IMPLANTABLE CARDIAC STIMULATOR WITH RATE-ADAPTIVE T-WAVE DETECTION

Abstract: An implantable device (10) for providing pacing and defibrillating stimuli to a heart, comprises: a power source (70), at least one electrode (60) electrically connected to said power source, a sensor (40) capable of providing data that can be used to determine a physiological pacing demand, and a control device (12). The control device (12) controls the application of electrical stimuli to the heart. The control device (12) calculates (106) an adjusted pacing interval based on said demand, calculates (112) an adjusted ventricular pace refractory period (VPRPcurrent) that is a function of said pacing interval, and calculates a T-wave monitoring window (108, 110). The end time, and optionally the start time also, of the T-wave window is a function of the pacing interval. The control device is also capable of adjusting the sensitivity of an amplifier (40) based on signals received during the T-wave window.

Description: Description Implantable Cardiac Stimulator with Rate- Adaptive T-Wave Detection Technical Field The present invention relates generally a cardiac stimulating device that combines the abilities and objectives of implantable pacemakers and defibrillators. More particularly, the present invention relates to a implantable cardioverter/defibrillator that is capable of both rate responsive pacing and detection of fibrillation. Still more particularly, the present invention relates to a cardiac stimulating device that includes a rate-responsive adjustment of the T-wave window, which is in turn used to automatically adjust the sensitivity, or gain of the device. Background Art In the normal human heart, the sinus (or sinoatrial (SA)) node generally located near the junction of the superior vena cava and the right atrium constitutes the primary natural pacemaker by which rhythmic electrical excitation is developed. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers (or atria) at the right and left sides of the heart. In response to excitation from the SA node, the atria contract, pumping blood from those chambers into the respective ventricular chambers (or ventricles). The impulse is transmitted to the ventricles through the atrioventricular (AV) node, and via a conduction system comprising the bundle of His, or common bundle, the right and left bundle branches, and the Purkinje fibers. The transmitted impulse causes the ventricles to contract, the right ventricle pumping unoxygenated blood through the pulmonary artery to the lungs, and the left ventricle pumping oxygenated (arterial) blood through the aorta and the lesser arteries to the body. The right atrium receives the unoxygenated (venous) blood. The blood oxygenated by the lungs is carried via the pulmonary veins to the left atrium. This action is repeated in a rhythmic cardiac cycle in which the atrial and ventricular chambers alternately contract and pump, then relax and fill. Four one-way valves, between the atrial and ventricular chambers in the right and left sides of the heart (the tricuspid valve and the mitral valve, respectively), and at the exits of the right and left ventricles (the pulmonic and aortic valves, respectively, not shown) prevent backflow of the blood as it moves through the heart and the circulatory system. The sinus node is spontaneously rhythmic, and the cardiac rhythm it generates is termed normal sinus rhythm ("NSR") or simply sinus rhythm. This capacity to produce spontaneous cardiac impulses is called rhythmicity, or automaticity. Some other cardiac tissues possess rhythmicity and hence constitute secondary natural pacemakers, but the sinus node is the primary natural pacemaker because it spontaneously generates electrical pulses at a faster rate. The secondary pacemakers tend to be inhibited by the more rapid rate at which impulses are generated by the sinus node. Disruption of the natural pacing and propagation system as a result of aging or disease is commonly treated by artificial cardiac pacing, by which rhythmic electrical discharges are applied to the heart at a desired rate from an artificial pacemaker. An artificial pacemaker (or "pacer" as it is commonly labeled) is a medical device that delivers electrical pulses to an electrode that stimulates the heart so that it will contract and beat at a desired rate. If the body's natural pacemaker performs correctly, blood is oxygenated in the lungs and efficiently pumped by the heart to the body's oxygen-demanding tissues. However, when the body's natural pacemaker malfunctions, an implantable pacemaker often is required to properly