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DEEPAK NANDAN

Principles of haemodynamic measurements: Pressure and flow measurements calculation of cardiac output Calculation of shunts valve area. DEEPAK NANDAN. Definition of Hemodynamics.

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DEEPAK NANDAN

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  1. Principles of haemodynamic measurements:Pressure and flow measurementscalculation of cardiac outputCalculation of shunts valve area DEEPAK NANDAN

  2. Definition of Hemodynamics Hemodynamics is concerned with the physical and physiological principles governing the movement of blood through the circulatory system. • The forces involved with the movement of blood throughout the human circulatory system include: 1. Kinetic and potential energy provided by the cardiac pump 2. Gravity 3. Hydrostatic Pressure 4. Pressure gradients, or differences, between two any points

  3. Properties of blood itself that affect its flow: 1. Viscosity 2. Inertial mass 3. Volume of blood to be moved • Factors that affect the motion of blood through the vascular conduits include: 1. Size of blood vessel 2. Condition of blood vessel 3. Smoothness of lumen 4. Elasticity of muscular layer (tunica media) 5. Destination of blood (distal vascular bed)

  4. Definition of Physical Concepts PRESSURE: the ratio of a force acting on a surface to the area of the surface (force per unit area). Units : Newtons/m², pascal(Pa), atmospheres(atm), mmHg. FLOW RATE: Amount of fluid passing a given point over a given period of time Described as either flow volume or flow velocity. Flow volume is measured in mI/mm or cm3/sec -defined by Poiseuille’s law. Flow velocity is measured in cm/sec or m/sec -described by Bernoulli’s principle. VISCOSITY: The internal friction between adjacent layers of fluid. Blood is 1.5 times as viscous as water and its viscosity is directly related to hct level

  5. KINETIC ENERGY: active energy , the energy of motion. In hemodynamics-described as the forward movement of blood. POTENTIAL ENERGY: stored energy. Kinetic energy is transferred into potential energy when it produces a lateral pressure or stretching of vessel walls during systole. The potential energy is converted back into kinetic energy when the arterial walls rebound during diastole.

  6. Energy in the blood stream exists in three interchangeable forms: 1)pressure arising from cardiac output and vascular resistance, 2)hydrostatic pressure from gravitational forces, 3)kinetic energy of blood flow • TE = (perpendicular pressure + gravitational pressure) + kinetic energy TE = (P per + P grav) + ½ ρ V² • BP—lateral pressure, kinetic energy (also known as the impact pressure or the pressure required to cause flow to stop), and gravitational forces. • Kinetic energy- highest in aorta (< 5% contribution) • Gravitational forces - important in a standing person, minimal impact while supine • The intravascular pressure is responsible for transmural pressure(i.e. vessel distention) and for longitudinal transport of blood.

  7. Pressure PRESSURE IN STATIC FLUIDS Rest- the pressure caused by liquid is α to the depth of the liquid & density pressure is exerted equally in all directions in a static liquid External pressure exerted on an enclosed liquid changes the overall pressure. Pascal’s Principle states that any change of pressure in an enclosed fluid is transmitted undiminished to all parts of the fluid. ΔP = ρg Δh magnitude of the gravitational effect (ΔP) is the product of the specific gravity or density of the blood (ρ), the acceleration due to gravity (980 cm/s/s) (g), and the vertical distance above or below the heart (Δh) which is 0.77 mm Hg/cm at the density of normal blood. HYDROSTATIC PRESSURE The pressure at a given depth in a static liquid is a result of the weight of the liquid acting on a unit area at that depth plus any pressure acting on the surface of the liquid. The pressure in any blood vessel below the level of the heart is increased, and the pressure in any vessel above heart level is decreased by the effect of gravity. Eg:-in the upright position, when the MAP 100 mm Hg, the mean pr in a large artery in the head (50 cm above ) is 62 mm Hg (100 - [0.77 x 50]) an the pressure in a large artery in the foot (105 cm below ) is 180 mm Hg (100 + [0.77 x 105]).

