Cerebral Perfusion Monitor

• US 20080275352A1
• Filed: 12/05/2005
• Published: 11/06/2008
• Est. Priority Date: 01/15/2002
• Status: Active Grant

• Current Assignee: Orsan Medical Technologies, Ltd.
• Original Assignee: Orsan Medical Technologies, Ltd.
• Inventors: Rappaport, Alon, Shapira, Aharon, Ben-Ari, Shlomi, Reichman, Yosef, Barnea, Ofer
• Granted Patent: US 8,187,197 B2
• Time in Patent Office: Days
• US Class Current: 600/506
• CPC Class Codes: A61B 2562/164 the sensor is mounted in or on a conformable substrate or carrier
• CPC Class Codes: A61B 5/0261 using optical means, e.g. infrared light
• CPC Class Codes: A61B 5/0265 using electromagnetic means, e.g. electromagnetic flowmeter
• CPC Class Codes: A61B 5/0295 using plethysmography, i.e. measuring the variations in the volume of a body part as modified by the circulation of blood therethrough, e.g. impedance plethysmography
• CPC Class Codes: A61B 5/0535 Impedance plethysmography for measuring blood flow A61B5/0295
• CPC Class Codes: A61B 5/245 specially adapted for magnetoencephalographic [MEG] signals
• CPC Class Codes: A61B 5/6814 Head
• CPC Class Codes: A61B 5/6817 Ear canal

Abstract

A method of estimating cerebral blood flow includes obtaining a measure of time-varying blood volume in the head, using an impedance plethysmography (102 and 104), obtaining a measure of time-varying blood volume in the scalp, and using the time-varying blood volume in the head and scalp to estimate cerebral blood flow.

Claims

1. A method of estimating cerebral blood flow, comprising:

a) obtaining a measure of time-varying blood volume in the head, using impedance plethysmography;

b) obtaining a measure of time-varying blood volume in the scalp; and

c) using the measure of time-varying blood volume in the head and time-varying blood volume in the scalp to estimate the cerebral blood flow.

2. A method according to claim 1, wherein obtaining a measure of time-varying blood volume in the scalp comprises using photoplethysmography.

3. A method according to claim 1, wherein estimating the cerebral blood flow comprises estimating the relative cerebral blood flow as it changes over time.

4. A method according to claim 1, wherein using the measures of time-varying blood volume comprises finding a difference between weighted measures of time-varying blood volume.

5. A method according to claim 4, wherein the measures of time-varying blood volume are weighted to have at least approximately the same value at a time in the cardiac cycle when the blood pressure is falling.

6. A method according to claim 4, wherein the measures of time-varying blood volume are weighted to have approximately equal power spectra at frequencies for which the cross-power spectrum between the measures of time-varying blood volume is relatively high.

7. A method according to claim 1, wherein obtaining a measure of blood volume in the head using impedance plethysmography comprises:

a) passing a current through the head using two current-carrying electrodes; and

b) measuring a voltage across the head, associated with the current, using two voltage-measuring electrodes.

8. A method according to claim 7, also including applying to the head an annular electrode surrounding at least one of the current-carrying electrodes, and maintaining the annular electrode at a same voltage as the current-carrying electrode it surrounds, thereby suppressing radial current from said current-carrying electrode.

9. A method according to claim 7, wherein the voltage-measuring electrodes are distinct from, and substantially electrically decoupled from, the current-carrying electrodes.

10. A method according to claim 7, wherein obtaining a measure of blood volume in the head using impedance plethysmography comprises placing the two current-carrying electrodes on the left and right temples respectively.

11. A method according to claim 7, wherein obtaining a measure of blood volume in the head using impedance plethysmography comprises placing each of the two voltage-measuring electrodes on the head in a position adjacent to a different one of the current-carrying electrodes.

12. A method according to claim 11, wherein obtaining a measure of blood volume in the scalp using photoplethysmography comprises placing a photoplethysmography sensor on the head adjacent to one of the current-carrying electrodes and to the voltage-measuring electrode which is adjacent to said current-carrying electrode.

13. A method of estimating cerebral blood flow, comprising:

a) measuring an impedance across the head as a function of time in a cardiac cycle; and

b) estimating the cerebral blood flow from a rate of change of the impedance during a time in the cardiac cycle when the blood pressure is rising.

14. A unit for estimating cerebral blood flow, adapted for placing on the head, the unit comprising:

a) at least one electrode adapted to pass current through the head for impedance plethysmography, and adapted to be placed on the head so that a significant part of the current goes through the brain; and

b) a plethysmography sensor adapted for measuring blood flow in a scalp.

15. A unit according to claim 14, wherein said plethysmography sensor adapted for measuring blood flow in a scalp comprises a photoplethysmography sensor.

16. A unit according to claim 15, also comprising a signal processor configured to process one or both of data from the photoplethysmography sensor and impedance plethysomography data from the electrode.

17. A unit according to claim 15, wherein the at least one electrodes comprise:

a) a current-carrying electrode adapted for injecting current through the head when it is placed on the skin; and

b) a voltage-measuring electrode adapted for measuring voltage across the head when it is placed on the skin, and when the current-carrying electrode is injecting current.

18. A unit according to claim 17, wherein the current-carrying and voltage-measuring electrodes are configured such that the voltage measuring electrode will measure a potential substantially equal to a potential at the dermis, largely excluding the voltage drop across the epidermis, when the current-carrying electrode is injecting current.

19. A unit according to claim 18, adapted for use in patients of a range of degree of maturity, wherein the current-carrying electrode comprises an annulus surrounding the voltage-measuring electrode, and the radial thickness of the annulus and the gap between the current-carrying and voltage-measuring electrodes are each at least twice as great as a typical thickness of the epidermis in patients of said range of degree of maturity.

20. A unit according to claim 19, wherein the radial thickness of the annulus and the gap between the current-carrying and voltage-measuring electrodes are each at least 1 mm.

21. A unit according to claim 20, wherein the radial thickness of the annulus and the gap between the current-carrying and voltage-measuring electrodes are each at least 2 mm.

22. A unit according to claim 17, also including an annular electrode surrounding the current-carrying electrode, thereby suppressing radial current from the current-carrying electrode when the annular electrode is maintained at the same voltage as the current-carrying electrode.

23. A system for estimating cerebral blood flow, comprising:

a) at least one unit according to claim 15;

b) an impedance measuring unit comprising at least one electrode adapted for placing on the head and performing impedance plethysmography;

c) a power supply adapted for passing current across the head between one of the at least one electrodes of the one unit and one of the at least one electrodes of the impedance measuring unit, when said units are placed on different sides of the head; and

d) a data analyzer which calculates a cerebral blood flow using impedance data obtained from a voltage difference measured between one of the at least one electrodes of the one unit and one of the at least one electrodes of the impedance measuring unit, and from photoplethysmography data generated by the photoplethysmography sensor.

24. A system according to claim 23, wherein the impedance measuring unit is also a unit according to claim 15.