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