17.2. Examine the data provided from a co-oximeter and a simultaneous pulse oximeter recording from patient A and B. List three (3) causes in each patient for the discrepancy between the two oximeters.
|
Patient A: |
Co-oximeter Oxy Hb 85% |
Pulse oximeter oxygen saturation 95% |
|
Patient B: |
Co-oximeter Oxy Hb 98% |
Pulse oximeter oxygen saturation 88% |
Patient A:
CoHb
Met Hb
Radiofrequency interference
Patient B:
Tricuspid regurgitation
Ambient light
Poor peripheral perfusion Dyes- Methylene blue
Poor probe contact
The pulse oximeter is a dumb machine, whereas the co-oximeter will measure lots of different subtypes of haemoglobin simultaneously.
In general terms, the co-oximeter is correct, and the pulse oximeter is frequently confused.
Thus, in Patient A, the co-oximeter reads 85% (the true saturation of haemoglobin) while the pulse oximeter reads 95%. Clearly, there is some haemoglobin here which closely resembles normal oxygenated haemoglobin, but is in fact carrying no oxygen.
The causes of that could be:
Interestingly, only mild methaemoglobinaemia should be on that list. Pulse oximetry measurement of mixed normal haemoglobin and methaemoglobin will usually be depressed. When one's methaemoglobin level is in excess of 35%, the pulse oximeter will usually read 85% (Barker et al, 1989). The pulse oximeter will then continue to read this value, whether the oxygenation deteriorates or improves, i.e. at 60% methaemoglobin concentration and a fractional oxygen saturation of 30% or 100%, it will still give you an SpO2 of 85%. This is why highly respected resources report that "methemoglobinemia typically causes the pulse oximeter to report a [pulse oximeter] saturation of ~82-86% (even if the PaO2 is very high)."
In Patient B, the co-oximeter confirms a normal oxygen saturation of haemoglobin; however, something is confusing the pulse oximeter.
Here is the operations manual for an AVOXimeter 4000.
Barker, Steven J., et al. "Measurement of carboxyhemoglobin and methemoglobin by pulse oximetry: a human volunteer study." Anesthesiology105.5 (2006): 892-897.
Mathews Jr, P. J. "Co-oximetry." Respiratory care clinics of North America 1.1 (1995): 47-68.
Watcha, Mehernoor F., Michael T. Connor, and Anne V. Hing. "Pulse oximetry in methemoglobinemia." American Journal of Diseases of Children 143.7 (1989): 845-847.
Barker, Steven J., Kevin K. Tremper, and John Hyatt. "Effects of methemoglobinemia on pulse oximetry and mixed venous oximetry." The Journal of the American Society of Anesthesiologists 70.1 (1989): 112-117.