pulse ox lag

4 Sep


Your next patient is a 42 yo M with COPD and severe respiratory distress, who rolls in with EMS and has an 02 sat of 77%.   You quickly decide to intubate, but immediately after doing so, the 02 sat is still sitting in the 80’s.   Your ETCO2 is working and shows a good waveform, so you should be in the right spot (trachea).  So what’s going on?



(other than Airway, Breathing, and Circulation)



After changing someone’s oxygenation (supplemental 02, in whatever way/shape/form from nasal cannula to ETT, or removal thereof), there seems to be a lag time between their actual oxygenation (centrally) to the detection peripherally, as with an 02 sat monitor.



Pulse ox study (most pertinent) by Gruber et al.

  • 51 patients with nasal cannula O2 (43 monitored while O2 treatment was initiated, 18 monitored when it was discontinued)
  • Patients were placed on and/or taken off O2 via nasal cannula set at 2 or 4 L/min based on clinical indications. 
  • 02 saturation was measured at 1-minute intervals over a 30-minute period
  • Most (95%) of the patients placed on O2 attained equilibration of O2 saturation within 3.5 minutes.
  • Most (95%) of the patients taken off O2 attained equilibration of O2 saturation within 4.5 minutes.

ABG study on COPD-ers by Weinrich et al:

  • 12 patients with severe COPD on long term 02
  • FiO2 increased or decreased, and ABGs checked at increasing intervals over 34 min
  • time required to reach clinically stable values (within 1% of equilibrium values)
    • FiO2 increased: median time was 2.6 minutes and in worst case 6.8 minutes. 
    • FiO2 reduced: median time was 7.4 minutes, and in the worst case 15.6 minutes.


References: pulse ox study; abg/COPD study; picture


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