Electrotherapy circuit having controlled current discharge based on patient-dependent electrical parameter

Claims

What is claimed is:

1. An electrotherapy circuit for administering to a patient a current waveform, comprising:

a charge storage device;

at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device;

a variable impedance connected between the charge storage device and one of the electrodes, the variable impedance being capable of being set at more than two discrete impedance levels;

a sensor that senses a patient-dependent electrical parameter; and

a control circuit, connected to the sensor and the variable impedance, that controls the impedance of the variable impedance during discharge of the charge storage device based on the patient-dependent electrical parameter sensed by the sensor.

2. The electrotherapy circuit of claim 1 wherein the patient-dependent electrical parameter comprises impedance of the patient.

3. The electrotherapy circuit of claim 1 wherein the control circuit controls the variable impedance in a manner so as to reduce dependence of peak discharge current on the patient-dependent electrical parameter.

4. The electrotherapy circuit of claim 3 wherein the control circuit selects the impedance of the variable impedance inversely with respect to the impedance of the patient as sensed by the sensor.

5. The electrotherapy circuit of claim 4 wherein the peak discharge current if the impedance of the patient is 15 ohms is no more than about twice the peak discharge current if the impedance of the patient is 125 ohms.

6. The electrotherapy circuit of claim 3 wherein the discharge of the charge storage device comprises a current waveform having a sensing pulse portion during which the sensor senses the impedance of the patient between the electrodes, and a therapeutic discharge portion during which the control circuit selects the impedance of the variable impedance inversely with respect to the impedance of the patient as sensed by the sensor.

7. The electrotherapy circuit of claim 1 wherein the sensor is a current sensor.

8. The electrotherapy circuit of claim 7 wherein the sensor is a current sense transformer.

9. The electrotherapy circuit of claim 1 wherein the variable impedance comprises a variable resistive circuit.

10. The electrotherapy circuit of claim 9 wherein the resistive circuit comprises a plurality of discrete resistors.

11. The electrotherapy circuit of claim 10 further comprising a switching circuit connected to the plurality of resistors that selectively provides at least one path for flow of electric current from the charge storage device through a subset of the plurality of resistors to one of the discharge electrodes.

12. The electrotherapy circuit of claim 11 wherein the control circuit controls the switching circuit to select the subset of the resistors through which the electric current flows, the control circuit selecting different subsets of the resistors during different portions of discharge of the charge storage device so as to produce a rectilinear current waveform.

13. The electrotherapy circuit of claim 11 wherein the resistors are connected together in series.

14. The electrotherapy circuit of claim 13 wherein the switching circuit selectively provides the path for flow of electric current by shorting out resistors not in the subset through which the path extends.

15. The electrotherapy circuit of claim 10 wherein the resistors are stepped in a binary sequence.

16. The electrotherapy circuit of claim 6 wherein the therapeutic discharge portion of the current waveform is rectilinear.

17. The electrotherapy circuit of claim 1 further comprising at least one switch connected between the charge storage device and one of the electrodes that, when closed, creates a closed circuit for flow of current from the charge storage device to the electrodes.

18. The electrotherapy circuit of claim 17 wherein the at least one switch comprises a plurality of switches arranged as an H-bridge.

19. The electrotherapy circuit of claim 1 wherein the current waveform comprises a first phase and a second phase having a polarity opposite to the first phase.

20. The electrotherapy circuit of claim 1 wherein the control circuit comprises a microprocessor.

21. The electrotherapy circuit of claim 1 wherein the control circuit is hard-wired.

22. The electrotherapy circuit of claim 1 wherein the current waveform comprises a defibrillation pulse.

23. The electrotherapy circuit of claim 1 wherein the electrodes are non-implanted.

24. The electrotherapy circuit of claim 1 wherein the charge storage device comprises at least one capacitor.

25. The electrotherapy circuit of claim 24 wherein the charge storage device is a single capacitor.

26. A method of forming an electrotherapy current waveform, comprising the steps of:

charging a charge storage device;

sensing a patient-dependent electrical parameter; and

discharging the charge storage device through at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device, and through a variable impedance connected between the charge storage device and one of the electrodes, the variable impedance being capable of being set at more than two discrete impedance levels; and

controlling the impedance of the variable impedance during discharge of the charge storage device based on the sensed patient-dependent electrical parameter.

27. The method of claim 26 wherein the patient-dependent electrical parameter comprises impedance of the patient.

28. The method of claim 26 wherein the step of controlling the variable impedance is performed in a manner so as to reduce dependence of peak discharge current on the patient-dependent electrical parameter.

29. The method of claim 28 wherein the step of controlling the variable impedance comprises setting the variable impedance inversely with respect to the sensed impedance of the patient.

