GB2234351A

GB2234351A - Pressure transducer system

Info

Publication number: GB2234351A
Application number: GB8912879A
Authority: GB
Inventors: Norman Barrie Jones; George L Papageorgion
Original assignee: Medical & Diagnostic Systems L
Current assignee: Medical & Diagnostic Systems L
Priority date: 1989-06-05
Filing date: 1989-06-05
Publication date: 1991-01-30
Also published as: GB8912879D0

Abstract

To avoid base line drift of a pressure transducer which may be positioned at a site which is not readily accessible, e.g., within a patient, pressure is detected by a first transducer at the site and also by a second external transducer 5 in communication with said site. The output signal of the first transducer 1 is subjected to high-pass filtering and that of the second to low-pass filtering, the filters having a matched transfer function. The high and low frequency signals are summed by amplifier 15 to generate a combination signal representative of the pressure at the site. Transducer (1) may comprise means for modulating light reflected back through an optical fibre (3) and a liquid filled tube (4) may communicate with transducer (2). <IMAGE>

Description

PRESSURE TRANSDUCER SYSTEM This invention relates to pressure transducer systems and in particular to a method and apparatus capable of compensating for base line drift of signals derived from small pressure transducers. The invention has particular utility in biomedical applications.

In the medical and veterinary treatment of humans and animals it is often desirable to closely monitor the pressure changes in the vascular system. Various devices have been proposed for pressure monitoring which generally fall into two categories; pressure transducers positioned externally of the body and small pressure transducers which are positioned within the body.

External pressure transducers e.g. a silicon foil gauge, are connected to the vascular system of a patient via a fluid filled conduit. Generally, a catheter is inserted into a blood vessel and connected to the external pressure transducer via plastics tubing which is filled with saline solution. Whilst such a system is capable of faithfully recording the mean pressure, the presence of any air bubbles in the tubing deleteriously affects the ability of the system to accurately monitor the high frequency pressure fluctuations. Furthermore the external pressure transducer must be positioned at the level of the heart in order to ensure accurate monitoring and movement of the patient can lead to inaccuracies.

Various constructions of internal pressure and temperature transducers suitable for insertion into blood vessels are known and are disclosed for example in U.S.

Patent Nos. 3,273,447, 3,911,902, 3,942,387 and 4,176,551. The transducers typically record a change in temperature as pressure by modulating an optical, electrical or ultrasonic signal which is processed by an electrical circuit to provide a visual indication of the pressure e.g. on a screen, paper recorder etc. One of the problems associated with such small transducers is the tendency for signals corresponding to the base line or mean pressure to drift in an unpredictable manner thereby causing inaccurate pressure readings. Some base line drift appears to be inherent with most transducers but whilst external transducers may readily be recaliberated when necessary the internal transducers are inaccessible.Sources of base line drift in pressure transducer systems also arise in the electronic circuits e.g. changes in the value of resistors as the temperature changes, and in optical systems e.g. fluctuation in output of a light source and in signals generated by a photodetector as temperature varies. Attempts to alleviate this problem have resulted in low-drift catheter tip pressure transducers with temperature compensation. However, such devices suffer from the disadvantage that they are too large for use in the smaller blood vessels and are expensive to manufacture and must be repeatedly used in the interests of economy.

Sterilisation between successive use is required.

It is an object of the present invention to provide a system which will at least substantially reduce the base line drift of pressure and temperature transducers.

According to one aspect of the present invention there is provided a method of compensating for the base line drift of a pressure transducer which may be positioned at a site which is not readily accessible, which method comprises; (a) obtaining a first electronic signal representative of the output signal of the pressure transducer positioned at a desired site, (b) providing a second pressure transducer external to the desired site and in communication with said site such that the second pressure transducer provides an output signal indicative of the pressure at that site, (c) obtaining a second electronic signal representative of the output signal of the second pressure transducer, (d) subjecting the first electronic signal to highpass filtering to obtain a high frequency signal, (e) subjecting the second electronic signal to lowpass filtering to obtain a low frequency signal such that the low and high-pass filters have a matched transfer function, and, (f) passing the high and low frequency signals to a summation amplifier so that a summed output signal is generated, by the combination of the high and low frequency signals from (d) and (e) which is representative of the pressure at the site.

