Onset and progression of calcification in porcine aortic bioprosthetic valves implanted as orthotopic mitral valve replacements in juvenile sheep.

The purpose of this study was to characterize the onset and progression of mineralization in porcine bioprosthetic valves implanted in sheep and to test the hypothesis that such valves simulate calcification that is observed clinically and in other experimental models. Hancock I porcine aortic bioprosthetic valves (Medtronic Heart Valve Division, Irvine, Calif.) were implanted as orthotopic mitral valve replacements in juvenile sheep, retrieved after 1 to 124 days, and analyzed as follows: gross inspection, radiography, light, transmission, and surface scanning electron microscopy, and calcium analysis by absorption spectroscopy. Mineralization increased with increasing time after implantation in both valve cusps and adjacent aortic wall. Mean cuspal calcification was 80 micrograms/mg in valves removed after 3 to 4 months. Nevertheless, considerable variability among valves was apparent in the level of calcification noted at specific time intervals. Virtually all aspects of the morphologic characteristics were identical to those previously noted for clinical explants and experimental specimens, both subcutaneous and circulatory. In particular, ultrastructural examination revealed that the earliest calcific deposits were associated with devitalized cuspal connective tissue cells and their fragments. Collagen calcification was sparse. Both surface scanning and transmission electron microscopy indicated a lack of endothelial or blood-derived cells on the valves at all sampling times. We conclude that porcine bioprosthetic valves implanted as mitral valves in sheep provide a useful calcification model, simulating morphologic and pathobiologic events that occur clinically and in noncirculatory models. However, sufficient specimen replicates must be done to overcome variability in calcification among valves and sampling sites.

Comparison of catecholamine effects on canine myocardial metabolism and regional blood flow during and after cardiopulmonary bypass.

The acute metabolic and hemodynamic effects of dopamine, dobutamine (both at 10 micrograms . kg-1 . min), and isoproterenol (at 0.05 or 0.1 micrograms . kg-1. min) were determined in dogs following 20 minutes of normothermic global myocardial ischemia. The catecholamines were started 10 minutes before cardiopulmonary bypass (CPB) was discontinued and were continued for 1 hour after bypass. Regional myocardial and systemic blood flow distribution was measured by means of the radioactive microsphere technique. On bypass all catecholamines sharply increased heart rate, myocardial oxygen consumption, and left ventricular blood flow (p less than 0.01). Because the hearts were unloaded, these data suggest that velocity of contraction is an important component of myocardial oxygen consumption. Although these drugs did not lower myocardial adenosine triphosphate (ATP) and creatine phosphate (CP) levels, the significant rise in oxygen consumption suggested that inotropic treatment on bypass may not be beneficial. Furthermore, renal blood flow was diminished in dobutamine-treated dogs (p less than 0.01) and tended to decrease with isoproterenol infusion. No change was seen with dopamine infusion. After bypass, dobutamine treatment increased cardiac output (p less than 0.01) and stroke volume (p = 0.017) with no change in heart rate, myocardial oxygen consumption, high-energy phosphate levels, and total or transmural distribution of left ventricular blood flow. Dopamine infusion did not change cardiac output but did increase oxygen consumption (p less than 0.01). Isoproterenol showed a slight inotropic effect, but frequent ventricular arrhythmias were present during weaning from bypass. In all treatment groups, blood flow in the other systemic beds (cerebral, gastrointestinal, and renal) was similar to that in control dogs. These data suggest that dobutamine is the most efficient of the drugs tested for support of the heart following global myocardial ischemia but, when given during bypass, it appears to decrease renal blood flow.

Antimineralization treatments for bioprosthetic heart valves. Assessment of efficacy and safety.

Since calcification limits the durability of contemporary bioprosthetic heart valves, antimineralization treatments are being widely investigated. Potential antimineralization treatments must have sustained prevention of mineralization without adverse effects. The preclinical investigation of efficacy and safety of antimineralization treatments comprises four essential steps: (1) subcutaneous implantation in small animals, (2) in vitro biomechanical studies of hemodynamics and durability, (3) morphology of unimplanted valves, and (4) circulatory implants in large animals.

Brain damage in profound hypothermia. Perfusion versus circulatory arrest.

