Variable Rate Pacemaker, Counter-controlled, Variable Rate Pacer

Abstract

A variable rate pacer having two flip-flops for defining four possible states. Each state determines the magnitude of a charging current which in turn determines the rate of the pacer. The flip-flops, arranged in a counter configuration, are cycled by a monostable magnetic reed switch which is pulsed by placing an external magnetic field in the vicinity of the patient's chest. The magnetic reed switch is used only for cycling the counter to establish the proper rate of operation; the reed switch is not required for maintaining the charging current once it is established. Reliability of operation is improved because, unlike the prior art, mechanical switches are not required for maintaining the selected charging current level.

Description

This invention relates to pacers, and more particularly to a variable rate pacer whose rate of operation can be adjusted after implantation in a patient.

Early pacers generally operated continuously at a fixed rate. Following the use of this type of pacer for many years, the demand pacer was introduced. In a demand pacer, electrical heart-simulating impulses are provided only in the absence of natural heartbeats. If only a single natural heartbeat is absent, only a single electrical impulse is generated. If more than one natural heartbeat is missing, the pacer fills in beats at the pacer rate as long as the natural beats fail to occur. No matter how many electrical stimuli are generated, they occur at essentially the same time spacing from each other and from previous natural heartbeats. The result is an overall "integrated" operation, i.e., a mutually exclusive cooperation of natural heartbeats and stimulating impulses. The demand pacer of this type is disclosed in U.S. Pat. No. 3,345,990 issued on Oct. 10, 1967 to Barouh V. Berkovits.

Generally, a demand pacer is primed to generate an impulse at a predetermined time after the last natural heartbeat. If another heartbeat occurs during the timing interval of the pacer, an impulse is not generated and the timing period starts all over again. On the other hand, if a natural heartbeat does not take place by the end of the timing period a stimulating impulse is generated. For the proper operation of a demand pacer, the pacer circuitry must determine if a natural heartbeat has occurred. The largest magnitude electrical signal generated by the heart activity is the QRS-complex corresponding to a ventricular contraction. To determine whether a natural heartbeat has occurred, an electrode is generally coupled to a ventricle. Since in most cases ventricular stimulation is required, the same electrode can be used for both stimulating the ventricles and detecting a natural heartbeat, as disclosed in the aforesaid Berkovits patent.

In the presence of noise, erroneous operation of a demand pacer of this type can take place. The noise may result in the generation of an electrical signal on the ventricular electrode, and the pacer circuitry may treat this noise as indicative of a natural heartbeat and inhibit the generation of a stimulating impulse even if one is required. In the copending application of Barouh V. Berkovits, Ser. No. 727,129 filed on Apr. 11, 1968, which has matured into U.S. Pat. No. 3,528,428, an improved demand pacer is disclosed. In this improved demand pacer, in the presence of the noise the pacer timing period is not interrupted. Continuous stimulating impulses are generated, even if they are not required. It is better to provide an impulse even if it is not required than it is not to provide an impulse if it is required.

In the pacer disclosed in the above-identified copending Berkovits application, there is a single switch and two potentiometers which can be manually adjusted to change the pacer operation. Depending on the setting of the switch, the pacer operates in either a continuous mode or a demand mode. The first potentiometer controls the width of each generated pulse. The second potentiometer controls the interpulse interval. If the switch and the potentiometers cannot be adjusted after they are implanted in the patient, it is apparent that the pacer can operate in only one of the two possible modes, and with only a single pulse width and only a single interpulse interval.

In some demand pacers marketed in recent years there is provision for switching between the continuous and demand modes of operation without requiring access to the pacer after it is implanted. The switch described above consists of a monostable magnetic reed. When a magnetic pole of either polarity is placed adjacent to the patient's chest, the reed is closed and the pacer operates in the continuous mode. When the magnet is removed, the reed opens and the pacer functions in the demand mode. One of the most important reasons for switching to the continuous mode is to allow the physician to check the pacer operation. If the patient's heart is beating normally, no pulses are generated by the pacer. Even if the patient is monitored by an electrocardiographic machine, there is no way to determine that the pacer is still functional--if the patient's heart is beating normally, there are no pacer pulses in the ECG-signal. However, when the magnet controls the closing of the reed switch, the pacer pulses appear in the ECG-signal and the physician can determine that the pacer is still functional and will pulse properly if and when it is required to do so.

