Oxygen changes may not be detected by the pulse oximeter until how many seconds after application?

• A. 15-30 seconds
• B. 30-90 seconds
• C. 90-120 seconds
• D. 2-3 seconds

Answer: (B)

When you change the amount of oxygen a patient is receiving, the pulse oximeter doesn’t update instantly. The reading depends on arterial blood that has to circulate from the lungs to the measurement site and the device’s averaging to produce a stable value. That combination creates a short lag, typically tens of seconds. In practice, you usually see a new SpO2 reading settle about 30 to 90 seconds after applying or changing oxygen. If perfusion is poor or the device is averaging longer, it can take a bit longer, but 30–90 seconds is the standard window for this change to be reflected.