To investigate brain changes in induced deep core hypothermia (18 degrees C) with or without circulatory arrest, four groups of dogs were subjected to cardiopulmonary bypass (CPB) under the following conditions: (1) differential head perfusion with pulsatile flow and simultaneous circulatory arrest to the rest of the body; (2) differential perfusion to the head with a nonpulsatile flow; (3) total circulatory arrest; and (4) continuous hypothermic perfusion. Parameters analyzed were: (1) blood flow distribution; (2) creatine kinase isoenzyme (CK-BB) elevation in the cerebrospinal fluid (CSF) and in the brain venous return; and (3) microscopy of the brain in animals killed at 30 minutes, 24 and 48 hours, 1 and 2 weeks, and 1 month. Although minor brain tissue flow differences were found at 37 degrees C among the groups, flows equalized at 18 degrees C. A significant seven-fold brain flow increase followed the period of circulatory arrest in Group III. Rise of CK-BB levels occurred in brain venous return but not in CSF in all groups. Microscopic cellular damage appeared in all groups with an equal degree of severity, regardless of the method of hypothermia and perfusion implemented.

Adequacy of ascending aorta-descending aorta shunt during cross-clamping of the thoracic aorta for prevention of spinal cord injury.

The effectiveness of various sized shunts placed between the ascending and the descending aorta to prevent paraplegia in dogs with the thoracic aorta cross-clamped for 1 hour was tested. Three tapered shunts sizes were used with tip dimensions of 3.8, 5.2, and 6.3 mm inner diameter, with cross-sectional areas of 11.34, 21.23, and 33.18 mm2, respectively, and with an equal midportion diameter of 10 mm (3/8 inch). These shunts carried 40%, 60%, and 72% respectively, of baseline descending aortic flow during the cross-clamping period. Flow distribution was measured with radioactive microspheres in the spinal cord (gray and white matter) and kidneys. All dogs without shunts (Group I) developed paraplegia, severe proximal circulatory embarrassment, and severe ischemia of the spinal cord (mainly gray matter) that was followed by marked hyperemia persisting up to 24 hours following the experiment. Mortality was 33%. Only animals treated with large shunts (Groups III and IV) avoided paraplegia and postischemic injury. An effective shunt was characterized as carrying 60% or more of baseline descending aortic flow, having a cross-sectional area at its tip equal to or larger than 29% of the descending aorta, and equaling at least 54% of its diameter. Porportionately, the size of the tridodecylmethylamonium-heparin shunts being used in human beings (even the largest 9 mm inner diameter) is significantly inadequate to maintain distal flows and pressures for the prevention of spinal cord injury. Four clinical options are discussed.

Composite and plain tubular synthetic graft conduits in right ventricle-pulmonary artery position: fate in growing lambs.

Our goal was to identify the most appropriate material for right ventricle-pulmonary artery conduits in growing animals. We used 100 lambs that were 3 to 4 weeks old (mean weight 11.7 kg). Follow-up was up to 24 months. Group I received plain tubular conduits: (1) Dacron knitted fabric, (2) collagen-coated knitted fabric, (3) Milliknit and Microknit material, (4) woven Dacron fabric, (5) three-dimensional Dacron fabric (crossweave 500 and 800), or (6) polytetrafluoroethylene. Group II received either a (1) woven Dacron fabric conduit with a built-in tissue valve or (2) polytetrafluoroethylene graft with a built-in St. Jude Medical valve. We did angiograms and catheterizations every 3 to 6 months and killed the lambs at 6, 12, 18, or 24 months. Tubular Dacron fabric woven or knitted grafts, regardless of matrix, pore size, thickness, or coating, caused formation of a thick acellular pseudointima buildup, which led to progressive obstruction starting as early as 3 months. Polytetrafluoroethylene grafts in groups I and II showed the formation of thin inner and outer capsules (0.5 mm) and none developed obstruction despite wall calcification. Conduits of woven Dacron fabric with a built-in tissue valve degenerated rapidly, leading to calcification thrombosis and obstruction within 3 months; no lamb survived 12 months. Polytetrafluoroethylene conduits with a St. Jude Medical valve in lambs receiving anticoagulants remained free of obstruction and continued to function well. It appears that synthetic conduits of polytetrafluoroethylene perform well in either of the situations here tested and may be the best choice at present.

Blood flow to the latissimus dorsi muscle pouch during chronic counterpulsation stimulation.

