7 —1
7 — SpO
2
Theory of Operation
The monitor uses Digital Oximetry (DOX™) technology to measure
blood oxygen saturation (SpO
2
).
Heart Rate
The SpO
2
pulse rate is used if the ECG and the IBP pulse rates are
not available. See “Heart Rate” on page 5-1 for more information.
Definition
Hemoglobin exists in the blood in several forms:
• Oxygenated (Oxyhemoglobin)
• Reduced (Deoxyhemoglobin)
• Dyshemoglobins (carboxyhemoglobin and methemoglobin.)
In the monitor, SpO
2
(pulse arterial oxygen saturation) is the ratio of
oxygenated hemoglobin to the sum of oxygenated hemoglobin plus
hemoglobin which is available for binding to oxygen, as expressed in
the following formula:
Dyshemoglobins, such as carboxyhemoglobin and methemoglobin,
are not directly measured and therefore are not factored into the
measurement.
DOX™ Digital Oximetry
The monitor does not use analog circuitry for signal processing.
Digital signal processing in the microprocessor results in lower noise
from circuitry components, resulting in a cleaner signal and better
performance under low perfusion conditions. There is also improved
rejection of noise from the patient and environment, due to the
availability of the “true,” unfiltered sensor signal for digital signal
processing.
Method
The digital pulse oximeter measures oxygen saturation and pulse rate
using the principles of spectrophotometry and plethysmography. The
sensor is completely non-invasive, and there is no heat source that
could burn the patient.
The pulse oximeter sensor contains two types of LEDs. Each type
emits a specific wavelength of light. Since oxygenated hemoglobin
and deoxygenated hemoglobin absorb light selectively and
predictably, the amounts of these two compounds can be determined
by measuring the intensity of each wavelength that passes through
the measuring site.
percent oxygen saturation
oxyhemoglobin
oxyhemoglobin
deoxyhemoglobin
+
-------------------------------------------------------------------------------------------------------
100
×
=
