Oximeter sensor with encoded temperature characteristic

• US 6,356,774 B1
• Filed: 09/28/1999
• Issued: 03/12/2002
• Est. Priority Date: 09/29/1998
• Status: Expired due to Term

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: Mallinckrodt Incorporated (Mallinckrodt plc)
• Inventors: Fein, Michael E., Chew, Bradford, Shea, William L., Crone, Willem A., Mannheimer, Paul D., Merchant, Adnan, Bernstein, Michael J.
• Primary Examiner(s): Winakur, Eric F.
• Assistant Examiner(s): KREMER, MATTHEW J
• Application Number:
  US09/407,469
• Time in Patent Office: 896 Days
• Field of Search: 600/309-312, 600/316-317, 600/322-329, 600/331-332, 600/334, 600/336, 600/364, 250/205, 250/238, 356/39-42
• US Class Current: 600/323
• CPC Class Codes: A61B 2560/0252 using ambient temperature
• CPC Class Codes: A61B 2562/085 combined with means for recording calibration data
• CPC Class Codes: A61B 5/14552 Details of sensors specially adapted therefor
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes

Abstract

A method for operating an oximeter sensor, and corresponding apparatus, which includes an encoded temperature characteristic of a light emitter in the sensor. The encoded temperature characteristic is read, and is used to modify a drive of the light emitter in the sensor. This enables a light emitter to be operated at its maximum allowable intensity to maximize a signal to noise ratio, without burning a patient, in accordance with the particular characteristics of that light emitter.

Claims

1. A method for operating an optical medical sensor, comprising the steps of:

applying a driving current to a light emitter in said sensor,

encoding in the sensor a forward voltage temperature characteristic of the light emitter,

reading said encoded forward voltage temperature characteristic,

measuring an electrical characteristic of said emitter which is indicative of its junction temperature, and

modifying said driving current in accordance with said electrical characteristic and said encoded temperature characteristic to maintain the junction temperature of said emitter at or near a maximum value.

2. The method of claim 1 further comprising the steps of:

driving said light emitter with a known current;

measuring a resulting voltage across said light emitter; and

adjusting said driving of said light emitter in accordance with a variance of said voltage from a desired voltage, the desired voltage being dependent on the encoded temperature characteristic.

3. The method of claim 2 wherein said measuring step is performed at a lower current than used in normal operation of said sensor.

4. The method of claim 1 wherein said step of modifying said driving current comprises the step of:

estimating a temperature at the skin of a patient based upon said maintained junction temperature; and

modifying said driving current using said estimated skin temperature of said patient to maintain the temperature of the skin of said patient at or near a desired maximum value.

5. The method of claim 4 further comprising the steps of:

estimating a resistance of connections to said sensor from measured forward voltages; and

providing an error signal if said resistance varies by more than a predetermined value.

6. The method of claim 1 further comprising the steps of:

using said encoded temperature characteristic to select a stored temperature calibration curve of said light emitter; and

using said temperature calibration curve in a control system to limit the drive current for said light emitter to a safe value.

7. The method of claim 1 wherein said step of modifying said driving current comprises the steps of:

providing a table of encoded temperature characteristics and corresponding temperature responses for a junction temperature of said light emitter;

providing a maximum drive current for each temperature response, said maximum drive current being based on an assumed heat dissipation of said sensor and a maximum temperature at the skin of a patient.

8. The method of claim 7 further comprising the steps of:

providing in said sensor an encoding of a sensor type; and

reading said encoded sensor type and selecting said assumed heat dissipation of said sensor based on said sensor type.

9. The method of claim 1 wherein said encoding step comprises a calibration process, applied during manufacture of said sensor, of:

placing said sensor in an environment at a first controlled temperature;

applying a predetermined current to said light emitter;

measuring a voltage across said light emitter; and

incorporating in said sensor an encoding means to represent said voltage.

10. The method of claim 9 wherein said encoding means further contains information representing a rate of change of said voltage as a function of temperature.

