Apr 252013
 

image: wikimedia
Pulse Oximetry: The 30-Second Time Machine

Why does it seem to take so long to re-oxygenate your crashing patient? Because your pulse oximeter is lying to you, no matter how good it is.

Telescopes show us how a star looked millions or billions of years ago; pulse oximeters create a similar, though tiny time warp, revealing the patient's blood oxygenation was about 30 seconds ago, or more. (It can easily take that long for newly oxygenated blood to get out to the skin where you have the probe, especially if cardiac output is low.) In the steady state, the lag is irrelevant, but in crashing or unstable patients, it can mean a lot.

For example, many patients have been unsafely extubated pre-hospital for "failed" intubations that were in fact correctly done, with an ET tube in proper position -- just because the pulse oximeter lag fooled the first responders into believing the patient was still hypoxic a minute or two after intubation, and that ET tube misplacement was to blame.

In the hospital, pulse-ox lag needs to be considered in the decision-making on any hypoxemic patient, especially those in respiratory or cardiac arrest (being intubated and or getting CPR), who are just before or after these dire states, or who are so unstable they require minute-by-minute attention and management.

The above linked case studies (for falsely "failed" intubations) highlight the benefits of continuous quantitative CO2 waveform analysis (capnography) as the ideal standard of care, with colorimetry (the litmus paper inside plastic that changes color with the patient's exhalations through an ET tube) a poor second choice. Exhaled CO2 is high immediately in a properly intubated patient -- no lag time.

Scott Weingart's EMCrit gave me the idea for this post. You can listen to his great podcast with Dan Davis below, or check it out at EMCrit.

Clinical practice guideline on Capnography/Capnometry During Mechanical Ventilation, 2011 (Respiratory Care)

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Pulse oximetry as time machine: Lag times confuse doctors, complicate intubations (EMCrit)