The cyclic contraction of a skeletal muscle ventricle (SMV) stimulated in counterpulsation results in phasic perfusion of the muscle. Perfusion will occur primarily during cardiac systole when the muscle is relaxed. However, the resting preload of the SMV will be systolic arterial pressure, which will impede blood flow to the relaxed muscle. To determine the effect of chronic counterpulsation stimulation on the blood flow to an SMV and identify stimulation regimens that prevent the risk of chronic ischemia, SMVs were created in four mongrel dogs by implementing an implantable mock circulation device. The SMV was stimulated in counterpulsation for 4 weeks after a 2-week vascular delay period and 2 weeks of low-frequency muscle conditioning. During biweekly studies, the muscle was stimulated in four modes against preloads varying from 20 to 120 mm Hg. Resting blood flow decreased significantly at preloads greater than 60 mm Hg. Normalized blood flow increased between 10% and 30% during stimulation; greater increases corresponded to more demanding stimulation modes. The elevated blood flow during stimulation, however, decreased with increasing preload. Stroke work increased with increasing preload until preload exceeded 100 mm Hg. The decreased blood flow and increased stroke work occurring at higher preloads indicate that the supply/demand ratio becomes compromised with increasing preload. A hyperemic response occurred during the resting beats after a stimulated beat, increasing the volume blood flow by as much as 80%. This response occurred regardless of preload or stimulation rate. If the SMV relaxed before the onset of systole, a hyperemic response occurred within the stimulated beat.(ABSTRACT TRUNCATED AT 250 WORDS)

Recovery of adenine nucleotide levels after global myocardial ischemia in dogs.

Loss of myocardial adenosine triphosphate (ATP) during ischemia can result in decreased cardiac function. Postischemic ATP levels remain low, and the reason for this is poorly understood. Previous attempts to enhance ATP recovery after ischemia have been only partially successful. To determine the long-term dynamics of ATP recovery and evaluate the effect of providing ATP precursors, we devised a method of obtaining sequential ventricular biopsies in dogs after 20 minutes of normothermic global ischemia on cardiopulmonary bypass. Our kinetic data show adenine (A) is metabolically favored over adenosine to regenerate ATP levels when adequate ribose (R) is present to produce phosphoribosylpyrophosphate. Therefore A (20 mM) plus R (80 mM) or saline (NS) was infused (1.0 ml X min-1) into the right atrium of dogs for 48 hours after ischemia. During A infusion myocardial tissue A was 0.19 +/- 0.07 nmol X mg-1, arterial A was 18.3 +/- 1.3 microM, coronary sinus A was 11.0 +/- 1.6 microM, and extraction of A by the myocardium was 38% +/- 10%. We found that while the decrease in ATP levels during ischemia was at least 50% in both groups, the postischemic ATP recovery rate in A/R dogs was more than eightfold greater than de novo synthesis (2.8 +/- 0.59 versus 0.34 +/- 0.06 nmoles X mg-1 X day). ATP levels in NS dogs were only 54% +/- 8% of preischemic values by 48 hours and required 9.9 +/- 1.4 days for full recovery. Recovery in A/R dogs required 1.2 +/- 0.2 days. Our results reveal that ATP recovery after a significant ischemic insult is slow, precursor availability is an important limiting factor in ATP recovery, and recovery time can be greatly shortened with precursor infusion even when started after the ischemic insult.

Alterations in amino acid clearance during ischemia predict hepatocellular ATP changes.

Although decreases in hepatic adenosine triphosphate (ATP) levels during ischemia are thought to reflect loss of hepatic energy reserves and decreased viability, such changes have not been correlated with a clinically relevant index of hepatic function or viability. Ability to clear amino acids from plasma has been shown to correlate with function of the allograft in hepatic transplantation and survival after portal decompression in patients with hepatic cirrhosis. The effects of 60 minutes of warm ischemia in two groups of mongrel dogs were studied to assess the relationship between loss of ATP and amino acid clearance. One group (shunted) had portal decompression during the ischemic period and the other (portal stasis) did not. There was a significant correlation between loss of ATP and amino acid clearance after ischemia. Although the effects of ischemia on the liver were similar in both groups, the portal stasis group demonstrated significantly elevated SGOT levels during reperfusion that were related to impaired net adenine monophosphate synthesis and suggestive of ongoing injury. These data support the contention that loss of ATP during ischemia is associated with reduced functional capacity. In addition, they suggest that portal stasis produces toxic products that can impede hepatic recovery from ischemia.

Conformational adaptation of muscle: implications in cardiomyoplasty and skeletal muscle ventricles.

