2004B11 Explain how a pulse oximeter works. Do NOT discuss the potential sources of error.

· Physics

· Device

Physics:

Beer-Lambert principle

· Absorption ∝ concentration of the absorbing substance in the medium (Beer’s law)

· Absorption ∝ distance travelled through the medium (Lambert’s law)

· Describes the basis for, but not the functioning of, the pulse oximeter

Known absorption spectra

· 940nm: O2Hb > HHb

· 660nm: O2Hb < HHb

Components

· Probe on the finger, nose, ear

· Two monochromatic LEDs on one side, rapid on and off

· Photodetector on the other side

· Electrical cable

· Processor

· Display: SpO₂ and plethysmograph

Function

(1)Pulsatile signal isolated

· Excludes venous and tissue absorption

(2)Ratio of absorbances calculated

· Ratio = (pulsatile₆₆₀ / non-pulsatile₆₆₀) / (pulsatile₉₄₀ / non-pulsatile₉₄₀)

(3)Ratio correlated with SaO₂ derived from experiments

· R 0.4 -> 100%

· R 1 -> 85%

· R 2 -> 50%