This method is based on the direct Fick principle and utilizes a sudden increase in the arterial saturation of oxygen as the tracer. The amount which accumulates in cerebral tissue is measured over time (t). Cerebral blood flow is calculated from the ratio of the amount of tracer accumulated to the amount introduced during time t:
where K = 0.89 is a constant which reflects the ratio of cerebral to large-vessel hematocrit (0.69), the molecular weight of hemoglobin (64 500), and cerebral tissue density.
When there is a small sudden increase in arterial saturation, the initial increase in cerebral HbO 2 concentration [HbO2] represents the accumulation of tracer. The quantity introduced is given by the product of the integral of D SaO2 with respect to time and the arterial total hemoglobin concentration [tHb]. It is clear that a number of assumptions must be made during measurement of cerebral blood flow using this method, including constant cerebral blood flow, cerebral blood volume, and oxygen extraction. Early work in adults suggests that cerebral blood flow changes minimally for PaO2 between 6 and 13 kPa. Similarly, oxygen extraction and blood volume have been observed to vary little (the latter is also measurable by near-infrared spectroscopy).
The equipment required to make this measurement includes a near-infrared spectroscopy monitor and a pulse oximeter capable of beat-by-beat measurement of SaO2. Even if an ear probe is used, the lung-brain circulation time is 1 to 2 s faster than the lung-ear circulation time and an adjustment must be made. Early work in pediatric practice (transmission near-infrared spectroscopy) ( Edwards,...eia/ 1988) has been extended to adults (reflectance near-infrared spectroscopy), but further development of these emerging technologies is required before the widespread use of this method can be recommended.
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