GB2238389A - Respirometer - Google Patents

Abstract

A respirometer for recording exhalation parameters has a duct (12) with a restricted outlet (14), through which, in use, expired air is blown to create a pressure within the duct representative of the flow rate. A pressure sensitive transducer (15) communicates with the interior of the duct (12) and produces electrical output signals which vary with the pressure of expired air in the duct. An automatic signal processing unit (20, 21, 22) produces from the varying electrical signal generated by the pressure transducer (15) signals representing one or more parameters characterising the exhalation which can be displayed on a screen (16). These parameters can also be stored in a memory (25) from which they can be read by external equipment. <IMAGE>

Description

A RESPIROMETER The present invention relates generally to a respirometer, and particularly to an improved respirometer suitable not only for clinical use, but also for more general use in monitoring fitness and general health.

Prior art clinical respirometers fall into two main types.

For long term monitoring of the patients pulmonary function the only instrument currently readily available is a rather crude peak flow meter which is designed only to produce a single mechanical reading representing the peak expired flow upon exhalation. These instruments are produced cheaply so that they can be given to patients to use over a period of time, and then discarded. Because of the necessary economies effected in their production such instruments are inaccurate and unreliable. This inherent mechanical inaccuracy due to the wide manufacturing tolerances, is exacerbated by the manner of use since it requires the patient, after having performed one or a number of trial exhalations, to read a scale himself and record the reading on paper, either as a numerical record or as a point plotted on a graph.This adds further to the inaccuracy since, not only is there the unreliability of the observer in that the patient himself may make errors in reading the scale, but there is also a lack of certainty in the fidelity of the result since a patient plotting figures representing his own state of health may well, even subconsciously, be tempted to set down on record figures which are perhaps slightly more favourable than a truly dispassionate observation of the reading would indicate.

More sophisticated electronic instruments are available to the medical profession, but these involve relatively large structures which are not easily portable, and which are far too expensive for use with individual patients. Such instruments are normally formed as hospital installations, and although they have some additional electronic sensing and processing means, including chart plotters for recording instantaneous exhalation volumes, flow rates and the like, cannot offer long term monitoring for individual patients except by repeated regular visits of the patient to the apparatus. This, clearly, is not practical if readings are required several times each day and the patient resides anything more than the most minimal distance from the hospital.Typically, a full set of observations would require exhalation tests to be taken three times per day, morning, noon and night, in order to assess what diurnal variation and changing pattern of behaviour may be experienced.

The technical problem which the present invention seeks to solve, therefore, is how to achieve accurate dispassionate, reliable readings of exhalations over a long term in such a way that only the judgment of medical practitioners can be involved in assessing the readings whilst avoiding any subjective judgment from the patient.

The solution to this problem, according to the present invention, is provided by means of a small, portable, battery or mains powered electronic respirometer provided with a memory and means for recording automatically the values of exhalations when used by a patient.

According to the present invention, therefore, a respirometer comprises a duct for receiving expired air and having an interior cavity therein, at least one outlet from the duct of smaller cross-sectional area than the inlet to the duct, and a pressure sensitive transducer in communication with the said cavity and operating to produce electrical output signals having a predetermined relationship with the pressure of expired air in the cavity.

In a preferred embodiment there is further provided an automatic signal processing unit operable to produce, from the said varying electrical signal generated by the pressure transducer, signals representing at least one calculable parameter characterising the exhalation, and means for displaying the result of such calculations. This display may make the information available to a user graphically or alpha-numerically, that is it may be an analogue or a digital display.

The said signal processing unit may be operable to calculate a plurality of different parameters characterising the exhalation, there being further provided means for selecting a parameter for display.

A plurality of such parameters are already known to the medical profession, and include, without limitation, the peak expired flow rate (that is the rate of flow of the exhaled air when at its maximum), the total expired volume of air (effectively the patient's vital capacity), the volume of air expired within a given unit of time (typically one second) and the volume of air expired during the main portion of the exhalation, that is ignoring the first 25% and the last 25% of the exhalation time. This is known as the 25/77 exhalation volume.

