Clinical meaning
Pulse oximetry (SpO2) measures functional oxygen saturation by transmitting two wavelengths of light (660 nm red and 940 nm infrared) through a pulsatile vascular bed and calculating the ratio of oxyhemoglobin (HbO2) to deoxyhemoglobin (Hb). The oxyhemoglobin dissociation curve describes the relationship between PaO2 and SpO2: the sigmoid shape means SpO2 remains > 90% until PaO2 drops below ~60 mmHg, below which saturation drops precipitously (the 'cliff'). Critical limitations include: (1) Carboxyhemoglobin (COHb): CO absorbs light at 660 nm similarly to HbO2, causing falsely NORMAL/HIGH SpO2 readings in carbon monoxide poisoning — always obtain co-oximetry ABG in suspected CO exposure; (2) Methemoglobinemia: methemoglobin absorbs equally at both wavelengths, driving the ratio toward 1.0 and SpO2 reading toward 85% regardless of true saturation — does not rise above ~85% or fall below ~82% on pulse oximetry; (3) Severe anemia: SpO2 measures SATURATION (percentage of hemoglobin carrying O2), not oxygen content — a patient with Hb 4 g/dL can have SpO2 99% but severely inadequate oxygen delivery; (4) Dark skin pigmentation: pulse oximeters overestimate SpO2 by 2-4% in patients with darker skin, leading to missed hypoxemia (occult hypoxemia); (5) Poor perfusion states: hypothermia, shock, vasopressor use, and peripheral vascular disease reduce pulsatile flow and produce unreliable readings; (6) Nail polish and synthetic nails: dark nail polish (especially blue, black, green) can interfere with light transmission; (7) Motion artifact: patient movement creates false readings.