Determining heart condition statuses using subcutaneous impedance measurements

1 . A system for detecting statuses of heart failure of a patient, the system comprising:

an implantable medical device (IMD) comprising a housing and a plurality of electrodes on the housing, the housing and the plurality of electrodes configured for subcutaneous implantation, wherein the IMD is configured to determine one or more impedance signals of subcutaneous interstitial fluid measured using only the plurality of electrodes on the housing, wherein the plurality of electrodes are positioned within 5 centimeters (cm) apart on the housing; and

processing circuitry configured to periodically:

determine, based at least in part on the one or more impedance signals, at least one first tissue impedance value that corresponds to a first time period;

determine, based at least in part on the one or more impedance signals, at least one second tissue impedance value that corresponds to a second time period different from the first time period;

determine, based at least in part on the at least one first tissue impedance value, one or more reference impedance values;

determine one or more fluid index values based at least in part on the one or more reference impedance values and the at least one second tissue impedance value;

determine an adaptive threshold based on an absolute impedance value of at least one of the one or more impedance signals;

perform a comparison of the one or more fluid index values and the adaptive threshold;

determine an impedance score based at least in part on the comparison; and

determine a heart failure status of the patient based at least in part on the impedance score.

2 . The system of claim 1 , wherein the plurality of electrodes are configured to contact the subcutaneous interstitial fluid.

3 . The system of claim 1 , wherein the processing circuitry is further configured to:

determine the one or more reference impedance values by at least determining a statistical representation of the at least one first tissue impedance value corresponding to the first time period.

4 . The system of claim 1 , wherein the processing circuitry is further configured to:

maintain a buffer of relative changes in the one or more impedance signals over time; and

determine the one or more fluid index values based at least in part on the buffer, the one or more reference impedance values and the at least one second tissue impedance value.

5 . The system of claim 4 , wherein the processing circuitry is further configured to:

perform a comparison of the at least one second tissue impedance value to the one or more reference impedance values; and

modify the buffer based at least in part on the comparison.

6 . The system of claim 1 , wherein the processing circuitry is further configured to:

identify a first fluid index calculation to determine at least a first subset of the one or more fluid index values; and

identify a second fluid index calculation to determine at least a second subset of the one or more fluid index values,

wherein identification is based at least in part on a number of fluid index values that have satisfied a fluid index threshold over time.

7 . The system of claim 1 , wherein the processing circuitry is further configured to:

modify the impedance score in response to the one or more fluid index values satisfying one or more scoring thresholds for at least one of: a predetermined amount of time or a predetermined number of times.

8 . The system of claim 1 , wherein the processing circuitry is further configured to:

identify a resolution parameter for determining at least one of: the at least one first tissue impedance value, the one or more reference impedance values, or the at least one second tissue impedance value.

9 . The system of claim 1 , wherein the processing circuitry is further configured to:

exclude a subset of impedance signals from the one or more impedance signals when determining at least one of: the at least one first tissue impedance value or the at least one second tissue impedance value.

10 . The system of claim 1 , wherein the processing circuitry is further configured to:

compare the impedance score to one or more risk thresholds; and

generate an alert in response to the impedance score satisfying at least one of the one or more risk thresholds.

11 . The system of claim 1 , wherein at least two electrodes of the plurality of electrodes are positioned on the housing to face a skin layer of the patient when the IMD is subcutaneously implanted.

12 . The system of claim 1 , wherein at least two electrodes of the plurality of electrodes are separated by a fixed distance.

13 . The system of claim 1 , wherein the processing circuitry is further configured to:

exclude a subset of impedance signals from the one or more impedance signals based on a determination that the subset of impedance signals includes a particular characteristic.

14 . The system of claim 1 , wherein to determine, based at least in part on the one or more impedance signals, at least one second tissue impedance value that corresponds to a second time period different from the first time period, the processing circuitry is configured to determine, based at least in part on the one or more impedance signals, a plurality of second tissue impedance values that correspond to the second time period different from the first time period, and wherein the processing circuitry is further configured to:

determine a variability of the plurality of second tissue impedance values; and

adjust the one or more fluid index values based on the variability of the plurality of second tissue impedance values.

15 . The system of claim 1 , wherein to determine the adaptive threshold the processing circuitry is further configured to:

determine the adaptive threshold to be proportional to the absolute impedance value and an intra-day variation in the one or more impedance signals.

