Body Organ Threshold Analyzer

Abstract

A current control device having particular application as a body organ threshold analyzer is described comprising pulse current selection means by which a desired output pulse current from a pulse generating means, such as a cardiac pacer, can be selected, pulse current control means by which the pulse current can be controlled, and a pulse current sensing and correcting means by which the pulse current is sensed and when deviations from the selected current occur, sends corrective signals to the current control means.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a current control device. In particular, it relates to a current control device or threshold analyzer for controlling the output of a pulse generating means such as a body organ stimulating device and for determining the threshold requirements for organ stimulation as measured on the patient. The invention will be described for the most part as applied to cardiac pacers and cardiac thresholds analysis as it is in this area where most of the organ stimulation work has been developed. However, the invention may be readily adapted for use with stimulators for many other body organs, muscular tissue, etc.

2. Description of the Prior Art

The extension of human life by the use of implanted heart stimulating devices has been carried out with great effectiveness for approximately a decade. In order to simplify the implanting operation, in order to be sure that the heart stimulating electrodes are properly located, and particularly to insure that the implantable stimulator is in proper operating condition, it has been found desirable to measure the minimum heart stimulating impulse required by the patient and compare it to the output of the stimulating device with which he will be supplied. Several devices for making these measurements have been heretofore described in the prior art. The generic name of threshold analyzers has been used for such devices and will be used in the following discussion. In general, these threshold analyzers have measured the minimum stimulating requirement in terms of fractions of the energy output of the stimulator to be used with an individual patient. Thus, with the threshold analyzers available, there is little opportunity for obtaining statistical information on actual energy requirements. Further, when a particular stimulator-electrode combination does not appear to have a sufficient factor of safety for a reasonably long implant, there is no conclusive way with the prior art devices to determine whether it is the electrode system or the stimulator device that is at fault.

In my co-pending application Ser. No. 140,360, filed May 5, 1971, a form of threshold analyzer comprising a complete circuit for determining the peak voltage and current of a stimulating pulse is combined with a heart stimulator to enable a surgeon to make complete and exhaustive tests of the stimulating electrodes when implanted in the patient as well as enabling the surgeon to fully check out the implantable stimulator to be used with a particular patient. This form of threshold analyzer is ideal for use in large hospitals where there may be several stimulator implant operations in a week. However, for smaller hospitals where implanting operations may not be held oftener than about once per month or so or for applications where a low cost or disposable device is preferred, a less costly and less complicated device, yet one that will indicate true current values independent of the current or voltage capabilities of the generator, is needed.

In general, current controlling devices have been used extensively in the past. One such device is the iron wire ballast tube. This has the fault of being very slow in its action, requiring several minutes to reach equilibrium and hence, is not suited to pulse type operations. Certain power supply devices also embody current control means. In general, these devices are large or contain numerous components.

SUMMARY OF THE INVENTION

The present invention provides a simplified current control device having particular application as a body organ stimulator threshold analyzer means comprising: a current selection means by which the pulse current of a body organ stimulator can be selected, a current controlling means by which the pulse current can be controlled, and a current sensing means by which the pulse current can be sensed and when deviations from the selected current occur sends corrective signals to the control means. The sensing and correcting means include a transistor the forward base emitter junction potential of which serves as a reference potential against which deviations of the pulse current are measured. A threshold analyzer device in accordance with the invention is small in size and low in cost. A first advantage over prior art threshold analyzers is that the threshold current as controlled by the threshold analyzer of the invention is indicated directly in terms of true current values, i.e., milliamperes.

Another advantage of the device when used as a threshold analyzer is that it requires no external power in its operation. In addition, it is of almost universal application and is suitable for use with most conventional heart pacers presently available. Further, the current control device of the invention provides a low impedance reverse path when not conducting current in a forward direction. This is particularly advantageous when using the analyzer with a synchronous pacer in which the natural heart rhythm is sensed and returned to the pacer. The total voltage drop across the threshold analyzer of the invention is low due to the voltage reference selected so that controlled pulses therefrom have sufficient voltage to give proper stimulation in spite of limited voltage available from the implantable stimulating pulse generating means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in block form the several functional portions of the threshold analyzer of the invention; and

