2.11 Cerebral Oximetry

Question 1

Key points

Answer 1

Cerebral oximeters enable continuous non-invasive monitoring of cerebral oxygenation.

Cerebral oximeters utilize similar physical principles to pulse oximeters.

Cerebral oximeters use the Beer–Lambert law and spatial resolution to provide estimates of cerebral haemoglobin oxygen saturation.

Baseline cerebral oximetry values should be obtained before induction of anaesthesia.

Cerebral oximetry values represent a balance between cerebral oxygen delivery and consumption.

Question 2

Physics

Answer 2

Uses Near-infrared spectroscopy (NIRS)

Monitor that is connected to oximeter probes.

Adhesive pads attach probes
to the patient’s scalp.

Probes are most commonly applied
to the scalp overlying the frontal lobe.
-contain fibreoptic light source and light detectors

light-emitting diodes
Emitted light in the infrared range is
able to penetrate the skull to reach
underlying cerebral tissue.

Infrared light contacts haemoglobin,
a change in the light spectrum occurs,

depending upon the oxygenation status of the haemoglobin molecule.

Reflected light returns towards the surface
and is detected by the light detectors
within the oximetry probes

Cerebral oximeters calculate
cerebral oxygenation using the Beer–Lambert Law

According to these laws,
an amount of a substance, that is, oxygen,

can be determined by how
much light the substance absorbs.

Question 3

Beer’s law

Answer 3

Beer’s law

The intensity of transmitted light
decreases exponentially as the
concentration of a substance
the light passes through increases.

As the concentration of a substance increases,
the amount of light absorbed by the
substance increases
and the amount of light
detected by the photodetector decreases

Two containers of equal size are filled with identical volumes of a solution
The concentration of solution in Figure 1 is less
than the concentration of solution in Figure 1.

Light from identical light sources are
shone through the containers.

The amount of light passing through
each container is detected by a photodetector.

The amount of light reaching the photodetector in
Figure 1 is greater than the amount of light reaching the detector in Figure 1

Question 4

Lambert’s law

Answer 4

Lambert’s law

The intensity of transmitted light decreases exponentially as the distance travelled by the
light through a substance increases

As the distance a light travels through
a substance increases,

the amount of light absorbed increases,
and the amount of light detected
by the photodetector decreases

Two containers of differing size are each
filled with volumes
of solution of identical concentration.

Light from identical light sources are shone through each container.

The amount of light passing through
each container is detected by a photodetector.

Light passing through the container in
Figure 2 has less distance
to travel through the substance,
than light passing through the
container in Figure 2.

The amount of light reaching the photodetector
in Figure 2 is greater than that in Figure 2.

Question 5

Wavelengths

Isobestic point

Answer 5

Near-infrared light with a wavelength
of 650–940nm is able to penetrate
the skull to underlying cerebral tissue

Haemoglobin exists in either an
oxygenated or deoxygenated form.

The absorption spectra for each
haemoglobin state are different

The absorption spectrum for deoxygenated haemoglobin is 650–1000 nm

Oxygenated haemoglobin 700–1150 nm.

The isobestic point where the absorption spectra
for oxygenated and deoxygenated haemoglobin
are the same can be used to
calculate total tissue haemoglobin concentration

Question 6

Source of error

How is it mitigated

Answer 6

Extracranial blood is a potential
source of error in cerebral
oximetry measurements.

In order to limit this,
cerebral oximeters
utilize multiple probes

and a process of spatial resolution

Spatial resolution is based on a principle
that the depth of tissue investigated
is directly proportional to the distance between the
light emitter and light detector

Increasing the distance between the emitter and detector will increase the depth of tissue sampled.

Cerebral oximeters use mathematical algorithms

involving subtraction of values obtained from the emitters near and far from the photodetector to limit
contamination from extracranial blood,

and obtain a reading representative of cerebral oxygenation values.

Variability occurs as a result of
different wavelengths of light
emitted by the probes,

different light sources,

different mathematical algorithms
used to obtain cerebral oxygenation values.

Question 7

Are they different form Pulse Oximeters

Answer 7

Cerebral oximetry values are derived

mainly from venous blood,

and in contrast to pulse oximeters
are independent of pulsatile blood flow.

