PATENT ABSTRACT
Exertion levels of a patient are measured by monitoring signals from adaptive-rate sensors such as an accelerometer and or a minute ventilation sensor; sensor data is collected for conversion into metabolic equivalent measurements. The data obtained can be used to evaluate patient physical activity levels and can be used to assess the patient&#39;s condition and change pacing therapy or other treatments accordingly. An automatic adjustment of the adaptive-rate pacing therapy may be based on the activity levels detected by the metabolic equivalent measurements made by the pacemaker.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates to monitoring a patient&#39;s metabolic need over time, and more particularly to measuring metabolic equivalent (NETS) levels from sensors used in adaptive rate cardiac stimulators including, but not limited to, minute ventilation sensors and accelerometers, whereby patient metabolic equivalent rates and exercise events can be tracked and monitored. 
     II. Description of the Related Art 
     In order to measure metabolic equivalents it has been necessary in the past to measure the oxygen uptake (VO 2 ) of a patient. This is difficult to do, particularly outside of a laboratory setting, in that it is necessary to have the patient equipped with respiratory monitoring devices, such as a breathing mask and sample tube, a gas analyzer, ekg leads, an electronics module for processing the parameters being monitored, etc. 
     Some models of calculating metabolic equivalents are based on the patient&#39;s intrinsic heart rate, but chronotropically incompetent pacemaker patients, of course, need a different model. 
     A method is therefore needed to monitor a patient in order to provide a physician with METS data and exertion levels, during the patient&#39;s normal living activity, without performing difficult exercise-based, oxygen uptake measurements. The measurements taken are needed to assess the lifestyle, exertion level, exercise capacity, cardiovascular functional capacity, quality of life and wellness of a patient for overall therapy management. 
     A method is also needed for changing the pacing parameters of a pacemaker based on current METS. 
     SUMMARY OF THE INVENTION 
     The invention provides metabolic equivalents data (METS) derived from an accelerometer (XL) and/or minute ventilation sensor (MV) used in rate-responsive pacemakers implanted in patients. Pacemakers commonly used today already have these sensors for providing rate adaption so patients need not be subjected to wearing a breathing mask or other devices to obtain the exertion level data needed to assess their well being. 
     The data collected is presented to a physician to show rates of excursion and exertion levels experienced by a patient using the pacemaker. The physician can then vary the therapy being provided by, for example, adjusting the pacemaker&#39;s rate, AV delay or other programmable quantity accordingly. 
     The accelerometer and minute ventilation sensor data obtained by the pacemaker can be stored in memory and a microprocessor can be programmed to manipulate the data into forms useful for the physician. Such useful forms include daily maximum exertion levels, average daily exertion levels, moving average exertion levels, exertion levels above a certain threshold, the number of times per day that the exertion levels are above the threshold and the duration of time above a threshold. The diagnostic reports to the physician can be transmitted to the physician and presented as daily, weekly, monthly or yearly data in graphic or tabular form. 
     The method employed for assessing patient well-being in accordance with the present invention is carried out by implanting in the patient a cardiac rhythm management device having a cardiac depolarization sensor, a physiologic sensor that produces electrical signals proportional to patient activity, a pulse generator for applying stimulating pulses to the heart and a microprocessor-based controller that is coupled to receive the output from the cardiac depolarization sensor along with the electrical signals from the physiologic sensor for producing delta rate signals for the pulse generator. The microprocessor-based controller is equipped with a memory whereby the delta rate signals may be stored for later readout. The microprocessor in the microprocessor based controller, is programmed to compute an average of the stored delta rate signals over a first pre-determined time interval. This average is used as an operand in a linear regression formula whereby a metabolic equivalent (METS) may be computed. 
     In accordance with a further feature of the invention, the physiologic sensor may be one or both of an accelerometer for sensing body motion and a transthoracic impedance sensor from which a minute ventilation signal can be derived. On a daily or weekly basis the maximum METS value and the average MET value for the interval in question can be computed and stored. 
     OBJECTS OF THE INVENTION 
     It is a principal object of the invention to provide a physician with metabolic need, physical activity and lifestyle information about a patient to evaluate the pacing parameters for the patient. 
     It is another object of the invention to measure and record maximal and average METS for various time periods. 
     It is yet another object of the invention to provide a method and apparatus for recording maximal MET, average MET, exercise frequency, and duration for storage whereby trended daily or weekly variations can be followed. 
     It is a further object of the invention to provide ambulatory activity monitoring and assessment in pacemaker patients, especially those suffering from CHF. 
     It is an object of the invention to improve management of pacing therapy. 
     It is an object of the invention to optimize rate responsive pacing. 
     It is still another object of the invention to change pacing therapy based on the MET measurements automatically. 
     It is an object of the invention to determine a patient&#39;s exertion level and exercise capacity. 
