Abstract:
A system and method are provided for compensating for the drop in blood pressure upon standing. Upon transition from prolonged sitting, lying down, or standing position, the pacemaker abruptly increasing its pacing rate upon postural transition. The pacing rate is abruptly increased to about 80-100 bpm in 20 seconds-one minute. The pacing rate then decreases slowly to another high rate in two or three minutes and then follows the metabolically indicated rate. This pacing rate is triggered whenever a patient stands after a prolonged reclined or supine/prone position as indicated by activity variance measurements.

Description:
FIELD OF INVENTION 
     This invention relates to implantable cardiac stimulation devices which monitor the activity level of a patient to detect changes in activity that indicate changes in body position and metabolic need and varies the stimulation rate as needed. 
     BACKGROUND OF THE INVENTION 
     A pacemaker is an implantable stimulation device that delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia, a condition in which a patient cannot normally maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate, typically a rate deemed to be appropriate for the patient at rest. 
     Early advances in pacemakers included the ability to sense a patient&#39;s cardiac rhythm. This led to the development of demand pacemakers, so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are able to detect spontaneous, hemodynamically effective cardiac contractions that occur within an acceptable time period. This extends the life of the pacemaker&#39;s battery as well as avoids competition with the heart&#39;s intrinsic rhythm. 
     The next major advance in pacemakers included the rate-responsive pacemaker which automatically adjusts the patient&#39;s heart rate in accordance with metabolic demands. An implanted rate-responsive pacemaker typically operates to maintain a predetermined base rate when a patient is engaged in physical activity at or below a threshold level and gradually increases the paced heart rate in accordance with increases in physical activity until a maximum rate is reached. These pacemakers typically correlate measured activity physical activity to an appropriate heart rate and define a transition slope between the minimum and maximum heart rate. This transition slope can be telemetrically adjusted to meet patient needs. A common rate-responsive sensor is an activity sensor that transduces mechanical forces associated with physical activity into an electrical signals. Typically, these activity sensors generally contain a piezoelectric transducing element which generates a measurable electrical potential. The pacemaker then analyzes this signal to determine the stimulation rate. 
     A variety of signal-processing techniques have been used to process the raw activity sensor signals. In one approach, the raw signals are rectified and filtered. Also, the frequency of the highest signal peaks can be monitored. Typically, the end result is a digital signal indicative of the level of sensed activity at a given time. The activity level is then applied to a transfer function which defines the pacing rate (also known as the sensor indicated rate) for each possible activity level. Attention is drawn to U.S. Pat. No. 5,074,302 to Poore et al., entitled “Self-Adjusting Rate-Responsive Pacemaker and Method Thereof”, issued Dec. 24, 1991, which is hereby incorporated by reference in its entirety. This transfer function can be modified telemetrically by the patient&#39;s physician. It can also be modified within the pacemaker based upon the stored history of the patient&#39;s activity levels to define a new transfer function. 
     While the rate-responsive pacemaker has very closely mimicked the function of a normal heart during exercise, it was discovered that some patients could not sleep well because either the base rate was too high or they were experiencing short bursts of increased stimulation rate, possibly from sleep movement, that would waken them. This base rate did not accommodate the patient&#39;s need for a lower stimulation rate during sleep or sustained rest. A 10-20 beats per minute (bpm) difference can result in difficulty sleeping as well as unnecessarily depleting the pacemaker battery. An example of a rate-responsive pacemaker, which determines when a patient is sleeping and adjusts its base rate accordingly, is set forth in U.S. Pat. No. 5,476,483 to Bornzin et al., entitled “System and Method for Modulating the Base rate During Sleep for a Rate-Responsive Cardiac Pacemaker”, issued Dec. 19, 1995, which is hereby incorporated by reference in its entirety. This reference sets forth methods of modulating the base rate based upon the monitoring of activity variance. By monitoring the variance of an activity signal, it has been shown that one can distinguish between sleep (low variance in the activity signal) and exercises (high variance in the activity signal). This modulated base rate is also known as the circadian base rate. Otherwise, the processor uses the activity transfer function as defined above to determine the stimulation rate. 
     Unfortunately, there is another group of patients who are not fully assisted with the above stimulation methods. These patients are typically long-term sufferers of diabetes. These individuals have a tendency to gradually lose the function of their autonomic nerves. This is caused by a widespread degeneration of the neurons in the brain and spinal cord due to long term exposure to excess levels of blood sugar. This condition is characterized by a marked decrease in blood pressure upon standing caused by an inability to increase the heart rate and constrict the systemic resistance and capacitance vessels. Healthy individuals, in contrast, can increase their heart rate immediately when they are standing from a prolonged reclined or sitting position. This normal response is called orthostatic compensation. As a result, this patient group has a need for a pacemaker which detects their change in body position from lying or sitting to standing and compensates with an abrupt increase in the pacing rate. 
