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

List:

· 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

Components

· 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

Components

· 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:

Method

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

Intrinsic:

· 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

Probe:

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

· Electrical interference: esp diathermy

Patient

· 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