According to a further aspect of the present invention there is provided a system for pressure measurement comprising; (a) a pressure transducer capable of being positioned at a desired site which may not be readily accessible and means for generating an electronic signal representative of the output signal of the pressure transducer, (b) a second pressure transducer and means for providing communication between the second pressure transducer and the desire site such that the second pressure transducer will produce an output signal representative of the pressure at the desired site, the second pressure transducer being associated with means for producing a second electronic signal representative of said output signal, (c) a high-pass filter, for filtering electronic signal from (a) to provide a high frequency signal, (d) a low pass filter for filtering electronic signal from (b) to provide a low frequency signal, the low and high-pass filters have a matched transfer function, and, (e) a summation amplifier receiving said high and low frequency signals and capable of generating an output signal which is the sum of said low and high frequency signals and is representative of the pressure at the desired site.

The invention also provides apparatus suitable for use in the above described method and system comprising the high-pass and low-pass filters adapted to receive the electronic signals of the two transducers, in an electronic circuit comprising a summation amplifier to combine the signals from the high-pass and low-pass filters.

It has been found that accurate pressure readings may be obtained and base line drift abolished by taking that component of the signal derived from an internal pressure transducer which is representative of the high frequency pressure fluctuations i.e. dynamic pressure, and combining that component with the component of a signal derived from an external pressure transducer in communication with the same site which is representative of the mean static or base line pressure and by combining these components to form a signal which is an accurate representation of the pressure. Thus, the signal representative of the base line pressure from the internal pressure transducer is filtered out by the highpass filter and any base line drift of the internal pressure transducer may be ignored.The high frequency component of the signal from the external pressure transducer which may be deleteriously affected by air bubbles is similarly filtered out by the low-pass filter.

Any base line drift in the signal from the external pressure transducer may readily be compensated for by recalibration since this transducer is accessible. Thus the invention effectively combines only the accurate components of the signals from internal and external pressure transducer to provide an accurate pressure reading.

In addition to providing the advantage of accuracy the system of the invention allows simple, small catheter tip pressure transducers to be employed which are relatively inexpensive and may be disposed of after use.

Similarly the external pressure transducer and associated conduits may be cheap and disposable. The high and lowpass filters and associated electrical circuit may be readily fabricated as a unit for repeated use.

Whilst the invention is specifically described with reference to the measurement of vascular pressure with the internal transducer within the body of an animal or human it will be readily appreciated that the invention is applicable to the use of transducers in other areas of technology where it may be necessary to install a transducer in an inaccessible site e.g. chemical and manufacturing installations, fuel delivery systems etc.

Furthermore, whilst the invention is described with reference to pressure measurement it will be readily appreciated that many transducers by virtue of their construction will provide an indication of temperature fluctuations and the invention extends to the use of signals derived from a transducer for providing a measurement of temperature.

The internal pressure transducer may comprise any transducer known to the art. For medical purposes the transducer chosen must be physiologically safe and suitably sized for internal delivery to the patient. In a preferred embodiment, the internal pressure transducer comprises a catheter tip transducer. In use the internal pressure transducer is inserted into the area where the pressure is to be measured e.g. the transducer may be mounted in a flexible cannula and inserted into; the vascular system via the lumen of a blood vessel, a lymph vessel or the lungs etc.

The external pressure transducer may comprise any pressure transducer known to the art and is positioned external to the area where the pressure is to be measured e.g. outside the body. Communication between the external transducer and the internal small transducer is provided by means of fluid filled conduit e.g. liquid filled tube, to complete an external manometer. The tube may terminate adjacent to the internal pressure transducer, in approximately the same area e.g. the lumen of the same blood vessel where the pressure is likely to be the same or the internal transducer may be positioned within the tube for protection.

For non-medical purposes the manometer tubes may contain air or liquid. For medical purposes air is not acceptable as the release of even relatively small amounts of air bubbles to a patient's bloodstream will be deleterious. The liquid preferably comprises a physiologically acceptable saline buffer. In use with a patient, liquid filled manometer tubes require accurate positioning at substantially the same level as regards the small pressure transducer e.g. in monitoring the blood pressure of the heart it is common practice to position the manometer at approximately mid sternum level. The manometer line is desirably adapted to flush the region of the internal pressure transducer to remove deposits such as blood clots.

The output signal from the pressure transducer of the external manometer is electronically low-pass filtered such that only the D.C. component and very low frequencies are able to progress to the summing amplifier. The filter attenuates higher frequencies above a critical threshold while amplifying those frequencies below.