FIG. 2 is a complete electrical circuit of a particular embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates in block form the principal functional portions of the threshold analyzer of the invention. In this diagram, a stimulating pulse generating means, as for example a heart pacer, is indicated at 10, and a stimulating electrode already implanted in the patient is shown at 12. The heart pacer is adapted to generate a series of heart stimulating pulses at the normal pulse rate and of a strength sufficient to properly stimulate the heart of the patient. The stimulating electrode is adapted to deliver the stimulating pulses from the pacer to the heart of the patient in such a manner that the patient's heart is stimulated to beat at the rate provided by the pacer. As described above, the stimulator 10 is intended to be implanted in the patient after its performance with the implanted electrode 12 has been thoroughly analyzed by means of the analyzer.

The analyzer comprises three functional blocks, a stimulating pulse current selection means 14, stimulating current controlling means 16 and a current sensing and corrective means 18. As shown, the current selection means 14 and the current controlling means 16 are connected in series between the stimulator 10 and the electrode 12. The current sensing means 18 is connected between the current selection means 14 and the current controlling means 16. The current sensing means senses the pulse current and when the pulse current deviates from the selected current, its sends corrective signals to the current control means.

The operation of the device can be understood by following its use in a typical implantation operation as follows: a typical heart block patient, after the immediate crisis is over, is put on a temporary external heart stimulator. As soon as possible, arrangements are made for the implantation of a permanent stimulator. An implantable stimulating pulse generator is selected and an output or stimulating electrode is selected and located in a suitable cardiac location. The threshold analyzer with current selector set for maximum current is connected between the output of the stimulating generator and the electrode. In certain cases, i.e., when used with stimulators having an unpolar electrode, it is also necessary to provide a temporary return lead from the patient to the generator. This is shown at 20. At the same time that the implantable stimulating circuit is completed, it is necessary to break the circuit of the temporary external stimulator, otherwise, the heart of the patient would receive a double quantity of pulses which would, of course, be undesirable. A sufficient time is then allowed to pass for the heart of the patient to get accustomed to the stimulating pulses from the new source. When this is achieved, the surgeon, by means of the current selection portion of the analyzer, gradually reduces the pulse current. This is continued until the surgeon observes through an electrocardiograph or other means that the stimulating current is at the threshold or the minimum required to stimulate the patient.

A particular feature of the threshold analyzer of the invention is that the currents passed by the analyzer are given in terms of milliampers so that they are reproduceable and suitable for correlation. If the threshold current, as determined by the threshold analyzer, is unduly higher than expected by the surgeon, the surgeon is advised that the stimulating electrode is not properly located or that other problems exist. The threshold current as determined can be entered on the patient's medical history sheet so as to be available for future checks.

In FIG. 2, a detailed circuit diagram of one form of the threshold analyzer is shown. In this diagram 10 represents the stimulating generator and 12 the implanted electrode. An input terminal 56 to which the generator lead is externally connected connects internally with the current selection means 14 comprising a variable resistor R6. In FIG. 2, a fixed resistor R1 is shown in series with R6. The purpose of R1 is to provide a safe limit to the current capabilities of the control. Although R1 is shown as being separate from R6, R1 can be combined with R6 as for instance, by providing a stop to limit the excursion of R6. It can also be pointed out that R1 is not a necessary part of the control device and may be omitted without in any way altering its operation as a current controlling device. It is desirable that resistor R6 has a logarithmic resistance characteristic so as to cover a broad range of resistance values with approximately equal accuracy at all dial settings.

The second terminal of R6 connects to the collector of a current controlling transistor Q1 and represents the current control means 14. It is desirable that Q1 has a low saturation voltage, low emitter-base potentials, and that in general it will operate in a voltage range less than 1 volt. To this end, transistor Q1 is chosen to be an NPN germanium transistor. The three components R1, R6 and Q1 form the principal current path through the analyzer. The portion of the circuit comprising the transistors Q2 and Q3, resistors R2, R3, and R5 and capacitor C1 forms the current sensing means 18. For best results, the emitter-base potential-current relationship of transistor Q3 should have a sharp and well-defined knee. This characteristic is found with the silicon tpe transistor and therefore Q3 is by preference a silicon type of PNP construction.

A branch of this circuit comprising the emitter and collector of a transistor Q3 and resistor R5 connects between the terminals 56 and 58.

