Biomedical Engineering - Biomachines

Course summary

Complete course

III - Haemolisys Erytrhop oietin : make red blood cells Haemolisys : pysiological process. Occurs in s pleen (+liver, reed bone narrow) by macrophages that eliminate older red blood cells slow continuous haemolytic state always p resent (1-2% of erythr ocytes , [Hbfree] close to 0.1 mg/dl ) Beyond the process is pathological → anaemia of varying severity - release of haemoglobin into plasma → toxic state if [Hbfree] > 160 mg/dl) → red urine. - torn stroma → can clog filters (liver, spleen and kidneys ) spectrophotometer: determine amount of aemolysis Fluid mosaic model : Cell membrane = lipid bilayer + p roteins immersed (significant degree of mobility) Lipid layer = double leaflet -> phospholipids: hydrophobi c tails face inward hydrophilic tails face outward Hydrophobic -> adhesion cellule ➔ causes of blood damage (haemolysis) : - mechanical: o Surface interaction (cell squashed between pumps wall) o Bulk flow (flow -induced haemolysis) o stretching of the membrane: Negative pressures of 50 -80 mmHg - Chemical: o Poison (snake, insect) o Bacteria toxins o Osmotic pressure effects (C° o f water) - Thermal: o Temperatures greater than 42 °C 0 °C -> freezing water -> tensile stress triggered in the stroma o high temperature variation ( right atrium (RA) -> right ventricle (RV) -> pulmonary artery (PA) -> lungs -> left atrium (LA) -> left ventricle (LV) -> aorta (Ao) -> different organs of the body - Diastole : => blood in open: mitral and tricuspid valve s close: pulmonic and aortic valves ➔ Preload and stroke volume CO = f . SV - Systole : => blood out (contraction) Open: pulmonic and aortic valves Close: mitral and tricuspid valves CO = cardiac output (left ventricule) f = heart rate [bpm] SV = strole volume [ml] Preload : initial stretching of the cardiac myocytes prior to contraction (cf. sarcomere length at the end of diastole ) longer preload length prior to contraction -> developed active tension increased not unique active tension curve: depends on inotropic state Sarcomere length cannot be determined ->indirect indices of preload (ventricular EDV or pressure ) Frank -Starling mechanism: ++venous retourn to heart (diastole) -> ++filling -> ++stretching of myocytes -> ++preload -> ++force generation -> ++heart ejection -> balancing the output of the two ventricle s length -tension diagram for isometric contractions V- Guyton model ➔ Guyton model: o CO -RAMP relationship RAMP : right atrium mean pre ssure Wiggers diagram Objecti ve : VR=f(RAMP), CO=f(RAMP) Animal heart -lung complex: - Very large reservoir mimicking -> RAMP (cf height) - Clamp at the outlet of left ventricule -> afterload - Peacemaker -> heart rate CO enhanced : increase heart rate and inotropy , decrease afterload o VR -RAMP relationship VR : veno us return o CO -RAMP & VR -RAMP relationship MSP : means systemic pressure upstream pressure of the venous return (reference pressure , pressure of one global organ ) if extracorporeal circuit: VI- Heart Valve Prosthesis Valve leafets = protein matrix of collagen fibres (circumferential direction) and elastin fibre (radial direction) ➔ Causes of heart valve disease: - Bicuspid aortic valve o Two aortic valve leaflets instead of three o Congenital (from birth) - Rheumatic heart disease from rheumatic fever (when strep throat and scarlet fever infections are not treated properly) - Age – sclerosis (thickening from aging) - Endocarditis (infection of the heart valve) : STENOSIS, REGURGITATION ➔ Criteria: - Unidirectional Valve - Passive (as natural valves are) - “Light” -> low density - Biocompatible (to enhance the interaction with the blood) - No Mechanical Failure • Good Fluid -Dynamics (to reduce clots formation) - Embolism ➔ Complications: +anticoagulant therapy (indefinitely f or mechanical) Biologicale valve: - Autografte: From the same individual, e.g. pulmonary to aortic - Homograft: From a different individual, e.g. cadaveric - Hetero/Xeno -graft autograft: From a