Hemodynamics “Dynamics of Blood” Prof. Dr. Metin TULGAR
Hemodynamics describes the physical principles governing • pressure, • flow, and • resistance of cardiovascular system.
Objectives • Explain the relationship between • pressure, • flow, and • resistance and describe laminar and turbulant flows. • Compare the systematic and pulmonary vascular resistances, • State the relation between blood flow and vessel radius using Poiseuille’s Law.
Describe the term viscosity and explain the relation between shear stress and cardiovascular function. • State the relation between • blood flow, • velocity, and • cross-sectional area. • Describe how Law of Laplace is applied to dilated hearts and aneurysm.
Pressure, Flow and Resistance • Most important factors governing function of circulatory system: • Volume • Pressure • Resistance • Flow.
Blood flow is determined by two factors • The pressure difference between two ends of the blood vessel • The impediment to blood flow through the vessel, which is called resistance. Q = ΔP / R Q: Blood flow, ΔP: Pressure difference, R: Resistance
Laminar and Turbulent Flow • When blood flows at a steady rate through a long, smooth vessel, it flows in streamlines. ‘laminar or streamline type of flow’
For the case of laminar flow, the velocity of flow in the center of the vessel is far greater than that toward the outer edges. (parabolic profile)
The flow may become turbulent; • when the rate of blood flow becomes too great, • when it passes an obstruction in a vessel, • when it makes a sharp turn, or • when it passes over a rough surface.
Reynold’s number: The tendency for turbulent flow increases • in direct proportion to • the velocity of blood flow, • the diameter of the blood vessel, and • inversely proportional to • the velocity of the blood vessel divided by its density
When Re rises above 2000, turbulent flow will occur in a straight smooth vessel.
Measuring Blood Flow • Blood Flow is the amount of blood that passes from a point of circulatory system at a specific amount of time. • Unit : mm/s, or ml/m • At rest blood flow is approx. 5000 ml/m, This value is also known as approx. pumped blood from the heart in a minute.
Electromagnetic Flowmeter • The Neumann Law (or Lenz Law) states that if a conductive wire is moving at right angle through a magnetic field, a voltage E [Volts] will appear at the end of the conductor(Fig.1):
Use a magnet and two electrodes to peek thevoltage that appear across the fluid movingin the magnetic field.
Impedance plethysmography • Impedance plethysmography is a method of determining changing tissue volumes in the body, based on the measurement of electric impedance at the body surface. • Based on the evaluation of the voltage change that occur as a consequence of blood flow variations in a particular tissue section. • Determine the cardiac stroke volume. • Monitor blood volume changes in the thorax
Measuring Impedance Plethysmography: • Made by introducing an electric current in the frequency range of 20-100 kHz into the volume conductor and measuring the corresponding voltage. • The ratio of voltage to current gives impedance, Z. • There is a relationship between stroke volume and net change in the thorax blood volume.
The impedance of the thorax is measured longitudinally by four band electrodes, shown in the figure:
The figure shows: • a typical thorax impedance curve (Z), • its first time derivative (dZ/dt), and \ • the simultaneous electrocardiogram (ECG), and • phonocardiogram (PCG) curves.
Other applications of Impedanceplethysmography: • Peripheral blood flow • Cerebral Blood flow • Intrathoracic Fluid Volume • Determination of Body Composition
Advantages of Impedance plethysmography: • easy to apply, • noninvasive, and • cheap. however, • The change of blood conductivity with • change in velocity has been entirely neglected in this model.
Photoplethysmography • Photoplethysmography satisfies most of the conditions for a non-invasive technique to estimate skin blood flow and is ideally suited to situations which require measurement to be made over long periods. • It provides a signal proportional to changes in skin blood volume but does not produce a quantitative measure.
The raised blood flow greatly increases the light absorption in well defined wavelengths; the exudation of fluids and cells in tissues modifies their diffusive properties (Staderini and Gigante, 1985).
The signal detected by photoplethysmography: Consists of: • a steady component (d.c.), *related to the relative vascularisation of the tissue, *representstotal red cell volume below the sensor plus some reflected components from within the skin. • a pulsatile component (a.c.), *related to changing blood pulse volume. *is produced by the fluctuations in the blood volume below the sensor.
