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When the probe is connected the system will turn on the laser, therefore
passing red light down the length of the probe's fi ber optics to the probe
head. Th e probe head has an optical lens that allows light to be transmitted
to the skin surface as well as allowing the transmission of backscattered light
via a separate optic fi ber. Th e returned or backscattered light travels up to the
probe cable to a photodiode assembly attached to the input connector. Th e
photodiode generates an electrical signal proportional to the light it receives.
Th e returned signal will be a combination of the original light and
backscattered light which results in a signal with a varying frequency
content. Th is varying signal content is the signal of interest as it represents
the Doppler shift as a result of light being backscattered off moving blood
cells.
Th is photodiode signal is digitized and a digital signal processor is used to
perform a spectral analysis of the signal to determine the mean velocity of
the blood cells and thus generate an LDF signal. Th e result is converted back
into an analog signal that is proportional to the blood perfusion in the area
of measurement.
Probe Operation
Th ere are several types of optic probes that can be used with the system,
each one having its own optical characteristics. In order to make sure that
diff erent probes will produce consistent results, each probe contains a means
of identifying itself to the system. In this way you can change probes without
having to recalibrate your system.
What the Blood FlowMeter Measures
Th e Blood FlowMeter is a laser Doppler fl owmeter whose primary purpose
is to measure real-time microvascular red blood cell (RBC) perfusion
(otherwise known as RBC fl ow or RBC fl ux). Laser Doppler signals are
recorded in BPU which is a relative units scale defi ned using a carefully
controlled motility standard.
The Blood Perfusion Unit (BPU)
Th e Blood FlowMeter has been factory calibrated with a constant, known
motility standard so that, for a given perfusion situation, all probes will
read the same value of BPU. However, it should be noted that BPU is not an
intrinsic physiological defi nition of blood perfusion.
Th e notion of a universal physiological standard, valid for all types of tissue
is scientifi cally inconceivable. Such activity rests on the assumption that
microcirculatory blood perfusion is essentially homogeneous for all tissue
structures over the human body. Th is assumption is seriously fl awed due to
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Blood FlowMeter Owner's Guide
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