Mallinckrodt NPB-190 Manual do Operador descarregar pdf (Página 74)

Appendix B: Principles of Operation

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

determine SpO

2. This resistor is read when the monitor is turned

on, periodically thereafter, and each time a new sensor is

connected.

Additionally, to compensate for differences in tissue thickness,

the intensity of the sensor’s LEDs are adjusted automatically.

Functional versus Fractional Saturation

This monitor measures functional saturation — oxygenated

hemoglobin expressed as a percentage of the hemoglobin that can

transport oxygen

. It does not detect significant amounts of

dysfunctional hemoglobin, such as carboxyhemoglobin or

methemoglobin

. In contrast, hemoximeters such as the IL482

report fractional saturation — oxygenated hemoglobin expressed

as a percentage of all measured hemoglobin, including measured

dysfunctional hemoglobins

. To compare functional saturation

measurements to those from an instrument that measures

fractional saturation, fractional measurements must be converted

as follows:

functional saturation =

fractional saturation

100 - (% carboxyhemoglobin + % methemoglobin )

x100

Measured versus Calculated Saturation

When saturation is calculated from a blood gas partial pressure of

oxygen (PO

2), the calculated value may differ from the SpO2

measurement of a pulse oximeter

. This usually occurs because the

calculated saturation was not appropriately corrected for the

effects of variables that shift the relationship between PO

2 and

saturation (Figure 13): pH, temperature, the partial pressure of

carbon dioxide (PCO

2), 2,3-DPG, and fetal hemoglobin.

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