Question 3

Highest mark

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.

[Click here to toggle visibility of the answers]

College Answer

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

	PaO2	SpO2
Advantages	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 measurement
Disadvantages	Invasive	Affected by peripheral perfusion, arrhythmias, motion artefact
	Prone to pre-analytic errors – air bubbles, heparin contamination	Affected by dyshemoglobins
	Needs a blood gas analyser
	Usually a time delay between sampling and result
	Provides only intermittent measurements

Discussion

An alternative tabulated answer would resemble the following:

	PaO2	SpO2
Advantages	accurate impression of oxygenation not confounded by dyshemoglobins allows accurate calculation of hemoglobin saturation	real-time monitoring Non-invasive requires no special expertise
Disadvantages	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

References

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

[Submit a comment or correction]