Monitoring of pacing capture using acceleration

1. An implantable medical device, comprising:

an activity sensor configured to generate a signal indicative of motion of a heart of a patient and physical activity of the patient;

a pulse generator configured to generate and deliver pacing pulses to the patient's heart via a pair of electrodes coupled to the implantable medical device; and

a control module coupled to the pulse generator and the activity sensor, the control module being configured to:

compute an activity metric from the activity sensor signal over an activity metric interval;

compare the activity metric to a loss of capture detection threshold;

detect loss of capture by the delivered pacing pulses in response to the activity metric being less than the loss of capture detection threshold; and

increase a pacing pulse output in response to detecting the loss of capture; and

wherein the control module is further configured to:

determine a sensor indicated pacing rate based on the activity metric; and

set a pacing pulse rate responsive to the sensor indicated pacing rate.

2. The device of claim 1 , wherein the control module is further configured to:

adjust at least one of the loss of capture detection threshold and the activity metric interval responsive to the pacing rate being set at a sensor indicated pacing rate that is greater than a predetermined base rate.

3. The device of claim 1 , wherein the control module is further configured to:

accumulate a plurality of activity metric values; and

establish the loss of capture detection threshold based on at least a portion of the plurality of activity metric values.

4. The device of claim 3 , further comprising a sensing module for sensing a cardiac electrical signal, wherein the control module is further configured to:

accumulate the plurality of activity metric values by:

performing a capture threshold search to determine a capture threshold indicated by the sensed cardiac electrical signal; and

accumulating the plurality of activity metric values during the capture threshold search, wherein the loss of capture detection threshold is established based on the at least one of the plurality of activity metric values accumulated during the capture threshold search.

5. The device of claim 1 , wherein the control module is further configured to:

perform a capture threshold search to determine a capture threshold in response to detecting the loss of capture;

compare the determined capture threshold to a previously determined capture threshold; and

adjust a loss of capture detection control parameter in response to the determined capture threshold being equal to or less than the previously determined capture threshold.

6. The device of claim 5 , wherein the control module is configured to adjust the loss of capture detection control parameter by decreasing the loss of capture threshold.

7. An implantable medical device, comprising:

an activity sensor configured to generate a signal indicative of motion of a heart of a patient and physical activity of the patient;

a pulse generator configured to generate and deliver pacing pulses to the patient's heart via a pair of electrodes coupled to the implantable medical device; and

a control module coupled to the pulse generator and the activity sensor, the control module being configured to:

compute an activity metric from the activity sensor signal over an activity metric interval;

compare the activity metric to a loss of capture detection threshold;

detect loss of capture by the delivered pacing pulses in response to the activity metric being less than the loss of capture detection threshold; and

increase a pacing pulse output in response to detecting the loss of capture; and

wherein the control module is configured to determine the activity metric by one of:

rectifying the activity sensor signal and counting a number of activity threshold crossings of the activity sensor signal during the activity metric interval, and

rectifying the activity sensor signal and summing all sample point amplitudes of the rectified activity sensor signal during the activity metric interval.

8. An implantable medical device, comprising:

an activity sensor configured to generate a signal indicative of motion of a heart of a patient and physical activity of the patient;

a pulse generator configured to generate and deliver pacing pulses to the patient's heart via a pair of electrodes coupled to the implantable medical device; and

a control module coupled to the pulse generator and the activity sensor, the control module being configured to:

compute an activity metric from the activity sensor signal over an activity metric interval;

compare the activity metric to a loss of capture detection threshold;

detect loss of capture by the delivered pacing pulses in response to the activity metric being less than the loss of capture detection threshold; and

increase a pacing pulse output in response to detecting the loss of capture; and

further comprising a sensing module configured to

receive a cardiac electrical signal from the heart developed across a sensing electrode pair coupled to the implantable medical device,

sense an intrinsic cardiac event from the cardiac electrical signal, and

pass a cardiac sense event signal to the control module in response to sensing the intrinsic cardiac event,

wherein the control module is further configured to:

receive cardiac sense event signals from the sensing module,

responsive to the activity metric being greater than the loss of capture threshold, determine if an intrinsic sensed event occurs during the activity metric interval based on the received cardiac sense event signals; and

responsive to occurrence of the intrinsic sensed event during the activity metric interval, adjusting a loss of capture detection control parameter.

9. The device of claim 8 , wherein the control module is configured to adjust the loss of capture detection control parameter by at least one of increasing the loss of capture detection threshold and increasing a number of activity metrics less than the loss of capture detection threshold that are required to detect the loss of capture.

10. An implantable medical device, comprising:

an activity sensor configured to generate a signal indicative of motion of a heart of a patient and physical activity of the patient;

a pulse generator configured to generate and deliver pacing pulses to the patient's heart via a pair of electrodes coupled to the implantable medical device; and

a control module coupled to the pulse generator and the activity sensor, the control module being configured to:

compute an activity metric from the activity sensor signal over an activity metric interval;

compare the activity metric to a loss of capture detection threshold;

detect loss of capture by the delivered pacing pulses in response to the activity metric being less than the loss of capture detection threshold; and

increase a pacing pulse output in response to detecting the loss of capture; and

further comprising a memory,

wherein the control module is further configured to:

detect an occurrence of capture subsequent to increasing the pacing pulse output;

if capture is not detected, adjust the pacing pulse output incrementally up to a maximum pacing pulse output until capture is verified; and

record a date and time stamp in the memory in response to the maximum pacing pulse output being reached without the detection of capture.