Extraction of hemodynamic pulse pressure from fluid and myocardial accelerations

A cardiac stimulating apparatus and method is described that non-intrusively determines an amount indicative of hemodynamic pulse pressure from an accelerometer signal. The amount indicative of pulse pressure is determined over several cardiac cycles and is used to optimize cardiac performance by evaluating the amount indicative of pulse pressure over varying timing intervals. The timing intervals are measured between at least one of intrinsic and paced stimulations of pre-selected chambers of the heart and a maximum pulse pressure indicates the optimum timing interval under manipulation. The cardiac stimulating apparatus and method may be used in any of several pacing modes including A-V pacing, V-V pacing, or A-A pacing.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
This invention relates generally to an implantable, programmable, cardiac 
stimulating apparatus and method that non-intrusively determines the pulse 
pressure of a patient's heart. The determined pulse pressure may be 
utilized, for example, to enhance cardiac performance. The cardiac 
performance may be enhanced by first manipulating the pacing rate and 
pacer timing intervals between intrinsic or paced stimulations of 
pre-selected chambers of the patient's heart over a plurality of periods, 
then determining the pulse pressure during each period, identifying the 
pacer timing interval and/or pacing rate that results in the greatest 
pulse pressure, and finally setting the cardiac stimulator to the timing 
interval associated with the greatest pulse pressure to thereby increase 
cardiac performance. 
II. Discussion of the Related Art 
The cardiac output of a patient's heart may be reduced as a result of 
defects, failure, disease, ageing, or other cardiac disorders or 
anomalies. Reduced cardiac output can lead to shortness of breath, 
restricted movement, and even death. Over the years various devices 
including pacers and defibrillators have been used to assist and/or alter 
the intrinsic contractions and pacing of the heart in order to increase 
the cardiac output of the heart. The pacer, for example, typically 
includes a pulse generator, power supply, microprocessor based controller, 
and an electrical lead of suitable construction coupled to the pulse 
generator for unipolar or bi-polar pacing. 
Various methods have been devised to maximize cardiac output, wherein the 
maximum cardiac output is correlated with a measured pulse pressure. 
Typically, the pulse pressure is measured via cardiac catheterization or 
through a pressure sensor positioned on a lead. A pacer of suitable 
construction is required in order to receive a signal from the pressure 
sensor and correlate the received signal with cardiac output. At times it 
may become necessary to replace an already implanted pacemaker with a 
pacer capable of correlating the maximum cardiac output with the measured 
pulse pressure. Ideally, replacement of the pacer would not require 
placement of additional leads or lead ablation and replacement. 
In U.S. Pat. No. 4,566,456 issued to Koning et al., a device is described 
that adjusts a pacer rate relative to right ventricular systolic pressure. 
The right ventricular systolic pressure is measured by a piezoelectric 
pressure sensor mounted on a lead. Koning et al. does not provide for a 
device or method that non-intrusively determines the pulse pressure of a 
patient's heart. The Koning et al. device requires a lead having a 
pressure sensor coupled thereto, and thus requires replacement or use of 
the specialized lead in conjunction with the disclosed pacer. 
In U.S. Pat. No. 5,549,650 issued to Bornzin et al., a device is described 
for providing hemodynamically optimal pacing therapy. The device 
apparently includes a cardiac wall motion sensor which must be 
incorporated into an implantable lead. The rate of pacing therapy is 
controlled by the Bornzin et al. device as a function of the cardiac wall 
velocity signals and cardiac wall displacement signals (mechanical 
activities of the heart generally) transmitted by the cardiac wall motion 
sensor. The Bornzin et al. device does not provide hemodynamically optimal 
pacing therapy by non-invasively measuring the hemodynamic pulse pressure 
of the heart. The Bornzin et al. device requires replacement or use of a 
specialized lead in conjunction with its pacing device. Hence, there is a 
need for a pacemaker that non-intrusively determines the pulse pressure of 
a patient's heart. The present invention addresses these and other needs. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a cardiac stimulation device is 
provided which is capable of non-intrusively determining a hemodynamic 
pulse pressure of the patient's heart. The cardiac stimulating device may 
be programmed to function in a preset pacing mode having a preset pacing 
rate and timing interval to optimize cardiac performance of a patient's 
heart. The cardiac stimulation device is capable of operating in any of a 
plurality of pacing modes, including A-V pacing, V-V pacing and A-A 
pacing, wherein the A-V pacing mode may include A.sub.R -V.sub.R pacing, 
A.sub.R -V.sub.L pacing, A.sub.L -V.sub.R pacing, A.sub.R -V.sub.RL 
pacing, A.sub.L -V.sub.RL pacing, and A.sub.L -V.sub.L pacing. 
