A cardiac pacer for pacing a human heart

Abstract: A cardiac pacer, which generates pacing pulses at a predetermined basic pacing rate, comprises a plu­ rality of body activity sensor means (7, 8, 9; 10, 11, 12; 13, 14, 15) for sensing body activity dependent on different physiological variables and for respectively generating a corresponding body activity output signal dependent thereon. It further includes means (19) for selecting single activity output signals and/or combi­ nations thereof to determine different exercise stages dependent on different physiological variables. Finally it incorporates means (5) connected to said means (4) for generating pacing pulses and to said means (19) for selecting for varying the predetermined basic pacing rate dependent on the selected body activity output signal or combination of signals.

Description: The invention relates to a cardiac pacer for pacing a heart dependent on body activity. The present application is related to the European applications 0 249 818, 0 249 819 and 0 249 821 all having the same priority date. Description of the prior art Conventional cardiac pacers of the respiration rate-responsive type are described for example in the US patent 3,593,718 and in the European Patent application 0 089 014. An accelerometer rate-responsive cardiac pacer is for example depicted in the US patent 4,428,378. One of the main difficulties with these rate-responsive cardiac pacers is that it is difficult to exactly determine different exercise stages, such as the start, duration and termination of an exercise for a patient wearing such a cardiac pacer. US patent 4,527,568 already describes a heart pacer, wherein the pacing rate is determined as a function of sensed atrial signals. It further incorporates an extra sensor mode, wherein the pacing rate is controlled as a function of at least one sensed body parameter. Nevertheless, exact determination of different exercise stages as mentioned before is impossible. EP patent application 0 222 681, published after the priority date of the present application, further describes a pacer for stimulating the heart depending on the physical activity, which pacer comprises means for generating a basic pacing rate, means for transmitting the pacing pulses to the heart for pacing, a plurality of body activity sensor means for sensing body activity dependent on different physiological variables and for respectively generating a corresponding body activity output signal dependent thereon, means for varying the basic pacing rate dependent on the body activity output signals, means for selecting single activity output signals and/or combinations thereof to determine different exercise stages dependent on different physiological variables, whereby the means for varying the basic pacing rate varies said rate dependent on the selected body activity output signals or combination of signals. SUMMARY OF THE INVENTION Objects It is an object of this invention to provide for an improved cardiac pacer comprising a sensor arrangement which is adequate for correctly determining different exercise stages, such as the start duration and termination of an exercise. Summary To achieve this object, the above mentioned pacer inventively comprises means for generating pacing pulses at a predetermined basic pacing rate, means for transmitting the pacing pulses to the heart for pacing, a plurality of body activity sensor means for sensing body activity dependent on different physiological variables and for respectively generating a corresponding body activity output signal dependent thereon, means for varying the predetermined basic pacing rate dependent on the body activity output signals, and means for selecting single activity output signals and/or combinations thereof to determine different exercise stages dependent on different physiological variables, said means for selecting comprise means for successively selecting single activity output signals and/or combinations thereof in predetermined time steps after the start of an exercise cycle, whereby the means for varying the predetermined basic pacing rate varies said rate dependent on the selected body activity output signals or combination of signals. According to this invention two or more different sensors for different physiological variables yield better quality information at different points in the exercise period. For example, a respiration (or e.g. temperature) sensor is not a very good sensor for determining the start of exercise, yet it is a very good sensor for determining whether or not the exercise has continued. Conversely, an accelerameter is very suitable for determining the start of exercise but is less-reliable in determining the duration of the exercise or the termination of the exercise. The subject invention also clearly distinguishes over the teachings of US patent 4,527,568 because this patent, although describing a number of different types of signals which can be obtained corresponding to different physiological characteristics of the patient, there is no teaching to selectively use one or a combination of such signals, with a possible weighting of the selected signals, in order to determine different exercise stages of the patient and to vary the basic pacing rate in accordance with the current exercise stage which has been determined. 