Calibration Of Electrical Blood-pressure Monitoring Equipment

Abstract

Producing calibrating voltage wave forms for electrical blood-pressure monitoring equipment. Such production includes (1) the generation of a nonrectangular monopolar voltage wave form having a leading edge with twice the slope of the trailing edge, and a constant amplitude portion between such edges, and (2) from this pulse the forming of a bipolar wave form which is the first derivative of the monopolar wave form. The monopolar wave form is specially configured to approximate closely the voltage wave form generated by a conventional blood-pressure monitoring catheter as such follows normal human arterial blood pressure.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

This invention pertains to the following of arterial blood pressure in a human being, and more particularly, to a method and apparatus for accurately calibrating an electrical instrument, such as a recorder or an oscilloscope, for monitoring such pressure.

In many medical procedures it is important, or at least desirable, to be able accurately to monitor changes in arterial blood pressure in a patient. In this connection, one usually wishes to observe not only the instantaneous changes (in time) of the actual pressure, but also the first derivative of these changes whereby even minor variations in the systolic and diastolic actions of the heart can readily be detected.

Usually, the actual sensing of such blood pressure is done by means of arterial catheterization, using a catheter equipped to generate an electrical signal which reflects blood pressure changes. For example, a catheter capable of performing extremely accurately in this respect is disclosed in U.S. Pat. No. 3,710,781. Such a catheter may be connected to a buffer amplifier which serves both to isolate the patient from electrical shock, and to amplify signals from the catheter to a usable level. Such an amplifier, for instance, is disclosed fully in my prior-filed copending application, Ser. No. 269,747, filed July 7, 1972, for "Electromedical Patient Monitoring System".

However, satisfactory means has not heretofore been available for accurately calibrating the monitoring device (recorder, oscilloscope, etc.) so that information derived from such a catheter and amplifier is knowingly accurately presented and/or recorded by the device.

A general object of the present invention, therefore, is to provide a novel method and apparatus for assuring the accurate calibration of such monitoring device.

A related object is to provide such a method and apparatus which is relatively simple to practice and use, and which is extremely reliable.

According to the invention, what is contemplated is the generation of a pair of carefully controlled voltage wave forms one of which, in configuration, closely approximates the electrical wave form usually obtained from a catheter following normal arterial blood pressure, and the other of which is an accurate first derivative of the first-mentioned wave form. The novel means used to accomplish such action includes a generating circuit which is operable to generate a monopolar first electrical voltage wave form having a leading edge with a positive slope similar to the slope found in the leading edge of a normal arterial blood pressure wave form, and a trailing edge with a negative slope similar to the negative slope of the trailing edge of the usual blood pressure wave form, and with these edges separated in time by a substantially common amplitude portion. Working in conjunction with this generating circuit is a differentiating circuit which, on receiving a wave form supplied by the generating circuit, produces a bipolar second voltage wave form having separated positive and negative pulses whose amplitudes are directly proportional to the slopes of the leading and trailing edges, respectively, of the wave form generated by the generating circuit. Means is provided in the generating circuit for adjusting, and accurately controlling, the respective slopes of the mentioned leading and trailing edges.

The various other objects and advantages attained by the invention will become more fully apparent as the description which follows is read in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram illustrating a blood-pressure monitoring system employing apparatus constructed according to the invention.

FIG. 2 is a circuit diagram of a generating circuit as contemplated.

FIG. 3 is a circuit diagram of a differentiating circuit employed in the invention.

And, FIG. 4 is a graph illustrating, on a common time scale, a number of voltage wave forms which are significant in the operation of the system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, and referring first to FIG. 1, here is shown generally at 10, in block form, a blood-pressure monitoring system employing apparatus constructed in accordance with the present invention. At its input end, so-to-speak, system 10 includes a blood-pressure monitoring catheter 12, and at its output end, a dual-input, dual-track, ink-stylus recorder 14. Both of these devices are conventional in construction, and thus are neither illustrated, nor described, in detail herein. Catheter 12 herein is one constructed in accordance with previously referred to U.S. Pat. No. 3,710,781.

