System and method for distinguishing among cardiac ischemia, hypoglycemia and hyperglycemia using an implantable medical device

Abstract

Techniques are described for detecting ischemia, hypoglycemia or hyperglycemia based on intracardiac electrogram (IEGM) signals. Ischemia is detected based on a shortening of the interval between the QRS complex and the end of a T-wave (QTmax), alone or in combination with a change in ST segment elevation. Alternatively, ischemia is detected based on a change in ST segment elevation combined with minimal change in the interval between the QRS complex and the end of the T-wave (QTend). Hypoglycemia is detected based on a change in ST segment elevation along with a lengthening of either QTmax or QTend. Hyperglycemia is detected based on a change in ST segment elevation along with minimal change in QTmax and in QTend. By exploiting QTmax and QTend in combination with ST segment elevation, changes in ST segment elevation caused by hypo/hyperglycemia can be properly distinguished from changes caused by ischemia.

1 . A method for use with an implantable medical device for detecting cardiac ischemia in a patient in which the device is implanted comprising:
 tracking changes in one or more electrical cardiac signal parameters relative to a corresponding baseline;   comparing tracked changes to a corresponding first threshold indicative of a moderate ischemia and a second threshold indicative of a more serious ischemia; and   determining an ischemic condition based on the comparisons.   
     
     
         2 . The method of  claim 1  wherein:
 the one or more electrical cardiac signal parameters comprise intervals between QRS complexes and peaks of corresponding T-waves (QTmax intervals); 
 the first threshold corresponds to a first amount of shortening of the QTmax interval; 
 the second threshold corresponds to a second amount of shortening of the QTmax interval, greater than the first amount 
 moderate ischemia is determined if changes in QTmax intervals exceed the first threshold; and 
 more serious ischemia is determined if changes in QTmax intervals exceed the second threshold. 
 
     
     
         3 . The method of  claim 1  wherein:
 the one or more electrical cardiac signal parameters comprise ST segments representative of segments of the cardiac signals between depolarization events and corresponding repolarization events; 
 the first threshold corresponds to a first amount of deviation of the ST segment; 
 the second threshold corresponds to a second amount of deviation of the ST segment, greater than the first amount 
 moderate ischemia is determined if deviations in ST segments exceed the first threshold; and 
 more serious ischemia is determined if deviations in ST segments exceed the second threshold. 
 
     
     
         4 . The method of  claim 1  wherein:
 the one or more electrical cardiac signal parameters comprise ST segments representative of segments of the cardiac signals between depolarization events and corresponding repolarization events having first and second thresholds and intervals between QRS complexes and ends of corresponding T-waves (QTend intervals) having a first threshold; 
 the first ST segment threshold corresponds to a first amount of deviation of the ST segment; 
 the second ST segment threshold corresponds to a second amount of deviation of the ST segment, greater than the first amount of deviation of the ST segment; 
 the first QTend interval threshold corresponds to a first amount of lengthening of QTend 
 moderate ischemia is determined if deviations in ST segments exceed the first ST segment threshold and changes in QTend do not exceed the first QTend threshold; and 
 more serious ischemia is determined if deviations in ST segments exceed the second ST segment threshold and changes in QTend do not exceed the first QTend threshold. 
 
     
     
         5 . The method of  claim 1  wherein the corresponding baseline comprises a running average of the one or more electrical cardiac signal parameters over a plurality of heartbeats. 
     
     
         6 . The method of  claim 1  wherein:
 tracking changes comprises calculating for each of a plurality of heartbeats, the difference between the one or more electrical cardiac signal parameters and the corresponding baseline; and 
 comparing tracked changes comprises calculating an average of the differences and comparing the average of the differences to the first and second thresholds. 
 
     
     
         7 . The method of  claim 1  wherein:
 tracking changes comprises calculating for each of a plurality of heartbeats, the difference between the one or more electrical cardiac signal parameters and the corresponding baseline; 
 comparing tracked changes comprises comparing each of the differences to the first and second thresholds; and 
 determining an ischemic condition comprises indicating ischemia when a predetermined number of differences exceed the first threshold or the second threshold. 
 
     
     
         8 . A system for use with an implantable medical device for detecting cardiac ischemia in a patient in which the device is implanted comprising:
 a timing unit operative to track paced QTmax values representative of intervals between QRS complexes induced by pacing pulses delivered by the implantable medical device, and peaks of corresponding T-waves within electrical cardiac signals and to separately track sensed QTmax values representative of intervals between intrinsic QRS complexes and peaks of corresponding T-waves within electrical cardiac signals; and   a cardiac ischemia detection unit operative to detect an episode of cardiac ischemia based on a decrease in at least one of the paced QTmax intervals and the sense QTmax intervals.   
     
     
         8 . A method for use with an implantable medical device for detecting cardiac ischemia in a patient in which the device is implanted comprising:
 tracking elevations of segments of cardiac signals between depolarization events and corresponding repolarization events within electrical cardiac signals;   detecting a possible ischemia based on a change in elevation of the ST segments;   tracking repolarization-based intervals representative of intervals between depolarization events and corresponding repolarization events within the electrical cardiac signals; and   verifying the detection of ischemia based on an examination of the repolarization-based intervals.