  8. PRESSURE IN FLOWING FLUIDS pressure in a flowing liquid depends on the details of the flow process, in contrast to the case of the static liquid pressure gradient, which is defined as a pressure drop/ unit length. For a uniform horizontal tube the pg will be the same at all points in the tube if a perfectly uniform flow pattern is maintained Pressure gradient = P1 – P2 /L ( P1 = entry pressure,P2 = exit pressure, L = length of tubing) Drops in pressure during flow represent losses in energy. These losses are largely attributable to friction effects Viscosity- force that oppose the flow

  9. Flow Rate POISEUILLE’S LAW The Bernoulli Effect When fluid flows steadily (without acceleration or deceleration) from one point in a system to another further downstream, its total energy content along any given streamline remains constant, provided there are no frictional losses. Bernoulli’s equation -rel between kinetic energy, gravitational potential energy, and pressure in a frictionless fluidsystem. predicts flow velocity and explains the presence of high-velocity jets obtained with Doppler instruments • predicts the volume of flow in moving fluids. • Q= (P¹-P²)π r4 8ήl (Q = volume flow P1 = entry pressure P2 = exit pressure l = length of tubing r = radius of tube ή = viscosity of fluid) As pressure difference or diameter of vessel ↑, volume flow ↑ As length or viscosity ↑, volume↓ predicts volume flow under laminar flow conditions in straight tubes where wall friction is not a significant factor.

  10. relationship bet bf , resistance, and pressure - modification of Ohm’s law for the flow of electrons in an electrical circuit: Flow(Q) = pressure gradient (P)/resistance(R) • Blood is not an “ideal fluid” and energy (and pressure) is lost as flowing blood overcomes resistance. • Resistance to blood flow is a function of viscosity, vessel radius, and vessel length. relationship is known as Poiseuille’s law Resistance = 8 × viscosity × length/ π× radius4 Blood flow = π × radius4 × difference in pressure/8 × viscosity × length Viscosity is also important in determining resistance. Exp relative to water. • plasma is 1.7 × viscosity of water and viscosity of blood is 3–4 × viscosity of water, the difference being due to blood cells and particularly hematocrit.

  11. In the mammalian circulation, resistance is greatest at the level of the arterioles. effective area is much larger (capillaries) • arteriolar resistance can be regulated

  12. Transition from laminar to turbulent flow can be predicted by calculatingtheReynold’s number, which is the ratio of inertial forces to viscous forces R = diameter × velocity × density/viscosity (viscosity (ή) of blood is 0.004 Pa s, density (ρ) of blood is approximately 1050 kg/m3, velocity (V) of blood is in m/s, and the diameter of the tube is in m. Reynold’s number is dimensionless.) • Aorta- laminar to turbulent flow occurs at Nr bet 2000 and 2500 • In atherosclerotic arteries &/or branch points,the critical Nr is much lower • In severe stenoses, turbulence can be initiated at Reynold’s numbers an order of magnitude less than in the theoretical, straight pipe. • Vessel diameter is doubly important for not only is it a direct variable in the equation, it also influences velocity • both velocity and diameter decrease in the microcirculation- laminar flow

  13. Methods of Measuring Blood Pressure • INTRAVASCULAR METHOD • AUSCULTATORY METHOD (Stethoscope and cuff) • PALPATION METHOD (Pulses) • Reverend stephen Hales measured blood pressure of a horse using a vertical glass tube - 1732 • Landois ( 1872 ) used a needle in an artery to direct spraying blood onto a moving paper surface-Hemautogram

  14. Pressure wave • A complex periodic fluctuation in force per unit area • A pressure wave is the cyclical force generated by cardiac muscle contraction • Its amplitude and duration are influenced by various mechanical and physiological parameters 1 .force of the contracting chamber 2.surrounding structures - contiguous chambers of the heart pericardium, lungs, vasculature 3.Physiological variables - heart rate, respiratory cycle