In dynamic cardiomyoplasty and other forms of muscle-powered cardiac assist, the stretch that should be applied to the skeletal muscle to obtain optimal resting tension remains unclear. To test the hypothesis that skeletal muscle is capable of conformational adaptation over time, the effect of altered resting tension on the chronic performance of a skeletal muscle ventricle was studied. In 7 mongrel dogs, skeletal muscle ventricles constructed from the lastissimus dorsi muscle were stimulated to contract for 12 weeks against an implantable mock circulation. The preload pressure was altered, thereby varying the resting tension of the latissimus dorsi. One group (group I; n = 5) was maintained at a preload of 80 mm Hg, whereas a second group (group II; n = 2) was maintained at 20 mm Hg. Adaptation to preload was observed. After 12 weeks, the pressure increase generated by the skeletal muscle ventricle at a preload of 20 mm Hg was only 35 +/- 2 mm Hg for group I compared with 44 +/- 5 mm Hg for group II. At a preload of 80 mm Hg, the pressure increase was 61 +/- 4 mm Hg for group I and only 34 +/- 6 mm Hg for group II. Adaptation of the latissimus dorsi to a new resting tension has important implications in the use of skeletal muscle for cardiac assist. Stretching the latissimus dorsi to its in situ length during cardiomyoplasty is not required for future muscle performance to be optimal.

Use of epicardial electrocardiograms for detecting cardiac allograft rejection.

Since the advent of cyclosporin A surface electrocardiograms have been unreliable for diagnosing cardiac allograft rejection. Although several noninvasive methods have been proposed, none have been sufficiently accurate to be considered for clinical use. We have studied the use of the QRS complex amplitude, the unipolar peak-to-peak amplitude, recorded from intramyocardial electrodes for detecting rejection. Ten adult mongrel dogs underwent placement of intramyocardial electrodes on each ventricle. After stabilization of signals the hearts were transplanted heterotopically into unmatched recipients receiving cyclosporin A, azathioprine and methylprednisolone. Endomyocardial biopsies were performed after stabilization of unipolar peak-to-peak amplitude, twice weekly thereafter, and when unipolar peak-to-peak amplitude fell significantly. This detected 13 of 14 episodes of rejection. There was one false-positive and one false-negative result. The false-negative study became positive the following day. Thus, analysis of unipolar peak-to-peak amplitude detected all episodes of rejection in a clinically relevant time frame and was able to detect mild forms of rejection and multiple episodes of rejection in the same heart even in the presence of therapeutic levels of cyclosporin A.

The Carbomedics "Oxford" Photofix stentless valve (PSV).

The hemodynamic superiority of first-generation stentless xenografts over their stented equivalents is now clearly defined. Issues of durability remain. In light of considerable early experience, we sought to provide an "ideal" stentless valve design with the potential for improved durability by eliminating glutaraldehyde. The Carbomedics Oxford Photofix stentless valve (PSV) is a composite xenograft root constructed from three selected porcine noncoronary cusps fixed by the photofix process. The valve inflow has a low pericardial cuff. Consequently there are no coronaries, cloth, or muscle bar in the structure. The valve was tested in the descending aorta and subcoronary positions of juvenile sheep to compare the influence on calcification of photofixation and the alpha-amino-oleic-acid-treated Freestyle valve. There was a substantial difference between the valves, both in radiographic imaging of calcification and the elemental analysis of calcium and phosphorus. In the valve cusp, calcium content of PSV was 3.98+/-5.77 mg/g versus 14.3+/-19.29 mg/g for the Freestyle valve. Equivalent calcium values for the aortic wall were 1.99+/-3.39 mg/g versus 130.79+/-56.29 mg/g. We consider the composite root configuration without cloth, coronaries, or muscle bar to be a promising valve design. The photofix process minimizes xenograft tissue calcification and appears superior to existing methods.

In vivo efficacy of silver-coated fabric against Staphylococcus epidermidis.

