Apparatus and method for identifying atrial arrhythmia by far-field sensing
In a subcutaneous implantable cardioverter/defibrillator, cardiac arrhythmias are detected to determine necessary therapeutic action. Cardiac signal information is sensed from far field electrodes implanted in a patient. The sensed cardiac signal information is then amplified and filtered. Parameters such as rate, QRS pulse width, cardiac QRS slew rate, amplitude and stability measures of these parameters from the filtered cardiac signal information are measured, processed and integrated to determine if the cardioverter/defibrillator needs to initiate therapeutic action.
1. A method of identifying atrial fibrillation in an implantable defibrillator comprising:
capturing cardiac signals from electrodes disposed beneath the skin and outside of a patient's ribcage;
identifying R-waves in the cardiac signals and observing whether intervals between R-waves are stable or irregular;
if the R-wave intervals are irregular, observing whether QRS morphology is similar on a beat-to-beat basis; and
if R-wave intervals are irregular and QRS morphology is similar on a beat-to-beat basis, determining that the patient is experiencing atrial fibrillation.
2. The method of claim 1 wherein the implantable defibrillator comprises a lead assembly coupled to a canister that houses operational circuitry for the cardioverter-defibrillator, and the method further comprises placing the canister and the lead assembly in the patient subcutaneously without directly contacting the patient's heart or entering the intrathoracic blood vessels of the patient.
3. The method of claim 1 further comprising withholding cardiac stimulus that would otherwise be delivered to the patient if it is determined that the patient is experiencing atrial fibrillation.
4. The method of claim 1 further comprising:
calculating a cardiac rate for the patient using intervals between the identified R-waves;
determining that the cardiac rate exceeds a predefined threshold; and
if it is determined that the patient is experiencing atrial fibrillation, withholding therapy that would otherwise be delivered.
5. The method of claim 1 wherein the implantable defibrillator comprises operational circuitry for analyzing cardiac signals and each of the steps of capturing, identifying, observing, and determining are performed by the operational circuitry.
6. The method of claim 1 further comprising filtering the captured cardiac signals by use of a first sensing channel and a second sensing channel, wherein the first sensing channel is subjected to narrow-band filtering to generate a signal for use in identifying R-waves in the cardiac signal and the second channel is subject to wide-band filtering to generate a signal for use in observing whether QRS morphology is similar on a beat-to-beat basis.
7. An implantable cardioverter defibrillator (ICD) comprising a housing containing operational circuitry and configured coupling to a plurality of electrodes for implantation in a patient, wherein the operational circuitry is configured to perform the following method of cardiac signal analysis:
capturing cardiac signals from the electrodes;
identifying R-waves in the cardiac signals and observing whether intervals between R-waves are stable or irregular;
if the R-wave intervals are irregular, observing whether QRS morphology is similar on a beat-to-beat basis; and
if R-wave intervals are irregular and QRS morphology is similar on a beat-to-beat basis, determining that the patient is experiencing atrial fibrillation.
8. The ICD of claim 7 wherein the operational circuitry is further configured to withhold cardiac stimulus that would otherwise be delivered to the patient if it is determined that the patient is experiencing atrial fibrillation.
9. The ICD of claim 7 in which the operational circuitry comprises a first sensing channel and a second sensing channel, both of which operate using a signal captured between a pair of electrodes, wherein the first sensing channel is subjected to narrow-band filtering to generate a signal for use in identifying R-waves in the cardiac signal and the second channel is subject to wide-band filtering to generate a signal for use in observing whether QRS morphology is similar on a beat-to-beat basis.
10. The ICD of claim 7 wherein the operational circuitry is configured for use with electrodes implanted subcutaneously which do not directly contact the patient's heart or reside in the intrathoracic blood vessels of the patient.