Rate responsive pacemaker with exercise recovery using minute volume determination

A rate responsive implantable pacemaker generates a metabolic demand parameter which is converted into a corresponding metabolic indicated parameter. The parameter is used to determine certain characteristics related to an exercise period and the patient, including exercise duration and intensity. These characteristics are used to dynamically calculate an exercise recovery period during which the pacing rate is elevated above a rate indicated by the metabolic parameter. Preferably, fuzzy logic circuitry is used for this purpose.

RELATED APPLICATIONS 
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RATE-RESPONSIVE EMAKER WITH 
08/850,557 
5/2/97 
RAPID MINUTE VOLUNE 
DETERMINATION 
RATE-RESPONSIVE EMAKER WITH 
08/848,968 
5/2/97 
NOISE-REJECTING MINUTE VOLUME 
DETERMINATION 
RATE-RESPONSIVE EMAKER WITH 
08/850,529 
5/2/97 
MINUTE VOLUME DETERMINATION AND 
EMI PROTECTION 
______________________________________ 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to rate-responsive pacemakers and, more 
particularly, to pacemakers that employ a minute volume metabolic demand 
sensor as a metabolic rate indicator, said sensor operating appropriately 
to thereby insure that the pacemaker reacts accurately to changes in the 
level of activity, especially to recovery from short or long-term 
exercise. 
2. Description of the Prior Art 
Many attempts have been made to control the heart rate of a pacemaker 
patient so that it will duplicate the intrinsic heart rate of a healthy 
person both when the patient is at rest and when the patient is involved 
in various levels of exercise. Metabolic demand related parameters 
heretofore proposed for controlling the pacing rate include physiological 
parameters such as the QT interval, respiration rate, venous oxygen 
saturation, stroke volume, venous blood temperature, and minute volume or 
ventilation, among others. (The terms minute ventilation and minute volume 
are used interchangeably). In addition, other metabolic demand parameters 
are also used based on physical activity as a criteria. More specifically, 
mechanical and electrical sensors are used to detect patient motion. Of 
the various parameters available, it has been found that pacemakers using 
minute volume as a parameter for controlling pacing rate are particularly 
advantageous. 
A problem with the current minute ventilation sensor is that for the 
recovery stage of patient exercise, no adjustment has been made for 
intensity of exercise, duration of exercise, which may cause 
unphysiological response. Publications in the field have identified these 
factors as important in affecting a fast or slow recovery from exercise. 
No current minute ventilation sensor contains means to actively adjust for 
the recovery stage of exercise. A patent by Bonnet, et al., "Method and 
Apparatus for Controlling the Pacing Rate of a Metabolic Demand 
Pacemaker," (U.S. Pat. No. 5,249,572), contains a state space approach in 
which during recovery a low-pass filter with different time constant than 
during the onset is utilized to adjust for recovery from exercise. 
However, the time constant for the filter is programmable and there is no 
physiological basis which takes into consideration factors such as 
exercise duration, and exercise in the selection of the programmable time 
constant of the filter used in recovery of exercise. 
Other prior art on fine-tuning paced rate during recovery from exercise 
include two patents, Bennett, et al., "Rate Responsive Pacemaker and 
Pacing Method" (U.S. Pat. No. 5,134,997), and Shelton, et al., "Rate 
Responsive Cardiac Pacemaker and Method for Work-Modulating Pacing Rate 
Deceleration," (U.S. Pat. No. 5,312,453). These prior patents were 
targeted for the activity-based sensor which did not respond well 
physiologically to the recovery phase of exercise, dropping almost 
immediately to the resting heart rate level at the cessation of exercise. 
Although these two patents incorporated the duration and intensity of 
exercise as criteria for recovery, the algorithms do not model physiologic 
observations in the most appropriate manner. As a result, the algorithms 
are contrived, complicated, and difficult to understand. 
OBJECTIVES AND SUMMARY OF THE INVENTION 
In view of the above mentioned disadvantages of the prior art, it is an 
objective of the present invention to provide a pacemaker which 
dynamically and physiologically responds to the recovery stage of patient 
exercise. 
Another further objective is the utilization of fuzzy logic methodology to 
provide for the exercise recovery determination. 
