Simulated surface-induced thrombin generation in a flow field.

A computational model of blood coagulation is presented with particular emphasis on the regulatory effects of blood flow, spatial distribution of tissue factor (TF), and the importance of the thrombomodulin-activated protein C inhibitory pathway. We define an effective prothrombotic zone that extends well beyond the dimensions of injury. The size of this zone is dependent on the concentrations of all reactive species, the dimensions of TF expression, the densities of surface molecules, and the characteristics of the flow field. In the case of tandem sites of TF, the relationship between the magnitude of the effective prothrombotic zone and the interval distance between TF sites dictate the net response of the system. Multiple TF sites, which individually failed to activate the coagulation pathway, are shown to interact in an additive manner to yield a prothrombotic system. Furthermore, activation of the thrombomodulin-activated protein C pathway in the regions between sites of TF downregulate the thrombin response at subsequent TF sites. The implications of prothrombotic effects, which extend downstream beyond the discrete site of injury to interact with subsequent lesions are critical given the systemic nature of atherosclerotic disease.

Thrombomodulin improves early outcomes after intraportal islet transplantation.

Primary islet nonfunction due to an instant blood mediated inflammatory reaction (IBMIR) leads to an increase in donor islet mass required to achieve euglycemia. In the presence of thrombin, thrombomodulin generates activated protein C (APC), which limits procoagulant and proinflammatory responses. In this study, we postulated that liposomal formulations of thrombomodulin (lipo-TM), due to its propensity for preferential uptake in the liver, would enhance intraportal engraftment of allogeneic islets by inhibiting the IBMIR. Diabetic C57BL/6J mice underwent intraportal transplantation with B10.BR murine islets. In the absence of treatment, conversion to euglycemia was observed among 29% of mice receiving 250 allo-islets. In contrast, a single infusion of lipo-TM led to euglycemia in 83% of recipients (p = 0.0019). Fibrin deposition (p < 0.0001), neutrophil infiltration (p < 0.0001), as well as expression TNF-alpha and IL-beta (p < 0.03) were significantly reduced. Significantly, thrombotic responses mediated by human islets in contact with human blood were also reduced by this approach. Lipo-TM improves the engraftment of allogeneic islets through a reduction in local thrombotic and inflammatory processes. As an enzyme-based pharmacotherapeutic, this strategy offers the potential for local generation of APC at the site of islet infusion, during the initial period of elevated thrombin production.

Lipopeptide conjugates: biomolecular building blocks for receptor activating membrane-mimetic structures.

A simple method for the derivatization of phospholipid subunits with short peptide sequences is presented. Amphiphilic conjugates of distearoylphosphatidylethanolamine (DSPE) and the minimal human thrombin-receptor peptide agonist SFLLRN were synthesized via coupling of the bromoacetyl derivative of DSPE (PEBr) with a thiol-terminated decapeptide of SFLLRN. Bromoderivatization of DSPE was performed by condensation of bromoacetic acid and DSPE, with an overall yield of 40%. Coupling of PEBr and the unprotected thiol-terminated decapeptide was performed in chloroform/methanol/water in the presence of triethylamine and afforded 25.5% of the lipopeptide. The compound was characterized by TLC, 1H-NMR (600 MHz) and mass spectrometry. Lipopeptide bioactivity was confirmed by a platelet aggregation assay. The EC50 of the all-L amino acid lipopeptide was 38 +/- 3 microM. The all-D conjugate was inactive. Model receptor-activating systems, including well-ordered assemblies of amphiphilic phospholipid-peptide conjugates, represent the first step in the construction of bioactive surfaces for tissue engineering based on a membrane-mimetic approach.

Nontraumatic perforation of the small bowel.

Small intestinal perforation is an infrequent but often fatal complication of a variety of disorders. Treatment of 76 adults with nontraumatic perforation occurring during a 21 year period was reviewed. The majority of perforations were due to mechanical causes, malignancy, and Crohn's disease. An abdominal mass (53 percent, p less than 0.05) and fever (81 percent, p less than 0.01) were most frequently documented in patients with Crohn's disease, whereas a positive result on fecal occult blood testing (77 percent, p less than 0.025) was common in those with malignant perforations. Otherwise, clinical and laboratory manifestations, including radiographic evidence of free air, were inconstant. The overall mortality rate was 29 percent, with 65 (86 percent) of the patients undergoing operation. Survival was independent of diagnostic delay (p greater than 0.10). In general, despite various causes and delays in diagnosis, resection and primary anastomosis remains an effective treatment for perforation of the small bowel.