The volume changes recorded sequentially reflect the variations in flow. Thus the a.c. component is a measure of changing flow. • The signal produced by thephotoplethysmography depends upon • the location, and • the properties of the subject's skin at that site, including • the skin structure, • the blood oxygen saturation , • blood flow rate, and • the skin temperatures
Uses of photoplethysmography • In therapy: e.g. photodynamic therapy of cancer, and photodynamic therapy for jaundice in newborns. • In diagnosis: • as a pulse counter in operating rooms by the anaesthetist • in sport centres incorporated in heart rate meters and • detect the onset of vasodilatation in both the forearm and the fingers in the hand. • measurements of vascular changes in the skin capillaries
Ultrasonic Blood Flowmeter There are two different types: • Transit time • Reflection (Doppler)
Transmit time Flowmeters • It is applied for clean or nearly clean fluids, as well as natural gas pipe systems. • work by measuring the time difference between an ultrasonic pulse sent in the flow direction and a ultrasound pulse sent opposite the flow direction. • This time difference is a measure for the speed of the fluid in the path of the ultrasound beam in terms of the speed of sound, c, in the fluid.
Doppler Flowmeters • If you are moving relative to the source of a sound wave, or vice versa, the frequency of sound you hear will change by an amount that depends on how fast the source is moving. • this is called the Doppler effect, and the change in frequency is the Doppler shift.
Blood Pressure • Systolic (high) Pressure,and • Diastolic (Low) Pressure of blood. Blood pressure is different for each one, and for different conditions. It depends on the age, physiological and psychological conditions of a person
Measuring Blood Pressure • Recording arterial pressure using mercury manometer
Operation of Sphygmomanometers • The cuff is placed around the upper arm, • The cuff is inflated until the artery is completely occluded. • Listening with a stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff.
As the pressure in the cuffs falls, a pounding sound is heard (Korotkoff sounds), when bloodflow first starts again in the artery. • The pressure at which this sound began, is the systolic blood pressure. • until the sound can no longer be heard and this is recorded as the diastolic blood pressure.
Electronic Transducers for recording rapidly changing blood pressure • A: a simple metal plate is placed above the membrane.Whenthe membrane bulges, the membrane comes closer tothe plate, which increases the electrical capacitancebetween these two.
B,a small iron slug rests on the membrane,and this can be displaced upward into a centerspace inside an electrical wire coil. Movement of theiron into the coil increases the inductanceof the coil. • C, a very thin, stretched resistancewire is connected to the membrane.When this wireis stretched greatly, its resistance increases; when it isstretched less, its resistance decreases.
Resistance to Blood Flow • The resistance is measured indirectly, from blood flow and pressure difference of two different points. • Unit: pru (peripheral resistance unit) 1 pru = 1 mmHg / 1 ml/s
Total Peripheral Resistance The resistance constructed, as the blood flows through the systemic circulation. • The pressure difference between systemic arteries and systemic veins is about 100mmHg • Blood flow, at rest, is 100ml/sn Therefore, TPR = 100 mmHg / 100 ml/sn = 1 pru
Total Pulmonary Resistance Total resistance in the pulmonary circulation: • Pressure of pulmonary artery: 16 mmHg • Pressure of left atrium: 2 mmHg PR = ΔP / Q = 14 mmHg / 100 ml/sn = 0.14 pru
Conductance of Blood • it is reciprocal of resistance, that is, • known as Blood flow to a corresponding pressure difference. C = Q / ΔP Therefore C = 1 / R
Conductance and Vessel Diameter conductance α (diameter)4 A small change in diameter cause tremendous change in conductance.
Poiseuille’s Law • The velocity of flow in each ring is different. • The blood touching the wall of vessel flows extremely slowly. • The flow is the fastest at the center of vessel
Derivation of Poiseuille’s Law • By integrating the velocities of all rings, and • Multiplying these by the areas of the rings • Flow rate Q is also dependent upon fluid viscosity η, radius r, pipe length L and the pressure difference between the ends P
Blood Viscosity Remember • Viscosity is the resistance to flow caused by the friction of molecules in a fluid. • greater the hematocrit, the more friction there is between successive layers of blood. • The effect of plasma proteins are much less important than the effect of hematocrit.
According to Poiseuille’s equation, Blood flow inversely related to the viscosity of blood: Q α 1/η *Blood is a Newtonian Fluid
Bernoulli Equation • Describes the behavior of a fluid moving along a streamline. • States that the sum of all forms of energy in a fluid flowing along an enclosed path is the same at any two points in that path.
The work done by the forces in the fluid + decrease in potential energy = increase in kinetic energy
The work done by the forces is • The decrease of potential energy is • The increase in kinetic energy is