Question 3

Describe the advantages  and disadvantages  of PaO2 (partial  pressure  of oxygen in the arterial  blood) and SpO2 (oxygen saturation  measured by a pulse oximeter) as indicators of arterial oxygenation.

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College Answer

This question lends itself to an answer in table form. Such a table should include the following type of information:




Allows calculation of P:Fratio and A-a gradient. Calculating SaO2 allows calculation of DO2/VO2/shunt fraction

Also a reliable indicator of

oxygenation particularly in the clinical relevant range

Accurate indicator of oxygenation, not influenced by position of Oxyhaemoglobin Dissociation Curve

Easy to use, non invasive,

reliable in most clinical situations both in ICU and on the wards

Measurement accurate evenin the presence of dyshaemoglobins.

Provides a continuous




Affected by peripheral

perfusion, arrhythmias, motion artefact

Prone to pre-analytic errors

– air bubbles, heparin contamination

Affected by


Needs a blood gas analyser

Usually a time delay between sampling and result

Provides only intermittent




An alternative tabulated answer would resemble the following:




  • accurate impression of oxygenation
  • not confounded by dyshemoglobins
  • allows accurate calculation of hemoglobin saturation
  • real-time monitoring
  • Non-invasive
  • requires no special expertise


  • Invasive
  • requires arterial access expertise
  • requires blood gas analyser
  • confounded by collection error, eg. bubbles in the syringe
  • Measurement delay exists
  • confused by dyshemoglobins
  • does not reflect level of oxygenation in hyperoxic patients
  • not a direct measurement of hemoglobin saturation - instead, uses a signal intensity and a look-up table derived from empirical data
  • no absolute method for calibration exists - only empirical data collected from hypoxic volunteers
  • unreliable in severely hypoxic patients
  • unreliable in poorly perfused patients
  • unreliable in arrhythmia
  • positional
  • unreliable in confused patients, confounded by mostion artifact



Hutton, P., and T. Clutton-Brock. "The benefits and pitfalls of pulse oximetry."BMJ: British Medical Journal 307.6902 (1993): 457.