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

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%

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%

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