PACER FOR STIMULATING A HEART SYNCHRONOUS TO ATRIAL ACTIVITY

Abstract: An impedance measurement, used for controlling the pulse generation rate of the pacer, is obtained by placing an electrode in the ventricular portion, rather than the atrial portion, of the heart. The characteristic p-wave in the electrocardiogram is detected as a rapid inflection in the impedance signal.

Description: Field of the Invention The present invention relates to a pacer for stimulating the heart of a patient synchronous to atrial activity. Description of the Prior Art P-synchronous pacers require a sensing element in the atrium to measure the atrial contraction. This requires, in turn, the use of a separate lead in the atrium or a special ventricular lead with a separate floating electrode in the atrium. Either approach requires a skilled surgeon to successfully implant such a system. It is furthermore subject to sensing problems at a later date. A p-synchronous pacer as the one referred to above is, for example, described in the Journal "Thoracic and Cardiovascular Surgery", Vol. 46, No. 4 (October, 1963) in an article entitled "Chronic Postsurgical Complete Heart Block" by Lillehei et al. on pages 436-456. Further, pacers relying on impedance measurements for their control are also known. One such pacer is, for example, described in GB-A-2 070 282. The '282 pacer includes circuitry for determining a trend in (stroke volume indicating) peak-to-peak impedance swing for subsequent heart cycles. If a trend is present, the AV-time of the pacer can be adjusted to maximise stroke volume. In addition to the normal pacing electrodes, this pacer makes use of separate atrial and ventricular impedance measurement electrodes. SUMMARY OF THE INVENTION It is an object of the invention to provide a pacer for heart stimulation synchronous to atrial activity which allows for a simpler and more dependable electrical connection with the heart. The invention is defined in claim 1. According to the invention, instead of measuring the electric potential evoked by the atrium, the impedance, including the ventricular impedance, is monitored continuously. This impedance measurement indirectly detects the intraventricular volume as one component of the overall body impedance. As the atrium contracts, the ventricular impedance is lowered when the atrial blood enters the ventricle. This results in a rapid inflection (notch) in the overall impedance signal. Thus a signal corresponding to the p-wave can be detected from any ventricular electrode without requiring additional sensing in the atrium. This signal can then be used to trigger the ventricular stimulus (VVI pacing). 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 Figure 1 is a schematic block diagram of a heart pacer synchronous to atrial activity according to the principles of the present, invention, for a unipolar impedance measurement in the right ventrical of a patient's heart. Figure 2 is an impedance diagram corresponding to an impedance measurement as utilized by the heart pacer synchronous to atrial activity according to the principles of the present invention. Figure 3 is a detailed circuit diagram of the delta modulator and the event detector. DESCRIPTION OF THE PREFERRED EMBODIMENTS In Figure 1 the pacer 1 comprises a metallic housing 2 encapsuling electrical components. The illustrated components are a stimulation pulse generator 3, which generates stimulation pulses SP, an impedance measuring unit 4, a delta modulator 5, comprising a comparator 6, an event detector 7, a signal delay unit 8, and a blanking control unit 9. The impedance measuring unit 4 includes a high-frequency (e.g. 4 kHz) current generator 10, with oscillator 11, a pre-amplifier 12, a demodulator 13, which can be blanked by the blanking control unit 9, and a band-pass filter 14 (0,1 Hz - 20 Hz). The signal output of the stimulation pulse generator 3 is electrically connected with the right ventrical RV of the patient's heart 15 through pacing lead 16 and pacing electrode 17. The indifferent electrode (indicated by reference number 18) is formed by metallic housing 2 of the pacer 1. The left ventricle of the heart 15 is indicated with LV. The right and left atriums are indicated by RA and LA, respectively. According to Figure 1, a high frequency current IHF is introduced by the high frequency current generator 10 through lead 16 to pacing electrode 17 and, from there, back to indifferent electrode 18 (metallic housing 2 of pacer 1). The current signal received from the body is a measure of the overall impedance between pacing electrode 17, located in the right ventricle, and metallic housing 2 of the pacer (uni-polar impedance measurement). Instead of this unipolar impedance measurement, a bipolar impedance measurement could equally well be used, by locating the indifferent electrode 18, for example, close to the pacing electrode 17 at another point on lead 16, e.g. by fabricating lead 16 as a bipolar catheter. The overall impedance is measured by the elements 12 to 14 of the impedance measuring unit 4. The impedance signal at the output of the impedance measuring unit 4 is generally designated by IS. In the case of the unipolar system the impedance includes intraventricular (e.g. end systolic, end diastolic volume) and extraventricular components (e.g. respiratory cavitary changes). For bipolar electrodes this signal is almost entirely due to intraventricular components. In both cases, there is a characteristic change in the overall impedance resulting from blood entering the ventricle from the atrium. This change appears as a rapid up-down inflection 19 of the overall impedance measurement signal IS (see Figure 2). Such an inflection 19 is detected by delta modulator 5 in conjunction with event detector 7. The delta modulator 5 is tuned to a maximum slope, which exceeds less steep non-atrial components of the impedance signal IS. The delta modulator 5, in conjunction with the event detector 7, generates the detector signal DS when a rapid inflection in the impedance signal IS exceeds the maximum slope of the signal MS from the delta modulator 5. This signal DS, which corresponds to the occurance of a p-wave, after a given delay time t D , triggers the stimulation pulse generator 3 to generate a stimulation pulse SP. At the same time it triggers the blanking control unit 9 to blank the demodulator 13 of the impedance measuring unit 4 (blanking time: e.g. 40 ms) while the stimulation pulse SP is generated. The delay time t D is given by the signal delay unit 8. It corresponds to the atrium-ventricle delay time of the patient's heart (e.g. t D = 150 ms). The trigger signal for triggering the stimulation pulse generator 3 and the blanking control unit 9 is generally designated TS. In Figure 2 an electrocardiogram (ECG) comprising p-waves is shown above the impedance signal IS, as a function of time t. As can be seen from Figure 2, the deflections 19 of the impedance signal IS correspond with the p-waves in the ECG on the time axis. In Figure 3 the delta modulator 5 comprises comparator 6, a bistable multivibrator 20, current generators 21, an integration capacitor 22, and an exclusive-OR-gate 23. The inverting input of comparator 6 is connected to a reference voltage V REF . The bistable multivibrator 20 is clocked by a clock frequency f₁ (e.g. 16 kHz). The event detector 7 comprises a counter 24, which is clocked by a frequency f₂ (e.g. 4 kHz), and a comparator 25. The comparator 25 compares the output signal S₁ of the counter 24 with a time reference signal S₂. In case that the output signal S₁ meets the time reference signal S₂, a detector signal DS is generated. The operation of the circuit of Figure 3 is as follows. The impedance signal IS, including inflections 19, is supplied to the delta modulator 5, as indicated in Figure 3. As soon as an inflection 19 occurs, the opposite side of integration capacitor 22 transfers the signal to the non-inverting input of comparator 6 with a slope of either +I/C or -I/C, wherein I is the current of current generator 21 (programmable) and C is the capacity of the integration capacitor 22. Whenever the bistable multivibrator 20 switches from one state to the other, the exclusive-OR-gate 23 zeros the counter 24 in the event detector 7. When the slope of the impedance signal IS exceeds +I/C or falls below -I/C (namely, upon the occurrence of an inflection 19), the bistable multivibrator 20 does not switch. As a result, the time counter 24 of event detector 7 is not zeroed. The output signal S₁ of the time counter 24 increases until it meets the time reference signal S₂. A detection signal DS is generated, as indicated in Figure 2, by pulses 26.

