M.G. Walker

Shear‐stress preconditioning and tissue‐engineering‐based paradigms for generating arterial substitutes

In situ tissue engineering using shear‐stress preconditioning and adhesive biomolecules is a new approach to autologous tissue engineering. In the present study, novel tissue‐engineering grafts (TEGs) were preconditioned within an in vitro pulsatile flow circuit, with and without the addition of fibronectin (FN), to establish whether low‐shear‐stress conditions promoted endothelial cell (EC) retention and differentiation. TEGs (n=24) were generated by the contraction and compaction of collagen(I) by porcine aortic smooth‐muscle cells (SMCs) on to a compliant polyester graft scaffold. ECs were radiolabelled with [111In]indium tropolonate and seeded on to the luminal surface of the TEGs. Following organ culture in a bioreactor (7 days), TEGs were split into four groups (n=six TEGs per group): Group A acted as controls with TEGs unmodified and seeded with radiolabelled ECs; Group B underwent luminal pre‐coating with FN (75 μg/ml) prior to EC seeding; Group C underwent preconditioning within a pulsatile flow circuit at 10–20 μN (1–2 dyn)/cm2 for 7 days prior to EC seeding, and Group D TEGs were preconditioned for 7 days at 1–2 dyn/cm2, followed by luminal pre‐coating with FN prior to EC seeding. The resistance to physiological shear stress of the seeded ECs was assessed using a γ‐radiation counter within a physiological flow circuit producing an arterial waveform with a mean shear stress of 93.2 μN (9.32 dyn)/cm2. Environmental scanning electron microscopy (ESEM) was used to determine the distribution and degree of differentiation of the attached Ecs, and tissue‐type‐plasminogen‐activator (tPA) assays provided a measure of function and viability. EC resistance to shear stress at 93.2 μN/cm2 was significantly enhanced by a period of preconditioning (Group C) at 10–20 μN/cm2, surface modification with FN (Group B), or both (Group D) when compared with control grafts (Group A). However, TEGs coated with FN whether preconditioned (Group D) or not (Group B) demonstrated the best results for EC retention. ESEM demonstrated near‐confluent differentiated flattened ECs in both these cases. EC function was demonstrated by a steady increase in tPA production. Low‐shear‐stress preconditioning of TEGs enhances EC retention in vitro with an additional advantage demonstrated by pre‐treatment with FN prior to endothelialization. These findings may be exploited in the development of tissue‐engineered constructs to maintain a confluent endothelial lining.

Access for Endovascular Aneurysm Repair

Despite advancement in stent-graft technology, access-related problems continue to occur during endovascular repair of aortic aneurysms. Various techniques have been adopted to overcome difficult access situations, however. To survey these developments in arterial access, we performed a systematic literature review from 1994 through 2005 to identify relevant articles pertaining to endovascular access techniques and complications. Excessive iliac tortuosity, circumferential vessel wall calcification, significant occlusive disease, and small caliber vessels account for the majority of access problems, most of which are readily apparent with adequate baseline imaging. Even with careful preoperative assessment, however, some access problems may not be foreseen; nonetheless, the majority can be overcome using todays array of ancillary procedures, such as an iliac conduit, a brachiofemoral wire, or arterial reconstruction. Alternatively, other approach routes, such as the common carotid artery or direct aortic access, may be used to facilitate endovascular aneurysm repair.

A systematic review of open versus endovascular repair of inflammatory abdominal aortic aneurysms.

