System and method for detecting cardiac ischemia using an implantable medical device

• US 7,274,959 B1
• Filed: 06/24/2003
• Issued: 09/25/2007
• Est. Priority Date: 06/24/2003
• Status: Expired due to Fees

• Current Assignee: Pacesetter Incorporated (Abbott Laboratories Incorporated)
• Original Assignee: Pacesetter Incorporated (Abbott Laboratories Incorporated)
• Inventors: Fard, Mohssen, Snell, Jeffery D., Wang, Zifei
• Primary Examiner(s): Evanisko; George R.
• Assistant Examiner(s): KAHELIN, MICHAEL WILLIAM
• Application Number:
  US10/603,429
• Time in Patent Office: 1,554 Days
• Field of Search: 600/508, 600/513, 600/516, 600/517, 600/509, 600/5, 600/16, 607/25, 607/26, 607/4, 607/5, 607/6, 607/9, 607/1, 607/14
• US Class Current: 600/509
• CPC Class Codes: A61B 5/349 Detecting specific parameters of the electrocardiograph cycle
• CPC Class Codes: A61B 5/352 Detecting R peaks, e.g. for synchronising diagnostic apparatus; Estimating R-R interval

Abstract

A technique is provided for detecting episodes of cardiac ischemia based on an examination of post-T-wave signal segments. Since cardiac ischemia is often a precursor to acute myocardial infarction (AMI) or ventricular fibrillation (VF), the technique thereby provides a method for predicting the possible onset of AMI or VF so that a warning may be delivered to the patient. The warning preferably includes both a perceptible electrical notification signal applied directly to subcutaneous tissue and a warning signal delivered via short range telemetry to a handheld warning device external to the patient. In one example, the onset of cardiac ischemia is identified by detecting a sharp falling edge within post-T-wave signals by filtering the signals using a high-pass filter having a cutoff frequency of at least 1 Hz. The total amount of energy in the filtered signal is calculated and compared against various thresholds.

Claims

1. In an implantable medical device for implant within a patient, a method comprising:

receiving electrical cardiac signals having a series of cycles, each cycle including a ventricular repolarization followed by a ventricular depolarization;

identifying segments of the cardiac signals subsequent to a ventricular repolarization and prior to the ventricular depolarization following the ventricular repolarization;

taking a difference between the total amount of energy in one of the identified segments and the running average of the total amount of energy in a plurality of the identified segments and comparing said difference to a first threshold;

comparing just the running average to a second threshold; and

detecting cardiac ischemia if either the first threshold or the second threshold is exceeded for a predetermined number of heart beats.

2. The method of claim 1 further comprising routing the cardiac signals through a high-pass filter operative to pass all signals above a cutoff frequency, to yield a high-pass filtered signal.

3. The method of claim 2 wherein the cutoff frequency is in the range of 0.1 to 5.0 Hz.

4. The method of claim 2 wherein the cutoff frequency is at least 1 Hz.

5. The method of claim 1 wherein the total amount of energy in an identified segments is derived by calculating:

$E_{\mathrm{PostT}}=\underset{k=\mathrm{Tstart}}{\overset{\mathrm{Tend}}{\sum <br><br>}}<br><br><br><br>s<br><br>\left(k\right)$

wherein s(k) is a digitized version of the filtered cardiac signal, T_start and T_end are start and end points, respectively, of the identified segment, and k represents individual samples of the digitized signal.

6. The method of claim 5 further comprising initially calculating T_start and T_end by:

identifying a pair of consecutive ventricular depolarizations (S1 and S2) within the cardiac signals;

determining a time interval (S_to_S_Interval) between S1 and S2;

setting T_start equal to S1+S_to_S_Interval/4; and

setting T_end equal to S2−

S_to_S_Interval/4.

7. The method of claim 1 wherein the running average of a plurality of identified segments is derived by calculating:

E<sub>PostT—

Ave</sub>(i)=α

·

E<sub>PostT—

Ave</sub>(i−

1)+(1−

α

)·

E_Post

at time increment “

i”

where α

is a predetermined value and wherein E<sub>PostT—

Ave</sub>(0) is set to a default value.

8. The method of claim 7 wherein α

is equal to 15/16.

9. The method of claim 1 wherein, during comparing, the absolute value of the difference between the total amount of energy in one of the identified segments and the running average of the total amount of energy in a plurality of the identified segments is compared to the first threshold, and the absolute value of the running average is compared to the second threshold.

10. The method of claim 1 further comprising detecting the end of an episode of ischemia if both the first threshold and the second threshold are not exceeded for a predetermined number of heart beats.

11. The method of claim 1 further comprising:

generating a warning signal if ischemia is detected.

12. The method of claim 11 wherein generating a warning signal comprises:

applying a perceptible electrical notification signal to subcutaneous tissue.

13. The method of claim 11 wherein generating a warning signal comprises:

transmitting a notification signal to a warning device external to the patient.

14. An implantable medical device comprising:

a sensing system operative to receive electrical cardiac signals having a series of cycles, each cycle including a ventricular repolarization followed by a ventricular depolarization, and to identify segments of the cardiac signals subsequent to a ventricular repolarization and prior to the ventricular depolarization following the ventricular repolarization;

a signal integration unit operative to determine energy values representative of a total amount of energy within identified segments; and

a threshold comparison unit operative to;

take a difference between the total amount of energy in one of the identified segments and the running average of the total amount of energy in a plurality of the identified segments and compare said difference to a first threshold;

compare just the running average to a second threshold; and

detect cardiac ischemia if either the first threshold or the second threshold is exceeded for a predetermined number of heart beats.

15. In an implantable medical device for implant within a patient, a system comprising:

means for receiving electrical cardiac signals having a series of cycles, each cycle including a ventricular repolarization followed by a ventricular depolarization;

means for identifying segments of the cardiac signals subsequent to a ventricular repolarization and prior to the ventricular depolarization following the ventricular repolarization;

means for deriving energy values representative of a total amount of energy within identified segments;

means for taking a difference between the total amount of energy in one of the identified segments and the running average of the total amount of energy in a plurality of the identified segments and comparing said difference to a first threshold;

means for comparing just the running average to a second threshold; and

means for detecting cardiac ischemia if either the first threshold or the second threshold is exceeded for a predetermined number of heart beats.