The cardiac stimulation device includes a pulse generator for stimulating a 
patient's heart in a preselected pacing mode, a power supply, a 
microprocessor-based controller, and an accelerometer sensor, all of which 
are enclosed in an implantable casing. An internal or external cardiac 
electrogram or other conventional device for identifying cardiac cycles of 
a patient's heart is coupled to the microprocessor based controller. The 
microprocessor-based controller is coupled to both the accelerometer and 
the pulse generator for receiving an input from the former and providing 
control signals to the latter. 
The accelerometer sensor is electrically coupled to the microprocessor 
based controller and the accelerometer transmits a signal to the 
controller associated with fluid and myocardial accelerations of the 
patient's heart. A filtering means is coupled to the accelerometer for 
filtering and conditioning the signal transmitted by the accelerometer to 
produce a waveform related to a pulse pressure within the patient's heart. 
The microprocessor based controller includes a linear prediction means 
(utilizing Levinson's Algorithm well known to those skilled in the art) 
which predicts values associated with the waveform, wherein the linear 
prediction means includes a means for auto-regressive analysis of the 
preconditioned accelerometer energy signal using an algorithm which is 
described below in greater detail. The microprocessor based controller 
also includes: a bandwidth determining means for determining a bandwidth 
from predicted values of the waveform, center frequency determining means 
for determining a center frequency from predicted values of the waveform, 
means for calculating an amount indicative of pulse pressure from the 
determined bandwidth and center frequency, and analyzing means for 
analyzing the amounts indicative of pulse pressure over corresponding 
cardiac cycles. 
The calculated value associated with pulse pressure may be analyzed by the 
microprocessor over a preselected number of cardiac cycles and for a 
plurality of preselected timing intervals, wherein the timing interval is 
a measured time between at least one of intrinsic and paced stimulations 
of pre-selected chambers of the heart. The value associated with pulse 
pressure corresponding with each timing interval is then compared to 
determine which timing interval results in the greatest pulse pressure. 
The pacer may then automatically reset the timing interval to this 
"maximum" timing interval. 
The analysis and comparison of the accelerometer signal preferably occurs 
when the patient is at rest, the quiescent period. The accelerometer 
signal may also be used to determine the period of quiescent activity. 
Analyzing the accelerometer signal during the period of quiescent activity 
minimizes motion artifact in the accelerometer signal. Further, analyzing 
the signal during the period of quiescent activity allows the measurements 
to be taken during relative steady state hemodynamic conditions. 
Those skilled in the art will recognize that the accelerometer and other 
components may be mounted externally, linking these components with the 
microprocessor by telemetry. However, without limitation, a single 
self-contained implantable cardiac stimulating device including all of 
these components is preferred. 
OBJECTS 
It is accordingly a principal object of the present invention to provide a 
cardiac stimulation device capable of dual chamber pacing which 
non-intrusively determines a value indicative of the pulse pressure for a 
selected cardiac cycle. 
Another object of the present invention is to provide a cardiac stimulator 
which maximizes cardiac performance through non-invasive means by 
determining a value indicative of the cardiac pulse pressure from a signal 
of an accelerometer. 
A further object of the present invention is to optimize cardiac 
performance based on an analysis and comparison of non-intrusively 
measured values indicative of pulse pressure over a plurality of 
preselected timing intervals, to thereby determine the optimum timing 
interval of the cardiac stimulator. 
Still another object of the present invention is to provide a method for 
optimizing cardiac performance by non-intrusively determining the optimal 
timing interval based on the mechanical performance of the patient's 
heart. 