     Many different methods have been attempted to determine the physical position of the patient. An example is U.S. Pat. No. 5,354,317 to Alt, entitled “Apparatus and Method for Cardiac Pacing Responsive to Patient Position”, issued Oct. 11, 1994, in which the controller monitors a motion sensor to produce a static output which represents the static position of the patient, i.e. lying down or upright. This static output is used to determine which of the predetermined base rates should be used, i.e. the sleep base rate or the awake base rate. This reference, however, depends upon the detection of a static and stable position. The DC accelerometer, for example, cannot differentiate between the patient lying on the left or right side and standing. It also cannot detect the difference between standing and sitting. 
     What is needed for this patient population, suffering from a lack of orthostatic compensation, is an abrupt increase in the pacing rate. The &#39;317 reference only teaches to increase the pacing rate upon standing from lying down supine or prone, but does not include standing from sitting or standing from lying on one&#39;s side. In addition, the &#39;317 reference depends upon the detection of a static position from a position sensor to determine when to implement the stimulation therapy. The &#39;317 reference also fails to teach the adjustment of the pacing rate to accommodate the patient&#39;s sleep cycle. 
     Accordingly, it is desirable to develop an implantable cardiac stimulation device which maintains the patient&#39;s heart rate in relation to the activity level or other metabolic indicator and detects the need for an abrupt increase in the pacing rate upon standing from sitting or lying, thereby mimicking the normal heart&#39;s response to orthostatic compensation. 
     SUMMARY OF THE INVENTION 
     The present invention is directed towards an implantable stimulation device having rate-responsive and demand abilities. In addition, this invention monitors the patient&#39;s activity level and variances in activity level to determine when a patient has a sudden increase in activity after an extended period of inactivity. Meeting these conditions indicates that the patient must be standing, and the stimulation device compensates for the sudden drop in blood pressure upon standing (also known as the orthostatic compensation pacing). 
     To this end, the present invention is directed toward any stimulation device that uses a sensor to detect an indicator of patient activity over time. While the preferred embodiment is directed toward a single AC accelerometer, other types of sensors may be used, such as oxygen saturation sensors; impedance sensors that measures the change in blood volume; and sensors that detect the change in IEGM or evoked response, etc. The signals from the AC accelerometer are then used to derive the activity level signal and the long term variance in activity. These two indicators are used to determine when a patient must be standing after a prolonged period of time in the sitting or lying position. 
     The orthostatic pacing rate method is a specific pacing regimen wherein the system abruptly increases the pacing rate and then slowly decreases the pacing rate to maintain the blood volume and the blood pressure upon standing. 
     To determine when orthostatic compensation pacing is necessary, the system monitors two factors: if the activity signal is above a first threshold, and there has been a sufficiently extended period of inactivity preceding the higher level of activity. If these conditions are met, the patient must have been inactive an extended period of time and is now active. In this case, the system uses the orthostatic compensation pacing rate. 
     As such, the pacemaker monitors the activity level and the variance in activity to determine when to implement the orthostatic compensation rate method based upon the level of and duration of patient activity, in addition to its rate-responsive and demand pacing capabilities. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The above and other aspects, features, and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
     FIG. 1 shows a simplified functional block diagram of an implantable stimulation device, according to the present invention; 
     FIG. 2 shows a manner in which the pacing rate can be modulated in light of activity levels; 
     FIG. 3 shows the response of a normal heart upon standing, 
     FIG. 4 shows a pacing rate for the orthostatic compensation pacing method, according to the present invention; and 
     FIG. 5 shows a flow chart of determining whether the pacing rate follows the activity indicated rate, circadian base rate, or the orthostatic compensation pacing rate method. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, the implantable cardiac stimulation device in accordance with this invention is shown as a dual sensor rate-responsive pacemaker  10 . It is well within the scope of this invention to operate this stimulation device in a demand mode as is well known within the art. While the preferred embodiment is directed towards a stimulation device which uses an activity sensor for determining the pacing rate, it is well within the scope of this invention to apply the principles of this invention for use with other physiologic sensors that measure metabolic demand. 