Interconnecting these two devices are a buffer amplifier 16, a selector 18, an amplifier 20, a calibration generating circuit 22, and a differentiating circuit 24. Amplifier 16 herein comprises a circuit such as that disclosed in the above-mentioned patent application. Selector 18 may simply be a switch through the actuation of which it is possible to feed amplifier 20 with signals coming either from amplifier 16 or from circuit 22. Amplifier 20 is unity-gain, low output impedance amplifier of any suitable design.

Signals from catheter 12 are supplied the buffer amplifier through a conductor 17, and are fed from this amplifier to selector 18 via a conductor 19. Circuit 22 supplies signals to the selector over a conductor 21--the selector feeding amplifier 20 through a conductor 23. The output of amplifier 20 couples to one of the inputs in recorder 14 through a conductor 25, and couples to the input of circuit 24 via conductor 25 and a conductor 27. Finally, the output of circuit 24 connects with the other input in the recorder through a conductor 29.

With catheter 12 and buffer amplifier 16 constructed as above indicated, and when the catheter is inserted for use in an artery, blood pressure change, in either direction, of 1 mm. Hg produces a voltage change of 1 mv. at the output of the buffer amplifier. An increase of pressure causes a rise in voltage, and vice versa. While the apparatus of the invention is adapted herein to work in conjunction with such a catheter and such a buffer amplifier, it should be understood, and it is believed it will become apparent, that the apparatus can easily be adapted for calibrating in conjunction with other types of catheters and/or buffer amplifiers, or like input devices.

Circuits 22, 24 are specially constructed in accordance with the invention. Details of circuit 22 are shown in FIG. 2. This circuit includes an oscillator 26, a current integrator 28, and an attenuating and level-setting circuit 31. The oscillator, also referred to as a first subcircuit, contains a conventional integrated-circuit operational amplifier 30, connected with other components in the circuit to perform as an oscillator with a frequency of about 2 Hz., and a duty cycle of about 20%. More specifically, connected between the output and the inverting input of this amplifier is the parallel combination of an adjustable resistor 32 and a pair of Zener diodes 34, 36 poled as indicated. Connecting the inverting input to ground is a capacitor 38. Further, the output of amplifier 30 is connected to ground through the resistance element 40a of a potentiometer 40. The wiper 40b of this potentiometer is connected to the noninverting input of amplifier 30.

In oscillator 26, its frequency of oscillation is determined by the relative values of the resistance of resistor 32 and the capacitance of capacitor 38. Through adjusting the resistance value of resistor 32, fine adjustments in the frequency of the oscillator are possible. Duty cycle is controlled by the position of wiper 40b on resistance element 40a. Diodes 34, 36 perform a clipping function--preventing undesirable saturation in amplifier 30.

When operating, oscillator 26 generates a rectangular voltage wave form such as wave form A in FIG. 4. Preferably, the oscillator is adjusted so that this wave form has a frequency of about 2 Hz., and a duty cycle of about 20%, with each positive pulse having a time width (t.sub.1) of about 100 milliseconds, and with adjacent pulses separated by about 400 milliseconds (t.sub.2). Wave form A may have an overall amplitude of about 20 volts, although this is not at all critical.

Integrator 28, also referred to as a second subcircuit, includes an operational amplifier 42 which is substantially the same in construction as amplifier 30. Connected as a feedback path between the output and inverting input of amplifier 42 is the parallel combination of a capacitor 44 and a pair of Zener diodes 46, 48 poled like diodes 34, 36. The noninverting input of the amplifier is grounded. The wave form produced by oscillator 26 is fed to the integrator through a dual-branch, parallel feeder circuit--one branch of which includes an adjustable resistor 50 and a diode 52, and the other branch of which includes a similar adjustable resistor 54 and a diode 56. As can be seen, diodes 52, 56 are poled in reverse directions, with diode 52 arranged to conduct away from the oscillator, the diode 56 disposed to conduct toward the oscillator.

