WO2005118063A1

WO2005118063A1 - Transcutaneous pacemaker

Info

Publication number: WO2005118063A1
Application number: PCT/US2005/018624
Authority: WO
Inventors: Kalpathi L. Venkatachalam
Original assignee: Mayo Foundation For Medical Education And Research
Priority date: 2004-05-27
Filing date: 2005-05-26
Publication date: 2005-12-15

Abstract

A transcutaneous pacemaker produces a pacing signal for a subject and applied it to the patient’s heart through a pair of externally attached electrodes. A high frequency carrier signal is employed to transport the pacing signal into the chest cavity where the low pass filter electrical characteristics serve as a demodulator that reconstitutes the pacing signal in the heart. The high frequency carrier signal does not stimulate muscles in the chest wall and does not, therefore, cause significant discomfort.

Description

TRANSCUTANEOUS PACEMAKER

BACKGROUND OF THE INVENTION

[0001] The field of the invention is therapies for cardiac arrhythmias, and more particularly, to a method and an apparatus for forcing cardiac muscle stimulation by delivering a pulsatile electrical signal to the heart in patients incapable of producing adequate electrical cardiac stimulation spontaneously.

[0002] Cardiac pacing is an established therapeutic approach used in treating patients with many types of electrical condition system defects of the heart including, but not limited to, sick-sinus syndrome, heart block and diffuse conduction system disease. This may be done using surgically implanted pacemakers with wires delivering pacing signals to various chambers of the heart as described in U.S. Pat. No. 6,269,268, transvenously using wires in various chambers of the heart with an external pacemaker as shown in U.S. Pat. No. 6,567,697 or transcutaneously using wires connected to the surface of the body with an external pacemaker.

[0003] One of the problems with using an external temporary cardiac pacemaker

(pacer) is that the pacer pulses stimulate skeletal muscle and this can be very painful. Patients have to be sedated before this can be done. The alternative is to use a transcutaneous pacer by placing a pacer wire through a vein (femoral or internal jugular) under fluoroscopy and then using a temporary pulse generator to pace the heart. Problems associated with this technique include 1) significant time to place the wire; 2) risk of cardiac perforation; 3) risk of pneumothorax; 4) risk of infection; 5) requirement for patient immobility; 6) very high procedural competence. Given these risks and limitations, this procedure is almost always done in a critical care unit and the patients have to stay in the critical care unit until they improve or more definitive therapy is performed. This is expensive and inconvenient. [0004] Techniques have been developed to reduce the pain associated with externally applied pacemaker signals, but these are only marginally effective. In U.S. Pat. No. 5,431,688 the current density applied to the subject's skin is reduced by employing multiple pairs of external electrodes. In U.S. Pat. Nos. 4,349,030 and 5,108,552 the pain associated with external non-invasive pacing is reduced by extending the duration and shaping the current pacing pulses.

SUMMARY OF THE INVENTION

[0005] The present invention is a transcutaneous pacing method and apparatus in which a high-frequency carrier signal is modulated by a pacing signal and this composite signal is applied across the chest wall of the subject. Since the pacing information is on a high-frequency carrier, it will not stimulate muscle and so pacing can take place in conscious, unsedated patients. The difficulty with this approach is that there needs to be a method to demodulate the composite signal once it has been delivered to the myocardium before it can be used as a pacing signal. Ostensibly, this requires an implant in the myocardium, but a further aspect of the present invention is to use a specific modulation method so as to use the electrical properties of the chest wall and pericardium to demodulate the signal, such that the final signal at the myocardium is the original pacing pulse. No additional hardware within the heart is required.

[0006] A general object of the invention is to provide a transcutaneous pacing method and apparatus which does not stimulate skeletal muscle and is therefore, painless. By using a high frequency carrier signal to "transport" the pacing signal through the chest wall, muscles in the chest wall are not significantly stimulated and do not result in pain. [0007] Another object of the invention is to provide a pacing device and method which is easy to use. No implants or transvenously placed electrodes are required to use the present invention. A device made according to the present invention is particularly well suited to provide temporary pacing in a low-level care setting with none of the previously mentioned risks or limitations. It may also be used in lieu of a permanent pacer in patients who are not pacemaker-dependent but needed the increased heart rate for improved cardiac function. It may also be used to determine if a patient would benefit from a permanent pacer before going through the trouble and expense of surgically installing one. All this can be done in normally functioning patients with no discomfort.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 is a block diagram of a preferred embodiment of a pacemaker which employs the present invention;

[0009] Fig. 2 is a circuit diagram of a preferred PWM circuit which forms part of the pacemaker of Fig. 1;

[0010] Fig. 3 is a graphic illustration of signals applied to and produced by the PWM circuit of Fig. 2; and

[0011] Fig. 4 is a pictorial representation of the filtering action of a subject's chest wall and pericardium which serves as a demodulator of the applied electrical signals according to the present invention.