  15. FOURIER ANALYSIS The complex waveform- (Fourier analysis)- Mathematical summation of a series of simple sine waves of differing amplitude and frequency- harmonics Fourier found that each pressure wave is a summation of a series of simple sine waves of differing amplitude and frequency

  16. Wiggers principle • The essential physiologic information is contained within the 1st 10 harmonics of the pressure wave’s Fourier series • HR-120/min • Fundamental frequency- 2 Hz, • 10th harmonic- 20 Hz • System with frequency response range upto 20 Hz suffice

  17. To record pressure accurately, a system must respond with equal amplitude for a given input throughout the range of frequencies contained within the pressure wave • If components in a particular frequency range are either suppressed or exaggerated by the transducer system, the recorded signal will be a grossly distorted version of the original physiologic waveform

  18. Transducer • Amplifier • Recorder • A pressure recording system is said to be adequate if the tenth harmonic of the fundamental frequency of a wave can be recorded with uniform sensitivity

  19. Sensitivity- ratio of the amplitude of the recorded signal to the amplitude of the input signal • Natural frequency- • Frequency at which the system oscillates when shock excited or • The frequency of an input pressure wave at which the ratio of output/input amplitude of a system is maximal • Directly proportional to lumen radius • Inversely proportional to cath length, √cath compliance, √liquid density • Highest natural frequency- short, wide-bore, stiff cath, low-density fluid without air bubbles- overdamped

  20. Damping- dissipation of the energy of oscillation of a pressure measurement system owing to friction • Optimal damping

  21. THE ELECTRICAL STRAIN GUAGE-WHEAT STONE BRIDGE

  22. Pressure Measurement Systems • Fluid-filled Systems • Micromanometer Catheters

  23. Fluid-filled Systems • fluid-filled catheter attached to a pressure transducer • pressure wave is transmitted by the fluid column within the catheter • Pressure measurement system should have the highest possible natural frequency and optimal damping • Data should be collected ,with the patient in steady state, in close proximity to one another and before introduction of radiographic contrast. • Accurate ‘zero’ reference is essential • Transducers must be caliberated frequently (before each rec)

  24. The pressure transducer -calibrated against a known pressure, and the establishment of a zero reference undertaken at the start of the catheterization procedure • To “zero” the transducer, the transducer is placed at the level of the atria, which is approximately midchest • If the transducer is attached to the manifold and is therefore at variable positions during the procedure, a second fluid-filled catheter system should be attached to the transducer and positioned at the level of the midchest

  25. Phlebostatic Axis Intersection of the 4th ICS and ½ the anterior-posterior diameter of the chest

  26. ERROR & ARTIFACT DETERIORATION OF FREQUENCY RESPONSE • During the course of study deterioration fr response and damping • Air bubbles-> compliance- excessive damping and lower natural freq • High freq components of the wave sets the system in oscillation-pressure overshoot( early syst & diastole of ventr pressure curve) • flushing

  27. Artifacts • Movement artifact (WHIP Artifact) Motion of tip of the catheter within the measured chamber→ Enhance the fluid oscillations of the transducer system May produce superimposed waves of ±10 mm Hg Particularly common in PA • Render systolic and to a lesser extent diastolic pressures unreliable • No way to fix it internally • Stabilize externally • If whip noted - should indicate that to the physician and consider using mean pressures which are usually not affected

  28. End hole artifact An end-hole catheter measures an artificially elevated pressure because of streaming or high velocity of the pressure wave Flowing bld- K.E- sudden halt- converted to pressure This added pressure may range from 2-10 mm Hg • Catheter impact artifact When the catheter is struck by the walls or valves of the cardiac chambers Common with the pigtail cath in the LV, where the MV hits the cath as they open in early diastole