30. The method of claim 28 wherein the peak discharge current if the impedance of the patient is 15 ohms is no more than about twice the peak discharge current if the impedance of the patient is 125 ohms.

31. The method of claim 26 wherein:

the discharge of the charge storage device comprises a current waveform having a sensing pulse portion and a therapeutic discharge portion;

the step of sensing the impedance of the patient is performed during the sensing pulse portion; and

the step of controlling the variable impedance comprises, during the therapeutic discharge portion, setting the variable impedance inversely with respect to the impedance of the patient as sensed during the sensing pulse portion.

32. The method of claim 26 wherein the variable impedance comprises a plurality of discrete resistors and the step of controlling the variable impedance comprises selectively providing at least one path for flow of electric current from the charge storage device through a subset of the plurality of resistors to one of the discharge electrodes.

33. The method of claim 32 wherein the step of providing the path for flow of electric current comprises selecting different subsets of the resistors during different portions of discharge of the charge storage device so as to produce a rectilinear current waveform.

34. The method of claim 32 wherein the resistors are connected together in series and the step of providing the path for flow of electric current comprises shorting out resistors not in the subset through which the path extends.

35. The method of claim 26 wherein the current waveform comprises a first phase and a second phase having a polarity opposite to the first phase.

36. The method of claim 26 wherein the current waveform comprises a defibrillation pulse.

37. The method of claim 26 further comprising the step of applying the electrodes externally to the patient prior to discharging the charge storage device.

38. The method of claim 37 wherein the electrodes are externally applied directly to the patient's heart during surgery.

39. An electrotherapy circuit for administering to a patient a current waveform, comprising:

a charge storage device;

at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device;

a variable impedance connected between the charge storage device and one of the electrodes;

a sensor that senses a patient-dependent electrical parameter; and

a control circuit, connected to the sensor and the variable impedance, that controls the impedance of the variable impedance during discharge of the charge storage device based on the patient-dependent electrical parameter sensed by the sensor,

wherein the control circuit selects the impedance of the variable impedance inversely with respect to the impedance of the patient as sensed by the

sensor, and

wherein the peak discharge current if the impedance of the patient is 15 ohms is no more than about twice the peak discharge current if the impedance of the patient is 125 ohms.

40. An electrotherapy circuit for administering to a patient a current waveform, comprising:

a charge storage device;

at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device;

a variable impedance connected between the charge storage device and one of the electrodes;

a sensor that senses a patient-dependent electrical parameter; and

a control circuit, connected to the sensor and the variable impedance, that controls the impedance of the variable impedance during discharge of the charge storage device based on the patient-dependent electrical parameter sensed by the sensor,

wherein the control circuit controls the variable impedance in a manner so as to reduce dependence of peak discharge current on the patient-dependent electrical parameter, and

wherein the discharge of the charge storage device comprises a current waveform having a sensing pulse portion during which the sensor senses the impedance of the patient between the electrodes, and a therapeutic discharge portion during which the control circuit selects the impedance of the variable impedance inversely with respect to the impedance of the patient as sensed by the sensor.

41. An electrotherapy circuit for administering to a patient a current waveform, comprising:

a charge storage device;

at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device;

a variable impedance connected between the charge storage device and one of the electrodes;

a sensor that senses a patient-dependent electrical parameter; and

a control circuit, connected to the sensor and the variable impedance, that controls the impedance of the variable impedance during discharge of the charge storage device based on the patient-dependent electrical parameter sensed by the sensor,

wherein the variable impedance comprises a variable resistive circuit, and

wherein the resistive circuit comprises a plurality of discrete resistors.

42. The electrotherapy circuit of claim 41 further comprising a switching circuit connected to the plurality of resistors that selectively provides at least one path for flow of electric current from the charge storage device through a subset of the plurality of resistors to one of the discharge electrodes.

43. The electrotherapy circuit of claim 42 wherein the control circuit controls the switching circuit to select the subset of the resistors through which the electric current flows, the control circuit selecting different subsets of the resistors during different portions of discharge of the charge storage device so as to produce a rectilinear current waveform.

44. The electrotherapy circuit of claim 42 wherein the resistors are connected together in series.

45. The electrotherapy circuit of claim 44 wherein the switching circuit selectively provides the path for flow of electric current by shorting out resistors not in the subset through which the path extends.

46. The electrotherapy circuit of claim 41 wherein the resistors are stepped in a binary sequence.

47. The electrotherapy circuit of claim 40 wherein the therapeutic discharge portion of the current waveform is rectilinear.