stimulate the heart. Implanted pacemakers can either pace continuously, or can be provided with sensors that are able to detect a natural pacing signal, allowing the implanted device to be inhibited from pacing when artificial pacing stimulus is not needed. Pacing in this manner is known as demand pacing, in that the pacer only provides a pacing stimulus when it fails to detect a natural pacing signal. In addition, pacers today are often rate responsive on the basis of sensor input. That is, one or more physiological parameters are measured and used as a basis for calculating an optimal pacing rate. Thus, the rate at which this type of rate responsive demand pacer looks for natural pacing signals depends on the value of the physiological parameter(s) being tracked. This produces an effect similar to that in dual chamber pacemakers in which the ventricular pacing rate increases as it "tracks" increases in the atrial rate. An in-depth explanation of certain cardiac physiology and pacemaker theory of operation is provided in U.S. Patent No. 4,830,006. Throughout the following discussion and claims, the concepts of "rate responsiveness" and "pacing demand" refer to and include both sensor-driven and "tracked" types of systems. Cardiac ventricular fibrillation is a condition characterized by rapid, chaotic electrical and mechanical activity of the heart's excitable myocardial tissue that results in uncoordinated activity of the heart tissue and consequent ineffectual quivering of the ventricles. This in turn causes an instantaneous cessation of blood flow from the heart. Unless cardiac output is restored almost immediately after the onset of ventricular fibrillation, tissue begins to die for lack of oxygenated blood and patient death can occur within minutes. Defibrillation is a technique involving the application of one or more high energy electrical stimuli to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections and to restore the synchronized contraction of the total mass of heart tissue. Successful defibrillation requires the delivery of a sufficient electrical pulse to the heart of the patient to terminate fibrillation and preclude immediate re-emergence of the condition. The use of implantable defibrillators for treating cardiac fibrillation is well known. Likewise, devices capable of delivering both demand pacing pulses and defibrillation shocks are known and are indicated for heart patients suffering from both irregular heart beat and occasional cardiac fibrillation. Conventional pacemakers and implantable cardioverter/defibrillators (ICD's) typically operate by analyzing the electrical output of the heart in a series of windows. Such implantable devices combining pacing, defibrillating and sensing functions are known, for example, from US 5,540,724. Referring now to Figure 2, a portion of an external electrocardiograph showing two complete pacing cycles is labeled so as to illustrate the component parts of each cycle. A cycle begins with a P-wave, which indicates an atrial event, followed by a QRS wave indicative of ventricular contraction. The cycle ends with a T-wave caused by repolarization of the ventricles in preparation for the next cycle. A ventricular demand pacemaker looks for the R-wave that indicates a natural pacing or "sensed" event. When a sensed event occurs, the pacer is inhibited from providing an unnecessary pacing stimulus. It is common for pacers and ICD's to use a ventricular pace refractory period (VPRP), during which ventricular activity is ignored. The VPRP helps prevent misinterpretation of post-pace electrical activity or T-waves as R- waves and creates a monitoring window for T-wave activity. Early pacers commonly used a fixed VPRP. Many dual-chamber and rate-responsive pacers still use a fixed VPRP, however, the VPRP in these devices is limited by the maximum possible ventricular pacing rate. That is, the VPRP must be shorter than the shortest possible pacing interval. Thus, the fixed VPRP may not be completely appropriate for all of the pacing rates that the pacer can deliver. To address this problem, it is known to dynamically adjust the VPRP in proportion to the pacing interval, allowing a longer VPRP to be used at slower pacing rates. On the other hand, because the amplitude of the signals generated by the ventricles during fibrillation are so much smaller than the amplitude of signals generated during normal operation (either paced or sensed), there is a possibility that device may not be able to detect fibrillation signals during rapid ventricular pacing. If the device cannot detect fibrillation signals and thereby