NU2ICU V3 logoPulse Oximetry

Image – http://drrajivdesaimd.com/wp-content/uploads/2015/07/Pulse-oximeter-sensor.jpg

The pulse oximeter is made up of a light emitter on one side with 2 LED’s (light emitting diodes), one emitting at 660nm and one at 940nm

Opposite is a photodetector/transistor

The LED’s are switched on and off 100’s of times per second so that the absorption of O2Hb and HHb (oxygenated and deoxygenated blood cells) is recorded during pulsatile and non pulsatile flow

A ratio is then calculated of the absorbance of the two wavelengths in the pulsatile phase divided by that in the non pulsatile phase

This ratio is then fed into the computer where it is converted to a saturation

The values in the table are determined by testing on healthy volunteers made hypoxic to saturations of ~ 75%.  All values below this level are estimated and unreliable

Limitations

  • low SaO2
  • improper probe placement, ambient light
  • motion
  • radiation

Patient-related

  • dyshaemoglobinemias – MetHb, COHb
  • venous pulsations and congestion
  • low perfusion and anaemia
  • intravascular dyes
  • nail polish

Also some things DON’T matter … like bilirubin and most skin pigmentation (REALLY dark skin has special units available)

 

INACCURACIES

  • saturations become less accurate under ~70% (under 60% with forehead sensors) as the data used by the microprocessor becomes extrapolated rather than actual
  • due to the flattening of the O2-Hb dissociation at saturations above 90%, oximetry is relatively insensitive to changes in PaO2 above this level
  • improper probe placement
    • poor positioning may result in some light not passing through tissue, thus inaccurate absorption reading result
    • finger probes are no longer the most accurate site
    • BP cuff on the same side will effects the pulsatile flow
  • ambient light
    • especially infra-red, strong incandescent or fluorescent light can give a falsely low reading
  • motion
    • like shivering, seizures
  • venous congestion and pulsations eg tricuspid regurgitation, obstructed venous flow, tourniquets or bandaging
    • artificially low readings as the detector includes the less oxygenated pulsatile venous blood in the arterial reading
    • ear and forehead probes are more sensitive to this so are more accurate
    • ideally the probe should be, if on the fingers, at heart level
    • check this picture – is there better blood supply in the fingers or the forehead?  (red = hot & blue = cold comparatively)

With most forehead probes there is an alignment indicator and in this version the line (on blue oval detector) is meant to be vertical and above the pupil on either side of the forehead

Image – https://www.cpapusa.com/nellcor-oximax-max-fast-forehead-sensor-10-kg.html

In Summary

  • pulse oximetry is easily used as a non-invasive measure of a patients’ arterial oxygen saturation
  • it works on the principles that oxygenated and deoxygenated haemoglobin absorb different amounts of light at different frequencies
  • the technology of oximeters has much improved since first invented, however it still has its limitations which need to be remembered when interpreting results
  • finally pulse oximetry isn’t a true substitute for arterial O2 measurement if clinically concerned

 

Nursing Care

  • finger probes should be rotated 3 – 4 hourly (with full body Pressure Area Care)
  • taped forehead sensors should be redressed / observed at the same interval to observe for pressure injury (we have nursed patients with really obvious black dots / pressure areas from these on their foreheads
    • the supplied “maxfast” headband or similar used to help attach / cover the probes is unnecessary on quiet, intubated non-diaphoretic patients
    • forehead sensors should be orientated with the marked line above the pupil this should locate it over a vessel supplied by the internal carotid artery
  • any issues please talk to your T/L or Shift Coordinator
    • never be satisfied with a poor quality reading on an unstable patient, if in doubt perform an ABG (or if really.. stuck a VBG)
  • your patient is your responsibility
    • please raise any concerns as you see them

 


Humidification

All wall gases in ICU like O2 and air are perfectly dry and contain 0% water vapour, supplying these gases on a continuous basis to your patient can overcome their inherent humidification abilities and dry and damage the airways, as well as cause extreme pain, whether given as at pressure or at high flow rates or even bypassed completely by an endotracheal tube.

Most air we breathe is already partially saturated with water and we just add to it on inspiration.  To maintain normal airway function (cilia, mucous, epithelia etc) we must add “normal”levels of moisture to the gases wherever possible

Definitions

  • relative humidity (%)             = the ratio of the mass of water vapour in a given volume of air to the mass required to fully saturate that volume of air at a given temperature 370C
  • absolute humidity (mg/L)   = the mass of water vapour (g) present in a given volume of air (m3)
  • humidity may also be expressed as the pressure exerted by water vapor in a gas mixture (mmHg)

We supply 2 types;

  • Essential – normally at entry to our sinus passages air is being warmed to 310C however this is with atmospheric gases that already contain a level of usually about 10mg per litre of humidity
  • Absolute – on passing through the rest of the respiratory passages gases are now carrying 100% of possible water vapour – 44mg per litre

Image – From Fisher & Paykel™ Information package 2016

To heat and humidify gases the body must expend energy and fluid – both of which in the critically or chronically unwell patient may be overwhelming.

Humidity in the airways facilitates improved mucocilliary clearance therefor improving expectoration (however always watch for excessive fluids collecting in any ventilation tubing which may be caused by the cooling of humidified air by fans, air conditioning or bad temperature settings these large fluid boluses to the poorly protected airway can, in the worst case scenario, kill)

HiFlow / NIPPV humidification heater pots are set at either mask (for automatic temperature pots) or 37oc minus 3-6oc at the heater wire (or patient end of hoses) with manual temperature pots, always attempt to run the temperature as close to 37oc as possible if the patient will tolerate it during NIV / NIPPV.   There is no use having high humidity and temperature if the patient hates it and is refusing the treatment.

Heat Moisture Exchange and Filter units (HME / HMEF) are relatively cheap, usually cellulose based, filters applied to the dual lumen circuit of a ventilator to trap exhaled moisture for humidifying the following inhalation.  They are moderately effective and usually only used for short term humidification (like surgery or overnight use for drug overdoses etc).   As most NIV / NIPPV is performed via single lumen circuits in most ICUs the use of a HME / HMEF is unfortunately very limited.

 

REFERENCES

Oh’s Intensive Care Manual 7th Edition 2014 by Andrew Bersten, Neil Soni

The ICU Book 4th Edition 2015 by Paul L Marino
Pilbeam’s Mechanical Ventilation: Physiological and Clinical Applications 6th Edition 2016 by J M Cairo

 

More reading

ARTERIAL BLOOD GASES

VENTILATION ASSISTS

VENTILATION SEDATION and POSITIONING

INVASIVE MECHANICAL VENTILATION CARE