01-02-2013, 01:02 PM
MEASURMENT OF FLOW & BLOOD VOLUME
MEASURMENT OF FLOW.pptx (Size: 821.55 KB / Downloads: 42)
INTRODUCTION
A measurement of paramount importance: concentration of O2 and other nutrients in cells Very difficult to measure
Second-class measurement: blood flow and 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? Standard flow meters, such as turbine flow meters, obviously cannot be used!
Indicator-dilution method: cont./rapid injection, dye dilution, thermo dilution
Electromagnetic flow meters
Ultrasonic flow meters / Doppler flow meters
Plethysmography: Chamber / electric impedance / photoplethysmography
INDICATOR DILUTION WITH RAPID INJECTION
A known amount of a substance, such as a dye or radioactive isotope, is injected into the venous blood and 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.
ELECTROMAGNETIC FLOWMETER
Based on Faraday’s law of induction that a conductor that moves through a uniform 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 and passes through the magnetic field B, the induced emf e measured at the electrodes is.
ELECTROMAGNETIC FLOWMETER PROBES
Comes in 1 mm increments for 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!!!
ULTRASONIC FLOWMETER
Based on the principle of measuring the time it takes for an acoustic wave launched from a transducer to bounce off red blood cells and reflect back to the receiver.
All UT transducers, whether used for flowmeter or other applications, invariably consists of a piezoelectric material, which generates an acoustic (mechanical) wave when excited by an electrical force (the converse is also true)
UT transducers are typically used with a gel that fills the air gaps between the transducer and 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 and far fields of the UT transducer, in which the acoustic wave exhibit different properties
The near field extends about dnf=D2/4λ, where D is the transducer diameter and λ is the wavelength. During this region, the beam is mostly cylindrical (with little spreading), however with nonuniform intensity.
In the far field, the beam diverges with an angle sinθ=1.2 λ/D, but the intensity uniformly attenuates proportional to the square of the distance from the transducer