distinguish fibrillation from bradycardia, there is a possibility that the needed defibrillation therapy will not be administered. One solution to this problem is to adjust the gain of the sensing amplifier in response to changes in the amplitude of T-wave. Thus, it is desirable to use T-wave amplitude and/or timing data collected during the T-wave monitoring window within the VPRP to calculate, among other things, the degree to which gain should be adjusted. Because the physiologic delay from pace to T-wave is often proportional to the pacing interval, at slower ventricular pacing rates, the T-wave may occur too late in the fixed VPRP to be completely detected if the fixed VPRP is too short. Thus, many of the approaches taken to address the problems related to rate-responsive pacing also have the undesired effect of reducing the reliability of the T-wave data. Hence, it is desired to provide a technique for ensuring that collection of T-wave data is optimized for all possible pacing/defibrillating modes and at all foreseeable pacing rates. Disclosure of the Invention The present invention comprises an implantable device capable of both rate-responsive pacing and defibrillation as needed. The device includes a dynamically adjustable VPRP that corresponds to the adjusted pacing rate, which in turn corresponds to the measured physical activity level of the patient (demand). The present invention further includes a dynamically adjustable T- wave monitoring window that is designed to optimally collect the T-wave data within the adjusted VPRP. The T-wave window is adjusted in response to changes in the pacing rate. The present device further includes a T-wave sensor that provides a measurement of the T-wave amplitude, which in turn can be used to adjust the gain or sensitivity of the device. The rate-responsive adjusted pacing rate is calculated in a known manner, based on one or more measured physiological indicators. Brief Description of the Drawings Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompany drawings, wherein: Figure 1 is a schematic representation of an implantable pacemaker-defibrillator, suitable for implementing this invention; Figure 2 shows an exemplary external electrogram during normal sinus rhythm and during fibrillation; Figure 3 is a plot showing the variation of the VPRP and T-wave window in accordance with one embodiment of the present invention; and Figure 4 is a flow chart representing the steps for ventricular pacing in accordance with the present invention. Best Mode for Carrying Out the Invention According to the present invention an implantable rate-responsive pacer/cardioverter/ defibrillator is provided with an adjustable ventricular pace refractory period (VPRP) that varies as a function of the adjusted pacing rate and an adjustable gain control that adjusts the sensitivity of the device in response to changes in the amplitude of the T-wave signal. As stated above, a preferred embodiment includes a rate-responsive feature that calculates an adjusted pacing rate that is greater than the programmed base pacing rate, based on measured physiological demand data, such as activity or blood oxygen level. FIG. 1 is a block diagram illustrating a rate adaptive pacemaker/defibrillator 10. A microprocessor 12 preferably provides pacemaker control and computational facilities. It will be appreciated that other forms of circuitry, such as analog or discrete digital circuitry can be used in place of microprocessor 12. However, a microprocessor is preferred for its miniature size and its flexibility, both of which are of critical importance in the implantable systems in which it is envisioned the invention will find use. A particularly energy efficient microprocessor which is designed specifically for use with implantable medical devices is fully described in Gordon, et al. , U.S. Patent No. 4,404,972. The microprocessor 12 has input/output ports connected in a conventional manner via bidirectional bus 14 to a memory 16, an A-V interval timer 18, and a pacing interval timer 20. In addition, the A-V interval timer 18 and pacing interval timer 20 each has an output connected individually to a corresponding input port of the microprocessor 12 by lines 22 and 24 respectively. Memory 16 preferably includes both ROM and RAM. The microprocessor 12 may also contain additional ROM and RAM as described in the Gordon, et al. U.S. Patent No. 4,404,972. The pacemaker operating routine is stored in ROM. The RAM stores various programmable parameters and variables. The A-V and pacing interval timers 18 and 20 may be external to the microprocessor 12, as