Cardiac pacer for pacing a human heart

Abstract: A cardiac pacer which generates pacing pulses at a predetermined pacing rate, comprises a device (8) for generating an alternating signal which is unable to pace the heart (1). The pacing pulses and the alternating signal are transmitted to the heart. The alternating signal after transmittal to the heart is measured and processed (9) in a manner that a respiratory signal is obtained. The predetermined pacing rate is then varied dependent on the respiratory signal.

Description: This invention relates to a cardiac pacer for pacing a human heart, wherein the pacing rate is controlled by the respiration of the patient. The present application is related to the European Patent Applications 0 249 818, 0 249 820 and 0 249 821 all having the same priority date as the present application. Description of the Prior Art Conventional cardiac pacers usually comprise a pacing electrode and an indifferent electrode. The pacing electrode is placed in the human heart by means of a pacing lead. The metallic housing of the cardiac defines the indifferent electrode. The US Patent 3,593,718 describes such a conventional cardiac pacer which in addition utilizes an impedance pneumograph for obtaining a respiratory signal from thoracic impedance variations. The impedance penumograph comprises a first and a second impedance measuring electrode each connected with a separate first and second lead, respectively. Both impedance measuring electrodes of the impedance pneumograph are placed on the surface of the patient's chest. The European Patent Application 0 089 014 depicts a conventional cardiac pacer which also employs an impedance pneumograph for obtaining a respiratory signal. Again, the impedance pneumograph comprises a first and a second impedance measuring electrode. However, only the first impedance measuring electrode is connected to a lead while the second electrode is defined by the conductive (metallic) housing which contains the cardiac pacer and the impedance pneumograph. Furthermore, the first impedance measuring electrode and, since the conductive (metallic) housing is implanted, also the second electrode are subcutaneously placed in the thorax. Under the circumstances, the complete pacing electrode on a first lead, a first impedance measuring electrode on a second lead and the metallic housing as both the indifferent electrode of the cardiac pacer and the second electrode of the impedance pneumograph. In the European Patent Application 0 135 911, an improvement of the cardiac pacer according to the 0 089 014 application is described, whereby an alternating signal is generated between the first impedance measuring electrode and the metallic housing of the pacer. After transmittal between the impedance measuring electrodes, the alternating signal is measured and processed for obtaining a respiratory signal, which is used to control the pacing rate. Further, the European Patent Application 0 218 007, now patent, filed prior to the priority date of this application and published after that date is comprised in the state of the art. The 0 218 007 document depicts a cardiac pacer with means for generating pacing pulses and transmitting them to the heart. By measuring the pulse voltage and the pulse current and dividing them, an electrode impedance signal is obtained, the respiration-dependent signal component of which is used to control the pacing rate. US-A-4 303 075 describes an apparatus for maximizing stroke volume. The apparatus has an atrial and a ventricular pacing electrode. Furthermore there are impedance measuring electrodes. The impedance measuring electrodes are connected to an impedance processor. The impedance processor produces a signal from which information regarding the stroke volume is obtained. The impedance measuring electrodes may be of the type disclosed in US-A-4 030 509. This document discloses defibrillation electrodes which are placed around the heart. The defibrillation electrodes may be provided with separate insulated pacing electrodes. The leads of the pacing electrodes are insulated from the leads of the defibrillation electrodes. Monitoring equipment may be connected to the pacing electrodes. EP-A-0 151 689 describes a cardiac pacer which obtains a respiratory signal from a pulsed signal transmitted between two subcutaneously positioned electrodes. The electrodes may be positioned on leads and/or on the casing of the pacemaker. The respiratory signal is used to control the pacing rate. There is also a heart stimulation lead. Object It is an object of this invention to provide a cardiac pacer for pacing a human heart, wherein the pacing rate is controlled by the respiration rate of the patient and wherein the number of necessary electrodes and therefore also the number of necessary leads for obtaining a respiratory signal from thoracic impedance variations is reduced to a minimum. Summary According to this invention, an improved cardiac pacer in accordance with claim 1 for pacing a human heart is provided. In a preferred embodiment of the invention said alternating signal generating means are designated for generating an alternating current having an amplitude, said amplitude being dependent on the frequency of the alternating signal and lying substantially below the fibrillation threshold of the heart (e. g. < 10 µA, preferably at 2 µA). The frequency lies above 150 Hz, e. g. in the range of 1 kHz to 10 kHz, preferably at 4 kHz. 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 shows the invention in a schematic block diagram; and Fig. 2 illustrates the invention in a more detailed block diagram. Description of the Preferred Embodiment In Fig. 1 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. An impedance pneumograph 7 comprises an AC source 8 generating a continuous alternating current, and a device 9 for measuring and evaluating a respiratory signal from the continuous alternating current after transmittal to the heart. The AC source 8 is connected with the pacing lead 3 through leads 10, 11. The device 9 is connected with the pacing lead 3 through leads 11, 12. Under the circumstances the current of the AC source 8 is supplied to the pacing electrode 2 together with the pacing pulses 13 according to this invention. The impedance pneumograph 7 controls the time base unit 5 through line 14 in a manner that a predetermined basic pacing rate of the pacing pulse generator 4 is varied dependent on the respiratory signal. 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 15 which is the housing of the cardiac pacer according this invention. The conductive (metallic) housing 15 defines both the indifferent electrode for pacing and the second electrode for impedance measurement as indicated in Fig. 1 with reference numeral 16. Under the circumstances the cardiac pacer according to this invention operates with only two electrodes and one lead, namely electrodes 2 and 16 and lead 3 for both pacing and impedance measurement. Fig. 2 depicts the schematic block diagram of Fig. 1 in more detail. The complete impedance including the heart is generally designated with Z. The pacing pulse generator 4 comprises an output capacitor 17 which is switchable by means of switch 18 between battery 19 (switch position A) and pacing lead 3 (switch position B). In switch position A the output capacitor 17 is charged by the battery 19 to a voltage V1. In switch position B the output capacitor 17 is discharged through pacing lead 3 as pacing pulse 13. The device 9 comprises a demodulator 20 for the alternating current picked up from the patient, a filter 21, a non-linear amplification (e.g. squaring) circuitry 22, an integrator 23 and a voltage to pulse rate converter 24. The non-linear amplification circuitry 22 amplifies the output signal of filter 21 such that that signal portions having higher amplitudes are more amplified than signal portions having lower amplitudes. Under the circumstances signal portions of interest including the respiration signal are enhanced with respect to low amplitude noise for further processing. Non-linear amplification circuits of this kind are well known in the art and need not be described in more detail. The output signal of the non-linear amplification circuitry 22 is integrated in integrator 23 over a period of time, e. g. in the range of 5 to 30s. By integrating, high-frequency noise is significantly reduced. The voltage to pulse rate converter 24 converts the integrated signal into a pulse rate according to the breathing rate. The time base unit 5 comprises a zero decoder 25, a pulse rate down counter 26, a time base register 27 and an analog signal to digital control word converter 28. The latter one converts the pulse rate signal of the voltage to pulse rate converter 24 into a digital control word. This digital control word is supplied to the time base register 27. It controls the time base register 27 such 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 27 increases the counting speed of down counter 26 so that it reaches zero faster than at the basic rate. Under these conditions the zero decoder 25 generates switching signals at higher rates, so that the output capacitor 17 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.

