Low power pulse oximeter

Details Low power pulse oximeter Low power pulse oximeter

2002-06-26

2004-02-24

Winakur, Eric F. (Department: 3736)

Surgery

Diagnostic testing

Measuring or detecting nonradioactive constituent of body...

C600S336000

Reexamination Certificate

active

06697658

ABSTRACT:

BACKGROUND OF THE INVENTION
Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of a person's arterial blood, an indicator of their oxygen supply. Oxygen saturation monitoring is crucial in critical care and surgical applications, where an insufficient blood supply can quickly lead to injury or death.
FIG. 1
illustrates a conventional pulse oximetry system
100
, which has a sensor
110
and a monitor
150
. The sensor
110
, which can be attached to an adult's finger or an infant's foot, has both red and infrared LEDs
112
and a photodiode detector
114
. For a finger, the sensor is configured so that the LEDs
112
project light through the fingernail and into the blood vessels and capillaries underneath. The photodiode
114
is positioned at the finger tip opposite the fingernail so as to detect the LED emitted light as it emerges from the finger tissues. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled “Low Noise Optical Probe,” which is assigned to the assignee of the present invention and incorporated by reference herein.
Also shown in
FIG. 1
, the monitor
150
has LED drivers
152
, a signal conditioning and digitization front-end
154
, a signal processor
156
, a display driver
158
and a display
159
. The LED drivers
152
alternately activate the red and IR LEDs
112
and the front-end
154
conditions and digitizes the resulting current generated by the photodiode
114
, which is proportional to the intensity of the detected light. The signal processor
156
inputs the conditioned photodiode signal and determines oxygen saturation based on the differential absorption by arterial blood of the two wavelengths emitted by the LEDs
112
. Specifically, a ratio of detected red and infrared intensities is calculated by the signal processor
156
, and an arterial oxygen saturation value is empirically determined based on the ratio obtained. The display driver
158
and associated display
159
indicate a patient's oxygen saturation, heart rate and plethysmographic waveform.
SUMMARY OF THE INVENTION
Increasingly, pulse oximeters are being utilized in portable, battery-operated applications. For example, a pulse oximeter may be attached to a patient during emergency transport and remain with the patient as they are moved between hospital wards. Further, pulse oximeters are often implemented as plug-in modules for multiparameter patient monitors having a restricted power budget. These applications and others create an increasing demand for lower power and higher performance pulse oximeters. A conventional approach for reducing power consumption in portable electronics, typically utilized by devices such as calculators and notebook computers, is to have a “sleep mode” where the circuitry is powered-down when the devices are idle.
FIG. 2
illustrates a sleep-mode pulse oximeter
200
utilizing conventional sleep-mode power reduction. The pulse oximeter
200
has a pulse oximeter processor
210
and a power control
220
. The power control
220
monitors the pulse oximeter output parameters
212
, such as oxygen saturation and pulse rate, and controls the processor power
214
according to measured activity. For example, if there is no significant change in the oxygen saturation value over a certain time period, the power control
220
will power down the processor
210
, except perhaps for a portion of memory. The power control
220
may have a timer that triggers the processor
210
to periodically sample the oxygen saturation value, and the power control
220
determines if any changes in this parameter are occurring. If not, the power control
220
will leave the processor
210
in sleep mode.
There are a number of disadvantages to applying consumer electronic sleep mode techniques to pulse oximetry. By definition, the pulse oximeter is not functioning during sleep mode. Unlike consumer electronics, pulse oximetry cannot afford to miss events, such as patient oxygen desaturation. Further, there is a trade-off between shorter but more frequent sleep periods to avoid a missed event and the increased processing overhead to power-up after each sleep period. Also, sleep mode techniques rely only on the output parameters to determine whether the pulse oximeter should be active or in sleep mode. Finally, the caregiver is given no indication of when the pulse oximeter outputs were last updated.
One aspect of a low power pulse oximeter is a sensor interface adapted to drive a pulse oximetry sensor and receive a corresponding input signal. A processor derives a physiological measurement corresponding to the input signal, and a display driver communicates the measurement to a display. A controller generates a sampling control output to at least one of said sensor interface and said processor so as to reduce the average power consumption of the pulse oximeter consistent with a predetermined power target.
In one embodiment, a calculator derives a signal status output responsive to the input signal. The signal status output is communicated to the controller to override the sampling control output. The signal status output may indicate the occurrence of a low signal quality or the occurrence of a physiological event. In another embodiment, the sensor interface has an emitter driver adapted to provide a current output to an emitter portion of the sensor. Here, the sampling control output determines a duty cycle of the current output. In a particular embodiment, the duty cycle may be in the range of about 3.125% to about 25%.
In another embodiment, the sensor interface has a front-end adapted to receive the input signal from a detector portion of the sensor and to provide a corresponding digitized signal. Here, the sampling control output determines a powered-down period of the front-end. A confidence indicator responsive to a duration of the powered-down period may be provided and displayed.
In yet another embodiment, the pulse oximeter comprises a plurality of data blocks responsive to the input signal, wherein the sampling control output determines a time shift of successive ones of the data blocks. The time shift may vary in the range of about 1.2 seconds to about 4.8 seconds.
An aspect of a low power pulse oximetry method comprises the steps of setting a power target and receiving an input signal from a pulse oximetry sensor. Further steps include calculating signal status related to the input signal, calculating power status related to the power target, and sampling based upon the result of the calculating signal status and the calculating power status steps.
In one embodiment, the calculating signal status step comprises the substeps of receiving a signal statistic related to the input signal, receiving a physiological measurement related to the input signal, determining a low signal quality condition from the signal statistic, determining an event occurrence from the physiological measurement, and indicating an override based upon the low signal quality condition or the event occurrence. The calculating power status step may comprise the substeps of estimating an average power consumption for at least a portion of the pulse oximeter, and indicating an above power target condition when the average power consumption is above the power target. The sampling step may comprise the substep of increasing sampling as the result of the override. The sampling step may also comprise the substep of decreasing sampling as the result of the above power target condition, except during the override.
Another aspect of a low power pulse oximetry method comprises the steps of detecting an override related to a measure of signal quality or a physiological measurement event, increasing the pulse oximeter power to a higher power level when the override exists, and reducing the pulse oximeter power to a lower power level when the override does not exist. The method may comprise the further steps of predetermining a target power level for a pulse oximeter and cycling between the lower power level and the highe

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