Control of an active implantable medical device
The invention concerns a control process of an implantable active medical device, more particularly for a device of the type that is functionally responsive to a sensed physiological parameter such as minute ventilation. 
     FIELD OF THE INVENTION 
     The family of implantable active medical devices includes, for example, and without limitation, cardiac pacemakers and cardiac defibrillators. 
     An implantable active medical device can be perturbed by the presence of electromagnetic waves emanating from various sources, such as, for example, radio-telephones, anti-theft devices or televisions. In the case of a rate responsive pacemaker, in which the delivered stimulation frequency is functionally related to a measure of a physiological parameter, the perturbing electromagnetic waves can be perceived by the physiological parameter sensor as variations of the physiological parameter. As a result, the stimulation frequency may be accelerated without valid reason. This is a particular problem for physiological sensors that depend on processing sensed electrical signals, such as occur in devices that sense minute ventilation, respiration rate, QT interval, etc. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an apparatus and a process of controlling an implantable active medical device that makes a count of noise events occurring due to parasitic electromagnetic signals, so as to allow the implantable active medical device to decide whether to operate in accordance with a defined function of the sensed physiological parameter, or to suspend operation according to the function when the noise exceeds a preselected (or programmable) threshold. 
     Broadly, the invention concerns a control process for an implantable active medical device that has an operating mode responsive to a sensed physiological parameter, which device is of the type including an electronic circuit that senses a physiological parameter and provides an output signal representative of the magnitude of the sensed physiological parameter, and a control device that controls whether or not the implantable active device operates as a function of (i.e., in response to) the output signal. One such process includes: 
     a) measuring the magnitude of the output signal to determine a value of the noise; 
     b) comparing the value of the noise to a threshold; and, 
     c) when the value of the noise exceeds the threshold, indicating an unacceptable level of noise exists, taking this threshold crossing into account by the control device, in view of a possible suspension of operation as a function of the output signal. 
     In one embodiment of the present invention, the implantable active medical device includes cardiac stimulation, and the function of the output signal for the implantable device is the adjustment of a cardiac stimulation frequency as a function of the physiological parameter. One example of a suitable physiological parameter is the minute ventilation, in which case the representative magnitude is the voltage sensed between a sensing electrode and the case (reference electrode) of the medical device, e.g., a pacemaker. More particularly, the voltage is preferably first measured during an injection of current to determine the dynamic pulmonary impedance, and then measured in the absence of current injection to determine the noise. The voltage corresponding to the noise is then compared to a threshold value. When the voltage corresponding to the noise exceeds the threshold value, this crossing is taken in account by the control device, in view of a possible suspension of the rate responsive pacing function. 
     In one embodiment, if the noise value is considered too high or if too much noise at an unacceptable level has occurred, e.g., during a defined time period, then the acquired pulmonary impedance last acquired (or acquired during the time interval) may be disregarded as unreliable. In such case, the stimulation frequency is provided according to a different function which is not affected by such noise. 
     The dynamic pulmonary impedance is defined by difference between the measured pulmonary impedance and a reference or &#34;static&#34; pulmonary impedance, which is obtained by integrating preceding measures of the dynamic pulmonary impedance. Methods and circuits for calculating minute ventilation and selecting a pacing frequency as a function of sensed minute ventilation are known. See, e.g., U.S. Pat. Nos. 5,249,572 and 5,303,702, which are commonly assigned and incorporated herein by reference in their entireties. See also, e.g., U.S. Pat. Nos. 4,702,253, 4,596,251, and 3,593,718. 
     In one implementation of the invention, the voltage can be measured during an injection of current to determine the dynamic pulmonary impedance, and the voltage can be measured outside of a current injection to determine the noise, using the same electronic sensing circuit and electrodes. 
     The step of taking into account a crossing of the threshold value preferably is implemented by incrementing a counter. In this case, the counter and the control device cooperate to suspend operation according to the function when the counter reaches a predetermined total. The predetermined total may be an absolute value, or it may be a total for a given period of time. In one embodiment, the counter is reset to zero if no noise (or an insufficient number of noise events) is encountered in a given time period or if the count does not exceed the threshold during that time period. Alternatively, the counter may be decremented, rather than reset. 