GENERAL DESCRIPTION OF THE INVENTION

[0012] Referring particularly to Fig. 4, the human heart 1 is located in the chest cavity 2 that is surrounded by a chest wall 3 comprised of skin, muscles, bones and cartilage. The heart 1 is contained in a pericardium 4. The pericardium 4 is comprised of an outer fibrous pericardium layer which is lined by a double-layered membranous sac called the serous pericardium. The serous pericardium in turn is comprised of an outer parietal layer and an inner visceral layer, or epicardium, that covers the heart walls and great vessels. A thin film of fluid between the visceral and parietal layers allow the heart to move within the sac. The heart walls, or myocardium 5, are muscle tissue that is stimulated by electrical signals from the nervous system to drive the beating action of the heart. These nerves are in electrical contact with the myocardium that they stimulate.

[0013] It has been discovered that when an alternating current is applied across the chest by a pair of electrodes 6 in contact with the subject's skin, the higher frequencies are attenuated by the chest wall 3 and pericardium 4. This was measured by placing an electrode in the heart 1 and in contact with the heart wall while applying an AC voltage of constant amplitude across electrodes 6. The amplitude of the resulting voltage measured in the heart wall dropped as a function of frequency. The chest wall 3 and pericardium 4 thus act as a low pass filter to stimulation signals applied transcutaneously across the subject's chest. [0014] Referring still to Fig. 4, the present invention employs this low pass filter characteristic to demodulate a pacing signal contained in a high frequency carrier. If the pulse width of a high frequency square wave (e.g., 100 kHz) is modulated by a pacing signal, the low pass filtering action discussed above demodulates the applied carrier signal and reconstitutes the pacing signal in the heart walls. For example, if the carrier signal has an amplitude of -10 volts and a 10% duty cycle, the average voltage at the output of a low pass filter indicated at 9 is approximately -1 volt. On the other hand, if the duty cycle is changed to 70%, for example, the output of the low pass filter is -7 volts. Thus, modulating the high frequency carrier between a 10% and 70% duty cycle with the pacing signal will result in a reconstituted pacing signal at the output of the low pass filter 9 which ranges from -1 to -7 volts in amplitude. The location of the electrodes 6 and the amplitude of the applied modulated carrier signal is selected such that this reconstituted pacing signal will stimulate the heart 1 in the desired manner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [0015] Referring particularly to Fig. 1, the pacemaker according to the present invention includes an ECG, or cardiac activity detector 10 having electrodes 12 which are placed on the patient 14 to monitor ECG signals. The cardiac activity detector 10 includes amplifiers for amplifying the cardiac signals picked up by electrodes 12 and P wave and R wave detector circuits for sensing the pace at which the heart is functioning. [0016] The detected P waves and R waves are output to a microcomputer 16 which is programmed in a manner well known in the art to analyze these signals and produce an appropriate pacing signal at output 18. Many different pacing signals can be produced depending on the location of the ECG electrodes 12, the actual electrical signals produced by the patient's own cardiac conduction system, and the processing performed by microcomputer 16. For example, the pacing signal can be derived from different ECG signals such that one part of the heart (e.g., right atrium) can be used to pace other parts of the heart (e.g., right or left ventricle). Of course, many other pacing signals known to those skilled in the art may be produced and no pacing signal is produced when conditions indicate it is not needed. [0017] Rather than applying the pacing signal on output 18 directly to electrodes, the pacing signal is input to a pulse width modulator (PWM) circuit 20. The PWM 20 also receives carrier signals from a carrier frequency generator 22 which is set to a frequency above the response frequency of skeletal muscle, and in the preferred embodiment is set to 100 kHz. This carrier signal is pulse width modulated by the pacing signal in the PWM circuit 20 and output through a pair of leads 24. The leads 24 connect to conventional electrodes 26 that make electrical connection to the patient's skin. The electrodes 26 are positioned such that the electrical current flowing between them will stimulate the heart in the desired manner. For example, one electrode 26 may be placed over the right ventricle on the anterior chest and the other electrode 26 is placed on the posterior chest near the left scapula. Other arrangements are possible depending on which part of the heart is to be paced with the particular pacing signals being produced.

[0018] The advantage of PWM 20 is that the demodulation process only requires a low-pass filter. Since the chest wall and pericardium act as a low-pass filter to electrical signals, a square wave may be used as a carrier signal at a frequency well above the cut-off frequency of the chest wall. This carrier is pulse-width modulated using any of a number of established techniques using a baseband pulse with the amplitude and frequency characteristics of the desired pacemaker pulse (e.g., -5 volts at 60 beats per minute). Once the high-frequency carrier has been pulse-width modulated, the low frequency pacing information is encoded in the carrier and no low-frequency energy is present in the final modulated signal to stimulate muscle. However, in passing through the chest-wall, the low-pass filtering action of the chest wall and pericardium allows full recovery of the baseband pacing signal at the myocardium with consequent capture by the myocardium. This capture by the myocardium results in the application of the pacing signal thereto.

