Implantable device for monitoring aerobic capacity of patients

An implantable electronic circuit for monitoring a patient's aerobic capacity includes an accelerometer for detecting the onset and termination of a period of exercise and a means for measuring the patient's minute volume. By computing the time interval from the cessation of exercise to the point where the patient's minute volume reaches a predetermined value at or near an at-rest value thereof, a measure of aerobic capacity relating to a patient's physical fitness can be determined.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
This invention relates generally to implantable electronic monitoring 
apparatus, and more particularly an implantable electronic device for 
monitoring aerobic capacity of patients. 
II. Discussion of the Prior Art 
A typical prior art pacemaker of the rate adaptive type has commonly 
incorporated one or more sensors for detecting a physiologic function and 
for producing an electrical control signal proportional thereto. The 
electrical control signal is then applied to the timing circuitry of the 
pacemaker for adjusting the rate at which cardiac stimulating pulses are 
produced from a programmed lower rate limit to a similarly programmed 
upper rate limit. Physiologic sensors have included accelerometers for 
detecting body motion and, in this regard, reference is made to the 
Meyerson et al. U.S. Pat. No. 5,179,947. Other physiologic parameters that 
change with hemodynamic need include blood temperature (Cook et al. U.S. 
Pat. No. 4,436,092), oxygen saturation (Wertzfeld et al. U.S. Pat. No. 
4,202,339), the heart's preejection interval (Citak et al. U.S. Pat. No. 
4,773,401) as well as respiratory factors including tidal volume (Alt U.S. 
Pat. No. 4,919,136), respiration rate (Krasner U.S. Pat. No. 3,593,718) 
and the product thereof, minute ventilation (Plicchi et al. U.S. Pat. No. 
4,596,251). These respiratory factors can be derived from the 
accelerometer output signal using signal processing techniques to isolate 
components of body motion due to breathing. Alternatively, electrodes may 
be provided for measuring variations in electrical impedance between 
electrodes placed in the thoracic cavity. 
Implantable cardiac pacemakers are now also being used in treating patients 
suffering from congestive heart failure (CHF). It has been found that the 
efficiency of the heart as a pump can be improved by pacing the ventricles 
with an optimum AV delay between the occurrence of an intrinsic atrial 
depolarization and the application of a ventricular stimulating pulse. 
From the foregoing, it is clear that the technology exists for 
incorporating within an implantable, fluid impermeable, body compatible 
housing electronic circuitry including a microprocessor-based controller 
that is adapted to receive the output from a variety of physiologic 
sensors including accelerometers and means for sensing respiratory 
parameters for controlling a pulse generator whose output is used to 
stimulate the atrium, the ventricles or both. 
In addition to the pacing function, implantable cardiac rhythm management 
devices are increasingly taking advantage of the increase in memory 
capacity of microprocessors to monitor and store a variety of parameters 
for later read-out over a telemetry link to an external monitor/programmer 
module. For example, heart rate variability over a period of many days may 
be calculated and stored for later read-out. See Heemels et al. U.S. Pat. 
No. 5,603,331. Heart rate variability has been found to be a significant 
indicator of the progress of CHF. 
Another measure of the efficacy of treatment by drugs, pacing or a 
combination thereof is the change in aerobic capacity resulting from the 
therapy. During the recovery stage following exercise, the oxygen 
consumed, or equivalently the oxygen uptake, is above normal resting 
levels. The total oxygen consumed from onset of recovery until the 
pre-exercise level is reached is referred to as the recovery oxygen 
uptake. There are two components to recovery oxygen uptake: a fast 
component indicative of a quick drop in elevated oxygen consumption 
immediately after the cessation of activity, and a slower phase of 
recovery referred to as the slow component. For a given work level, the 
recovery oxygen uptake is less for more aerobically fit individuals and 
the total time of elevated oxygen uptake is an important measure of a 
patient's level of physical fitness. Recovery oxygen uptake duration may 
also be evaluated in determining the efficacy of therapy in CHF patients. 
It is accordingly a principal purpose of the present invention to provide 
an implantable electronic device capable of measuring and storing the 
aerobic capacity of the patient in whom the device is implanted. 