Other objectives and advantages of the invention shall become apparent from 
the following description. Briefly, a pacemaker constructed in accordance 
with this invention includes sensing means for sensing a metabolic demand 
parameter of the patient indicative of his or her instantaneous physical 
activity. Preferably, the metabolic demand parameter is minute volume 
which can be determined, for example, from impedance measurements. Minute 
volume has been found to be an accurate representation of the physical 
activity and the corresponding blood flow and oxygen demand of a patient. 
This parameter is converted into a corresponding metabolic indicated rate 
(MIR), which rate may be used to define the interval between the pacer 
pulses. The mapping of minute volume to metabolic indicated rate (MIR), 
preferably, uses a preselected curve which may be, for example, an 
exponential curve, or another type of monotonic curve. Next, an exercise 
recovery determination is applied to the curve to compensate for changes 
in the baseline due to exercise. The resulting rate is then used to 
calculate a optimal paced pulse interval.

DETAILED DESCRIPTION OF THE INVENTION 
Details of a pacemaker in accordance with the present invention are shown 
in FIGS. 1-6. FIG. 1 shows a block diagram of the pacemaker. The pacemaker 
10 is designed to be implanted in a patient and is connected by leads 12 
and 13 to a patient's heart 11 for sensing and pacing the heart 11 as 
described for example in U.S. Pat. No. 5,441,523 by T. Nappholz, entitled 
FORCED ATRIOVENTRICULAR SYNCHRONY DUAL CHAMBER EMAKER, and incorporated 
herein by reference. Briefly, the atrial cardiac lead 12 extends into the 
atrium of the heart 11 and the ventricular cardiac lead 13 extends into 
the ventricle of the heart 11. Leads 12 and 13 are used for both sensing 
electrical activity in the heart and for applying pacing pulses to the 
heart. The pacemaker 10 includes a pace and sense circuit 17 for the 
detection of analog signals from leads 12 and 13 and for the delivery of 
pacing pulses to the heart; a microprocessor 19 which, in response to 
numerous inputs received from the pace and sense circuit 17, performs 
operations to generate different control and data outputs to the pace and 
sense circuit 17; and a power supply 18 which provides a voltage supply to 
the pace and sense circuit 17 and the microprocessor 19 by electrical 
conductors (not shown). The microprocessor 19 is connected to a random 
access memory/read only memory unit 81 by an address and data bus 122. A 
low power signal line 84 is used to provide to the microprocessor 19 a 
logic signal indicative of a low energy level of the power supply 18. The 
microprocessor 19 and the pace and sense circuit 17 are connected to each 
other by a number of data and control lines including a communication bus 
42, an atrial sense line 45, an atrial pacing control line 46, an atrial 
sensitivity control bus 43, an atrial pace energy control bus 44, a 
ventricular sense line 49, a ventricular pace control line 50, a 
ventricular sensitivity control bus 47, and a ventricular pacing energy 
control bus 48. 
FIG. 2 shows details of the pace and sense circuit 17. The circuit 17 
includes an atrial pacing pulse generator 24, a ventricular pacing pulse 
generator 34, an atrial heartbeat sensor 25, a ventricular heartbeat 
sensor 35, and a telemetry circuit 30. The preferred embodiment of the 
pace and sense circuit 17 also includes an impedance measurement circuit 
14 for measuring a physiological parameter indicative of the patient's 
metabolic demand. The pace and sense circuit 17 also includes a control 
block 39 which is interfaced to the microprocessor 19. 
In operation, the atrial and ventricular heartbeat sensor circuits 25 and 
35 detect respective atrial and ventricular analog signals 23 and 33 from 
the heart 11 and convert the detected analog signals to digital signals. 
In addition, the heartbeat sensor circuits 25 and 35 receive an input 
atrial sense control signal on a control bus 27 and an input ventricular 
sense control signal on a control bus 37, respectively, from the control 
block 39. These control signals are used to set the sensitivity of the 
respective sensors. 
The atrial pacing pulse generator circuit 24 receives from the control 
block 39, via an atrial pacing control bus 28, an atrial pace control 
signal and an atrial pacing energy control signal to generate an atrial 
pacing pulse 22 at appropriate times. Similarly, the ventricular pacing 
pulse generator circuit 34 receives from the control block 39, via a 
ventricular pacing control bus 38, a ventricular pace control signal and a 
ventricular pacing energy control signal to generate a ventricular pacing 
pulse 32. The atrial and ventricular pace control signal determine the 
respective timing of atrial and ventricular pacing that take place, while 
the atrial and ventricular pacing energy control inputs determine the 
respective magnitudes of the pulse energies. 
