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Principles of Pulse Oximetry
FIG. 1
Introduction
Pulse oximeters provide a spectrophotometric assessment of functional arterial hemoglobin oxigenation (SpO2). Pulse Oximetry is based hemoglobin (Hb) and oxygenated hemoglobin (HbO2) differ in their absorption of red and infrared light. Second, the volume of arterial blood in tissue (and therefore light absorption by the hemoglobin) changes during the pulse. A pulse oximeter passes red and infrared light into Photo Diodes an arteriolar bed, measures changes in light FIG. 2
How Pulse Oximeters Work
Pulse oximeter sensors have red and infrared low voltage
light emitting diodes (LEDs) wich serve as light sources. The emitted light is transmitted through the tissue, then detected by the photodetector and sent to the microprocessor of the pulse oximeter (Figure 1) All constituents of the human body, venous and arterial blood, and tissue absorb light (Figure 2). The pulsating of arterial blood results in changes Absorptionin the absorption to to added hemoglobin (Hb) and oxygenated hemoglobin (HbO2) in the path of the light. Since HbO2 and Hb absorb light to varying degrees, this varying absorption is translated into plethysmographic waveforms at both red and infrared wavelengths (Figure 3). The relationship of red and infrared plethysmographic signal amplitude can be directly related to arterial oxygen saturation. For example, when the plethysmographic amplitude ad 660nm and 910nm are equal and the ratio R/IR=1, the SpO2 is approximately 85% (Figure 4).
FIG. 3
FIG. 4
Calibration of Pulse Oximeters
red and infrared wavelengths are tightly Validation of Accuracy
Mediaid Inc. pulse oximeters and sensors are tested for accuracy at the Anesthesia Research
Laboratory at the University of California Medical Center in San Francisco. Validation consists
of inducing hypoxemia in healthy subjects and comparing pulse oximeter readings (SpO2) using
arterial samples. Figure 5 & 6 compare results from a typical Mediaid pulse oximeter and a
Nellcor N-200. Both instruments show a small bias and similar distribution of sampling points.
FIG. 5
FIG. 6
Linear Fit
Linear Fit
Summary of Fit
Summary of Fit
Clinical use of Pulse Oximetry
Pulse oximeters may be used in a variety of situations that call for monitoring oxygenation and
pulse rates. Pulse oximeters increase patient safety by alerting the hospital staff to the onset of
h y p o x i a d u r i n g o r f o l l o w i n g s u r g e r y. O x i m e t e r s c o n f i r m a d e q u a t e o x y g e n a t i o n d u r i n g
mechanical ventilation. Physician and dental offices utilize pulse oximetryfor spot checking
respiratory status, as well as for monitoring during procedures that call for sedation. Truly, pulse
oximetry is the fifth vital sign, essential to complete patient monitoring.
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