16 . The system of claim 1 , wherein the processing circuitry is further configured to:

modify the impedance score in response to the one or more fluid index values satisfying one or more scoring thresholds for a predetermined amount of time.

17 . The system of claim 1 , wherein the processing circuitry is further configured to:

determine the at least one second tissue impedance value is less than the one or more reference impedance values;

in response to the determination that the at least one second tissue impedance value is less than the one or more reference impedance values:

determine the adaptive threshold based on at least one of a mean or median of an absolute impedance value of the one or more impedance signals of subcutaneous interstitial fluid during a particular period of time, the particular period of time being within 30 days prior to a measurement of the one or more impedance signals of subcutaneous interstitial fluid; and

modify the impedance score based on the comparison of the one or more fluid index values to the adaptive threshold.

18 . A method for controlling operation of processing circuitry of a system, the method comprising:

determining, by the processing circuitry and for a first time period, at least one reference impedance value based at least in part on one or more impedance signals of subcutaneous interstitial fluid received from an implantable medical device (IMD) comprising a housing and a plurality of electrodes on the housing, the one or more impedance signals of subcutaneous interstitial fluid being measured using only the plurality of electrodes on the housing, wherein the plurality of electrodes are positioned within 5 centimeters (cm) apart on the housing, and the housing and the plurality of electrodes are implanted in a subcutaneous layer of a patient;

determining, by the processing circuitry and for a second time period different from the first time period, at least one other impedance value based at least in part on the one or more impedance signals received from the plurality of electrodes;

determining, by the processing circuitry and based at least in part on the at least one reference impedance value and the at least one other impedance value, one or more fluid index values of the patient;

determining, by the processing circuitry, an adaptive threshold based on an absolute impedance value of at least one of the one or more impedance signals;

performing, by the processing circuitry, a comparison of the one or more fluid index values and the adaptive threshold;

determining, by the processing circuitry, an impedance score based at least in part on the comparison; and

determining, by the processing circuitry, a heart failure status of the patient based at least in part on the impedance score.

19 . The method of claim 18 , wherein the plurality of electrodes are implanted in a thoracic region of the patient.

20 . The method of claim 18 , wherein determining the one or more fluid index values further comprises determining a statistical representation of the at least one reference impedance value.

21 . The method of claim 18 , wherein determining the one or more fluid index values further comprises accessing a buffer that includes relative changes in the one or more impedance signals over time.

22 . The method of claim 18 , further comprising:

modifying the impedance score in response to the one or more fluid index values satisfying one or more scoring thresholds for at least one of: a predetermined amount of time or a predetermined number of times.

23 . The method of claim 18 , further comprising:

determining a statistical representation of at least one of: the at least one reference impedance value or the at least one other impedance value; and

modifying the impedance score in response to the statistical representation satisfying an impedance threshold amount.

24 . The method of claim 18 , further comprising:

identifying a first fluid index calculation when determining at least a first subset of the one or more fluid index values; and

identifying a second fluid index calculation when determining at least a second subset of the one or more fluid index values.

25 . The method of claim 18 , further comprising:

identifying a resolution parameter for determining at least one of: the at least one reference impedance value or the at least one other impedance value.

26 . A non-transitory computer-readable storage medium having stored thereon instructions that, when executed, cause one or more processors to at least:

determine, for a first time period, at least one reference impedance value based at least in part on one or more impedance signals of subcutaneous interstitial fluid sensed via a plurality of electrodes disposed on a housing of an implantable medical device (IMD) implanted in a subcutaneous layer of a patient, the one or more impedance signals of subcutaneous interstitial fluid being measured using only the plurality of electrodes disposed on the housing, wherein the plurality of electrodes are positioned within 5 centimeters (cm) apart on the housing;

determine, for a second time period different from the first time period, at least one other impedance value based at least in part on the one or more impedance signals received from the plurality of electrodes;

determine, based at least in part on the at least one reference impedance value and the at least one other impedance value, one or more fluid index values;

determine an adaptive threshold based on an absolute impedance value of at least one of the one or more impedance signals;

perform a comparison of the one or more fluid index values and the adaptive threshold;

determine, based at least in part on the comparison, an impedance score of the patient; and

output a heart failure status of the patient based at least in part on the impedance score.