Cerebral oximetry values reflect a balance
between oxygen consumption
and oxygen delivery to the brain.

Question 8

Clinical interpretation of cerebral oximetry measurements

Answer 8

Baseline cerebral oximetry values
should be obtained before induction
of anaesthesia.

Normal values range from 60% to 80%;
however, lower values of 55–60% are
not considered abnormal in
some cardiac patients.

Anatomical variations, for example,
an incomplete Circle of Willis,
or severe carotid artery stenosis
can create errors in cerebral oximetry values;

therefore, it is recommended that cerebral
oximetry is performed bilaterally.

Cerebral oximetry values must not be
interpreted in isolation;

alterations in cerebral oximetry measurements
must take into consideration
all available clinical information
and physiological state of the patient

One of the most common limitations
in cerebral oximetry monitoring has
been the absence of an intervention
protocol to treat a decrease in regional brain oxygenation

Question 9

Factors resulting in reduced cerebral oxygenation values

Answer 9

1. Cerebral blood flow:

Cardiac output

Acid–base status

Major haemorrhage

Arterial inflow/venous outflow obstruction

2. Oxygen content:

Haemoglobin concentration

Haemoglobin saturation

Pulmonary function

Inspired oxygen concentration

Question 10

Treatment algorithm based on optimizing
cerebral oxygen delivery and consumption
to treat a reduction in cerebral oximetry values

Answer 10

Baseline Vale - Desat >20%

1. Check head position
   Ensure Neutral
2. Check ETT Ties
   Venous / art obstruction
3. Optimise O2 Delivery
4. Optimise O2 Consumption

Question 11

Optimise O2 Delivery

Answer 11

1. Ensure adequate cardiac output
   Hr + SV
2. Optimise MAP
   ?Vasopressors
3. O2 Sats
   Increase Fio2
   ?Rx Hypoxia
4. PacO2
   Ventilation
5. Rule out Anaemia
   ?Transfusion

Question 12

Optimise O2 Consumption

Answer 12

1. Ensure Adequate depth
2. Temp
   Avoid High
3. Rule out seizures
   AED

Question 13

Limitations in cerebral oximetry measurements

Answer 13

All monitoring devices have limitations.

Limitations associated
with cerebral oximetry include:

(i) Blood from an extracranial source can create erroneously low measurement

(ii) Electrosurgical equipment,
that is, diathermy, can affect the
accuracy of measurement

(iii) Cerebral oximeters only measure
regional cerebral oxygenation.
Large areas of the brain remain unmonitored

(iv) Cerebral oximeters are unable to identify a cause for the desaturation

Question 14

Clinical applications

Answer 14

Questions have been raised with regard to
the clinical utility of
cerebral oximetry monitoring.

An increasing number of studies are
demonstrating the ability of cerebral oximetry monitoring to detect clinically silent episodes of cerebral ischaemia.

Cerebral oximeters have the potential to be an important safeguard for cerebral function

Cardiac surgery

Vascular surgery

Paediatrics

Additional uses

Question 15

Cardiac surgery
CABG

Answer 15

Patients undergoing cardiac surgery
are at risk of adverse perioperative
neurological events.

Cerebral oximetry monitoring can be used,
potentially reducing the incidence
of these devastating events.

CABG:
Salter and colleagues carried out a study
+ found an association between

cerebral desaturation and
early postoperative cognitive dysfunction.

However, the study did not identify
an association between

the use of a cerebral oximetry-guided
intervention protocol,
and a reduction in the
incidence of postoperative cognitive dysfunction.

Persistent postoperative cognitive dysfunction after cardiac surgery is controversial.

Meta-analyses have identified that
persistent cognitive decline is not as common as previously thought.

Some patients may even show
an improvement in cognitive
function after CABG surgery

Question 16

Cardiac Surgery
DHCA

Answer 16

Deep hypothermic circulatory arrest

A number of cardiac surgical procedures are
performed using cardiopulmonary bypass (CPB).

Certain complex procedures, however,
require a cessation of all blood flow.

Deep hypothermic circulatory arrest
describes the rapid reduction
in core body temperature,
followed by the cessation of CPB.