These and other objects as well as these and other features and advantages 
of the present invention will become readily apparent to those skilled in 
the art from a review of the following detailed description of the 
preferred embodiment especially when considered in conjunction with the 
claims and accompanying drawings in which like numerals in the several 
views refer to corresponding parts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, there is generally shown in block diagram the 
cardiac stimulator 10 of the present invention. The cardiac stimulator 10 
includes a pulse generator 12 having a power supply 14, an accelerometer 
16, a microprocessor based controller (represented by a dotted line) 18, 
and an electrocardiogram (ECG) 20. The ECG 20 may be a surface or internal 
electrogram of known suitable construction. A portion of the electrical 
components of the microprocessor based controller 18 are shown enclosed by 
the dotted line representing the microprocessor based controller 18. 
Atrial and ventricular leads 22 and 24 are shown coupled to the 
microprocessor based controller 18. 
The microprocessor 18 controls the cardiac stimulating pulses delivered by 
pulse generator 12 to one or both of the leads 22 and 24, depending upon 
the pacing mode selected. Further, the microprocessor based controller 18 
establishes the optimal timing interval between at least one of intrinsic 
or paced stimulations of pre-selected chambers of the heart. The timing 
interval to be optimized may include the timing interval between any of 
the following pacing modes: A-A pacing, the V-V pacing and A-V pacing. A 
timing interval for A-A pacing refers to the timing between the right and 
left atrial contractions (either intrinsic or paced), a timing interval 
for V-V pacing refers to the timing between the right and left ventricular 
contractions (either intrinsic or paced), and a timing interval for A-V 
pacing refers to the timing between atrial and ventricular contractions 
(either intrinsic or paced), when sensing/pacing in any one of the 
following configurations: A.sub.R -V.sub.R pacing, A.sub.R -V.sub.L 
pacing, A.sub.L -V.sub.R pacing, A.sub.R -V.sub.RL pacing, A.sub.L 
-V.sub.RL pacing, and A.sub.L -V.sub.L pacing. 
Cardiac stimulating devices capable of telemetering various status 
information including selecting a pacing mode and other parameters 
including the timing interval (determined by the physician), are 
commercially available from, for example Cardiac Pacemakers, Inc., St. 
Paul, Minn. An external programmer having a microprocessor and associated 
memory may transmit information in a conventional way through a telemetry 
link and transmission receiver 48 of the cardiac stimulator's 
microprocessor. Using the programmer and the telemetry link, operating 
parameter values for the cardiac pacer can be delivered to it by a 
cardiologist for setting the cardiac cycle pacing parameter values to be 
utilized, including the timing interval. 
The microprocessor 18 further has both RAM (random access memory), and ROM 
(read only memory) for storing programs and data, which allows generally: 
the processing of a signal from an electrogram, processing of signals 
transmitted from the accelerometer 16, storing various information derived 
from the processing, and changing the preset constants of the program. 
The accelerometer 16 is positioned within the casing of the cardiac 
stimulator or pacer and is coupled to the microprocessor based controller 
18 through an analog/digital convertor 26 and filters further described 
below. The accelerometer 16 provides a signal that is processed to provide 
a non-intrusive measure of pulse pressure during a cardiac cycle. The 
casing of the cardiac pacer 10 is implanted in a surgically made pocket, 
typically in either the left or right shoulder region of the patient. By 
positioning the accelerometer 16 in the casing (not shown) of the cardiac 
pacer 10, the accelerometer 16 generates a global signal associated with 
various atrial and ventricular events. A globalized signal is preferred 
over a localized signal (a signal transmitted from an accelerometer in 
direct contact with an outer wall of the heart). The signal from the 
accelerometer 50 may also be used to evaluate levels of physical activity, 
thereby identifying periods in which physical activity is low. 
An analog signal of the accelerometer 16 comprises events associated with 
heart sounds, compressions, blood fluid motions and/or cardiac wall 
accelerations and decelerations caused from cardiac activity along with 
motion artifacts and respiratory events. Intermediate the accelerometer 16 
and the microprocessor based controller 18 a preconditioning filter 28, 
low pass filter 30, bandpass filter 32 and analog-to-digital (A/D) 
converter 26 are electrically coupled therebetween. The raw or analog 
signal transmitted from the accelerometer 16 passes through the 
preconditioning filter 28 and low pass filter 30 to produce a first 
derivative of the low pass filter signal. 
