PaO2SpO2

PaO2 and SpO2 are two key measures of arterial oxygenation. PaO2, the partial pressure of oxygen in arterial blood, reflects the amount of dissolved oxygen available to tissues and is obtained from an arterial blood gas test. SpO2, the peripheral oxygen saturation, is the percentage of hemoglobin binding sites occupied by oxygen as estimated noninvasively by pulse oximetry. The term PaO2SpO2 is not a single standardized metric, but it refers to the relationship between these two values: how well the oxygen dissolved in blood (PaO2) aligns with the oxygen carried by hemoglobin as indicated by SpO2.

The connection between PaO2 and SpO2 is described by the oxygen-hemoglobin dissociation curve, a sigmoidal relationship that is nonlinear. In general, an SpO2 around 90% corresponds to a PaO2 of about 60 mmHg under typical conditions, while SpO2 in the high 90s usually maps to PaO2 well above 60 mmHg. However, the exact mapping shifts with factors such as pH, temperature, PaCO2, 2,3-DPG, and other conditions that alter hemoglobin’s affinity for oxygen. Therefore, small changes in PaO2 may produce modest changes in SpO2 at higher saturations, and larger changes in SpO2 can occur with small PaO2 shifts at lower saturations.

Measurement and clinical use: PaO2 is obtained from arterial blood gas analysis, while SpO2 is monitored continuously with a pulse oximeter. SpO2 provides a noninvasive, convenient estimate of oxygenation and is often used to titrate supplemental oxygen, with ABG data used for confirmation or precise assessment. Limitations include inaccuracies from poor perfusion, motion, nail polish, ambient light, and dyshemoglobinemias (e.g., methemoglobinemia, carboxyhemoglobinemia) that can skew SpO2 readings.