With either a demand or continuous pacer, it is often necessary to adjust the rate of operation after implantation. Of course, it would be most desirable to accomplish a change in rate without requiring physical access to the pacer. Magnetic reed switches can be used for this purpose. An example of the use of a magnetic reed switch for controlling the rate of a demand pacer is disclosed in the copending application of Barouh V. Berkovits, Ser. No. 862,695 filed on Oct. 1, 1969.

The magnetic reed switches which have been used in the prior art to control the rate of a pacer have been of the bistable type. The magnetic reed is set in one or another of two states depending on the polarity of the magnetic field brought into the vicinity of the patient's chest. Each state of the reed controls a different rate. For example, in the Berkovits pacer disclosed in application Ser. No. 862,695, a resistor in the pacer-timing circuit is shorted out when the reed switch is closed, and it is placed in the circuit to decrease the rate when the reed switch is open. In general, a problem encountered with the use of bistable magnetic reed switches, or other types of mechanical switches, in this manner is one of reliability. It has been found that if a mechanical contact is required to maintain a selected rate, shock and vibration can cause inadvertant changes. This is true not only of bistable magnetic reed switches, but also of many other mechanical contact switches used for the same purpose. It is apparent that it is highly undesirable for the pacer rate to accidentally change after setting by the physician.

It is a general object of my invention to provide a variable rate pacer (of the demand or continuous type, although the illustrative embodiment of the invention is disclosed in the context of a demand pacer) whose rate of operation can be changed, with the selected rate being independent of the state of a mechanical contact.

It is another object of my invention to provide a variable rate pacer whose rate of operation is not affected by mechanical shock or vibration, and whose rate can be adjusted without requiring physical access to the pacer.

In most pacers, the interpulse interval is determined by an RC-timing circuit. A capacitor charges from a voltage source through a resistor, the charging rate being dependent upon the magnitude of the resistor. In the illustrative embodiment of my invention, the capacitor is charged directly from a constant current source, the magnitude of the constant current delivered by this source is dependent upon the state of a two-bit counter. The counter can be set in any one of four states, and thus four different pacer rates are possible. The counter is constructed of bistable electronic circuits which do not rely upon mechanical contacts for maintaining them in particular states once they are set.

Coupled to the counter is a monostable magnetic reed switch. The switch is normally open. When a magnetic field is brought into the vicinity of the patient's chest, the switch closes. The closing of the switch pulses the counter to advance its state. This in turn results in a change in pacer rate. Once the counter is set in a particular state, this state is maintained independent of the condition of the reed switch. When the magnetic field is removed, the reed switch opens but the counter state and current level remain the same. Because the pacer rate is not maintained by the reed switch (the reed switch remains open), the pacer rate is not affected by mechanical shock or vibration. Of course, it is possible for other types of devices to be used for pulsing the electronic circuits to change their states as required. However, it has been found that a monostable magnetic reed switch is particularly advantageous for use as the rate-changing control mechanism.

It is possible to include two monostable magnetic reed switches in a pacer, one for controlling a change in the pacer rate, and another for controlling the continuous operation as described in the two above-identified Berkovits applications. In such a case, if both reeds close under the influence of a magnetic field, the pacer would switch to the continuous mode at the same time that its rate is changed. In most case this is of no concern because as soon as the proper rate is established the pacer switches back to the demand mode. However, it is also possible to control the closing of only one of the two switches at any time by placing them in perpendicular orientations within the pacer. In such a case a magnetic field in one direction will close only one of the reeds and a magnetic field in another direction will close only the other.

It is a feature of my invention to provide a multistate electronic circuit which is maintained in any selected state independent of mechanical contacts for controlling the pacer rate.