The pacemaker 10 makes an impedance measurement when the microprocessor 19 
sends a signal on the impedance control bus 21 to activate the impedance 
measurement circuit 14. The impedance measurement circuit 14 then applies 
a current to the ventricular cardiac lead 13 via lead 20 and measures a 
voltage resulting from the applied current, as discussed in more detail 
below. These current and voltage signals define an impedance 
characteristic of the patient's metabolic demand, and more particularly, 
of the instantaneous minute volume. This instantaneous minute volume is 
then filtered and further modified by subtracting from it a long term 
average value, as discussed above. The resulting parameter is the minute 
volume parameter. 
The telemetry circuit 30 provides a bidirectional link between the control 
block 39 of the pace and sense circuit 17 and an external device such as a 
programmer. It allows data such as the operating parameters to be read 
from or altered in the implanted pacemaker. An exemplary programmer is the 
9600 Network Programmer manufactured by Telectronics Pacing Systems, Inc. 
of Englewood, Colo., U.S.A. 
FIG. 3 shows the microprocessor 19 having a timer circuit 51 for generating 
several timing signals, a controller 53, a vectored interrupts circuit 54, 
a ROM 55, a RAM 56, an external memory 57 and an interface port 41. 
Signals between these elements are exchanged via an internal 
communications bus 40. The RAM 56 acts as a scratch pad and active memory 
during execution of the programs stored in the ROM 55 and used by the 
microprocessor 19. ROM 55 is used to store programs including system 
supervisory programs, detection algorithms for detecting and confirming 
arrhythmias, and programming for determining the rate of the pacer, as 
well as storage programs for storing, in external memory 57, data such as 
that concerning the functioning of the pulse generator 10 and the 
electrogram provided by the ventricular cardiac lead 13. The timer circuit 
51, and its associated control software, implements some timing functions 
required by the microprocessor 19 without resorting entirely to software, 
thus reducing computational loads on, and power dissipation by, the 
controller 53. 
Signals received from the telemetry circuit 30 permit an external 
programmer (not shown) to change the operating parameters of the pace and 
sense circuit 17 by supplying appropriate signals to the control block 39. 
The communication bus 42 serves to provide signals indicative of such 
control to the microprocessor 19. 
The microprocessor 19 through its ports 41 receives status and/or control 
inputs from the pace and sense circuit 17, including the sense signals on 
the sense lines 45 and 49 previously described. Using controller 53, it 
performs various operations, including arrhythmia detection, and produces 
outputs, such as the atrial pace control on the line 46 and the 
ventricular pace control on the line 50, which determine the type of 
pacing that is to take place. Other control outputs generated by the 
microprocessor 19 include the atrial and ventricular pacing energy 
controls on the buses 44 and 48, respectively, which determine the 
magnitude of the pulse energy, and the atrial and ventricular sensitivity 
controls on the buses 43 and 47, respectively, which set the sensitivities 
of the sensing circuits. The rate of the atrial and/or ventricle pacing is 
adjusted by controller 53 as set forth below. 
The pacemaker 10 of the present invention will function properly using any 
metabolic indicator rate system, so long as that system is able to 
reliably relate the sensed parameter to an appropriate matching of 
metabolic demand with the paced rate. However, the preferred embodiment of 
the invention employs the impedance measurement circuit 14, shown in FIG. 
5, which measures the thoracic impedance to determine the respiratory 
minute volume as described generally in U.S. Pat. No. 4,901,725 to T. A. 
Nappholz, et al., issued Feb. 20, 1990 for "Minute Volume Rate-Responsive 
Pacemaker," incorporated herein by reference. 
FIG. 4 shows the block diagram of the controller 53 of FIG. 3. The 
controller 53 includes a pacer 53C, which is preferably a state machine, a 
minute volume processor 53A and an atrial rate monitor 53B. The minute 
volume processor 53A uses the data supplied via the internal bus 40 and 
the communication bus 42 from the impedance measurement block 14 to relate 
the minute volume indicated by the impedance measurement to the Metabolic 
Rate Interval (MRI). This interval is then used by the pacer 53C to 
determine the length of each interval in the timing cycle. While the 
pacemaker 10 is preferably operating in a DDD mode, it should be 
understood that it can operate in other modes as well. The atrial rate 
monitor 53B generates an Automatic Mode Switching (AMS) signal upon 
detection of a non-physiological atrial rate and rhythm. This AMS signal 
automatically switches the pacemaker 10 to a pacing mode which is 
non-atrial tracking. When a physiological atrial rate resumes, the AMS 
signal is deactivated and the pacemaker returns to an atrial tracking 
mode. 