     The invention also is directed to an implantable active medical device of the type comprising an electronic circuit for sensing a physiological parameter and producing an output signal representative of the magnitude of the sensed physiological parameter, and a device to insure a function of the implantable active medical device, wherein the electronic circuit comprises a feedback amplifier system comprising a digital-to-analog converter which returns to the input of the feedback amplifier a reference signal corresponding to an average of the preceding measures of the amplifier output. Preferably, the physiological parameter sensed is the minute ventilation, and the function is the provision of the cardiac stimulation frequency as a function of the measurement of minute ventilation. More specifically, the function is responsive to a dynamic pulmonary impedance and the analog signal feedback signal is the static pulmonary impedance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention will become more apparent from the accompanying drawings and the following detailed description of the invention, in which like reference numbers refer to like elements, and in which: 
     FIG. 1 is a schematic diagram of an electronic circuit for measuring the dynamic pulmonary impedance in accordance with a preferred embodiment of the invention, in a cardiac pacemaker in which the stimulation frequency is controlled responsive to a physiological parameter that, in this example, is the minute ventilation; 
     FIG. 2 is a schematic diagram of an electronic circuit for measuring the noise according to a preferred embodiment of the invention; and 
     FIG. 3 a schematic diagram of an electronic circuit for measuring the noise according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the implantable active medical device is illustrated as an implanted cardiac pacemaker including a case (reference electrode) 1, a current injection electrode 2, for example, at an endocardial lead tip electrode, and the sensing electrode 3, for example, an endocardial lead ring electrode. The injection of current is insured at electrode 2. The pulmonary impedance Z also is symbolized as impedance 4 between the sensing electrode 3 and the case 1. The voltage V is measured between the case 1 and the sensing electrode 3, and allows determination of the value Z of the pulmonary impedance 4 variations, thus translating the respiratory volume into sensed variations of impedance. 
     The electronic circuit measuring the pulmonary impedance includes a high-pass filter 5, an amplifier 6, an amplifier 7 with a feedback input comprising an integrator 8 placed between an analog-to-digital converter (&#34;ADC&#34;) 9 and a digital-to-analog converter (&#34;DAC&#34;) 10, and a low-pass filter 11, whose output signal (a digital code) is read by a microprocessor 14. Such sensing circuits are commercially available in the model CHORUS RM 7034 rate-responsive cardiac stimulator, which is available from ELA Medical S.A., Montrouge, France. 
     The functioning of the circuit is as follows. A current is injected at injection electrode 2. The voltage which is linked to the pulmonary impedance Z is measured between the other (sensing) electrode 3 and the case 1. The current injection and voltage sensing electrodes 2 and 3 can be located in the atrium or in the ventricle. 
     The sensed voltage is high-passed filtered at filter 5 and amplified by amplifier 6. The obtained voltage is then decreased by a reference voltage corresponding to the static pulmonary impedance, determined by a numerical integration of obtained voltage samples over the preceding voltage sampling cycles. More specifically, the sampled voltage and the static voltage inputs are differentially amplified by amplifier 7. The consequent voltage at the output of the amplifier 7 corresponds then to the dynamic pulmonary impedance. The output of amplifier 7 is digitally sampled by ADC 9. An integration by integrator 8 of the voltage in the digital form provided by ADC 9 allows the determination of the static impedance also in the form of the digital code output by integrator 8. The DAC 10 converts this digital code to an analog signal corresponding to the static impedance determined by integrator 8, which analog signal is input to the reference input of amplifier 7. The integrator 8 is preferably of the inverter type, which suppresses the input signal low frequency component (F≦0.05 Hz) and presents, according to the amplification of the signal, time-constants of increasing rapid values so as to allow the circuit to follow most closely near to the signal. This is particularly necessary in case of important (large) variations in the static impedance. 
     After the analog-to-digital conversion by ADC 9, the digital voltage also is filtered by low-pass filter 11, and the consequent digital code is read by the microprocessor 14. Microprocessor 14 implements the function responsive to the physiological parameter, namely an algorithm that selects the stimulation frequency as a function of the read digital code (the sensed physiological parameter) or an average of recent read digital codes. Thus, at each injection of current the microprocessor 14 executes a complete measuring cycle with injection enabling the &#34;enslavement&#34; function, i.e., sensing the physiological parameter minute ventilation and adjusting a stimulation frequency as a function of the sensed minute ventilation. 
     In accordance with the present invention, a noise measure also is obtained. In this regard, outside of a current injection, the voltage measured between the sensing electrode 2 and the case 1 corresponds to the noise sensed at electrode 2. This noise perturbs, i.e., interferes with, the pulmonary impedance during a current injection, and hence perturbs the enslavement of the pacemaker to minute ventilation, in this example. 
     Referring now to FIG. 2, according to the present invention, a measure of the noise is thus undertaken by using the electronic circuit for measuring the pulmonary impedance (FIG. 1), as in the condition of measuring of the pulmonary impedance, but without any injection of current. The signal processed by the circuit illustrated in FIG. 2 is the signal present on the electrode 3 during a sampling without current injection. A cycle for measuring the noise comprises the same operations as a cycle for measuring the pulmonary impedance, except for the lack of any current injection. It comprises notably the determination of a static impedance due to the noise and a dynamic impedance due to the noise. For each measuring cycle, the digital voltage corresponding to the dynamic impedance due to the noise is compared to a threshold corresponding to an unacceptable level of noise, which is preferably programmable. The threshold can be positive or negative, and there can be a positive threshold and a negative threshold. Each crossing of the threshold causes, for example, the increment of a counter. In a preferred embodiment, the counter is maintained in software or a memory device, and is read and reset to zero periodically by the microprocessor 14. When the counter reaches a predetermined value, the microprocessor 14 decides to suspend the enslavement (i.e., the stimulation frequency is not further determined as a function of the sensed physiological parameter). 
