Biomedical Engineering Reference
In-Depth Information
Because the emboli are moving past an angled detector, they appear to
grow gradually shallower (or deeper) as they pass towards (or away from)
the closest portion of the detector (depending on the blood fl ow
direction). 37
Other embolic detection devices, such as transcranial doppler, monitor
only the end organ and are therefore unable to identify the source of
emboli. In contrast, novel air detection technology has a large range of
count and fl ow rates with predetermined alarm thresholds, thus displaying
consistent results and preserving the integrity of the embolic documenta-
tion with reliable count measurements from a fl ow rate of 0.2 to 6 L/min.
Novel air detection devices are very new in the market and there is a long
way to go.
6.6.4 Non-invasive brain oximetry
Cerebral oximeter is a continuous wave spatially resolved spectrometer
that measures change in regional oxygen saturation (rS O 2 ). The rS O 2 value
is thought to refl ect the balance between cerebral oxygen supply and
demand in a mixed vascular bed dominated by gas exchanging vessels,
especially venules. As with the measurement of systemic arterial oxygen
saturation by pulse oximetry, cerebral oximetry takes advantage of the fact
that the hemoglobin absorption spectrum changes with the degree of oxy-
genation. With two infrared wavelengths, the oxyhemoglobin fraction may
be determined. Measurement within cerebral blood vessels is possible
because the skull is translucent to infrared light. Selective measurement of
brain tissue is achieved through the principle of spatial resolution. The
intracranial depth of photon penetration from a skin-mounted infrared
source is infl uenced by the distance of its separation from an adjacent detec-
tor. Through the use of a pair of detectors with differing separation from
the infrared source, surface refl ections may be subtracted from those involv-
ing underlying brain vasculature. 38
Invariably, new technologies invite criticism and misunderstanding; cere-
bral oximetry is no exception. Criticisms have focused on the absence of
absolute measures of oxy- and deoxyhemoglobin, concern that intravascu-
lar oxygenation may be an inadequate surrogate for intracellular oxygen-
ation, incorporation of a fi xed arterial/venous ratio in the calibration of the
rS O 2 computational algorithm, large hematocrit shifts that occur during CPB
leading to large variations in photon scattering, infl uences of temperature
and hydrogen ion concentration changes on oxyhemoglobin dissociation
and inconsistencies with alternative estimates of brain oxygenation or
blood fl ow. Currently, the cerebral oximeter is used in half of the US institu-
tions that perform paediatric cardiac surgery and 10% of those that perform
adult cardiovascular procedures. 39
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