SUMMARY OF THE INVENTION 
The implantable medical device of the present invention includes means for 
assessing a patient's aerobic capacity. The device comprises a fluid 
impervious, tissue compatible housing containing a battery power supply 
and electronic circuitry powered by the battery. The electronic circuitry 
preferably includes an accelerometer for sensing physical movements of a 
patient's body and for producing electrical signals relating thereto. The 
circuitry also includes means for sensing respiratory parameters of the 
patient. The information from the accelerometer and from the respiratory 
sensing means (which may also comprise the accelerometer) may then be used 
to compute the time required following a period of physical exercise and 
the cessation thereof for the respiratory parameter to return to a 
predetermined level associated with the patient's rest state. As 
mentioned, this time interval corresponds to the patient's aerobic 
capacity. Alternatively, rather than measuring the time for the heavy 
breathing to subside, it is also possible to compute the rate of change of 
the respiratory parameter following a period of physical exercise and the 
cessation thereof. For example, the rate of change or slew rate of minute 
ventilation with respect to time is also an indicator of the patient's 
fitness level. 
The slew can be calculated immediately post-exercise and closer to the end 
of recovery allowing for calculation of the fast and slow components of 
the recovery phase. In addition, the integral of the respiratory 
parameters could also be determined and stored for reference. 
The measured time value and/or time rate of change may be computed and 
stored at periodic intervals for later read-out, via a telemetry link, 
when the implanted device is interrogated by an external 
monitor/programmer module. Such history becomes valuable in assessing the 
efficacy of any therapy which may be rendered at various points in time. 
The means for sensing respiratory parameters may include the accelerometer 
along with signal processing circuits for extracting from the 
accelerometer signal motion artifacts occasioned by breathing. 
Alternatively, a transthoracic impedance signal may be demodulated to 
derive tidal volume and respiratory rate information from which minute 
volume may be calculated.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is indicated generally by numeral 10 an 
implantable medical device, such as a pacemaker or defibrillator, that 
includes a hermetically sealed housing 12 often times referred to as a 
"can" because it is made of a suitable metal such as titanium. Contained 
within the housing or can 12 is electronic circuitry as well as a battery 
power supply. Affixed to the header 14 of the can 12 is a lead barrel 16 
containing contacts (not shown) that are connected via feedthrough pins to 
the circuitry within the can 12. Those skilled in the art of manufacturing 
implantable cardiac rhythm management devices, such as pacemakers and 
defibrillators, are thoroughly familiar with techniques for creating 
moisture impermeable, body-compatible devices for housing electronic 
circuitry. The implant device 10 is shown as being connected by a 
conventional pacing lead 18 to an electrode 20. The can 12 is preferably 
coated with a suitable insulator, such as a Silastic coating over its 
entirety except for one or more small predetermined areas 24 where the can 
is exposed to body tissue and can function as a return electrodes. 
Referring next to FIG. 2, there is shown a block diagram of the circuitry 
contained within the housing 12 for implementing the aerobic capacity 
monitoring feature of the present invention. In FIG. 2, the housing 12 is 
represented by a broken line box 12'. Shown included within the housing is 
a battery 26, preferably a lithium iodide battery which exhibits long life 
with no out-gassing. The battery is connected to an integrated circuit 
microprocessor 28 to provide the requisite operating potential thereto. 
The microprocessor 28 includes an address bus 30 and a data bus 32 and 
connected across these two buses are a ROM 34, a RAM 36 and an 
input/output interface module 38. As is also conventional, the ROM will 
typically store a program of instructions executable by the microprocessor 
38 for performing arithmetic and logical functions on operands applied to 
it. The RAM memory 36 stores various programmable parameters and 
intermediate computations carried out by the microprocessor. The 
input/output interface (I/O) 38 provides buffering of data read out from 
the RAM memory under control of the microprocessor 28 for delivery 
external to the body via a telemetry link 40. A programmer/monitor module 
42, which may itself comprise a personal computer having a keyboard for 
data entry and a display can be used to further process and present 
information read out from the implanted device. It also permits medical 
personnel to alter programmable parameters stored in RAM 36, again via the 
telemetry link 40. 
Also contained within the housing 12 is an accelerometer chip 44 that 
provides an electrical signal train to an input of the microprocessor 
proportional to body movements sensed by the accelerometer. While not 
shown in FIG. 2, the accelerometer module 44 may include suitable 
filtering and signal processing circuitry as well as an A/D converter. The 
A/D converter may also be a part of the microprocessor 28. 
