Biomechanics Cardiovascular - Bruce

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Biomechanics Cardiovascular - Bruce

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What is atherosclerosis
What are consequences
Artery wall thickens - fatty tissue build up
Reduced blood flow and nutrients
Amputation, stroke, heart attack, kidney failure
Cellular Mechanisms of Plaque
1. Excess LDL in arterial wall
2. LDL stimulate endothelial cells - adheasion molecules - latch to monocytes = inflammation
3. monocytes - macrophages = inflammation mediators
4. Inflammatory mediators - plaque growth, fibrous cap (proliferation of SMCs tough glue fibrous matrix)
5. Plaque - stenosis/thrombosis clot rupture
6. Heart attack/blocked blood vessel
Arterial Imaging Methods
1. Angiogram - x-ray based (0.25mm)
2. Intravascular Ultrasound IVUS - planning and post op analysis
3. Optical Coherence Tomography (OCT) near infrared light (+/- 0.015mm)
Atherosclerosis Treatment Options
Angioplasty, atherectomy, force focused angioplasty, cryoplasty, laser, radiation, local drug delivery, stents, bypass surgery, blunt dissection
PTCA - percutaneous transluminal coronary angioplasty
POBA - plain old balloon angioplasty
medical devie that is inserted into clogged artery and inflated to high pressures to clear the blockage and allow blood to flow
Angioplasty lumen enlargement happens from what combination?
1. plaque reduction (compression/embolisation)
2. axial plaque redistribution
3. vessel expansion
Describe two types of medical ballons
1. high pressure balloons - molded to inflated geometry, non/low compliant materials that retain size/shape even in high pressure
2. low pressure ballon - molded to tubular shape, expanded to several time original size (fizxation/occlusion)
Balloon Definitions
Diameter: diameter at specific pressure
Length: working length/length of body section
burst pressure: avg pressure to rupture ballon
rated pressure: statisically guaranteed pressure to inflate ballon without failing
profile: max diamter of wrapped ballon on cathether
compliance: change in balloon diameter as a function of inflation pressure
Balloon materials
PET: polyethylene terephthalate - high tensile strength, low-medium compliance, high stiffness, low profile, max rated pressure 254psi, 20 atm, radiation Etd
Nylon - not as strong but softer = easier to withdraw from guide catheter
Desired Ballon Qualities
1. smallest possible wall thickness
2. burst pressure > 15 bar
3. defined pressure & diameter characteristics
How balloons are made?
extrusion of material into a tube shape - blow molding
1. heating jaws to melt plastic
2. tubing kept in balloon form for a time - at constant pressure
3. after warm up - high pressure applied with compressed air supply
4. strecth of distal and proximal ends - prodcued exact form and optimum wall thickness
Balloon mechanics
stress circumferential = pd/2t
stress axial = pd/t
coronary d = 2.5 mm
valvuplasty d = 26 mm
would expect much greater stresses in valvuplasty
Design Requirments for Balloon Catheter
1. low profile
2. flexible
3. pushable
4. high burst pressure
5. tensile strength
6. kink resistance
7. pressure transfer properties
8. fast inflation/deflation times

Two main catheter designs
1. Over-the-wire (OTW) catheter (co-axial/dual-lumen design) peripheral procedures
2. Rapid exchance - long SS hypotube & short flexible distal shaft - exit port located along distal shaft
Testing Requirements
1. Cyclic testing 10-40 cylcles of RPB (rated balloon pressure?)
2. Burst pressure - must tear longitudinally along balloon
3. inflation/deflation (30s max for peripheral catheters)
4. Profile - must not exceed inner diamter of recomment introducer sheath/guide catheter
5. tensile strenght ob bonds must exceed certain minimum value
Drug Eluting Balloons (DEBs)
what are they coated with?
what is concept?
coated with proprietary solution or dipped in paclitaxol
assumption rapid release of drug into tissue is more effective than gradual release of drug (DES)
Application of DEB?
Coronary arteries - bifurcations, small vessels <2.5mmd, left main disease
Peripheral arteries - arterias arms, legs, abdomen - pain/gangrene

Angioplasty vs Surgery in Lower Extremity Lessions
Angioplast - short occlusions
< 2 cm tibal, < 5cm iliac, < 10 cm superficial femoral artery
Surgery favored - long occlusions
failure post angioplasty
What is RAS
- renal artery stenosis - arteries that feed kidneys!
- occurs in 5% of pop
- can lead to renal failure if untreated - angioplasty/stenting effective in halting/retarding
What is CAS
what can happen?
Caratoid arterial stensis
- can develop stoeks
- angioplasty as alternative to invasive carotid endarterectomy?
- not suitable of reg SS stent
- intracerebral embolism (plaque fragments)
- emboli from catheter manipulatino/balloon deflation
- protection devices during stenting?
Protection devices during carotid arterial stenting
1. distal occlusion ballon - obstructs bloodflow as well as distal emboli, require aspiration of blood and emboli from artery prior to deflation
2. proximal occlusion -
3. filtration device - positioned distal to stenosis in interal carotid area, filter trapped emboli
What is atherectomy?
removal of atheroslerotic tissue by cutting/shaving tissue off inside of the artery
Problems with Balloon Angioplasty
1. restenosis ~30% cases - elastic recoil, negative vascular remodling maintain acute luminal gain
Stent manufacturing steps
1. alloy tubes
2. laser cut stent pattern
3. grist blast - remove laser burrs
4. electropolish to remove surface debris
5. inspect and clean
6. crimp on to catheter
7. package and sterilize
Stent requirements
1. high expandibility (ratio collapsed to expanded diameter)
2. wall thickness as small as possilbe to offer low profile before and after deployment + increase flexibility
3. postimplant metal surface should be small - potentially determines degree of thrombosis formation/degree of vessel injury
4. high radia strength to prevent vessel elastic recoil
5. radiopaque for precise positioning
6. scaffolding properties
7. minimal shear of stent over vessel during expanision
8. good trackability - follow a tortuos path
9. minimal foreshortening
10. minimal acute elastic recoil
11. biocompatible
Stent Materials
1. plastically deformed alloys (316L/304 SS or CoCr alloy)
2. self expanding nitinol stents 
3. biodegradable polymers

(superelastic 20x more that SS, spring loaded stent - biocompatible + corrosion resistant + reshape after externally applied force, physiological compatibility + dynamic interface as expands with pre-programmed diamter with no recoil
- small opening force to artery wall
nitinol disadvantages - spring loaded - perhapds no sufficient radial force, difficult to place, may need to be oversize to secure stent
superelastic and thermal shape memory predetermine shape through heating
USED: where kink/crush resistance is essential
Problems after stenting
1. cannot abolish restenosis - endothelial injury can create neointimal proliferation
2. ISR can occure up to 40% of the time