Claims: An atrial activity synchronized ventricular heart stimulator (1) for stimulating the heart (15) of a patient, comprising: a) generating means (3) for generating stimulation pulses (SP); b) a pacing electrode means (17) electrically connected with said generating means (3), said pacing electrode means (17) being designated for being located in the ventricular portion (RV) of the heart (15); c) means (4) electrically connected to said pacing electrode means for executing an impedance measurement, whereby said impedance includes a ventricular impedance, and for generating an impedance signal (IS) dependent upon the measured impedance; d) means (5, 7) for detecting a rapid inflection (19) in said impedance signal (IS) and for generating a corresponding detection signal (DS); and e) means (8) for time-delayed triggering of said means (3) for generating stimulation pulses, dependent on the occurrence of said detection signal (DS) such that said means (3) for generating stimulation pulses (SP) generate a stimulation pulse (SP), wherein the time delay (t₀) corresponds with the atrium-ventricle delay time of the patient's heart (15). A pacer (1) according to claim 1, wherein said means (5, 7) for detecting a rapid inflection (19) comprises a delta modulator (5) tuned to a maximum slope, which exceeds less steep non-atrial components of said impedance signal (IS) and comprises a comparator (6) which generates said detection signal (DS) when a rapid inflection (19) in said impedance signal (IS) exceeds said maximum slope of an output signal (MS) from said delta modulator (5).

Inventors: AMUNDSON, DAVID, HEDIN, ASA, NOREN, KJELL
Applicants: SIEMENS AKTIENGESELLSCHAFT, SIEMENS ELEMA AB
Application year: 1988
Main classification: A61N1
IPC codes: A61N1