Pulse Oximetry

A pulse oximeter reads 97% — but the patient is a heavy smoker with carbon monoxide exposure. That number is a lie, and trusting it could cost a life.

Pulse oximetry (SpO2) uses infrared and red light to measure the percentage of hemoglobin saturated with oxygen. Normal SpO2 is 95–100%; values below 90% indicate hypoxemia and correspond roughly to a PaO2 of 60 mmHg — the steep part of the oxyhemoglobin dissociation curve where small PaO2 drops cause rapid desaturation. The sensor reads pulsatile arterial blood, so anything that reduces pulsatile flow — hypotension, hypothermia, peripheral vasoconstriction, or tight restraints proximal to the sensor — produces unreliable readings or no signal. The device cannot distinguish oxyhemoglobin from carboxyhemoglobin, so carbon monoxide poisoning produces falsely normal or elevated SpO2 readings. Dark nail polish (especially blue, black, or green) and heavy skin pigmentation can also skew accuracy. For the NCLEX, place the sensor on a warm, well-perfused finger or earlobe, remove nail polish if readings seem inconsistent, and correlate SpO2 with the patient's clinical picture — cyanosis, respiratory effort, and mental status always take priority over a number on a screen.

Key Distinctions

SpO2 measures hemoglobin saturation, not oxygen delivery — a severely anemic patient can read 98% yet have dangerously low tissue oxygenation because total hemoglobin is inadequate. Don't confuse SpO2 with PaO2: SpO2 is noninvasive and continuous; PaO2 comes from an ABG and reflects dissolved oxygen in plasma. Students often treat the oximeter as infallible — always assess the waveform quality and the patient before trusting the number.

Clinical Pearl

SpO2 tells you how full the buses are, not how many buses are running. A full bus on an empty highway (anemia) still means tissues starve.