Systems for Detecting Cardiac Arrhythmias

Abstract

A system for calculating a variability value that is indicative of AF by obtaining a signal sequence of a plurality of RR intervals by monitoring electrical activity of a patient's heart. Each RR interval is converted into an instantaneous heart rate value and sorted into ascending order. The difference between each successive heart rate is calculated, discarding the two largest differences. The variability value is calculated by adding the retained differences.

1 . A method for calculating a variability value that is indicative of AF using a sequence of RR intervals, comprising the steps of:
 obtaining a signal sequence comprising a plurality of RR intervals wherein said signal sequence is obtained by monitoring the time intervals between successive heart beats of a patient's heart by measuring at least one of electrical activity; acoustic activity; trans-thoracic impedance; blood pressure; blood velocity; blood oxygenation; heart movement and/or deformation;   converting each RR interval in said sequence into an instantaneous heart rate value;   sorting said instantaneous heart rate values into ascending order;   calculating the difference between each successive heart rate;   discarding the two largest differences generated in the previous step; and   calculating a variability value by adding the retained differences.   
     
     
         2 . The method of  claim 1 , further comprising the steps of segmenting said RR interval sequence into a plurality of segments and generating a variability value for each said segment. 
     
     
         3 . The method of  claim 2 , further comprising the step of filtering said generated variability value for at least one segment using previous or successive segment variability values. 
     
     
         4 . The method of  claim 3  wherein said filtering is implemented by a 7-sample median filter. 
     
     
         5 . The method of  claim 2 , further comprising the steps of comparing at least one said variability value to at least one threshold. 
     
     
         6 . The method of  claim 5  wherein said step of comparing at least one said variability value to at least one threshold comprises comparing at least one said variability value to an AF detection threshold or an AF start/end threshold. 
     
     
         7 . The method of  claim 6 , wherein after said step of comparing at least one said variability value to at least one threshold, a corresponding segment is flagged as ‘probably containing AF’ when said detection threshold is exceeded and an AF event is detected when a pre-determined number of consecutive segments are flagged as ‘probably containing AF’ and a corresponding segment is flagged as ‘AF extension permitted’ when said start/end threshold is exceeded. 
     
     
         8 . The method of  claim 7 , wherein once an AF event has been detected, previous and/or following segments are tested for the AF extension permitted flag and, if found, the detected AF event is extended to include said segment(s). 
     
     
         9 . The method of  claim 1  wherein RR intervals shorter than 180 milliseconds or longer than 2 seconds are discarded from the variability calculation. 
     
     
         10 . The method of  claim 1 , further comprising the step of reducing said variability value if said sequence contains no heart rate interval greater than a heart rate threshold. 
     
     
         11 . The method of  claim 10  wherein said heart rate threshold is 60 beats per minute. 
     
     
         12 . The method of  claim 10  wherein said reduction is implemented by halving said variability value. 
     
     
         13 . The method of  claim 1 , further comprising the step of limiting said variability value if said sequence is identified as containing a heart rate trend. 
     
     
         14 . The method of  claim 13  wherein said heart rate trend is identified by, for each instantaneous heart rate of an RR interval, if the heart rate is significantly greater or less than the previous interval heart rate, defining the start of a trend. 
     
     
         15 . The method of  claim 14  wherein a trend is determined to persist until an RR interval with a significant heart-rate change in an opposite direction is detected. 
     
     
         16 . The method of  claim 15  wherein, once a new trend is determined on a RR interval, all intervals in a previous trend are flagged as being of the previous trend type. 
     
     
         17 . The method of  claim 16  wherein each RR interval in a trend is flagged as being a member of a long heart rate if a consecutive number of RR interval trend flags exceeds a long heart rate threshold. 
     
     
         18 . The method of  claim 17  wherein each RR interval in a trend is flagged as being a member of a short heart rate if a consecutive number of RR interval trend flags exceeds a lower short heart rate threshold. 
     
     
         19 . The method of  claim 18  wherein each RR interval in a trend is flagged as being not trended if no threshold conditions apply. 
     
     
         20 . The method of  claim 19  where, for a plurality of RR intervals, trend flags are tested to establish whether said plurality is dominated by heart trend behavior. 
     
     
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