Claims: A cardiac pacer for pacing a human heart comprising a) means (4) for generating pacing pulses (13) at a predetermined pacing rate; b) means (8) for generating a continuous alternating current signal which is unable to pace the heart (1); c) a single lead (3) connected with a single electrode (2) for transmitting the pacing pulses together with the alternating signal to the heart; d) means (9) connected through leads (11, 12) to said lead (3) for measuring and processing the alternating signal after transmittal to the heart for obtaining a respiratory signal; and e) means (5) for varying the predetermined pacing rate dependent on the respiratory signal, the cardiac pacer having a conductive housing (15) which defines both the indifferent electrode for pacing and the second electrode for the alternating signal generating means. The cardiac pacer according to claim 1, wherein said alternating signal generating means (8) are designated for generating an alternating current having an amplitude, said amplitude being dependent on the frequency of the alternating signal lying substantially be low the fibrillation threshold of the heart. The cardiac pacer according to claim 2, wherein said alternating signal generating means (8) are designated for generating an alternating current having an amplitude, said amplitude being dependent on the frequency of the alternating signal lying substanially below the fibrillation threshold of the heart at frequencies above 150 Hz. The cardiac pacer according to claim 3, wherein said alternating signal generating means (8) are designated for generating an alternating current in the frequency range of 1 kHz to 10 kHz, preferably at 4 kHz The cardiac pacer according to claim 4, wherein said alternating signal generating means (8) are designated for generating an alternating current having an amplitude which is smaller than 10 µA, preferably at 2 µA. The cardiac pacer according to one of the preceding claims 1 to 5, wherein said means (9) for measuring and processing the alternating signal comprise a demodulator (20) for the picked up alternating signal. The cardiac pacer according to one of the preceding claims 1 to 6, wherein said means (9) for measuring and processing the alternating signal further comprises means (22) for non-linearly amplifying the respiratory signal in a manner that signal portions having higher amplitudes are more amplified than signal portions having lower amplitudes, means (23) for integrating the non-linearly amplified respiratory signal over a period of time, e. g. 5 to 30s. and means (5) for varying the predetermined pacing rate dependent on the integrated signal. The cardiac pacer according to one of the preceding claims 1 to 7, wherein said means (9) for measuring and processing the alternating signal further comprises a voltage to pulse rate converter (24) for the integrated signal. The cardiac pacer according to claim 8, wherein said means (5) for varying the predetermined pacing rate comprises an analog signal to digital control word converter (28) for the output signal of the voltage to pulse rate converter (24), a time base register (27) for the output signal of the analog signal to digital control word converter (28), a down counter (26) which is set to higher zero counting speed when the respiration rate increases, and a zero decoder (25) at the output of the down counter (26), said zero decoder (25) being connected with and controlling the pacing pulse generator (4) in a manner that a pacing pulse is generated at each zero count.

Inventors: AMUNDSON DAVID C, ELMQVIST HAKAN, HEDBERG SVEN-ERIK, LEKHOLM ANDERS, AMUNDSON, DAVID C., ELMQVIST, HAKAN, HEDBERG, SVEN-ERIK, LEKHOLM, ANDERS
Applicants: PACESETTER AB
Application year: 1987
Main classification: A61N1
IPC codes: A61N1