It is another feature of my invention to provide a pulsing element which may be operated without direct access thereto for changing the state of the electronic circuit.

It is a more particular feature of my invention, in the illustrative embodiment thereof, to provide a monostable magnetic reed switch for the pulsing element.

Further objects, features and advantages of my invention will become apparent upon a consideration of the following detailed description in conjunction with the drawing, in which:

FIG. 1 is the same as FIG. 1 in the copending application of Barouh V. Berkovits, Ser. No. 727,129 which has matured into U.S. Pat. No. 3,528,428, and depicts a demand pacer;

FIG. 2 depicts a typical electrocardiogram; and

FIG. 3 depicts the illustrative embodiment of the present invention.

Referring to the pacer of FIG. 1, electrodes E1 and E2 are implanted in the patient's heart, electrode E2 being the neutral electrode and electrode E1 being positioned to stimulate the ventricles. When switch S is open, current flows between electrodes E1 and E2 to stimulate the ventricles only when an electrical stimulus is required.

Capacitor 65 serves to provide a source of current when an impulse is required. At that time, transistor T9 conducts and the capacitor discharges through the electrodes. Capacitor 57 charges through potentiometers 35 and 37 until the voltage across it causes transistors T7 and T8 to conduct. At that time, capacitor 57 discharges through potentiometer 37 and transistors T7 and T8, transistor T9 conducts, and an impulse is delivered to the patient's heart from capacitor 65. The setting of potentiometer 37 controls the time taken for capacitor 57 to discharge, that is, the width of each impulse. The setting of potentiometer 35 (along with the setting of potentiometer 37) controls the time required for capacitor 57 to charge to that level which causes conduction in transistors T7 and T8, that is, the interpulse interval. Ordinarily, in the absence of conduction of transistor T6, capacitor 57 would continuously charge and discharge, and impulses would be supplied to the patient's heart at fixed intervals determined by the setting of the potentiometers.

Electrode E1 is coupled over conductor 11 to the base of transistor T1 through capacitor 17. A representative ECG-trace is shown in FIG. 2, and transistors T1 and T2 amplify the R-wave signal on electrode E1 resulting from a ventricular contraction. (Excessive signals are shorted through Zener diode 67 to prevent damage to transistor T1.) Following detection of an R wave, a positive pulse is delivered to the base of transistor T6. Transistor T6 turns on and capacitor 57 discharges through it. Thus although the capacitor was previously charging to the level which would have resulted in the generation of an impulse, it is discharged and a new timing interval begins. This arrangement insures that an impulse is not generated if a natural heartbeat has occurred. The timing interval is such that impulses are generated with an interpulse interval slightly in excess of the desired natural interbeat interval. Only if a natural heartbeat is missing is a stimulating impulse generated.

The remaining transistors in the circuit serve to prevent conduction of transistor T6 in the presence of noise. In the presence of noise it would otherwise be possible for transistor T6 to conduct and prevent the generation of an impulse even though one is required. For this reason, when the pacer detects extraneous noise, transistor T6 is prevented from operating and impulses are delivered at a fixed rate. A more complete description of the operation of the circuit of FIG. 1 is set forth in the above-identified Berkovits U.S. Pat. No. 3,528,428.

With switch S open, as shown, the pacer operates in the demand mode as just described. However, with switch S closed, the base of transistor T6 is held at the potential of conductor 9. In such a case, pulses transmitted through capacitor 53 do not turn on the transistor. Capacitor 57 is not discharged through transistor T6 and each time the capacitor voltage rises to the point where transistors T7 and T8 conduct, a stimulating pulse is generated. The pacer thus operates in the continuous mode.

