Constant charge pacemaker

Abstract: A pacemaker comprises a pulse source for delivering heart stimulation pulses to at least one heart stimulation electrode. The pulse source comprises a source (2, 4) for charging a charge storage means (6) with a prescribed amount of charge for each heart stimulus. The charge storage means are then disposed to discharge a predetermined amount of charge through the stimulation electrode (8) to form a heart stimulation pulse.

Description: The present invention relates to a pacemaker comprising a pulse source for delivering heart stimulation pulses to at least one heart stimulation electrode. In biomedical therapy of heart diseases electric stimulation of the tissue is often utilized. In presence of normal spontaneous activity of the heart stimulation is inhibited or synchronized to the sensed natural event. It is important that contraction of the heart is achieved as a result of stimulation, so called stimulation capture. When a stimulation apparatus is implanted the minimum stimulation voltage for capture is normally determined. It is well-known that the threshold value of stimulation capture first is temporarily increased to a maximum during a number of weeks. It is also well-known that this threshold value will vary with time later on too. Because of these effects the stimulation level must be set rather high to secure stimulation capture. Often a stimulation voltage of twice or more the measured threshold value is selected as a standard procedure. As a result the current consumption is increased with a factor of 4 above the measured threshold value for stimulation capture, with a shortened longevity of the stimulator as a consequence. This is a serious inconvenience and it is a permanent ambition within this technical field to reduce the necessary stimulation energy and lengthen the re-operation intervals. A larger contact area with the tissue of the stimulation electrode normally results in a more stable stimulation threshold, whereas a smaller contact area of the stimulation electrode, below about 3 mm 2 , gives rise to larger spreads and variations in the threshold value, cf. eg. Stokes et al, The Mythology of Threshold Variations as a Function of Electrode Surface Area, PACE, volume 14, November 1991, Part II, pp. 1748 - 1751. On the other hand, a small electrode contact area shows a higher electrical contact impedance and consequently losses of energy in electric wires and output switches are reduced. Further, the current density seems to be one factor of major importance for obtaining stimulation capture and a high current density is obtained for a lower stimulation voltage for a small electrode contact area with the tissue. Fibrotic tissue growth and fat-cells will increase the contact impedance between electrode and tissue and results in large local variations. This effect gives rise to problems when using small electrode surfaces but is equalized over larger electrode surfaces. Higher energies will also overcome this difficulty, a higher energy can stimulate cells located a distance away and will thus "bridge over" a nonconductive layer. Implantation of a pacemaker is normally followed up by monitoring the value of the stimulation threshold, measured in voltage peak amplitude, and stimulation impedance variations, as seen from the pacemaker. This impedance is normally of the order of 500 ohm. Larger deviations in these values indicate inappropriate pacing conditions. Since the use of small stimulation electrodes after all has indisputable advantages, the interest in such electrodes has increased. Stimulation pulses are today determined by voltage amplitude and duration, sometimes by stimulation current and duration. If the contact impedance then is increased, which normally occurs when reducing the contact area, the stimulation energy is, however, decreased with an obvious risk for loss of capture. The purpose of the present invention is to solve the problem of loss of stimulation capture because of high or unstable contact impedances when using pacemakers with small lead area electrodes. This purpose is obtained with the pacemaker according to claim 1. Thus according to the present invention a pacemaker is proposed with which the problem of loss of stimulation capture at high or unstable contact impedances is overcome by delivering a predetermined amount of stimulation charge, instead of using a predetermined stimulation voltage. When stimulating with a predetermined, programmed amount of charge the output voltage will automatically be increased if the contact impedance is increased. Thus with the pacemaker according to the invention the problem of e.g. decreased conductivity at the edge of an electrode with small surface caused by fat-cells or fibrotic growth is eliminated. If a loss of capture should be detected when using a pacemaker according to the invention the predetermined stimulation charge will be increased. According to an advantageous embodiment of the pacemaker according to the invention at least two condensers are charged in parallel across the battery and then discharged in series to the charge storage means. In this way a capacitive voltage multiplier is obtained which makes it possible to get output voltages which are several times higher than the voltage of the battery. According to a further advantageous embodiment of the pacemaker according to the invention the charge storage means is charged by a so called charge pump. To obtain a constant stimulation charge it is then sufficient to have a charge pump