Pacemaker comprising a back-up pulse generator

Abstract: A back-up pulse generator, for a pacemaker having a first energy storing means (C1) for storing energy to be delivered in a stimulation pulse and connectable to means for conducting the stimulation pulse to the heart (2), in which the back-up pulse generator comprises back-up pulse energy storing means (C3) for emitting a back-up pulse (Pb) after a failure to sense an evoked response from a preceding stimulation pulse (Ps) wherein the back-up pulse energy storing means (C3) substantially permanently stores energy at a higher voltage than the energy stored in the first energy storing means (C1).

Description: The invention relates to pacemakers comprising back-up pulse generators. Pacemakers stimulate the heart by means of stimulation pulses produced in a pulse generator. In the case of an implantable pacemakers the pulse generator uses energy from the pacemaker battery to charge up a stimulation circuit containing energy storing means, usually in the form of one or more capacitors. The charging is under the control of a control circuit, which controls the voltage supplied to the energy storing means. Control means also control when the stimulation circuit will discharge a stimulation pulse, the pulse current amplitude and the pulse duration. In order to extend the battery life of pacemakers as long as possible, it is desirable to use the lowest possible stimulation pulse output energy which results in heart capture. Pacemakers are known which have automatic output regulation, that is, a system for testing for the lowest possible stimulation pulse output energy which results in heart capture. The energy in a pulse is changed by varying the pulse current and the pulse duration while the pulse initial voltage, that is the voltage that is stored in the energy storing means, is maintained at a programmable amplitude, for example 1.5 volts. The system has means for detecting heart contractions. In order to use the stimulation electrodes to detect the evoked response, it is usual to use a compensating pulse after the stimulation pulse, which is transmitted in the opposite direction to the stimulation pulse to neutralise the electrodes. When a stimulation pulse has been discharged to the heart and a successful capture results, the contraction is detected and the system uses an algorithm to decide if the stimulation energy in subsequent pulses is to be reduced. If a pulse does not result in capture, then the algorithm can decide that the stimulation energy in subsequent pulses should be increased. Some pacemakers have means which not only test for successful capture but also, in the event of loss of capture, are capable of generating a back-up pulse shortly after the failure to sense an evoked response from a preceding stimulus. The back-up pulse is supplied from the stimulation circuit, which contains a stimulation capacitor. In order to ensure capture, the energy supplied in the back-up pulse is discharged at the maximum permissible voltage, usually around 4.5 volts. Thus immediately after each stimulation pulse has been discharged, the stimulation capacitor is charged to its maximum voltage of around 4.5 volts, so as to be able to produce a back-up pulse if one is required. If no back-up pulse is required or if the voltage of the energy remaining in the capacitor after a back-up pulse has been discharged is above the level programmed for the next stimulation pulse, typically 1.5 volts, then some of the stored energy in the capacitor has to be dumped to lower the voltage of the energy stored in the capacitor to the programmed stimulation voltage. This wastes energy which is detri-mental to battery life. A pacemaker which can generate a back-up pulse as described above is known from US-A-4,878,497. Document US-A-3 920 024 discloses a device according to the preamble of claim 1. An object of the invention is to provide a high voltage back-up pulse as well as lower voltage stimulation pulses with a minimum waste of energy. The invention is defined in claim 1. The invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is an example of a pacemaker output stage which includes a back-up generator in accordance with the invention, Figure 2 depicts a stimulation pulse, Figure 3 depicts a stimulation pulse followed by a heart contraction, Figure 4 depicts a stimulation pulse followed by a back-up pulse. In one embodiment of the invention, shown in Figure 1, the output stage 1 of the pacemaker has, in addition to the usual stimulation capacitor C1 and compensation capacitor C2, at least one back-up pulse capacitor C3. In order to stimulate the heart 2, the stimulation capacitor C1, which typically has a capacitance of 10 microfarads, is charged by a charge pump to a voltage determined by the physician or the manufacturer, which could be, for example, 1.5 volts. When the pulse is to be sent to the heart 2, switch S1 is closed and current flows through the output circuit to the heart via the compensation capacitor C2. Compensation capacitor C2 has a capacitance of typically 5 microfarads, and it is charged by the current passing through it. As shown in Figure 2, after the programmed pulse duration t1 has been reached, for example 0.5 milliseconds, switch S1 is opened and current ceases to flow. After a delay t2 of typically 1 millisecond, switch S2 is closed which causes the compensation capacitor C2 to discharge a pulse Pd of opposite polarisation to that of the stimulation current through the heart for a duration t3 of typically 10 milliseconds. As shown in Figure 3, if the stimulation pulse Ps causes the heart 2 to contract, then a signal H corresponding to the contraction will be detected by the contraction detector means well within a period t4 of 45 milliseconds. If no contraction is sensed in this period, as shown in Figure 4, then a back-up pulse Pb is required. The back-up pulse capacitor C3 is used to supply the back-up pulse and is kept charged by the pacemaker battery via a charge pump to the programmable maximum permissible voltage, typically 4.5 volts. Back-up pulse capacitor C3 typically can have a capacitance similar to that of the stimulation capacitor C1. When the pacemaker requires a back-up pulse to be sent to the heart 2, it closes the back-up pulse switch S3 which allows the energy stored in the back-up pulse capacitor C3 to start to flow to the heart 2 via compensation capacitor C2. The switch S3 is opened and the flow of energy to the heart 2 is stopped after a predetermined pulse has been sent. After a delay of typically 1 millisecond, switch S2 is closed which causes the compensation capacitor to discharge a pulse Pdb of opposite polarisation to that of the back-up current through the heart for a duration of typically 10 milli-seconds. The back-up pulse capacitor C3 is then recharged by the charge pump to the maximum permissible voltage and the stimulation capacitor C 1 is charged to the programmed stimulation voltage ready for the next stimulation. The stimulation capacitor C1 only produces the stimulation pulse and is charged to the programmed voltage, typically 1.5 volts. As the stimulation capacitor C1 is no longer used to produce a back-up pulse, it is no longer necessary to charge it to the maximum voltage after every stimulation pulse; it is just charged to the programmed stimulation voltage. This avoids the requirement to dump energy from the output capacitor before the following stimulation pulse and hence reduces the waste of energy. In another embodiment of the present invention the back-up pulse energy storing means is a battery having a voltage greater than 1.5 volts, perferably 4.5 volts. In a further embodiment of the invention the back-up pulse energy storing means is a rechargable battery or an accumulator. In yet another embodiment of the invention the back-up pulse energy storing means is connected in series with the stimulation pulse energy storing means. Examples of this embodiment are conceivable in which the back-up pulse energy storing means can be two capacitors in series each charged with energy at 1.5 volts or one capacitor charged to eg 3.0 volts. The stimulation pulse energy storing means could be charged to eg 1.5 volts. Thus when a back-up pulse is required the combined back-up pulse voltage from the series connected back-up pulse and stimulation pulse energy storing means will be the sum of the voltages eg 4.5 volts. Although the examples above have described stimulation pulses which have a voltage of 1.5 volts and back-up pulses with voltages of 4.5 volts, any suitable voltages and pulse durations can be used which fulfill the purpose of stimulating the heart. Thus the ratio between the back-up pulse voltage and the stimulation pulses voltage can be any suitable ratio and is not intended to be limited to a ratio of 3:1.