[0019] Referring particularly to Figs. 2 and 3, while many different PWM circuits may be used, a preferred PWM circuit 20 receives two 100 kHz carrier signals 30 and 32 at the inputs of an analog switch 34. The carrier signal 30 has a 70% duty cycle and the carrier signal 32 has a 10% duty cycle. The pacing signal 18 is applied to the control input of the analog switch 34 and it switches between the two carrier signals 30 and 32 to provide an output signal 36 having a frequency of 100 kHz and a duty cycle which is modulated by the pacing signal (either 10% or 70%).

[0020] The pulse width modulated output signal 36 is applied to an amplifier 38 which drives an output transistor 40 to produce current pulses from 0 to 80 ma at the output leads 24. In the alternative, the output transistor 40 may be driven to produce voltage pulses from 0 to 10 v at the output leads 24.

[0021] Since the chest wall impedance may vary widely among people due to patient size, chest wall surgery or pulmonary disease, the carrier frequency for the pulse width modulator (PWM) may require adjustment for some patients. In most cases, using a frequency well beyond the expected cutoff frequency (»10 - 20 kHz) will work to provide a low-pass response that can be used to demodulate the PWM signal. A method to ensure that this technique will work in all patients is to measure the frequency response of each individual patient after the device is attached to them. This is done by applying known voltages at several frequencies and measuring the resulting current to determine impedance

(frequency response). This information is then used to determine the cutoff frequency of the chest wall for that particular patient and a PWM carrier frequency is selected that is about 10 times that cutoff frequency.

[0022] While the PWM modulation method is preferred, other methods are possible.

Another modulation method is to amplitude modulate (AM) a high-frequency carrier

(preferentially 100 kHz but at any frequency above the sense frequency of skeletal muscle) with the baseband pacing signal and applying the modulated signal via the output amplifier to two output leads 24. Simultaneously, the unmodulated carrier signal is applied to another pair of output leads also connected to the patient. The AM signal and the unmodulated carrier signals interact at the level of the cardiac muscle (which has anisotropic or nonlinear properties to electrical signals), producing sum-and-difference frequencies of the carrier signal and the baseband pacing signal. In effect, the cardiac muscle itself acts as a demodulator and the difference frequency output, which is the baseband pacing signal, is recovered at the cardiac tissue and is used to electrically stimulate the heart.

Claims

1. A pacemaker which comprises: a pacing signal generator for producing a pacing signal; a carrier frequency generator for producing a carrier signal having a frequency above the response frequency of skeletal muscle; a modulator connected to receive the carrier signal from the carrier frequency generator and being operable to produce an output signal at the carrier frequency which is modulated by the pacing signal received from the pacing signal generator; and an electrode connected to receive the output signal and being operable to conduct the output signal to the skin of a subject.

2. The pacemaker of claim 1 in which the modulator is a pulse width modulator and the electrical properties of the subject's chest wall and pericardium are used as a low pass filter to recover the pacing signal in the heart.

3. The pacemaker as recited in claim 2 in which the carrier frequency is substantially 100 kHz.

4. The pacemaker as recited in claim 1 in which the pacing signal generator includes electrodes attached to the skin of the subject to receive signals indicative of the subject's heart beat.

5. The pacemaker as recited in claim 1 in which the electrode includes a pair of electrodes for applying the output signal across the chest of the subject.

6. The pacemaker as recited in claim 5 in which the modulator is a pulse width modulator.

7. The pacemaker as recited in claim 6 in which the pacing signal has two amplitudes and the modulator produces pulses at the carrier frequency with two different pulse widths that are determined by the pacing signal amplitude.

8. The pacemaker as recited in claim 7 which includes an amplifier connected to receive the output signal from the modulator and apply an amplified output signal across the pair of electrodes.

9. A method of pacing the heart of a subject, the steps comprising: producing a pacing signal; producing a carrier signal having a frequency sufficiently high to avoid stimulating muscles in the chest wall of the subject; modulating the carrier signal with the pacing signal; and applying the modulated carrier signal to the chest wall of the subject to stimulate the subject's heart with the pacing signal contained therein.

10. The method as recited in claim 9 in which the modulating is performed by: producing pulses at the carrier signal frequency; and modulating the width of the pulses in response to the amplitude of the pacing signal.

11. The method as recited in claim 10 in which the carrier frequency is substantially 100 kHz.

12. The method as recited in claim 9 in which the modulated carrier signal is applied to the chest wall by attaching a pair of electrodes to the surface of the chest wall at two different locations.

13. The method as recited in claim 12 in which the locations are selected such that the subject's heart is disposed substantially between them.

14. The method as recited in claim 9 in which the modulating is performed by amplitude modulating the carrier signal with the pacing signal.

15. The method as recited in claim 9 which includes determining the frequency of the carrier signal by measuring the cutoff frequency of the subject's chest wall.