different species, e.g. porcine, bovine VII I- Extracorporeal Circulation ➔ To empty the heart from the blood Heart stopped -> no nutrients -> ischemia -> cellular death ➔ Cardiac function remplacement -> ExtraCorporeal Circulation ( ECC ) or CardioPulmonary Bypass ECC: patient connected to the extracorporeal circuit, creating a new circuit -> no blood donor, NaCl 0.9 % + drugs -> Haematocrit reduction 1 pump is enough: ECC = Total bypass of both heart & lungs Bernoulli Relationship between Atrium and Reservoir IX - Exctracorporeal circulation – connections Alternative to ECC: circulatory arrest = cooling cells down to 18 °C -> ↓oxygen requirement (connection to ECC ->Temp to 18 °C -> circulatory arrest -> surgery -> restard ECC -> Temp brougt back to normal conditions) Temp < 30 -32 °C -> stop beating but cells still consum a lot -> circulate blood in // -> ECC ➔ BSA nomogram Calculate CO based on patient charecteristics ➔ Priming volume Donors blood is impossible (quantity available, risks of advers reaction) ❖ Cannulation Cannula=most haemolytic element in E CC Pos sible on: - Right atrium : only if no work on the right atrium (ex. Øwork on tricuspidale valve) - Venae cavae : allo ws the heart to be emptied , ↓size of operation field - Pulmonary artery : complex: just above the aorta, tube are annoying for operation - Aortic arch : pb: lumbs bypassed -> need to oxygenate the blood Where Ht Pecc = 0 % -> ↓Ht = Dangerous Priming volume is important and must be minimized ➔ Aortic cannulae CO + amount of blood s up llied are known -> cannula must allo w a given flow rate haemolysis cf. pressure drop at 100mmHg -> solution = ↑Dcannula if D cannula = D aorta -> afterload (very large resistance) -> heart “stresses” by surgeon won’t be able to restart beating = restart barrier Solutions in cannula configuration: - Insert cannula as lo w as possible in the blood vessel risk of slip out - Direct cannula in the vessel in an appropriate way - Design the cannula: tip cut into a flut bake -> ↑outflow section + flow correctly directed -> ↓haemolysis ➔ “Tobacco purse ” created on the aorta -> must tighten flaps of the artery after it has been tighten Air entry must be avoided: Embolism very risky o On the cannula, branch for air elimination o clamped cannula → tube and cannula facing each other → connected when blood overflows. ➔ Alternative connection: fermoral artery For a orta surgery / surgery in patients with adhesions (already operated) descending aorta -> renal problem (renal arteries face downward) -> low er flow rate Plastic - Pro: o Deformable -> ↑circuitry comfort - Cons: o not possible to decrease wall thickness much Metal : - Pro: o ↑resistan ce to mechanical stress o ↓wall thicknesse s - Cons: o High costs used for paediatric patients where pb of size is important (lower V patient -> ↑haemolysis) ➔ Venous cannulae: Fewer problems than in aorta (P= 2mmHg). ↑D to ↓Haemolysis Minimize pressure drop -> ensure venous return Dcannual = 2/3 D vena cava -> blood can enter right atrium at restart Tube in plastic -> tub mustn’t “ kneel ” (bending -> flow restriction = kinking effect ) -> metal helix in the thickness - Surgery outside the heart → cannulation into right atrium (auricle or appendage) - Surgery inside the heart → cannulation of the venae cavae Most used method ❖ Air Removal from the Extra Corporea Circulation Circuit Mostly import in arterial cannula: avoid embolism - Closed loop : circuit closed on itself and filled with saline solution Continuous operation for bubble elimination ( filter and/or free surface reservo ir) - Transparent circuit for bubble detection tapped using clamps to remove any bubbles that may be attached to the tubes if difficult to remove bubble: circuit fill ed with CO2 (Cf. more soluble with bool and water) and then proceed to fill with priming fluid X- Extracorporeal circulation – reservoir pump Aortic cannula: most haemolitic part of the ECC circuit (cf. dimensions) ➔ Reservoir Cardiotomy reservoir (= vents) : Collection place for