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Pulse Oximeter

By Ruby Watson,2014-12-11 14:50
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Pulse Oximeter

Pulse Oximeter

22/11/10

    FANZCA Part I Notes

    FANZCA Part II Notes

    USES measurement of oxygen-haemoglobin saturation (SaO2)

DESCRIPTION

(1) Diodes

- within the probes produce light of the required wavelengths, usually in the red or infra-red

    range as absorbance by body tissue is small.

    - emitted light may alternate between wavelengths at several hundred Hz.

(2) Photodetector

- on opposite side of probe

    - detects transmitted light

(3) Signal converter

- converted to a DC component = tissue background, venous blood & the constant part of

    arterial blood flow.

    - converted to a AC component = pulsatile arterial blood flow. - former disregarded

    - latter amplified & averaged over a few seconds

(4) Display unit

    - signal is displayed ideally as a continuous trace - shows quality of signal & numbered value of SpO2

    Jeremy Fernando (2011)

METHOD OF INSERTION AND/OR USE

- theory = pulsatile blood absorbance of IR or red light changes with regard to degree of

    oxygenation

    - 2 wavelengths of light - red (660nm) & infrared (930nm) - signal is divided into two components

(1) ac = pulsatile arterial blood

    (2) dc = tissue + capillary blood + venous blood + non-pulsatile arterial blood

-> all pulse oximeters assume that only the pulsatile absorbance is arterial blood

    - for each wavelength, the oximeter determines the ac/dc ratio - the ratio (R) of these is calculated:

    R

    =

     (ac absorbance/dc absorbance) red

    /

     (ac absorbance/dc absorbance )IR

R corresponds to SaO2

SaO2 R

100% 0.4

    85% 1

    0% 3.4

OTHER INFORMATION

Limitations

    1. SpO2 does not indicate oxygenation (unless [Hb] & Q known 2. insenstive to directional changes in PaO2 above 80mmHg 3. relatively insensitive to perfusion -> due to gain 4. reading failure

Sources of Error

- motion artefact

    - signal to noise ratio (shocked, hypothermia, vasoconstrictors) - light artefact

    - dysHbaemias (COHb indistinguishable from HbO2, Met Hb absorbance high -> R = 1.0)

    - anaemia

    - intravenous dyes

    - pigmented skin

    - nail polish

    - abnormal pulses -> venous waves & ventilation - probe position

    - low saturations (progressive inaccuracy below 80%) - non-pulsatile flow (bypass)

    - ambient light

    - radiofrequency interference (MRI)

    Jeremy Fernando (2011)

COMPLICATIONS

    Causes of HIGH Co-oximetery and LOW Pulse Oximetry Readings

Poor peripheral perfusion

    Ambient light

    Poor probe contact

    Dyes methylene blue

    TR

    Causes of LOW Co-oximetry and HIGH Pulse Oximetry Readings

COHb

    MetHb

    Radiofrequency interference

    Jeremy Fernando (2011)

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