Wireless cardiac pulsatility sensing

1. A method, comprising:

delivering a non-stimulating electrical current using a first dipole located along a first body location and comprising a first electrode and a second electrode;

measuring a voltage signal using a second dipole located at a second body location spaced apart from the first body location, the measured voltage signal corresponding to the electrical current being conducted through a portion of the patient's body, the second dipole comprising a third electrode and a fourth electrode different than the first electrode and the second electrode;

computing a pulsatility metric in response to the measured voltage signal;

detecting a physiological condition in response to the pulsatility metric; and

sensing a physiological signal using a transducer at the first body location, wherein delivering the non-stimulating electrical current comprises wirelessly transmitting a data signal correlated to the physiological signal through the portion of the patient's body to a processor configured to receive the data signal.

2. The method of claim 1 , further comprising comparing the pulsatility metric to a threshold and withholding a therapy in response to the pulsatility metric exceeding the threshold.

3. The method of claim 1 , further comprising:

sensing cardiac electrical signals using a pair of electrodes;

detecting a tachycardia in response to the sensed cardiac electrical signals;

computing the pulsatility metric in response to detecting the tachycardia;

comparing the pulsatility metric to a threshold; and

withholding a defibrillation shock in response to the pulsatility metric exceeding the threshold.

4. The method of claim 1 , wherein the voltage signal comprises a magnitude and a phase and computing the pulsatility metric comprises determining a phase of the voltage signal.

5. The method of claim 1 , further comprising:

transmitting a wireless request signal from a first device carrying the second dipole to a second device carrying the first dipole, the request signal requesting the second device to transmit the non-stimulating electrical current;

sending the current signal from the second device using the first dipole;

measuring the voltage signal at the first device using the second dipole.

6. The method of claim 1 , further comprising:

deploying a first device carrying the first dipole at a first location;

deploying a second device carrying the second dipole at a second location;

the first and second devices being configured for wireless communication with each other.

7. The method of claim 1 , wherein delivering the non-stimulating electrical current comprises:

wirelessly transmitting a data communication signal from a first device carrying the first dipole to a second device carrying the second dipole;

receiving the data communication signal at the second dipole; and

extracting the voltage signal from the data communication signal.

8. The method of claim 7 , further comprising:

transmitting information corresponding to the non-stimulating electrical current in the data communication signal;

computing a transimpedance signal in response to the information and the extracted voltage signal; and

computing the pulsatility metric in response to the transimpedance signal.

9. A method, comprising:

delivering a non-stimulating electrical current using a first dipole located along a first body location and comprising a first electrode and a second electrode;

measuring a voltage signal using a second dipole located at a second body location spaced apart from the first body location, the measured voltage signal corresponding to the electrical current being conducted through a portion of the patient's body, the second dipole comprising a third electrode and a fourth electrode different than the first electrode and the second electrode;

computing a pulsatility metric in response to the measured voltage signal; and

detecting a physiological condition in response to the pulsatility metric, wherein computing a pulsatility metric comprises:

determining a peak amplitude using the measured voltage signal;

computing a moving average using the measured voltage signal; and

computing a ratio of the peak amplitude and the moving average.

10. An implantable medical device system, comprising:

a first dipole to deliver a non-stimulating electrical current, the first dipole comprising a first electrode and a second electrode adapted to be deployed along a first body location;

a second dipole to measure a voltage signal resulting from the non-stimulating electrical current being conducted through a portion of a patient's body; the second dipole comprising a third electrode and a fourth electrode different than the first electrode and the second electrode and adapted to be deployed along a second body location spaced apart from the first body location;

a processor configured to compute a pulsatility metric in response to the measured voltage signal and to detect a physiological condition in response to the pulsatility metric;

a transducer sensing a physiological signal at the first body location; and

a data transmission module configured to wirelessly transmit a data signal correlated to the physiological signal through the portion of the patient's body via the first dipole, wherein the processor is configured to extract the measured voltage at the second dipole from the data signal.

11. The system of claim 10 , further comprising a therapy delivery module,

wherein the processor is further configured to compare the pulsatility metric to a threshold and withhold a therapy to be delivered by the therapy delivery module in response to the pulsatility metric exceeding the threshold.

12. The system of claim 10 , further comprising:

a pair of electrodes to sense cardiac electrical signals; and

an event detector to detect a tachycardia in response to the sensed cardiac electrical signals, wherein the processor is configured to compute the pulsatility metric in response to detecting the tachycardia, compare the pulsatility metric to a threshold, and withhold a defibrillation shock in response to the pulsatility metric exceeding the threshold.

13. The system of claim 10 , wherein the voltage signal comprises a magnitude and a phase and computing the pulsatility metric comprises determining a phase of the voltage signal.

14. The system of claim 10 , further comprising:

a first device carrying the first dipole and comprising a first telemetry module to transmit a wireless request signal requesting the non-stimulating electrical current; and

a second device carrying the second dipole and comprising a second telemetry module for receiving the request signal and for transmitting the non-stimulating electrical current in response to the request signal.

15. The system of claim 10 , further comprising:

a first device carrying the first dipole; and

a second device carrying the second dipole, the first and second devices being configured for wireless communication with each other.

16. The system of claim 10 , further comprising:

a first device comprising a first telemetry module and carrying the first dipole, the first device configured to wirelessly transmit a data communication signal using the first dipole; and

a second device comprising a second telemetry module and carrying the second dipole, the second device configure to receive the data communication signal using the second dipole, wherein the processor is configured to extract the voltage signal from the data communication signal.

17. The system of claim 16 wherein the first device is further configured to transmit information corresponding to the non-stimulating electrical current in the data communication signal;

the processor further configured to compute a transimpedance signal in response to the information and the extracted voltage signal, and compute the pulsatility metric in response to the transimpedance signal.

18. An implantable medical device system, comprising:

a first dipole to deliver a non-stimulating electrical current, the first dipole comprising a first electrode and a second electrode adapted to be deployed along a first body location;

a second dipole to measure a voltage signal resulting from the non-stimulating electrical current being conducted through a portion of a patient's body; the second dipole comprising a third electrode and a fourth electrode different than the first electrode and the second electrode and adapted to be deployed along a second body location spaced apart from the first body location; and

a processor configured to compute a pulsatility metric in response to the measured voltage signal and to detect a physiological condition in response to the pulsatility metric, wherein computing the pulsatility metric comprises:

determining a peak amplitude using the measured voltage signal;

computing a moving average using the measured voltage signal; and

computing a ratio of the peak amplitude and the moving average.

19. A computer-readable medium storing a set of instructions which cause a processor of a medical device system to:

control the delivery of a non-stimulating electrical current using a first dipole located along a first body location and comprising a first electrode and a second electrode;

measure a voltage signal using a second dipole located at a second body location spaced apart from the first body location, the voltage corresponding to the electrical current being conducted through a portion of the patient's body, the second dipole comprising a third electrode and a fourth electrode different than the first electrode and the second electrode;

compute a pulsatility metric in response to the measured voltage;

detect a physiological condition in response to the pulsatility metric; and

sensing a physiological signal using a transducer at the first body location, wherein delivering the non-stimulating electrical current comprises wirelessly transmitting a data signal correlated to the physiological signal through the portion of the patient's body to a processor configured to receive the data signal.