Datex-Ohmeda CAPNOMAC ULTIMA, Руководство по обслуживанию

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CAPNOMAC ULTIMA
TM
SERVICE MANUAL
4-10
4.4.3
Principle of SpO
2
measurement
S p O
2
Oxygen is the most acutely necessary substrate for survival.
A
major
concern during and after anesthesia is prevention of tissue hypoxia.
Pulse Oximetry provides immediate and direct information on tissue
oxygenation and, therefore, it is at present seen as a prerequisite of
patient safety in anaesthesia departments.
Most of oxygen needed by the body is transported bound to
hemoglobin. The total hemoglobin of blood is composed of oxygenated
oxyhemoglobin (HbO
2
), reduced or deoxygenated hemoglobin (Hb),
and other forms of hemoglobin such as carboxyhemoglobin (HbCO)
and methemoglobin (MetHb).
The absorption of light of normal human blood at different
wavelengths is mainly determined by HbO
2
and Hb (see Figure 4.2). A
Pulse Oximeter measures the relative absorption of light of blood at
two wavelengths, one in the near infrared (about 900 nm) and the
other in the red region (about 660 nm) of light spectrum. These
wavelengths are emitted by LEDs in the SpO
2
probe, the light is
transmitted through peripheral tissue and is finally detected by a PIN-
diode opposite to LEDs in the probe. Pulse Oximeter derives the
oxygen saturation SpO
2
using empirically determined relationship
between the relative absorption at the two wavelengths and the arterial
oxygen saturation
SaO
2
.
The total relative absorption of blood can be divided into components
of tissue, venous blood, arterial blood, and the pulse added volume of
arterial blood.
In order to focus the measurement on the arterial blood and thus to
measure the arterial saturation accurately, Pulse Oximeters use the
component of light absorption giving variations synchronous with heart
beat as primary information on the arterial saturation. In fact, this
invention was most essential for Pulse Oximetry and eventually made
feasible the measurement of oxygen saturation noninvasively.
A general limitation of the above pulse oximetry principle is that due
to only two wavelengths used only two hemoglobin species can be
discriminated by the measurement.
The modern Pulse Oximeters are empirically calibrated either against
fractional saturation
SaO
2
frac,
SaO
2
frac: = HbO
2
/(HbO
2
+Hb+Dyshemoglobin),
or against functional saturation SaO
2func
,
January 15th, 1997/6