Covidien Nellcor OxiMax N-600x, Operator'S Manual

Page: 121 / 146

Operator’s Manual
115
10 Theory of Operations
Overview
This section explains the theory behind OxiMax N-600x™ pulse oximeter operations.
Understanding Pulse Oximetry
Theoretical Principles
The OxiMax N-600x pulse oximeter uses pulse oximetry to measure functional
oxygen saturation in the blood. Pulse oximetry works by applying an OxiMax™ pulse
oximetry sensor to a pulsating arteriolar vascular bed, such as a finger or toe. The
sensor contains a dual light source and a photodetector.
Bone, tissue, pigmentation, and venous vessels normally absorb a constant amount
of light over time. The arteriolar bed normally pulsates and absorbs variable
amounts of light during the pulsations. The ratio of light absorbed is translated into a
measurement of functional oxygen saturation (SpO
2
).
Ambient conditions, sensor application, and patient conditions can influence the
ability of the oximeter to accurately measure SpO
2.
Reference Performance
Considerations , page 95.
Pulse oximetry is based on two principles: oxyhemoglobin and deoxyhemoglobin
differ in their absorption of red and infrared light (measured using
spectrophotometry), and the volume of arterial blood in tissue (and hence, light
absorption by that blood) changes during the pulse (registered using
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 (LED) in the
sensor serve as light sources; a photo diode serves as the photo detector.
Since oxyhemoglobin and deoxyhemoglobin differ in light absorption, the amount
of red and infrared light absorbed by blood is related to hemoglobin oxygen
saturation.
The oximeter uses the pulsatile nature of arterial flow to identify the oxygen
saturation of arterial hemoglobin. 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 pulse oximeter
bases its SpO
2
measurements on the difference between maximum and minimum
absorption (measurements at systole and diastole). By doing so, it focuses on light