BACKGROUND AND AIM OF THE STUDY: Prosthetic valve endocarditis (PVE) is a serious complication of heart valve replacement. The use of silver-coated polyester fabric in sewing cuff fabrication is intended to reduce the incidence of PVE. In this study, three pathogenic strains of Staphylococcus epidermidis (two PVE and one peripheral vasculature pathogen) were used to evaluate infection of silver-coated and uncoated fabric. METHODS: Fabric was inoculated by preincubation for 30 min in bacterial suspensions containing 10(4), 10(5), 10(6), 10(7) or 10(8) CFU/ml, and implanted subdermally in mice for up to seven days. Bacteria adherent to fabric implanted for zero, one, three, or seven days were enumerated by sonicating the fabric and plating serial dilutions of the resulting suspension. Percent infection was assessed by implanting samples, subdermally, for seven days, then incubating explanted samples in growth media for three days and calculating the percent of fabric showing bacterial growth, for each concentration of inoculum. A logistic regression model was used to estimate curves relating percent infection to log concentration of the bacterial inoculum. These curves were used to estimate the concentration of inoculum required to produce 50% infection (ID50) for the three strains of S. epidermidis, for silver-coated and uncoated fabric. RESULTS: Direct enumeration showed no difference in bacteria adherent to silver-coated and uncoated fabric, and no bacteria present in either fabric type in samples implanted for seven days. Nevertheless, incubation of those samples in growth media showed that many of the samples were infected. The calculated ID50 was significantly lower for silver-coated fabric than for uncoated fabric, for all three strains of S. epidermidis tested. CONCLUSION: Silver-coated fabric increases resistance to infection by S. epidermidis in this direct-contamination, mouse subdermal model.

Current state of in vivo preclinical heart valve evaluation.

The use of animals to predict prosthetic valve performance in humans has now been in operation for over 25 years. Studies have focused on improvements in structural integrity, anti-thrombogenicity and anticalcification, and provide extensive in vivo analysis and validation of a valve before its clinical evaluation. Many animal models have been developed for such investigations, including calves, goats, pigs, baboons, dogs and adult sheep. Recent models used to assess prosthetic mitral valves have involved site-specific testing in several species; the present study represents the largest series of orthotopically implanted stentless bioprosthetic aortic valves and the first to utilize a juvenile sheep model for the aortic position.

Feasibility evaluation of a new pericardial bioprosthesis with dye mediated photo-oxidized bovine pericardial tissue.

BACKGROUND AND AIMS OF THE STUDY: In an attempt to overcome the problem of calcification of bioprostheses, a novel bovine pericardial tissue valve preserved with a non-aldehyde, dye mediated photo-oxidation process (PhotoFixTM) developed by CarboMedics, Inc. The device was evaluated by implantation in the mitral position of juvenile sheep with a mean age of 3.5 months. MATERIALS AND METHODS: Thirteen valves were evaluated; six experimental valves with photo-oxidized tissue, five identically designed valves with glutaraldehyde-fixed tissue, and two Carpentier-Edwards pericardial valves as controls. RESULTS: Four of the six animals in the photo-oxidized group were free of calcification when electively sacrificed at 152 days, 152 days, one year and 1.5 years respectively. One animal was electively sacrificed at 98 days. Pathologic findings indicated minimal calcification of the valve due to uneven stress distribution from two bent stent posts that most likely occurred during surgical implantation, but the device was still functional. The sixth animal with photo-oxidized tissue died at 131 days due to massive calcification of a single leaflet. There was no pathologic evidence of infection. Explants at five months from additional and still continuing sheep studies per FDA guidelines have not reproduced single leaflet calcification in over forty explanted valves with photo-oxidized tissue. The glutaraldehyde-fixed valves all exhibited some calcification at explant; two of these animals died early due to the mineralization. Both control animals with Carpentier-Edwards valves died early from mitral stenosis due to extensive device calcification. CONCLUSIONS: The fact that four of six valves with photo-oxidized tissue remained free of any signs of calcification for up to 1.5 years, while none of the other valves did, suggests that photo-oxidation is a promising method of preserving and fixing tissue for use in bioprostheses. These results suggest that photo-oxidized bioprosthetic valves may be clinically valuable and warrant further study.

In-vivo assessment of a photofixed bovine pericardial valve.