In the integrator, diodes 46, 48 perform the same function as do diodes 34, 36 in the oscillator. Capacitor 44 works with resistors 50, 54 to perform current integration.

Circuit 31 includes a pair of potentiometers 49, 51 whose wipers 49a, 51a are connected together through series resistors 53, 55. One end of the resistance element 49b in potentiometer 49 is connected to the output of amplifier 42, and the other end is grounded. With respect to the resistance element 51b in potentiometer 51, its upper end is connected to the positive terminal of a suitable floating DC voltage supply, and its lower end is connected to the negative terminal of the supply. The junction between resistors 53, 55 connects with conductor 21.

The integrator receives wave form A, and, together with circuit 31, produces a nonrectangular, monopolar wave form, such as that shown at B in FIG. 4. It should be understood that the integrator alone generates a wave form identical in configuration with wave form B, but with a considerably larger overall amplitude. For example, the actual wave form produced at the output of amplifier 42 might typically have an overall amplitude of about 14 volts, whereas wave form B preferably has an overall amplitude of about 150 millivolts. Wiper 49a is adjusted to control the final overall amplitude of wave form B, and wiper 51a is adjusted to assure that the lowest voltage portions of this wave form are at ground potential.

As can be seen, each pulse in wave form B has a positively sloped leading edge, a negatively sloped trailing edge, and a substantially constant voltage portion between these edges. The value of the positive slope of the leading edge is determined by the relative values of the capacitance of capacitor 44 and the resistance of resistor 50--the latter being adjustable to change the value of this slope. Similarly, the value of the negative slope of the trailing edge is determined by the capacitance of capacitor 44 and the resistance of resistor 54. Resistor 54 may be adjusted to change the value of the slope.

Preferably, and as is in fact the case with wave form B, the leading edge slope value is twice that of the trailing edge slope--the former being about +3,000 millivolts/sec, and the latter being about -1,500 millivolts/second. The leading edge extends over about 50 milliseconds (t.sub.3), and the trailing edge extends over about 100 milliseconds (t.sub.5)--these two slopes being separated by the constant amplitude portion which lasts about 50 milliseconds (t.sub.4). Such a shape is preferred, since it closely approximates the actual shape of a typical voltage wave form generated by devices such as a catheter 12 and buffer amplifier 16 when following normal arterial blood pressure. Such a wave form is illustrated at D in FIG. 4.

Adjustability of resistors 50, 54 is a very advantageous feature of the integrator, since it permits accurate controlling of the slopes mentioned.

FIG. 3 shows details of differentiating circuit 24. This circuit includes a pair of operational amplifiers 58, 60 which are essentially the same in construction as amplifiers 30, 42. The noninverting inputs of both amplifiers are grounded. The inverting input of amplifier 58 is connected to previously mentioned conductor 27 through a resistor 62 in series with a capacitor 64. Forming a feedback path between the output and the inverting input of amplifier 58 is the parallel combination of a resistor 66 and a capacitor 68. The output of amplifier 58 is coupled to the inverting input of amplifier 60 through a resistor 70. An adjustable resistor 72 is connected as a feedback path between the output and the inverting input of amplifier 60. Amplifier 58, along with resistors 62, 66 and capacitors 64, 68, performs signal differentiation with respect to any signal supplied it via conductor 27. Amplifier 60, along with resistors 70, 72, performs as a low output impedance unity-gain amplifier.

When selector 18 is set to pass signals from circuit 22, wave form B is supplied over conductor 27 to the differentiating circuit. From this signal, the differentiating circuit produces bipolar wave form C in FIG. 4 which it feeds to recorder 14. As will be noted, for each pulse in wave form B, wave form C contains a pair of pulses, the first one in time being positive, and the second one in time being negative, The first, positive pulse lasts throughout the duration of the leading edge of a pulse in wave form B. Its amplitude is essentially constant, and directly reflects the value of the slope of the leading edge. Similarly, the second, negative pulse lasts throughout the duration of the trailing edge of the B wave form pulse, having an amplitude directly reflecting the value of the slope of the trailing edge.