The base of transistor Q3 is connected via resistor R3 to the collector of transistor Q1. The base of transistor Q1 is connected to the collector of the transistor Q2, of PNP construction.

The requirements for this transistor are similar to transistor Q1, i.e., it should have low saturation voltage, low emitter-base potentials and in general operate in voltage ranges less than 1 volt. For this reason, Q2 is desirably of the germanium type. The emitter of transistor Q2 is connected to the collector of transistor Q1. The base of transistor Q2 is connected to the collector of transistor Q3. The resistor R2 connects the collector of transistor Q2 to the output terminal 58. A capacitor C1 in series with a resistor R4 is connected between the base and collector of transistor Q3.

Although the actual operation of this circuit is to control current pulses, for purposes of explanation, it is easier to speak in terms of continuous direct currents as the operation is the same in either case.

Current flowing through resistor R1 and R6 causes a voltage drop between point 56 and point 60. This same voltage will appear between the collector and base of transistor Q3.

It is well known that the voltage drop across the emitter junction of a silicon transistor is practically constant, regardless of the current through it and that this voltage is approximately 0.7 volts. It is to be noted that the forward base emitter junction potential of transistor Q3 is used in the present circuit as a reference potential.

If the voltage drop across the emitter junction of transistor Q3 should be greater than 0.7 volts, transistor Q3 will rapidly increase its conductivity and increase the voltage drop across resistor R5.

When the voltage drop across R5 increases, the conductivity of transistor Q2 is decreased. Since transistor Q2 is a PNP type, this increases the negative bias on Q1, and since Q1 is a NPN type transistor, this reduces its conductivity which in turn reduces the current flow through R1 and R6, until the voltage drop across the two resistors balance the normal 0.7 volts drop across Q3. This action occurs in a very short time and well within the period of a single heart stimulating pulse.

If, on the other hand, the voltage drop across the emitter junction of transistor Q3 should be less than 0.7 volts, transistor Q3 will rapidly decrease its conductivity and decrease the voltage drop across resistor R5. This will result in an increase in the current flow through Q1, again resulting in a balance between the voltage drop across R1 and R6 and the 0.7 volts drop of Q3. Thus, the net effect of resistors R1 and R6 plus transistors Q1, Q2 and Q3 is to stabilize a voltage drop across resistors R1 and R6 and by so doing hold a constant current from input terminal 56 to output terminal 58. It will also be seen that the value of the controlled current will be adjustable by the value given resistor R6.

Transistor Q3 is chosen to be a silicon device because it has a large voltage drop than germanium transistors and is therefore a more stable voltage reference. Transistors Q1 and Q2 are preferably germanium for minimum voltage drop. The germanium transistors do not react as fast as silicon. In order to match the silicon transistor reaction time to that of the germanium transistors, a retarding circuit is included. This retarding circuit is comprised of resistor R3, R4 and capacitor C1 which slow down the response time of transistor Q3 so as to match transistors Q1 and Q2. Resistors R2 and R5 serve to set the several transistors at desirable working voltages. Resistor R3 also serves to limit the curernt flow from Q3 to point 60. Without R3, the circuit may be unstable.

The device as described in detail above is designed to pass a pulse of negative electricity as generated by a negative emitting pulse generator. If by chance a positive pulse should be passed through it, the device will not be damaged but will not control the current flow. However, if the device is to be used for controlling a positive pulse, it is only necessary to connect terminal 58 to the generator and terminal 56 to the load. When so connected, the controller becomes a positive current controller.

It has been stated that transistor Q1 should be an NPN germanium type for best results. The device will operate quite successfully if Q1 is a PNP transistor provided that transistors Q2 and Q3 are NPN type. This changes the polarity of the device so that when connected as shown in FIG. 2, it will control positive currents passing from terminal 56 to terminal 58.

If transistors Q1 and Q2 are silicon, the device will function but have a higher minimum control voltage. Also, it was stated that transistor Q3 should be a silicon type for best results. The device will operate successfully even if Q3 is a germanium transistor, but with somewhat less accuracy of control. In these cases, there may be no particular need for the retarding circuit R4 and C1.