The output signal from the small pressure transducer is electronically high-pass filtered to complete the pressure spectrum at frequencies above the threshold frequency of the low-pass filter. The high-pass filter attenuates the low frequencies and amplifies the high frequencies. The filters may comprise any electronic filters known to the art but preferably can be adjusted such that the overall transfer characteristics (i.e.

frequency response) relating the pressure to be measured and the output of the summing amplifier is flat from D.C.

to any appropriate high frequency. In use, the transfer function of the filters is accurately matched so as to obtain a realistic reproduction of all the components present in the overall pressure signal being monitored.

In a preferred embodiment the post-filtered outputs for each transducer may additionally comprise a balance system, for example, a variable resistor to allow equalisation of respective output signals.

The low and high filtered output signals may be summed in an analogue or digital summing amplifier. If a digital amplifier is incorporated as a summing amplifier, means to digitise the signals e.g. the respective outputs of each filter, must be included before signal input to the amplifier. Preferably the amplifier comprises an analogue summary amplifier providing a summed output substantially free of drift. Re-calibration of the external pressure transducer may easily be achieved when required to ensure accuracy of the signal.

The invention will now be described with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a catheter tip pressure having drift compensation according to the present invention, and, Figure 2 is a theoretical- plot of the matched transfer function of the high and low-pass filters.

Referring to Figure 1, a catheter (7) comprising a catheter tip pressure transducer (1) modulating light reflected back to photodetector (2) through optical fibre (3) is inserted into an area of dynamic pressure change.

The modulated optical signal is converted to an electronic output signal by electronic circuit (6). The catheter tip pressure transducer (1) positioned within liquid filled tube (4) and is flushed to remove debris which may obstruct the lumen or transducer during insertion and use. Tube (4) communicates with external pressure transducer (5) to produce an output signal which is converted by circuit (8) to an electronic output signal. The electronic output signal of the catheter tip transducer (1) is filtered by high-pass filter (11) to produce a high frequency output signal. The electronic output signal of the external pressure transducer is filtered by low-pass filter (12) to prod a low frequency signal. The high and low frequency signals are balanced by variable resistors (13), (14) respectively and fed into a summing amplifier (15).In the embodiment shown summing amplifier (15) is an analogue summer, but with the inclusion of a digitising circuit (not shown) converting the analogue signal from each filter to a digital signal, a digitising amplifier may be optionally substituted. Output from summing amplifier (15) comprises a combination of the low and high frequency signals to provide a reading of the pressure source being monitored.

Referring to Figure 2, the respective transfer function of each filter and the summed output corresponding to the pressure source being monitored are graphically plotted. Curve (21) depicts the transfer function (modules) of the low-pass filter, curve (22) depicts the transfer function (modules) of the high-pass filter. Dotted line (23) represents a theoretical flat spectrum output corresponding to the present source, by combining the low and high filtered signal components produced by each filter. As shown both the low and highpass filters have a matched transfer function i.e. the cut off frequency (Fc) above or below which attenuation/amplification occurs is the same.

The output signal may be used to generate a visual display e.g. on a screen or graph plotter etc.#0. 9 Our co-pending British Patent Application#of even date discloses a transducer for measuring pressure or temperature comprising; a fibre optic, and, a substantially cylindrical sleeve positioned over and in sealing engagement with one end of the fibre optic and extending beyond said end, wherein the end of the sleeve extending beyond the end of the fibre optic has a seal to entrap a volume of gas within the sleeve, between the seal and the end of the fibre optic, the seal and/or the sleeve being flexible and capable of deformation upon changes in temperature of pressure. The cylindrical sleeve may conveniently comprise a portion of the protective cladding of the fibre optic and the seal may comprise silicon rubber. Such fibre optic transducers are particularly useful in the present invention.

Claims

1. A method of compensating for the base line drift of a pressure transducer which may be positioned at a site which is not readily accessible, which method comprises; (a) obtaining a first electronic signal representative of the output signal of the pressure transducer positioned at a desired site, (b) providing a second pressure transducer external to the desired site and in communication with said site such that the second pressure transducer provides an output signal indicative of the pressure at that site, (c) obtaining a second electronic signal representative of the output signal of the second pressure transducer, (d) subjecting the first electronic signal to highpass filtering to obtain a high frequency signal, (e) subjecting the second electronic signal to lowpass filtering to obtain a low frequency signal such that the low and high-pass filters have a matched transfer function, and, (f) passing the high and low frequency signals to a summation amplifier so that a summed output signal is generated, by the combination of the high and low frequency signals from (d) and (e) which is representative of the pressure at the site.