The first derivative of the low pass filter signal then passes through a 
bandpass filter and is digitized by an analog to digital (A-D) converter 
at 26. Preconditioning and filtering of the accelerometer signal enhances 
the pre-ejection accelerometer signature portion of the signal that is due 
to the on-set of ejection and further filters out other extraneous events. 
In this manner, a waveform is produced representative of pulse pressure, 
eliminating non-essential frequencies utilizing the bandpass filter 32 and 
eliminating the higher frequency components utilizing the low pass filter 
30. The digital waveform is then transmitted from the A/D converter 26 to 
the microprocessor based controller 18, wherein the waveform first passes 
through an auto regressive analysis 34 using well-known Levinson or the 
Yule-Walker algorithms from autocorrelation lags of the accelerometer 
signal derived during the time of the main lobe of the energy signal. A 
discrete set of reflection coefficients representing the reference signal 
result. This coefficient set is fed to an inverse linear filter predictor 
36. 
The resulting coefficients then pass through an inverse linear predictor 
for which produces an output indicative of the intensity or ongoing 
intensity energy level of the accelerometer signal as a function of time. 
The absolute value of this output is then subjected to a multiplier 40. 
The time of maximum absolute value of the spike corresponds to the time of 
minimum error with respect to the beginning of ejection. The preferred 
implementation of the inverse filter is that of an FIR lattice filter. 
This implementation results in a structure that is maximally numerically 
stable, since stable filters result in coefficient multiplication 
operations that are bounded in absolute value by 1. 
The peak detector 42 is enabled and then a bandwidth and the center of 
frequency detectors identify values associated with the bandwidth and 
center of frequency (F.sub.c). Once a value for the bandwidth and F.sub.c 
have been determined, then a mathematical manipulator 46 determines a 
value indicative of pulse pressure from these values. The value indicative 
of pulse pressure is repeatedly determined over a predetermined number of 
respiratory cycles and then a mean, maximum, or average value indicative 
of pressure may be determined. An analyzer 48 compares for example, the 
maximum value indicative of the pulse pressure for several preset pacing 
intervals and thereby determines the pacing interval which results in the 
greatest pulse pressure immediately before ejection. The microprocessor 
based controller 18 then sets the timing interval to the determined 
interval. Those skilled in the art will appreciate that the adjustment in 
timing interval may be programmed to occur periodically or at a specific 
time each day. 
In further explaining the invention, and especially the flow chart of FIG. 
2, it is assumed that the timing interval of the cardiac stimulation 
device 10 is preset to correlate the pulse pressure with the 
atrial-ventricular (A-V) interval. It should be emphasized that the 
invention is not to be limited to use in a system where only the A-V delay 
interval is adjusted, and the results of the adjustment on pulse pressure 
noted. Those skilled in the art will recognize that the algorithm 
described equally applies to other timing intervals for any of a number of 
pacing modes. For example, the lower rate limit interval (R-R), the 
interval between right and left atrial stimulations (A.sub.R -A.sub.L 
interval), the interval between right and left ventricular stimulations 
(V.sub.R -V.sub.L interval), A.sub.R -V.sub.R interval, A.sub.L -V.sub.R 
interval, A.sub.R -V.sub.RL interval, A.sub.L -V.sub.RL interval, A.sub.L 
-V.sub.L interval etc. may be subjected to periodic changes with the 
effects on the pulse pressure being noted and stored. 