INTRODUCTION Inflammatory abdominal aortic aneurysms (IAAAs) have traditionally been treated by open surgical repair (OSR). Over the last decade, endovascular aneurysm repair (EVAR) has been increasingly employed. The optimal treatment option for IAAA remains unclear. This article aims to evaluate and compare outcomes of OSR and EVAR in IAAA repair. METHODS All publications in the English language relating to IAAA were sought electronically using OVID and MEDLINE (1972-2008). Studies identifying 30-day mortality were considered. Periaortic inflammation (PAI), hydronephrosis and 1-year mortality were obtained from studies with at least 1-year computed tomography (CT) follow-up. Outcomes of OSR and EVAR were compared and analysed for statistical significance using Fishers exact test. RESULTS The results were obtained from 35 studies comprising 999 patients and 21 studies with 121 patients who underwent OSR and EVAR, respectively. One-year CT follow-up was available for 124 and 52 patients from the two groups, respectively. Thirty-day mortality after OSR was 6% (95% confidence interval (CI); 6-13) and 2% (95% CI; 0-7) after EVAR (p=0.1). At 1 year, PAI regressed in 73% (95% CI; 64-80) in the OSR group compared to 65% (95% CI; 49-77) of the EVAR group (p=0.7). Conversely, inflammation progressed in 1% and 4%, respectively (p=0.1). Forty-five patients undergoing OSR and 29 EVAR were found to have preoperative hydronephrosis. This regressed postoperatively in 69% (95% CI; 53.3-81.8) and 38% (95% CI; 20.6-57.7), respectively (p=0.01). Hydronephrosis progressed in 9% of patients after OSR and in 21% after EVAR (p=0.1). New-onset hydronephrosis developed in 6% undergoing OSR compared to 2% with EVAR (p=0.2). One-year all-cause mortality after OSR was 14% (95% CI; 6-18) compared to 2% (95% CI; 0-13) after EVAR (p=0.02). CONCLUSION Either OSR or EVAR may be considered based on patient suitability. EVAR is associated with lower 1-year mortality compared to OSR. However, OSR may be preferred in those patients who have hydronephrosis and are deemed low risk.

Contemporary Management of Aorto-iliac Aneurysms in the Endovascular Era

Up to 40% of abdominal aortic aneurysms have co-existing unilateral or bilateral iliac artery ectasia or aneurysm. These are associated with an increased risk of endoleak, morbidity and mortality following endoluminal repair. To reduce the adverse sequelae of internal iliac artery (IIA) occlusion, various open, endovascular and hybrid measures have been described to maintain perfusion to the pelvis. This review discusses the contemporary management of aorto-iliac aneurysm in the endovascular era with reference to the sequelae of IIA occlusion and the strategies to preserve IIA perfusion. Particular consideration is given to iliac bifurcation devices.

True Profunda Femoris Aneurysms: Are They More Dangerous than Other Atherosclerotic Aneurysms of the Femoropopliteal Segment?

Three cases of true aneurysms of the profunda femoris artery are reported along with a review of 17 other cases in the literature. These aneurysms are rare and commonly present with rapid enlargement or rupture (9/20), the risk of rupture being higher than those affecting the femoral or popliteal arteries. All patients underwent successful surgical treatment except for one who required amputation. The diagnosis of an aneurysm of the profunda femoris artery must be considered in all patients with a pulsatile swelling in the groin. Surgical treatment is mandatory, and it carries a low mortality as well as a low risk of amputation.

Effects of shear stress on endothelial cell monolayers on expanded polytetrafluoroethylene (ePTFE) grafts using preclot and fibronectin matrices

Animal studies have shown that endothelial seeding of vascular prosthetic grafts reduces thrombogenicity and improves their patency. However, for endothelial seeding to be of clinical benefit in humans, it must withstand shear stress of blood flow. Endothelial cells labelled with Indium-111-oxine were seeded in supra-confluent densities on preclot or fibronectin coated ePTFE graft segments over a period of 90 min. These grafts with rapidly formed endothelial cell monolayers were then exposed to varying shear stresses up to a flow rate of 300 ml/min, using tissue culture medium in an artificial flow circuit. Grafts coated with preclot matrix showed 2 h cell retentions of 82.4 +/- 6.8% at 25 ml/min, 79.9 +/- 8.2% at 100 ml/min, 75.4 +/- 9.5% at 200 ml/min and 58.3 +/- 15.5% at 300 ml/min whilst those for the fibronectin matrix were 57.8 +/- 9.9%, 55.2 +/- 13.3%, 55.4 +/- 12.9% and 56.5 +/- 15.2% respectively. Overall the preclot matrix was found to be better than fibronectin (P less than 0.001). Light and scanning electron microscopy revealed well-formed endothelial cell monolayers retained on preclot matrix up to a flow rate of 200 ml/min whereas uncovered patches were seen at 300 ml/min and at all flow rates on fibronectin matrix.

