Third heart sound activity index for heart failure monitoring

Abstract

A cardiac rhythm management system provides for the trending of a third heart sound (S 3 ) index. The S 3 index is a ratio, or an estimate of the ratio, of the number of S 3 beats to the number of all heart beats, where the S 3 beats are each a heart beat during which an occurrence of S 3 is detected. An implantable sensor such as an accelerometer or a microphone senses an acoustic signal indicative heart sounds including S 3. An S 3 detector detects occurrences of S 3 from the acoustic signal. A heart sound processing system trends the S 3 index on a periodic basis to allow continuous monitoring of the S 3 activity level, which is indicative of conditions related to heart failure.

1 . (canceled) 
     
     
         2 . A medical system, comprising:
 a heart sound detector configured to:
 determine a second heart sound (S 2 ) timing estimate using a received heart sound signal and heart rate information; 
 determine a third heart sound (S 3 ) window with respect to the determined S 2  timing estimate; and 
 measure an S 3  amplitude using at least a portion of the heart sound signal within the determined S 3  window; and 
   a heart failure detector configured to detect a condition indicative of worsening heart failure (WHF) using the measured S 3  amplitude.   
     
     
         3 . The system of  claim 2 , wherein the S 3  amplitude includes signal energy of at least the portion of the heart sound signal within the determined S 3  window. 
     
     
         4 . The system of  claim 2 , wherein the S 3  window begins at a first delay with respect to the determined S 2  timing estimate, wherein the first delay is programmable between 100 and 200 milliseconds. 
     
     
         5 . The system of  claim 2 , wherein the heart sound detector, to detect the S 2  timing estimate, is configured to:
 determine a S 2  window from the heart sound signal, the S 2  window starting at a second delay with respect to a ventricular event, the second delay dynamically adjusted according to the heart rate information; and 
 determine the S 2  timing estimate corresponding to a timing when the heart sound signal exceeds a S 2  threshold during the determined S 2  window. 
 
     
     
         6 . The system of  claim 2 , wherein the second delay is linearly related to a heart rate. 
     
     
         7 . The system of  claim 2 , further comprising a therapy control circuit configured to deliver a heart failure therapy based on the detected condition indicative of WHF. 
     
     
         8 . The system of  claim 2 , wherein the heart failure detector is configured to:
 generate an S 3  index trend using measurements of S 3  amplitude within a specified time period; and   detect the condition indicative of WHF using the S 3  index trend.   
     
     
         9 . The system of  claim 8 , wherein the S 3  index trend includes a plurality of S 3  indices indicative of prevalence of S 3  activity, the S 3  indices each generated using measurements of S 3  amplitude within a respective measurement period that includes a plurality of heart beats. 
     
     
         10 . The system of  claim 9 , wherein the S 3  indices each includes a ratio of (1) a count of S 3  amplitude measurements, within the respective measurement period, that exceeds an S 3  amplitude threshold to (2) a count of the plurality of heart beats within the respective measurement period. 
     
     
         11 . The system of  claim 10 , wherein the S 3  amplitude threshold is determined based on signal energy of at least a portion of the heart sound signal within a cardiac cycle. 
     
     
         12 . The system of  claim 11 , wherein the S 3  amplitude threshold is determined based on signal energy of the heart sound signal during systole of the cardiac cycle. 
     
     
         13 . The system of  claim 2 , comprising an accelerometer configured for sensing the heart sound signal including a cardiac vibration signal. 
     
     
         14 . The system of  claim 2 , comprising a microphone configured for sensing the heart sound signal including a cardiac audio signal. 
     
     
         15 . A method for operating a medical system, the method comprising:
 determining, via a heart sound detector, a second heart sound (S 2 ) timing estimate using a received heart sound signal and heart rate information;   determining, via the heart sound detector, a third heart sound (S 3 ) window with respect to the determined S 2  timing estimate;   measuring, via the heart sound detector, an S 3  amplitude using at least a portion of the received heart sound signal within the determined S 3  window; and   detecting, via a heart failure detector, a condition indicative of worsening heart failure (WHF) using the measured S 3  amplitude.   
     
     
         16 . The method of  claim 15 , further comprising:
 generating an S 3  index using measurements of S 3  amplitude over a specified measurement period that includes a plurality of heart beats, the S 3  index indicative of prevalence of S 3  activity; and   generating an S 3  index trend over a plurality of the specified measurement periods, the S 3  index trend including a plurality of S 3  indices;   wherein the detecting the condition indicative of worsening heart failure (WHF) is at least based on the generated S 3  index trend.   
     
     
         17 . The method of  claim 15 , wherein the S 3  amplitude includes signal energy of at least the portion of the heart sound signal within the determined S 3  window. 
     
     
         18 . The method of  claim 15 , wherein the S 3  window begins at a first delay with respect to the determined S 2  timing estimate, and
 wherein the determining the S 2  timing estimate includes:
 determining an S 2  window from the heart sound signal, the S 2  window starting at a second delay with respect to a ventricular event, the second delay dynamically adjusted according to the heart rate information; and 
 determining the S 2  timing estimate corresponding to a timing when the heart sound signal exceeds a S 2  threshold during the determined S 2  window. 
 
 
     
     
         19 . The method of  claim 16 , wherein the generating an S 3  index includes producing a ratio of (1) a count of S 3  amplitude measurements, within the specified measurement period, that exceeds an S 3  amplitude threshold to (2) a count of the plurality of heart beats within the specified measurement period. 
     
     
         20 . The method of  claim 19 , further comprising determining the S 3  amplitude threshold based on signal energy of at least a portion of the heart sound signal within a cardiac cycle. 
     
     
         21 . The method of  claim 16 , further comprising delivering a heart failure therapy based on the detected condition indicative of WHF.