with a maximum output voltage of two times the stimulation output voltage. The amount of charge delivered to the output capacitor is proportional to the time the charge pump is in operation and to select a certain output stimulation charge the time of operation for the charge pump after a stimulation is fixed. In a situation where the energy drawn from the energy source of the pacemaker is mainly used for heart stimulation, i.e. losses are negligible, a constant amount of energy supplied per stimulus, would result in a constant stimulation charge. According to another advantageous embodiment of the pacemaker according to the invention the charge storage means comprises a high inductance coil which discharges stored magnetic energy directly through the electrode as a stimulation pulse. The prescribed amount of charge will then be discharged and the output voltage will be automatically adapted to necessary values. Thus if the transition impedance is increased the voltage will also increase such that a sufficient current for the prescribed amount of charge is delivered. According to one more advantageous embodiment of the pacemaker according to the invention the charging source comprises a battery charging a coil which in its turn charges the charge storage means. In such an embodiment charging losses are reduced to a minimum. According to another advantageous embodiments of the pacemaker according to the invention the charge storage means comprises an output condenser and a comparator is comparing the voltage of this output condenser with a predetermined security level during the charging operation to stop the charging if the voltage of the condenser should reach this security level. As examples chosen embodiments of the pacemaker according to the invention will now be described in more detail with reference to the enclosed drawings, on which fig. 1 illustrates the principles of one embodiment of the pacemaker according to the invention, fig. 2 shows how charge storage means of the pacemaker according to the invention can be charged to a voltage exceeding the voltage of the battery, fig. 3 illustrates the principle of charge pumping used in one embodiment of the pacemaker according to the invention, fig. 4 illustrates how the output voltage of the circuit in fig. 3 is stepwise increased during the charging procedure, fig. 5 shows a coil serving as a charge storage means, and fig. 6 shows an alternative to the charge pump exhibited in fig. 3. Figure 1 illustrates the principal layout of a pulse source of the pacemaker according to the invention. The pulse source comprises a charging source having a battery 2 and charging circuitry 4 for charging a charge storage means in the form of an output condenser 6. When the condenser 6 is charged to a predetermined level, it is discharged by means of a switch 7 through a stimulation electrode 8 inserted in a heart 10. The voltage of the battery 2 is normally about 3 volts. To be able to obtain higher stimulation voltages the charging circuitry 4 can comprise a capacitive voltage multiplier or "charge pump" of the type illustrated in figure 2. During charging the condensers 12 and 14 are connected in parallel to the battery, the switches 16, 18, 20, 22 being in the position shown in figure 2. The switches 16, 18, 20, 22 are then switched in the directions of the arrows into their other positions in which the condensers 12 and 14 are connected in series for discharge to the output condenser. Thus the output condenser will be charged from the condensers 12 and 14 with a voltage which is twice the battery voltage. Of course the circuit of figure 2 can include more than two condensers thus realizing a corresponding higher voltage multiplication. As an alternative the output condenser 6 can be charged by a charge pump the principle of which is illustrated in figures 3 and 4. In this latter case the charge circuitry comprises a condenser 24 which is first connected to the battery 2 through the switch 26. Thus condenser 24 is first charged to the voltage U bat of the battery 2. The position of the switch 26 is then changed and the output condenser 6 is charged from the condenser 24 to the voltage U bat /2, the two condensers 6 and 24 then having equal voltage. The switch 26 is then switched to its first position again and the condenser 24 is again charged to the voltage U bat . The switch 26 is switched to its second position to discharge the condenser 24 to the condenser 6 until the voltages of the two condensers are the same and equal to 0,75 U bat . The condenser 24 is again charged to voltage U bat , again connected to the condensers 6 to again charge this condenser until the voltage of the two condenser are equal, 0,875 U bat , etc. until the output condenser 6 has been charged to desired level. This gradual step by step charging of the output condenser 6 is illustrated in figure 4. As appears from this figure the voltage of the output condenser 6, U out , approaches asymptomatically the voltage U bat . It should be noted that the capacitances of the condensers 6 and 24 are equal in the example described in figures 3 and 4. To avoid that the voltage on the output condenser 6 exceeds a predetermined security level a comparator (not shown) is provided to compare the output voltage with the predetermined security value. If this level is reached the charging is stopped. For stimulation with a constant charge it is in practice sufficient to have a charge pump with a maximum output voltage of twice the stimulation output voltage. The amount