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 an embodiment of a cardiac pacer comprising the invention in a schematic block diagram; Fig. 2 a diagram illustrating a pacing rate R as a function of time t during an exercise period; and Fig. 3 an embodiment of a cardiac pacer according to Fig. 1 including two sensors and following the diagram according to Fig. 2 in more detail. 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. A first sensor 7 for physical activity dependent on a first physiological variable is connected with a first body activity sensor circuitry 8 through line 9. A second sensor 10 for physical activity dependent on a second physiological variable is connected with a second body activity sensor circuitry 11 through line 12. Both sensors 7, 10 are also preferably implanted in the patient's body. The sensor arrangement, if desired, may be extended by more body activity sensors and associated bodt activity sensor circuitries as indicated in Fig. 1 in broken lines by an n-th body activity sensor 13 for an n-th physiological variable which is connected with an associated n-th body activity sensor circuitry 14 through line 15. The different body activity sensors 7, 10, 13 are designed to sense body activity in different exercise stages, e.g. start, duration and termination of exercise, dependent on different physiological variables, e.g. acceleration, respiration, temperature, pH, pO 2 etc.. The associated body activity sensor circuitries 8, 11, 14 generate corresponding body activity output signals thereon which are supplied through lines 16, 17, 18 to a selector device 19. The selector device 19, which may comprise e. g. a microprocessor 20 or any kind of appropriate selective logic circuitry in combination with input selectors 21 to 24 and for example a weighting coefficient generator 25, is designed for selecting single activity output signals and/or combinations thereof to determine different exercise stages dependent on different physiological variables. For example the input selector 21 may serve for successively selecting single activity output signals and/or combinations thereof in predetermined time steps after the start of an exercise cycle. Accordingly the input selectors 22, 23 and 24 may for example be designed for selecting single activity output signals and/or combinations thereof dependent on predetermined pacing rates, dependent on activity output signals which fluctuate least and/or dependent on whether or not a single activity output signal and/or combinations thereof fall(s) below a threshold. The output of the selector device 19 is connected with the time base unit 5 through line 26. Under the circumstances the time base unit 5 is designed for varying the predetermined basic pacing rate dependent on the selected single or the combination of selected single output signals of the body activity sensor circuitries 8, 11, 14. In Fig. 1 the pacing pulse generator 4, the time base unit 5, the body activity sensor units 8, 11, 14 and the selector device 19 are all encapsuled in an implantable conductive (metallic) housing 26 which is the housing of the cardiac pacer according to this invention. Figures 2 and 3 illustrate an embodiment of the invention according to Fig. 1, wherein an acceleration sensor 10 and an associated acceleration sensor circuitry 11 are utilized to determine the start of exercise (time period A in Fig. 2) and a respiration sensor (pacing electrode 2) and an associated respiration sensor circuitry 8 are used to determine the duration and termination of exercise (time periods B and C in Fig. 2) according to predetermined time steps Δ t1, Δ t2, Δ t3 after start of exercise. The time steps may be predetermined by means of time steps input selector 21 of the selector device 19. They may for example be selected in the ranges of Δ t1 = 10-60s, Δ t2 = 30s - 20 min, Δ t3 = 30s - 3 min. The peak value of the pacing rate is indicated in Fig. 2 with Rp. In Fig. 3 the respiration sensor circuitry 7 is an impedance pneumograph which is connected with the pacing pulse generator 4 through line 27 for processing the pacing pulses 28 in a manner that a respiratory signal is obtained from the pacing pulses 28 when evaluating the amplitude decays D of pacing pulses 28. The amplitude decay D changes according to alternating body impedance during respiration. The pacing pulse generator 4 comprises an output capacitor 29 which is switchable by means of switch 30 between battery 31 (switch position a) and pacing lead 3 (switch position b). In switch position a the output capacitor 29 is charged by the battery 31 to a voltage V1. In switch position b the output capacitor 29 is discharged through pacing lead 3 as pacing pulse 20. The amount of discharge depends on the impedance variations of the patient's thorax during respiration. According to Fig. 3 the pacing pulse 28 discharges from V1 to V2 (amplitude decay D). The conductive housing 26' provides for both the indifferent electrode for pacing and the electrode for impedance measurement (indicated by reference numeral 32). The impedance measurement may also be done by a division of voltage and current. The impedance pneumograph 7 includes a sample and hold circuitry 33, a difference former 34, a filter 35, a non-linear (e.g. squaring) amplification circuitry 36, an integrator 37 and a voltage to pulse rate converter 38. The sample and hold circuitry 33 samples and holds the voltages V1, V2 