Body stability measurement using pulse transit time

1. An insertable cardiac monitoring device configured to be subcutaneously inserted into a patient comprising:

a housing;

accelerometer circuitry configured to generate at least one accelerometer signal;

at least one of a) a plurality of electrodes, at least one of the plurality of electrodes being disposed on a proximal portion of the housing and at least another one of the plurality of electrodes being disposed on a distal portion of the housing, orb) a plurality of light detectors, at least one of the plurality of light detectors being disposed on the proximal portion of the housing and at least another one of the plurality of detectors being disposed on the distal portion of the housing;

sensing circuitry configured to monitor sensed signals via the at least one of the plurality of electrodes or the plurality of light detectors;

a memory; and

processing circuitry communicatively coupled to the accelerometer circuitry, the sensing circuitry, and the memory, the processing circuitry being configured to:

determine, based on the sensed signals, a first plurality of pulse transit times of the patient prior to a Sit-to-Stand transition of the patient;

determine, based on the at least one accelerometer signal, whether the Sit-to-Stand transition of the patient occurs;

determine, based on the sensed signals and in response to the Sit-to-Stand transition occurring, a second plurality of pulse transit times after the Sit-to-Stand transition of the patient;

calculate a first metric comprising a median, mean or mode of the first plurality of pulse transit times;

calculate a plurality of second metrics based on the second plurality of pulse transit times, the plurality of second metrics comprising a slope of the second plurality of pulse transit times and at least one of a minimum pulse transit time of the second plurality of pulse transit times, maximum pulse transit time of the second plurality of pulse transit times, or a median, mean, or mode of the second plurality of pulse transit times; and

determine a likelihood the patient will fall based on the first metric and the plurality of second metrics.

2. The insertable cardiac monitoring device of claim 1 , wherein the processing circuitry is configured to determine the likelihood the patient will fall at least in part by calculating difference metrics, wherein the difference metrics comprise at least one difference between the first metric and one or more of the plurality of second metrics.

3. The insertable cardiac monitoring device of claim 2 , wherein the processing circuitry is configured to determine the likelihood the patient will fall at least in part by calculating tendency metrics, the tendency metrics comprising at least one of a) a mean, median, or mode of at least one of the difference metrics or the slope of the second plurality of pulse transit times over time and at least one of b) a standard deviation or range of at least one of the difference metrics or the slope of the second plurality of pulse transit times over time.

4. The insertable cardiac monitoring device of claim 3 , wherein the processing circuitry is configured to determine the likelihood the patient will fall at least in part by comparing at least one of the difference metrics or the slope of the second plurality of pulse transit times to the tendency metrics.

5. The insertable cardiac monitoring device of claim 1 , wherein the processing circuitry is further configured to determine that the patient has been inactive for a predetermined period of time prior to the Sit-to-Stand transition and wherein the processing circuitry is configured to calculate the first metric and calculate the plurality of second metrics based on the determination that the patient has been inactive for the predetermined period of time prior to the Sit-to-Stand transition.

6. The insertable cardiac monitoring device of claim 1 , further comprising communication circuitry, the communication circuitry being operable to transmit an alert to an external device upon determining the likelihood the patient will fall has increased.

7. A method for controlling operation of an insertable cardiac monitoring device configured to be subcutaneously inserted into a patient to determine a likelihood that the patient will fall comprising:

determining, by processing circuitry of the insertable cardiac monitoring device, a first plurality of pulse transit times prior to a Sit-to-Stand transition of the patient based on sensed signals, the sensed signals being sensed via at least one of a) a plurality of electrodes, at least one of the plurality of electrodes being disposed on a proximal portion of a housing of the insertable cardiac monitor and at least another one of the plurality of electrodes being disposed on a distal portion of the housing, or b) a plurality of light detectors, at least one of the plurality of light detectors being disposed on the proximal portion of the housing and at least another one of the plurality of detectors being disposed on the distal portion of the housing;

determining, by the processing circuitry and based on at least one accelerometer signal from an accelerometer of the insertable cardiac monitoring device, whether the Sit-to-Stand transition of the patient occurs;

determining, by the processing circuitry and in response to the Sit-to-Stand transition occurring, a second plurality of pulse transit times after the Sit-to-Stand transition of the patient based on the sensed signals;

calculating, by the processing circuitry, a first metric comprising a median, mean or mode of the first plurality of pulse transit times;

calculating, by the processing circuitry, a plurality of second metrics based on the second plurality of pulse transit times, the plurality of second metrics comprising a slope of the second plurality of pulse transit times and at least one of a minimum pulse transit time of the second plurality of pulse transit times, a maximum pulse transit time of the second plurality of pulse transit times, or a median, mean, or mode of the second plurality of pulse transit times; and

determining, by the processing circuitry, the likelihood the patient will fall based on the first metric and the plurality of second metrics.

8. The method of claim 7 , wherein the determining the likelihood the patient will fall further comprises calculating difference metrics, wherein the difference metrics comprise at least one difference between the first metric and one or more of the plurality of second metrics.

9. The method of claim 8 , wherein the determining the likelihood the patient will fall further comprises calculating tendency metrics, the tendency metrics comprising at least one of a) a mean, median, or mode of at least one of the difference metrics or the slope of the second plurality of pulse transit times over time and at least one of b) a standard deviation or range of at least one of the difference metrics or the slope of the second plurality of pulse transit times over time.

10. The method of claim 9 , wherein the determining the likelihood the patient will fall further comprises comparing at least one of the difference metrics or the slope of the second plurality of pulse transit times to the tendency metrics.

11. The method of claim 7 , further comprising determining, by the processing circuitry, that the patient has been inactive for a predetermined period of time prior to the Sit-to-Stand transition, and wherein calculating the first metric and calculating the plurality of second metrics is based on the determination that the patient has been inactive for the predetermined period of time prior to the Sit-to-Stand transition.

12. The method of claim 7 , further comprising transmitting, by communication circuitry and based on determining the likelihood the patient will fall increased, an alert to an external device.