oxygen saturation
Summary
Oxygen saturation (SaO2 or SpO2) measures the percentage of hemoglobin molecules bound to oxygen. Normal values are 95-100%, with values <90% indicating hypoxemia requiring immediate attention. Pulse oximetry provides non-invasive continuous monitoring.
Detail
Oxygen saturation represents the ratio of oxyhemoglobin to total hemoglobin capacity, expressed as a percentage. It's measured via pulse oximetry (SpO2) non-invasively or arterial blood gas analysis (SaO2) invasively. The oxygen-hemoglobin dissociation curve is sigmoidal, showing cooperative binding - once one oxygen molecule binds, subsequent binding is facilitated. The curve shifts right (decreased affinity) with increased temperature, CO2, 2,3-DPG, or decreased pH, facilitating oxygen release to tissues. Left shifts occur with opposite conditions, increasing oxygen affinity. Clinically, SpO2 <90% corresponds to PaO2 <60 mmHg due to the steep portion of the dissociation curve, indicating severe hypoxemia. Limitations include poor perfusion, motion artifacts, nail polish, and inability to detect carbon monoxide poisoning (carboxyhemoglobin appears as oxyhemoglobin). Methemoglobinemia causes falsely low readings. Normal variations occur with altitude and certain populations.
Sources
- First Aid for the USMLE Step 1
- Harrison's Principles of Internal Medicine
- West's Respiratory Physiology
- Marino's The ICU Book
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