Abstract:
An implantable cardiac device is described in which therapeutic pulses for application to a patient&#39;s heart are generated by a controllable current source. Thus, the amplitude, duration and/or other characteristics of the pulses can be readily selected. The device can be used to apply brachy-or-tachy-pacing pulses, cardioversion pulses or even defibrillation shocks.

Description:
BACKGROUND OF THE INVENTION 
     a. Field of Invention 
     This invention pertains to an implantable cardioversion device having circuitry for shaping and controlling precisely the therapeutic pulses delivered to a patient. Alternatively, the device may be used to induce tachycardia or fibrillation. 
     b. Description of the Prior Art 
     In the present invention the term implantable cardioversion device or ICD is used to generically cover all implantable devices capable of delivering therapeutic pulses or shocks. That is, the term is intended to cover not only devices arranged and constructed to deliver cardioversion pulses, defibrillation shocks or other similar antitachycardia therapy but pacing pulses on demand for bradycardia therapy. 
     ICDs presently available on the marked deliver either pulses of defined duration and magnitude having relatively small energy content (in the order of 50 Microjoules) or high voltage energy content of up to 40 Joules. In either case, the energy for these pulses or shocks is derived from one or more capacitors. These capacitors are first charged to a nominal voltage, and when pulses are required, they are switched to the delivering electrodes and are discharged through the body tissues. While the defibrillation pulses are applied the capacitors cannot be charged back to their nominal value. 
     A further disadvantage of the present devices is that once the capacitors are switched to the electrodes, the currents generated are strictly depending on the impedance of the body and any parasitic resistivity of the electrodes. The magnitude of these currents is not otherwise limited or controlled. 
     OBJECTIVES AND SUMMARY OF THE INVENTION 
     In view of the above disadvantages of the prior art, it is an objective of the present invention to provide an ICD which is capable of delivering shocks or other types of therapeutic pulses at a predetermined controlled current magnitude. 
     Yet another object is to provide an ICD constructed and arranged to provide cardioversion pulses. 
     Other objectives and advantages of the invention shall become apparent from the following description. 
     Briefly, an ICD constructed in accordance with this invention includes a sensor for sensing cardiac activity in a patient&#39;s heart, a signal generator generating control signals, a power supply, and a controllable current source receiving power from the power supply, and delivery current pulses through electrodes to the patient&#39;s heart. The controllable current source is activated and controlled by the commands whereby both the timing and the shape of the current pulses is precisely selected. In one specific embodiment a controlled current source configuration is used for generating the current pulses. In a second embodiment, a controlled voltage source configuration is used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of an ICD constructed in accordance with this invention; 
     FIG. 2 shows a simplified elementary diagram for a prior art charging and discharging circuit; 
     FIG. 3 shows a simplified elementary diagram of a discharge circuit in accordance with the present invention; 
     FIG. 4 shows a detailed circuit diagram of a first embodiment of the invention; and 
     FIG. 5 shows a detailed circuit diagram of a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, typically an ICD 10 consists of a housing or case 12 which holds the various electrical circuitry such as a sensor 14, a pacing pulse generator 16, a cardioversion pulse generator 18 and a defibrillation shock generator 20. Power for the generators and the other circuitry is derived from a power supply 22. A microprocessor 24 is used to control the various circuits as described in more detail below. Communication to the outside world takes place through a telemetry circuit 26. 
     The housing 12 may be used as an electrode or it may be electrically isolated. Pulses from generator 16 or 18 are sent to the heart 28 via lead 30 and intrinsic cardia signals are sensed by sensor 14. In addition, in many instances, current delivered by the electrodes may also flow from the heart through the intermediate tissues to the housing 12. 