     The advantages of the invention include the following. First, the same analog circuit elements can be used to realize all steps of sensing the physiological parameter used in the function, and all steps of measuring the noise, which two sets of steps may be offset in time. Stated otherwise, the circuit for measuring the physiological parameter at one time also can be used for sensing noise at a different time. This allows the implementation of noise detection without the need to integrate new circuit components. This saves space and cost, and reduces battery consumption, which are important for implanted medical devices. Further, measuring noise during a sampling of the same type as the sampling during the current injection allows for the detection of parasitic noise even of weak values due to electromagnetic interference. 
     In addition, the frequency of the cycles for measuring noise can be modified. For example, the frequency can be set to be the same as the frequency of current injection or to another frequency (faster or slower). Similarly, the time between the injection and the sampling of the noise measurement can be constant or can vary. If the measurement of noise takes place at each cycle of injection, and with a delay that is fixed relative to the injection, the beating due to a periodic parasitic signal will be seen in the same manner during the measurement of pulmonary impedance and during the measurement of noise. The parasitic noise perturbing the measure of pulmonary impedance will therefore be well detected. 
     In one embodiment, the taking of the noise measurement is temporarily halted when the physiological parameter sensor detects a level of patient activity that corresponds to a resting state. For example, when a patient is asleep, the activity level decreases and remains decreased until the patient wakes up. During this time, there is little need to measure noise, and consequently, the measurement can be halted. This has the advantage of reducing the drain on the battery during substantial periods of time when the patient is at rest, and noise is not likely to be a problem with pacing activity. The noise measurement is then resumed when the sensed patient activity level rises above the level corresponding to rest. 
     In one implementation, the system provides a threshold that is used to control the taking of the noise measurement. The threshold is selected to correspond to a sensed signal representative of an activity level where the patient is no longer at rest. For a description of how such an activity threshold may be set and adjusted over time to provide a threshold suitable for identifying an activity level distinguishing rest from a more active state, reference is made to U.S. Pat. No. 5,303,702, and in particular the discussion therein of the threshold VE LOW  and its adjustment. The disclosure of U.S. Pat. No. 5,303,702 is hereby incorporated herein by reference in its entirety. 
     Another advantage is the fact that the thresholds for comparing the sensed voltage during the measure of noise are programmable. This renders the system flexible, and the criterion of detection is easily modifiable. Similarly, the reading of the counter count by the microprocessor can be set to an adjustable frequency (time interval). 
     The ability to vary parameters such as the comparison thresholds, the frequency of measurement, the frequency of reading of the number of passings, allows to adapt the sensing system to disruptive noises. 
     The principle of detection of noise due to electromagnetic interference can be generalized, such that the above-mentioned embodiment represents only one of the possibilities. The voltage obtained at the output of the sensing circuit during the measurement of noise can be processed (for example, by averaging) before the comparison to the threshold is made. The processing circuit of the sensed voltage can be of the type represented on FIG. 3. The voltage amplified by amplifier 6 is digitized by ADC 12, filtered by a filter 13 and the corresponding digital code is read by the microprocessor 14. 
     The intervention (i.e., the sequence of operation) of the microprocessor can be modified. Thresholds of detection of the noise can be hardwired (fixed) or programmable. 
     The processing of the voltage obtained at the circuit output and the suspension of the enslavement (function) in case of perturbation can be realized by the microprocessor, by the hardware, or by a combination of the two. The length of the suspension can be determined by the absence of sensed noise for a set time period (which assumes that the system continues to measure the dynamic pulmonary impedance but does not use perturbed signals) or for a set time period. 
     The example described concerns a rate responsive cardiac pacemaker enslaved (i.e., responsive) to the determined minute ventilation. It should be understood that without departing from the scope of the present invention, the principle can apply equally to other types of enslavements (functional relationships) where the representative magnitude of the sensed physiological parameter (i.e., a useful signal) is susceptible to be perturbed by electromagnetic interferences where the noise perturbations can be sensed separate from the useful signal. Thus, it can be determined when the noise level is too high that the sensed useful signal is deemed too unreliable to use, so that the function of the device during such noise is suspended. It is the case, for example, the enslavement to parameters such as the ejection of blood volume, the temperature, the concentration of a gas (e.g., oxygen saturation) in the blood, the QT interval, etc. which parameters are well known in the art. It should be understood that during the time of suspension of the function, some alternate function that is not susceptible to the noise interference may be provided. 
     Furthermore, the representative magnitude of the physiological parameter can fulfill other functions than the enslavement of a stimulation frequency, as, for example, the confirmation of the presence of a rhythmic disturbance in an implantable defibrillator or cardioverter. 
     One skilled in the art will appreciate that the present invention can be practiced by other than the foregoing embodiments, which are presented for purposes of illustration and not of limitation.