Artificial Heart valves - Fluid Dynamics

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Fluid dynamics of mechanical heart valves

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Artificial Heart valves: Influence of fluid dynamics on their success :

Artificial Heart valves: Influence of fluid dynamics on their success Muraleedharan CV Sree Chitra Tirunal Institute for Medical Sciences & Technology, Trivandrum, INDIA

Heart and circulatory system:

Heart and circulatory system Transport system of the body Blood as the media Oxygen / CO 2 Nutrients to cells Waste removal Heart acts as the pump Positive displacement pump Rhythmic contraction and expansion of muscles controlled by electrical signals Blood vessels acts as the pathways – tubular structures

Heart - anatomy:

Heart - anatomy Has four chambers Two pumping chambers Four non return valves Two valves on either side Right ventricle pumps through the lungs (pulmonary circulation) Left ventricle pumps through the entire body ( systemic circulation) Left heart & its valves are more susceptible to damage, since it works against the entire vascular resistance of systemic circulation

Circulatory system - revisited:

Circulatory system - revisited LV RV LA RA 1 3 2 4 LUNGS BODY LA and RA are the collection chambers LV and RV are the pumping chambers Two valves, at the inlet and outlet of the pumping chambers Tricuspid valve Pulmonary valve Mitral valve Aortic valve

Heart valve defects:

Heart valve defects The heart valves, normally due to certain diseases, fail to function as unidirectional check valves; they either become too leaky in the closed state ( Regurgitation ) or very narrow and offer resistance to blood flow in the open state ( Stenosis ) Mostly, rheumatic fever and associated complications cause valvular defects Many modes of management are feasible for valvular defects – depending on the state of the valve and the patient LV RV LA RA 1 3 2 4 LUNGS BODY

Heart valve defects - management:

Heart valve defects - management Corrections : The state of the valve is improved, either using a catheter and balloon or using surgical intervention on valve using annulaoplasty rings Balloon expansion is employed in the case of stenosis Annularplasty rings are mostly used for valve leaks Correction procedure cannot be performed in all cases Annularplasty ring

Artificial heart valves:

Artificial heart valves Artificial heart valves are devices used for replacing damaged or diseased natural valves of the heart. The natural valves are excised out and replacements are implanted

Pioneers:

Pioneers Hufnagel : 1952 Harken: 1960 Starr: 1960

Artificial heart valve - types:

Tilting Disc Bileaflet Caged Ball Tissue Valve Artificial heart valve - types Tissue valves : Fabricated from materials of biological origin Homorafts / Xenografts Mechanical valves : Made from materials of synthetic origin – metals, ceramics and polymers

Evolution – based on fluid dynamics:

Evolution – based on fluid dynamics Ball at the centre of fluid pathway made the caged ball design inferior leading to high pressure gradients and increased thromboembolic complications Substantially improved the gradients and provided improved flow characteristics Improved the hemodyanmics further providing a near central flow which also eliminated the complications due to the asymmetry in the flow characteristics of the tilting disc design

Flow profiles:

Flow profiles Clin Exp Pharmacol Physiol. 2009 February ; 36(2): 225–237.

Flow profiles:

Flow profiles Clin Exp Pharmacol Physiol. 2009 February ; 36(2): 225–237.

Effect of occluder shape:

Effect of occluder shape Ball shaped occluders cause substantial flow separation, turbulance and regions of stasis at the distal end, leading to thrombus formation and hemolysis Disc shaped occluders have shown improved flow characteristics and lower propensity for thrombus formation .

Fluid dynamics of heart valves:

Fluid dynamics of heart valves Study of fluid dynamics of artificial heart valves are essential to assess their Functional performance characteristics like Forward pressure drop Effective orifice area Closing volume Leak Potential for thrombus formation Platelet activation and aggregation Shear induced cell damage Shear induced damage to endothelial lining Potential for cavitation induced damage To valve components Formed elements of blood Closing mechanism Impact forces Valve sound

Functional Performance Characteristics:

Functional Performance Characteristics Clinicians have developed two parameters to quantify the degree of stenosis / regurgitation to assess valve performance: the effective orifice area (EOA), which is a measure of the effective valve opening during the forward flow phase the regurgitant volume, which is a measure of the back flow (or regurgitation) during the leakage flow phase.