The illustrative embodiment of my invention is shown in FIG. 3. The circuit is identical to that of FIG. 1 except for the elements shown within heavy outlines--switch S and box 35'. The left half of the circuit of FIG. 1 is not repeated on FIG. 3, it being understood that to the left of transistors T3 and T5 the two circuits are identical. Switch S is shown as a monostable magnetic reed switch. The switch includes two reeds encased in a glass enclosure. Such a monostable magnetic reed switch has been used in pacers for several years. When a magnet is brought into the vicinity of the monostable magnetic reed switch, the two reeds engage if the magnet flux is parallel to them. As long as the magnet is held in position, the base of transistor T6 is shorted through the reed switch to conductor 9 and the pacer operates in the continuous mode. As soon as the magnet is removed, the switch opens and the pacer reverts to the demand mode of operation.

A departure from the circuit of FIG. 1 is the substitution of box 35' for potentiometer 35. In FIG. 1, potentiometer 35 is connected at one end to the junction of resistor 33 and battery 7, and at the other end to the junction of potentiometer 37 and the emitter of transistor T7. Box 35' on FIG. 3 has three input conductors, two of which are connected between the same two junctions. The third input to the box couples the base of transistor T14 to the junction of batteries 5 and 7. In the circuit of FIG. 1, potentiometer 35 serves to control the current (an exponential waveform) which flows from battery 7 to capacitor 57. In the circuit of FIG. 3, box 35' serves as a constant current source which delivers current through potentiometer 37 to capacitor 57. (It is also possible to connect the collector of transistor T14 directly to capacitor 57 so that potentiometer 37 is not in the charging current path.)

The constant current is delivered through potentiometer 37 to capacitor 57 from the collector of transistor T14. The current flows through the potentiometer and the capacitor, and through resistors 61 and 63 to ground conductor 9. The charging current is small so that the voltage developed across resistor 63 is small enough to prevent conduction of transistor T9. It is only when capacitor 57 discharges through transistors T7 and T8 that the current through resistor 63 is large enough to turn on transistor T9.

There are two requirements for the current source represented by box 35'. The first is that the current source function over the entire voltage range of capacitor 57. The maximum voltage across the capacitor is that which fires transistors T7 and T8. Capacitor 57 is connected at one end through resistors 61 and 63 to ground conductor 9. The voltage across resistors 61 and 63 during charging of the capacitor is negligible so one end of the capacitor may be considered to be grounded during charging. The base of transistor T7 is held at a potential V.sub.bb determined by batteries 3 and 5. Transistor T7 conducts when its base-emitter voltage is 0.4 volts. Consequently, the maximum voltage across capacitor 57 is equal to V.sub.bb + 0.4 volts. The constant current source must function over the voltage range from 0 to V.sub.bb + 0.4 volts.

The second requirement of the constant current source is that the voltage at the emitter of transistor T14 not change when transistors T7 and T8 conduct. If the voltage changes, it is possible for the bistable circuits within box 35' to be switched. To prevent changes in the charging current level, the voltage at the emitter of transistor T14 should not change even when capacitor 57 discharges through transistors T7 and T8.

The base potential of transistor T14 is held constant by batteries 3 and 5 and is the same as the base potential of transistor T7, V.sub.bb. Transistor T14 starts to conduct when its emitter voltage is approximately 0.4 volts greater than its base voltage. Although an emitter-base forward bias of 0.4 volts is sufficient to cause the transistor to start conducting, the emitter-base voltage drop is 0.5 volts when the transistor operates linearly and is conducting the charging current. Since the base of transistor T14 is held at a constant potential of V.sub.bb volts, the emitter of transistor T14 is held at a constant potential of V.sub.bb volts, the emitter of transistor T14 is at a potential 0.5 volts greater than the base potential essentially independent of the magnitude of the emitter current. The collector current equals the emitter current multiplied by the parameter .alpha. of the transistor (typically, a value of 0.99). Transistor T14 functions as a current source with the emitter potential being greater than the base potential by 0.5 volts only if the emitter-collector voltage drop is greater than approximately 0.1 volts, that is, only if the collector voltage does not exceed V.sub.bb + 0.4 volts. (Although transistor T14 functions as a constant current source independent of the emitter-collector voltage drop, it is necessary that there be a drop.) Since transistors T7 and T8 turn on to discharge capacitor 57 when the emitter voltage of transistor T7 rises to V.sub.bb + 0.4 volts, it is apparent that the collector voltage of transistor T14 cannot rise higher than that value which would prevent the transistor from operating as a constant current source. Thus transistor T14 functions as a constant current source over the entire voltage range of capacitor 57. Furthermore, the emitter impedance of transistor T14 is relatively small compared with resistors 70-73 which, as will be described below, determine the emitter current of transistor T14. The emitter of transistor T14 thus appears as a voltage source to the counter circuitry in box 35', that is, the transistor conducts any of the four possible currents without any changes in its emitter potential. Transistor T14 thus prevents spurious switching of the bistable circuits within box 35' as a result of the operation of the timing circuit.