     It is also an object of the invention to monitor a patient to improve his quality of life and wellness. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing in which FIG. 1 is a block diagram of an implantable cardiac rhythm management device in which the present invention may be implemented. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 there is illustrated by means of an electrical schematic block diagram the hardware platform whereby the method of the present invention can be carried out. Shown enclosed by a broken line box  10  is an implantable CRM device having dual indifferent electrodes  12  and  14  disposed thereon. The electrode  12  may comprise an uninsulated portion of the metal (titanium) hermetically sealed housing while electrode  14  may be disposed on the device&#39;s header. The CRM device  10  is adapted to be coupled to a patient&#39;s heart via an electrical lead assembly  16  comprising an elongated flexible plastic tubular body member  18  having a distal tip electrode  20  and a ring electrode  22  affixed to the surface thereof. Extending the length of the lead are electrical conductors  24  that connect through electrical contacts in the lead barrel to the internal circuitry of the CRM device. 
     Contained within the hermetically sealed housing is a R-wave sensing amplifier  26  which picks up and amplifies ventricular depolarization signals picked up by the electrode  20 . The output from the sense amplifier is applied as an input to a microprocessor circuit  28  by way of conductor  30 . The microprocessor, following a stored program, provides a control signal on line  32  to a pulse generator  34  whose output signal is applied over one of the conductors  24  to the tip electrode  20  for stimulating and thereby evoking a paced response from the heart. 
     In accordance with the present invention, circuitry is also provided for measuring impedance changes within at least one chamber of the heart due to the influx and outflow of blood. In this regard, there is provided an oscillator  36  which, when activated, produces an alternating current of a predetermined frequency, typically in a range of from about 2000 Hz to 5000 Hz and of an amplitude below about  10  microamperes, which insures that the output from the oscillator will not stimulate heart tissue. This signal is preferably applied between the indifferent electrode  12  on the implanted CRM device and the tip electrode  20  on the lead and comprises an AC carrier signal that is modulated by the influx and outflow blood from the right ventricle. The modulated carrier signal is developed between the ring electrode  22  and the indifferent electrode  14  on the device&#39;s header and is amplified by sense amplifier  38  and then demodulated by demodulator circuit  40  to remove the modulating envelope from the carrier. The envelope signal is a measure of instantaneous impedance as a function of time. 
     The impedance vs. time (Z vs. t) is then applied to a signal processing circuit  42  which comprises a peak/valley/zero cross detector. When a zero cross is detected, the circuit  42  calculates the preceding peak-to-valley amplitude and issues an interrupt signal to the microprocessor  28 . Upon receiving this interrupt, the microprocessor fetches the peak-to-valley amplitude from the signal processing circuit  42  and sums the absolute values of the peak-to-valley amplitudes over an eight-second interval. This eight-second sum of the peak-to-valley amplitudes comprises the sensor input that is used in establishing the minute ventilation delta signal fed over line  32  to the pulse generator  34  for adjusting the rate at which the pulse generator issues cardiac stimulating pulses to the heart. 
     The pacemaker  10  also includes an activity sensor in the form of an integrated silicon accelerometer  44  that is bonded to a ceramic circuit board contained within the housing of the CRM device. The accelerometer includes a mass suspended by four leaf spring elements from a frame. The springs each include a piezoresistive element forming the four legs of a Wheatstone bridge which becomes unbalanced from displacement of the mass due to acceleration forces in a direction perpendicular to the frame. 
     To conserve battery power, the Wheatstone bridge is energized in a pulse mode where a predetermined voltage is applied across it for only a short period of time, typically 15 microseconds, and at a repetition rate of about 146 Hz. The raw accelerometer output from device  44  is then amplified by amplifier  46  before being applied to a switched capacitor bandpass filter  48 . The pass band of the filter  48  effectively excludes motion artifacts due to external noise while allowing passage of signal components whose frequencies are related to body motion due to exercise. 
     The output from the bandpass filter  48  is further signal processed by circuit  50  and then converted to a digital quantity by A/D converter  52  before being applied to the microprocessor  28 . 
     The CRM device  10  further includes a telemetry circuit  54  of known construction which allows information stored in the microprocessor&#39;s RAM memory banks to be read out transcutaneously to an external monitor  56  for viewing by a medical professional. Moreover, the telemetry link  58  allows programmable operands of the pacemaker to be altered following implantation of the CRM device. 
     One way to measure the activity level of a person is to measure the amount of oxygen the person is consuming. However, as explained above it is difficult to obtain accurate measurement of the amount of oxygen a person consumes unless the person is evaluated with somewhat cumbersome metabolic rate measuring equipment. For people going about their normal activities in a non-laboratory setting, a different method of measuring the person&#39;s activity level is required. 
     Metabolic equivalents (METS) are a unit of energy expenditure that is proportional to work load or oxygen uptake (VO 2 ).1METS=3.5 ml/(kg min). At rest, a person uses approximately 1 MET. Walking at 3 miles per hour, a person uses approximately 3.3 METS. Although METS are used in this application for the units of energy expenditure, any units measuring the energy used by the body may be applicable. 
     A pacemaker having an accelerometer and/or a minute ventilation sensor such as that described above, when installed in a patient, can conveniently be used to gather data which can then be used to calculate the metabolic equivalent (MET) in the patient due to patient activity. 
     The data collected by the sensors may be averaged over a period of from about 8 seconds to about 16 seconds. Then the data is converted into MET data using a formula which accurately correlates the minute ventilation and/or the accelerometer data to MET data. Time averaged data over a period of 1 to 5 minutes, or over other time periods, may also be used to provide a running time change comparing MET data in a given time period to the pervious ones. 
     In the present invention the MET level is calculated by the microprocessor solving the following linear equation: 
     