of charge delivered to the output condenser is then porportional to the operation time of the charge pump. The operation time of the charge pump after a stimulation is thus selected for getting the desired output stimulation charge, as the amount of charge in the output condenser is lineary increasing with the charging time. From an energetic point of view an inductive charge storage means is better than a capacitive one. In a capacitive circuit 50 % of the energy are lost in charging resistances. Use of inductances is not associated with any such losses. Charging losses will be reduced to a minimum and the efficiency will be high, about 90 % or even more. In figure 5 an inductance in the form of a high inductance coil 28, preferably with a ferrite core, can be charged to a selected level by connection to the battery through a switch 30. The magnetic energy stored in the coil 28 is then discharged through the stimulation electrode as a stimulation pulse by switching the switch 30 to its second position. The amount of charge stored in the coil 28 and transferred to the stimulation electrode is proportional to the time for which the coil 28 is connected to the battery. The current through the coil 28 is increasing approximately lineary with time, and with the charging voltage for short charging times. Alternatively for the charging of the coil 28 a fixed charging time and a variable charging voltage could be used. When using an inductance as a charge storage means you only have to check that necessary energy or charge is delivered from the coil for stimulation and during this procedure the voltage may assume those values which are needed. Thus if the transition impedance between electrode and tissue is increased the voltage will be higher to maintain a sufficient current or the predetermined stimulation charge. In figure 6 an alternative to the charge pump shown in figure 3 is disclosed wherein the condenser 24 is replaced by a coil for charging the output condenser 6. In this embodiment the inductance is not directly discharged for stimulation but is forming an output stage together with an output condenser. The coil 32 is thus charged from the battery 2 and the charge is then transferred to the condenser 6 prior to stimulation. In this way an "inductive" charge pump analogous to the capacitive charge pump described in connection with figures 3 and 4 is obtained. In this case a smaller coil can be used than if the coil is used as a charge storage means for direct stimulation as shown in figure 5. The condenser 6 can be charged in a one step or multistep procedure, the amount of charge transferred to the condenser 6 being determined by the operation time of the circuit. If the coil 32 in a multistep procedure is charged with a constant charge for each step the amount of charge transferred to the condenser 6 will again be proportional to the number of charging steps (or the time of active operation of the circuit. It is also possible to charge a plurality of coils connected in parallel to a charging source and then connect the charged coils in series and discharge them to a condenser which thus is charged to a voltage which can be considerably higher than the voltage of the charging source. In this way an inductive voltage multiplier analogous to the capacitive voltage multiplier described in connection with figure 2 is realized.

Claims: Pacemaker comprising a pulse source for delivering heart stimulation pulses to at least one heart stimulation electrode, characterized in that said pulse source comprises a source (2, 4) for charging a charge storage means (6, 28) with a prescribed amount of charge for each heart stimulus, said charge storage means being disposed to discharge a predetermined amount of charge through the stimulation electrode (8) to form a heart stimulation pulse. Pacemaker according to claim 1, characterized in that said charging source comprises a battery (2) and a charge pump (24, 26; 32) connected to said charge storage means (6). (Fig. 3 and 6) Pacemaker according to claim 1, characterized in that said charging source comprises a battery (2) charging at least two condensers (12, 14) connected in parallel across the battery, which condensers then are connected in series and discharged to the charge storage means. (Fig. 2). Pacemaker according to claim 1 or 2, characterized in that said charging source comprises a battery (2) charging a coil (32) which in its turn charges said charge storage means (6). (Fig. 6). Pacemaker according to claim 4, characterized in that a switch is provided to alternatively connect the coil (32) to the battery (2) and to the charge storage means for stepwise charging of the charge storage means (6), preferably in a multi-step procedure. Pacemaker according to any of the claims 1 - 5, characterized in that said charge storage means comprises an output condenser (6). Pacemaker according to any of the claims 1 - 3, characterized in that said charge storage means comprises a high inductance coil (28). (Fig. 5) Pacemaker according to claims 4, 5 or 7, characterized in that means are provided to fix the time of charging the coil (28; 32) to a predetermined length. Pacemaker according to claim 6, characterized in that a comparator is comparing the voltage across the output condenser (6) and stops the charging of the output condenser as the condenser voltage reaches a predetermined security level.

Inventors: EKWALL CHRISTER, EKWALL, CHRISTER
Applicants: PACESETTER AB
Application year: 1992
Main classification: A61N1
IPC codes: A61N1