Nellcor OxiMax N-550, Service Manual

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Technical Discussion
100
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
integrated circuit in the sensor.
During monitoring, the N-550's software reads this integrated circuit and
selects coefficients that are appropriate for the wavelength of that individual
sensor's red LED; these coefficients are then used to determine SpO
2
. This
integrated circuit is read when the N-550 is turned on, periodically thereafter,
and each time a new sensor is connected.
Additionally, to compensate for differences in tissue thickness, the light
intensity of the sensor's LEDs is adjusted automatically.
Functional versus Fractional Saturation
This N-550 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:
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 SpO
2
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 pH, temperature, the partial pressure of carbon dioxide
(PCO
2
), 2,3-DPG, and fetal hemoglobin. See Figure 44.
functional saturation =
fractional saturation
100 - (% carboxyhemoglobin + % methemoglobin )
x100