Fatal overwhelming postsplenectomy infection.

A total of 776 patients underwent splenectomy at the Massachusetts General Hospital between 1962 and 1972. Follow-up information was obtained on 637 patients (82 percent), including 584 adults and 53 children. There was a total of 4,837 person-years of follow-up with a mean observation interval of 8.4 years. Four cases of fatal overwhelming postsplenectomy infection were identified. In our pediatric population, the incidence of fatal overwhelming postsplenectomy infection was 3.77 percent, which was significantly higher than the incidence of 0.34 percent in our asplenic adults. Overwhelming postsplenectomy infection is a unique clinical entity distinguishable from other infections. It may occur during the lifetime of any asplenic patient and especially in those patients who have had a splenectomy in childhood. In asplenic adults, the incidence is low. The aggressive approach to splenic preservation in the adult should be tempered by these results.

Delayed rupture of aortic aneurysms following endovascular stent grafting.

BACKGROUND: Deployment of transfemoral, endovascular stent grafts for treatment of abdominal aortic aneurysms is appealing for several reasons: avoidance of abdominal incision, lack of aortic cross-clamping, potential for regional anesthesia, and shortened hospital stay. Concerns remain, however, regarding the ability of these devices to completely exclude the aneurysm and prevent aneurysm rupture and the long-term integrity of the device. The availability of endografts and the likely development of percutaneous devices have also raised the delicate issue of personnel training for patient selection, endograft implantation, and postoperative follow-up. PATIENTS AND METHODS: The cases of 2 patients are reported in which Dacron endovascular grafts, anchored proximally and distally by Palmaz stents, were deployed for treatment of infrarenal abdominal aortic aneurysms. RESULTS: In a patient with and absent distal cuff, choosing this procedure represented a clear error in patient selection. The endograft failed to reach the aortic bifurcation and the aneurysm ruptured, with the death of the patient 4 months postimplantation. In a patient with anatomy suitable for endograft placement, a perigraft leak persisted at the distal anastomosis following device placement. The aneurysm ruptured 14 days postprocedure. Although the patient survived emergent aneurysm repair, he developed acute renal failure. CONCLUSION: Careful preoperative assessment of aortic anatomy is crucial in selection of patients for transfemoral endovascular graft placement. Lack of a distal cuff of at least 1 cm precludes tube graft implantation. Patients with a perigraft leak are not protected by the endograft from aneurysm rupture. Vascular surgeons must be involved in the preoperative evaluation of these patients and are the only specialty group who can provide the prerequisite care in evaluation and management of postoperative complications.

Peripheral vascular complications of aortic dissection.

BACKGROUND: The incidence and management of peripheral vascular complications of aortic dissection is unsettled. PATIENTS AND METHODS: Peripheral vascular complications of spontaneous aortic dissection were examined in a 5-year retrospective review. Patients who had peripheral vascular complications were categorized as group A; those without as group B. RESULTS: Thirty-eight major vessels were affected in 18 patients. No patient underwent a peripheral vascular procedure for complications of the carotid, subclavian, celiac, mesenteric, or renal arteries. Three patients underwent femorofemoral bypass for acute iliofemoral occlusion due to dissection. A fourth patient had repair of an iliac aneurysm that developed as a complication of chronic dissection. The mortality rate was 17% for group A, 9% for group B, and 10% overall. Following repair of the aortic dissection, the majority of the peripheral vascular complications resolved. CONCLUSIONS: Peripheral revascularization is infrequently required in aortic dissection following primary dissection repair.

Embryology, anatomy, and surgical exposure of the great abdominal vessels.

This article describes the embryology of the abdominal aorta and the anatomic features of its major visceral branches, including the celiac, superior mesenteric, and inferior mesenteric arteries. The common anatomic variants of these visceral vessels also are reviewed. Various operative techniques to gain surgical exposure to these vessels are described.

Endovascular treatment of vascular injuries.

