Principles of Biomedical Systems & Devices 0909.504.04 / 0909.402.02 WEEK 10: Fl
Lamarre, Tom, Contributing Editor has reference to this Academic Journal, PHwiki organized this Journal Principles of Biomedical Systems & Devices 0909.504.04 / 0909.402.02 WEEK 10: Flow & Volume of Blood Measurement of Flow & Volume of Blood A measurement of paramount importance: concentration of O2 in addition to other nutrients in cells Very difficult to measure Second-class measurement: blood flow in addition to changes in blood volume correlate well with concentration Third-class measurement: blood pressure correlates well with blood flow Fourth class measurement: ECG correlates adequately with blood pressure How to make blood flow / volume measurements St in addition to ard flow meters, such as turbine flow meters, obviously cannot be used! Indicator-dilution method: cont./rapid injection, dye dilution, thermodilution Electromagnetic flowmeters Ultrasonic flowmeters / Doppler flowmeters Plethysmography: Chamber / electric impedance / photoplethysmography Indicator Dilution with Continuous Injection Measures flow / cardiac output averaged over several heart beats Ficks technique: the amount of a substance (O2) taken up by an organ / whole body per unit time is equal to the arterial level of O2 minus the venous level of O2 times the blood flow Blood flow, liters/min (cardiac output) Consumption of O2 (mL/min) Arterial in addition to venous concentration of O2 (mL/L of blood) Change in  due to continuously added indicator m to volume V
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Ficks technique How is dm/dt (O2 consumption) measured Where in addition to how would we measure Ca in addition to Cv (Exercise) Indicator Dilution with Rapid Injection A known amount of a substance, such as a dye or radioactive isotope, is injected into the venous blood in addition to the arterial concentration of the indicator is measured through a serious of measurements until the indicator has completely passed through given volume. The cardiac output (blood flow) is amount of indicator injected, divided by average concentration in arterial blood. Indicator Dilution Curve After the bolus is injected at time A, there is a transportation delay be as long as e the concentration begins rising at time B. After the peak is passed, the curve enters an exponential decay region between C in addition to D, which would continue decaying alone the dotted curve to t1 if there were no recirculation. However, recirculation causes a second peak at E be as long as e the indicator becomes thoroughly mixed in the blood at F. The dashed curve indicates the rapid recirculation that occurs when there is a hole between the left in addition to right sides of the heart.
An Example Dye Dilution In dye-dilution, a commonly used dye is indocyanine green (cardiogreen), which satisfies the following Inert Safe Measurable though spectrometry Economical Absorption peak is 805 nm, a wavelength at which absorption of blood is independent of oxygenation 50%of the dye is excreted by the kidneys in 10 minutes, so repeat measurements is possible Thermodilution The indicator is cold saline, injected into the right atrium using a catheter Temperature change in the blood is measured in the pulmonary artery using a thermistor The temperature change is inversely proportional to the amount of blood flowing through the pulmonary artery
Measuring Cardiac Output Several methods of measuring cardiac output In the Fick method, the indicator is O2; consumption is measured by a spirometer. The arterial-venous concentration difference is measure by drawing simples through catheters placed in an artery in addition to in the pulmonary artery. In the dye-dilution method, dye is injected into the pulmonary artery in addition to samples are taken from an artery. In the thermodilution method, cold saline is injected into the right atrium in addition to temperature is measured in the pulmonary artery. Electromagnetic Flowmeters Based on Faradays law of induction that a conductor that moves through a uni as long as m magnetic field, or a stationary conductor placed in a varying magnetic field generates emf on the conductor: When blood flows in the vessel with velocity u in addition to passes through the magnetic field B, the induced emf e measured at the electrodes is. For uni as long as m B in addition to uni as long as m velocity profile u, the induced emf is e=BLu. Flow can be obtained by multiplying the blood velocity u with the vessel cross section A. Electromagnetic Flowmeter Probes Comes in 1 mm increments as long as 1 ~ 24 mm diameter blood vessels Individual probes cost $500 each Made to fit snuggly to the vessel during diastole Only used with arteries, not veins, as collapsed veins during diastole lose contact with the electrodes Needless to say, this is an INVASIVE measurement!!! A major advantage is that it can measure instantaneous blood flow, not just average flow