With the leading edge having the slope value indicated, the amplitude of the positive pulse in wave form C directly indicates this value--i.e., +3,000 millivolts/sec. While the "absolute" amplitude of the positive pulse is a matter of choice, resistor 72 herein is set to make this amplitude about +150 millivolts. With such a setting, it will be apparent that each millivolt above or below zero volts generated by the differentiating circuit represents 20 millivolts/sec. slope value in wave form B. And, since wave form B has been shaped like wave form D, and with about the same overall amplitude, each millivolt from the differentiating circuit can be directly interpreted as a change rate of 20 mm. Hg/sec. of blood pressure.

The amplitude of the negative pulse in wave form C, of course, has the same relationship to the slope of the trailing edge of a pulse in wave form B.

When selector 18 is set to pass signals from buffer amplifier 16, and assuming that catheter 12 senses normal bloodpressure changes, differentiating circuit 24 feeds wave form E in FIG. 4 to recorder 14. It will be noted that above-described wave form C closely approximates wave form E.

Explaining now the operation of system 10, under circumstances of monitoring the arterial blood pressure of a patient, two wave forms derived from catheter 12 are provided the two inputs of recorder 14. One of these wave forms-- that supplied via conductor 25, is a voltage whose amplitude follows the actual instantaneous changes in arterial pressure, with a change in pressure of 1 mm. Hg. producing an actual voltage change of 1 millivolt in this wave form. The other wave form, supplied the recorder via conductor 27, comprises the differentiated form of what is supplied through conductor 25, with a rate of blood-pressure change of 20 mm. Hg/sec. producing a 1 millivolt change in such wave form. Conductors 25, 29 are referred to collectively herein as an output means.

It is naturally desirable that this information be accurately recorded by the recorder in easily readable and interpretable form. Circuits 22, 24 cooperate to calibrate the recorder to assure such performance.

More specifically, to calibrate recorder 14, selector 18 is set to pass signals from circuit 22 to amplifier 20. With circuit 22 adjusted as previously described to produce closely controlled wave form B, this wave form passes through amplifier 20 (a unity-gain amplifier), and over conductor 25, to the recorder, whereupon it is recorded. Since it is known what the actual amplitude and shape of wave form B is, it is a relatively simple matter for the operator to adjust the usual sensitivity control associated with the input connected to conductor 25, whereby to adjust the recorder to draw a wave form having an actual amplitude in the drawing which can easily be directly read in terms of actual blood pressure. In other words, since it is known that the actual amplitude of wave form B is 150 millivolts, and since it is known that this is related to the expected performances of catheter 12 and amplifier 16 whereby each millivolt represents 1 mm. Hg. of blood pressure, it should be a simple matter to adjust the actual amplitude of the wave drawn by the recorder so that it can easily be read off (on the usual graph-like recorder paper) in terms of actual blood pressure.

Wave form B is also supplied to differentiating circuit 24. The latter performs differentiation and supplies the first derivative of wave form B to the other input in the recorder. The sensitivity control associated with this input is then also adjusted to produce a recorded drawing which can easily be read to indicate the rate of blood-pressure changes.

With respect to the recording of wave form B, any appreciable nonlinearity in the operation of the recorder's circuitry associated with the input receiving this wave form will be immediately apparent. The same is also true with respect to the recording of wave form C. Here, the fact that there is a time separation between the positive and negative pulses, resulting from the presence of the constant amplitude portion in each pulse of wave form B, greatly assists in determining the accuracy and linearity of performance of the recorder. The actual time separation used is not critical, so long as it is long enough to be easily viewable in a recording of wave form C. A time separation of as little as 1 millisecond, for example, has been found to be usable. 50 milliseconds as used herein is very satisfactory, for it clearly enables one to observe whether the monitoring device accurately settles to a zero voltage level, as it should.