The endovascular management of hemodynamically stable patients with traumatic vascular lesions is an appealing concept. In principle, many of the injuries detected at the time of diagnostic angiography can be treated at the same setting. Moreover, lesions that occur at the base of the skull or at infraclavicular and pelvic locations pose far less difficulty when managed by transcatheter techniques than by traditional surgical exposure. Even among more accessible injuries, standard surgical dissection is often complicated by the presence of hematoma or pseudoaneurysm, which causes obliteration of natural tissue planes, or arteriovenous fistulas that may complicate dissection because of associated regional venous hypertension. Thus, endovascular approaches may provide easier access to the target lesion, limit the morbidity often associated with surgical exploration, and reduce transfusion requirements. Nonetheless, the long-term consequence of placing an intravascular foreign body in a young patient is undefined, and the potential risk for a device infection cannot be ignored. Definitive answers to these issues await the outcome of longitudinal follow-up studies. Until that time, a prudent approach in the use of this new technology is appropriate.

Anatomic changes after endovascular grafting for aneurysmal disease.

Long-term follow-up of controlled clinical trials of endovascular grafting for aortic aneurysms will provide data on the safety and efficacy of this new treatment to reduce morbidity and mortality compared with standard treatment. It will be several years before this information will be available, and careful analysis of surrogate markers for clinical success has value for predicting long-term outcome. One essential surrogate marker is aortic aneurysm diameter, which has traditionally been the most important variable in calculating rupture risk, and multiple studies have shown that aneurysms shrink after complete endovascular exclusion. Furthermore, measurements of aortic neck size and aortic length has shown interesting patterns that may affect the durability of endovascular repair and, thus, may suggest potential strategies for the design of future devices.

The synthesis of neoglycophospholipid conjugates via reductive amination of omega-oxoalkylglycosides and phosphatidylethanolamines.

Phospholipid conjugates of mono- and disaccharides tethered with an n-decanyl spacer were efficiently synthesized via an improved reductive amination of deprotected omega-oxodecanyl beta-glycosides and phosphatidylethanolamines with or without alkenyl groups. The omega-oxodecanyl beta-glycosides were prepared by stereoselective glycosidation of glycosyl halides with 1, 10-decanediol followed by pyridinium dichromate oxidation. The acetyl groups of the omega-oxodecanyl beta-glycosides were removed with sodium methoxide prior to their conjugation with phosphatidylethanolamines.

A membrane-mimetic barrier for islet encapsulation.

BACKGROUND: Enhanced control of both transport properties and surface physiochemical characteristics will be important steps in the development of an effective immunoisolation barrier critical to the success of pancreatic islet cell transplantation. We hypothesize that the cell membrane establishes an important paradigm for the design of a biomimetic immunoisolation barrier with improved performance characteristics because of its capacity to control interfacial mass transport, as well as its ability to act as a template for more complex structures with other immunoregulatory macromolecules. METHODS: Islets were isolated from Wistar rats using collagenase digestion and a discontinuous Ficoll-Histopaque gradient and subsequently encapsulated in 2% alginate. After coating with a polyelectrolyte multilayer of polylysine and alginate, a polymeric membrane-mimetic coating was applied to the capsule surface. Individual islet viability was evaluated at each stage of the encapsulation procedure by use of a two-color live/dead cell assay. Preservation of islet function was determined by transplanting 1000 encapsulated islets into the peritoneal cavity of streptozotocin-induced diabetic nonobese diabetic NOD/Scid mice. RESULTS: At the end of the coating procedure, the proportion of viable cells within each islet was >50% in 88% of encapsulated rat islets and >75% in over half of the encapsulated cohort. Nonfasting blood glucose levels normalized within 24 hours after transplantation (n = 8). Normoglycemia has been maintained in all mice with the longest time course being 73 days thus far. CONCLUSIONS: We have demonstrated that microencapsulated islets coated with a membrane-mimetic thin film can be generated with high viability in vitro and persistent function in vivo.

High-resolution analysis of engineered type I collagen nanofibers by electron microscopy.

Collagen nanofibers were generated at ambient temperature and pressure by electrospinning a 1 wt% solution of type I collagen and polyethylene oxide. Products were imaged with high-resolution scanning electron microscopy (HRSEM) at medium (approximately 30,000 x) and high magnifications (approximately 100,000 x) and with transmission electron microscopy (TEM). The capacity to produce collagen nanofibers may lead to the generation of extracellular matrix-based fabrics with applications in the fields of wound healing and tissue engineering.

Engineering and material considerations in islet cell transplantation.

The successful application and optimization of cell transplantation will require quantitative engineering design and analysis of cells and materials in which relevant biological processes remain complex and incompletely defined. This report primarily reviews the engineering and material considerations in islet cell transplantation, including established biological constraints and biohybrid devices for cell delivery, as well as available barrier materials and the associated processing strategies directed at the control of solute transport, barrier permeability, and host responses at the biological-material interface. Also described are current areas of investigation with particular promise as enabling technologies for accelerating the clinical effectiveness of islet cell transplantation.

