Systems and methods for estimating central arterial blood pressure of a patient

Abstract

In specific embodiments, a method for estimating a patient's central arterial blood pressure (CBP) for use with an implantable system, comprises (a) using an implanted sensor at a first site to obtain a first signal indicative of changes in arterial blood volume at the first site, the first site being along one or more peripheral arterial structures of the patient, (b) using an implanted sensor at a second site to obtain a second signal indicative of changes in arterial blood volume at the second site, the second site being a distance from the first site downstream along an arterial path of the peripheral arterial structure of the patient, and (c) using implanted electrodes to obtain a signal indicative of electrical activity of the patient's heart. The method further comprises (d) determining a time t 1 from a predetermined feature of the signal indicative of electrical activity to a predetermined feature of one of the first and second signals, the time t 1 being a first pulse arrival time (PAT 1 ) indicative of how long it takes a pulse wave to travel from the patient's aorta to one of the first and second sites, (e) determining a time t 2 from a predetermined feature of the signal indicative of electrical activity to a predetermined feature of the other of the first and second signals, the time t 2 being a second pulse arrival time (PAT 2 ) indicative of how long it takes a pulse wave to travel from the patient's aorta to the other of the first and second sites, and (f) estimating the patient's central arterial blood pressure (CBP) based on the first pulse arrival time (PAT 1 ) and the second pulse arrival time (PAT 2 ).

1 . For use with an implantable system, a method for estimating a patient's central arterial blood pressure (CBP), the method comprising:
 (a) using an implanted sensor at a first site to obtain a first signal indicative of changes in arterial blood volume at the first site, the first site being along one or more peripheral arterial structures of the patient;   (b) using an implanted sensor at a second site to obtain a second signal indicative of changes in arterial blood volume at the second site, the second site being a distance from the first site downstream along an arterial path of the one or more peripheral arterial structures of the patient;   (c) using implanted electrodes to obtain a signal indicative of electrical activity of the patient's heart;   (d) determining a time t 1  from a predetermined feature of the signal indicative of electrical activity of the patient's heart to a predetermined feature of one of the first and second signals, the time t 1  being a first pulse arrival time (PAT 1 ) indicative of how long it takes a pulse wave to travel from the patient's aorta to one of the first and second sites;   (e) determining a time t 2  from a predetermined feature of the signal indicative of electrical activity of the patient's heart to a predetermined feature of the other of the first and second signals, the time t 2  being a second pulse arrival time (PAT 2 ) indicative of how long it takes a pulse wave to travel from the patient's aorta to the other of the first and second sites; and   (f) estimating the patient's central arterial blood pressure (CBP) based on the first pulse arrival time (PAT 1 ) and the second pulse arrival time (PAT 2 ).   
     
     
         2 . The method of  claim 1 , wherein step (f) comprises:
 (f.1) estimating a peripheral pulse wave velocity (PPWV), indicative of a pulse wave velocity between the first and second sites, based on the pulse arrival times (PAT 1  and PAT 2 ) and the distance from the first site to the second site;   (f.2) estimating a central pulse wave velocity (CPWV), indicative of a pulse wave velocity proximal the patient's aorta, based on the first pulse arrival time (PAT 1 ) and the peripheral pulse wave velocity (PPWV); and   (f.3) estimating the patient's central arterial blood pressure (CBP) based on the estimated central pulse wave velocity (CPWV).   
     
     
         3 . The method of  claim 2 , wherein step (f.2) comprises estimating the central pulse wave velocity (CPWV) using the following equation: 
       
         
           
             
               
                 v 
                 cpw 
               
                
               
                   
               
               ≈ 
               
                 
                   
                     v 
                     ppw 
                   
                   * 
                   
                     d 
                     c 
                   
                 
                 
                   
                     
                       v 
                       ppw 
                     
                     * 
                     
                       Pat 
                       x 
                     
                   
                   - 
                   
                     d 
                     p 
                   
                 
               
             
           
         
       
       where
 v cpw  is the estimated central pulse wave velocity (CPWV), 
 v ppw  is the estimated peripheral pulse wave velocity (PPWV), 
 PAT x  is one of PAT 1  and PAT 2 , 
 d c  is a distance travelled in central arterial structures, and 
 d p  is a distance traveled in peripheral arterial structures to the measurement location of PAT x . 
 
     
     
         4 . The method of  claim 2 , wherein step (f.3) comprises estimating the patient's central arterial blood pressure (CBP) based on the square of the central pulse wave velocity (CPWV). 
     
