Impedance measurement

#Bio impedance measurements are known to be used to monitor fluid content in a subject's thorax.

#Some bio impedance measurement implementations suggest using implanted electrodes. For example, bio impedance measurement is utilized in some pacemakers, where the bio impedance associated with tissues/organs located in-between the pacemaker electrodes and/or case is measured. For example, Medtronic Plc of Dublin Ireland offers OptiVol®, which provides fluid status monitoring through intrathoracic impedance measurements. OptiVol® Fluid Status Monitoring is utilized with implantable cardioverter defibrillator (ICD) or cardiac resynchronization therapy device (CRT-D). External devices, which are able to perform Bio impedance measurements, are also used, for example the Zoe fluid status monitor from NMT medical. The electrical principle behind bio impedance is Ohm's law. When a sinusoidal electrical current passes through a medium, a sinusoidal voltage drop in magnitude is generated. The electrical resistance gives the relationship between the electrical current and the corresponding voltage. An electrical current that passes through biological tissue is impeded by biological tissue resistance, which causes a phase shift between the sinusoidal electrical current and the sinusoidal voltage. Therefore, the tissue impedance that is calculated depends on both the magnitude of the signal and the phase shift that is generated. Depending on the frequencies that are used, different parameters can be measured within the body. High frequency signals are able to pass through the cell membrane and as a consequence both the intracellular and extracellular impedance is measured. Low frequency signals on the other hand only measure the impedance of the extracellular compartment. These properties can be used to measure different parameters. Depending on the desired output, two different bio impedance measuring techniques can be distinguished, i.e. single-frequency measurements and multi-frequency measurements. Multi-frequency measurements, also known as bio impedance spectroscopy (BIS), employ electrical currents of multiple frequencies through the body and can be used to determine static parameters (e.g. body composition, fluid status, etc.). Single-frequency signals on the contrary, can be used to measure more dynamic parameters, such as respiration (2). Single-frequency bio impedance devices that measure respiration can be used in patients suffering from pulmonary problems such as chronic obstructive pulmonary disease, sleep apnea or asthma.

#Bio impedance technology has limitations due to the use of electrical currents for the measurements, causing accuracy, specificity and stability issues limiting the use of bio-impedance in relative and trend analysis.

#Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced.

#In the drawings:

#FIG. 1 is a schematic illustration of a bio impedance measuring device worn on an exemplary subject and adapted to determine target thorax area values based on a calibration function calculated according to readings of an RF sensor, according to some embodiments of the present invention; and

#There is provided in accordance with an aspect of the presently disclosed subject matter a device that is capable of determining a fluid level in the subject's thorax.

#According to examples of the presently disclosed subject matter, there are provided methods and devices for using a calibration function calculated based on RF measurements and bio impedance measurements for adjusting subsequent bio impedance measurements in the process of calculating thorax area values such as fluid level values. The calibration function is optionally used in a bio impedance measuring device, for example implanted and/or wearable bio impedance measuring device based on a calibration function calculated according to bio impedance measurements and signals acquired using an RF sensor comprising radiofrequency (RF) signal generator and signal analyzer system. The impedance measuring device may include one or more electrodes and RF signal generator (e.g. RF transceiver that includes EF antenna(s)) and a processing unit, for example one or more processors for computing the bio impedance values and the calibration function. The impedance values may be obtained from impedance level measurement(s) acquired from electrical current(s) passing between at least the electrodes of the pair. Optionally, the calibration function is set to adjust subsequent impedance level measurement(s) for the calculation of thorax area values. The impedance level measurement(s) may be indicative of fluid content in the subject's thorax and/or fluid content change in the subject's thorax. The RF sensor provides an RF interaction measurement(s), for example interception(s) of RF radiation reflected or passing through the subject's thorax after interacting with the subject's body. Optionally, a sequence of pairs of measurements is gathered during a learning phase. Each pair includes an RF measurement and a bio impedance measurement which are correlated with one another, for instance taken at the same time or about the same time. The sequence of pairs of measurements may be used for deducing a calibration function such as a calibration model and/or calibration values. At a later phase, operational phase, thorax area values which are calculated based on subsequent bio impedance measurements can be adjusted.

#Using calibration function calculated based on the readings of RF sensors allows improving the accuracy and consistency of thorax area values such as fluid level values, reducing possible false negative and possible false positive alerts. In use, bio impedance devices may be used for monitoring patients hospitalized for a few days or more. During the monitoring period bio impedance measurements are collected. In some embodiments of the present invention bio impedance measurements and RF measurements are gathered for calculating calibration function. The measurements are from an area with changing fluid levels. A calibration function for adjusting subsequent bio impedance measurements taken at a later time, for instance when the patient is at home, can now be calculated to acquire improved thorax area values. The subsequent bio impedance measurements may be gathered by the same bio impedance measuring device used for capturing the bio impedance measurements or by another bio impedance measuring device. In some examples the RF sensor is integrated with the bio impedance device for example in an implantable device and the RF measurements are only triggered periodically to perform the calibration process while on a regular basis only bio impedance measurements are performed and output is calculated using the calculated calibration function. This provides for saving on energy used from a limited implanted energy source. As mentioned this method may also be used for corrections of estimates which are based on bio impedance measurements, for instance air content values and/or respiration values. Fluid content in the target thorax area, for instance the lungs includes blood content and so any other parameters based on such fluid content related bio impedance measurement can also be corrected and/or improved and/or made more accurate and/or made more quantitative using this calibration process.