1. Pulse Oximetry Non-invasive detection of arterial blood gases using optical and colorimetric techniques

2. Definition • All atoms and molecules reflect specific wavelengths of light. • Spectrophotometry is an optical detection method that uses light refection properties of molecules to measure the concentration of chemical species in a gaseous or liquid environment. • When applied to detection of oxygenated and deoxygenated haemoglobin, method known as oximetry.

3. Pattern of light refection by oxygenated haemoglobin (HbO2) and deoxygenated haemoglobin (Hb) • At wavelengths of660 nm, HbO2 reflects light more effectively then Hb. • Relationship is reversed at 940 nm. • Concentrations of HbO2 and Hb are expressed in relative terms i.e. as the fraction of haemoglobin that is in the oxygenated form (% saturation). • % Saturation = (HbO2/HbO2 + Hb) X 100

4. Limitations • Use of two wavelengths of light to derive fractional concentration of HBO2 is based on assumption that other forms of haemoglobin (metHb) and (COHb) have negligible contributions to total haemoglobin. • In most situations, < 5% of haemoglobin is present as COHb and metHb. • Increase in COHb or metHb leads to falsely high estimates of HbO2.

5. Photodetector senses only light of alternating intensity

6. Accuracy of Pulse Oximetry Recording

7. Questions ?