blood suctioned from the surgical field Blood from the cardiotomy reservoir is filtered before entering the venous reservoir (cf. presence of tissue particles, fat, b one particles, suture material, calcium, fibrin, blood clots, microaggregates or bubbles ) Depth Filter and Screen Filter (also called direct interception filter ) - To entrap small (> 20 µm) and light particles (not entrapped by depth filter ) - = further safety element (if depth filter is saturated ) Made of PC -> rigid, strong, transparent, cheaper (than HE, OXY) Filters are disposable ➔ Pumps ➔ Continuous VR Pulsatile flowrate XI - Extracorporeal Circulation – Roller pumps ➔ Fumero -Parenzan Pump / PULSAMATIC pulsatil pump , 1 st one for infant and paediatric cardiopulmonary bypass segment of elastic tubing compressed by pneumatically driven pushing plate under control of a microprocessor (Pulse rate and stroke volume can be set , pump synchroni zed with patient's ECG for counterpulsation heart assist ) ↑cooling and rewarming rate ↓vascular resistance , intensive care unit time and need for blood transfusion => mortality significantly lower pump performance characteristic = relationship pump developed head and pump discharge flow (v=cte) ➔ Classifications of pumps: - Positive displacement (volumetric) : Theoretically : constant flow at a given speed (rpm) no matter ΔP must not operate against a closed valve on the discharge side of the pump (cf. ø shutoff head ) o Rotatory type: Screw, gear, lobe, peristaltic o Reciprocating -type Piston, diaphragm, syringe o Linear type : Rope, chain - Rotodynamic: kinetic machine -> energy continuously imparted to the pumped fluid by rotating impeller, propeller, or rotor Three classes: radial flow , axial flow and mixed flow operated with very low differential head -> flow and absorbed power will increase dramatically Diaphragm pump Rope pump ➔ Peristalt ic pump / roller pump - n = Rotational Speed (rpm) - N = number of rollers - V= volume between two consecutive rollers - Ψ = eccentricity of the pump (related to the tolerance of the manufacturing of the pump) ΔR = roller to pump wall distance shear stress = f ( RPM, ΔR ) -> large and small meatus can create haemolysis Effect of altering perfusate temperature on the output XII- Extracorporeal Circulation – Centrifugal pump Ie. Non -volumetr ic - Centrifugal direction of flow changes by 90 ° as it flows over an impeller -> bladed impeller, cones impeller (viscous drag principle) - Axial direction of flow is unchanged ➔ Centrifugal pump lost in velocity -> gain in pressure * If well designed: less haemolytic than a roller -type pump Velocity triangles : Blood axial velocity V = relative velocity W (of the blood, referred to the rotor velocity) + tangential velocity U Design will influence the velocity XII - Extracorporeal Circulation – Oxygenation difficulty in designing an oxygenating system -> mimic human anatomy - Lungs = modular structure with alveolus (cf. different type of activity) - Blood -> contact with the alveolar atmosphere (cf. partition of several biological membranes) - Oxygene transportation by blood: o Dissolved in plasma o Chemically bound to haemoglobin (Hb) exchange mechanism = 50 to 70 m2 But oxygenator: - less area (cf. very limited exchange of a patient in the operating room) - gas composition = oxygen -rich mixture ➔ partial pressure dalton’s law of the partial pressure Henry’s law Mixing with "exhaust gas" previous breath driving forces (=Δp): 60 mmHg for O2 and 6 mmHg for CO2 correct gas exchange : A = 60/70 m 2 Arterial blood: • pO2 = 100 mmHg (99% of saturation) • pCO2 = 40 mmHg Venous blood : • pO2 = 40 mmHg (75% of saturation) • pCO2 = 46 mmHg Dgas = diffusion coefficient Dgas.α : Characteristic of the single compartment ➔ Fick’s law ➔ Oxygenator design Limit: pO2=760 mmHg -> ΔpO2 720 mmHg ( *10) -> A/10 Dissociation curve During haemoconcentration or dilution of the blood, THE SATURATION DOESN’T CHANGE XII - Extracorporeal Circulation – Oxygenation (2) ➔ DESIGN an Oxygenator