US3894535A

US3894535A - Blood pressure monitor system with pushbutton zero-offset compensation

Info

Publication number: US3894535A
Application number: US438818A
Authority: US
Inventors: Robert L Cannon; Robert A Mceachern
Original assignee: American Optical Corp
Current assignee: Warner Lambert Co LLC
Priority date: 1974-02-01
Filing date: 1974-02-01
Publication date: 1975-07-15
Anticipated expiration: 1992-07-15

Abstract

A blood pressure monitor system with pushbutton zero-offset compensation. There is disclosed a blood pressure monitor system for monitoring blood pressure of a patient. The system includes a manual control which when operated, automatically provides a feedback signal to compensate for zero-offset of the pressure transducer when zero pressure is applied to the transducer. Zerooffset compensation is thus rapidly and easily obtained, and the signal is maintained thereafter for a long period of time. The present invention thus greatly facilitates monitoring blood pressure of a patient.

Description

United States atent Cannon et al.

[ 1 July 15, 1975 1 1 BLOOD PRESSURE MONITOR SYSTEM 3,765,239 10/1973 01550 128/208 x WITH PUSHBUTTON ZERO OFFSET 3,771,038 11/1973 RUbiS 330/9 X COMPENSATION FOREIGN PATENTS OR APPLICATIONS [75] Inventors: Robert L. Cannon, Waltham; Robert 1,508,303 11/1967 France 128/208 A. McEachern, Billerica, both of Mass- Primary Examinerl(yle L. Howell [73] Assigneez American Optical Corporation, Attorney, Agent, or FirmJoel Wall; W1llIam C.

southbridge Mass Nealon; H. R. Berkenstock, Jr.

[22] Filed: Feb. 1, 1974 [57] ABSTRACT [21] Appl' 438318 A blood pressure monitor system with pushbutton zero-offset compensation. There is disclosed a blood [52] U.S. Cl. l28/2.05 A; 128/205 D pressure monitor system for monitoring blood pres- [Sl] Int. Cl. A61B 5/02 sure of a patient. The system includes a manual con- [58] Field of Search 128/205 R, 2.05 F, 2.05 G, trol which when operated, automatically provides a 128/205 A, 2.08, 2.06 B, DlG. 29, 205 D, feedback signal to compensate for zero-offset of the 205 M; 330/9, 85; 73/194 R pressure transducer when zero pressure is applied to the transducer. Zero-offset compensation is thus rap- [56] References Cited idly and easily obtained, and the signal is maintained UNITED STATES PATENTS thereafter for a long period of time. The present in- 3 237 116 2/1966 Skinner et a1 330/9 Vemion thus greatly facilitates monitoring blood Pres 3,516,002 6/1970 Hillis 330/9 X Sure of a patlem' 3,694,760 9/1972 Loessi 330/9 3.726.271 4 1973 Mondshine et al. 128/208 1 Clam" 2 Drawmg Fgures 13 b to 1:1

17 (-1 P S 1 l l i TRANSDUCER SI 1H) 50 S 0 FL; 1

l- T t l I I I9 ,8!) HN/lLOG K MEMORY I 21 20 I 16 X B BLOOD PRESSURE MONITOR SYSTEM WITH PUSHBUTTON ZERO-OFFSET COMPENSATION BACKGROUND OF THE INVENTION ments thereto are continually being developed to fur ther advance this important area of human endeavor. Over the years, many electrical equipments have been proven to be quite effective in assisting human patients. For example, heart defibrillators, heart pacers, respiration monitors, blood pressure monitors and other devices can be included in this group of equipment. Continual focused attention in this area provides improvements to these pieces of equipment and it is indeed fortunate for the patient that this is the case.

The present invention relates to an improvement to those electro-medical devices that utilize transducers for sensing a patients vital signs and for converting them into electrical signals. More specifically, the present invention relates to an improved blood pressure monitoring system. In the prior art, blood pressure monitors would typically use a pressure transducer to convert a patients blood pressure to an electrical signal. The signal would then be operated upon by electrical circuitry to provide an indication of blood pressure on some electronic or mechanical indicating device. In blood pressure monitoring systems, accuracy of measurement is important. Accuracy is to-a greatextent dependent upon linearity, repeatability, zero-offset, and other characteristics of the transducer itself. When zero pressure, which is hereafter intended to herein mean atmospheric pressure. is applied to the' transducer, ideally a zero signal should be generated by the transducer. However. in reality, there are mechanical and electrical zero-offsets generally found in transducers so that a signal is generated even when zero or atmospheric pressure is applied. This, of course. caused an erroneous blood pressure reading in certain prior art devices.