The algorithm 60 used to non-intrusively extract the hemodynamic pulse 
pressure from an accelerometer signal is shown in FIG. 2. Initially, 
signals from both the accelerometer 16 and ECG 20 are initiated to produce 
signals corresponding to the cardiac motion and cardiac cycles (see Block 
62). The signal produced by the ECG is used to correlate a measured pulse 
pressure with the cardiac cycle. The accelerometer's 16 signal is then 
transmitted through a series of filters as described above, to remove 
ancillary information contained in the accelerometer signal (see block 
64). The filtered analog accelerometer signal is then converted to a 
digital signal (see block 66). The digital signal and ECG signal are 
transmitted to the microprocessor based controller 18 (see block 68) for 
processing and analysis. The microprocessor based controller 18 then 
performs a linear prediction from the digital signals resulting in K.sub.1 
and K.sub.2 and thereafter determines the F.sub.B (bandwidth) and F.sub.C 
(central frequency) of the linear prediction (see blocks 70 and 72), where 
F.sub.C is determined from the following: 
##EQU1## 
and F.sub.B is determined from the following equations: 
##EQU2## 
The determined bandwidth and center frequency are added to obtain a value 
associated with the pulse pressure (see block 74). As previously 
recognized, the non-intrusively determined pulse pressure may be utilized 
to enhance cardiac performance. By means of example, immediately below is 
a description of one method of utilizing the calculated pulse pressure to 
enhance cardiac performance. 
An ordered set of pre-set A-V interval values may be programmed into the 
memory of the microprocessor based controller 18 at the time of implant by 
the physician. This timing interval set would contain a range of A-V 
interval values over which the unit will automatically switch. Oftentimes, 
the sequence of the set may comprise alternation between a baseline 
without pacing (intrinsic) and a randomly selected A-V interval (having a 
value somewhat less than the intrinsic A-V interval). This alternation 
reduces hysteresis and other effects that a previous A-V interval value 
may have on the next A-V interval. 
The microprocessor receives a digitized accelerometer signal from the 
accelerometer. A portion of this signal represents the level of physical 
activity of the patient. An initial test may be made to determine whether 
the physical activity is less than a predetermined amount X, which is 
indicative of a patient at rest. When the patient is resting, the 
accelerometer readings are less subject to noise and motion artifacts. 
When the physical activity is less than the predetermined amount X, the A-V 
interval index m is then set to 1. The A-V interval is periodically 
changed, determining the value indicative of pulse pressure over several 
cardiac cycles for each A-V interval. The microprocessor 18 simultaneously 
analyzes the electrogram 20 signal to thereby correlate the determined 
value indicative of pulse pressure with the respiratory cycles determined 
from the ECG signal. The microprocessor based controller 18 then compares 
values indicative of pulse pressure for each iterated A-V interval to 
determine which A-V interval setting results in the greatest value 
indicative of pulse pressure. The A-V interval setting is then set by the 
microprocessor based controller 18 to the optimum A-V interval value. The 
A-V interval remains at this optimum setting until a predetermined time 
period Z has passed. The analysis is then repeated to determine a new 
optimum A-V interval. 
FIGS. 3-8 are various graphs illustrating graphically that a feature 
filtered from an accelerometer signal correlates with an independently 
measured pulse pressure. By filtering the accelerometer signal, a waveform 
80 of a specific event feature associated with pulse pressure is separated 
out from the accelerometer signal and is shown in FIG. 3 plotted over 
several cardiac cycles. The peaks in FIGS. 3-7 correspond with the 
ejection of blood fluid. FIG. 4 is a plot 82 of the pulse pressure values 
measured by a pulse pressure sensor. From a comparison of FIGS. 3 and 4 
those skilled in the art will appreciate that the frequency domain feature 
derived from the accelerometer signal 80 correlates with the measured 
pulse pressure 82. Likewise, a comparison of FIGS. 5 to FIGS. 6 and 7 
illustrates that the peaks of a filtered accelerometer feature 84, 86, and 
88 correspond with the peaks of a measured pulse pressure signal 90. Also, 
it can be seen that the maximum observed pulse pressure 92 in FIG. 5 
corresponds with the maximum accelerometer feature 94 in FIG. 6. 
FIG. 8 substantiates that for different pacing modes the maximum pulse 
pressure 100 may be used to identify an optimal timing interval 102. The 
timing interval 102 observed in FIG. 8 was the A-V interval, however those 
skilled in the art will appreciate that other timing intervals may also be 
maximized through correlation with the maximum pulse pressure 100. 
This invention has been described herein in considerable detail in order to 
comply with the patent statutes and to provide those skilled in the art 
with the information needed to apply the novel principles and to construct 
and use such specialized components as are required. However, it is to be 
understood that the invention can be carried out by specifically different 
equipment and devices, and that various modifications, both as to the 
equipment and operating procedures, can be accomplished without departing 
from the scope of the invention itself