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 O 2 and other nutrients in cells Very difficult to measure
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0909.504.04 / 0909.402.02
Flow & Volume of Blood
Change in  due to continuously added indicator m to volume V
Consumption of O2 (mL/min)
Blood flow, liters/min(cardiac output)
Arterial and venousconcentration of O2 (mL/L of blood)
After the bolus is injected at time A, there is a transportation delay before the concentration begins rising at time B. After the peak is passed, the curve enters an exponential decay region between C and D, which would continue decaying alone the dotted curve to t1 if there were no recirculation. However, recirculation causes a second peak at E before 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 and right sides of the heart.
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 and in the pulmonary artery. In the dye-dilution method, dye is injected into the pulmonary artery and samples are taken from an artery. In the thermodilution method, cold saline is injected into the right atrium and temperature is measured in the pulmonary artery.
When blood flows in the vessel with velocity u and passes through the magnetic field B, the induced emf emeasured at the electrodes is.
For uniform B and uniform velocity profile u, the induced emf is e=BLu. Flow can be obtained by multiplying the blood velocity uwith the vessel cross section A.
Higher frequencies and larger transducers should be used for nearfield operation. Typical operating
frequency is 2 ~ 10 MHz.
Zero-crossingdetector / LPF
Effective velocity of sound in blood: velocity of sound (c) +velocity of flow of blood averaged along the path of the ultrasound (û)
û=1.33ū for laminar flow, û=1.07ū for turbulent flowū: velocity of blood averaged over the cross sectional area, this is differentthan 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 and downstream directions
The quantity ∆T is typically very small and very difficult to measure, particularly in the presence of noise. Therefore phase detection techniques are usually employed rather then measuring actual timing.
Source signal frequency
Speed of blood flow
Angle between UT beamand flow of blood
Speed of sound in blood
(a) Quadrature-phase detector. Sine and cosine signals at the carrier frequency are summed with the RF output before 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.