Chapter 6- Heart Valves
Mitral Valve (Human)
- see slide 7
Clinical Significance
-5 million diagnosed with heart valve disease annually in the US ($200 billion)
Tissue deterioration
-5-15 year life span -Replacement - 11.5% - 32% mortality rate for patients age 70+
ThermaFix
-A pH temperature-controlled fixation technique mitigates the formation of residual glutaraldehyde moieties -> targets calcium binding sites for calcium mitigation; increases the life-time to ~15 yr
Stenotic Aortic Valve
-A view of a stenotic aortic valve form the aorta. Deposits that inhibit normal opening and closing can be seen on all three cusps. -see slide 14
Mechanical Aortic Valve
-A view of mechanical aortic valve from the left ventricle. (porcine) -see slide 17
Key features from engineering standards
-ANSI/AAMI ISO 5840:2005 Cardiovascular Implants - Cardiac Valve Prostheses 1. Hydrodynamic performance : minimum requirements of effective orifice area (AEO) and regurgitate fraction for various tissue annulus diameter (TAD) 2. Component fatigue 3. One-year clinical study 4. Clinical study long-term follow-up 5. Biocompatibility : cytotoxicity, sensitization, irritation, toxicity, genotoxicity, implantation, hemocompatibility, and carcinogenicity.
Endocarditis
-Bacteria/infection induced inflammation of the endocardium -Glutaraldehyde treatment
Bacterial endocarditis
-Bacterial endocarditis is a bacterial infection that can affect the valves of the heart causing deformity and damage to the leaflets of the valve(s).
Heart valve design
-Ball and cage valve (Starr-Edwards valve design) -Tilting disk valve (Bjork-Shiley valve) -Bileaflet valve (St. Jude Medical) -see slide 16
Regurgitation
-Blood leaking back across the valve --Inability to close fully -occurs when blood flows back into the valve as the leaflets close or leaks through the leaflets after they are closed. This condition causes the heart to work harder to pump the same amount of blood
Edwards SAPIEN
-Bovine pericardium prosthesis mounted on a balloon-expandable stent
Major Problems
-Calcification -Tissue deterioration -Thromboembolism -Endocarditis
Calcification due to aging
-Calcification is the process that refers to the accumulation of calcium on the heart's valves. The aortic valve is the most frequently affected. This build-up hardens and thickens the valve and can cause aortic stenosis, or narrowing of the aortic valve. As a result, the valve cannot open completely as the valve function is limited and blood flow is hindered. This blockage forces the heart to work harder causing limited physical capacities. Calcification comes with age as the calcium amasses in the heart over the course of a lifetime.
Coronary artery disease
-Coronary artery disease affects the blood flow between the heart and its systems due to an accumulation of plaque in the arteries. This build up causes hardening of the coronary arteries also known as atherosclerosis and restricts maximum blood flow to the heart muscle. Without an ample supply of oxygen-rich blood, the heart suffers. This may be evidenced by chest pain or shortness of breath in the patient. If the arteries become completely blocked, the blood will not flow and will clot, leading to a heart attack. If a patient is exhibiting symptoms of coronary artery disease, a physician will run a series of tests including, but not limited to, an electrocardiogram or EKG/ECG, cardiac catheterization, blood work, and/or chest x-ray.
Heart valve disease causes
-Degenerative valve disease (age-related) -Calcification due to aging (most common) -Coronary artery disease (papillary muscle rupture) -Rheumatic fever -Congenital abnormalities (bicuspid aortic valve) -Bacterial endocarditis
Cardiac mechanics
-Each beat of the cardiac cycle begins with electrical stimulation and ends with mechanical valve closure. -The cardiac action potential (electrical stimulation by SA node) stimulate muscle contraction. -The goal of electrical stimulation and muscle contraction is to transport oxygenated blood to the tissues and deoxygenated blood to the lungs. -The heart valves prevent back flow, or regurgitation, of blood between a ventricle and atrium or between a large blood vessel and ventricle.
Transcatheter valve
-Edwards SAPIEN -Medtronic CorValve -Both are deployed via catheter delivery -see slide 21
Transcatheter Aortic Valve Replacement (TAVR)
-For patient who are either at high risk or too sick for open-heart surgery, TAVR may be an alternative -This less invasive procedure allows the aortic valve to be replaced with a new valve while the heart is still beating. -Although it may slightly increase a risk of stroke, TAVR is durable and cost effective procedure.
