SpO2
Formal Definition
Peripheral oxygen saturation: a non-invasive measurement of arterial hemoglobin oxygen saturation obtained by pulse oximetry, reported as a percentage — the fraction of oxygenated hemoglobin relative to total hemoglobin.
How It's Used on the Ward
"Sat" or "O2 sat" — checked on every patient, every encounter. "Sat is 88 on room air" is a red flag.
Example
""Overnight cross-cover: nursing called for sat of 84% on 2L NC, up from baseline 96%. Patient breathing 26 times per minute. Went immediately — new bilateral infiltrates on portable CXR. CPAP started, MICU called.""
Clinical Context
Normal SpO2: 95–100%. Below 90% = hypoxemia requiring intervention. Pulse ox is non-invasive and continuous — but has limitations: poor signal in vasoconstriction, nail polish, motion, severe anemia, carbon monoxide poisoning (falsely normal), and in dark-skinned patients (systematic bias documented in literature). An ABG is required when pulse ox reliability is in question or when PaO2 matters (FiO2 titration, ARDS).