Respironics 920M Service Manual download pdf (Page 42)

VII. Theory of Operation

The pulse oximeter shines light (red and infrared) through the tissue

and detects the fluctuating signals caused by arterial blood pressure

pulses. Well oxygenated blood is bright red, while poorly

oxygenated blood is dark red. The pulse oximeter determines

oxygen saturation from this color difference by measuring the ratio

of absorbed red and infrared light. Since steady conditions (steady

venous blood flow, skin thickness, bone, finger nails, etc.) do not

cause fluctuations, they do not affect the saturation reading.

Mathematically:

SpO

infrared

(

)

min

max

red

(

)

min

max

Note that anything that affects the intensity of the light such as thick

or colored skin will affect the maximum and minimum proportionally

and thus the ratio max/min does not change. However, if too little

light gets through, the pulse oximeter will not work and will signal

sensor alarm.

CAUTION

• The Respironics 920 Pulse Oximeter is

calibrated to determine the percentage of

arterial oxygen saturation of functional

hemoglobin. Significant levels of dysfunctional

hemoglobin such as carboxyhemoglobin or

methemoglobin may affect the accuracy of the

measurement.

• Cardiogreen and other intravascular dyes,

depending on the concentration, may affect the

accuracy of the SpO

measurement.

Pulse oximeters use two different wavelengths of light (colors) and

thus have the ability to determine one component of blood. The

Respironics 920 Hand Held Pulse Oximeter is calibrated to closely

approximate functional oxygen saturation values. The 920 oxygen

saturation values will closely approximate laboratory instrument

fractional saturation values if the dysfunctional hemoglobin

saturation levels are negligible. If the dysfunctional hemoglobin is

carboxyhemoglobin or methemoglobin, then the difference between

the oxygen saturation value displayed by the 920 and the oxygen

saturation values determined by the laboratory instrument will be

greater as the dysfunctional hemoglobin levels rise approximately in

accordance with the following formulas:

1 2 ... 37 38 39 40 41 42 43 44 45 46 47 ... 51 52

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Respironics 920M Service Manual Page 42

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