TISSUE OXYGENATION
Arterial oxygenation as an indicator of respiratory function may
be controversial, because an acceptable arterial O2
content may not necessarily
be associated with adequate tissue oxygenation because of abnormalities of blood
flow. A more appropriate monitor of O2
delivery would measure tissue
oxygenation. Tissue PO2
electrodes have
been designed and implemented but have associated problems of sampling bias and tissue
destruction. Because of heterogeneity of local tissue blood flow and O2
consumption, there exists a distribution of tissue PO2
values rather than a single value as in blood PO2
.
Nevertheless, changes in tissue oxygenation have been demonstrated under various
clinical conditions.[153]
Another approach is the noninvasive monitoring of O2
saturation of blood within the tissue. This can be accomplished by transilluminating
the tissue with at least two appropriately chosen wavelengths of light.[154]
Light within the near-infrared band (650 to 1100 nm) can penetrate tissue reasonably
well. Incident light is applied to the scalp, where it enters the tissue; a small
proportion is scattered by the tissue and returned to the analyzer through a fiberoptic
bundle. Instruments capable of monitoring
blood O2
saturation in vivo within brain tissue have been commercially
available for some years. Measured saturation within the illuminated volume includes
arterial, capillary, and venous blood but is heavily weighted toward venous values,
[155]
which reflect the adequacy of tissue oxygenation
under normal circumstances.[156]
Reduction of O2
to water occurs at the terminal end
of the cytochrome chain, and monitoring of the cytochrome redox state is therefore
more likely to provide a better estimate of O2
availability at the cellular
level than current clinical monitoring parameters. By using a similar technique
with four wavelengths, it is possible to obtain information about the state of oxygenation
of intracellular chromophores, including myoglobin and cytochrome a,
a3
.[157]
[158]
This technology has been used to monitor intracellular changes in muscle and brain
due to respiratory acidosis in anesthetized cats,[155]
in canine hearts during and after coronary occlusion,[159]
in human forearm ischemia,[160]
and in human brain
during hypoxia[161]
or cardiopulmonary bypass.[162]