Anatomy of Heart Valves
-Four valves --Atrioventricular (AV) valves : Mitral and Tricuspid valves --Semilunar valves : Aortic and Pulmonary valves --Open and close due to pressure gradient -Leaflets -Chorade tendineae -see slide 4
Mechanical Valve
-Frame --Titatium --Co-Cr alloys -Occluding disc/ball --UHMWPE (ultra-high-molecular-weight polyethylene) --Silicon elastomer --Pyrolytic carbon (PyC) : great thrombus-resistance -Suture ring --Dacron --E-PTFE
Congenital abnormalities
-Generally, congenital heart defects affect the flow of blood through the cardiovascular system. Blood can flow in the wrong direction, in abnormal patterns, and can even be blocked, partially or completely, depending on the type of heart defect present. Ranging from mild defects such as a malformed valve to the more severe such as an absent heart valve, congenital heart abnormalities require different treatments. In some cases, medicine can be used to treat the condition; in others, surgery may be necessary.
Stented porcine valves treaded with glutaraldehyde treatment
-Glutaraldehyde (colorless tanning agent) prevents collagen denaturation by intermolecular cross linkages and reduces thromboembolic risk. -A tissue valve --Leaflets : treated porcine (pig) pericardium or bovine (cow) pericardium --Stent : Teflon-covered stainless steel stent --Swing cuff : Dacron (Polyethylene terephthalate; PET) fabric covering the stent -Calcification issue (younger than 60 yr) - less calcium absorption and greater calcium circulation in blood due to glutaraldehyde fixation processes and intrinsic tissue components
Calcification
-Mainly in tissue valves -Ca2+ + P on tissue : Ca-P mineral deposits -Can cause stenosis
Material used
-Mechanical Valve -Tissue Valve
Qualification of heart valve
-Minimal pressure drop -Relatively small regurgitation volumes -Minimum production of turbulence and turbulent shear stresses -Not create stagnation or flow separation regions in the immediate vicinity
Future of Artificial Heart Valves
-Minimally invasive heart valve surgery --Totally endoscopic aortic valve replacement (TEAVR) --Compressed valve inserted through vein or artery in the leg --Inflates once in position -Transplant cells from the kidneys and bone marrow to tissue valve --Improves biocompatibility of valve -Tissue valve with living cells --Self-healing --Slow degradation rate
Biological Valves
-Natural material -No prolonged anticoagulant medication -Limited lifespan of 10-15 years -Calcification occurs -Patient over 40
Material Specifications
-Non-thrombogenic -Non-hemolytic -Infection resistant -Biological inert -Durable -Facilitate healing -Non-obstructive
Mechanical valve design
-Occluder -Occluder retention mechanism -Sewing cuff
Medtronic CorValve
-Porcine pericardial prosthesis mounted in a shelf-expanding nitinol frame
Mechanical valve biocompatibility
-Prosthetic valve -Biocompatible material -All patients who choose mechanical valve replacements are placed on continued warfarin anticoagulation therapy for the lifetime of the valve (30-40 yr), Patient of any age =Anticoagulant medication required for life to prevent thrombosis. -Constant "clicking" noise -see slide 15
Biocompatible material
-Pyrolytic carbon (thermal decomposition of hydrocarbons, such as propane, propylene, acetylene, and methane) > Silastic rubber
Heart valve disease effects
-Regurgitation -Stenosis
Rheumatic fever
-Rheumatic fever is caused by an infection of the Group A Streptococcus bacteria and can detrimentally affect the heart and cardiovascular system, especially the leaflet tissue of the valves. Rheumatic fever is most common in children ages 5 - 15, but can develop in adults. Rheumatic heart disease is common in developing countries.