2. A method as claimed in Claim 1 in which the second pressure transducer is calibrated by a reference manometer.

3. A method as claimed in Claim 1 or Claim 2 in the summed output signal is presented on a screen.

4. A method as claimed in any one of Claims 1 to 3 in which the first pressure transducer comprises a catheter tip pressure transducer.

5. A method as claimed in any preceding claim in which the second pressure transducer communicates with the site of the first pressure transducer by means of a fluid filled tube.

6. A method as claimed in Claim 5 in which the fluid filled tube is filled with liquid.

7. A method as claimed in Claim 5 or Claim 6 in which the first pressure transducer is positioned within the lumen of the fluid filled tube.

8. A method as claimed in any one of Claims 5 to 7 in which the first pressure transducer is flushed by fluid from said tube.

9. A method as claimed in any preceding claim in which the high and low frequency signals are adjusted by one or more balance controls.

10. A method as claimed in any preceding claim in which the summing amplifier is an analogue amplifier.

11. A method as claimed in any one of Claims 1 to 9 in which the high and low frequency signals are digitised and the summing amplifier is a digital amplifier.

12. A method as claimed in any preceding claim in which the site is within a human or animal body.

13. A method as claimed in any preceding claim in which the site is a blood vessel and the first pressure transducer measures blood pressure.

14. A system for pressure measurement comprising; (a) a pressure transducer capable of being positioned at a desired site which may not be readily accessible and means for generating an electronic signal representative of the output signal of the pressure transducer, (b) a second pressure transducer and means for providing communication between the second pressure transducer and the desired site such that the second pressure transducer will produce an output signal representative of the pressure at the desired site, the second pressure transducer being associated with means for producing a second electronic signal representative of said output signal, (c) a high-pass filter, for filtering electronic signal from (a) to provide a high frequency signal, (d) a low pass filter for filtering electronic signal from (b) to provide a low frequency signal, the low and high-pass filters having a matched transfer function, and, (e) a summation amplifier receiving said high and low frequency signals and capable of generating an output signal which is the sum of said low and high frequency signals and is representative of the pressure at the desired site.

15. A system as claimed in Claim 14 additionally comprising means to calibrate the second pressure transducer.

16. A system as claimed in Claim 14 or Claim 15 additionally comprising a balance control for adjustment of the high and low frequency signals.

17. A system as claimed in Claims 14 to 16 in which the summing amplifier is an analogue amplifier.

18. A system as claimed in Claims 14 to 16 comprising means for digitising the high and low frequency signals and which the summing amplifier is a digital amplifier.

19. A system as claimed in Claims 14 to 15 comprises a fibre optic, the system additionally comprising means to an optical signal to an electronic signal.

20. A system as claimed in any one of Claims 14 to 19 in which the first pressure transducer comprised a catheter tip pressure transducer.

21. A system as claimed in any one of Claims 14 to 20 which additionally comprises means for displaying the output signal from the summing amplifier.

22. Apparatus suitable for use in a system as claimed in Claim 14 which comprises; (a) an electronic high-pass filter adapted to receive an electronic signal from a pressure transducer positioned at a site which is not readily accessible, which filter produces a high frequency signal, (b) an electronic low-pass filter adapted to receive an electronic signal from a second pressure transducer external to the site of the first pressure transducer and in communication with that site, which filter produces a low frequency signal wherein the low and high-pass filters have a matched transfer function, (c) a summing amplifier in communication with the high and low-pass filters to generate a signal which is the sum of the high and low frequency signals.

23. Apparatus as claimed in Claim 22 which additionally comprises a balance control to adjust the high and low frequency signals.

24. Apparatus as claimed in Claim 22 or Claim 23 in which the summing amplifier is an analogue amplifier.

25. Apparatus as claimed in Claim 22 or 23 additionally comprising means for digitising the high and low frequency signals and which the summing amplifier is a digital amplifier.

26. Apparatus as claimed in any one of Claims 22 to 25 additionally comprising means to visually display the output signal of the summing amplifier.