Pressure-sensing Signal Generator

Abstract

Extremely sensitive apparatus for detecting effort of a patient to inhale and to produce a powerful output signal of very short duration in response for initiating action of a respirator, with means immunizing the apparatus against adverse temperature change effects, means for stabilizing operation of the apparatus, and means for varying the sensitivity of the apparatus to pressure differential.

Description

BRIEF SUMMARY OF THE INVENTION

a. Background of the Invention

In certain applications of pressure change sensing, it is imperative to very quickly and positively sense pressure decrease below ambient or a determined level or value, and to generate a signal indicative of the change for use in or by an apparatus. For example, in respirator apparatus adapted for use in relief of hyaline membrane syndrome and other breathing difficulties evidenced by neonatal infants, it is of importance to sense very weak efforts to inhale by the infant and to immediately aid the infant by supplying air or a special mixture of gases under superambient pressure to the infant via nasal mask means, and to thereafter permit free exhalation by the infant at a determined subsequent time. The effort to inhale causes a drop in pressure, however slight, at the nostril entrance; and this pressure change has been utilized, by use of pressure change sensing means, to initiate action of the respirator to supply or deliver an accurately measured volume of air, oxygen enriched or otherwise, under regulatable pressure and at a regulatable speed, to the infant patient. The respirator is so devised that following delivery of the air, valve means are operated and exhaling is accomplished by natural contraction of the chest cavity of the patient. The same factors are noted in respect of respirator mechanisms employed for other than neonates. Heretofore various kinds of pressure transducers have been employed to provide an output for initiating actuation of respirator means, such transducers including pressure-sensitive electric switches. Since the pressure sensor is most desirably situated as close as is practicable to the nostrils of the patient, whereby sensitivity may be maximized, and since switch means in proximity to a patient, and especially near to oxygen or oxygen-enriched air, are undesirable, designers of respiratory aid systems have had to compromise between optimum sensitivity and attendant undesirable factors. One compromise has resulted in the use of sealed reed switch means operated by a pressure-sensitive means, located distant from the nasal mask and having a sensitivity much less than is desirable. No fully satisfactory solution to the problem of providing an extremely sensitive safe means for rapidly initiating action of the respirator means incident to an effort to inhale by the patient has been attained prior to advent of the present invention. Thus, it is a prime object of the invention herein disclosed in a presently preferred exemplary form to obviate the noted disadvantages of prior art devices of the indicated class; and it is a broad object of the invention to provide improvements in means for initiating action of respiration-augmenting means. A more specific object is to provide an extremely sensitive, extremely fast-acting regulatable means for producing a powerful signal indicative of an effort of a patient to inhale and which signal can be utilized to initiate action of other means such as a respirator. Other objects and advantages of the present invention are hereinafter stated or made evident in the appended claims and disclosure of the preferred exemplary embodiment.

b. Brief Description of the Invention

According to the invention, presence of electric switch means in proximity to the patient is avoided, and extreme sensitivity of sensor means is concurrently attained, by using electrical capacitor means, a movable plate or electrode of which is disposed on a thin resilient membrane or diaphragm which in turn is exposed on one face to the ambient and on the other to the interior of a passage closely communicating with the nasal passages of the patient, as sensor means sensitive to any effort of the patient to inhale. The capacitive means is connected in the circuit of an electronic oscillator and is effective to initiate oscillation of the oscillator incident to very slight change of capacitance as the diaphragm moves in response to very slight change in differential pressure as the patient makes an effort to inhale. The oscillator circuit is so devised that commencement of oscillation results in transmission of an electric signal which is used to activate or to initiate activation of means in the respirator system. The exemplary system comprising the oscillator circuit is further devised to be self-quenching, whereby the oscillator is quenched from oscillatory to quiescent status at the end of a brief interval of time (e.g., 5 milliseconds) next following initiation of oscillation.