illustrated, or internal thereto, as described in the Gordon, et al. U.S. Patent No. 4,404,972. The timers 18, 20 are suitable conventional up or down counters of the type that are initially loaded with a count value and count up to or down from the value and output a roll-over bit upon completing the programmed count. The initial count value is loaded into the timers 18, 20 on bus 14 and the respective roll-over bits are output to the microprocessor 12 on lines 22, 24. The microprocessor 12 preferably also has an input/output port connected to a telemetry interface 26 by line 28. The pacemaker when implanted is thus able to receive pacing, arrhythmia therapy, and rate control parameters from an external programmer and send data to an external receiver if desired. Many suitable telemetry systems are known to those skilled in the art. One such system and encoding arrangement is described in Armstrong and Cox, U.S. Patent No. 5,383,912. The microprocessor 12 output ports are connected to inputs of an atrial stimulus pulse generator 30 and a ventricle stimulus pulse generator 32 by control lines 34 and 36 respectively. The microprocessor 12 transmits pulse parameter data, such as amplitude and width, as well as enable/disable and pulse initiation codes to the generators 30, 32 on the respective control lines. The microprocessor 12 also has input ports connected to outputs of an atrial sense amplifier 38 and a ventricular sense amplifier 40 by lines 42 and 44 respectively. The atrial and ventricular sense amplifiers 38, 40 detect occurrences of P-waves and R- waves. The atrial sense amplifier 30 outputs a signal on line 42 to the microprocessor 12 when it detects a P-wave. This signal is latched to the microprocessor 12 input port by a conventional latch (not shown). The ventricular sense amplifier 40 outputs a signal on line 44 to the microprocessor 12 when it detects an R-wave. This signal is also latched to the microprocessor 12 input port by a conventional latch (not shown). The input of the atrial sense amplifier 38 and the output of the atrial stimulus pulse generator 30 are connected to a first conductor 46, which passes through a conventional first lead 48. Lead 48 is inserted into a patient's heart 50 intravenously or in any other suitable manner. The lead 48 has an electrically conductive pacing/sensing tip 52 or tip and ring at its distal end which is electrically connected to the conductor 46. The pacing/sensing tip 52 is preferably lodged in the right atrium 55. The input of the ventricular sense amplifier 40 and the output of the ventricular stimulus pulse generator 32 are connected to a second conductor 54. The second conductor 54 passes through a conventional second lead 56 which is inserted intravenously or otherwise in the right ventricle 58 of the heart 50. The second lead 56 has an electrically conductive pacing/sensing tip 60 or tip and ring at its distal end. The pacing/sensing tip 60 is electrically connected to the conductor 54. The pacing/sensing tip 60 is preferably lodged on the wall of the right ventricle 58. The conductors 46, 54 conduct the stimulus pulses generated by the atrial and ventricular stimulus pulse generators 30, 32 respectively, to the pacing/sensing tips 52, 60. The pacing/sensing tips 52, 60 and corresponding conductors 46, 54 also conduct cardiac electrical signals sensed in the right atrium and right ventricle to the atrial and ventricular amplifiers, 38, 40 respectively. The sense amplifiers 38, 40 enhance the electrical signals. The amplifiers 38, 40 have an automatic gain control feature, as described in U.S. Patent 4,903,699 to Baker, et al. The implantable cardiac stimulator 10 also has a defibrillator circuit 62. If fibrillation is detected through the atrial or ventricular sense amplifiers 38, 40, a high energy shock can be delivered through defibrillation leads and electrodes 64, 66. Detection algorithms for detection of tachycardias and fibrillation are described in Pless, et al , U.S. Patent 4,880,005. The shock is controlled by a shock driver circuit 68, which will be more particularly described hereafter. All of the aforementioned components are powered by a power supply 70. The power supply 70 may comprise either standard or rechargeable batteries or both, which may be dedicated to the operation of different parts of the stimulator 10. Referring now to Figures 2 and 3, according the present invention, an implantable device is provided with an adjustable VPRP that is a function of the adjusted pacing rate. Thus: VPRP = /^adjusted pacing rate) While it will be understood that the principles described