Near Infrared Spectroscopy (NIRS) user guide

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:

• Inadequate tissue perfusion – A poor cardiac output may reduce the delivery of oxygenated blood to the tissue to a critical level sufficient enough to threaten cellular metabolism and risk cell death. The cells of the tissue will extract as much oxygen as needed to stay alive but if its availability is so poor because blood flow is so marginal, the ‘reserve’ contained within the venous blood will become increasingly deplete, relative to the inadequacy of the supply. The more marginal the oxygen delivery, the lower the residual oxygenation of the venous blood. 

OR

• Excessive tissue oxygen consumption (cellular catabolism is augmented such that oxygen demand exceeds supply).

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. 

Acceptable parameters: 

• Acyanotic (usual) circulation
  • Arterial sats >95% •   SvO₂ >65% 
  • Cerebral NIRS >55
  • Renal NIRS >65
• Cyanotic circulation (more at risk of ischaemia and hypoxia because of the reduced oxygen supply capabilities).
  • Arterial sats ~ 75% •SvO₂ >50%
  • Cerebral NIRS >45 
  • Renal NIRS >55

The trend of all these markers is the most important part of the data. 

A Cerebral NIRS < 40 is associated with neurological morbidity. 

Clinical indications for NIRS

Whilst its use should be at the discretion of the Consultant Intensivist, we suggest two cohorts of patients that merit specific consideration: 

• Infants less than 6 months of age with either:
  • Low cardiac output state or potential for it to develop (eg postcardiopulmonary bypass cases, Coarctation repair, etc)
  • Cyanotic congenital heart disease patients who are dependent upon a ‘balanced’ circulation.

• Any patient with Cardiomyopathy or Myocarditis
• Any patient supported on ECLS (please refer to ECLS manual)

It may also prove useful in trauma and sepsis cases. 

NIRS Set up guide

NIRS monitors are stored in the store room on the right hand side, next to the ventilators.

 There are two sizes of probes available:

• Infant probes for patients under 5kg 
• Paediatric probes for patients between 5-40kg 

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.  

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.

Skin care:

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.

Place the NIRS monitor at the head end of the bed, next to suction  Plug the monitor into a power supply Connect the probes into the correct modules
Place the NIRS Intellivue link into a doc in the Phillips monitor Connect the grey cable from the NIRS monitor to the Philips Intellivue doc.
The on/off button (green) is located at the bottom left hand side of the NIRS monitor. When depressed, the unit is turned on(off), the NIRS system automatically runs a self check  Data is automatically downloaded onto CIS. It will be recorded onto CIS every minute as rSO2-1 (Cerebral) or rSO2-2 (Renal). It is essential that the cerebral probe is placed into port 1 on the module and renal into port 2 so readings can be reviewed appropriately The alarm mute button is on the  bottom of the NIRS monitor.  The signal strength indicator (SSI) gives a guide to the level of NIRS signal. This varies from 0 to 5 green bars. Ideally 5 green bars should be displayed at the top left of the screen showing a good signal.

Baseline Value:

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: Press the ‘HOME’ key to display the Main Screen  Then press ‘BASELINE MENU’ Then press ‘SET BASELINES’

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. 

Clinical scenarios

1. ↓ Cerebral NIRS and Renal NIRS 

Global oxygen delivery fallen or global increase in oxygen consumption 

Confirm with paired arterial and venous blood gas

• ? ↑ oxygen consumption suspected, treat by:
  • analgesia, sedation, cooling, paralysis
• ? ↓  oxygen delivery suspected, treat by:
  • Red cell transfusion
  • Consider whether the arterial blood is optimally saturated (care in patients with parallel  circulations as the ventilation changes may adversely upset the ‘balance’ - recruitment manoeuvres, ↑PEEP, ↑PIP,  ↑FiO₂)
  • Augmentation (inotropes) and/or manipulation (vasoactive agents) of cardiac output

• If on ECLS:
  • VA-ECMO = diagnostic oxygenator membrane checks + check optimal circulating blood flow
  • VV-ECMO = diagnostic oxygenator membrane checks + exclude evidence of recirculation + exclude evidence of right heart failure and the need for inotropic support

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

• ? ↑ oxygen consumption, suspect if:
  • awake, seizure, hyperthermia

• ? ↓ oxygen delivery, 
  • ↓ PaCO₂ = powerful cerebro-vascular vasoconstrictor. (Excessive ventilation; if on ECMO, consider reducing sweep gas flow or adding CO₂ into the ECMO circuit).
  • If on ECMO - check arterial cannula position

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.

• ? ↓ oxygen delivery,
  • If on ECMO - check cannula position
  • Secondary to systemic vasoconstriction (vasopressin/noradrenaline)
  • Secondary physiological compensation to reduced cerebral blood flow (Treatment strategy = see 1 above)
    
    
•  Renal NIRS can potentially be used as a warning sign for acute kidney injury. 
  
  
• Low renal NIRS infers reduced infra-diaphragmatic blood flow and the risk of developing necrotising enterocolitis (NEC), but as the renal blood flow far exceeds GIT demand, the threshold fall in renal NIRS that is strongly correlated with the development of NEC is unknown.