Thus, unless the capacitor means has returned to the neutral attitude during the signal generation period, the oscillator is again triggered into oscillation, generates another signal, and again quenches, and repetitively does so until the capacitor means returns to the initial neutral attitude and capacitance value. The signal, in the form of an electric wave, is integrated, preferably following wave shaping and/or amplification in circuitry of the system, to produce an output signal for transmission to and utilization in the respirator. In the respirator, arrival of the output signal is effective to set in motion cyclical means which then operates through one cycle during which a measured volume of air is supplied under moderate superatmospheric pressure to the nasal mask on the patient over a regulatable period of time which is followed by a period during which valve means are operated and the patient exhales. The respirator is so devised that in one mode of operation the cyclical operation is automatically repeated and in another mode of operation the cycle is again initiated by attempted inspiration by the patient. An exemplary circuit according to the present invention permits variation of the sensitivity of the detecting means, whereby action is initiated only in response to subatmospheric pressure in the mask of any selected value in the range from 0.1 mm. to 5.0 cm. of water column. Since the oscillator is set into oscillation as the result of a very small movement of a diaphragm-supported capacitor electrode, the circuit can be employed to detect a very small mechanical movement of a part connected to the electrode. The circuit is so arranged that capacitive means including the pressure-sensitive capacitor means, herein termed the principal capacitor, are in shunt with the oscillatory circuit of the oscillator, the capacitive means further comprising a variable capacitor in the form of a voltage-variable capacitor whereby the effective value of the shunt capacitance may be varied by varying the potential applied across the later capacitor. The circuit preferably includes an amplifier which provides a strong output signal during oscillation of the oscillator; and a portion of the output signal is fed back via signal-delay means to the voltage-variable capacitor to change the shunt capacitance to a value that causes rapid decay and extinction or quenching of the oscillation of the oscillator after a determined period of time. Since return of the capacitor to a capacitance value at which oscillation is initiated will not generally cause the oscillator to stop oscillating, the feedback quenching of the oscillator permits of greatly increasing sensitivity of the signal generator to change of pressure at the movable portion of the principal capacitor. To enhance stability of operation and sensitivity, the active elements of the oscillator are temperature stabilized by enclosing them in an oven that is maintained at a temperature somewhat above room temperature, e.g., at 80.degree. C.

The preferred voltage-variable capacitor is a voltage-sensitive diode; and the preferred active element of the oscillator is a field-effect transistor. A trimming or adjusting capacitor is provided for initial adjustment of the oscillator. A preferred form and arrangement of components of an exemplary pressure-sensitive signal generator is depicted in schematic form in connection with a known respirator in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a functional block diagram of an exemplary system according to the invention;

FIG. 2 is a fragmentary sectional view of the exemplary pressure-sensitive capacitor comprised in the capacitive shunt connected to the oscillator of the system; and

FIG. 3 is a detailed circuit diagram of the signal generator part of the system.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 2, there is illustrated in section a pressure-sensitive capacitor device or sensor adaptable to delicately sense change of pressure from ambient atmospheric pressure to a lower value. The drawing is essentially schematic and to not specific scale. The sensor comprises means in the form of a box 10 providing a chamber sealed at the peripheral juncture between the body 10b and cover 10c of the box. Disposed in body 10b is a rigid stop device 12 formed of molded material. The stop and box body 10b are bored to provide an opening for reception of a conduit 14 which is supported by the body and communicates with the mentioned nose mask (not shown) which is per se not of the present invention and may be of the type illustrated in U.S. Pat. No. 3,357,428 or that illustrated in U.S. Pat. No. 1,206,045, for examples. The cover 10c is of insulation and is similarly bored to receive and support an conduit 16 which is open to the ambient atmosphere. The box is preferably of circular plan form, and has secured thereto at the interior periphery along the juncture of the cover and body a thin resilient diaphragm 20. The diaphragm overlies a centrally perforated circular metal electrode 18a which is affixed to cover 10c and forms one electrode of a principal capacitor herein denoted C1 in the circuit diagram. A second metal plate 18b, complementary to plate 18a and which may be formed of a film of metal formed on the diaphragm, serves as a movable electrode of capacitor C1. Flexible conductors 22a and 22b, connected as indicated in the drawings, serve as respective terminal connectors for the capacitor plates. An extension of cover 10c serves as a base for support of an enclosure 24 in which other components of the signal generator may be housed, as on a circuit board.

