Cerebral Oximetry (NIRS)

Cerebral oximetry uses near infrared spectroscopy to estimate regional cerebral oxygen saturation (rSO2) non-invasively. Sensors on the forehead emit near infrared light, which penetrates the scalp, skull and brain. The ratio of oxygenated to deoxygenated haemoglobin in the underlying tissue is calculated.

The signal comes mainly from venous blood, so it reflects the balance between oxygen delivery and consumption in the frontal cortex. It is a trend monitor, not an absolute measure of cerebral oxygenation.

Usage. It is used in cardiac surgery, carotid endarterectomy, neonatal intensive care, and in selected critical care and anaesthetic settings.

Preparation and technique. Adhesive sensors are placed on the right and left forehead, between the hairline and the eyebrows. A baseline is recorded before induction of anaesthesia or before the procedure, while the patient breathes room air or a known oxygen concentration.

Reference values. Baseline values in healthy adults are commonly 60 to 75%, with wide individual variation and differences between devices.

Interpretation.

• A fall of more than 20% from baseline, or an absolute value under 50%, indicates cerebral desaturation and prompts action.
• Causes of desaturation include hypotension, hypoxaemia, anaemia, hypocapnia, raised intracranial pressure, and reduced cardiac output.
• Asymmetry between hemispheres during carotid clamping indicates inadequate collateral flow.

Critical values and findings needing immediate action. A sustained fall in rSO2 over 20% from baseline requires immediate assessment of arterial pressure, oxygenation, carbon dioxide, haemoglobin and head position.

Limitations and artefacts. Extracranial contamination from scalp blood, haematoma, dark skin pigmentation, hair and ambient light affect values. Devices from different manufacturers give different absolute numbers. Cerebral oximetry samples the frontal cortex only and misses ischaemia in other regions.

Follow up tests. Arterial blood gases, haemoglobin, cardiac output measurement, transcranial Doppler, and imaging of the brain where injury is suspected.

Special groups. Neonates and infants have thinner skulls and a stronger signal, and values differ from adults. Patients with cerebral oedema or skull defects have altered signals.