In the Field
Pulse ox is the field number that most providers reach for first. It is non-invasive, fast, and gives a continuous trend that catches deterioration before the patient looks bad. In TCCC, the doctrinal triggers are clear: SpO2 below 90 percent in trauma, below 92 percent in moderate to severe TBI, drives airway and ventilation intervention. The probe goes on a finger, ear, or toe and reads within 30 seconds. Continuous monitoring with audible alarms catches changes the provider would otherwise miss while doing other tasks.
Common Mistake
Trusting the pulse oximetry reading without considering the conditions that make it unreliable. Severe shock with poor peripheral perfusion produces falsely low or unreadable values. Hypothermia does the same. Carbon monoxide poisoning produces falsely normal readings because carboxyhemoglobin absorbs light similarly to oxyhemoglobin. Nail polish (particularly dark colors) and motion artifact corrupt readings. Methemoglobinemia tends to drive the reading toward 85 percent regardless of true oxygenation.
Technical Detail
Pulse oximetry uses two wavelengths of light (typically 660 nm red and 940 nm infrared) transmitted through tissue. Oxyhemoglobin and deoxyhemoglobin absorb these wavelengths differently. The pulsatile component of the signal isolates arterial blood from venous and tissue absorption. The ratio of absorptions at the two wavelengths correlates with arterial oxygen saturation via an empirically derived calibration curve. Normal SpO2 95 to 100 percent at sea level; values 90 to 94 percent indicate hypoxemia requiring intervention; values below 90 percent represent significant hypoxemia. TCCC 2026 specifies SpO2 monitoring for all moderate or severe TBI casualties; supplemental oxygen target greater than or equal to 92 percent in TBI; consideration of nasopharyngeal airway and BVM if SpO2 falls below 90 percent (below 92 percent with moderate or severe TBI). Limitations include unreliability in shock, hypothermia, motion, carbon monoxide poisoning, methemoglobinemia, and certain nail polishes.