In other prior art blood pressure systems, this offset signal was partially removed by generating an opposite compensating signal. This compensating or nulling signal was typically determined in a hospital environment by the nurses on duty at the time. Ordinarily, nurses do not receive any training in this technical area. The compensating signal was generated by a manuallyoperated hunt-and-seek method, where the nurses would vary a potentiometer to provide voltage generated by the transducer. This was a time consuming process. In this area of electro-medical life support systems, time is a commodity which usually cannot be wasted. And, in addition to being time consuming there were certain inherent inaccuracies and other disadvantages in this prior art approach to zero pressure compensation.

The present invention provides a solution to these prior art problems. The present invention provides a manual push-button, which when operated or depressed causes generation of a dc compensating signal zeroing the blood pressure monitor. This compensating signal can be stored or maintained for a long period of time, as long as 24 hours or more. If rezeroing is desired, it can be achieved merely by repeating this simple procedure. Thus. inaccuracies and time losses inherent in prior art blood pressure monitoring systems are eliminated by the present invention.

SUMMARY or. THE INVENTION The present invention relates to a'manually-operated automatic zero-offset transducer compensator. Zero pressure is first applied to the transducer. Then, a pushbutton switch is operated which automatically increases amplifier gain. generates a transducercompensating Zero-pressure-offset dc signal, and extends that signal to the input of the amplifier. The pressure indicator then displays Zero pressure. The present invention includes an analog memory for maintaining the offset signal for an extended period of time after release of the push-button. Advantages of the present invention over the prior art include increased accuracy and efficiency.

It is thus an object of the present invention to provide an improved blood pressure monitoring system.

It is another object of the present invention to provide a blood pressure monitoring systenrwhjc h incorporates an automatic transducer-zero-offset compensator. y i

It is yet another object of the present invention to provide compensation for inherent offsets in transducers using a manually operated servoloop and an analog memory. v '7 Other objects and advantages of-thepresent invention will become apparent to one having reasonable skill in the art after referring to a detailed description of the appended drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 depicts a block diagram illustrative embodiment of the present invention. Transducer l0 senses pressure P at its input and provides a differential input signal to operational amplifier 11 onamplifier input terminals 17() and 17(+). Amplifier output 18 provides an output signal to indicator 12 which is connected between output 18 and ground, and this signal is'also fed back to the input of amplifier 11 in the following manner.

Feedback element 13b is connected between output 18 of amplifier 11 and inverting or negative input 17() of amplifier 11. Feedback element 13a is connected between output '18 and input 17() through switch contact Sa. Feedback elements 15a and 13b in part control gain of amplifier 11. Switch contacts Sn and Sb are ganged together and are likewise ganged to other contacts Sc and Sd shown in FIG. 2. This interconnection is depicted by dashed line 19. Push-button switch B operates allfour switch contacts together.

Output .18 of amplifier 11 is also connected through switch contact Sb, amplifier 14, conductor 20, analog memory 15, conductor 2l,amplifier 16,'and resistor 50 to non-inverting or positive input 17(+) of amplifier 11. This-feedback path provides the automatic zerooffset compensation. The detailed operation of these feedback circuits will be discussed below.

FIG.- 2 depicts a circuit diagram of analog memory 15. Drain terminal D of field effect transistor (FET) 22 is connected to +15 volts DC. Source terminal S of FET 22 is connected to conductor 21 as well as one end of resistor 27. The other end of resistor 27 is connected to l volts DC. Source terminal S is also connected to the cover or case of FET 22 and to shield 29 of capacitor'28. Source terminal S of FET 22 is likewise connected to normally closed

switch contacts

40 and 41. Gate terminal G of FET 22 is connected to one end of capacitor 28, the other end of the capacitor being connected to ground. Gate terminal G is also connected to normally opened contact 42.

Shield 29 of capacitor 28 is connected to normally closed contact 43. Contact 43 is depicted as being connected to relay switching element 24 and one end of resistor 23. The other end of resistor 23 is connected to conductor 20. Relay coil 25 is depicted asbeing magnetically operative with respect to switching element 24 indicated by dashed line 44. One end of relay coil 25 is connected to one end of resistor 26, the other end of resistor 26 being connected to switch element St: The other end of relay coil 25 is connected'to switch element Sd.

