Patent ID: 8900140

Claim:
An apparatus comprising: a first interface, adapted to be coupled to implantable first electrodes adapted to define a first measurement vector, within a subject, the first measurement vector comprising a first excitation vector for providing a first excitation signal and a first response vector for sensing a first response to the first excitation signal, wherein the first excitation vector is non-orthogonal to the first response vector; a second interface, adapted to be coupled to implantable second electrodes adapted to define a second measurement vector, within the subject, the second measurement vector comprising a second excitation vector for providing a second excitation signal and a second response vector for sensing a second response to the second excitation signal, wherein the second excitation vector is substantially orthogonal to the second response vector; a tissue impedance measurement circuit, selectively communicatively coupled to the first electrodes and to the second electrodes, and adapted to repeatedly perform a first tissue impedance measurement using the first measurement vector and to perform a second tissue impedance measurement using the second measurement vector; and a processor, communicatively coupled to the tissue impedance measurement circuit, the processor configured to: compute a change in the first tissue impedance measurement over a period of time and a change in the second tissue impedance measurement over a period of time; generate an indication associated with how much fluid is present at a first location in the subject relative to how much fluid is present at a second location in the subject using a comparison between the change in the first tissue impedance measurement and the change in the second tissue impedance measurement.