       
         MET=ax+b 
       
     
     Where a is a conversion factor, b is the resting MET level, which is usually defined as 1, and x is the averaged sensor signal from either the accelerometer sensor XL or from the minute ventilation sensor MV or from a blended or weighted value of these two sensors. 
     In studies conducted on a significant number of patients we have empirically determined that for an accelerometer based rate responsive pacer, the values of a and b should be about 0.0576 and 1 respectively. Hence, the formula for accelerometerbased METS is: 
     
       
         XL METS=0.0576 * XL+1 
       
     
     For a pacemaker having a minute ventilation sensor it was found to be preferred to use the value of a as a=0.0172 and the value for b of b=1 to calculate METS such that the formula for minute ventilation derived METS is: 
     
       
         MV METS=0.0172 * XL+1 
       
     
     The values used for a and b in the above formulas may change with the type of sensor used, the amplification of the sensor and the placement of the sensor in the body; however, the principle of operation will remain the same. 
     If the patient is on a treadmill, or otherwise has a known walking speed, V, then METS may be calculated as follows: 
     
       
         METS=a * XL *V+b 
       
     
     In one test sequence performed on a selected number of patients METS was found to be: 
     
       
         METS=0.0123* XL * V+1. 
       
     
     If the average sensor signal is sampled, for example, every 10 seconds, then there are 6 MET calculations per minute using the above formula. The data from each calculation can be stored for future reference. Of particular interest is data showing the daily maximum MET level, which comprises the maximum activity level sustained by the patient during the day. This information is useful to a physician for setting pacing parameters of the pacemaker for the patient. 
     Also of interest is a daily moving average of MET levels of the patient. This information may also be calculated and stored. The moving average is calculated as the average over the last n number of measurements. For example n may be 50 or 100 to provide a moving average over recent measurements. 
     In order to record exercise events, a 1 to 8 minute moving average and an amplitude threshold may be applied to the sensor signal, such that exercise events are counted and stored. 
     In accordance with the invention, the daily maximum data, daily moving average, 1-8 minute moving average, and exercise events totals and times may be compiled in any combination of useful statistical manner, for daily, weekly, monthly or yearly reports, or for whatever need there is for MET data to aid in the treatment of the patient. 
     The data can be programmed to be reported to the physician or other health care provider in any manner desired to give useful information about the patient&#39;s activity levels. The data may be displayed or printed in tabular form, as a graph, a histogram chart, or as a simple listing or data as collected chronologically. The long term history of the MET levels show the patient&#39;s activity patterns and the physician may use the data as a diagnostic tool to assess the efficacy of a treatment protocol. 
     The MET data may also be used in a rate adaptive CRM device to automatically adjust the rate of pacing in a pacemaker. 
     The accelerometer data and the minute ventilation data may be combined in a blending algorithm to provide METS values. 
     The microprocessor  10  may be programmed to average received signals from the sensors over a time period of on the order of 8 to 16 seconds, and then to calculate the METS from the signals received, according to the above conversion formula e programmed into the microprocessor. 
     The microprocessor is further programmed to store the data from each MET time period calculation along with the time it occurred and can compare the MET for each time period to determine the daily maximal MET level and then store the value and time thereof in a memory register for later readout. The microprocessor is also preferably programmed to calculate a daily 24 hour moving average MET value and to store that value and the date thereof in a predetermined memory register. For example, the microprocessor may be programmed to calculate a 1 to 8 minute moving average MET value and further, an amplitude threshold can be applied to the XL and/or MV sensor signals to detect when the threshold is exceeded, indicative of exercise events of a given intensity. The microprocessor can also be programmed to count and store the total number of exercise events and the time of occurrence and MET values of each such exercise event. 
     The microprocessor can also be programmed to provide daily, weekly, monthly or yearly reports and average the MET values over any time periods to suit the report data desired by the physician or health care provider to better monitor the patient. The microprocessor may determine minimum averages of exertion during rest periods, and average exertion for the entire day or portions of the day. 
     The maximum and averaged MET levels and exercise frequency data can be retrieved from the pacemaker by telemetry methods well known in the art. 
     The METS data can be presented to the physician or health care provider in various forms including, but not limited to, time charting, graphs and tables. The physician can then use the data to determine what the patient&#39;s activity patterns are and to what degree of exertion the patient has reached during an exercise regimen and how frequently the exercise events take place. From this, the physician can then readily determine the degree of wellness of the patient and change the treatment of the patient accordingly. Such treatment may include changes in pacemaker pacing setting, drug delivery, etc. 
     Alternatively the moving average MET values calculated by the pacemaker may be used to adaptively adjust the pacemaker automatically for the activity level currently being experienced by the patient. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.