Systems and methods for monitoring hemodynamic status

1. A system for use in assessment of a patient's hemodynamic status comprising:

one or more devices comprising:

a first sensor to measure a first signal from tissue of a patient, and

a second sensor to measure a second signal from tissue of the patient from a different location along a circulatory path from the patient's heart than the first sensor, wherein the one or more devices are configured to periodically measure the first and second signals over a monitoring time period; and

an external monitoring device wirelessly operatively coupled to the one or more devices and configured to:

receive data representative of the first and second signals from the one or more devices;

determine a pulse transit time based on the data representative of the first and second signals for each measurement of the first and second signals resulting in a plurality of pulse transit times for the monitoring time period; and

determine a surrogate hemodynamic status value based on the plurality of pulse transit times representative of the patient's aggregate hemodynamic status for the monitoring time period.

2. The system of claim 1 , wherein the monitoring time period is greater than or equal to 1 hour.

3. The system of claim 1 , wherein the external monitoring device is further configured to:

monitor the hemodynamic status values for a plurality of monitoring time periods; and

determine whether the hemodynamic status of the patient has changed based on the hemodynamic status values over the plurality of monitoring time periods.

4. The system of claim 3 , wherein the external monitoring device is further configured to issue an alert if the hemodynamic status of the patient has changed based on the hemodynamic status values.

5. The system of claim 1 , wherein the external monitoring device is further configured to:

receive a plurality of blood pressure values of the patient synchronized to the plurality of pulse transit times over the monitoring time period,

model a relationship between the plurality of blood pressure values and the plurality of pulse transit times;

monitor the relationship between the plurality of blood pressure values and the plurality of pulse transit times for at least two monitoring time periods; and

determine whether the hemodynamic status of the patient has changed based on a change in the relationship between the plurality of blood pressure values and the plurality of pulse transit times over the at least two monitoring time periods.

6. The system of claim 1 , wherein the one or more devices comprises:

a first device comprising the first sensor; and

a second device comprising the second sensor.

7. The system of claim 1 , wherein the one or more devices comprises a single implantable device comprising the first sensor and the second sensor.

8. The system of claim 1 , wherein the one or more devices are subcutaneously implantable devices.

9. The system of claim 1 , wherein the first signal comprises one or more of an electrocardiography signal and an impedance signal.

10. The system of claim 1 , wherein the data representative of the first signal comprises a digital representation of one of an electrocardiography signal, an impedance signal, an acoustic signal, an optical signal, and an accelerometer signal.

11. The system of claim 1 , wherein the data representative of the first signal comprises a time stamp corresponding to a fiducial point within one of an electrocardiography signal, an impedance signal, an optical signal, an acoustic signal, and an accelerometer signal.

12. The system of claim 1 , wherein the second signal comprises one or more of an impedance signal, an optical signal, an acoustic signal, and an accelerometer signal.

13. A method for use in assessing a patient's hemodynamic status comprising:

periodically measuring a first signal from tissue of a patient and a second signal from tissue of the patient in a different location along a circulatory path from the patient's heart than where the first signal is measured over a monitoring time period;

receiving data representative of the first and second signals;

determining a pulse transit time based on the data representative of the first and second signals for each measurement of the first and second signals resulting in a plurality of pulse transit times for the monitoring time period; and

determining a hemodynamic status value based on the plurality of pulse transit times representative of the patient's aggregate hemodynamic status for the monitoring time period.

14. The method of claim 13 , wherein the monitoring time period is greater than or equal to 1 hour.

15. The method of claim 13 further comprising:

monitoring the hemodynamic status values for a plurality of monitoring time periods; and

determining whether the hemodynamic status of the patient has changed based on the hemodynamic status values over the plurality of monitoring time periods.

16. The method of claim 15 further comprising issuing an alert if the hemodynamic status of the patient has changed based on the hemodynamic status values.

17. The method of claim 15 further comprising:

receiving a plurality of blood pressure values of the patient synchronized to the plurality of pulse transit times over the monitoring time period;

modelling a relationship between the plurality of blood pressure values and the plurality of pulse transit times;

monitoring the relationship between the plurality of blood pressure values and the plurality of pulse transit times for at least two monitoring time periods; and

determining whether the hemodynamic status of the patient has changed based on a change in the relationship between the plurality of blood pressure values and the plurality of pulse transit times over the at least two monitoring time periods.

18. The method of claim 13 , wherein the first signal comprises one of an electrocardiography signal, an impedance signal, an optical signal, an acoustic signal, and an accelerometer signal.

19. The method of claim 13 , wherein the data representative of the first signal comprises a digital representation of one of an electrocardiography signal, an impedance signal, an optical signal, an acoustic signal, and an accelerometer signal.

20. The method of claim 13 , wherein the data representative of the first signal comprises a time stamp corresponding to a fiducial point within one of an electrocardiography signal, an impedance signal, an optical signal, an acoustic signal, and an accelerometer signal.

21. A system for use in assessment of a patient's hemodynamic status comprising:

one or more devices comprising:

a pulse generator to generate a pulse in tissue of a patient, and

a sensor to measure a signal from tissue of the patient from a different location along a circulatory path from the patient's heart than the pulse generator, wherein the one or more devices are configured to periodically deliver a pulse and measure the signal over a monitoring time period; and

an external monitoring device wirelessly operatively coupled to the one or more devices and configured to:

receive a time stamp corresponding to the delivery of the pulse and data representative of the signal from the one or more devices;

determine a pulse transit time based on the time stamp and the data representative of the signal for each pulse delivery and measurement of the signal resulting in a plurality of pulse transit times for the monitoring time period; and

determine a surrogate hemodynamic status value based on the plurality of pulse transit times representative of the patient's hemodynamic status for the monitoring time period.