11. The method of claim 1 wherein said optical medical sensor is provided as a pulse oximeter sensor.

12. A method for operating an optical medical sensor, comprising the steps of:

encoding a forward voltage temperature characteristic of an LED junction in said sensor;

reading said forward voltage temperature characteristic;

driving said LED with a known calibration current different from an operating current used in normal operation of said sensor;

measuring a forward voltage across said LED which is indicative of its junction temperature;

providing a table based upon the read forward voltage temperature characteristics, which table relates forward voltage to junction temperature of said light emitter;

determining a maximum drive current in accordance with said measured forward voltage and said table, the maximum drive current being based on an assumed heat spreading behavior of said sensor and a maximum temperature at the skin of a patient; and

adjusting a driving of said LED in normal operation to be at or substantially at, but not to exceed, said maximum drive current.

13. An optical medical sensor system comprising:

a sensor including

a light emitter and

a coding element indicative of a forward voltage temperature characteristic of said light emitter; and

a monitor including

a driver configured to provide a driving current to said light emitter,

a circuit configured to decode said coding element,

a reader configured to measure an electrical characteristic of said emitter which, together with the decoded element, is indicative of a junction temperature of the light emitter, and

a control circuit configured to modify said driving current of said light emitter in accordance with said coding element and said measured value of said electrical characteristic to maintain the junction temperature of the light emitter at or near a maximum value.

14. The optical medical sensor system of claim 13 wherein the control circuit is configured to estimate a temperature at the skin of a patient based upon said maintained junction temperature and to modify said driving current of said light emitter based upon said estimated skin temperature of said patient to maintain the temperature of the skin of said patient at or near a maximum value.

15. A method for evaluating an optical medical sensor, comprising the steps of:

measuring a voltage across a light emitting element in said sensor for a given drive current while maintaining a switch coupled in parallel with the light emitting element ON; and

estimating a resistance of connections to said sensor from said measured voltage.

16. A method for evaluating an optical medical sensor, comprising the steps of:

providing drive current to a light emitter through first and second leads;

measuring a voltage across said light emitter in said sensor for a given drive current through third and fourth leads; and

estimating a resistance of connections to said sensor from said measured voltage.

17. An optical medical sensor system comprising:

a sensor including

optical elements including a light emitter and a light detector;

a coding element indicative of a forward voltage temperature characteristic of one of said optical elements; and

a monitor including

a driver configured to provide a driving current to said light emitter,

a circuit configured to read said coding element,

a reader configured to measure an electrical signal which is indicative of a junction temperature of said light emitter, and

a control circuit configured to modify said driving current of said light emitter in accordance with said coding element and said electrical signal to maintain the junction temperature of said light emitter at or near a maximum value.

18. The system of claim 17 wherein said maximum value of the junction temperature is based on an assumed heat dissipation of said sensor and a maximum temperature at the skin of a patient.

19. The system of claim 17 further comprising a heater element in said sensor.

20. An optical medical sensor system comprising:

a sensor including

a light emitter and a light detector; and

a monitor including

a driver configured to provide a driving current to said light emitter,

a reader configured to determine a junction temperature of said light emitter and produce a junction temperature signal indicative of said junction temperature, and

a control circuit configured to modify said driving current of said light emitter in accordance with said junction temperature signal to maintain said junction temperature of said light emitter at or near a maximum value that is based on a maximum temperature at the skin of a patient.

21. The system of claim 20 wherein said monitor further includes a perfusion monitor configured to provide, in response to said junction temperature signal, a perfusion signal corresponding to blood perfusion of a portion of a patient where said sensor is connected.

22. The system of claim 20 wherein said monitor further includes:

a table of calibration coefficients; and

a selector configured to select a set of said calibration coefficients at least partially based on a value of said junction temperature signal.

23. The system of claim 20 wherein:

said sensor further includes a coding element indicative of a temperature characteristic of one of said optical elements;

said monitor further includes

a circuit configured to read said coding element, and

a control circuit configured to modify said junction temperature signal in accordance with said coding element.