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Vascular Grafts, Stents, and Meshes Introduction • Arterial diseases – Major medical problem world-wide – One of the main causes of death in the US • Surgical reconstruction – Does not deal with the causes of disease • Not fully understood – Solve problems caused by symptoms Characteristics of Vascular Grafts • Porosity – Essential component – Long-term patency – Permit ingrowth of cells (fibroblasts) • Necessary for uniform and satisfactory bonding of the internal lining • Compliance – Must be matched to the properties of the artery • Occlusion of the replacement • High shear stress at suture line • Turbulence of blood flow with local stagnation • Biodegradability – Control hemorrhage – Low porosity during implantation/high porosity during healing – Enables patient’s cells to replace the graft with natural tissue Graft Failure • Dilitation – Permanent enlargement of graft diameter due to pulsing stresses – Most frequent mechanical failure – 35 months after implantation • Suture failure – Mismatch of compliances – Suture material failure – 30-50 months after implantation • • Defects in the graft – – – – Rare Holes, perforations, rents, slits Occur during manufacturing and handling Usually hard to detect Bleeding and infection – – – – Rare Usually within first 10 months Suture line Interstices of the graft History – Natural Materials • 1906 to 1916 – First documented case of veins used to replace human arteries • 1940s – Arterial grafts from young, dying persons used (allograft/homograft) – Degenerative changes – Rejection – Abandoned in the 1950s • 1950’s – Carotid artery from cow (heterograft/xenograft) was initially successful – Development of complications • 1976 – Tanned umbilical vein graft – Still used for lower extremity revascularization History - Synthetics • During WW I (1914-1918) paraffin-coated Ag tubes introduced • Also, paraffin-coated glass, aluminum, polyethylene, and steel mesh – Inert – Non-porous – Compliance Cardiovascular System • Blood vascular system – Distributes nutritive materials, oxygen, and hormones – Removes cellular waste products of metabolism and carbon dioxide – 60,000 miles (http://www.cardio.bayer.com/en/heart_vascular/vascular/) Heart • Modified blood vessel serving as a double pump – 2 sides, left and right heart – Atria and ventricles • Atria – reservoirs • Ventricles - pumps Arteries, Capillaries, and Veins • Arteries – Carry blood from the heart to the extremities – Pulsating pressure • Capillaries – Major location of biological interchange – Meshwork of fine tubules • Veins – Return blood from extremities to heart – Constant pressure Pressure/Area Profile Muscular Arteries • Parts of the body under varying conditions require different amounts of blood • Supply arteries must be able to vary the size of their lumina – Walls consist of smooth muscle fibers – Controlled by autonomic nerve system Arterioles • Blood delivered to capillaries under reduced pressure because walls thin to allow nutrient/waste transfer • Narrow arteries (<100 μm or less) with thick, muscular walls Microscopic Structure of Arteries • Single layer of endothelial cells – Capillaries – major wall component • Structure and thickness of other walls depends upon function “Elastic Arteries” • • Wall thickness is relatively thin for the size of the vessel Large arteries Blood • Belongs to group of tissues called connective tissue • 7% of total body weight • 5 ℓ in average adult • Formed elements (55%) – Red cells, white cells, platelets • Plasma (45%) – Imparts fluid properties to blood Plasma • Fluid that transports nutritive materials Component Water Protein (fibrinogens, globulins, albumins) Percentage 91-92 7-8 Other solutes: Small electrolytes (Na+, K+ , Ca2+, Mg2+, Cl-, HCO3 -, PO4 3-, SO4 2-) Nonprotein nitrogen substances [NPN] (urea, uric acid, creatine, creatinine, ammonium salts) Nutrients (glucose, lipids, amino acids) Blood gasses (oxygen, carbon dioxide, nitrogen) Regulatory substances (hormones, enzymes) 1-2 Blood Component Number or percentage Red blood cells (erythrocytes) 4-5x106/mm3 White blood cells (leukocytes) 6000-9000/mm3 Agranular leukocytes: Lymphocytes 30-35% (of leukocytes) Monocytes 3-7% (of leukocytes) Granular leukocytes: Neutrophils 55-60% (of leukocytes) Eosinophils 2-5% (of leukocytes) Basophils 0-1% (of leukocytes) Platelets (thrombocytes) 2-4x105/mm3 Blood Clotting • Discontinuity in endothelial lining – Leads to deposition of proteins – Leads to platelet aggregation – Followed by adhesion of other platelets • Coagulation initiated by factors in plasma • Cascade of at least 13 plasma proteins • Last step is conversion of monomer fibrinogen to fibrin through action of plasma enzyme thrombin creating a fibrous network that traps blood cells Angioplasty • Opening up plaquenarrowed artery without doing major surgery • Catheter w/balloon tip inserted into coronary or major leg or arm artery • Inflation of balloon – often repeatedly – stretches artery wall – disrupts plaques Stents • wire mesh • Used in 70-90% of all angioplasty procedures • keeps the vessel open after widening • placed onto balloon prior to insertion • permanently attached as balloon inflates Uses of the Procedure • Open narrowed or blocked coronary artery in patients suffering from angina – alternative to bypass surgery • Open a blocked artery in the pelvis, leg, or arm – peripheral arterial disease • Control blood pressure in renal hypertension – caused by narrowing of one or both arteries supplying kidneys • To keep blood vessel grafts open in patients undergoing hemodialysis – most have a graft between a artery and vein in the arm to easily draw and replace blood • maintain blood flow to the brain by keeping open the carotid artery