Automatic Calibration; Functional Versus Fractional Saturation - Nellcor N-200 Operator's Manual

Principles of Operation
The delay between the electrical ECG pulse and the optical
pulse at the sensor site is relatively stable for a given patient
and sensor site. C-LOCKprocessing takes advantage of this
temporal relationship, using the QRS complex as a reference
point for identifying the oximetry pulse and for timing SpO
measurements. This enhances "good" pulses and minimizes the
effect of random artifacts associated with motion and low
perfusion.

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
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 determine SpO
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, laboratory hemoximeters 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 =
12
. This resistor is read when the monitor
2
fractional saturation
100 - (% carboxyhemoglobin + % methemoglobin)
. During manufacturing,
2
2
x100