Leaflets
-Sheets of connective tissue
Tissue Valve
-Stent --Polypropylene --Elgiloy (Co-Cr-Ni allow) -Stent covering --PTFE (polytetrafluroethylene) --Polyester --Acetyl polymer -Leaflets --Porcine pericardium --Bovine pericardium -Suture ring --Darcon --e-PTFE --Silicon rubber
Right Ventricle Outflow Track (RVOT)
-The anterior papillary muscle seen trough the pulmonary valve from the pulmonary tank. -see slide 6
Fluid mechanics
-The cylinder of fluid in the tube --Newtonian (or uniform) --Fluid with laminar (or smooth) flow -see slide 11
The cardiac cycle
-The pressure driving systole and diastole in the right ventricle are much lower than the pressure in the left ventricle. -> The semilunar valve opens in the left ventricle above 80 mmHg, but only above 8 mmHg in the right ventricle. -see slide 8
Shear stress for a Newtonian fluid
-The stress encountered at the boundary condition of the total radius, which is equal to the product of the viscosity and velocity gradient -see slide 12 -However, .... this model is limited because: --Blood flow does not involve rigid cylindrical tube (smooth, stretch, flexible) --The pulsatile flow with heart valves opening and closing is not smooth.
Degenerative valve disease
-This is a common cause of valvular degeneration. Most commonly affecting the mitral valve, it is a progressive process that represents slow degeneration from mitral valve prolapse (improper leaflet movement), a condition that affects 4-5 percent of the general population. Over time, the attachments of the valve thin out or rupture and the leaflets become floppy and redundant. This leads to leakage through the valve.
Thromboembolism
-Tissue death, stroke, heart attack -Lifetime anticoagulant treatment
Pericardial valve
-To minimize flow turbulence -Bovine pericardial tissue that has been shaped and stented into leaflets -ThermaFix -A view of a stentless bioprosthetic porcine tissue valve form the left ventricle. -see slide 20
Comparison of velocities and turbulent shear stresses
-Uniform and low reverse flow velocity : Ball and cage < Bileaflet; porcine < pericardial -Turbulent shear stresses : Bileaflet < Ball and cage; pericardial < porcine -see slide 25
Stenosis
-Valve becomes constricted --Inability to open fully (narrowing of the valve leaflets) --Decreased blood flow, high blood pressure -Stenosis is the narrowing of a valve opening that causes lower blood flow through the valve. This in turn increases the risk of blood clots and causes the heart to work harder.
Tilting disk valve (Bjork-Shiley valve)
-a disk tilts within struts at an angle in the open position and totally occludes the valve orifice in the closed position.
Ball and cage valve (Starr-Edwards valve design)
-a silastic rubber ball, silicon-coated stainless steel cage, and Teflon sewing
The AV valves also have fiber strands called
-chrodae thedineae that connect their leaflets to papillary muscles on the ventricular wall. -see slide 3
Chorade tendineae
-cord-like tendons -prevent valve eversion -Connect valves to heart muscle -80% collagen, while the remaining 20% is made up of elastin and endothelial cells.
During systole
-isovolumetric contraction period and ejection period occur --Cardiac out put (CO) = Stroke volume (SV) x Heart rate (HR) --Ejection fraction (EF) = Stroke volume (SV)/120 = ~60 % --Normal CO at rest = 5 L/min --Left ventricular ejection time (LVET) = 150-300 ms
During diastole v
-isovolumetric relaxation period and filling period occur --Initial 50 ms, both valves remain closed --The intraventricular pressure falls rapidly to almost zero --When the pressure is lower than the atrial pressure, the mitral valve opens, causing the ventricle to fill in preparation for the next systole.
Each valves is passively opened or closed through a
-pressure gradient
Semilunar valves
-pulmonary and aortic valves -see slide 3
Bileaflet valve (St. Jude Medical)
-pyrolitic carbon leaflets allow blood flow thought a central rectangular orifice and two semicircular lateral orifices in the open position, and some degree of leakage flow through small gaps in the closed position. -> Most commonly used mechanical valve
Tricuspid Valve (Human)
-see slide 5 -The tricuspid valve viewed from the superior vena cava. The septal cusp is on the left, the anterior cusp is on the right, and the posterior cusp is on the bottom. The right fibrous ring is also visible.
Prosthetic valve
-the host response occur directly in the bloodstream stimulating thrombosis and thromboembolic events.
A valve consists of two or three ..
-tissue leaflets attached to an annulus, or ring, that separates the two cardiac regions
Atrioventricular (AV) valves
-tricuspid and mitral valves -see slide 3
Phases of the cardiac cycle
1. Atrial Contraction 2. Isovolumetric Contraction 3. Ventricular Ejection 4. Isovolumetric Relaxation 5. Atrial Filling 6. Ventricular Filling -see slide 9
