Ambulatory detection of QT prolongation

1 . A medical-device system for monitoring a patient, comprising:

a controller circuit, including a long QT syndrome (LQTS) detector configured to:

receive a subcutaneous cardiac signal sensed from the patient;

measure a Q wave-to-T wave (QT) time interval from the subcutaneous cardiac signal; and

detect an indication of QT prolongation using the measured QT time interval and a programmable threshold; and

dynamically adjust a memory space of a memory circuit allocated for storing the subcutaneous cardiac signal based on a type of cardiac event detected from the patient or the detected indication of QT prolongation.

2 . The medical-device system of claim 1 , comprising a sensing circuit configured to sense the subcutaneous cardiac signal including a subcutaneous electrocardiogram (S-ECG) via one or more subcutaneously implanted electrodes in the patient.

3 . The medical-device system of claim 2 , wherein the LQTS detector is configured to generate a representative S-ECG segment from an ensemble of segments of the S-ECG in multiple cardiac cycles, and to measure the QT time interval using the representative S-ECG.

4 . The medical-device system of claim 1 , wherein:

the LQTS detector is configured to generate a QT time interval trend under different postures or physical activities of the patient, and to detect the indication of QT prolongation at different postures or physical activities; and

an output unit configured to display the QT time interval trend and the indication of QT prolongation at the different postures or physical activities.

5 . The medical-device system of claim 1 , wherein:

the LQTS detector is configured to generate a QT time interval trend during a course of treatment of the patient, and to detect the indication of QT prolongation during the course of treatment; and

an output unit configured to display the QT time interval trend and the indication of QT prolongation during the course of treatment.

6 . The medical-device system of claim 1 , wherein the LQTS detector is configured to measure the QT time interval and to detect the indication of QT prolongation in response to cardiac arrhythmia, or frequent non-sustained arrhythmia episodes exceeding a frequency threshold.

7 . The medical-device system of claim 1 , wherein the LQTS detector is configured to measure the QT time interval and to detect the indication of QT prolongation at a specific posture state.

8 . The medical-device system of claim 1 , wherein the LQTS detector is configured to measure the QT time interval and to detect the indication of QT prolongation in response to an initiation of a therapy or a change of therapy.

9 . The medical-device system of claim 1 , comprising the memory circuit configured to store at least a portion of the subcutaneous cardiac signal in response to the indication of QT prolongation.

10 . The medical-device system of claim 1 , comprising a first ambulatory medical device operatively in communication with a second medical device separated from the first ambulatory medical device,

wherein the first ambulatory medical device includes the controller circuit configured to communicate a control signal to the second medical device to adjust a cardiac sensing parameter of the second medical device.

11 . The medical-device system of claim 1 , wherein the controller circuit is configured to dynamically adjust a size of an onset buffer to store a longer period of the subcutaneous cardiac signal sensed from the patient in the memory circuit based on the detected indication of QT prolongation.

12 . The medical-device system of claim 11 , wherein the LQTS detector is configured to generate an ensemble average subcutaneous cardiac signal using the subcutaneous cardiac signal in the dynamically adjusted onset buffer and to measure the QT time interval using the ensemble average subcutaneous cardiac signal in the dynamically adjusted onset buffer.

13 . The medical-device system of claim 1 , comprising:

a user interface configured to receive a user input of the programmable threshold, the user interface including an output unit configured to generate a programmable alert of the detected indication of QT prolongation corresponding to the user input of the programmable threshold.

14 . A method for monitoring a patient, comprising:

receiving a subcutaneous cardiac signal sensed from the patient;

measuring, via a controller circuit, a Q wave-to-T wave (QT) time interval from the subcutaneous cardiac signal;

detecting, via the controller circuit, an indication of QT prolongation using the measured QT time interval and a programmable threshold; and

dynamically adjusting a memory space of a memory circuit allocated for storing the subcutaneous cardiac signal based on a type of cardiac event detected from the patient or the detected indication of QT prolongation.

15 . The method of claim 14 , wherein the subcutaneous cardiac signal includes a subcutaneous electrocardiogram (S-ECG) sensed using one or more subcutaneously implanted electrodes in the patient, the method comprising:

generating a representative S-ECG segment from an ensemble of segments of the S-ECG in multiple cardiac cycles; and

measuring the QT time interval using the representative S-ECG.

16 . The method of claim 14 , wherein measuring the QT interval and detecting the QT prolongation is triggered by one or more triggering events including:

a cardiac arrhythmia;

frequent non-sustained arrhythmia episodes;

a specific posture state; or

an initiation of a therapy or a change of therapy.

17 . The method of claim 14 , comprising:

storing at least a portion of the subcutaneous cardiac signal in response to the indication of QT prolongation.

18 . The method of claim 14 , wherein the indication of QT prolongation is detected by a first ambulatory medical device, the method comprising:

communicating a control signal from the first ambulatory medical device to a second medical device separated from and communicatively coupled to the first ambulatory medical device; and

adjusting a cardiac sensing parameter of the second medical device.

19 . A medical-device system for monitoring a patient, comprising:

a controller circuit, including a long QT syndrome (LQTS) detector configured to:

receive a subcutaneous cardiac signal sensed from the patient;

measure a Q wave-to-T wave (QT) time interval from the subcutaneous cardiac signal; and

detect an indication of QT prolongation using the measured QT time interval and a programmable threshold; and

adjust a cardiac sensing parameter based on the indication of QT prolongation, including to extend a post-ventricular event refractory period by an amount based on the detected indication of QT prolongation.

20 . The medical-device system of claim 19 , comprising:

a user interface configured to receive a user input of the programmable threshold, the user interface including an output unit configured to generate a programmable alert of the detected indication of QT prolongation corresponding to the user input of the programmable threshold,

wherein the controller circuit is configured to dynamically adjust a memory space of a memory circuit allocated for storing the subcutaneous cardiac signal based on a type of cardiac event detected from the patient or the detected indication of QT prolongation.