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Methods and systems described herein are especially useful wherein monitoring for atrial fibrillation (AF) is based on RR interval variability as measured from an electrocardiogram (ECG) signal. An activity threshold, which can be patient specific, is obtained. Patient activity is monitored. Based o

Methods and systems described herein are especially useful wherein monitoring for atrial fibrillation (AF) is based on RR interval variability as measured from an electrocardiogram (ECG) signal. An activity threshold, which can be patient specific, is obtained. Patient activity is monitored. Based on the monitored patient activity and the activity threshold, there is a determination of when it is likely that AF monitoring based on RR interval variability is adversely affected by patient activity. When it has been determined that it is likely that AF monitoring based on RR interval variability is adversely affected by patient activity, whether and/or how AF monitoring is performed is modified.

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1. A method for use with a system that monitors for atrial fibrillation (AF) based on RR interval variability as measured from an electrocardiogram (ECG) signal, the method comprising: (a) obtaining a patient specific activity threshold;(b1) monitoring patient activity with an activity sensor not ba

1. A method for use with a system that monitors for atrial fibrillation (AF) based on RR interval variability as measured from an electrocardiogram (ECG) signal, the method comprising: (a) obtaining a patient specific activity threshold;(b1) monitoring patient activity with an activity sensor not based on a cardiac electrical signal;(b2) monitoring for AF based on RR interval variability in said ECG signal;(c) determining, based on the monitored patient activity and the patient specific activity threshold, when it is likely that AF monitoring based on RR interval variability is adversely affected by patient activity, wherein step (c) includes: (c.1) determining activity correlation values based on both the monitored patient activity and heart rate reserve values corresponding to the monitored patient activity; and(c.2) determining that it is likely that AF monitoring based on RR interval variability is adversely affected, when the activity correlation values exceed the patient specific activity threshold; and(d) modifying whether and/or how AF monitoring is performed, when it has been determined that it is likely that AF monitoring based on RR interval variability is adversely affected by patient activity. 2. The method of claim 1, wherein when it has been determined at step (c) that it is likely that AF monitoring based on RR interval variability is adversely affected, step (d) comprises one of the following: (d.1) inhibiting AF monitoring;(d.2) ignoring detected episodes of AF; and(d.3) storing information about detected episodes of AF along with an indication that the detected episodes of AF were detected during elevated patient activity. 3. The method of claim 1, wherein when it has been determined at step (c) that it is likely that AF monitoring based on RR interval variability is adversely affected, step (d) comprises: storing information about detected episodes of AF along with an indication that the detected episodes of AF were detected during elevated patient activity;analyzing the stored information about episodes of AF that were detected during elevated patient activity, in order to determine which detected episodes were actually AF episodes and which detected episodes were detected due to patient activity; andadjusting a variability threshold, used to detected episodes of AF, based on results of the analyzing. 4. The method of claim 1, wherein step (c) further includes, after it has been determined at (c.2) that it is likely that AF monitoring based on RR interval variability is adversely affected: determining that it is no longer likely that AF monitoring based on RR interval variability is adversely affected, when the activity correlation values do not exceed the activity threshold for at least a specified duration. 5. The method of claim 1, wherein step (a) includes: (a.1) determining, based on the monitored patient activity, a value indicative of long term average patient activity; and(a.2) determining the patient specific activity threshold by adding an offset to the value indicative of long term average patient activity. 6. The method of claim 5, wherein step (a.1) includes: (i.) determining, based on the monitored patient activity, a value indicative of short term average patient activity; and(ii.) determining, based on the value indicative of short term average patient activity and a previous value indicative of long term average patient activity, the value indicative of long term average patient activity. 7. The method of claim 5, wherein the offset is at least twice the value indicative of long term average patient activity. 8. The method of claim 1, wherein step (a) includes determining the patient specific activity threshold based on historical patient heart-rate data and historical patient activity data. 9. The method of claim 8, wherein step (a) includes: calculating a plurality of time-period activity thresholds over a period of time; andcalculating the patient specific activity threshold value based on the plurality of time-period activity thresholds;wherein the patient specific activity threshold is based on the average of the plurality of time-period activity thresholds. 10. The method of claim 9, wherein the calculating a plurality of time-period activity thresholds over a period of time comprises: calculating a plurality of correlation values derived from patient heart-rate, patient activity, a heart-rate offset parameter and an activity offset parameter over a plurality of portions of the period of time;identifying a peak correlation value for each of the plurality of portions; andfor each of the plurality of portions, obtaining a time-period activity threshold as a function of the peak correlation value corresponding to the portion. 