In operation, consider both FIG. 1 and FIG. 2. Blood pressure P is applied to the pressure input of transducer 10, and the output electrical signal is fed to amplifier 11. The output from amplifier 11 is registered on indicator 12 which can be an analog or digital meter, or other indicator which indicates the value of blood press'ure. Normally closed contact Sa maintains feedback element 130 in the system, which in part determines gain of the-system. (Amplifier 11 is an operational amplifier whose gain is determined by the well known relationship of feedback impedance divided by'input =impedance. In FIG. 1, input impedances are included within transducer and therefore are not specifically shown.)

ln orderto automatically zero-offset compensate the system, switch B is operated. In the preferred embodiment, switches Sa, Sh, Sc; and Sd are mechanically linked or ganged together so that these-switch elements all change state when push-button B is operated. However, just prior to depressing push-button B, pressure P is reduced to zero or atmospheric by decoupling transducer 10 from the patients pressure P or by other means. Thereafter. switchB is operated and simultaneously switch element Sa opens, switch element Sb closes, switch element Sc connects to volts, and switch element Sd connects to ground. The effect .of these switch element changes is as follows.

The gain of amplifier 11 is increased. Amplifier 11 is a typical operational amplifier whose feedback loop as earlier explained determines its gain. By the opening of switch element Su, feedback element l3a is removed from the feedback loop thereby increasing the gain of amplifier 11. The reason for providingthis gain increase is to cause a more accurateresponse of amplifier contact 43 to normally open contact 42. The signal appearing on conductor 20 thus is passed through resistor 23, element 24, to gate G of PET 22, while charging capacitor 28. The voltage to whichcapacitor 28charges extends through F ET 22 and appears on source terminal S of PET 22 because of the source-follower circuit configuration of PET 22. That voltage is thus extended via conductor 21 and amplifier 16 back to input l7(+) of amplifier 11. The effect of this feedback signal is to reduce output 18 of amplifier 11 to zero. The value of voltage to achieve the zeroing is maintained on capacitor 28 after the push-button is restored to its initial state. Accordingly, this error correction is maintained on capacitor28 and at input 17(+) and leaks or discharges extremely slowly through the extremely high input impedance of PET 22. In fact, thisvoltage can be substantially maintained on capacitor 28 for days.

In this connection, the purpose of connecting source S of PET 22 to the case of FET 22, to the shield of ca.- pacitor 28, to normally closed contact 43, as .well as to relay coil 25, is to maintain all of these conductive elements at the same potential and substantially equal to the potential on capacitor 28. The voltage on sources is the driving potential for this arrangement. This arrangement minimizes leakage from capacitor 28 and is generally known as fguarding the capacitor. FET 22 is a metal oxide semiconductor field effect transistor (MOSFET) which has a lower leakage characteristic than a junction FET. However in this specification, the

designation PET was intended to mean MOSFET as well. v I

Recapitulating, when push-button B is operated capacitor 28 charges to a voltage necessary to correct transducer offset when the transducer has zero or at mospheric pressure applied thereto. Since capacitor C 28 can be charged in either direction relative to ground as required, this system corrects for both positive and negative transducer offsets.

This type of zeroing circuitry could be used with any transducer that has an offset situation. The types of transducers utilized in the preferred embodiments are strain gauge or semiconductor strain gauge bridge types. Also, blood pressure transducers are not the only transducerswith which this invention can be utilized but it can be utilized with transducers for converting other physiological parameters to electrical signals as well.

The invention may be embodied in yet other specific forms without departing from the spirit or essential characteristics thereof. Thus, the present embodiments are to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appendedclaims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed is:.

l. A blood pressure monitor system for monitoring blood pressure of a patient saidsystem comprising:

transducer means adapted to be operatively connected to said patient for sensing said pressure and for converting said pressure into an analogous electrical signal; amplifying means;

means for extending said electrical signal to input terminals of said amplifying means; indicator means responsive to output of said amplify- Y ing means for displaying value of said pressure; means for disconnecting said transducer means from said patient thereby applying zero pressure to said transducer; a manual control;

disconnecting means.

Claims

1. A blood pressure monitor system for monitoring blood pressure of a patient said system comprising: transducer means adapted to be operatively connected to said patient for sensing said pressure and for converting said pressure into an analogous electrical signal; amplifying means; means for extending said electrical signal to input terminals of said amplifying means; indicator means responsive to output of said amplifying means for displaying value of said pressure; means for disconnecting said transducer means from said patient thereby applying zero pressure to said transducer; a manual control; feedback means including said manual control; means responsive to operation of said manual control for simultaneously increasing gain of said amplifying means, generating a transducer-compensating zero-pressure-offset signal during operation of said disconnecting means, and extending said offset signal to said input terminals of said amplifying means to cause said indicator means to display zero pressure; and means for maintaIning said offset signal for an extended period of time after release of said manual control and after termination of operation of said disconnecting means.