     FIG. 1 sets forth a simplified block diagram of the stimulation device  10 . The stimulation device  10  is coupled to a heart  5  by way of two leads  12 ,  14 . The first lead  12  has at least one electrode  18  in contact with the atrium of the heart  5 , and the second lead  14  has at least one electrode  20  in contact with the ventricles of the heart  5 . The leads  12 ,  14  are electrically and physically connected to the stimulation device  10  through a connector  16  which forms an integral part of the housing (not shown) in which the circuits of the stimulation device  10  are housed. The connector  16  electrically protects circuits within the stimulation device  10  via a protection network  17  from excessive shocks or voltages that could appear on electrodes  18 ,  20  in the event with contact with a high voltage signal, e.g., from a defibrillator shock. 
     The leads  12 ,  14  carry the stimulating pulses to the electrodes  18 ,  20  from an atrial pulse generator  22  and a ventricular pulse generator  24 , respectively. Further, the electrical signals from the atrium are carried from electrode  18  through the lead  12  to the input terminal of an atrial channel sense amplifier  26 . The electrical signals from the ventricle are carried from the electrode  20  through the lead  14  to the input terminal of the ventricular channel sense amplifier  28 . Similarly, electrical signals from both the atrium and ventricle are applied to the inputs of the IEGM (intracardiac electrogram) amplifier  30 . The stimulation device  10  detects an evoked response from the heart in response to an applied stimulus, allowing the detection of capture with a suitable broad bandpass filter. The IEGM amplifier  30  is also used during transmission to the external programmer&#39;s  60  state machine or other control logic. 
     The stimulation device  10  is controlled by a controller  32  that typically includes a microprocessor to carrying out the control and timing functions. The controller  32  receives output signals from the atrial amplifier  26 , the ventricular amplifier  28 , and the IEGM amplifier  30  over the signal lines  34 ,  36 ,  38 , respectively. 
     The controller  32  also generates trigger signals that are sent to the atrial pulse generator  22  and the ventricular pulse generator  24  over the signal lines  40 , 42 , respectively. 
     The stimulation device  10  also includes a memory circuit  48  that is coupled to a control system  32  over a suitable data/address bus  50 . This memory circuit  48  allows certain control parameters, used by the controller  32  in controlling the operation of the stimulation device  10 , to be stored and modified, as required, in order to customize the stimulation device&#39;s operation to suit the needs of a particular patient. Further, the data sensed by the IEGM amplifier  30  during the operation of the stimulation device  10  may be stored in the memory circuit  48  for later retrieval and analysis. 
     A clock circuit  52  directs appropriate clock signal(s) to the controller  32  as well as any other circuits throughout the stimulation device  10 , e.g., to the memory circuit  48 , by the clock bus  54 . 
     The stimulation device  10  also includes a telemetry communications circuit  56  which is connected to the controller  32  by way of a suitable command/data bus  58 . In turn, the telemetry circuit  56  is selectively coupled to an external programming device  60  by an appropriate communication link  62  such as any suitable electromagnetic link. Advantageously, through the external programmer  60  and the communication link  62 , desired commands may be sent to the control system  32 . Other data measured within or by the stimulation device  10  such as IEGM data, etc. may be stored and uploaded to the programmer  60 . 
     The stimulation device  10  additionally includes a battery  64  which provides operating power to all the circuits of the stimulation device  10  via a POWER signal line  66 . 
     The stimulation device  10  also includes a sensor  68  that is connected to the controller  32  over a suitable connection line  72 . In the preferred embodiment, this sensor detects patient activity via an AC accelerometer, but could be any appropriate sensor which can indicate patient activity. Attention is directed to &#39;483 for further examples of other suitable activity sensors. In addition, any sensor which indicates metabolic need over time could be used in place of the activity sensor. Such sensors could be an oxygen saturation sensor, temperature sensor, etc. In the case of these alternative sensors, the sensor would be placed on the lead  14  as shown by alternative sensor  69 . 
     The operation of the above described stimulation device  10  is similar to the conventional manner to provide pacing pulses at a rate that comfortably meets the patient&#39;s metabolic demands. In this case, the controller  32  uses signals generated by the sensor  68  to determine the activity level of the patient. The measured level of activity is indicative of metabolic need. Many methods of determining the activity level are well known in the art. Attention is drawn to &#39;483, which is hereby incorporated by reference. 
     To regulate the pacing rate, the controller  32  provides a trigger signal to the A-pulse generator  22  and/or the V-pulse generator  24 . The timing of this signal (to a large extent) is determined by the activity level. 
