A CARDIAC PACER FOR PACING A HEART

Abstract: A cardiac pacer, which generates pacing pulses (15) at a predetermined pacing rate comprises a device (7) for realizing a body activity measurement. The measured body activity signal is non-linearly amplified in a manner that signal portions having higher amplitudes are more amplified than signal portions having lower amplitudes. Furthermore, the non-linearly amplified body activity signal is integrated over a period of time. Finally the predetermined pacing rate is varied dependent on the integrated signal.

Description: Field of the invention This invention relates to a cardiac pacer for pacing a heart with a pacing rate dependent on body activity, which pacer comprises means for generating pacing pulses at a predetermined pacing rate, means for transmitting the pacing pulses to the heart, means for realizing a body activity measurement, means for obtaining a body activity signal depending on the body activity measurement, said signal varying according to the body activity level and means for varying the predetermined pacing rate dependent on the body activity signal. The present application is related to European patent applications 0 249 818, 0 249 819 and 0 249 821 all with the same priority date. Description of the prior art Such a cardiac pacer is described in the European patent application EP-A 0 089 014, where the signal received from an impedance measurement is compared with two threshold levels. Signal portions which lie outside the width of the two threshold levels are discarded from further signal processing since they are considered impedance variations which do not have a certain consistency and therefore cannot attribute to a possible evolution of a respiratory act. Only those portions which lie inside the width of the two threshold levels are further utilized for evaluation. SUMMARY OF THE INVENTION Objects It is an object of this invention to provide for an improved cardiac pacer, which allows for a simpler signal processing procedure for obtaining a body activity signal for controlling the pacer rate. Summary To achieve this object, the above defined pacer inventively further comprises means for non-linearly amplifying the body activity signal in a manner that signal portions having higher amplitudes are more amplified than signal portions having lower amplitudes and means for integrating the non-linearly amplified body activity signal over a period of time, the integrated signal being used to vary the pacing rate. By non-linearly amplifying and integrating the body activity signal, low amplitude and/or high-frequency noise are automatically eliminated. A threshold evaluation is no longer necessary. In a preferred embodiment, the means for non-linearly amplifying the body activity signal comprising means for squaring the body activity signal. In another preferred embodiment, the body activity signal is a respiratory signal. It can, however, also be a signal obtained from a body accelerometer, core temperature sensor, pH sensor or pO₂ sensor. The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings. Brief Description of the Drawings Fig. 1 a first embodiment of a cardiac pacer comprising the invention in a schematic block diagram; Fig. 2 a second embodiment of a cardiac pacer comprising the invention in a schematic block diagram; Fig. 3 an energy measuring device including the invention of an impedance pneumograph of the embodiments of Figures 1 and 2 in more detail; and Fig. 4 the time base unit of the embodiment of Figures 1 and 2 in more detail. Description of the Preferred Embodiment In Figures 1 and 2 a human heart which has to be paced is generally designated with 1. A pacing electrode 2 is inserted in the human heart 1 in a manner and position that the heart can most efficiently be paced. The pacing electrode 2 is connected through a pacing lead 3 with a pacing pulse generator 4. A time base unit 5 controls the pacing rate of the pacing pulse generator 4 through line 6. According to Fig. 1 an impedance pneumograph 7 comprises an AC source 8 for generating a continuous alternating current, a demodulator 9, an energy measuring device 10 comprising the invention and a voltage to pulse rate converter 11. The AC source 8 is connected with the pacing lead 3 by means of lines 12, 13. The demodulator 9 is connected with the pacing lead 3 by means of lines 13, 14. Under the circumstances the current of the AC source 8 is supplied to the pacing electrode 2 together with the pacing pulses 15. Nevertheless this embodiment can be modified in a manner that instead of utilizing the pacing electrode 2 a separate electrode is provided for impedance measurement. In this case the AC source 8 and the demodulator 9 would have to be disconnected from pacing lead 3 and instead be connected through additional lead 16 with the separate impedance measuring electrode 17 as indicated in Fig. 1 by broken lines. In both cases the output signal (body activity signal) of the demodulator 9 is a measure of breathing rate and tidal volume. In Fig. 1 the pacing pulse generator 4, the time base unit 5 and the impedance pneumograph 7 are all encapsuled in an implantable conductive (metallic) housing 18 which is the housing of the cardiac pacer. The metallic housing 18 defines both the indifferent electrode for pacing and the second electrode for impedance measurement as indicated in Fig. 1 with reference numeral 19. In Fig. 2 an impedance pneumograph 20 includes a sample and hold circuitry 21 and a difference former 22 in addition to an energy measuring device 10 comprising this invention and a voltage to pulse rate converter 11. The sample and hold circuitry 21 of the impedance pneumograph 20 is connected with the pacing lead 3 of the pacing pulse generator 4 through line 23 for processing the pacing pulses 15 in a manner that a respiratory signal is obtained from the pacing pulses 15 when evaluating the amplitude decays D of pacing pulses 15. The amplitude decay D changes according to alternating body impedance during respiration. The measurement may also be done by a division of voltage and current. Again the pacing pulse generator 4, the time base unit 5 and the impedance pneumograph 20 are all encapsuled in an implantable conductive (metallic) housing 18 which is the housing of the cardiac pacer. The housing 13 again defines both the indifferent electrode for pacing and the second electrode for impedance measurement as indicated in Fig. 2 with reference numeral 19. As illustrated in Figures 1 and 2 the pacing pulse generator 4 comprises an output capacitor 24 which is switchble by means of switch 25 between battery 26 (switch position A) and pacing lead 3 (switch position B). In switch position A the output capacitor 24 is charged by the battery 26 to a voltage V1. In switch position B the output capacitor 24 is discharged through pacing lead 3 as pacing pulse 15. In the embodiment of Fig. 2 the amount of discharge depends on the impedance variations of the patient's thorax during respiration. According to Fig. 2 the pacing pulse 15 discharges from V1 to V2 (amplitude decay D). The sample and hold circuitry 21 samples and holds the voltages V1, V2 of output capacitor 24. The difference former 22 forms the difference V1-V2 which is again a measure for the breathing rate, i.e. a respiratory signal. In both embodiments each respiratory signal is supplied to the energy measuring device 10, which according to Fig. 3 comprises a filter 28, a non-linear amplification (e.g. squaring) circuitry 29 and an integrator 30. The non-linear amplification circuitry 29 amplifies the filtered respiratory signal in a manner that signal portions having higher amplitudes are more amplified than signal portions having lower amplitudes. Under the circumstances interesting signal portions including the respiration signal are enhanced with respect to low amplitude noise for further processing. Non-linear amplification circuitries of this kind are well known in the art. They do not need described in more detail. The output signal of the non-linear amplification circuitry 29 is integrated in integrator 30 over a period of time, e.g. in the range of 5 to 30 s. By integrating, high-frequency noise is significantly reduced. The voltage to pulse rate converter 11 in Figures 1 and 2 converts the integrated signal into a pulse rate according to the breathing rate. The voltage to pulse rate converter 11 controls the time base unit 5 through line 31 in a manner that a predetermined (e.g. programmable) basic pacing rate of the pacing pulse generator 4 is varied dependent on the respiratory signal. In Fig. 2 the line 32 is a control line from the time base unit 5 to the sample and hold circuitry 21 of impedance pneumograph 20. According to Fig. 4 the time base unit 5 comprises a zero decoder 33, a down counter 34, a time base register 35 and an analog signal to digital control word converter 36. The down counter 34 includes a reset input 37. The analog to digital control word converter 36 converts the analog pulse rate signal of the voltage to pulse rate converter 11 into a digital control word. This digital control word is supplied to the time base register 35. It controls the time base register 35 in a manner that a basic pacing rate, e.g. 60 beats/min., is varied dependent on the respiration rate. When the breathing rate increases the time base register 35 increases the counting speed of down counter 34 so that it reaches zero faster than at basic rate. Under the circumstances the zero decoder 33 generates switching signals at higher rates, so that the output capacitor 24 of the pacing pulse generator 4 charges and discharges at higher rates. As a result the pacing rate increases dependent on increasing breathing rate as desired. The impedance measurement electrodes do not need to be implanted. They can also be secured on the patient's chest, if desired. Such a possibility is for example illustrated in US patent 3,593,718.