Soluble heparin-binding peptides regulate chemokinesis and cell adhesive forces.

The ability of a soluble heparin-binding peptide sequence derived from fibronectin to modulate the adhesion and chemokinetic migration behavior of arterial smooth muscle cells was assessed using a novel glass microsphere centrifugation assay and automated time-lapse fluorescence videomicroscopy, respectively. Treatment of cells grown on fibronectin-coated substrates with the soluble heparin-binding peptide resulted in the disassembly of focal adhesions, as assessed by immunohistochemical staining. These observations were consistent with an observed dose-dependent two- to fivefold reduction in cell-substrate adhesive strength (P < 0.001) and a biphasic effect on migration speed (P < 0.05). Moreover, heparin-binding peptides induced a twofold reduction (P < 0.01) in two-dimensional cell dispersion in the presence of a non-heparin-binding growth factor, platelet-derived growth factor-AB (PDGF-AB). Heparin-binding peptides were unable to mediate these effects when cells were grown on substrates lacking a heparin-binding domain. These data support the notion that competitive interactions between cell surface heparan sulfates with heparin-binding peptides may modulate chemokinetic cell migration behavior and other adhesion-related processes.

Neoglycophospholipids with alkyl spacers: synthesis via an improved reductive amination and monolayer properties.

An efficient synthesis of neoglycophospholipids with variable length alkyl spacer chains is described. Neoglycophospholipids tethered by alkyl chains of 3, 5, 7, 10, and 16 methylene units were synthesized in good overall yields in four steps. The key intermediates, omega-oxoalkyl glycopyranosides, were synthesized in two steps by glycosidation of chloro (or ethylthio) glycosides with a diol followed by oxidation of the remaining hydroxy group to an aldehyde functionality. Conjugation of the omega-glycoalkyl aldehyde with distearoylphosphatidylethanolamine via an improved reductive amination procedure significantly enhanced efficiency and yields with respect to those from traditional procedures. The amphiphilic properties of the neoglycophospholipids were characterized at the air-water interface. While the carbohydrate head group had relatively little effect, the length of the alkyl spacer profoundly influenced surface area-pressure isotherms.

Continued expansion of aortic necks after endovascular repair of abdominal aortic aneurysms. EVT Investigators. EndoVascular Technologies, Inc.

BACKGROUND: Longitudinal studies have revealed that the aortic segment proximal to an infrarenal abdominal aortic aneurysm (AAA) is at risk for continued enlargement after a standard aneurysm repair. Similarly, preliminary reports have shown expansion of one or both aortic necks after endovascular repair. Although some investigators have suggested that this may be a transient effect, continued dilatation at the endograft attachment site could effect the overall device stability. METHODS: As part of a multi-institutional trial of endovascular grafting for the treatment of AAA, 59 patients were successfully implanted with straight endografts between February 1993 and January 1995. A morphometric analysis of aortic neck size was undertaken with serial review of computed tomography scans available through April 1997. The neck sizes at both graft attachment sites were measured, with investigators blinded to patient identity and date of scan. Changes in minor diameter were defined, annual interval expansion rates were calculated, and the data were correlated with endoleak, device migration, aneurysm size change, endograft diameter, attachment system fractures, and initial preimplant neck size. RESULTS: Significant aortic neck enlargement, particularly at the level of the distal neck, was observed for at least 24 months after AAA repair. The annual interval dilation rates of the proximal aortic neck were 0.7 +/- 2.1 mm/year (P = .023) and 0.9 +/- 1.9 (P = .008) mm/year during the first and second years, respectively. Enlargement of the distal neck during the observation period was more marked, with corresponding annual expansion rates of 1.7 +/- 2.9 mm/year (P < .001) and 1.9 +/- 2.5 (P < .001) mm/year. In 5 patients (14%), the minor diameter of the distal neck was at least 6 mm larger than the preimplant diameter of the graft. Migration of the distal attachment system was observed in 3 of these 5 patients. Expansion rates did not have a statistically significant correlation with initial neck size, endograft dimensions, aneurysm size change, presence of endoleak, or attachment system fracture. CONCLUSIONS: Aortic neck enlargement was observed for at least 2 years after endovascular grafting. Close patient follow-up remains mandatory in lieu of the potential risk of late failure as a result of continued aortic expansion. The relative contribution of device design to this phenomenon will need to be defined.