As is evident, when air exits from the chamber above diaphragm 20 incident upon an effort by the patient to inhale consequent reduction of pressure in conduit 14, the diaphragm will be forced to move in the direction away from electrode 18a under the influence of the ambient air and its pressure in the chamber below the diaphragm. Thus the capacitance of capacitor C1 is lessened or decreased. The slight change in capacitance is employed to initiate action of circuitry connected to capacitor C1 as shown in detail in FIG. 3 and in block diagram form in FIG. 1.

Referring to FIGS. 1 and 3, the capacitor unit 30 comprising pressure-sensitive capacitor C1 is connected to serve as the primary control component, in shunt to an oscillator unit 40. The oscillator unit, herein termed the oscillator in the interest of brevity, comprises tapped inductor L1, capacitor C2, resistor R1 and field-effect transistor Q1, all connected as a Hartley oscillator. Closely regulated or constant-potential DC power is supplied as from a battery B by way of lead P1 (negative) and ground (positive) as indicated. An oscillator trimming unit 50 comprising variable capacitor C2 is connected as a feedback control in the oscillatory circuit of unit 40, as shown. Thus the oscillator capacitance can be adjusted to a proper value to permit quenching and oscillation initiation as will presently be described.

A temperature-regulating unit 60 which comprises a heater Rx of the negative resistance type such as is sold under the trade name KLIXON is housed, together with temperature-sensitive components of the oscillator, in an insulated chamber or over H. Thus the unit 60, connected as shown and arranged as described, if effective when energized to raise the temperature in which the temperature-sensitive components operate to a superambient value, e.g., 80.degree. C.; and to maintain those components at that constant temperature. Thus the oscillator is immunized against adverse changes due to change in ambient temperature. The oven H is not shown in detail since it may be one of many kinds commercially available or may be merely an insulated box. While in the drawing only the transistor Q1 is shown in the oven with the heater thermostat, it will be understood that all components of the oscillator may be so housed.

When the oscillator is permitted to oscillate, it produces a high-frequency wave output. The circuit component values are such that at each alternate half-cycle of the oscillation of the oscillatory circuit the transistor Q1 quickly becomes saturated, whereby the output to a coupling capacitor C4 is of substantially square wave form. An exemplary oscillator frequency is 30 megahertz. Oscillation of the oscillator is permitted only under special circumstances, and at other times is prevented by one or more actions of means presently described.

The square wave output signal passed through coupling capacitor C4 is subjected to rectification in a rectifier-integrator unit 70 which comprises diodes CR1 and CR2 and capacitor C5. The effect of the units 70 is to supply to the gate of the transistor Q2 of an amplifier unit 80 a bias which causes or permits Q2 to conduct during the duration of the square wave signal. Thus during that period a negative-going pulse appears on output signal lead P2. That output signal is transmitted to and utilized in the respirator unit 90. For example, the negative-going pulse may be of the order of 5 milliseconds duration and may be used to pull up a latching relay which initiates a cycle of action of the respirator. The respirator may be, for example, like or similar to those disclosed in U.S. Pat. Nos. 3,357,427 and 3,006,336; or similar breathing-augmentor apparatuses.

Once a cycle of operations has commenced or been initiated by the output signal, air under pressure is forced into the mask and the pressure in the chamber above the sensor diaphragm 20 (FIG. 2) increases to a superambient value and the capacitance of capacitor C1 is increased at least to its original value. However, that alone does not preclude continued oscillation of the oscillator; hence means are provided for quenching the oscillator after oscillation has continued for a period sufficient to insure relay pullup or other initiation of respirator operation. Such a period has herein been selected, for example, to be of the order of 5 milliseconds.