For this purpose the processor preferably includes, or has associated therewith, a timer producing a signal which can be used as a reference in performing computations on the varying pressure signals, whereby to generate the said signals representing the characterising parameter such as expired volume.

As referred to above, there may further be provided storage means for storing signals representative of the or each calculated parameter characterising an exhalation or a plurality of successive exhalations. Storage means may also be provided for storing successive digitised values representing an individual exhalation rather than merely the result of the computations thereon so that, at a subsequent time, a medical practitioner may review the stored information to make necessary medical judgments on the pulmonary condition of the patient.

Of course, for this purpose, the storage means must also operate to store a timer signal representing the time of recording each set of signals representing a stored exhalation in order to be able properly to put the figures in context.

The present invention also provides, according to another aspect, a respirometer having automatic monitoring means sensitive to recorded signals representing a plurality of exhalations and operable to effect comparisons between different sets of such stored signals whereby to generate an alerting output signal if any stored signal differs by more than a predetermined threshold variation value from another or a predetermined other said stored signal.

The duct through which expired air is directed in use of the respirometer is preferably removably attachable to a self-contained portable respirometer body housing the said processor, storage and display means. There may further be provided an interface port for communicating with further display and/or recording instruments whereby to allow a medical practitioner to extract recorded signals from the portable instrument for further processing, display or study.

The present invention thus provides a small portable instrument for capturing data relating to an individual patient, and for storing this data until such time as it may be examined by a medical practitioner suitably qualified to make appropriate judgments thereon. At the same time a display of the characterising parameters can also be made available to the patient so that selfchecking can be undertaken.

This allows the instrument of the present invention to be applied to different purposes from the merely medical, and in particular makes it possible for embodiments of the present invention to be produced for fitness and health monitoring. Such embodiments may be more rugged and perhaps less accurate than clinical models, but nevertheless could be produced more economically and therefore find application in fields where respirometers have until now not been used.

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 schematic perspective view illustrating the outer casing of a respirometer formed as an embodiment of the present invention; Figure 2 is a graph illustrating the usual relationship between expired flow rate and time; and Figure 3 is a block schematic diagram of the respirometer of the present invention.

Referring now to the drawings the respirometer 10 shown in Figure 1 comprises a body 11 housing the sensor and electronic computing elements which will be described in more detail below, to which is removably attachable a generally cylindrical mouthpiece 12 having an inlet opening 13 at one end and a plurality of small outlet openings 14 adjacent the other end which is in communication with a pressure transducer 15 fixed to the body 11 at the end to which the mouthpiece 12 is attachable.

The body 11 has on its upper face a display screen 16 and two push buttons 17, 18 for control of the respirometer.

At the end of the body 11 is a connector 19 for interfacing with other equipment as will be described in more detail below. In use the push button 17 is depressed to put the respirometer 10 into the "ready" state, whereupon the message "blow" appears on the display screen. The user then places the open end of the mouthpiece duct 12 into his mouth and exhales sharply and as forcefully as he is able. The pressure within the duct builds up as the exhalation reaches its peak flow rate and then dies away in a curve represented in Figure 2, which follows substantially the same pattern of variation as the rate of flow of exhaled air itself, although there is in fact a square root relationship between them. As will be seen the peak pressure occurs about one third of the way through the total exhalation.

Within the instrument the analogue pressure signal produced by the pressure transducer 15 is converted into a train of digital signals and processed to produce output signals to the display, which are stored in memory. The output signals represent the result of computations performed on the raw digital signals to produce parameters characterising the exhalation, namely the peak expired air flow rate, which can be displayed on the screen 16 upon depression of the push button 18; the total volume of expired air, or vital capacity, obtained by integrating the curve to obtain a signal representing the area under it, which again can be brought onto the display by a second touch on the push button 18; the volume expired in one second (a normalised expired air value) obtained by integrating the curve up to the point in time, one second after the commencement of exhalation, indicated tl in Figure 2, and again brought onto the screen by a third touch on the button 18; and the so-called 25/75 expired volume which is obtained by integrating the curve values between point t25 and t75 in Figure 2. These computed parameters are stored automatically in the memory against a time value generated by a clock which is started by a signal applied through the interface port 19, for example by a Doctor or hospital technician.

