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Anaesthesia - cerebral NIRS management in cardiac patients

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Principles

  • Monitoring of the cerebral circulation is poorly done in anaesthesia and intensive care.
  • NIRS allows for noninvasive continuous monitoring of a small portion of the cerebral circulation and in neonates and infants it can be a reliable surrogate measure for mixed venous saturation though there are important exceptions. It is less reliable in adults.
  • The NIRS value will vary with the arterial saturation so instead of a "normal range" use a NIRS value that is acceptable for the arterial saturation (similar to the A-V difference a reading less than 30% less would be acceptable).
  • In anaesthesia it is useful to note what the baseline NIRS reading is and be guided by the amount of variation from this (aiming for <20% change). Obviously if the patient has a seriously deranged circulation, severe hypoxia etc preoperatively then the NIRS should be vastly improved on CPB and post repair. A greater than 20% decrease in the NIRS should prompt you to look for a reason and restore to baseline if possible. It is debatable what to do if the NIRS reads very high as is often the case on CPB - most places tend to do nothing while some attempt to decrease the cerebral blood flow. NIRS almost always drops during rewarming from hypothermia.
  • NIRS can be used to monitor the systemic circulation by placing a sensor over the abdomen or kidney but this is less well validated.
  • NIRS only looks at a small portion of the brain - usually the frontal lobe on 1 or both sides- so it is possible to miss important neurological insults.
  • Premature neonates and neonates with CHD may not have intact cerebral autoregulation and cerebral blood flow may be totally MAP dependent.

Practice

  • Apply the appropriate sensor to 1 side of the forehead after preparing the skin to ensure good contact and turn the monitor on.
  • After achieving a steady signal take a central venous blood gas if able and compare the 2 values.
  • Use the algorithm below to alter variables that will affect the NIRS value.
  • Do not respond to the NIRS value in isolation.

Factors affecting cerebral oxygenation (NIRS)

Supply

Arterial oxygen content  - PaO2
- Hemoglobin
Cerebral blood flow - Cardiac output - PaCO2
- MAP
- Cerebral vascular resistance
- Intracranial pressure
- Venous pressure
- Shunts eg ductus arteriosus, MBTS
- CPB flow rate

Demand

Assumed to be constant but will alter with temperature and cerebral activity e.g. temperature, seizures etc.

NIRS Algorithm

NIRS algorithm for low cerebral saturation (either >30% less than SaO2 or absolute value <40 or drop from baseline >20%)

If cerebral saturation is low, check arterial and central venous saturations as the low NIRS value and SvO2 may be due to a low arterial oxygen saturation.

NIRS
Notes

  1. The most common cause of a low NIRS value with a normal SvO2 is a low PaCO2.
  2. Balancing Qp:Qs in single ventricle can be difficult and often patients are able to do this best themselves i.e. intubating and ventilating to control pulmonary blood flow may make the situation worse. Some centres are using NIRS preoperatively to dictate timing of surgery in their SV neonates.
  3. It is unclear what if anything should be done about a high NIRS value - if there is suspicion of ICP being raised and NIRS is high then it may be reasonable to decrease cerebral blood flow by decreasing the PaCO2 but do not make the patient hypocapnic.
  4. Selective Cerebral Perfusion. This is used during arch repairs in the operating room to protect the brain. Usually the innominate artery is cannulated. Flow, MAP (R arm) and NIRS are monitored during the procedure to try and ensure adequate cerebral blood flow. Flows are between 20-50ml/kg/min guided by NIRS and MAP. A drop in the NIRS or a rise in the MAP when initiating SCP should be notified to the surgeon and the perfusionist immediately.
  5. NIRS values tend to be unreliable in postop BDG patients - it is unclear why but the NIRS is often high and may even read higher than the arterial oxygen saturation.

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Document Control

  • Date last published: 29 November 2018
  • Document type: Clinical Guideline
  • Services responsible: Paediatric Intensive Care Unit, Paediatric Anaesthesia
  • Owner: D Buckley
  • Editor: Michael Tan
  • Review frequency: 2 years

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