     In FIG. 2, the activity indicated rate and the circadian base rate methods are illustrated together via a single transfer function. The transfer function is used by the control system to correlate the activity level measurements shown along the horizontal axis to the activity indicated pacing rates shown along the vertical axis. The controller  32  then triggers the appropriate pulse generator  22 ,  24  at the activity indicated rate. It should be noted that an appropriate transfer function can be used based upon individual patient need. In addition, different modes of pacing (i.e. DDD, VVI, etc.) can be accommodated by this method. 
     Two activity levels are noted on the horizontal axis of the transfer function: a low activity threshold  202  and a high activity threshold  204 . For activity level measurements above&#39; the high activity threshold  204 , the pacing rate is maintained at a maximum pacing rate  200 . For activity level measurements between the low activity threshold  202  and the high activity threshold  204 , the activity indicated pacing rate varies according to the programmed transfer function  208 . In this case, the activity indicated pacing rate varies linearly between a circadian base rate  210  and a maximum pacing rate  200 . However, this transition can be programmed to meet patient&#39;s needs by the physician or adjusted by the processor periodically as set forth in U.S. Pat. No. 5,514,162 to Bomzin et al, entitled “System and Method for Automatically Determining the Slope of a Transfer Function for a Rate-Responsive Cardiac Pacemaker”, issued May 7, 1996, hereby incorporated by reference. 
     For activity level measurements below the low activity threshold  202 , the processor sets the pacing rate to a rate defined by the circadian base rate  210 . The circadian base rate  210  is defined through monitoring the activity variance measurements as described more fully in &#39;483. A patients activity levels are monitored and activity variance measurements are calculated and monitored to determine when and how long a patient typically sleeps. These two terms are used to define a stimulation rate which is below the programmed base rate of the stimulation device  10  such that the patient receives a lower pacing rate during sleep. This lower pacing rate more closely mimics the natural cardiac rhythm. 
     As discussed above, the patient group which suffers from long term diabetes tends to develop neuropathy from the long term exposure of their nerves to excessive blood sugar levels. This condition erodes the ability to adequately control the heart rate. In particular, this condition renders the patient unable to compensate for the dramatic drop in blood pressure upon standing after sitting or lying down due to an inability to increase the heart rate and constrict the system resistance and capacitance vessels. 
     To overcome this condition, the controller  32  compensates for the change in patient position by triggering at an orthostatic compensation pacing rate. 
     FIG. 3 sets forth the normal heart response to provide the orthostatic compensation. Upon standing after a prolonged period of sitting or lying down, the normal autonomic nervous system abruptly increases the natural heart rate to approximately 80-100 bpm in 3-8 seconds at  302 . Then, the natural heart rate then slowly decreases to a rest rate in 2 seconds to one minute, typically approximately 60-70 bpm at  304 . 
     FIG. 4 sets forth the orthostatic compensation pacing method that provides the orthostatic compensation to a patient that does not naturally have it. The controller  32  abruptly increases the stimulation rate to approximately 85-100 bpm in 8 seconds at  402 . The upper orthostatic compensation pacing rate is determined by the clinician for each individual patient. Then, the stimulation rate is slowly decreased to a high base rate in 20-60 seconds, typically approximately 70 bpm at  404 . The controller can override the orthostatic compensation pacing method if the patient activity level indicates an immediate stimulation rate higher than that of the immediate orthostatic compensation pacing rate. In other words, if the activity indicated pacing rate or the circadian base pacing rate should exceed the orthostatic compensation pacing rate at a particular time, then the controller  32  would follow the maximum of the three indicated stimulation rates. Once the orthostatic compensation pacing rate has finished, the controller  32  uses the transfer function  208  as set forth in FIG. 2 to determine the immediate stimulation rate. 
     To determine when a patient is in need of orthostatic compensation, the controller  32  uses the activity level signals and the activity variance measurements to determine when the patient has stood after prolonged time of sitting or lying down. 
     To this end, the controller  32  monitors two main variables in relation to two preset thresholds: the immediate activity level and the long term variance in activity (also known as the activity variance measurement). The clinician programs the activity threshold for a patient appropriate activity level. The activity variance threshold is predetermined based upon the patients activity. To determine the activity variance threshold, the activity variance measurements are stored in a histogram for a week. Further information regarding histograms can be found in &#39;483. 
     To determine when a patient is standing after an extended period of inactivity, a night time activity variance threshold and a day time activity variance threshold are determined. The night time activity variance threshold is predetermined to be the bins of the histogram which contain counts of less than 17.5% of the total histogram counts. The day time activity variance is predetermined to be the bins in the histogram which contains counts of less than 67.5% of the total bin count. These two activity variance thresholds are then averaged to determine an activity variance threshold that indicates a patient is resting. 