Functional Performance Characteristics:

Functional Performance Characteristics The regurgitant volume corresponds to the total volume of fluid that leaks back across after closure and is related to valve shape and leaflet closing dynamics. A high regurgitant volume indicates that the net cardiac output is reduced and the heart has to contract more to meet the demands of the body. For an artificial heart valve, the total regurgitation is composed of two components namely, Closing volume Leakage volume

Steady flow test :

Steady flow test

Flow visualization:

Flow visualization Size #27 Flow channel profile Input velocity profile

Flow visualization:

Flow visualization Flow rate ( Lpm ) 10 15 20 25 30 Experimental p (mm Hg) 1.2 3.7 6.8 7.8 17.6 CFD p (mm Hg) 2 4.6 8 9.6 16.8 Uncertainty in EOA Estimate (%) -8.2 -6.5 -6.1 -7.5 6.7

Pulse Duplicator:

Pulse Duplicator Simulates the systemic circulatory pathways of the heart. Pumping action is simulated using hydro/pneumatic drives. Fluid pathway resistances and compliance chambers are employed to emulate the systemic resistance and compliance . Many functional parameters could be assessed using the system Pressure drop across the valve Effective orifice area Regurgitation Velocity profiles Closing dynamics Impact forces

Potential for thromboembolism:

Potential for thromboembolism Non-physiological blood flow patterns in the vicinity of heart valvesmay initiate thrombus formation by: imposing forces on cell elements (regions of high shear stress cause tearing of the blood elements, thus leading to haemolysis and platelet activation) changing the frequency of contact (recirculation and flow stagnation regions increase the contact time between blood elements, in particular activated platelets, thereby promoting thrombus formation)

Potential for thromboembolism:

Potential for thromboembolism Schematic of a bileaflet mechanical heart valve implanted in the aortic position during the leakage flow phase. Shown are the blood cells damaged from the high shear environment experienced within the leakage gaps (not to scale). Top panel: forward flow phase; bottom panel: leakage flow phase. Clin Exp Pharmacol Physiol. 2009 February ; 36(2): 225–237.

Shear induced platelet aggregation:

Shear induced platelet aggregation Under low shear rates (typically wall shear rates ~ 1000 s − 1 ), tethering of platelets to the surface of immobilized platelets takes place This adhesive interaction is rapidly reversible and at shear rates above10 000 s −1 does not readily support stable platelet-platelet adhesion Under conditions of very low shear rates (<1000 s −1 ), the platelet adhesion and aggregation progresses Involvement of multiple adhesion receptor-ligand interactions in platelet aggregation under high shear flow ( Jackson S P Blood 2007;109:5087-5095) .

Velocity profiles and tissue overgrowth :

Velocity profiles and tissue overgrowth

Cavitation in heart valves:

Cavitation in heart valves Cavitation is the rapid formation of vaporous microbubbles in the fluid due to a local drop of pressure below the vaporization pressure at a given temperature. When conditions for cavitation are present bubbles will form and at the time of pressure recovery they will collapse or implode. This event will cause pressure or thermal shockwaves and fluid microjets which can damage a surface.

Cavitation in PHV- causes:

Cavitation in PHV- causes Squeeze flow - as the occluder approaches the housing during closure and fluid is squeezed between the occluder and the valve housing causing a low pressure formation. Water hammer - caused by the sudden stoppage of the valve occluder as it contacts the valve housing. This sudden retard of the fluid retrograde inertia put the fluid under tension causing cavitation. Squeeze flow forms a cloud of bubbles at the circumferential lip of the occluder whereas water hammer is seen as transient bubbles at the occlude housing.

Cavitation potential:

Cavitation potential Bubble visualization photographs show no evidence of cavitation on polymeric occluder valves This is confirmed by the maximum negative pressure transients (less than 250 mm in polymer occluder valves compared to 550 – 620 mm Hg for valves with PyC occluders ) CS Lee et al, ASAIO (2001)

Negative pressure transients:

Negative pressure transients KB Chandran et al, Ann Biomed Engg 1998 CH 27 MH 27 Negative pressure transients during valve closure. Mechanical valves with flexible (soft) occluders minimize the potential for the valves to cavitate since the negative pressure transients are well above the vapor pressure of blood.

Closing impact forces:

Closing impact forces Closing Impact Force (N) 0 20 40 60 80 TTK-Chitra Medtronic Hall BS Standard BS Monostrut Lower impact forces  lower sounds  Bulk of the impact energy dissipated in the disc  lower cavitation potential

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