Cerebral oximetry Flashcards
What law do cerebral oximeters use?
The Beer-Lambert Law
What is the physics behind cerebral oximeters?
They use NIRS (near infrared spectroscopy)
Pads attached to scalps over frontal lobe. Probes contain a fibreoptic light source and light detectors.
Light is emitted via stimulated emission of radiation/light emiting diodes.
The emitted light in infrared is able to penetrate the skull to reach cerebral tissue. Emitted light is either absorbed, redirected, scattered or reflexted
When light reaches Hb it will change the light spectrum depending on the oxygenation status of the Hb. Reflected light returns to the light detector. The light detector calculates cerebral oxygenation based on Beer-Lambert Law
What is Beer’s Law?
The intensity of transmitted light decreases exponentially as the concentration of a substance increases.
What is Lambert’s law?
The intensity of transmitted light decreases exponentially as the distance travelled by the light through a substance increases
What are the primary light absorbing molecules in the tissues?
Metal complex chromophores:
- Hb
- Bilirubin
- Cytochromes
What is the absorption of dexyHb?
650-1000 nm
What is the absorption spectrum of oxyHb?
700 - 1150 nm
What is a potential source of error in cerebral oximetry measurement?
Extracranial blood.
This is minimized by utilizing multiple probes and a process of spacial resolution.
What is spatial resolution?
The 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.
What type of blood does cerebral oximetry measure?
Venous blood - they are independent of pulsatile blood flow. They reflect a balance between O2 consumption and O2 delivery to the brain.
What are the normal values for baseline cerebral oximetry prior to induction of anaesthesia?
60-80%
(although lower values of 55% are not considered abnormal in some cardiac patients)
What factors will result in reduced cerebral oxygenation?
Cerebral blood flow
- cardiac output
- acid-base status
- major haemorrhage
- arterial inflow/venous outflow obstruction
Oxygen content
- Hb concentration
- Hb saturation
- Pulmonary function
- Inspired O2 concentration
What are the limitations of cerebral oximetry?
- blood from extracranial source can create erroneously low readings
- diathermy can affect accuracy
- they only measure regional cerebral oxygenation - large areas of the brain remain unmonitored
- cerebral oximeters can’t identify a cause for desaturation
What are transcranial dopplers?
They provide an indirect measure of cerebral blood flow by measuring blood velocity in a cerebral artery. They have to obtain this through transcranial windows
Where are transcranial windows?
Thinnest parts of skull - temporal bone or where bone is absent, the orbit