Ultrasonic Flowmeters Based on the principle of measuring the time it takes as long as an acoustic wave launched from a transducer to bounce off red blood cells in addition to reflect back to the receiver. All UT transducers, whether used as long as flowmeter or other applications, invariably consists of a piezoelectric material, which generates an acoustic (mechanical) wave when excited by an electrical as long as ce (the converse is also true) UT transducers are typically used with a gel that fills the air gaps between the transducer in addition to the object examined Near / Far Fields Due to finite diameters, UT transducers produce diffraction patterns, just like an aperture does in optics. This creates near in addition to far fields of the UT transducer, in which the acoustic wave exhibit different properties The near field extends about dnf=D2/4, where D is the transducer diameter in addition to is the wavelength. During this region, the beam is mostly cylindrical (with little spreading), however with nonuni as long as m intensity. In the far field, the beam diverges with an angle sin=1.2 /D, but the intensity uni as long as mly attenuates proportional to the square of the distance from the transducer Higher frequencies in addition to larger transducers should be used as long as near field operation. Typical operating frequency is 2 ~ 10 MHz. UT Flowmeters Zero-crossing detector / LPF Determine direction High acoustic impedance material
Transit time flowmeters Effective velocity of sound in blood: velocity of sound (c) + velocity of flow of blood averaged along the path of the ultrasound (û) û=1.33 as long as laminar flow, û=1.07 as long as turbulent flow : velocity of blood averaged over the cross sectional area, this is different than û because the UT path is along a single line not over an averaged of cross sectional area Transit time in up/down stream direction: Difference between upstream in addition to downstream directions Transit Time Flowmeters The quantity T is typically very small in addition to very difficult to measure, particularly in the presence of noise. There as long as e phase detection techniques are usually employed rather then measuring actual timing. Doppler Flowmeters The Doppler effect describes the change in the frequency of a received signal , with respect to that of the transmitted signal, when it is bounced off of a moving object. Doppler frequency shift Speed of sound in blood (~1500 m/s) Angle between UT beam in addition to flow of blood Speed of blood flow (~150 cm/s) Source signal frequency
Doppler Flowmeters Problems Associated with Doppler Flowmeters There are two major issues with Doppler flowmeters Unlike what the equations may suggest, obtaining direction in as long as mation is not easy due to very small changes in frequency shift that when not in baseb in addition to , removing the carrier signal without affecting the shift frequency becomes very difficult Also unlike what the equation may suggest, the Doppler shift is not a single frequency, but rather a b in addition to of frequencies because Not all cells are moving at the same velocity (velocity profile is not uni as long as m) A cell remains within the UT beam as long as a very short period of time; the obtained signal needs to be gated, creating side lobes in the frequency shift Acoustic energy traveling within the beam, but at an angle from the bam axis create an effective , causing variations in Doppler shift Tumbling in addition to collision of cells cause various Doppler shifts Directional Doppler Directional Doppler borrows the quadrature phase detector technique from radar in determining the speed in addition to direction of an aircraft. Two carrier signals at 90º phase shift are used instead of a single carrier. The +/- phase difference between these carriers after the signal is bounced off of the blood cells indicate the direction, whereas the change in frequency indicate the flowrate
Directional Doppler (a) Quadrature-phase detector. Sine in addition to cosine signals at the carrier frequency are summed with the RF output be as long as e detection. The output C from the cosine channel then leads (or lags) the output S from the sine channel if the flow is away from (or toward) the transducer. (b) Logic circuits route one-shot pulses through the top (or bottom) AND gate when the flow is away from (or toward) the transducer. The differential amplifier provides bi-directional output pulses that are then filtered.
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