Claims: Pacemaker having a first energy storing means (C1) for storing energy to be delivered in a stimulation pulse and connectable to means for conducting the stimulation pulse to a heart (2), said pacemaker comprises a separate back-up pulse generator, characterized in that said separate back-up pulse generator has a second energy storing means which is a back-up pulse energy storing means (C3) for emitting a back-up pulse (Pb) after a failure to sense an evoked response from a preceding stimulation pulse (Ps) wherein the back-up pulse energy storing means (C3) substantially permanently stores energy at a higher voltage than the energy stored in the first energy storing means (C1), and in that when a back-up pulse is required the back-up pulse energy storing means (C3) is coupled to be partially or wholly discharged through the means for conducting the stimulation pulse to the heart (2). Pacemaker according to claim 1, characterized in that the back-up pulse energy storing means (C3) is coupled in parallel with the first energy storing means (C1). Pacemaker according to claim 1 or 2, characterized in that the back-up pulse energy storing means (C3) comprises one or more capacitors (C3). Pacemaker according to claim 1 or 2, characterized in that the back-up pulse energy storing means (C3) comprises one or more batteries. Pacemaker according to any of the preceding claims, characterized in that the back-up pulse energy storing means contains energy stored at at least 4.5 volts.

Inventors: ANDERSSON PETER, HOLMSTROEM NILS, SKOG GUNBRITT, ANDERSSON, PETER, HOLMSTROEM, NILS, SKOG, GUNBRITT, HOLMSTRÖM, Nils
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1996
Main classification: A61N1
IPC codes: A61N1