     As shown in FIG. 2, prior to the present invention, a pulse was generated by cardioversion pulse generator 18 or defibrillator shock generator 20 as follows. First, a capacitor 40 was charged by a charging circuit 42 by closing a switch 44. (In FIG. 2, capacitor 40 may represent a plurality of capacitors hooked in parallel). It should be understood that this process may involve using a resonant tank circuit, especially if the required voltage V on capacitor 40 was relatively high, i.e. in the order of 700-vc. Next, switch 44 was opened and switch 46 was closed. Switch 46 connected the capacitor 40 to the leads and electrodes leading to the heart 28. Closing switch 46 allowed the capacitor 40 to discharge through the heart 28, schematically represented herein by resistance 48. If required, a plurality of switches arranged in a bridge could be used to provide a multiphase pulse to heart 28. However, the current through the heart had an initial peak value V/R where V was the voltage of capacitor 40 and R was the impedance of resistor 48, and decayed exponentially in a manner characteristic of RC circuits. 
     FIG. 3 shows how one or more current pulses are applied in accordance with this invention. In this FIG., charge circuit 50 charges a capacitor 52 through a diode 54. The capacitor 52 is discharged through a current source 56. Importantly, the voltage across the current source 56 is monitored by the microprocessor 24. Additionally, the circuit is also provided with a current control circuit 58, and a charge control circuit 62, all of these circuits operating under the control of microprocessor 24. The current control circuit 58 is adapted to control current source 56 so that the latter generates a current of an amplitude determined by the microprocessor 24. The current control circuit 58 turns the current source 56 on or off for a pulse duration specified by the microprocessor. Finally, the charge control circuit 58 controls the charge circuit 50, again under the control of microprocessor 24. The current from source 56 flows through the cardiac impedance 48 and then returns. 
     The microprocessor sets the level of the charge voltage to be applied to the capacitor 52 by charge circuit 50 in accordance with the therapy selected by the microprocessor. For example, for defibrillation pulses the circuit 50 charges the capacitor 52 to a very large amplitude. The charge required for cardioversion is of course much smaller. Thus, the same circuitry and same electrodes could be used to apply different types of therapies. Alternatively, different electrodes can be provided, in which case a switching network is used (not shown) to switch the current source 56 to different electrodes. 
     FIG. 4 shows an embodiment of current control circuit 58 wherein a constant voltage source is used. In this embodiment the current control circuit 58 generates a control signal V1. While V1 is low, transistor Q2 is off and the capacitor C1 is charged to about 750 Vdc. When V1 goes on, Q2 turns on and applies a constant voltage to the cardiac tissues (represented by resistor 48 having a value which has been previously determined). During this time, the capacitor Cl is slowly discharging; however, this has no effect on the voltage across 48. 
     In this configuration, a rectangular pulse, (as shown in FIG. 4) or any other type of pulse may be applied to the heart merely by shaping control signal V1 appropriately. 
     For example, if Q2 is an IGBT (Insulated Gate Bipolar Transistor) and for a value for 48 of 700 ohms, the control voltage V1 may be about 100 Vdc (peak value) to turn Q2 on. The resulting Vh applied to the heart may then be about 95 Vdc resulting in a constant current of about 1.4 amps. 
     Referring now to FIG. 5, a constant current generator could be used as described herein. In this configuration, transistor Q6 is connected with its collector in series with heart tissue 48 and its emitter in series with a load resistor R10. In this configuration, the current control 58 generates a control signal V1. When signal V1 is off, transistor Q6 is off and capacitor C1 charges up to about 750 Vdc. When signal V1 goes high (to about 25V), transistor Q6 turns on and is biased so that the current I flowing through the heart is constant. For example, R10 could be about 2 ohms resulting in a current pulse through the heart of about 10 amps. In this configuration, the voltage across the heart tissue 48 is about 25 volts lower than the voltage across capacitor C1. Again, by shaping control signal V1, the defibrillation pulse applied to heart tissue 48 may be similarly shaped to any desired form. 
     In either case the current can be shaped to any desired waveform by an appropriate waveform applied to the base of the transistors Q2 or Q10. 
     Although reference to several particular embodiments, it is to be understood that these embodiments are merely illustrative of the application of the principles of the invention. Accordingly, the embodiments described in particular should be considered exemplary, not limiting, with respect to the following claims.