Claims: A cardiac pacer for pacing a heart (1) with a pacing rate dependent on body activity, which pacer comprises: means (4) for generating pacing pulses (15) at a predetermined pacing rate, means (2, 3) for transmitting the pacing pulses (15) to the heart (1), means (2, 8, 19; 17, 8, 19; 2, 4, 15, 19) for realizing a body activity measurement, means (9, 21, 22) for obtaining a body activity signal depending on the body activity measurement, said signal varying according to the body activity level, and means (5) for varying the predetermined pacing rate dependent on the body acitivity signal, characterized by means (29) for non-linearly amplifying the body activity signal in a manner that signal portions having higher amplitudes are more amplified than signal portions having lower amplitudes and means (30) for integrating the non-linearly amplified body acitivity signal over a period of time, the integrated signal being used to vary the pacing rate. Cardiac pacer according to claim 1, wherein said means (29) for non-linearly amplifying the body activity signal comprises means for squaring the body activity signal. Cardiac pacer according to claim 1 or 2, wherein said integrating means (30) integrate said body activity signal over a time period in the range of 5 - 30 seconds. Cardiac pacer according to one of the preceding claims 1 - 3, wherein said body activity signal is a respiratory signal. Cardiac pacer according to claim 4, wherein said means (2, 8, 19; 17, 8, 19) for realizing a body activity measurement comprises means for realizing a body impedance measurement. Cardiac pacer according to claim 4 or 5, further comprising a sample and hold circuitry (21) for measuring the amplitude (V1) of the pacing pulse (15) at the beginning and the amplitude (V2) of the same pacing pulse (15) at the end of the pacing pulse (15) and a difference former (22) for obtaining a measure for the decay (D) between the two amplitudes (V1, V2), which is a measure for the respiration. Cardiac pacer according to one of the preceding claims, wherein said means (5,11) for varying the pacing rate comprises a voltage to pulse rate converter (11) for the integrated signal. Cardiac pacer according to claim 7, wherein said means (5,11) for varying the predetermined pacing rate comprises an analog signal to digitial control word converter (36) for the output signal of the voltage to pulse rate converter (11), a time base register (35) for the output signal of the analog signal to digital control word converter (36), a down counter (34), which is set to higher zero counting speed when the respiration rate increases and a zero decoder (33) at the output of the down counter (34), said zero decoder (33) being connected with and controlling the pacing pulse generator (4) so that a pacing pulse is generated at each zero count.

Inventors: AMUNDSON, DAVID C., LEKHOLM, ANDERS
Applicants: SIEMENS AKTIENGESELLSCHAFT
Application year: 1987
Main classification: A61N1
IPC codes: A61N1