Principles of Measurement and Parameters
Operator´s Manual PiCCO
2
Page
Version 3.1
B-9
5
Pulse oximetry and pulse dye densitometry
5.1
Principle of pulse oximetry
Pulse oximetry measures the percentage oxygen saturation of the haemoglobin. The principle of pulse
oximetry is based on the transmission and absorption of light waves in the visible and near-infrared spectrum
by haemoglobin. Light of different wavelengths is sent through the tissue non-invasively and is subsequently
detected by a sensor. The received light signal is used to determine the oxygen saturation. This measurement
is entirely non-invasive.
Pulse oximetry is used to measure the partial oxygen saturation of haemoglobin i.e. only oxygenated and
deoxygenated haemoglobin is included. However, not included is e.g. carboxyhaemoglobin or
methaemoglobin.
WARNING:
External influences: carboxyhaemoglobin can result in incorrect high values for SpO
2
.
The degree of elevation is approximately equal to the amount of carboxyhaemoglobin present.
Dye (e.g. Indocyanine Green) or other substances which contain dyes which usually modify the
light absorption capacities, can lead to faulty measurement values of the oxygen saturation.
WARNING:
The measurement of SpO
2
is not recommended for patients weighing less than
20 kg (44lbs).
5.2
Principle of pulse dye densitometry
The principle of pulse dye densitometry and with it, the measurement of the plasma disappearance rate of
Indocyanine Green (ICG) is based on pulse oximetry. The difference is that other light wavelengths are used
than with pulse oximetry.
Figure 10: Absorption spectra of O
2
Hb, Hb and ICG (According to Britton Chance, University of Pennsylvania)