  29. Systolic P amplification in the periphery • Peak SBP in radial,brachial,femoral > peak SBP in central Ao---- -20-50 mmHg • Mean arterial P remains same • Largely as a consequence of reflected wave from Aobifurc, art branch, small perivessls • Reinforce the peak and trough of the anterograde P wave

  30. Micromanometer –Tipped Catheters • Fluid filled system-distortion of wave forms- artifacts, amplification of syst pressure in periphery, damping or augmentation of frequency response system. • For precise undistorted high fidelity pressure recordings • Micromamometer chips at the end of catheters • Interposing fluid column is eliminated • Have higher natural frequencies and more optimal damping characteristics • To assess pressure waveform contours in a tachy situation, rate of ventricular pressure rise(dp/dt) etc • Limitation- additional cost, fragility , time needed for properly calibrating and using the system

  31. Techniques for determination of cardiac output • Fick Oxygen technique • Indicator dilution technique • Thermodilution technique • Angiographic technique

  32. Adolph Fick 1870 • The total uptake or release of any substance by an organ is the product of blood flow to the organ and arteriovenous oxygen difference of the substance • If no intracardiac shunt PBF=SBF • Pulmonary blood flow = oxygen consumption/ arteriovenous oxygen difference across the lungs • CO = O2 consumption (ml/min) ------------------------------------------------------------- arterial O2 content - mixed venous (PA) O2 content • O2 consumption varies according to individual, by age and sex

  33. Fick Method- (CONSERVATION OF MASS) cardiac output rate of O2 consumption 250 ml/min Flow = = = 5 litres/min [O2] leaving – [O2] entering 190 – 140 ml/litre rate of O2 consumption O2 concentration of blood entering lung O2 concentration of blood leaving lung lung

  34. Assumptions made • Uptake of oxygen by the lungs from the blood is measured instead of measuring the rate at which oxygen is taken up from the lungs by the blood • Left ventricular or systemic arterial blood is sampled instead of pulmonary venous blood • Due to bronchial venous and thebesian venous drainage oxygen content of systemic arterial blood is 2 to 5ml/l lower than pulmonary venous blood

  35. Douglas bag method Polarographic method Metabolic rate meter by Waters instruments Parts: oxygen hood /mask Polarographic oxygen sensor cell V o2=O2 content in the room air – O2 content in the air flowing past the polarographic cell Respiratory quotient is assumed Paramagnetic method Paramagnetic sensor for measuring O2 Adjusts for temperature and partial pressure of water vapour Calculates respiratory Q for each patient Deltatrac II made by sensormedics • Older • A timed sample of patients expired air is collected in a Douglas bag & analyzed for O2 content and ( Beckman oxygen analyzer) and volume • O2 content of room air is also measured • Oxygen consumption per l per minute is calculated

  36. Assumed • O2 consumption index – 125 ml / m2 • BSA Dubois, nomograms • 0.007184 × wgt.425 (kg) × height .725 (cm) • Mosteller = √(HT cm x Wt Kg )/ 60 Kendrick AH.DirectFick CO: are assummed values of O2 consumption acceptable?Eur Heart J 1988;9:37

  37. Arteriovenous difference and extraction reserve • The extraction of a given nutrient from the circulation is expressed as arteriovenous difference. • The factor by which arteriovenous diff can increase at constant flow -- extraction reserve • normal extraction reserve for oxygen - 3. • AV difference of O2 -30-40 ml/L

  38. SOURCES OF ERROR • Assumes prevalance of steady state • Reflectance oximetry accurate only for a range 45%-98% • Improper collection of the mixed venous sample • O2 con- error-6%. A-V O2 diff -error -5% • The total error Fick CO– about 10 % • Resting O2 consumption is 125 mL/m2, or 110 mL/m2 for older patients • 6% error by assumption in trials • Cardiac output age and BSA dependent • Age 7 to 70 yrs -4.5 to 2.5 litre /min /m2

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