You have just intubated a 65 year old woman with sudden respiratory failure. On arrival, she had a pulse but cool extremities. She was obtunded and agonal with SpO2 was in the high 70’s. After pre-oxygenating with BVM to 100%, the intubation went smoothly. You had a clear view of the cords and passed a cuffed 8-0 through easily with qualitative capnography and bilateral breath sounds. Now, post-intubation, her SpO2 is 86% and steadily dropping.

Your attending doubts your skills and wants to have a peek at your tube.

Hold up. Now the SpO2 is rising and you didn’t do a thing. That’s because SpO2 monitors are technically not a “real-time” reading. The number you see on the monitor represents the patient’s recent past. For healthy adults, this time could be 20-30 seconds ago, but for sick patients, particularly those with a high-degree of peripheral vasoconstriction (as in this patient), this time can be 90 seconds or higher. ┬áSo if you see a low SpO2 immediately following intubation, but you KNOW you are in trachea and that you are giving O2, keep giving breaths for a minute before removing or repositioning the tube. You are likely to see a lag time rise in SpO2.