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The NIRS equipment provides information regarding regional tissue perfusion depending upon the site of the probe. Currently we also use systemic venous saturations to inform us about total body perfusion.
The INVOS system is designed to non-invasively, directly and continuously measure regional tissue haemoglobin (venous) oxygen saturations (%rSO2 value), to indicate the adequacy of the balance between regional tissue oxygen delivery and extraction. Its interpretation is analogous to the systemic venous saturations (%SvO2), indicating the balance between oxygen delivery and extraction, only more reflective of the local tissue balance. The %rSO2 reflects the amount of haemoglobin still saturated after passing through the local tissue bed.
A low number (> 40%) may indicate either an:
The trend in value is more significant than the actual number.
NIRS probes are placed onto the skin as directed below. Near-infrared light is emitted from one end of the probe, passes through the superficial tissues, where it is either scattered or absorbed. The amount of absorption correlates to the degree of saturation of haemoglobin. The detector in the skin probe senses the light that has not been absorbed, measured as relative deoxy-haemoglobin, and converts this data into a number which indicates the regional venous saturations (%rSO2 also called NIRS value).
We routinely use a two probe technique with one cerebral probe (rSO2-1) and one renal probe (rSO2-2). Continually assessing the regional venous saturations of both the brain and below the diaphragm not only provides an indication of the adequacy of total cardiac output, but also the simultaneous adequacy of local perfusion to these areas.
In a normal physiological state the cerebral (brain) saturations are lower than the systemic venous saturations, whereas the renal saturations are higher. This is reflective of the combination of both a higher cardiac output delivered to the renal bed and lower oxygen extraction by the kidneys, compared to the brain. When the venous return from the upper body (cerebral) and lower body (renal) meet they unite to form the systemic venous oxygen saturation.
The trend of all these markers is the most important part of the data.
A Cerebral NIRS < 40 is associated with neurological morbidity.
Whilst its use should be at the discretion of the Consultant Intensivist, we suggest two cohorts of patients that merit specific consideration:
It may also prove useful in trauma and sepsis cases.
NIRS monitors are stored in the store room on the right hand side, next to the ventilators.
There are two sizes of probes available:
NB the two sizes of probes cannot be used simultaneously.
The probes are stored in the main store room next to the cannulae.
Infant probe position:
We use 2 probe sites: cerebral and renal (left renal bed). This will indicate the regional oxygenation status of the brain and the left kidney.
The probes should be plugged in as detailed:
Older child probe position:
We use only the cerebral probe in this age range. The smaller infant probe will not give adequate penetration to give accurate readings in the larger child.
This probe requires an extension cable to be attached to the module to allow the connection. This is stored in the NIRS tray in the Store room. Plug into port 1 in the NIRS box connection.
Attaching a probe:
NIRS probes should be attached using a piece of wide hypafix as seen above. The adhesive backing should not be removed to allow easy skin inspection every 6 hours. Attachment of NIRS should be documented in CIS on the event list.
The skin where the probe is attached with should be inspected similar to the arterial saturation probe practice - every 6 hours by gently peeling back the hypafix to show the full area under the probe.
NIRS Intellivue link & CIS:
Each NIRS unit has a grey cable and Phillips intellivue cable attached, all of which is labelled PICU NIRS.
Ideally the NIRS should be connected pre-operatively when the patient is awake, well and stable. This allows for a baseline NIRS value to be set which is the patient’s own control. Subsequent NIRS value changes from baseline during operative procedure can be measured from the initial values when the patient was awake and well.
However, if NIRS is connected for example after cardiac surgery, a baseline value should still be taken as it allows an immediate appreciation as to how therapy has influenced regional tissue oxygenation.
To set a baseline:
The baseline value(s) will be set at the current value(s) and the menu will return to the Main Screen.
NIRS values will turn red if above or below set alarm thresholds.
Baseline status will turn red if reading is 20% below set baseline.
A 20% fall in a NIRS value is significant and the patient should be assessed carefully.
If the NIRS value (%rSO2) are worrying, check arterial and patient mixed venous blood gases and inform a senior clinician.
Absolute Cerebral NIRS <40% may mean significant neurological ischaemia / hypoxia and may be associated with a poor neurological outcome. Seek senior medical advice immediately.
1. ↓ Cerebral NIRS and Renal NIRS
Global oxygen delivery fallen or global increase in oxygen consumption
Confirm with paired arterial and venous blood gas
2. ↓ Cerebral NIRS (cranial blood supply cannot meet demand = risk of neurological injury)
Exclude thrombo-embolic event / intra-cranial haemorrhage
Consider need for neurological imaging
3. ↓ Renal NIRS (infra-diaphragmatic blood supply cannot meet demand = ↑ risk of renal or gut ischaemia / infarction). Note: ↓ renal NIRS may reflect the presence of a PDA – lower infra-diaphragmatic arterial saturations becoming lower renal tissue saturations.
Last reviewed: 01 May 2016
Next review: 01 May 2018
Author(s): C Cairney, M Davidson & C Kidson
Approved By: Clinical Effectiveness
Reviewer Name(s): PICU Guideline Group