The brain is vulnerable to ischaemia during this time.

Cerebral oximetry monitoring may
provide a means of monitoring and detecting
the onset of cerebral ischaemia

However, there is insufficient
evidence surrounding the sensitivity of cerebral oximetry monitoring during profound hypothermia (temperatures <25°C).

Question 17

Vascular surgery

Answer 17

Vascular surgery
Carotid endarterectomy

Carotid endarterectomy is associated with postoperative stroke

Monitoring - Ischaemi
Common monitoring devices include
transcranial
Dopplers,
EEGs, and
monitoring of somatosensory evoked potentials (SSEPs).

SSEPs and EEG monitoring are affected
by anaesthetic agents and surgical diathermy.

Cerebral oximetry monitoring can be used
as a tool for
detection of cerebral ischaemia

A reduction in cerebral oximetry values >12%
from a baseline preoperative value has
been identified as a reliable, sensitive, and
specific threshold for detection of brain ischaemia

Reduction in cerebral oximetry values
after cross-clamping of the internal
carotid artery may indicate the
need for shunt placement during
the procedure.

Moritz and colleagues compared different
monitoring modalities in identifying
cerebral ischaemia during carotid surgery.

Results highlighted similar accuracy for the
detection of onset of ischaemia with
Transcranial Doppler and
cerebral oximetry monitoring,

least accuracy was identified for SSEP monitoring.

Question 18

Transcranial Doppler

Answer 18

Transcranial Dopplers provide an
indirect measure of cerebral
blood flow by measuring

blood velocity in a cerebral artery.

Measurements are obtained through
transcranial windows.

Transcranial windows are found across the thinnest parts of the skull—the temporal bone,
or where bone is absent—the orbit.

One-fifth of patients lack a transcranial window,
and as a result,
transcranial Doppler studies cannot be used

Question 19

Carotid endarterectomy hyperperfusion syndrome

Answer 19

Carotid endarterectomy hyperperfusion syndrome

Carotid endarterectomy hyperperfusion syndrome
is caused by an increase in cerebral blood flow
after repair of carotid stenosis.

It occurs as a result of
impaired cerebral auto-regulation.

The syndrome is characterized by headache,
cerebral oedema, seizures,
intracerebral haemorrhage, and death.

A correlation exists between cerebral oxygen saturation values and changes in cerebral blood flow after de-clamping of the internal carotid artery.

Cerebral oximetry could be used to
identify patients at risk of cerebral hyperperfusion syndrome

Question 20

Paediatrics

Answer 20

Paediatrics

Neonates born prematurely have impaired
cerebral auto-regulation
and are at risk of
intraventricular haemorrhage and periventricular
leukomalacia

Periventricular leukomalacia is usually
diagnosed by transcranial ultrasound.

Areas of ischaemia are identified in white matter surrounding the lateral ventricles.

By the time a diagnosis of
periventricular leukomalacia has been made,

permanent neurological damage
such as visual disturbance
and cerebral palsy has occurred.

Changes in cerebral oxygen
values as detected by cerebral oximeters provide an indirect measure of alterations in cerebral blood flow.

Continuous cerebral oxygenation monitoring may enable the early detection and prevention of periventricular leukomalacia and intraventricular
haemorrhage

Question 21

Additional uses

Answer 21

Additional uses

Cerebral oximetry monitoring is being increasingly used to monitor the adequacy of tissue and organ perfusion when placed on sites other than the scalp

NIRS is being investigated as a potential marker of perfusion for
hepatic, renal, and splanchnic tissues

NIRS is further being evaluated as a potential screening tool for the need for blood transfusion in trauma patients at risk of haemorrhagic shock

Question 22

Conclusion

Answer 22

Cerebral oximetry is a simple,
non-invasive monitoring methodology
that may improve patient outcome in a variety of different clinical situations;

evidence for its use beyond cardiac surgery is
continuously emerging.

This article has highlighted some of the
increasing roles and evidence for cerebral oximetry in clinical practice, further research is required to validate cerebral oximetry monitoring in improving patient outcomes in both cardiac and non-cardiac surgical patients