Amplifier transistor Q2 is normally biased off by bias supplied by bias unit 100 which comprises resistors R2 and R3 connected as indicated in FIG. 3. Thus the amplifier unit will not provide an output signal until the bias provided by the bias unit 100 is overcome by the input signal from the unit 70. When an amplifier unit output signal is produced and transmitted to the respirator unit 90, a quenching signal unit 110 also receives the amplifier output signal, via branch line P2'. The quench pulse unit comprises resistor R4, capacitor C6, transistor Q3, and connections as shown. The negative-going pulse signal is effective to build up a charge on capacitor C6, via resistor R4 connected to line P2'. When the potential across C6 reaches a determined value, transistor Q3 is biased to conduction. Transistor Q3 is normally biased off by a biasing unit 120 which comprises resistors R5 and R6 and connections as indicated.

When the quenching signal unit transistor Q3 has thus been induced to conduct following an amplifier output signal duration of the noted character, a current signal is produced which is transmitted via line P3 and the resultant potential change is transmitted via radiofrequency choke coil L2 to a voltage-sensitive capacitor CRX comprised in an oscillator quench unit 130. Capacitor CRX in series with auxiliary capacitor C7 forms a variable capacitive shunt to ground for the oscillatory circuit of unit 40. The arrangement is such that with substantially no DC potential applied across capacitor CRX, the oscillator will commence oscillating as soon as the capacitance of sensor capacitor C1 falls slightly incident to the patient's attempting to inhale. Thus, with the oscillator oscillating, a current signal is produced by Q3 after about 5 milliseconds oscillation. The current signal produces a potential-drop signal which has the effect of increasing the capacitance exhibited by CRX to the extent that oscillation of the oscillator is damped and quenched, even if sensor capacitor C1 has not yet returned to maximum value. THus the oscillation ceases and the amplifier output signal to the respirator unit decays to substantially zero volts value. If the respirator has not responded, decay of the amplifier output signal and consequent decay of the quench signal permits immediate resumption of oscillation if subatmospheric pressure prevails in the nose mask on the patient. Repetition of the oscillation-signaling cycle within a few milliseconds is of no significance if the respirator is proceeding through a cycle but has not yet produced superambient pressure on the upper surface of the sensor diaphragm 20, since the latching relay in the respirator has previously been pulled in and is holding. If for some reason the respirator failed to respond to the initial amplifier output signal, a new oscillation is promptly initiated and a repetitive signal transmitted to the respirator. Thus operation of the respirator is insured.

To prevent operation of the oscillator until the components have reached the desired constant operating temperature, a warmup timer circuit or unit 140 is incorporated and is effective to maintain a high capacitance value at CRX to bar oscillation in unit 40 for a determined period of time sufficient for the oscillator to become temperature-stable. Unit 140 comprises a transistor Q4, capacitor C8 and resistor R8. When the power circuit is closed as by switch S to energize the system, Q4 immediately conducts, current flowing from lead P1 via a selected portion of a variable resistor R9, through Q4 to ground. Thus potential is supplied via resistor R7 and choke L2 to voltage-sensitive capacitor CRX, raising that capacitance to a level sufficient to prevent oscillation of the oscillator of unit 40. After a time determined by the values of R8 and C8, transistor Q4 is biased off by the potential of the charge accumulating on C8, the capacitance of CRX falls to a value permitting oscillation to commence, and the system is ready to sense inspiratory effort of a patient and initiate respirator operation. The values of C8 and R8 are selected such as to permit the heater of unit 60 to bring the oscillator to the desired temperature within, for example, 5 minutes. Levels at which CRX is caused to quench, or permit operation of, the oscillator of unit 40 are set by adjustment of the values of variable resistors R9 and R10 comprised in a sensitivity control unit 150.

The nose mask used in conjunction with respirator unit 90 may be of a type now commercially available, or may be such as is disclosed in U.S. Pat. No. 1,206,045. Exemplary electronic components for the detailed circuitry shown are as tabulated in table I at the conclusion of this specification.