Transistor T14 functions as a current summer; the emitter current is the sum of the currents flowing into common bus 97 from the conducting ones of transistors T10-T13, all of the emitter terminals of these transistors being connected to the emitter of transistor T14. Transistors T10 and T11 form a first flip-flop and transistors T12 and T13 form a second flip-flop. The four collector resistors 70-73 are connected to respective transistors. The collector of transistor T10 is coupled to the base of transistor T11 through resistor 78, and the collector of transistor T11 is coupled to the base of transistor T10 through resistor 79. Resistors 80 and 81 serve to cross-couple transistors T12 and T13 in a similar manner. Capacitors 74-77 are speed-up capacitors which, as in known in the art, increase the speed and reliability of the flip-flop switching.

Assume, that in the flip-flop including transistors T10 and T11, transistor T10 is conducting and transistor T11 is off. With a 0.1-volt drop across the collector and emitter of transistor T10, and no current flowing through resistor 78, the base-emitter drop across transistor T11 is similarly 0.1 volts. Since a 0.4-volt drop is necessary to cause the transistor to conduct, the transistor remains off. Current flows from battery 7 through resistors 71 and 79 to the base of transistor T10 to maintain the transistor conducting.

With transistor T10 conducting and its collector at 0.1 volts relative to the potential of bus 97, the cathode of diode 86 is similarly held at this potential. The base-emitter drop of transistor T10 is 0.5 volts, and consequently the cathode of diode 84 is held at a potential of 0.5 volts relative to the potential of bus 97. Application of a negative pulse to the anode of either diode has no effect since the diode does not conduct when the anode-cathode drop is negative. In order for either diode to conduct, its anode must be 0.5 volts higher in potential than its cathode. (Both anodes are held at the potential of bus 97 due to their connection to the bus through resistor 85.) Since the reverse bias across diode 84 is 0.5 volts, diode 84 will not conduct until a positive pulse greater that 1 volt is applied to its anode. Since the reverse bias across diode 86 is only 0.1 volts, this diode will conduct when a positive pulse exceeding 0.6 volts is applied to its anode. Consequently, a positive pulse transmitted through capacitor 83 with a magnitude between 0.6 volts and 1 volt is steered through diode 86 but not diode 84. A positive pulse transmitted through diode 86 causes transistor T11 to turn on, which in turn causes transistor T10 to turn off, thus switching the state of the flip-flop.

When magnetic reed switch S' closes, the positive potential of source 7 is applied to capacitor 83. The capacitor and resistor 85 from a differentiating circuit and thus a positive spike is applied to the anodes of diodes 84 and 86. The component values are such that the positive pulse has an amplitude greater than 0.6 volts but less than 1 volt so that the flip-flop switches state as described above. When the magnetic read switch opens, a negative pulse is applied through capacitor 83 to the anodes of the two diodes. The negative pulse, however, has no effect since it further reverse biases both diodes. Following the closing and opening of the contacts, capacitor 83 discharges through resistors 85 and 82.

With the flip-flop in the state in which transistor T11 conducts and transistor T10 is off, the cathode of diode 84 is at 0.1 volts and the cathode of diode 86 is at 0.5 volts. The next positive pulse produced by the closing of the reed contacts is steered through diode 84 to cause transistor T10 to turn on and transistor T11 to turn off. Successive positive pulses produced by the closing of switch S' switch the state of the flip-flop.