herein are generally applicable to a variety of devices, they are primarily advantageous in rate-responsive devices, i.e. those that vary the pacing rate in response to demand by the body. The V-V interval is measured from the beginning of each R-wave for either a sensed or a paced event and all other periods or intervals discussed hereinafter are calculated and expressed in terms of the same time interval, namely the interval beginning with a ventricular pace or sense event and ending at the next ventricular pace or sense event. When the pacemaker is pacing the heart, this interval will have a duration of (pacing rate) "1 . It will be understood that, when the pacing rate is given in ppm, the interval as referred to herein is calculated as 60,000 x (pacing rate) "1 , which gives an interval having units of msec. Within the span of the VPRP, the present invention includes an adjustable T-wave monitoring 5 window that begins at t s and ends at t e , with t s and t e being defined as functions of the adjusted pacing rate: t s = / 2 (adjusted pacing rate) t e = / 3 (adjusted pacing rate) and the duration of the T-wave monitoring window being equal to t e - t s . 10 According to one preferred embodiment, the adjusted VPRP for a given pacing rate, VPRP cum;nt , is calculated as follows: f VPRP - VPRP 1 V v P r R r *-P r current = V v P r RP r base -~ ( V7 1 base - T ^current ^/ I __£_„_-?_? T.--. ~ „T„„l- m -.!- I I ι c L A base ~ A max J where: I base is the interval of the baseline (lowest) bradycardia pacing rate, I max is the interval of the maximum pacing rate, 0 I current is th e interval of the current (adjusted) pacing rate, VPRP base is the programmed VPRP for the baseline (lowest) bradycardia pacing rate, and VPRP max is the programmed VPRP for the maximum pacing rate. In a pacer that has been programmed for a particular patient, I base , L^ , VPRP base , and VPRP raax will be programmed constants, while c j mm will vary with demand. Thus, in this 25 embodiment, VPRP current is linearly proportional to the difference between the baseline pacing interval and the current pacing interval. According to this embodiment, the physician initially programs the device with set values for the baseline (lowest) bradycardia pacing rate (I base ) " ', the maximum pacing rate (I max )' , the VPRP base corresponding to the baseline pacing rate, and the VPRP max corresponding to the maximum 30 pacing rate. From these four fixed values, plus the input from the activity /demand sensor(s), all of the remaining variables, I current , VPRP current , t e and t. can be calculated on a continuous basis by the device. According to one preferred embodiment, t s is programmed to be a constant, rather than a function of the pacing rate, such as for example 130 msec, and t e is programmed to be equal to 35 VPRP current minus a constant, such as for example: t e = VPRP current - 25.6 msec. Figure 3 shows the operation of a device in accordance with this embodiment. Line 10 indicates the V-V interval, or base pacing rate, on which the other intervals described herein are based. As stated above, the V-V interval is dependent on the physiological demands of the patient and is calculated in accordance with known rate calculation techniques. Line 12 indicates the duration of the VPRP, line 14 indicates the end of the T-wave monitoring window t e and line 16 indicates the start of the T-wave monitoring window t s . Line 16 is a constant in the illustrated embodiment, having the value assigned above. The values of each of the variables I current , VPRP current and t e can be found for a given pacing rate by taking the value of each line 10, 12 and 14 at the point where it intersects a vertical line drawn through that pacing rate. In Figure 3, the programmed values discussed above were assigned the following values: baseline bradycardia pacing rate = 45 ppm, maximum pacing rate = 180 ppm, r base = 665 msec and r,^ = 200 msec. This fixes the constant multiplier in Equation (1) at approximately one-half for this example. While the foregoing embodiment defines the VPRP current as proportional to the difference between the maximum and adjusted pacing intervals, it will be understood that the function relating VPRP current to the V-V interval need not be linear, and can be any other type of function that yields a VPRP current that is less than the corresponding V-V interval. Similarly, while the end of the T-wave monitoring window t e is described above as VPRP current minus a constant, it could alternatively be defined as any other linear or non-linear function of VPRP current or the adjusted pacing