Clinical meaning
Pulse oximetry is a non-invasive method for continuously monitoring peripheral oxygen saturation (SpO2) by measuring the absorption of red and infrared light through a pulsatile vascular bed, typically the fingertip. Hemoglobin bound to oxygen (oxyhemoglobin) absorbs more infrared light, while deoxygenated hemoglobin absorbs more red light. The sensor calculates the ratio to determine the percentage of hemoglobin saturated with oxygen. Normal SpO2 for healthy adults is 95-100%; readings below 90% indicate hypoxemia requiring immediate intervention. The oxygen-hemoglobin dissociation curve explains the relationship between oxygen levels in the blood and saturation readings: above PaO2 of 60 mmHg, hemoglobin stays well saturated (SpO2 > 90%), but below 60 mmHg, saturation drops rapidly. This means small decreases in SpO2 below 90% represent large drops in blood oxygen — a critical concept for patient safety. Proper sensor placement is essential: the light source and detector must be aligned across the vascular bed, the site must be well-perfused with pulsatile flow, and nail polish (especially dark colors) should be removed or the sensor placed on the earlobe or toe if fingertip readings are unreliable.