Referring now to FIG. 5, impedance measurement circuit 14 includes a 
thoracic impedance sensor 100 which is coupled by connection 20 to one or 
both of the patient's leads, such as lead 13. The sensor 100 generates a 
time-dependent signal ti indicative of the sensed thoracic impedance of 
the patient. The signal ti is fed to a delta mv generator 102 which 
converts this ti signal into a corresponding dmv signal. The signal dmv is 
fed to a mapping circuit 104 which uses a conformal mapping (discussed in 
more detail below) to generate a corresponding metabolic indicated rate 
MIR1. 
Signal MIR1 is fed to an exercise recovery correction circuit 106. The 
operation of circuit 106 is discussed more fully below in conjunction with 
FIG. 8. The correction circuit 106 generates a baseline corrected signal 
MIR2. This latter signal MIR2 is fed to the paced pulse interval 
calculator 110 to generate the metabolic rate interval (MRI). 
Referring now to FIG. 6, a known thoracic impedance sensor 100 includes a 
current generator 120 and a high pass filter 122 coupled to one of the 
patient leads, such as lead 13. (It should be emphasized that other leads 
may be used as well for determining the mv parameter as described for 
example in U.S. Pat. No. 5,562,712). The lead 13 includes a tip electrode 
124 and a ring electrode 125. As known in the art, at predetermined times, 
the current generator 120 applies current pulses between the ring 
electrode 125 and pacemaker case 126, and the corresponding voltage is 
sensed between the tip electrode 124 and case 126. Typically, each current 
pulse has a pulse width of about 7.5 .mu.sec, at repetition rate of about 
18 pulses per second and an amplitude of about 1 mA. This pulse repetition 
rate is chosen at well above twice the Nyquist sampling rate for the 
highest expected intrinsic heart rate, and is preferably chosen so that it 
can be easily differentiable from noise induced by a power line at 50 or 
60 Hz. 
The sensed voltage is passed through the high pass filter 122 selected to 
accept the 7.5 .mu.sec pulses and exclude noise signals. After filtering, 
the voltage signal is sampled by a sample and hold (S/H) circuit 130. 
Preferably, the S/H circuit takes samples before the start of the test 
pulses from generator 120 (to enhance the effectiveness of the filter 122) 
as well as toward the end of the pulse duration. 
The output of circuit 130 is passed through a band pass filter 132 which 
selects the signals in the range of normal respiration rate, which is 
typically in the range of 5-60 cycles/minute. 
The output of the BPF 132 is amplified by amplifier 134 to thereby generate 
the thoracic impedance signal ti. The amplifier raises the signal ti to a 
level sufficient so that it can be sensed and processed by the delta 
minute volume generator 102. 
Circuit 102 is prior art and details can be found in U. S. Pat. No. 
4,901,725 to T. A. Nappholz, et al., issued Feb. 20, 1990 for "Minute 
Volume Rate-Responsive Pacemaker," incorporated herein by reference. 
The output from Circuit 102, the dmv value, must be converted into a 
metabolic indicated rate (MIR) parameter by mapping circuit 104. Schemes 
for performing this function are well known in the art. One such scheme is 
disclosed in copending application Ser. No. 08/641,223 filed Apr. 30, 
1996, entitled RATE RESPONSIVE EMAKER WITH AUTOMATIC RATE RESPONSE 
FACTOR SELECTION incorporated herein by reference. As disclosed in this 
reference, a curvilinear mapping between minute ventilation and MIR is 
preferable because it can be modeled after physiological data on a wide 
range of normal subjects. 
More particularly, it has been found that an excellent fit can be generated 
if an exponential mapping function is used. One such function is shown in 
FIG. 7. To save computational time, the exponential function may be 
performed by an interpolated table look-up function. The logarithmic 
function used to compute max.sub.-- dmv is evaluated by the programmer, 
only at the time min.sub.-- hr or max.sub.-- hr is changed. The rate 
response factor (RRF) is defined so that one unit change in RRF relates to 
a 10% change in the peak value minute ventilation signal. It may be 
computed and displayed by the programmer, or may be entered by the user 
and used to initialize mv.sub.-- gain. 