The preceding description makes it evident that the capacitance change at capacitor C1 necessary to trigger the oscillator can be made very small by adjustment of the sensitivity unit, since that change can be very much less than the opposite change at C1 that would be necessary to quench the oscillator, and quenching is brought about by a powerful action of voltage-sensitive capacitor CRX initiated by the quench pulse unit 110. Thus even the very feeble effort of a neonate to inhale while suffering hyaline membrane syndrome or other respiratory distress is sufficient to initiate positive respirator action. Further, the system is extremely stable and immune to effects of changing temperature of the ambient. Also it is evident the negative-pressure level at which action is initiated is adjustable by varying the resistor means of the sensitivity unit. The time period between successive oscillator signals is regulatable by change of the feedback circuit component values such as C6-R4 and R6-R5. The pressure differential at the diaphragm 22, required to change the capacitance of C1 sufficiently to initiate oscillation can, by virtue of the adjustability of the sensitivity control unit 150, be varied over wide limits e.g., from 0.1 mm. to 5.0 cm. water column. Thus the system is of value not alone in initiating respiration aid in response to extremely feeble respiratory efforts, but also in promoting increasing effort on the part of the patient to breathe voluntarily. The latter promotion is effected by gradually increasing the inspiratory effort necessary to initiate oscillation of the oscillator, by gradually reducing the sensitivity of the system. --------------------------------------------------------------------------- TABLE I

C1 1-5 pfd. R1 100 K ohms C2 1.5-12 pfd. R2 220 ohms C3 22 pfd. R3 2.7 K ohms C4 3 pfd. R4 100 K ohms C5 0.02 mfd. R5 680 ohms C6 0.02 mfd. R6 2.7 K ohms C7 10 pfd. R7 2.7 K ohms C8 50 mfd. R8 2.2 megohms R9 2.5 K ohms R10 1 K ohms R11 22 K ohms Rx Klixon SST 1-2, 80.degree. C.

cr1 1n34 g.e. cr2 1n34 g.e. crx mv1620 motorola

Q1 MPF 102, Motorola Q2 MPF 102, Motorola Q3 2N1377, Motorola Q4 MPF 102, Motorola

L1 15 closed turns with tap at five turns -22, 0.25" D. L2 22 microhenries __________________________________________________________________________

Claims

I claim:

1. A pressure-sensing signal generator system adapted to respond to very small difference between pressures exhibited at two closely spaced points and to produce an electric signal only immediately following such response, said system comprising:

first means, including electronic oscillator means, effective incident to oscillation of the oscillator means to produce an output signal during such oscillation;

second means, including pressure-sensitive capacitor means connected to said oscillator means and effective to inhibit oscillation of the oscillator means during absence of pressure difference between two points therein and effective incident to establishment of a pressure difference between said points to promote oscillation of said oscillator means; and

third means, including feedback means connected to receive an output signal from said first means, said third means effective upon receipt of a signal from said first means to quench oscillation of said oscillator means,

whereby oscillation of said oscillator is quenched following production of a signal representative of a pressure difference between said two points, irrespective of continuation of the pressure difference between said two points.

2. A system as defined in claim 1, in which said third means comprises potential-sensitive capacitor means shunting said first-named capacitor means, and means rendered active by said output signal to apply voltage to said potential-sensitive capacitor means to increase the capacitance exhibited by the shunt combination of capacitor means to effect quenching of oscillation in said oscillator means.

3. A system as defined in claim 2, including adjustable means for applying an adjustable continuous potential to said potential-sensitive capacitor means, whereby sensitivity of said system to pressure differential between said two points may be adjusted.

4. A system ad defined in claim 1, including means for maintaining at least an active element of said oscillator means at superambient temperature, and means for inhibiting oscillation of said oscillator means pending attainment of said superambient temperature, whereby said system is inhibited from producing an output signal until said oscillator means is stable with respect to variables induced by temperature change.

5. A system as defined in claim 1, including respirator means connected to receive an output signal produced by said first means and effective in response thereto to proceed through a cycle of operation.