rate that yields a t e that is less than VPRP currem . Similarly, while the start of the T-wave monitoring window s t is described above as a constant time value, it could alternatively be defined as any linear or non-linear function of VPRP current or the adjusted pacing rate, so long as t. remains less than t . Examples of instances in which it may be desired to use a non-constant t s include occurrences of problematic post- pace lead polarization. Referring now to Figure 4, a program 100 implementing the concepts described above illustrates how the calculated start and end times for the dynamically adjustable T-wave window are used in the microprocessor 12. This adjustable T-wave window helps ensure that the T-wave window is timed so as to maximize the probability that the T-wave will be detected, regardless of the current pacing rate. The program 100 first detects 102 a time to pace the ventricle. If such a time has occured, a command to pace the ventricle is issued 104. The microprocessor 12 then calculates the adjusted pacing rate 106. The start of a T-wave window, as described above, is calculated 108, as is the end of the T-wave window 110. The program 100 then calculates the end of the VPRP 112, and tests 114 to determine if the start of the T-wave window has been reached. If the start of the window ahs been reached, the device tests 116 for sensing of the T-wave. If the end of the T-wave window is reached 118 without sensing the T-wave, the device determines 122 if the end of the VPRP has been reached. At the end of the VPRP, control returns to other conventional processing until a time to pace is again detected 102. On the other hand, if a T-wave is sensed 116 during the T-wave window, the program 100 registers 120 the detection and adjusts rate and other parameters as described above. The program 100 detects the end of the VPRP 122, and returns to other conventional processing until a time to pace is again detected 102.

Claims: WHAT IS CLAIMED IS: 1. An implantable device (10) for providing stimuli to a heart, comprising: a power source (70); at least one electrode (60) electrically connected to said power source; a sensor (40) capable of providing data that can be used to determine a physiological pacing demand; and a control device (12) , said control device controlling the application of electrical stimuli through said electrode, said control device calculating a physiological pacing demand based on said data, and calculating an adjusted pacing interval based on said demand, characterized by means (112) for calculating an adjusted ventricular pace refractory period (VPRP current ) that is a function of said pacing interval, and means (110) for calculating the end time of a T-wave monitoring window, said end time being a function of said pacing interval and being less than said V v -P r R x P r current* 2. The implantable device according to claim 1 wherein said control device also comprises means (108) for calculating the start time of a T-wave monitoring window. 3. The implantable device according to claim 2 wherein said means (108) for calculating said start time has means for deriving said start time as a function of said pacing interval and for limiting said start time to a time less than said end time. 4. The implantable device according to claim 2 wherein said start time is a constant. 5. The implantable device according to claim 1 further comprising means (112) for adjusting said VPRP current to be proportional to the difference between a baseline pacing interval and the adjusted pacing interval. 6. The implantable device according to claim 1 , further including a detector (116) that detects T-waves that occur before said end time. 7. The implantable device according to claim 6, further including an amplifier (40) with an adjustable gain, wherein said gain is adjusted in response to changes in the amplitude of signals detected by said detector. 8. The implantable device according to claim 1 wherein said means (110) for calculating end time uses a function of said VPRP current . 9. The implantable device according to any of the foregoing claims wherein said implantable device further comprises a pacing pulse generator (32). 10. The implantable device according to any of the foregoing claims wherein said implantable device further comprises a defibrillation shock generator (62).

Inventors: ARMSTRONG RANDOLPH K, COOK DOUGLAS J, VANDEGRIFF JOSEPH W, ARMSTRONG, RANDOLPH, K., COOK, DOUGLAS, J., VANDEGRIFF, JOSEPH, W.
Applicants: SULZER INTERMEDICS INC, SULZER INTERMEDICS INC.
Application year: 1998
Main classification: A61N1
IPC codes: A61N1