48. An electrotherapy circuit for administering to a patient a current waveform, comprising:

a charge storage device;

at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device;

a variable impedance connected between the charge storage device and one of the electrodes;

a sensor that senses a patient-dependent electrical parameter; and

a control circuit, connected to the sensor and the variable impedance, that controls the impedance of the variable impedance during discharge of the charge storage device based on the patient-dependent electrical parameter sensed by the sensor;

at least one switch connected between the charge storage device and one of the electrodes that, when closed, creates a closed circuit for flow of current from the charge storage device to the electrodes,

wherein the at least one switch comprises a plurality of switches arranged as an H-bridge.

49. A method of forming an electrotherapy current waveform, comprising the steps of:

charging a charge storage device;

sensing a patient-dependent electrical parameter; and

discharging the charge storage device through at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device, and through a variable impedance connected between the charge storage device and one of the electrodes; and

controlling the impedance of the variable impedance during discharge of the charge storage device based on the sensed patient-dependent electrical parameter,

wherein the step of controlling the variable impedance is performed in a manner so as to reduce dependence of peak discharge current on the patient-dependent electrical parameter, and

wherein the peak discharge current if the impedance of the patient is 15 ohms is no more than about twice the peak discharge current if the impedance of the patient is 125 ohms.

50. A method of forming an electrotherapy current waveform, comprising the steps of:

charging a charge storage device;

sensing a patient-dependent electrical parameter; and

discharging the charge storage device through at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device, and through a variable impedance connected between the charge storage device and one of the electrodes; and

controlling the impedance of the variable impedance during discharge of the charge storage device based on the sensed patient-dependent electrical parameter,

wherein the discharge of the charge storage device comprises a current waveform having a sensing pulse portion and a therapeutic discharge portion,

wherein the step of sensing the impedance of the patient is performed during the sensing pulse portion, and

wherein the step of controlling the variable impedance comprises, during the therapeutic discharge portion, setting the variable impedance inversely with respect to the impedance of the patient as sensed during the sensing pulse portion.

51. A method of forming an electrotherapy current waveform, comprising the steps of:

charging a charge storage device;

sensing a patient-dependent electrical parameter; and

discharging the charge storage device through at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device, and through a variable impedance connected between the charge storage device and one of the electrodes; and

controlling the impedance of the variable impedance during discharge of the charge storage device based on the sensed patient-dependent electrical parameter,

wherein the variable impedance comprises a plurality of discrete resistors and the step of controlling the variable impedance comprises selectively providing at least one path for flow of electric current from the charge storage device through a subset of the plurality of resistors to one of the discharge electrodes.

52. The method of claim 51 wherein the step of providing the path for flow of electric current comprises selecting different subsets of the resistors during different portions of discharge of the charge storage device so as to produce a rectilinear current waveform.

53. The method of claim 51 wherein the resistors are connected together in series and the step of providing the path for flow of electric current comprises shorting out resistors not in the subset through which the path extends.

BACKGROUND OF THE INVENTION

This invention relates to electrotherapy circuits and more particularly relates to external defibrillators that apply defibrillation shocks to a patient's heart through electrodes placed externally on the patient's body or externally on the patient's heart during surgery.

Normally, electrochemical activity within a human heart causes the organ's muscle fibers to contract and relax in a synchronized manner. This synchronized action of the heart's musculature results in the effective pumping of blood from the ventricles to the body's vital organs. In the case of ventricular fibrillation (VF), however, abnormal electrical activity within the heart causes the individual muscle fibers to contract in an unsynchronized and chaotic way. As a result of this loss of synchronization, the heart loses its ability to effectively pump blood.

Defibrillators produce a large current pulse that disrupts the chaotic electrical activity of the heart associated with ventricular fibrillation and provide the heart's electrochemical system with the opportunity to re-synchronize itself. Once organized electrical activity is restored, synchronized muscle contractions usually follow, leading to the restoration of effective cardiac pumping.

The current required for effective defibrillation is dependent upon the particular shape of the current waveform, including its amplitude, duration, shape (i.e., sine, damped sine, square, exponential decay), and whether the current waveform has a single polarity (monophasic) or has both positive and negative polarity (biphasic). It has been suggested that large defibrillation currents may cause damage to cardiac tissue, however.

It is known to construct an external defibrillator that can sense patient impedance and can set the durations of the first and second phases of a biphasic waveform as a function of the patient impedance. An example of such a defibrillator is described in PCT Patent Publication No. WO 95/05215. Fain et al., U.S. Pat. No. 5,230,336 discloses a method of setting pulse widths of monophasic and biphasic defibrillation waveforms based on measured patient impedance. Kerber et al., "Advance Prediction of Transthoracic Impedance in Human Defibrillation and Cardioversion: Importance of Impedance in Determining the Success of Low-Energy Shocks," 1984, discloses a method of selecting the energy of defibrillation shocks based on patient impedance measured using a high-frequency signal.

