2004B11 Briefly explain how oximetry can be used to estimate the partial pressure of oxygen in a blood sample.



·      Pulse oximetry to estimate saturation: device, physics, how it works

·      Co-oximetry to measures saturation: device, how it works

·      SpO2 to PaO2 conversion: how to do it, sources of error


Pulse oximetry: estimates saturation


·   Probe on the finger, nose, ear

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

·   Photodetector on the other side

·   Cable to processor/display

Physical basis

(1)Beer-Lambert principle: I = I0e-ax

·   Aborption 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

(2)Known absorption spectra:

·   940nm: O2Hb > HHb

·   660nm: O2Hb < HHb

How it works

(1)Pulsatile signal isolated

·   Exclude 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%


Co-oximetry: measures saturation


·   Haemolyser unit (uses high frequency vibration)

·   Multi-wavelength photo lamp

·   Filter

·   Lens

·   Sensing diodes

How it works

·   Absorption spectra known for HbO2, HHb, HbCO, SulfHb

·   Multiple wavelengths emitted

·   Degree and ratio of absorbances assessed

·   % Saturation calculated: [HbO2] / [total Hb] x 100%


Conversion of saturation to partial pressure:


SpO2 ≈ SaO2

SaO2 -> PaO2 via oxyhaemoglobin dissociation curve

Sources of error: extrapolation

Bohr effect (↓pH, ↑PaCO2, ↑temp, ↑2,3-DPG -> right shift)

Very high PaO2 (SpO2 cannot exceed 100%)

Sources of error: pulse oximetry


·   Accuracy: 70-100% +/-2, 50-70% +/-3, <50% inaccurate

·   Lag time for signal: signal averaged over 10-20 seconds

·   Lag time for problem: e.g. airway obstruction -> delay between problem and detection

·   Unable to discern dyshaemoglobins due to only 2 LEDs

·   HbCO: absorbance tends toward 96% (see graph)

·   MetHb: absorbance tends towards 85% (see graph)

·   May be inaccurate in arrhythmia


·   Physical interference: excessive movement or ambient light, nail varnish

·   Electrical interference: esp diathermy


·   No signal at all: low cardiac output, vasoconstriction, hypothermia

·   Falsely low signal: high venous pressure -> detection of venous pulsation, ICG

·   Falsely high signal: HbCO as above is the main cause

·   Variable: arrhythmia e.g. AF



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