Near Infrared Spectroscopy (NIRS)

NIRS is a non-invasive modality capable of continuously measuring regional tissue oxygenation. Initially used for cerebral monitoring its utility has now expanded to other areas such as renal, gastrointestinal and spinal cord oxygenation monitoring.

Indications

• During and following cardiac surgery
• ECMO (Extracorporeal membrane oxygenation)
• Post cardiac arrest management
• Traumatic brain injury
• Spinal cord ischaemia after aortic resection
• Sepsis

NIRS is similar to a saturation probe and uses the same principle of infrared spectroscopy to measure oxygenated and deoxygenated haemoglobin via the Beer-Lambert law. NIRS probes contain a fibreoptic light source and a light detector. Emitted light crosses the skull which is transparent to infrared light to then penetrate 2-3cm into cerebral tissue.

When infrared light meets haemoglobin it is absorbed at different wavelengths depending on the ratio of oxygenated to deoxygenated haemoglobin. The remaining light wavelengths are reflected and detected by the NIRS detector. The absorption for deoxy-haemoglobin is best detected at 660nm and oxy-haemoglobin is best detected at 940nm.

Ultimately the probe relays this information to the monitor which provides a percentage of oxygenated haemoglobin reflecting the balance between oxygen consumption and delivery to the tissue in question. Unlike pulse oximetry (saturation probes) NIRS does not require pulsatile flow and therefore can be utilised during cardiopulmonary bypass (CPB) or ECMO.

Positioning

Typically, when using NIRS for cerebral monitoring two probes are attached; one either side of the midline on the forehead of the patient. The NIRS sensors should not cross the midline as erroneous readings from the superior saggital vein can be obtained rather than cerebral tissue. Positioning in this manner will monitor tissue oxygenation in the superficial area of the frontal lobe. NIRS probes are single patient use.

The number generated must be interpreted carefully accounting for the limitations of NIRS. This number is rarely equivalent to the saturation probe number. The trend and deviation from a baseline reading obtained prior to induction of anaesthesia is more relevant than a one-off data point. It is generally accepted that SO2 < 60% is considered abnormal and SO2 < 40% is considered very low. 

Other trends to cause concern include:

1. A reduction in any baseline reading by 20%
2. A new difference of > 10% between left and right NIRS
3. Cerebral values > 95% suggest lack of oxygen extraction and possible cerebral injury

In essence a true reading showing any of the above can be caused by inadequate regional perfusion OR excessive consumption of oxygen in this region

NIRS baseline values should ideally be obtained prior to induction of anaesthesia to allow for meaningful trends and changes to be observed. Anatomical factors such as carotid artery stenosis or an incomplete circle of willis can create erroneous values and this is why monitoring should be bilateral and baselines recorded. NIRS is also affected by diathermy (causing artefact) and extracranial blood [1].

Management and Intervention

If low or abnormal NIRS readings are obtained, then first consider:

• Adherence of the monitoring probe
• Correlate with contralateral NIRS sensor, sats probe, ABG and VBG (SvO2 can be useful as an indicator of total body perfusion)
• Consider the clinical context – cardiac output, temperature, interventions (ECMO/CPB cannulae etc) and volume status.

If inadequate regional perfusion is suspected then aim to treat the cause:

• Remember DO2 = CO x (1.39 x Hb) x SaO2 + (0.003 x PaO2)
• Consider increasing FiO2, optimising shunt, optimising deadspace and/or inotropes
• Specific to cerebral NIRS - consider thrombo-embolic event or intracranial haemorrhage
• On CPB consider membrane oxygenation and optimising circulating blood flow
• During deep hypothermic circulatory arrest (DHCA) consider increasing antegrade cerebral perfusion flow rate and ensure adequate cerebral venous drainage

If excessive oxygen consumption is suspected e.g. sepsis then institute measures to reduce oxygen consumption by reducing sympathetic tone:

• Analgesia
• Anaesthesia/sedation
• Paralysis
• Cooling measures
• Seizure control

Many protocols have been developed to treat abnormal NIRS readings, the most cited being the “Murkin protocol” which describes taking the following actions in the event of reduced NIRS [2].

Aim to maintain NIRS > 70% of baseline or > 50% as an absolute value. If these targets are not achieved then:

1. Ensure adequate venous drainage (ensure head in a neutral position and ETT ties aren’t obstructing venous drainage)
2. Raise MAP to > 60mmHg
3. Increase FiO2 if systemic hypoxia
4. Raise PaCO2 >40mmHg/5.3kPa to encourage cerebral vasodilation
5. Transfuse if HCt <23% (and consider raising pump flow on CPB)
6. If none of the above works, increase depth of anaesthesia and reduce core body temperature

Evidence

Studies show that NIRS closely correlates to central venous sats [3], [4], [5], [6] ,[7] and that there may be some inverse correlation between regional oxygenation and lactate [8].

Studies have demonstrated that episodes of cerebral ischaemia were detectable using NIRS including episodes of “silent” ischaemia which may otherwise have gone unnoticed for some time [9], [10].

Prolonged cerebral desaturations have been associated with longer hospital stays and early cognitive decline [11]. Active treatment of declining cerebral O2 (as per NIRS) has been shown to reduce ICU stay and the incidence of morbidity and mortality in some studies [12], [13].

Meta-analysis has shown NIRS-guided algorithms can improve post-operative cognitive dysfunction and delirium in major cardiac surgery (although this effect was not translated into non-cardiac surgery) [14].

NIRS may have a role in detecting cerebral ischaemia during DHCA when EEG is less useful, but further evidence is required.

Other indications

In paediatric patients NIRS can be placed over either flank as a surrogate for renal perfusion during complex aortic surgery.

NIRS placed bilaterally in the paraspinal thoracic and lumbar regions has been used as a surrogate for spinal cord perfusion following TEVAR and complex aortic surgery. There is some debate whether measuring paraspinal perfusion correlates with “true” spinal cord perfusion (which occurs within the bony compartment of the spinal canal).

NIRS has been used to measure distal limb perfusion in ECMO patients where the cannulae can compromise blood flow. NIRS is well-suited to this scenario as a baseline reading from the non-cannulated limb can be used for comparison.

NIRS measuring splanchnic perfusion has been used in experimental settings in paediatric surgery where the incidence of necrotising enterocolitis is around 3%.

Contact: Dr Ed Elliot, Anaesthetic SpR Derriford Hospital, eelliott2@nhs.net

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