Figure 3 shows the main blocks in the circuit for performing these tasks. The signal from the transducer 15 is fed to an amplifier 20 and through a filter 21 to an integrated circuit micro-controller 22 incorporating a central processor unit, and programme memory, and which also receives input from a timing circuit 23 and a power supply unit 24. The micro-controller is in two-way communication with a patient trend analysis memory 25 and the output from the micro-controller 22 is connected to the liquid crystal display 16.This memory 25 stores the computed parameter signals from the microprocessor 22 at each test exhalation, together with timing signals from the timer 23, and at each exhalation the processor scans the contents of the memory 25 to check for any variations greater than a predetermined threshold value between any of the currently calculated parameters and those previously stored in memory 25. If such a variation is detected an output alerting signal is sent to the display screen 16. An audible indicator, such as a buzzer (not shown) may also be sounded.

A predetermined time after a test exhalation has taken place the processor shuts down to a quiescent state awaiting the next signal from the push button 17 to indicate that a new test exhalation is to be performed.

Other parameters may, of course, be calculated from the transducer output signal, especially in embodiments made not for clinical but for health or training use. Such embodiments may make use of a more robust and perhaps slightly less accurate transducer such as a pressure sensitive register.

Claims (12)

1. A respirometer comprising a duct for receiving expired air and having an interior cavity therein, at least one outlet from the duct of smaller cross-sectional area than the inlet to the duct, and a pressure sensitive transducer in communication with the said cavity and operating to produce electrical output signals having a predetermined relationship with the pressure of expired air in the cavity.

2. A respirometer as claimed in Claim 1, in which there is further provided an automatic signal processing unit operable to produce, from the said varying electrical signal generated by the pressure transducer, signals representing at least one calculable parameter characterising the exhalation, and means for displaying the result of such calculation.

3. A respirometer as claimed in Claim 2, in which the said signal processing unit is operable to calculate a plurality of different parameters characterising the exhalation, there being further provided means for selecting a parameter for display.

4. A respirometer as claimed in any of Claims 1 to 3, in which the processor includes or has associated therewith, a timer producing a signal used as a reference in performing computations on the varying pressure signals, whereby to generate signals representing the expired volume in unit time.

5. A respirometer as claimed in any preceding Claim, in which the processor is operable to calculate at least the peak flow rate of expired air from the signals derived from the pressure transducer.

6. A respirometer as claimed in any preceding Claim, in which the processor is operable to calculate one or more of the peak flow rates of expired air, the normalised expired volume (ie in one second), the vital capacity and the 25/75 volume as hereindefined.

7. A respirometer as claimed in any preceding Claim, in which there are further provided storage means for storing signals representative of the or each calculated parameter characterising a plurality of successive exhalations.

8. A respirometer as claimed in any of Claims 4 to 7, in which the storage means also operates to store a timer signal representing the time of recording each set of signals representing a stored exhalation.

9. A respirometer as claimed in Claim 7 or Claim 8, in which there are further provided automatic monitoring means operable to effect comparisons between different sets of stored signals and to generate an alerting signal if any stored signals differ by more than a predetermined threshold variation value.

10. A respirometer as claimed in any preceding Claim, in which the duct is removably attachable to a self-contained portable respirometer body housing the said processor, storage and display means.

11. A respirometer as claimed in Claim 10, further provided with an interface port for communications with further display and/or recording instruments.

12. A respirometer substantially as hereinbefore described with reference to and as shown in the accompanying drawings.