Elements 88-92 are comparable to elements 82-86 and serve to switch the state of the flip-flop comprising transistors T12 and T13 when a positive potential is applied to the junction of resistor 88 and capacitor 89. The positive potential is derived when transistor T11 turns off and its collector rises in potential. At this time, a positive step is transmitted through capacitor 87. The capacitor is provided to isolate the junction of resistor 71 and the collector of transistor T11 from the second flip-flop in order that the second flip-flop not affect the bias voltages of the first flip-flop. The second flip-flop changes state only when transistor T11 switches from a conducting to a nonconducting state. When the transistor switches from the nonconducting state to the conducting state, it is a negative voltage step which is transmitted through capacitor 87, and the resulting negative pulse transmitted through capacitor 89 simply further reverse biases diodes 90 and 92 and has no effect on the second flip-flop. As will be apparent to those skilled in the art, the two flip-flops function as a 4-state counter.

The four states can be summarized by the following conduction conditions of the transistors:

State No. T10 T11 T12 T13 __________________________________________________________________________ 1 off on on off 2 on off off on 3 off on off on 4 on off on off __________________________________________________________________________

Each closure of monostable read switch S' causes the counter state to advance, with state 1 following state 4. The reed switch is closed when a magnetic field is produced in the vicinity of the patient's chest.

The current flowing through the emitter of transistor T14 is the sum of the currents delivered to bus 97, and its magnitude depends upon the state of the two flip-flops. Four different currents may flow through the emitter of transistor T14. For the most part, the currents are determined by the magnitudes of resistors 70-73. As long as three of these resistors have different values, four distinct currents will result. As described above, the emitter potential of transistor T14 is V.sub.bb + 0.5 volts, where the base potential of transistor T14 is V.sub.bb volts. The inclusion of battery 7 in the circuit increases the potential at the upper ends of each of resistors 70-73 to V.sub.cc volts, where V.sub.cc is greater than V.sub.bb. Consequently, the potential drop across a conducting one of transistors T10-T13 in series with its collector resistor is V.sub.cc - (V.sub.bb + 0.5) volts. Assuming a collector-emitter drop of 0.1 volts, the drop across one of resistors 70-73 connected in series with a conducting transistor is V.sub.cc - V.sub.bb - 0.6 volts. Assume that this drop is 2 volts. Assume further that resistor 70 has a magnitude of 10.sup.6 ohms, resistors 71 and 72 both have a magnitude of 2(10.sup.6) ohms and resistor 73 has a magnitude of 4(10.sup.6) ohms. In such a case, and neglecting currents which flow through resistors 78-81, 2 microamperes flow through transistor T10 when it is conducting, 1 microampere flows through either of transistors T11 and T12 when it is conducting, and 0.5 microamperes flow through transistor T13 when it is conducting. Since the current into the emitter of transistor T14 is the sum of the four transistor currents, referring to the table above it is seen that when the counter is in state 1 the total current is 2 microamperes, when the counter is in state 2 the total current is 2.5 microamperes, when the counter is in state 3 the total current is 1.5 microamperes, and when the counter is in state 4 the total current is 3 microamperes.

Actually, the above analysis is only approximate because the effect of resistors 78-81 has been omitted. For example, when transistor T10 conducts a current of 2 microamperes flows through resistor 70 and the transistor to bus 97 as described. But current also flows through resistors 71 and 79, and the base-emitter junction of transistor T10 to the common bus. However, resistor 79, and the other three cross-coupling resistors, are very large in magnitude compared to the four collector resistors, and do not affect appreciably the current levels described above. In any event, what is of importance is the four current levels themselves, not the manner in which they are derived. The values of the collector and cross-coupling resistors can be selected such that the four possible emitter currents of transistor T14 are the desired values.

