Which condition most likely causes pulse oximetry readings to be unreliable due to low perfusion?

• A. Smoke Inhalation
• B. Hypovolemia
• C. ECMO
• D. Hyperoxia

Answer: (B)

Pulse oximetry relies on a clear arterial pulsatile signal at the sensor site to distinguish arterial blood from surrounding tissues and venous blood. When perfusion to the extremities is poor, as in hypovolemia, the arterial pulse at the fingers or toes becomes weak, and the signal the device uses is small or unstable. That reduces the signal-to-noise ratio and can produce unreliable, fluctuating, or even missed readings, since the meter can’t reliably separate arterial absorption from the background. In hypovolemia, decreased circulating volume lowers stroke volume and tissue perfusion, especially peripherally, making pulse oximetry readings much less trustworthy. Other conditions can affect SpO2 in different ways, but the scenario described specifically centers on low perfusion driving unreliability, which is most characteristic of hypovolemia.