of output capacitor 29. The difference former 34 supplies the difference V1 - V2 through filter 35 to the non-linear amplification circuitry 36. The non-linear amplification circuitry 36 amplifies the output signal of filter 35 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 th art. They do not need described in more detail. The output signal of the non-linear amplification circuitry 36 is integrated in integrator 37 over a certain time period, e.g. in the range of 5 to 30 s. By integrating high-frequency noise is significantly reduced. The voltage to pulse rate converter 38 converts the integrated signal into a pulse rate according to the breathing rate. The time base unit 5 comprises a zero decoder 39, a down counter 40 having a reset input 41, a time base register 42 and an analog signal to digital control word converter 43. The latter one converts the output signal of the selector device 19 into a digital control word. This digital control word is supplied to the time base register 42. It controls the time base register 42 in a manner that a basic pacing rate, e.g. 60 beats/min., is varied dependent on the acceleration rate in time period A and on the respiration rate in time periods B and C of the exercise cycle according to Fig. 2. When the acceleration or breathing rate increases the time base register 42 increases the counting speed of down counter 40 so that it reaches zero faster than at basic rate. Under the circumstances the zero decoder 39 generates switching signals at higher rates, so that the output capacitor 29 of the pacing pulse generator 4 charges and discharges at higher rates. As a result the pacing rate increases dependent on increasing acceleration or breathing rate. It also decreases when acceleration and breathing rate are decreasing. In Fig. 3 the acceleration sensor 10 includes e.g. a piezoelectric force sensor coupled to the housing 26′ of the pacer for converting vibrational energy at the pacer site into a sensed acceleration signal. The sensed aceleration signal is applied to the acceleration sensor circuitry 11, which for example according to US patent 4,428,378 comprises a bandpass amplifier 44, a slow wareform generator 45, a slope control 46 and a voltage to pulse rate converter 47.

Claims: A cardiac pacer for pacing a heart (1) dependent on body activity, which pacer comprises means (4) for generating pacing pulses (28) at a predetermined basic pacing rate, means (2, 3) for transmitting the pacing pulses to the heart (1) for pacing, a plurality of body activity sensor means (7, 8, 9; 10, 11, 12; 13, 14, 15) for sensing body activity dependent on different physiological variables and for respectively generating a corresponding body activity output signal dependent thereon, means (5) for varying the predetermined basic pacing rate dependent on the body activity output signals and means (19) for selecting single activity output signals and/or combinations thereof to determine different exercise stages dependent on different physiological variables, said means for selecting comprise means (20, 21) for successively selecting single activity output signals and/or combinations thereof in predetermined time steps after the start of an exercise cycle, whereby the means (5) for varying the predetermined basic pacing rate varies said rate dependent on the selected body activity output signals or combination of signals. A cardiac pacer according to claim 1, further comprising means (20, 25) for weighting single activity output signals and/or combinations thereof by different coefficients. A cardiac pacer according to one of the preceding claims, wherein said means for selecting comprise means (20, 22) for selecting single activity output signals and/or combinations thereof dependent on predetermined pacing rate values. A cardiac pacer according to one of the preceding claims 1 to 3, wherein said means for selecting comprise means (20, 23) for selecting single activity output signals and/or combination thereof dependent on activity output signals which fluctuate least. A cardiac pacer according to one of the preceding claims 1 to 4, wherein said means for selecting comprise means (20, 24) for selecting single activity output signals and/or combination thereof dependent on whether or not a single activity output signal and/or combinations thereof fall(s) below a threshold. A cardiac pacer according to claim 5, wherein the threshold is programmable. A cardiac pacer according to one of the preceding claims 1 to 6, wherein at least a first body activity signal determines the start of exercise. A cardiac pacer according to one of the preceding claims 1 to 7 wherein at least a second body activity output signal determines the duration of exercise. A cardiac pacer according to one of the preceding claims 1 to 8, wherein said second body activity signal also determines the termination of exercise. A cardiac pacer according to claim 7, wherein said body activity sensor means comprise an acceleration sensor means (10 to 12) for sensing and generating said first body activity output signal. A cardiac pacer according to claim 8, wherein said body activity sensor means comprise a respiration sensor means (7 to 9) for sensing and generating said second body activity signal.

Inventors: AMUNDSON DAVID C, AMUNDSON, DAVID C.
Applicants: PACESETTER AB
Application year: 1987
Main classification: A61N1
IPC codes: A61N1