It should be noted that the prior art monostable reed switch S can still be used as in the past. The physician can verify that the pacer is still operational if the electrocardiogram of the patient shows that the pacer has switched to the continuous mode of operation when a magnet has been brought into the vicinity of the patient's chest. The additional switch S' can be arranged so that the same magnet controls a change in the pacer rate. If the same magnetic field closes both reed switches, the pacer will function in the continuous mode at the same time that its rate is changed; when the magnetic field is removed the pacer switches to the demand mode and continues to operate at the new rate. However, it is preferable to operate the two reed switches independently. As shown in FIG. 3, the orientations of the two monostable magnetic reed switches are perpendicular to each other. In such a case, a magnetic field in one direction will cause one of the switches to close and a magnetic field in a perpendicular direction will cause the other reed to close. In some cases it may be desirable to provide for the independent operation of the two switches. Of course, for a continuous pacer there would be no need for reed switch S in the first place and the problem of independent reed operation would not even exist.

Since the flip-flops are stable and self-sustaining, no external transmitter is required to maintain a selected rate. Nor is the mechanical reed switch necessary to maintain the selected rate; it is only used to control the switching from one rate to another. Since the human body is permeable to magnetic flux, a rate adjustment may be made simply by holding a magnet against the patient's chest. However, other techniques can be used to pulse the counter. For example, the pacer may include an RF detector, with the counter being cycled each time an RF transmitter is operated in the vicinity of the patient's chest. Similarly, instead of using flip-flops, other multistate electronic circuits could be utilized, e.g., those incorporating tunnel diodes, 4-layer diodes, etc. It will also be apparent to those skilled in the art that the counter can be constructed to have any desired number of states. A single flip-flop could be used for only two states, i.e., if only two current levels are required. If more than four states are required, more than two flip-flops could be utilized.

Although the invention has been described with reference to a particular embodiment, it is to be understood that this embodiment is merely illustrative of the application of the principles of the invention. Numerous modifications may be made therein and other arrangements may be devised without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. A pacer comprising terminal means for connection to a patient's heart and pulse-generating means for applying periodic pulses to said terminal means, said pulse-generating means including multistate electronic circuit means comprising an electronic counter for representing at least two states, normally inoperative pulsing means responsive to the detection of an external signal for changing the state of said multistate electronic circuit means, and means responsive to the state of said multistate electronic circuit means for maintaining a respective rate of operation of said pulse-generating means when said pulsing means is inoperative.

2. A pacer in accordance with claim 1 wherein said pulse-generating means comprises a capacitor-charging timing circuit, and a constant current source for supplying charging current to said timing circuit.

3. A pacer in accordance with claim 1 wherein said respective rate-maintaining means comprises a plurality of individual current-determining sources connected in parallel to and for feeding said constant current source, and means for controlling the operation of respective groups of said individual current-determining sources responsive to respective states being represented by said multistate electronic circuit means.

4. A pacer in accordance with claim 3 wherein said constant current source includes means for summing the currents from said individual current-determining sources and for isolating said timing circuit from said multistate electronic circuit means to prevent the spurious switching thereof.

5. A pacer in accordance with claim 4 wherein said pulsing means includes means comprising a monostable magnetic reed switch for advancing the state of said counter responsive to the closing of said switch.

6. A pacer in accordance with claim 5 further including means for detecting the natural beating of said patient's heart and in response thereto for inhibiting the generation of the next pulse which would otherwise be generated by said pulse-generating means, and means including an additional monostable magnetic reed switch for selectively preventing the inhibiting of the operation of said pulse-generating means, said monostable magnetic reed switch and said additional monostable magnetic reed switch being oriented in said pacer such that each of said switches can be closed independent of the other responsive to the application of a magnetic field having a respective orientation.

7. A pacer in accordance with claim 2 wherein said pulsing means includes means comprising a monostable magnetic reed switch for advancing the state of said counter responsive to the closing of said switch.

8. A pacer in accordance with claim 1 wherein said pulsing means includes means comprising a monostable magnetic reed switch for advancing the state of said counter responsive to the closing of said switch.

9. A pacer in accordance with claim 1 wherein said pulsing means includes means for making mechanical contacts.

10. A pacer in accordance with claim 9 wherein said mechanical contacts making means comprises a monostable magnetic reed switch.