Cold Sweat Series 1: Oxygen Desaturation

Nurse A: Dr, Dr! Patient desat....
New Dr : >< ohhh..desat..desat...what to do? God, please help me save this patient

Below is the overview of the approach in this case


Treatment:
1. Identify underlying cause
2. Supplementary oxygen

  • maintain Pa02 at 60-80mmHg / SPO2 of 92-100% 
  • to avoid oxygen toxicity as PaO2 of >80% is not proven to improve oxygen delivery and hence unnecessary
  • Oxygen toxicity is concentration and time-dependent
  • Lowest FiO2 should be used as FiO2 >60% leads to inflammatory changes, alveoli infiltration and then pulmonary fibrosis
  • Nasal cannulae: 1-6L/min of oxygen (because 6L/min is adequate to fill the nasopharynx) - Fi02<40%
  • Fi02>40% is delivered via oxygen mask with a reservoir
Remember the oxygen dissociation curve?

In the context of anesthesiologists, there are a few important points in the curve. They are ICU point, mixed venous point, arterial point, and P50.

Definition of each term is as follows:
  • ICU point: The point that separate the steep lower part from the flat upper part of the curve. PaO2 of 60mmHg is the lowest allowed for any ICU patient as marked desaturation occurs at PaO2 below it. 
  • Mixed venous point: Symbolized mixed venous blood. It does not necessarily lies on the normal ODC. It could be shifted to the right due to high CO2 and decreased pH. 
  • P50: A reference point, where partial pressure of oxygen at which the oxygen carrying protein is 50% saturated. In other words, it is an index of oxygen affinity of the oxygen carrying protein. It is also the most useful point for specifying the curve’s position because it is on the steepest part of the curve. It is therefore the most sensitive point for detecting a shift of the curve. Specifying the P50 of a curve allows comparison with the position of other curves under different conditions
  • Arterial point: PAO2 100mmHg, SPO2 at 97.5%
What is oxygen saturation?

Functional Vs Fractional Saturation
  • Functional saturation = ( [HbO2] x 100 / ( [HbO2] + [DeoxyHb]) 
  • Fractional saturation = ( [HbO2] x 100 / Total [Hb]) where Total [Hb] = [HbO2] + [DeoxyHb]                                           + [MetHb] + [COHb].
Clinically it may be more useful to consider fractional saturation. This is because we tend to use the oxygen saturation value as an index of blood oxygen content (ie Sat x [Hb]). If large amounts of MetHb and/or COHb were present, then fractional saturation is useful in this way but functional saturation would be very misleading (eg 99% functional saturation could be associated with quite a low oxygen content if a high percent of Hb was HbCO or MetHb)

So what does our usual pulse oximeter measures?
Neither of fractional or functional saturation. It can detect values close to these saturations.
Note: As a two wavelength device, the pulse oximeter is limited to measuring only two Hb species - HbO2 and deoxyHb.

Well, it is an eye opening read for me and hope it is beneficial for you.

Signing out,
J&Fi

Reference:
  1. Merck Manual: Oxygen Desaturation
  2. Anaesthesia MCQ: Oxygen Dissociation Curve

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