Blood volume monitor

1. A blood volume monitor comprising:

a carrier mountable about a part of a body of a subject, the carrier comprising a sleeve of a resiliently flexible material;

at least two bands carried by the sleeve and circumscribing two parts of the part of the body underlying the sleeve, wherein the at least two bands comprise an electrically resistive material configured to expand and contract in unison with the sleeve, wherein the at least two bands are in fixed positions relative to each other on the sleeve with a known distance between the at least two bands and configured to approximate a truncated cone, wherein the at least two bands are configured to provide output resistivity response signals representative of the volume of the part of the body of the subject underlying the sleeve and circumscribed by the at least two bands and wherein a change in volume of the part of the body of the subject underlying the sleeve is determined by a difference in volume at two time points determined from the output resistivity response signals representative of the volume, and wherein the at least two bands are calibrated to provide volume estimation within a specified tolerance of specific calibration values; and

a controller in communication with the at least two bands, the controller being configured to process data output by the at least two bands to determine, using the volume data provided by the at least two bands and change in resistivity of the at least two bands, the volume of the part of the body of the subject underlying the sleeve and to determine a change in blood volume in the part of the body of the subject, the controller configured to use the output resistivity response signals received from the at least two bands and the determined volume of the part of the body to determine the change in blood volume, wherein the controller contains the specific calibration values as parameters to perform volume measurements.

2. The monitor of claim 1 , wherein the output resistivity response signals represent a blood impedance value of blood in the part of the body of the subject.

3. The monitor of claim 1 , further comprising a plurality of sets of sensors attached to the sleeve and configured to measure changes in impedance in the part of the body of the subject, such changes in impedance being indicative of blood volume changes in the part of the body of the subject, one set of the sensors operating at any one time as stimulators configured to stimulate the part of the body of the subject with at least one of the other sets of sensors sensing response signals evoked in the part of the body of the subject.

4. The monitor of claim 3 , wherein each sensor of the plurality of sets of sensors is an electrode, the electrodes being arranged at circumferentially spaced intervals about the sleeve and each set of electrodes comprising a pair of opposed electrodes.

5. The monitor of claim 4 , wherein one of the pairs of electrodes is configured to be used at any one time as stimulating electrodes and at least one other pair of electrodes, rotated about the carrier relative to the one pair of electrodes, is used as sensing electrodes.

6. The monitor of claim 5 , wherein the controller is operable to cycle through the pairs of electrodes periodically to vary which pair of electrodes is operating as the stimulating electrodes and which pair of electrodes is being used as sensing electrodes.

7. The monitor of claim 6 , wherein the controller is operable to cycle through the sets of electrodes to select the stimulating set of electrodes prior to the stimulating set of electrodes stimulating underlying tissue of the subject.

8. The monitor of claim 3 , wherein a stimulating signal output by the stimulating set of sensors is one of a sine wave signal and a uniform pulse waveform.

9. The monitor of claim 8 , wherein an amplitude of the stimulating signal is selected to be lower than a level at which the subject is aware of the stimulation.

10. The monitor of claim 9 , wherein the controller is operable to vary the amplitude of the stimulating signal to account for changes in at least one of contact impedance and muscle contraction/relaxation.

11. The monitor of claim 1 , which is configured to impart muscular stimulation to muscles of the subject.

12. The monitor of claim 1 , wherein an array is configured to provide both volume data and measure change in volume of the part of the body of the subject underlying the carrier to output the response signals representative of the change in volume.

13. The monitor of claim 1 , comprising a position detector configured to detect a position of the part of the body of the subject.

14. The monitor of claim 1 , comprising a pressure applicator, configured to impart pressure to the part of the body of the subject.

15. A method of monitoring blood volume in a subject, the method comprising:

mounting a blood volume monitor to a body part of the subject, wherein the blood volume monitor comprises:

a carrier mountable about the body part of the subject, the carrier comprising a sleeve of a resiliently flexible material;

at least two bands carried by the sleeve and circumscribing two parts of the part of the body underlying the sleeve, wherein the at least two bands comprise an electrically resistive material configured to expand and contract in unison with the sleeve, wherein the at least two bands are in fixed positions relative to each other on the sleeve with a known distance between the at least two bands and configured to approximate a truncated cone, wherein the at least two bands are configured to provide output resistivity response signals representative of the volume of the body part of the subject underlying the sleeve and circumscribed by the at least two bands and wherein a change in volume of the body part of the subject underlying the sleeve is determined by a difference in volume at two time points determined from the output resistivity response signals representative of the volume, and wherein the at least two bands are calibrated to provide volume estimation within a specified tolerance of specific calibration values; and

a controller in communication with the at least two bands, the controller being configured to process data output by the at least two bands to determine, using the volume data provided by the at least two bands and change in resistivity of the at least two bands, the volume of the body part of the subject underlying the sleeve and to determine a change in blood volume in the body part of the subject, the controller configured to use the output resistivity response signals received from the at least two bands and the determined volume of the body part of the subject to determine the change in blood volume, wherein the controller contains the specific calibration values as parameters to perform volume measurements;

determining an initial volume of the body part of the subject using volume data provided by the at least two bands that define the truncated cone;

outputting response signals representative of a change in volume of the body part of the subject; and

using the response signals to determine a change in volume of blood in the body part of the subject based on the initial volume of the body part of the subject, the sensed change in volume of the body part of the subject and the specific calibration values.

16. The method of claim 15 , wherein the monitor further comprises a plurality of sets of sensors attached to the sleeve and configured to measure changes in impedance in the body part of the subject, such changes in impedance being indicative of blood volume changes in the body part of the subject, a first set of the sensors operating at any one time as stimulators configured to stimulate the body part of the subject and at least a second set of sensors sensing response signals evoked in the body part of the subject, wherein the method comprises sensing the change in volume in the body part of the subject by sensing a change in impedance of blood within the body part of the subject.

17. The method of claim 16 , comprising sensing the changes in impedance by stimulating the body part of the subject with the first set of sensors and monitoring the evoked response with the second set of sensors.

18. The method of claim 17 , comprising cycling through the sets of sensors to vary which set of sensors is functioning as the set of stimulating sensors at any one time.

19. The method of claim 17 , comprising effecting stimulation of the body part of the subject with a stimulating signal comprising one of a sine wave signal and a uniform pulse waveform.

20. The method of claim 19 , comprising selecting an amplitude of the stimulating signal to be lower than a level at which the subject is aware of the stimulation.

21. The method of claim 20 , comprising varying the amplitude of the stimulating signal to account for changes in contact impedance or muscle contraction/relaxation.

22. The method of claim 15 , comprising imparting muscular stimulation to muscles of the subject.

23. The method of claim 15 , comprising detecting a position of the body part of the subject.

24. The method of claim 15 , comprising imparting pressure to the body part of the subject.

25. The method of claim 15 , wherein the volume of the body part of the subject is calculated according to the following formula:

V =(π/3)( h )( b 2 +ab+a 2 ),

wherein:

a and b are radii of two of the at least two bands calculated from the output resistivity response signals, and

h is a height of the truncated cone between the two of the at least two bands.