11. The method of claim 10, wherein the heart-rate offset parameter is one of: a minimum heart rate measured over an offset acquisition period of time; anda minimum activity value of a plurality of maximum activity values measured over an offset acquisition period of time. 12. The method of claim 1, wherein when it has not been determined that it is likely that AF monitoring based on RR interval variability is adversely affected, then monitoring for AF comprises: measuring RR intervals from an ECG signal;determining RR interval variability based on the measured RR intervals;comparing the RR interval variability to a variability threshold; anddetecting AF when the RR interval variability exceeds the variability threshold. 13. The method of claim 1, wherein when it has not been determined that it is likely that AF monitoring based on RR interval variability is adversely affected, then monitoring for AF comprises: obtaining a baseline RR interval variability;measuring RR intervals from an ECG signal;determining RR interval variability based on the measured RR intervals;determining an increase in RR interval variability by determining a difference between the determined RR interval variability and the baseline RR interval variability;comparing the increase in RR interval variability to a variability threshold; anddetecting AF when the increase in RR interval variability exceeds the variability threshold. 14. A system, comprising: an atrial fibrillation (AF) monitor configured to monitor for AF based on RR interval variability as measured from an ECG signal;memory to store information about detected episodes of AF;at least one activity sensor configured to monitor patient activity without using a cardiac electrical signal;at least one processor configured to determine:activity correlation values based on both the monitored patient activity and heart rate reserve values corresponding to the monitored patient activity; andthat it is likely that AF monitoring based on RR interval variability is adversely affected, when the activity correlation values exceed the patient specific activity threshold; andwherein whether to or how to monitor for AF is modified, when the at least one processor determines that it is likely that AF monitoring based on RR interval variability is adversely affected by patient activity. 15. The system of claim 14, wherein the AF monitor is implemented by the at least one processor. 16. The system of claim 14, wherein the system is an implantable system configured to obtain the ECG signal using implantable extracardiac sensing electrodes. 17. The system of claim 14, wherein the system is a non-implantable system configured to obtain the ECG signal using surface electrodes. 18. The system of claim 14, wherein when it has been determined that it is likely that AF monitoring based on RR interval variability is adversely affected, at least one of following occurs: the AF monitor is inhibited;episodes of AF detected by the AF monitor are ignored; andinformation about episodes of AF detected by the AF monitor are stored in the memory along with an indication that the detected episodes of AF were detected during elevated patient activity. 19. A method for use with a system that monitors for atrial fibrillation (AF) based on RR interval variability as measured from an electrocardiogram (ECG) signal, the method comprising: (a) obtaining a patient specific activity threshold;(b1) monitoring patient activity with an activity sensor not based on a cardiac electrical signal;(b2) monitoring for AF based on RR interval variability in said ECG signal;(c) determining, based on both the monitored patient activity and the patient specific activity threshold, when it is likely that AF monitoring based on RR interval variability is adversely affected by patient activity, wherein step (c) includes: (c.1) determining activity correlation values based on both the monitored patient activity and corresponding heart rate data; and(c.2) determining that it is likely that AF monitoring based on RR interval variability is adversely affected, when the activity correlation values exceed the patient specific activity threshold;(d) modifying whether and/or how AF monitoring is performed, when it has been determined that it is likely that AF monitoring based on RR interval variability is adversely affected by patient activity, wherein step (d) comprises: storing information about detected episodes of AF along with an indication that the detected episodes of AF were detected during elevated patient activity;analyzing the stored information about episodes of AF that were detected during elevated patient activity, in order to determine which detected episodes were actually AF episodes and which detected episodes were detected due to patient activity; andadjusting a variability threshold, used to detected episodes of AF, based on results of the analyzing. 20. The method of claim 19, wherein step (c.2) includes determining that it is likely that AF monitoring based on RR interval variability is adversely affected, when the activity correlation values exceed the patient specific activity threshold for at least a specified first duration. 21. The method of claim 19, wherein step (c) further includes, after it has been determined at (c.2) that it is likely that AF monitoring based on RR interval variability is adversely affected: determining that it is no longer likely that AF monitoring based on RR interval variability is adversely affected, when the activity correlation values do not exceed the patient specific activity threshold for at least a specified second duration,wherein the specified second duration is longer than the specified first duration.