Assessing autonomic activity using baroreflex analysis

Abstract

An apparatus capable of being implanted in a patient's body, including a posture sensor, a heart rate sensor, and a processor coupled to the posture sensor and the heart rate sensor. The processor is configured to measure the patient's heart rate via the cardiac signal sensor, detect a change in a posture of the patient's body via the posture sensor, and determine an autonomic tone of the patient's body based on the patient's heart rate measured during a time period corresponding to the change in posture.

1 . A method, comprising: implantably detecting changes in posture of a patient's body; determining baroreflex responses to the posture changes; and determining an autonomic tone of the patient based on the baroreflex responses. 
     
     
         2 . The method of  claim 1 , wherein determining baroreflex responses to the posture changes comprises measuring the patient's heart rate during time periods corresponding to the changes in posture. 
     
     
         3 . The method of  claim 2 , wherein determining baroreflex responses to the posture changes further comprises measuring the patient's blood pressure during the time periods corresponding to the changes in posture. 
     
     
         4 . The method of  claim 3 , wherein measuring the blood pressure of the patient's body comprises measuring the blood pressure via a force transducer sensitive to displacement of a blood vessel. 
     
     
         5 . The method of  claim 4 , wherein measuring the blood pressure of the patient's body via a force transducer comprises measuring the blood pressure via at least one of a blood-vessel-implantable stent-like transducer and a cuff transducer placed around a blood vessel. 
     
     
         6 . The method of  claim 5 , wherein measuring the blood pressure of the patient's body via the stent-like transducer comprises measuring the blood pressure from the stent-like transducer implanted in a vein that is paralleled by an artery. 
     
     
         7 . The method of  claim 1 , further comprising forming a baseline data set from long-term measurements of posture changes and baroreflex responses. 
     
     
         8 . The method of  claim 1 , further comprising forming a threshold for diagnostic alarm based on the baseline data set. 
     
     
         9 . The method of  claim 1 , wherein determining the autonomic imbalance of the patient comprises performing a baroreflex sensitivity analysis (BSA) based on blood pressure and cardiac signals measured during a time period corresponding to the posture change. 
     
     
         10 . The method of  claim 9 , wherein performing the BSA comprises measuring intervals between heartbeats at least during the time period corresponding to the posture change. 
     
     
         11 . The method of  claim 10 , wherein performing the BSA comprises performing a power spectral analysis on the intervals between the heartbeats. 
     
     
         12 . The method of  claim 10 , wherein performing the BSA comprises determining a rate of change of between systolic blood pressure relative to the intervals during the time period corresponding to the posture change. 
     
     
         13 . The method of  claim 1 , further comprising determining the patient's susceptibility to heart disease based on the autonomic tone. 
     
     
         14 . The method of  claim 13 , wherein determining the patient's susceptibility to heart disease comprises determining the patient's susceptibility to arrhythmia based on the autonomic tone. 
     
     
         15 . The method of  claim 13 , wherein determining the patient's susceptibility to heart disease comprises determining the patient's susceptibility to sudden cardiac death based on the autonomic tone. 
     
     
         16 . An apparatus capable of being implanted in a patient's body, comprising: a posture sensor; a heart rate sensor; and a processor coupled to the posture sensor and the heart rate sensor, the processor configured to, measure the patient's heart rate via the cardiac signal sensor; detect a change in a posture of the patient's body via the posture sensor; and determine an autonomic tone of the patient's body based on the patient's heart rate measured during a time period corresponding to the change in posture. 
     
     
         17 . The apparatus of  claim 16 , wherein the time period encompasses a first time period before the change in posture and a second time period after the change in posture. 
     
     
         18 . The apparatus of  claim 16 , further comprising a blood pressure sensor, and wherein the processor is further configured to measure the patient's blood pressure via the blood pressure sensor and determine the autonomic tone based on the blood pressure measured during the time period. 
     
     
         19 . The apparatus of  claim 18 , wherein the blood pressure sensor comprises a force transducer sensitive to displacement of a blood vessel. 
     
     
         20 . The apparatus of  claim 19 , wherein the force transducer comprises a stent-like element capable of being implanted within a blood vessel of the patient's body. 
     
     
         21 . The apparatus of  claim 19 , wherein the force transducer comprises a cuff transducer capable of being placed around a blood vessel. 
     
     
         22 . The apparatus of  claim 18 , wherein the processor is configured to determine the autonomic tone by performing a baroreflex sensitivity analysis (BSA) based on the heart rate and blood pressure measured during the time period. 
     
     
         23 . The apparatus of  claim 22 , wherein performing the BSA comprises determining a rate of change of between systolic blood pressure relative to heart beat intervals during the change in posture. 
     
     
         24 . The apparatus of  claim 16 , wherein the posture sensor comprises at least one of a piezoelectric sensor and an accelerometer. 
     
     
         25 . The apparatus of  claim 16 , wherein the posture sensor comprises: a case having a plurality of conductive surfaces disposed on an inner surface of the case; a conductive solid movably disposed along the inner surface of the case, wherein movement of the conductive solid within the case causes the conductive solid to create an electrical connection between at least two surfaces of the plurality of surfaces. 
     
     
         26 . The apparatus of  claim 16 , wherein the processor is configured to determine the autonomic tone by performing a baroreflex sensitivity analysis (BSA) based on the heart rate signals measured during the time period. 
     
     
         27 . The apparatus of  claim 26 , wherein the processor is configured to perform the BSA based on a power spectral analysis on heart beat intervals determined via the heart rate sensor. 
     
     
         28 . The apparatus of  claim 26 , wherein the processor is configured to perform the BSA based on a time rate of change of heart beat intervals during the change in posture. 
     
     
         29 . A posture sensing system comprising: means for detecting a change in posture of a patient's body; means for measuring the patient's heart rate during at least one of a first time period before the change in posture, a second time period during the change in posture, and a third time period after the change in posture; and means for determining an autonomic tone of the patient based on the heart rate measured during the at least one of the first, second, and third time periods. 
     
     
         30 . The system of  claim 29 , further comprising: means for measuring the patient's blood pressure; and means for determining the patients autonomic tone based on the blood pressure measured during the at least one of the first, second, and third time periods.