The mapping function of FIG. 7 is defined by the following: 
MIR1=a0-a1* exp(-dmv/a2) 
a0=the upper heart rate asymptote, typically 230 pulses per minute 
a1=a0-min.sub.-- hr (a1 determines min.sub.-- hr) 
a2=max.sub.-- dmv/ln(a1/a0-max.sub.-- hr) (a2 determines max.sub.-- hr) 
dmv=filtered delta minute ventilation from delta mv generator 102 
max.sub.-- dmv=the value of dmv which is mapped to max.sub.-- hr 
max.sub.-- hr=the programmed maximum value of paced heart rate 
RRF=rrf.sub.-- const+ln (mv.sub.-- gain/max.sub.-- dmv)/ln (1.1) 
mv.sub.-- gain=max.sub.-- dmv*1.1 (RRF-rrf.sub.-- const) 
rrf.sub.-- const is chosen to establish the nominal RRF values at a 
preselected point along the curve of FIG. 7. 
Next, the exercise recovery correction circuit 106 applies recovery 
correction to the signal MIR1 to model the observation that baseline heart 
rate is elevated following high level exercise. Circuit 106 is preferably 
implemented digitally as follows. 
The algorithm for the recovery correction circuit following exercise can be 
seen in FIGS. 8A and 8B. The metabolic indicated rate MIR1 is first 
inputted through the fuzzy input membership functions LOW 150 and HIGH 152 
defined in FIG. 8A. The output from HIGH 152 is then entered into a Delay 
154 whose function can be described below: 
if (y&gt;x) delay=delay+k1*(y-x) 
if (y&lt;x) delay=delay-k2*(x-y) 
x=input 
y=output 
k1=T/rise.sub.-- time 
k2=T/fall.sub.-- time 
rise.sub.-- time=10 minutes 
fall.sub.-- time=40 minutes 
T=iteration interval 
delay=delay accumulator (initialized to zero) 
Then the outputs from LOW 150 and HIGH 152 through Delay 154 are multiplied 
by multiplier 156 and multiplied by a second multiplier 158 with 30 pulses 
per minute (ppm) to derive the augmented rate. The output from multiplier 
158 is then summed by summer 160 with the original MIR1 to result in the 
output MIR2 which is utilized to convert to pacing pulse intervals in 
calculator 110 (FIG. 5). 
The rationale for this recovery algorithm is based upon the physiology that 
rate remains elevated following the cessation of exercise and that the 
rate of recovery is dependent upon exercise intensity, and exercise 
duration. From the Figures and the formulas, it can be seen that the rate 
augmentation occurs only when the present rate is low and the past rate 
has been high as in recovery from exercise. The delay function serves to 
elevate the present rate. The longer the delay function, the longer the 
overall heart rate remains elevated. The delay is a function of exercise 
duration, as indicated by the time constants k1 and k2. The delay is also 
a function of the exercise intensity, as indicated by the increase of the 
output of delay with heart rate increase (assuming that exercise intensity 
is a function of heart rate elevation). 
Getting back to FIG. 5, the parameter MIR2 is then used to generate a 
metabolic indicated rate interval (MRI) by calculator 110. The paced pulse 
interval is inversely related to the paced heart rate as indicated by the 
following equation. 
ppi=60000/phr where 
ppi=paced pulse interval, milliseconds 
phr=paced heart rate, pulses per second 
Other time intervals of the pacing cycle are computed by the state machine 
53C (FIG. 4) using the paced pulse interval and/or the heart rate. 
The subject invention has been described in the content of a 
rate-responsive pacemaker, using minute ventilation as the metabolic 
demand parameter. However, one skilled in the art will recognize that it 
is equally applicable to pacemakers using other metabolic demand 
parameters, including both the physiological and physical activity 
parameters discussed above. 
Although the invention has been described with reference to several 
particular embodiments, it is to be understood that these embodiments are 
merely illustrative of the application of the principles of the invention. 
Accordingly, the embodiments described in particular should be considered 
exemplary, not limiting, with respect to the following claims.