Endothelial Cell Seeding: A Review

The concept of endothelial cell seeding, designed to provide vascular grafts with a nonthrombogenic lining, has progressed from crude animal experiments during the past two decades to detailed in vitro functional studies using human cells. Although favorable results have been obtained in animal studies this has yet to be translated to humans, where current application of these techniques has been limited to a very few clinical trials. The history, current status and future directions are reviewed herein.

Endothelial cell seeding kinetics under chronic flow in prosthetic grafts.

Improved patency of endothelial cell seeded grafts relies on good initial adherence and cell retention when the circulation is restored. In this study human adult endothelial cells (HAECs) were used to evaluate the suitability of commercially available prostheses for seeding. Acutely seeded indum-111 oxine labeled HAECs were used to measure cell adherence to plain and fibronectin (FN)-coated expanded polytetrafluoroethylene (ePTFE), gelatin-impregnated Dacron (Gelseal), and collagen-impregnated Dacron (Hemashield) grafts. Cell loss from FN-coated prostheses, when exposed to a simulated human arterial blood flow of 200 ml/min in an artificial pulsatile circulation, was quantified from the loss of gamma activity from the graft over 24 hours, pressure in the circulation being reduced to 15 mm Hg to reduce fluid loss. Initial HAEC adherence (mean [SD]) to plain grafts was 3(1)%, 47(9)%, and 53(9)% for ePTFE, Gelseal, and Hemashield, respectively. This improved significantly with FN coating (78[6]%, 60[8]%, and 76[4]%). Cell retention after 24 hours of flow to FN-coated grafts was 16(10)%, 25(5)%, and 65(4)% and was confirmed qualitatively by scanning electron microscopy and environmental scanning electron microscopy. FN significantly improved initial cell adherence with Dacron grafts showing the better adherence. Cell retention after 24 hours of flow was better with FN-coated Dacron than with ePTFE but was best with Hemashield grafts.

Systemic endotoxaemia and fibrinolysis during aortic surgery

OBJECTIVE To determine whether endotoxaemia and activation of the systemic fibrinolytic system occurs during and after aortic surgery. DESIGN Prospective clinical study. SETTING University Hospital. MATERIALS 31 patients undergoing aortic surgery. CHIEF OUTCOME MEASURES Venous blood assay for endotoxin and plasminogen activator inhibitor-1 (PAI-1). Samples were obtained preoperatively, immediately before and 5 minutes after cross-clamp application and removal, and at 2, 4, 6 and 24 hours postoperatively. Tonometric sigmoid intramural pH was monitored throughout this period as a means of detecting colonic mucosal ischaemia. MAIN RESULTS Endotoxin levels increased after clamping of the aorta, peaking immediately before clamp removal, mean value 34.5 pg/ml, p < 0.01, but returning to preoperative levels by 24 hours. PAI-1 levels progressively increased after surgery, with persistently high levels remaining at 24 hours (p < 0.01). CONCLUSIONS Endotoxaemia does occur during aortic surgery and appears to be associated with activation of the systemic fibrinolytic system.

The preservation of renal function by isovolemic hemodilution during aortic operations

PURPOSE In an investigation of the effects of isovolemic hemodilution, 39 consecutive patients undergoing elective infrarenal aortic operation had detailed measurements of renal function, renal artery blood flow, and cardiac hemodynamics. METHODS The patients were randomly allocated to receive acute preoperative isovolemic hemodilution to a hematocrit of 28%, with 20 patients receiving hemodilution and 19 being control subjects. RESULTS Twelve (63%) of the control group had renal impairment, compared with only four (20% in the group receiving hemodilution (p < 0.01). Hemodilution also prevented the fall in cardiac output induced by cross-clamping (p < 0.01) and significantly reduced the need for transfusion of donor blood (p < 0.01). CONCLUSIONS Acute isovolemic hemodilution is clearly a useful adjunct in the management of patients undergoing elective aortic operation.