Transcript Pulse Oximetry - Mike Poullis - Consultant Cardiothoracic

Pulse Oximetry
Non-invasive detection of arterial
blood gases using optical and
colorimetric techniques
Definition
• All atoms and molecules reflect specific
wavelengths of light.
• Spectrophotometry is an optical detection
method that uses light refection properties of
molecules to measure the concentration of
chemical species in a gaseous or liquid
environment.
• When applied to detection of oxygenated and
deoxygenated haemoglobin, method known as
oximetry.
Pattern of light refection by oxygenated haemoglobin
(HbO2) and deoxygenated haemoglobin (Hb)
•
At wavelengths of 660 nm, HbO2 reflects
light more effectively then Hb.
•
Relationship is reversed at 940 nm.
•
Concentrations of HbO2 and Hb are
expressed in relative terms i.e. as the
fraction of haemoglobin that is in the
oxygenated form (% saturation).
•
% Saturation = (HbO2/HbO2 + Hb) X 100
Limitations
• Use of two wavelengths of light to derive
fractional concentration of HBO2 is based on
assumption that other forms of haemoglobin
(metHb) and (COHb) have negligible
contributions to total haemoglobin.
• In most situations, < 5% of haemoglobin is
present as COHb and metHb.
• Increase in COHb or metHb leads to falsely high
estimates of HbO2.
Photodetector senses only light of alternating intensity
Accuracy of Pulse Oximetry Recording
Questions ?