     
         5 . The method of  claim 4 , wherein step (f.3) comprises estimating the patient's central arterial blood pressure (CBP) using the equation:
   CBP≈k1*v cpw   2 +k2
   
       where
 CBP is the estimated central arterial blood pressure, 
 v cpw  is the estimated central pulse wave velocity (CPWV), 
 k1 is a constant indicative of a linearized scaling factor between pulse wave velocity and blood pressure, and 
 k2 is a constant indicative of an offset used with the scaling factor to make a linear approximation of blood pressure based on pulse wave velocity. 
 
     
     
         6 . The method of  claim 5 , further comprising determining the constants k 1  and k 2  by:
 while the patient is in an initial position 
 using a secondary technique to measure initial peripheral blood pressure of the patient, 
 using the implanted sensor at the first site to obtain a first calibration signal indicative of a change in arterial blood volume, and 
 using the implanted sensor at the second site to obtain a second calibration signal indicative of a change in blood volume; 
 while the patient is in a secondary position 
 using a secondary technique to measure a secondary peripheral blood pressure of the patient, 
 using the implanted sensor at the first site to obtain a third calibration signal indicative of a change in arterial blood volume, and 
 using the implanted sensor at the second site to obtain a fourth calibration signal indicative of a change in blood volume; and 
 calculating the constants k 1  and k 2  using the first calibration signal, the second calibration signal, the third calibration signal, the fourth calibration signal, the initial peripheral blood pressure, the secondary peripheral blood pressure, and the distance from the first site to the second site. 
 
     
     
         7 . The method of  claim 2 , further comprising:
 (g) estimating the patient's peripheral arterial blood pressure (PBP) based on the peripheral pulse wave velocity (PPWV).   
     
     
         8 . The method of  claim 7 , wherein step (g) comprises estimating the patient's peripheral arterial blood pressure (PBP) using the equation:
   PBP≈k3*v ppw   2 +k4
   
       where
 PBP is the estimated peripheral arterial blood pressure, 
 v ppw  is the estimated peripheral pulse wave velocity (PPWV), 
 k 3  is a constant indicative of a linearized scaling factor between pulse wave velocity and blood pressure (accounting for blood density, blood volume and vascular compliance), and 
 k 4  is a constant indicative of an offset used with the scaling factor to make a linear approximation of blood pressure based on pulse wave velocity. 
 
     
     
         9 . The method of  claim 8 , further comprising determining the constants k 3  and k 4  by:
 while the patient is in an initial position 
 using a secondary technique to measure initial peripheral blood pressure of the patient, 
 using the implanted sensor at the first site to obtain a first calibration signal indicative of a change in arterial blood volume, and 
 using the implanted sensor at the second site to obtain a second calibration signal indicative of a change in blood volume; 
 while the patient is in a secondary position 
 using a secondary technique to measure a secondary peripheral blood pressure of the patient, 
 using the implanted sensor at the first site to obtain a third calibration signal indicative of a change in arterial blood volume, and 
 using the implanted sensor at the second site to obtain a fourth calibration signal indicative of a change in blood volume; and 
 calculating the constants k 3  and k 4  using the first calibration signal, the second calibration signal, the third calibration signal, the fourth calibration signal, the initial peripheral blood pressure, the secondary peripheral blood pressure, and the distance from the first site to the second site. 
 
     
     
         10 . The method of  claim 1 , wherein:
 step (a) comprises using an implanted optical sensor at the first site to obtain a first photoplethysmography (PPG) signal indicative of changes in arterial blood volume at the first site;   step (b) comprises using an implanted optical sensor at the second site to obtain a second photoplethysmography (PPG) signal indicative of changes in arterial blood volume at the second site; and   step (c) comprises using implanted electrodes to obtain an intracardiac electrogram (IEGM) signal indicative of electrical activity of the patient's heart.   
     
     
         11 . The method of  claim 10 , wherein in steps (d) and (e):
 the predetermined features of the first and second PPG signals are selected from the group consisting of   the minimum amplitude of the PPG signal,   the maximum upward slope of the PPG signal,   the maximum amplitude of the PPG signal,   the maximum downward slope of the PPG signal prior to the dicrotic notch,   the dicrotic notch of the PPG signal, and   the maximum downward slope of the PPG signal following the dicrotic notch; and   the predetermined feature of the IEGM signal is selected from the group consisting of   a Q-wave,   a R-wave, and   a QRS complex.   
     
     
         12 . A system for estimating a patient's central arterial blood pressure (CBP), comprising:
 a first sensor arrangeable at a first site to obtain a first signal indicative of changes in arterial blood volume at the first site, the first site being along one or more peripheral arterial structures of the patient;   a second sensor arrangeable at a second site to obtain a second signal indicative of changes in arterial blood volume at the second site, the second site being a distance from the first site downstream along an arterial path of the one or more peripheral arterial structures of the patient;   one or more electrodes to obtain a signal indicative of electrical activity of the patient's heart; and   a monitor adapted to estimate the patient's central arterial blood pressure (CBP) based on the signal indicative of electrical activity, the first signal and the second signal.   
     
