Ambulatory detection of QT prolongation

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.

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.