GB2181555A - Respiration indicator - Google Patents

Abstract

A non-quantitative respiration indicator comprises a piezo-electric transducer 14 sandwiched between two substantially in extensible flexible layers 11,12 which form a belt fitted, in use, at least part way around a patient and secured by form of adhesive tape 27,29 to the clothing of the patient. In use the piezo-electric crystal 14 is compressed regularly with the breathing of the patient as a tension is applied to the layers 11,12 between which the transducer is sandwiched. This provides a regular indication of the patient's breathing eg for use in electroencephalography. <IMAGE>

Description

SPECIFICATION A respiration indicator The present invention relates to a respiration indica tor, and particularly to an indicator operable to pro- vide a non-quantitive indication of respiration in cer tain medical situations where itis required to mon- itor a patient breathing without accurately measuring the depth of inspiration or extent of exhalation.

One such requirement arises when conducting electroencephalography.

Various known devices for providing an indication of respiration are known. One prior art device comprises a rubber tube filled with mercury or a saline solution and closed at each end. This tube is fitted around the chest of a patient prior to the commencement of the medical investigation, and the indication of respiration is obtained by detecting a voltage across the tube when a fixed current is passed through it.Thefillingofthetubeactsasavariableresistor,the resistance of which changes as the tube is stretched or contracted upon breathing. Another prior art such device comprises a rubber tube filled with graphite powder which operates in the same way.Such devices have the disadvantage that they must be passed entirely around the chest of the patient, which means thatthey must be fitted beforethe patient is lying down if he or she is to be immobilised, such as by anaesthetic. Additional difficulties are encountered if the patient is unconscious since he or she must be raised in orderto allow the tube to be passed around his or her chest. The rubbertube is also not as robust as may be desired and can occasionally be ruptured allowing escape of the filling and thereby corrupting the information gained from the signal.

An alternative known prior art respiration sensor comprises two small electrodes which can be taped to either side of the chest ofthe patient in contact with the skin, and an electric current passed between them. Again, a varying voltage is detected, which varies with the breathing due to the varying resistance ofthe chest wall itself. Again, however, this device has the disadvantage that the patients clothes must be removed in orderto be fitted and it istherefore inconvenient to use in an emergency or casualty situation where monitoring of respiration is required at short notice.

When conducting electroencephalography it is not normal to monitor respiration and the majority of el ectroencephalograms are produced without respiration being monitored: however, a requirementfor monitoring respiration does occasionally arise and the use of prior art respiration sensors in such circu mstances leads to inconvenience due to the factthat partway through andencephalogram investigation the patient will already be lying down and may be fully clothed so that either of the two known types of respiration transducer cannot conveniently be used.

The present invention seeks to provide a respiration transducer for non-quantitive indication of breathing which can be fitted to a supine patient without requiring direct skin contact or passing right around the chest of the patient.

According to the present invention a nonquantitive respiration indicaorfor use, for example, in electroencephalographyorthe like, comprises a piezo-electric transducer mounted on a flexible support having attachment means at each end for securement to a patient or a patients clothing, the mounting being such that the transducer can be stressed by the flexible support to generate an electr ical signal in response to the displacement of a pat ients chest upon breathing.

The major advantages of the respiration indicator of the present invention lie in the fact that the flexible supportforthe piezo-electrictransducer does not have to pass right around the chest of a user, and can be attached outside the clothing at each end ofthe flexible support. This is possible because the degree to which the piezo-electric transducer is stressed, which would vary in dependence on the manner in which the flexible support is attached to the patient, is not of importance, but rather the rhythm or period in which the stress variations arise.

The electrical output from the piezo-electrictrans- ducer is preferably fed to a high freq u ency filter o per able to attenuate the frequency components in the signal in the vicinity of and above the 50 Hz mains frequency.

Conveniently the piezo-electric transducer is mounted between two layers of flexible material in the form of a belt having attachment means at each end. Such attachment means may be any convenient form, although in the medical field adhesive tape is often used and it is envisaged that attachment to a patients clothing by adhesive tape or the like will be found to be the most convenient.

The outputfrom the high frequency filter is preferably connected to a voltage divider operating to pro duceanoutputsignal in the microvolt region from the input from the filter which is in the millivolt region.

The output from the voltage divider is preferably supplied to a screened cable leading to a pair of plugtype connectors for connection to an electroencaphalogram.

One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a respiration trans- ducerformed as an embodiment of the present in- vention, partly cut away to show the interior; Figure2 is a circuit diagram illustrating the circuit of the present invention; and Figure 3 is a perspective view illustrating the respiration transducer in use.

Referring now to the drawings, the embodiment shown comprises a flexible belt comprising a lower layer 11 and an upper layer 12 secured together atthe edges with an interspace 13 between them.

Mounted on the lower layer 11 is a piezo-electric crystal transducer 14which is sandwiched between the lower layer 11 and the upper layer 12 of the belt.