     
         13 . The system of  claim 12 , wherein the monitor is further adapted to
 determine a time t 1  from a predetermined feature of the signal indicative of electrical activity to a predetermined feature of one of the first and second signals, the time t 1  being a first pulse arrival time (PAT 1 ) indicative of how long it takes a pulse wave to travel from the patient's aorta to one of the first and second sites,   determine a time t 2  from a predetermined feature of the signal indicative of electrical activity to a predetermined feature of the other of the first and second signals, the time t 2  being a second pulse arrival time (PAT 2 ) indicative of how long it takes a pulse wave to travel from the patient's aorta to the other of the first and second sites, and   estimate the patient's central arterial blood pressure (CBP) based on the first pulse arrival time (PAT 1 ) and the second pulse arrival time (PAT 2 ).   
     
     
         14 . The system of clam 13, wherein the monitor is further adapted to
 estimate a peripheral pulse wave velocity (PPWV), indicative of a pulse wave velocity between the first and second sites, based on the pulse arrival times (PAT 1  and PAT 2 ) and the distance d from the first site to the second site,   estimate a central pulse wave velocity (CPWV), indicative of a pulse wave velocity proximal the patient's aorta, based on the first pulse arrival time (PAT 1 ) and the peripheral pulse wave velocity (PPWV), and   estimate the patient's central arterial blood pressure (CBP) based on the estimated central pulse wave velocity (CPWV).   
     
     
         15 . The system of  claim 12 , wherein:
 the first sensor is an optical sensor capable of obtaining a first photo-plethysmography (PPG) signal indicative of changes in arterial blood volume at the first site;   the second sensor is an optical sensor capable of obtaining a second photo-plethysmography (PPG) signal indicative of changes in arterial blood volume at the second site; and   the one or more electrodes are implantable to obtain an intracardiac electrogram (IEGM) signal indicative of electrical activity of the patient's heart.   
     
     
         16 . A method for estimating a patient's central arterial blood pressure (CBP), the method comprising:
 (a) obtaining a first signal indicative of changes in arterial blood volume at a first site along one or more peripheral arterial structures of the patient;   (b) obtaining a second signal indicative of changes in arterial blood volume at the second site a distance from the first site downstream along an arterial path of the one or more peripheral arterial structures of the patient;   (c) obtaining a signal indicative of electrical activity of the patient's heart;   (d) determining, based on the signal indicative of electrical activity of the patient's heart and the first signal indicative of changes in arterial blood volume, a first pulse arrival time (PAT 1 ) indicative of how long it takes a pulse wave to travel from the patient's aorta to the first site;   (e) determining, based on the signal indicative of electrical activity of the patient's heart and the second signal indicative of changes in arterial blood volume, a second pulse arrival time (PAT 2 ) indicative of how long it takes a pulse wave to travel from the patient's aorta to the second site; and   (f) estimating the patient's central arterial blood pressure (CBP) based on the first pulse arrival time (PAT 1 ), the second pulse arrival time (PAT 2 ), and the distance from the first site to the second site.   
     
     
         17 . The method of  claim 16 , wherein step (f) comprises:
 (f.1) estimating a peripheral pulse wave velocity (PPWV), indicative of a pulse wave velocity between the first and second sites, based on the pulse arrival times (PAT 1  and PAT 2 ) and the distance from the first site to the second site;   (f.2) estimating a central pulse wave velocity (CPWV), indicative of a pulse wave velocity proximal the patient's aorta, based on the first pulse arrival time (PAT 1 ) and the peripheral pulse wave velocity (PPWV); and   (f.3) estimating the patient's central arterial blood pressure (CBP) based on the estimated central pulse wave velocity (CPWV).   
     
     
         18 . The method of  claim 17 , wherein step (f.3) comprises estimating the patient's central arterial blood pressure (CBP) based on the square of the central pulse wave velocity (CPWV). 
     
     
         19 . The method of  claim 17 , further comprising:
 (g) estimating the patient's peripheral arterial blood pressure (PBP) based on the peripheral pulse wave velocity (PPWV).   
     
     
         20 . The method of  claim 16 , wherein:
 the first signal indicative of changes in arterial blood volume comprises a first photoplethysmography signal;   the second signal indicative of changes in arterial blood volume comprises a second photoplethysmography signal; and   the signal indicative of electrical activity of the patient's heart comprises a intracardiac electrogram or an electrocardiogram.