2019A15 Explain the reasons why a pulse oximeter may give incorrect readings.

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: HbO₂ > HHb

·   660nm: HbO₂ < 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%

Intrinsic

·  Low extreme of range

o 70-100% ±2

o 50-70% ±3

o <50% inaccurate

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

·  Lag time for problem:

o e.g. airway obstruction -> delay between problem and detection

·  Unable to discern dyshaemoglobins due to number of LEDs

o HbCO: tends toward 96%

o MetHb: tends towards 85%

·  May be inaccurate in arrhythmias (variation in pulse amplitude)

Probe

·  Physical interference:

o Excessive movement

o Ambient light

o Nail varnish

·  Electrical interference: especially diathermy

Patient

·  Inadequate signal:

o ↓Cardiac output

o Vasoconstriction

o Hypothermia

·  Venous pulsation confused for arterial:

o ↑CVP

o Severe TR