5. Cerebral Oximetry Flashcards
NIRS
A reliable non-invasive monitor of cerebral
oxygenation would render these less important, on the general assumption that if the
brain is being perfused with oxygenated blood then other systems will also function.
There are such non-invasive devices available, and it remains somewhat surprising that
they have not found their way into mainstream anaesthetic practice.
Intro
Near infrared spectroscopy (NIRS) is based on physical principles similar to those
which underlie pulse oximetry, and it can be used in the same way to measure the
oxygen saturation of peripheral tissues. However, its particular value lies in its ability to
provide a non-invasive measure not only of cerebral oxygenation but also of other
deeper structures that are usually inaccessible, such as the spinal cord and the splanchnic
circulation.
Niroscope
A ‘niroscope’ is essentially an optical spectrophotometer
that uses wavelengths in the near infrared range,
from 700 to 1,100 nanometres (nm).
This utilizes the fact that tissues, including those of the bony cranium,
allow penetration of light at these wavelengths to depths as great as 8 centimetres.
Visible light, in contrast, with wavelengths that range from 450 to 700 nm,
s able to penetrate little more than 1 cm of tissue due to much
greater attenuation by absorption, reflectance and scattering.
How does the sensor work
The instruments use laser diodes to generate infrared (IR) light and employ
reflectance-mode spectroscopy.
The light is emitted and received by optodes
(the term used in this context for optical sensors).
The transmitter and receiver are on the same side but are separated by around 5 cm.
This is because the pathway of IR light from the emitter, through the tissue,
and back to the detector is elliptical; and its
maximal penetration is proportional to the separation between the two.
This is variously quoted as being one-third to one-half of that distance.
The emitted IR light is absorbed by chromophores
The emitted IR light is absorbed by chromophores,
of which oxygenated haemoglobin (HbO2) and
deoxygenated haemoglobin (Hb) are two such species
with absorption spectra of 800–850 nm and 650–800 nm, respectively.
Others include cytochrome P450 enzymes,
myoglobin (mainly in the frontalis muscle) and bilirubin (of significance
only in jaundiced patients).
Each different chromophore has a specific absorption,
or extinction, coefficient which is expressed as a function of wavelength.
The selected wavelengths generally are between 700 and 850 nm,
which is the range within which the absorption spectra of HbO2 and Hb
show maximal separation.
Law
Cerebral oxygen saturation is determined by application of the Beer-Lambert law,
which put at its simplest states that the
concentration of a substance can be measured by the amount of light that it absorbs.
Normal values
The available devices can determine changes in oxygenated and deoxygenated
haemoglobin but are unable to distinguish between arterial and venous concentrations.
The distribution of haemoglobin in cortical tissue is approximately
70% in the venous system and 30% in arterial blood
‘normal’ rSO2 is not 96% or
greater as would be expected from a peripheral oximeter probe, but is between 55%
and 80%, and so trend monitoring is of more value than absolute values
below 50%, or a fall of 20% from baseline suggests cortical hypoxaemia
Challenges
One potential difficulty is that in non-metabolizing or dead brain, HbO2 may be sequestered
in veins and capillaries which will lead to misleadingly normal rSO2 values.
This is more likely to be a problem when cortical oxygenation is being assessed after
brain injury than during surgery when a falling rSO2 will indicate hypoperfusion or
hypoxia before cell death has occurred.
Cerebral oxygenation
Much of the work on cerebral oxygenation has related to cardiac surgery, but a
systematic review that was published in 2014 looked at changes in regional (cerebral)
oxygen saturation (rSO2) in non-cardiac surgery and found significant falls
in onelung ventilation during thoracic surgery,
in major abdominal surgery, in carotid endarterectomy
with arterial cross-clamping, in hip surgery and in laparoscopic surgery
performed in the reverse Trendelenberg (head-up) position.
The review also noted ‘pronounced cerebral desaturation during shoulder arthroscopy in the beach chair
position’
(H.B. Nielsen. Systematic review of near infrared determined cerebral oxygenation during non-cardiac surgery. March 2014,
Importance
The clinical importance of maintaining cerebral oxygenation is self-evident,
and the studies so far done would seem to illustrate two important realities.
The first is that silent cerebral cortical desaturation can occur
without any obvious perturbations of routine systemic
vital signs such as heart rate, arterial blood pressure and peripheral
oxygen saturation.
The second is that such episodes of desaturation do appear to
correlate with the development of postoperative cognitive dysfunction
How does this affect practice?
Given that routine monitoring may be falsely reassuring, it is difficult to know exactly
how specifically to manage anaesthesia during those higher-risk procedures. But if
the beach chair position for shoulder surgery is taken as an example, there should be
a high index of suspicion for any factors that might potentially lead to cerebral
hypoperfusion. Hence, in addition to maintaining cardiac output, volaemic status,
normocapnia and an adequate haemoglobin concentration to ensure oxygen delivery,
the anaesthetist should perhaps tolerate only minimal divergence from the
patient’s baseline levels. (These can readily be determined in the anaesthetic room
in the sitting position prior to the induction of anaesthesia.)
