Pulse oximeter probe-off detector

DETAILED DESCRIPTION To compute peripheral arterial oxygen saturation, denoted SP.
sub.
a O.
sub.
2, pulse oximetry relies on the differential light absorption of oxygenated hemoglobin, HbO.
sub.
2, and deoxygenated hemoglobin, Hb, to compute their respective concentrations in the arterial blood.
This differential absorption is measured at the red and infrared wavelengths of the sensor.
In addition, pulse oximetry relies on the pulsatile nature of arterial blood to differentiate hemoglobin absorption from absorption of other constituents in the surrounding tissues.
Light absorption between systole and diastole varies due to the blood volume change from the inflow and outflow of arterial blood at a peripheral tissue site.
This tissue site might also comprise skin, muscle, bone, venous blood, fat, pigment, etc.
, each of which absorbs light.
It is assumed that the background absorption due to these surrounding tissues is invariant and can be ignored.
Accordingly, blood oxygen saturation measurements are based upon a ratio of the time-varying or AC portion of the detected red and infrared signals with respect to the time-invariant or DC portion.
This AC/DC ratio normalizes the signals and accounts for variations in light pathlengths through the measured tissue.
FIG.
1 illustrates the typical operating characteristics of a pulse oximeter.
During a calibration phase, the pulse oximeter input gain is adjusted higher to accommodate opaque skin and lower to accommodate translucent skin at the sensor site.
Variations in blood perfusion at the sensor site result in variations in input signal strength.
The graph 100 shows acceptable input sensitivity as a function of gain.
The y-axis 110 represents the signal strength (SS), which is the ratio of the peak-to-peak AC signal to the DC signal, expressed as a percentage.
The x-axis 120 represents the gain, which is shown with decreasing values along the x-axis.
The graph 100 has an unshaded region 130 representing the acceptable operating range of the pulse oximeter and a shaded region 140 representing conditions outside that operating range, which, when detected, will result in a pulse oximeter "probe off" alarm