Mallinckrodt NPB-190 Návod k obsluze stáhnout pdf (Strana 65)

APPENDIX A: PRINCIPLES OF OPERATION

Oximetry Overview

OXIMETRY OVERVIEW

Pulse oximetry is based on two principles: that oxyhemoglobin

and deoxyhemoglobin differ in their absorption of red and

infrared light (i.e., spectrophotometry), and that the volume of

arterial blood in tissue (and hence, light absorption by that blood)

changes during the pulse (i.e., plethysmography). A pulse

oximeter determines SpO

2 by passing red and infrared light into

an arteriolar bed and measuring changes in light absorption

during the pulsatile cycle. Red and infrared low-voltage light-

emitting diodes (LEDs) in the oximetry sensor serve as light

sources; a photodiode serves as the photo detector.

Because oxyhemoglobin and deoxyhemoglobin differ in light

absorption, the amount of red and infrared light absorbed by

blood is related to hemoglobin oxygen saturation. To identify the

oxygen saturation of arterial hemoglobin, the monitor uses the

pulsatile nature of arterial flow. During systole, a new pulse of

arterial blood enters the vascular bed, and blood volume and light

absorption increase. During diastole, blood volume and light

absorption reach their lowest point. The monitor bases its SpO

measurements on the difference between maximum and minimum

absorption (i.e., measurements at systole and diastole). By doing

so, it focuses on light absorption by pulsatile arterial blood,

eliminating the effects of nonpulsatile absorbers such as tissue,

bone, and venous blood.

Automatic Calibration

Because light absorption by hemoglobin is wavelength dependent

and because the mean wavelength of LEDs varies, an oximeter

must know the mean wavelength of the sensor’s red LED to

accurately measure SpO

2. During manufacturing, the mean

wavelength of the red LED is encoded in a resistor in the sensor.

During monitoring, the instrument’s software reads this resistor

and selects coefficients that are appropriate for the wavelength of

that sensor’s red LED; these coefficients are then used to

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Mallinckrodt NPB-190 Návod k obsluze Strana 65