It is also known to construct a defibrillator with a safety resistor in the defibrillation path (PCT Patent Publication No. WO 95/05215). Before application of a defibrillation waveform to a patient, a test pulse is passed through the safety resistor while a current sensor monitors the current. If the sensed current is less than a safety threshold representative of a short circuit, the safety resistor is removed and the defibrillation waveform is applied to the patient.

It is also known, in an implantable defibrillator, to use a biphasic waveform having a first phase consisting of multiple truncated decaying exponentials that form a sawtooth approximation of a rectilinear shape (Kroll, U.S. Pat. No. 5,199,429). This is accomplished by charging a set of energy storage capacitors and then successively allowing individual capacitors to discharge during the first phase, thereby creating the sawtooth pattern in the output current of the circuit. A more recent patent, Kroll, U.S. Pat. No. 5,514,160, describes a biphasic waveform, in an implantable defibrillator, having a rectilinear-shaped first phase created by placing a MOSFET current limiter in the defibrillation path. This patent states that the grossly non-linear current limiter looks like a small and declining resistance to the capacitor. Also, Schuder et al., "Transthoracic Ventricular Defibrillation in the 100 kg Calf with Symmetrical One-Cycle Bidirectional Rectangular Wave Stimuli" describes the use of biphasic waveforms having rectilinear first and second phases to reverse ventricular fibrillation in calves. Stroetmann et al., U.S. Pat. No. 5,350,403, discloses a waveform having a sawtooth ripple that is formed by periodically interrupting a non-continuous discharge of a charging circuit.

SUMMARY OF THE INVENTION

The invention features an electrotherapy circuit for administering to a patient a current waveform such as a defibrillation waveform. The electrotherapy circuit includes a charge storage device, at least two discharge electrodes connected by electrical circuitry to opposite poles of the charge storage device, a variable impedance connected between the charge storage device and one of the electrodes, a sensor that senses a patient-dependent electrical parameter (such as a patient impedance sensor), and a control circuit. The control circuit is connected to the sensor and the variable impedance and controls the variable impedance during discharge of the charge storage device based on a patient-dependent electrical parameter (such as the patient impedance) sensed by the sensor.

By sensing patient impedance and choosing the appropriate impedance to include in the defibrillation path, it is possible to limit the difference in the peak current that passes through a low-impedance patient as compared with a high-impedance patient. Thus, the current is made more constant over a range of patient impedances, and the electrotherapy circuit provides effective defibrillation while maintaining controlled current levels to reduce any possibility of damage to heart, skin, and muscle tissue.

In preferred embodiments the variable impedance includes a set of resistors connected together in series. During discharge of the charge storage device, the resistors are successively shorted out, thereby creating a sawtooth approximation to a rectilinear shape in the output current (output decays and then jumps up every time a resistor is shorted out).

In preferred embodiments the control circuit decides, based on the patient impedance sensed during an initial sensing pulse portion of the discharge of the charge storage device, how many (if any) resistors to include in the defibrillation path at the beginning of a therapeutic discharge portion of the discharge of the charge storage device (e.g., at the beginning of a biphasic defibrillation waveform). This may mean, depending on the sensed patient impedance, that the current level steps up from the sensing pulse to the beginning of the biphasic defibrillation waveform. Once the biphasic defibrillation waveform begins, the resistors that are present in the defibrillation path are successively shorted out to create a sawtooth pattern and compensate for the decreasing capacitor voltage.

The invention provides an improved, low-cost way of creating a biphasic waveform having a rectilinear first phase. Resistors are relatively inexpensive as compared with capacitors, and a total of N steps in resistance values can be obtained with log.sub.2 N resistors, as opposed to N capacitors, simply by connecting the resistors in series in a binary sequence (1-2-4-etc.). Because resistors are used instead of capacitors, no circuitry is required to equalize voltages on capacitors upon recharge or to prevent reversal of voltages on capacitors.

Certain embodiments include a variable resistor stage that tends to smooth out the sawtooth pattern. The variable resistor stage is a circuit that is reset to its maximum resistance value every time one of the fixed-value resistors is shorted out and then decreases to zero over the time interval before the next resistance step reduction.

Another advantage of the invention is that the resistors in the defibrillation path inherently protect against possible short circuits.

We believe that the use of a waveform having a substantially rectilinear positive phase tends to provide a lower threshold of average current required for effective defibrillation, and tends to avoid damaging the patient's tissue even if the total energy applied to the patient is relatively high. We also believe that any sawtooth ripple in either phase of the waveform should preferably have a height less than about one-quarter of the average height of the phase, and more preferably less than about one-sixth of the average height of the phase, in order to further minimize the threshold of average current required for effective defibrillation and