Hapnikusaturation

Hapnikusaturation, or oxygen saturation, is the fraction of hemoglobin binding sites carrying oxygen. It is usually expressed as a percentage, with SpO2 used for noninvasive measurements (pulse oximetry) and SaO2 for arterial blood obtained by co-oximetry.

Measurement and interpretation are distinct. Pulse oximetry estimates SpO2 by analyzing light absorption in the finger, earlobe, or other sites. Arterial blood gas analysis with co-oximetry provides SaO2. SpO2 readings can be affected by perfusion, motion, and external factors, while SaO2 is generally more precise and is preferred in critical care or when exact values are needed.

Normal values and clinical meaning. In healthy people at sea level, SpO2 typically ranges from about 95% to 100%. Values below 90% indicate hypoxemia and may require evaluation and intervention, including supplemental oxygen. SaO2 normally parallels SpO2, though discrepancies can occur under certain conditions.

Factors and measurement pitfalls. Altitude lowers the maximum achievable saturation. Anemia lowers oxygen content even when saturation is normal because there is less hemoglobin to carry oxygen. Carbon monoxide binding can cause a falsely normal SpO2 despite reduced oxygen delivery. Methemoglobinemia can fix SpO2 around 85% regardless of PaO2. External factors such as skin pigmentation, nail polish, lighting, and poor peripheral perfusion can distort readings.

Clinical significance. Oxygen saturation is a key parameter in assessing respiratory and circulatory function, guiding oxygen therapy and monitoring in settings ranging from emergency care to anesthesia and critical care. Oxygen content depends on both saturation and hemoglobin concentration, described by CaO2 ≈ (1.34 × Hb × SaO2) + (0.003 × PaO2).