Output lines from the piezo-electric transducer 14 lead to a signal modifying circuit generally indicated 10, also mounted between the layers 11 and 12, but with a rigid backing board to secure them in position, and the output from the signal modying circuit 10 is passed on two lines 23, 24to a sheathed cable 9 from which they emerge at the other end to be connected to respective jack plugs 25,26.

Referring now to Figure 2, the electrical circuit shown comprises two lines 15, 16 connected to the piezo-electric transducer 14. A resistor 17 is connected between the line 15 and a circuit node 18, and a capacitor 19 is connected between the circuit node 18 and the line 16. The capacitor 19 and resistor 17 are dimensioned to act as a filter attenuating all frequencies above a critical threshold which may be present on the output of the transducer 1 4 as noise. Itwill be appreciated thatthe desired signal varies at a very lowfrequency ofonlyfew samples per minute (between 10 and 20) and consequentlytheterm "high frequency" as used herein will be understood to refer to frequencies significantly greaterthan the signal of interest, for example anything more than a few Hz.

From the circuit node 18 the resistor 17 and capacitor 19 are connected across a voltage dividercomprising two series resistors 21,22 and the output lead 23 is taken from the junction between the resistors 21 and 22 whilst the output lead 24 is taken from the1un- ction between the resistor 22 and the capacitor 19.

In use of the transducer of the present invention it is not necessary to remove the patients clothing, it being sufficient to provide a firm contact in terms of mechanical stress between the piezo-electric trans- ducer 14 and the patients chest movement. This can be achieved adequately by attaching the ends ofthe housing beltto the patients clothing as illustrated in Figure 3, for example by means of strips of adhesive tape 27,29 each of which has an adhesive face 28,30 respectively.As the patient inhales the belt 11, 12 is drawn tight against the patients clothing thereby stressing the piezo-electric crystal 14and causing a voltage to appear across the lines 15,16, and upon exhalation the clothing is loosened, the tension in the belt released and the piezo-electric crystal allowed to relax so that the voltage signal disappears.

As explained above the output signal on the lines 15,16from the piezo-electric crystal 14 is in the millivolt region which is much higherthanthesignals for which an electroencephalogram such as the machine 31 illustrated in Figure 3 can handle, and it is forthis reason that the voltage divider 21,22 is provided so that the voltage eventually appearing on the terminals 25,26 is in the microvolt region.Because of variations in the manner in which the belt 11, may be attached either over the patients clothing or directlyto the patients chest (the adhesive tape 27,29 being used in exactly the same manner in each case) the precise degree of stressing of the piezo-electric crystal 14 during breathing will vary and therefore the amplitude ofthe signal appearing across the piezo-electric crystal 14 may vary substantially. How- ever, since it is the frequency of the signal rather than its amplitued which is of importance in determining the patients breathing rate (or even that the patient is breathing atall)thevariation in amplitude are uni mportant.

The values forthe components are, however, im portant in establishing the correct range of values to be handled by the EEG machine and for this purpose, in the specific embodiment described above, the resistor 17 has a value of 150 K homs, the capacitor has a value of 1.0 microFared, the resistor 21 has a value of 3.3 Megohm and the resistor 22 has a value of 330 Khom.

It will be appreciated, however, that circuits incorporating different circuitvalues for the components may achieve the same desired effects and are thus intended to be incorporated within the scope of the present invention.

Claims (10)

1. A non-quantitative respiration indicator for use in electroencephalography comprising a piezoelectric transducer mounted on a flexible support having attachment means at each end forsecure- ment to a patient or a patient's clothing, the mounting being such that the transducer can be stressed by the flexible support to generate an electrical signal in response to the displacement of a patient's chest du r ing breathing.

2. A respiration indicator as claimed in Claim 1, in which the electrical output signal from the piezoelectric transducer is fed to a high frequency filter op erating to attenuate the frequency components in the vicinity of and above the 50 Hz mains frequency.

3. A respiration indicator as claimed in Claim 1 or Claim 2, in which the piezo-electrictransducer is mounted between two layers offlexible material in the form of a belt.

4. A respiration indicator as claimed in Claim 3, in which the belt is provided with attachment means at each end allowing itto be secured to a patient's cloth- ing.

5. A respiration indicator as claimed in any of Claims 2,3 or4, in which the outputfrom the high frequency filter is connected to a voltage divider operating to produce an output signal in the microvolt region from the millivolt input.

6. A respiration indicator as claimed in Claim 5, in which the output from the voltage divider is taken via a screened cable to a pair of connectors for connection to an electroencephalogram machine.

7. A respiration indicator as claimed in any of Claims 1 to 6, in which the said attachment means are releasable fasteners for securing the associated end of the flexible support to the patient's body or clothing without contact with the other end of the flexible support.

8. A respiration indicator as claimed in Claim 7, in which the said releasable fasteners comprise adhesive elements orflexible hook-type fasteners (Velcro) which can be quickly and easily positioned on or released from the patient's clothing.

9. A respiration indicator as claimed in any of claims 3 to 8, in which the said layers are secured together along their edges whereby entirely to enclose the piezo-electric element.

10. A non-quantitative respiration indicator sub- stantially as hereinbefore described with reference to, and asshown in,the accompanying drawings.