     In addition to monitoring the patient activity and the activity variance measurements, the controller  32  monitors how long the activity variance measurement is below the activity variance threshold. This time duration (T rest(expired) ) is the necessary time for a patient to be resting and then in need of orthostatic compensation once the patient stands. It has been found that a T rest(expired)  time duration of 1 minute to 3 hours as programmed by the clinician is the time duration that adequately indicates when a patient will require orthostatic compensation pacing upon standing. 
     FIG. 5 sets forth the flow chart describing how the controller  32  determines the immediate pacing rate and if patient has changed body positions and has previously been sitting or lying down for an extended period of time. 
     Following Flow  1 , the controller  32  determines if the activity level signal is less than an activity threshold (Block  502 ), and if the activity variance measurement is greater than an activity variance threshold (Block  504 ). If these conditions are met, then the patient must be inactive, and the controller  32  adjusts the pacing rate to follow the circadian base rate (Block  506 ) as set forth in FIG.  2 . 
     Following Flow  2 , the controller  32  determines if the activity level signal is less than the activity threshold (Block  502 ), and the activity variance is also less than the activity variance threshold (Block  504 ). If these conditions are met, then the patient must be in a reclined or in a supine/prone position and is resting. At this point, the controller  32  starts a duration timer. This duration timer monitors the time the patient is at an extended period of inactivity (i.e. T rest ) (Block  508 ). The controller  32  adjusts the stimulation rate to follow the circadian base pacing rate (Block  506 ) as set forth in FIG.  2 . 
     Following Flow  3 , the controller  32  determines if the activity level signal is greater than an activity threshold (Block  502 ) and if the T rest  duration timer has not been started (i.e. T rest =0)(Block  520 ). If these conditions are met, then the patient must be active and must have been active. In this case, the controller  32  adjusts the stimulation rate to be the maximum of the activity indicated pacing rate and the circadian base pacing rate (Block  522 ) as set forth by the transfer function  208  in FIG.  2 . 
     Following Flow  4 , the controller  32  determines if the activity level signal is greater than an activity threshold (Block  502 ) and if the T rest  duration timer has been started (Block  520 ) but has not exceeded T rest(expired)  (Block  524 ). If these conditions are meet, then the patient has been resting a short period of time and has stood. In this case, the controller resets the T rest  duration timer (Block  526 ) and adjusts the pacing rate to the maximum of the activity indicated pacing and the circadian base pacing rate as set forth by the transfer function  208  (Block  522 ). 
     Following Flow  5 , the controller  32  determines if the activity level signal is greater than an activity threshold (Block  502 ), and the Trest duration timer is greater than the T rest(expired)  duration (Blocks  520 ,  524 ). If these conditions are met, then the patient must have just stood and is in need of orthostatic compensation pacing. The orthostatic compensation pacing method has a total regimen time (N)  406 , and the controller  32  starts a regimen duration timer to monitor how far into the orthostatic compensation pacing method the patient is (K)  408 . If the regimen duration timer indicates that the pacing method is not complete (K&lt;N) (Block  528 ), then the controller  32  adjusts the stimulation rate to be the maximum of the activity indicated pacing rate, circadian base pacing rate, and the orthostatic compensation pacing rate (Block  530 ), and increments the regimen duration timer (Block  532 ) (i.e. K=K+1). 
     Following Flow  6 , the controller determines if the activity level signal is greater than an activity threshold (Block  502 ), the T rest  duration timer exceeds T rest(expired)  time period(Blocks  520 ,  524 ), and the regimen duration timer exceeds the regimen time (K&gt;N) (Block  528 ). If these conditions are met, then the patient must have just stood and has finished orthostatic compensation pacing. At this point, the controller  32  resets the T rest  duration timer and the regimen duration timer (Block  540 ) and adjusts the stimulation rate to be the maximum of the activity indicated pacing rate and the circadian base rate (Block  522 ) as set forth in FIG.  2 . 
     Although the invention has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the application of the principles of the invention. For instance, this method can be used to detect instantaneous changes and long term variations in any signal indicative of metabolic need to vary the pacing rate and pacing method as described. Such sensors could be an oxygen saturation sensor, impedance sensors that measures the change in blood volume; and sensors that detect the change in IEGM or evoked response, etc. Accordingly, the embodiments described in particular should be considered exemplary, not limiting, with respect to the following claims.