When these calibrating steps are complete, selector 18 may be set to pass signals from catheter 12 and amplifier 16. Such information is then fed directly over conductor 25 to the recorder, and in differentiated form over conductor 29, to the recorder. And, after the calibration described, a person using system 10 can accurately read off blood pressure information directly from the recordings in the recorder. Obviously, the calibration of the recorder can be checked at any time simply by resetting selector 18 to couple signals coming from circuit 22.

Thus, there is provided relatively simple and reliable circuitry for producing a pair of carefully controlled signals which can be used, as described, accurately to calibrate a blood pressure monitoring instrument, such as recorder 14. By producing a wave form B which closely approximates wave form D, and similarly, by producing differentiated wave form C which is very much like differentiated wave form E, calibration is possible under circumstances subjecting a monitoring instrument to wave forms essentially like that to be expected from normal arterial blood-pressure changes. The adjustability provided in resistors 50, 54 in integrator 28 enables very close control over the slopes of the leading and trailing edges mentioned in wave form B. This, of course, is important in making wave forms B, C similar to wave forms D, E, respectively. The fact that the overall amplitudes of the wave forms produced by circuits 26, 28 are controlled by clipping actions in the Zener diodes results in extremely good amplitude stability with ambient temperature changes.

While a preferred embodiment of the invention has been described, it is appreciated that variations and modifications may be made without departing from the spirit of the invention.

Claims

It is claimed and desired to secure by Letters Patent:

1. Apparatus for producing electrical voltage wave forms usable especially for calibrating an electrical instrument for monitoring human arterial blood pressure, said apparatus comprising

generating circuit means operable to generate a monopolar first electrical voltage wave form having a leading edge with a predetermined positive slope of one value, and a trailing edge with a predetermined negative slope of another value which is one-half said one value, said leading and trailing edges being separated in time by a substantially constant amplitude portion,

differentiating circuit means operatively connected to said generating circuit means to receive such a first wave form, operable, on receiving the same, to differentiate it and produce a bipolar second electrical voltage wave form having a first-in-time rectangular pulse with one polarity, and one amplitude which is directly proportional to the value of the slope of said leading edge of said first wave form, and a second-in-time rectangular pulse with the opposite polarity, and another amplitude which is directly proportional to the value of the slope of said trailing edge of said first wave form, said two pulses being separated by a constant zero-voltage portion of said second wave form, which portion corresponds in time to said constant amplitude portion of said first wave form, and

output means for coupling said first and second wave forms to such a monitoring instrument whereby proper operation of said instrument is verified by said constant zero-voltage portion of said second wave form.

2. The apparatus of claim 1, wherein said generating circuit means includes a first subcircuit operable to produce a rectangular voltage pulse, and a second subcircuit operatively connected to said first subcircuit, operable to receive such a pulse, and to produce therefrom, for supplying to said differentiating circuit means, a nonrectangular voltage pulse with such having said leading and trailing edges and said constant amplitude portion.

3. The apparatus of claim 2, wherein said second subcircuit includes means for independently adjusting the values of the slopes of said leading and trailing edges.

4. A method of calibrating an electrical instrument which is for monitoring human arterial blood pressure comprising

generating a monopolar first electrical voltage wave form having a leading edge with a predetermined positive slope of one value, and a trailing edge with a predetermined negative slope of another value which is one-half said one value, and with said leading and trailing edges being separated in time by a substantially constant amplitude portion,

differentiating said first wave form to produce a bipolar second wave form having a first-in-time rectangular pulse with one polarity, and one amplitude which is directly proportional to the value of the slope of said leading edge of said first wave form, and a second-in-time rectangular pulse with the opposite polarity, and another amplitude which is directly proportional to the value of the slope of said trailing edge of said first wave form, said two pulses being separated by a constant zero-voltage portion of said second wave form, which portion corresponds in time to said constant amplitude portion of said first wave form, and

coupling said first and second wave forms into said instrument whereby proper operation of said instrument is verified by said constant zerovoltage portion of said second wave form.

5. The method of claim 4, wherein said generating is accomplished by first producing a rectangular voltage pulse, and then from such pulse producing a nonrectangular voltage pulse having said leading and trailing edges and said constant amplitude portion.