Vascular Grafts, Stents, and Vascular Grafts, Stents, and MeshesMeshes
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 Percentage
Water 91-92
Protein (fibrinogens, globulins, albumins) 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
BloodComponent 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 plaque-narrowed 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