BACKGROUND AND AIM OF THE STUDY: The study aim was to assess the preclinical safety of a bioprosthetic cardiac valve in a large-animal model. The experimental pericardial valve was treated with a dye-mediated photooxidation process (PhotoFix) to enhance durability, and to reduce bioreactivity and mineralization potential. METHODS: Ten experimental (Sulzer Carbomedics PhotoFix pericardial valves, 25 mm) and three control valves (Carpentier-Edwards 2700, 25 mm) were placed in the mitral position of 13 juvenile sheep. Assessment criteria included animal survival, hematological analysis, host response, valve calcification and tissue stability. RESULTS: No significant differences were seen between the two groups for hematological analysis, weight or age. Ease of surgical handling was equivalent or superior to currently available technology. The PhotoFix valves had minimal leaflet calcification (mean 0.20 +/- 0.12 microg/mg) at 150 days. Two animals implanted with control valves were sacrificed at 173 and 227 days due to poor health; the control valves had massive thrombus and extensive leaflet mineralization (mean calcium content 18.2 +/- 3.8 microg/mg). For comparison with controls, two sheep with Photofix valves were sacrificed electively at 218 and 235 days. At 235 days, the experimental valve had a surgical suture looped over one commissure, restricting the motion of two leaflets and leading to massive inflow surface thrombosis. Despite this restriction, the animal remained healthy, with no noticeable regurgitation. The mean calcium content of the 218-day experimental valve leaflets was 6.28 microg/mg. Two experimental valves remain in place for long-term evaluation. CONCLUSION: In-vivo testing in juvenile sheep showed the PhotoFix pericardial valve to have no device-related mortality, no significant tissue degeneration, and minimal mineralization. In contrast, the control valves had significant valve mineralization and device-related morbidity.

Development and evaluation of a swine model to assess the preclinical safety of mechanical heart valves.

BACKGROUND AND AIM OF THE STUDY: The current standard of in vitro and in vivo preclinical heart valve testing has recently been questioned because of its failure to reveal the thrombogenic potential of the Medtronic Parallel prosthetic valve. The aim of this study was to develop a swine model for the in vivo preclinical evaluation of mechanical heart valves, and to assess the ability of this model to identify mechanical heart valve design features that result in valve-related thrombosis. METHODS: Twenty-two swine underwent mitral valve replacement (MVR) using three different bileaflet mechanical valve designs (St Jude Medical, CarboMedics, Medtronic Parallel). Each animal was placed in an anticoagulation protocol (group I, INR 3.0-3.5; group II, INR 2.0-2.5; group III, no anticoagulation) and followed for up to 20 weeks. Terminal studies were performed on all animals surviving for more than 30 days. RESULTS: Twenty-one animals survived the immediate postoperative period. Four of six group I animals died from hemorrhagic (large wound hematoma; hemopericardium) complications early in the study. In the two long-term (61 and 89 days) survivors, INRs of 3.0 to 3.5 were never achieved (61-day survivor, mean INR 2.0 +/- 1.03; range: 0.8-5.4; 89-day survivor, mean INR 1.92 +/- 1.34; range: 1.0-7.9). Pathological analysis of explants from group I survivors revealed minimally obstructive fibrous sheathing on the inflow orifice without restriction of bileaflet motion (61 and 89 days), and two large perivalvular defects (61 days). Six of seven group II animals died from early hemorrhagic complications (hemopericardium) (mean INR 2.32 +/- 1.84; range: 0.8-8.2). Vegetations resulting in obstruction of both sides of the valve orifice and restriction of bileaflet motion were observed in a group II survivor (mean INR 2.33 +/- 1.58; range: 0.9-7.0). Group III animals (n = 8) survived for a mean of 106 +/- 60 days (range: 1-177 days). In group III, fibrous sheathing was present on all explanted valves and organized thrombi in six valves; orifice obstruction (seven valves) and restriction of bileaflet motion (three valves) were also observed. CONCLUSION: The use of MVR in swine as a preclinical model to evaluate the safety and performance of mechanical heart valves is limited by: (i) difficulty in maintaining safe levels of anticoagulation with warfarin, resulting in a high incidence of hemorrhagic complications; (ii) marked fibrous sheath formation and associated thrombosis; and (iii) an increased incidence of perivalvular defects, believed to result from normal somatic growth occurring in young swine.

A juvenile sheep model for the long-term evaluation of stentless bioprostheses implanted as aortic root replacements.