From this detailed description it will be seen that the variable resistor R6 forms a current selection means by which a desired pulse current value can be selected. Transistor Q1 with its associated circuitry is a current controlling means by which the pulse current is controlled at the value. Finally transistor Q3 and Q2 with their circuitry from a current sensing means by which the pulse current is sensed and when deviations occur, send corrective signals to the control means.

It is also to be noted that the operation of the device is quite different from a resistor. With a resistor, there is the well-known relationship I = E/R.

With the present device, the following relationships hold (once a particular current is selected): I = constant R = kE

It as been found that the reverse impedance of the circuit is considerably lower than a simple resistor having an equal current controlling capacity. This feature is very desirable when using the device as a threshold analyzer for synchronous type heart pacer. In this duty for each heart beat, a signal is fed back from the heart to the pacer, triggering the pacer and causing the stimulating pulse to then be sent to the heart of the patient. The low reverse impedance of the present device causes little alternation of the heart signal and permits the use of the analyzer with the synchronous type of pacer. With other forms of current controlling devices based upon simple resistive cir-cuits, the alteration of the feedback signal may prevent satisfactory operation of the synchronous pacers.

It may be pointed out the the calibrations of variable resistor R6 are chosen so as to control the current through the device at selected values and are indicated on the instrument in milliampere values. The range covered is approximately from 1 to 20 milliamperes.

Typical values of components used in the circuit of the invention are:

1/4W 10%: R1 -- 100 R6 -- 2.5K Pot.(external) R2 -- 20K C1 -- 0.001 mfd 1 KV ceramic R3 -- 10K Q1 -- 2N1304 R4 -- 1K Q2 -- 2N1305 R5 -- 200K Q3 -- 2N5447

the 10 components of the circuit are both small in size and low in cost. The total cost of a unit is approximately 1 percent of cost of a typical stimulator or heart pacer. Thus, the units are low enough in cost so that they can be supplied with each stimulator without being a burden to the purchaser.

Although the general and detailed description presented above depicts the use of the device solely with a heart stimulator, it is quite obvious and is included in the scope of the invention that the device is suitable to determine the threshold energy limits for other areas or organs of the human body that need artificial stimulation such as bladder, kidneys, operational muscles, etc.

Further, the current controlling circuit of the invention has novelty and utility in the non-medical electrical and electronics fields where a simple, quick-acting current control is required.

The device as described is assembled from individual components. However, it is a part of this invention that it can be made using the techniques of integrated circuitry, hybrid circuitry, etc. in order to reduce its size, reduce cost or improve reliability.

Claims

Having decribed my invention and given an example of its assembly in detail as well as a normal method of use, I hereby claim:

1. A threshold analyzer having an input terminal operably connectable to an implantable stimulating pulse generating means and an output terminal operably connectable to an implanted stimulating electrode which comprises:

a. a variable resistor having a first terminal and a second terminal, the first terminal being connected to the input terminal of the threshold analyzer;

b. a first NPN germanium transistor having a collector, an emitter and a base, the collector thereof being connected to the second terminal of the variable resistor and the emitter thereof being connected to the output terminal of the threshold analyzer;

c. a second PNP silicon transistor having an emitter, a collector and a base, the emitter thereof being connected to the input terminal of the threshold analyzer;

d. a first fixed resistor having a first terminal and a second terminal, the first terminal being connected to the collector of the second transistor and the second terminal thereof being connected to the output terminal of the threshold analyzer;

e. a second fixed resistor having a first terminal and a second terminal, the first terminal thereof being connected to the base of the second transistor and the second terminal thereof being connected to the second terminal of the variable resistor;

f. a third fixed resistor having a first terminal and a second terminal, the first terminal thereof being connected to the base of the first transistor and the second terminal thereof being connected to the output terminal of the threshold analyzer;

g. a third PNP germanium transistor having an emitter, a collector and a base, the emitter thereof being connected to the collector of the first transistor, the collector thereof being connected to the base of the first transistor and the base of the third transistor being connected to the collector of the second transistor;

h. a fourth fixed resistor having a first terminal and a second terminal, the first terminal being connected to the base of the third transistor; and

i. a capacitor having a first terminal and a second terminal, the first terminal being connected to the second terminal of the fourth fixed resistor and the second terminal of the capacitor being connected to the base of the second transistor.