Theory of Operation
47
Operating Ins
tructions V
amos, 1
. US ed.
Measuring CO2 and Volatile Anesthetics
Measuring SpO2
Theory of Operation
Measuring CO
2
and Volatile Anesthetics
CO
2
and volatile anesthetics absorb light in the infared
spectrum. By means of a sample pump, a small quantity of the
breathing gas is drawn through a measuring vessel, which is
penetrated by infrared light. With the aid of various filters,
frequency bands are chosen in which only one of the gases to
be tested absorbs light. The optical characteristics of the
sensor guarantees a quasi-continuous measurement of all
gases. Light absorption thus serves as a measurement of gas
concentration in the vessel. By simultaneously measuring
temperature and absolute pressure in the vessel, the
anesthetic gas concentration of the breathing gas can be
calculated.
Cross sensitivity of gas measurement:
Vapors of organic substances in the ambient air, sample
gas hose or T-piece, (such as are present in detergents or
disinfectants), can distort the measurement of the
concentration of volatile anesthetics.
Measuring SpO
2
Oxygenated, arterial blood (HbO
2
) shows different light
absorption properties to when compared to unsaturated,
venous blood (with reduced hemoglobin Hb).
The SpO
2
sensor consists of two light-emitting diodes, which
alternately send out infrared light at wavelenghts of 920 nm
and 660 nm. A photo detector facing the light source
measures radiation intensity. The sensor is placed on a part of
the body, where light can penetrate arterial blood vessels, e.g.
in a finger, toe, accross the bridge of the nose.
The two particular wavelengths were selected, because they
allow to have usable absorption values even with less
perfusion for both oxygenated and reduced blood. At the same
time the absorption characteristics at those wavelenghts differ
significantly for the two types of hemoglobin.
Total absorption of the light alternately emitted by the diodes is
determined by the pulsating arterial blood, the skin,
fingernails, muscle tissue, bones, and venous blood. As
opposed to the pulsating arterial blood, however, the
proportion of absorption of the other components is constant
during a defined unit of time both with regard to quantity and
optical density.
The arterial blood, which pulsates with every heartbeat,
causes a change of volume in the tissue penetrated in
synchrony with pulse and consequently a change in the
absorption of the light sent through it, again in synchrony with
pulse.
Light absorption is determined first when no pulsating blood is
present (diastole). This measurement indicates the amount of
light absorbed by the tissue and by the non-pulsating blood.
Normally the amount absorbed does not change during the
pulse phase. It represents the reference for the pulsating part
of the absorption.
Absorption is then measured after the next heartbeat, when
the pulsating blood enters the tissue. In this measurement the
light absorption of both wavelengths changes due to the
pulsating arterial blood. Whereas at 660 nm absorption and
also the amplitude of the pulse decreases with increasing
oxygen saturation, at 920 nm it increases. Since the
absorption coefficients of HbO
2
and Hb are known for both
wavelengths, the device calculates the amount of both types of
hemoglobin. The quotient of oxygenated hemoglobin (HbO
2
)
and the sum total from oxygenated and reduced hemoglobin is
termed functional saturation and refers to the hemoglobin
capable of transporting oxygen. Higher concentrations of
dyshemoglobin, HbCO and MetHb, that can normally be
ignored at low levels, can however influence measurement
accuracy.
Accuracy Limitations
Due to the response time of the sensor and the gas sample
flow rate, the stated accuracy of CO
2
, N
2
O and volatile
anesthetics is limited by respiratory rate and inspiratory to
expiratory (I/E) ratio settings.
For CO
2
measurement, the stated accuracy of the Vamos is
maintained to a respiratory rate of 40 BPM with an I/E ratio of
1:1 and to a respiratoy rate of 75 BMP with an I/E ratio of 1:2.
For N
2
O and volatile anesthetic gas measurement, the stated
accuracy of the Vamos is maintained to a respiratory rate of
75 BPM with an I/E ratio of 1:1 or 1:2.
The effects of respiratory rate and I/E ratio settings on
accuracy were determined in a simulated breathing system
using square gas concentration waveforms.
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