

A chromogenic Limulus amebocyte lysate assay was used to measure portal and peripheral venous endotoxin concentrations in ten medically managed dogs undergoing surgery for correction of a single extrahepatic portosystemic shunt. In all dogs, both peripheral and portal venous blood samples were obtained at the time of surgical manipulation of the anomalous vessel. In six dogs, peripheral venous samples were obtained an average of 8.0 months after surgery. Five physically normal dogs without biochemical or histologic evidence of liver disease served as controls. Data analysis failed to demonstrate significant differences in peripheral and portal venous endotoxin concentrations between the control and study groups. Postoperatively five of six dogs showed a measurable reduction in peripheral venous endotoxin concentration over intraoperatively obtained values, but the differences were not statistically significant (P = 0.06). Based on results of this study it was concluded that systemic endotoxemia was not present in dogs with a single extrahepatic portosystemic shunt that were medically stable prior to surgery.

Intrahepatic portosystemic venous anomaly in the dog. Perioperative management and complications.

Hypotension, hemorrhage, and acute hepatic congestion were common complications during surgical correction of intrahepatic portosystemic venous anomalies in 13 dogs. Acute splanchnic congestion and hyperthermia were the most serious postoperative complications. Three dogs died within 24 hours; 10 dogs are alive and well after 12 to 46 months.

Comparison of shunt fraction estimation using transcolonic iodine-123-iodoamphetamine and technetium-99m-pertechnetate in a group of dogs with experimentally-induced chronic biliary cirrhosis.

Portosystemic shunt fraction estimation using transcolonic iodine-123-iodoamphetamine (IMP) has been previously validated relative to portal vein macroaggregated albumin injections using an experimental model of cirrhosis. Transcolonic technetium-99m-pertechnetate (TcO4-) has been proposed as an alternative tracer to IMP to study portal circulation in cirrhotic patients. We compared shunt fraction estimates from paired transcolonic IMP and TcO4- studies performed on a group of dogs before and after common bile duct ligation surgery. Pertechnetate over-estimated shunt fraction in 6/7 postoperative studies relative to IMP. A good correlation between the two methods was demonstrated, however, the slope of the regression line was substantially less than 1.0 with TcO4- values reaching 100% at IMP shunt values of approximately 60%. This apparent inability to accurately assess high shunt flows may limit the quantitative aspects of TcO4- studies on patients with severe portosystemic shunting.

Pleurovenous shunting technique for treatment of chylothorax in three dogs.

The operative technique for placement of a pleurovenous shunt catheter is described. Pleurovenous shunting was used in the surgical treatment of 3 dogs with chylothorax refractory to medical management. Chylothorax in 2 of the dogs was palliated, but because of severe restrictive fibrous pleuritis, the third dog was euthanatized during follow-up surgery.

Congenital feline portosystemic shunts.

Five young cats with portosystemic communications, 2 with single intrahepatic and 3 with single extrahepatic portosystemic communications, were managed surgically. One cat with a ductus venosus was treated successfully by surgery. Ptyalism and behavioral changes were similar in all 5 cats. Biochemical abnormalities included low BUN values, increased blood ammonia values, and increased sulfobromophthalein retention. None of the cats had portal hypertension at the time of surgery. Seemingly, single portosystemic shunts should be considered a surgical disease in the cat.

Constrictive pericardial disease in the dog.

The clinicopathologic features of constrictive pericardial disease in 13 dogs were reviewed. The causes were infection (3 dogs), metallic foreign body (1 dog), and idiopathic (9 dogs). Owner complaints included abdominal enlargement, tachypnea, weakness or syncope, exertional fatigue, and weight loss. Ascites and jugular venous distention were consistently observed, whereas abnormalities of arterial pulses and heart sounds were variable and inconsistent. Diminished QRS voltages were common. Mild to moderate cardiomegaly, rounding of the cardiac silhouette, and variable and nonspecific angiographic findings were frequently observed. Cardiac catheterization consistently showed elevation and equilibration of atrial and ventricular diastolic pressures, but a prominent early diastolic (y) descent was uncommon. Fibrosis was confined to the parietal pericardium in 8 dogs, and included the epicardium in 5 dogs. Parietal pericardectomy was successful in relieving the syndrome in 6 of 10 dogs. Pulmonary thrombosis was the most common cause of early postoperative mortality.

alpha-Adrenoreceptor stimulation of the systemic circulation in dogs.

The influence of phenylephrine (an alpha-adrenoreceptor agonist) on the arterial and venous systems of the systemic circulation was studied in 10 anesthetized dogs during a right ventricular bypass procedure. Phenylephrine (1 microgram X kg-1 of body weight X min-1) produced a significant (P less than 0.001) increase in arterial resistance, no change in venous resistance, and a mild decrease in venous compliance. Seemingly in the clinically normal dog, a dose of phenylephrine sufficient to double total peripheral resistance will redistribute a small amount of blood volume from the venous to arterial systems with a slight decrease in venous return and concomitant attenuation of cardiac output.