BACKGROUND AND AIM OF THE STUDY: Orthotopic valve replacement in large animals is an important component of the preclinical assessment of bioprosthetic valves. To provide the most useful preclinical information, the development of models that parallel clinical practice patterns is essential. Therefore, we sought to develop a technically feasible and reproducible model for chronic evaluation of stentless bioprosthetic aortic valves implanted as aortic root replacements in juvenile sheep. METHODS: Juvenile domestic sheep (mean age 21+/-2.28 weeks; range: 17-26 weeks) underwent aortic root replacement using standard cardiopulmonary bypass (CPB) and surgical techniques. Animals were implanted with 19 mm (n = 21), 21 mm (n = 18) or 23 mm (n = 4) bioprostheses from two different manufacturers, and followed for 150 days. Animals surviving at least 150 days were considered long-term survivors; those which died prior to postoperative day (POD) 31 were considered operative deaths. RESULTS: Forty-three animals underwent aortic root replacement. The mean CPB time was 91+/-20 min (range: 62-149 min); mean cross-clamp time was 63+/-13 min (range: 39-95 min). Thirty-five animals (81%) survived the first 30 days of the study period. Five deaths occurred at POD 0 due to anastomotic complications. One death occurred each on POD 3, 6, and 26 as a result of prosthesis size mismatching, thromboembolic complications, and endocarditis, respectively. There were five late deaths. Twenty animals survived the minimum 150-day study period, and 12 were sacrificed at 183+/-17 days. Six animals remain alive at 151+/-0.98 days, and one animal died each on POD 184 and 190. The remaining 10 animals are not yet 150 days from their operation. Currently, all are well at 102+/-34 days (range: 33-140) days. CONCLUSION: These data suggest that long-term evaluation of stentless aortic bioprostheses implanted as aortic root replacements can be accomplished using juvenile sheep.

Chronic evaluation of orthotopically implanted bileaflet mechanical aortic valves in adult domestic sheep.

This study was intended to develop a technically feasible and reproducible model for chronic hemodynamic and mechanical evaluation of orthotopically implanted bileaflet mechanical aortic valves in adult domestic sheep. Three adult sheep (mean age 22 weeks, mean weight 76 kg) underwent aortic valve replacement using 19-mm bileaflet aortic valves. Standard cardiopulmonary bypass techniques were followed, including mild hemodilution, systemic hypothermia, and cardioplegic arrest. After performing a left fourth intercostal thoracotomy, the valves were placed using interrupted 3-0 Ticron (Davis + Geck) inverted mattress sutures through a transverse aortotomy. The average cardiopulmonary bypass time was 58+/-1 min. No chronic anticoagulation was used. There were no surgical complications. All three animals (100%) remained clinically well until elective sacrifice after postoperative day 150. The average cardiac output for the animals at sacrifice was 3.8+/-1.0 L/min. The mean aortic ejection velocity was 304.7+/-47.3 cm/s and the mean pressure gradient was 24.6+/-6.7 mm Hg. There was no clinically significant thrombus formation or paravalvular leaks. Thus, we have demonstrated that it is technically feasible to orthotopically implant mechanical aortic valves in sheep. There are several features that contribute to the success of this model, including use of a transverse aortotomy, adequate de-airing, and the use of mild hemodilution during bypass. We believe that this model is reproducible and can be used to study other valve designs. In addition, this model allows for site-specific preclinical assessment of new or modified mechanical heart valves.

Long-term evaluation of orthotopically implanted stentless bioprosthetic aortic valves in juvenile sheep.

The aim of this study was to develop a technically feasible and reproducible model for chronic evaluation of stentless bioprosthetic aortic valves implanted orthotopically using juvenile domestic sheep. This report summarizes the results of a study conducted to assess orthotopically placed 19-mm stentless aortic bioprosthetic valves. Twenty-seven juvenile sheep underwent aortic valve replacement. Standard cardiopulmonary bypass techniques were followed. The average cardiopulmonary bypass time was 73 min. No chronic anticoagulation was used. There were two deaths (7%) due to surgical complications. In the remaining 25 experiments, 11 animals (41%) died prior to the scheduled sacrifice on postoperative day 150. One early death occurred due to coccidiomycosis infection, one due to technical error, one due to pulmonary embolus, four due to prosthetic annular size disproportion, and four due to thrombi. The remaining 14 animals (52%) underwent left and right heart catheterization, angiography, echocardiography, and sacrifice after postoperative day 150. The average weight of the sheep at elective sacrifice was 60 kg (mean weight gain 12.5 kg). The average cardiac output for the sacrificed animals was 5.1 L/min. The mean velocity of blood across the aortic valve for the sacrificed animals was 317 cm/s and the mean pressure gradient was 26.2 mm Hg. Two features suggest that this model may have broad application. First, we have demonstrated that it is technically feasible to evaluate orthotopically placed stentless bioprosthetic aortic valves in growing sheep. Second, the aortic root size of the juvenile sheep allows for implantation and evaluation of a human size aortic valve (19 mm). We believe that this model is reproducible and can be used to study stentless valve designs.