FIG. 3 is a plot of work rate vs. time defining an exercise protocol for a 
patient. For the first six minutes, the patient is at rest, sitting in a 
chair or the like. At the six-minute point, the patient is placed on an 
ergometer and made to exercise at a 25 watt work rate for two minutes. At 
that point, the work rate is then increased to 50 watts for another two 
minutes before the work rate is elevated to 75 watts. The patient is made 
to exercise at the 75 watt rate for another two minutes and then the work 
rate is increased to 100 watts. At the conclusion of the two minute 
exercise at 100 watts, the patient again is put at rest. 
Referring to FIG. 4, the signal processed analog output from the 
accelerometer 44 corresponding to the above exercise protocol is 
illustrated. With the patient initially at rest, the amplitude of the 
accelerometer output excursions remains relatively low. At time, T=6 min., 
the patient is allowed to begin to exercise, such as by walking on a 
treadmill, working out on a stair climber or, in the case of severely 
infirmed CHF patients, by merely walking down a hallway or the like. The 
length of the exercise period will generally be sufficient to elevate the 
patient's respiration rate and depth. 
FIG. 5 is a plot of the patient's tidal volume over time and which is 
correlated timewise with the activity plot of FIG. 3. At the end of the 
exercise period, at T=14 min., the amplitude of the accelerometer output 
(FIG. 4) drops noticeably to a resting level as does tidal volume. 
Referring to FIG. 6, during exercise, the patient's minute ventilation 
increases to a maximum and, at the conclusion of the exercise, at T=14 
min., it begins to fall off. The time that it takes for the minute 
ventilation to return to a predetermined level, such as its steady state 
rest level or some percentage thereof, is an indicator of oxygen debt 
accrued over the period of exercise. 
Thus, FIGS. 4 and 5 represent two separate and independent measurements of 
physical activity. In the plot of FIG. 6, the time, T, for the minute 
ventilation curve 48 to drop to a predetermined percentage of the steady 
state rest value comprises the payback time and the length of the interval 
is an indicator of the relative physical fitness of the patient. For a 
given workload, the longer the interval, the less physically fit is the 
patient. In addition or alternatively, the total recovery minute 
ventilation can be calculated. This is equivalent to the integral of the 
minute ventilation parameter over time. Again, for a given workload, the 
larger the integral, the less physically fit the patient. 
Instead of measuring the time interval, T, as shown in FIG. 6, another 
indicator of fitness is the slope or rate of change of the curve 48. The 
steeper the slope, the more healthy the patient. Still another measure of 
the fitness of the patient would be the area under the curve segment 48 
with a large area being indicative of a lack of fitness. As mentioned 
earlier, the slew rate can be computed immediately post-exercise and then 
later close to the end of the recovery period so that calculations can be 
made of the fast and slow components comprising the recovery phase. In 
addition, the integral of the respiratory parameters during predetermined 
intervals can be calculated and stored for later readout and analysis in 
assessing the patients overall fitness. 
As those skilled in the art appreciate, respiratory parameters can be 
derived directly from the acceleration signal using appropriate band-pass 
filtering. By choosing a pass band in a range from 0.05 to 1.0 Hz, body 
movement due to respiration is isolated. In particular, the variations in 
the accelerometer output in this frequency band provides both respiratory 
rate and tidal volume information from which minute volume can be 
computed. Alternatively or as an option, an impedance sensor 50 may be 
included in the circuitry contained within the housing 12. The impedance 
sensor may be like that described in the Salo et al. U.S. Pat. No. 
4,686,987 wherein a high frequency carrier signal from an oscillator is 
applied between two spaced electrodes, here the electrode 24 which is 
typically located in the pectoral region of the body, and the electrode 20 
located at the apex of the right ventricle. This carrier signal becomes 
modulated by inspiratory and expiratory movements of the diaphragm. By 
appropriate filtering and other signal processing techniques, it is 
possible to obtain signals having a strong linear relation to tidal 
volume, respiratory rate and to compute minute ventilation from these 
parameters. As those skilled in the art appreciate, it is not necessary to 
calibrate the sensor relationship to tidal volume as the resting or 
non-exercise level can be determined by long term observation and the 
exercise state discerned by a relative increase over the resting level. 
The microprocessor 28 is programmed to sense the cessation of an exercise 
session and when this event is detected, a timer is initiated which 
continues to run until the respiratory signal (minute ventilation) drops 
either to a rest level that prevailed at the start of the test or to a 
predetermined percent thereof which may be programmed by the physician. As 
mentioned earlier, the length of the time interval is an indicator of 
oxygen debt. 
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.