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1 Department of Medical Physics
and Bioengineering,
Two near-infrared spectroscopy (NIRS) methods
are available for measuring changes (
) in total cerebral hemoglobin
concentration (CHC): 1) a continuous measurement of the
changes in total hemoglobin concentration
(
[Hb]tot)
and 2) the difference between two absolute measurements of
CHC, each derived from a small, controlled change in inspired
O2 fraction. This paper
investigates the internal consistency of these two methods by using an
experimental and theoretical comparison. NIRS was used to measure
[Hb]tot in five newborn piglets before and after a change in arterial
PCO2.
[Hb]tot
demonstrated a low coefficient of variation of 2.8 ± 2.8 (SD) % which allowed changes in
CO2-cerebral blood volume reactivity to be clearly discriminated. However, a high coefficient of
variation of 22.8 ± 3.5% on the
CHC measurements
obscured any CO2 reactivity
changes. A theoretical analysis demonstrates the effects of optical
pathlength, background absorption, scatter, and blood vessel diameter
on both methods. For more accurate monitoring of CHC, individual
measurements of optical pathlength and more accurate pulse oximetry are required.
cerebral blood volume; near-infrared spectroscopy
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