PEAK FLOW MONITORING DEVICE

Abstract: Portable peak expiratory flow rate (PEFR) monitoring devices are known that use whistles or reeds that are theshold-activated in order to register PEFR. This invention provides a PEFR monitoring device including a threshold-activated reed (16) located in an outlet formed in a body. The body comprises inlet and outlet body shells (12, 14) that are interengageable with one another in a plurality of rotational positions of the shells relatively to one another. A vent opening (26) extends axially along the length of the outlet shell (14). The inlet shell (12) includes a mouthpiece (18) that extends into a cylindrical occluder (20), the upper edge (20.1) of which is spirally shaped. When the shells are interengaged, the occluder (20) extends into the outlet shell to occlude the vent to a greater or lesser extend, depending on the rotational position of the shells (12, 14), thereby to direct a greater or lesser proportion of the flow through the reed assembly (16). The body shells (12, 14) that are lockable to one another to set the vent outler size, normally in accordance with a predetermined treatment protocol.

Description: TITLE: PEAK FLOW MONITORING DEVICE This invention relates to a flow monitoring device, notably a peak expiratory flow monitoring device. BACKGROUND TO THE INVENTION: Peak expiratory flow rate (PEFR) is a measure of lung function of a person which can be easily and accurately determined by several devices. Home monitoring of PEFR has gained acceptance as a means of monitoring the states of asthma. It can be monitored reliably by co-operative subjects without the help of technically skilled personnel and, since its original definition in 1959 (Wright BM, McKerrow CB: Maximum forced expiratory flow rate as a measure of ventilatory capacity - British Medicine, 2, 1041 - 1959) it has found wide application. A number of portable PEFR measuring devices are presently available commercially. They vary in portability, graduation and cost and work, essentially, on two different principles. The first involves the displacement of a spring loaded piston, the total displacement of which depends on both the pressure arising from the generated flow and the area of a vent located behind the piston. The second class of devices utilises whistles or reeds that are normally threshold-activated in order to register PEFR. The first class of devices includes well known devices such as the Wright Peak Flow Meter and the Vitalograph Pulmonary Monitor. One of the very early devices of second kind is described in British Patent No. 1,018,387 - De Bono, an evaluation of which is reported in The Lancet, 31 July 1965, 212 - Colley JRT, Holland WW. The devices of the second class include a peak flow whistle, a first prototype of which is described in Annals of Allergy, 47, August 1981 - 95 to 98 - Chiaramonte LT, Prabhu SL: Peak flow whistle: preliminary report - and a latter form of which is described in Annals of Allergy, 52 March 1984, 155 to 158 - Chiaramonte LT, Goldstein S, Rockwell W: Report of a newly redesigned peak flow whistle. This device also forms the subject of USA Patent No. 4,421.120 - Edwards. Another device of this kind is described in USA Patent No. 5,357,975 - Kraemer, which describes a whistle with a sound generator designed and constructed using general wind instrument characteristics. This devices employs time and sound measurement electronics with the aim of assessing the subject's flow volume curve. The De Bono whistle consists of a plastics tube with a kettle-type whistle at one end. A disposable cardboard mouthpiece fits over the other end and a leak hole is formed down one side of the tube, the size of the hole is adjusted by moving the mouthpiece. A critical level of airflow through the whistle is needed to produce sound and the disposable mouthpiece is progressively withdrawn to increase the size of the leak hole thereby to require a progressively higher rate of airflow in order to produce a whistle. This device suffers from the disadvantage that the whistle loses its intensity and sharpness at low flow rates. In addition, the flow of exhaust air and any expiratory wheezes produced by the subject tend to swamp or mimic the whistle. The Edwards peak flow whistle utilises a threshold activated reed to register peak flow in that the reed sounds only at openings which are equal to or less than the peak flow of the subject. The vent opening is determined by setting a rotatable closing plate, prior to use, at a known "critical" level of a predicted peak flow for the subject. The subject is then required to blow through the device, widening the opening with each successive expiratory effort until the whistle no longer sounds. The last position of the closing plate at which the whistle can just be heard gives the peak flow value. Both the De Bono and Kraemer devices suffer from the disadvantage that their sound generators do not have a clearly determinable threshold. SUMMARY OF THE INVENTION: According to this invention a flow monitoring device comprises a signal generator that is adapted to generate a signal in dependence on the achievement of a predetermined volume flow rate of fluid across the signal generator, the signal generator being located in a fluid flow passage extending between an inlet and a vent outlet formed in a body, the size of the vent outlet being variable to vary the fluid flow resistance characteristics of the passage and the variation of the vent outlet being settable at a predetermined variation of the vent outlet size. The size of the vent outlet is preferably incrementally variable and settable. In the preferred form of the invention, the sound generator is constituted by a reed located in an outlet formed in the body, the reed being threshold activated in that it is adapted to generate sound in dependence on the achievement of a predetermined volume flow rate of fluid across the reed and the flow rate of the fluid being predeterminable by the setting of the vent outlet. The reed and vent outlets are preferably separate outlets formed in the body and the reed outlet may conveniently be shaped to expand outwardly to atmosphere in a horn-like shape, thereby substantially improving the signal to noise ratio of the device. The static and dynamic flow thresholds of the sound generator or the reed are preferably close to one another in value, to minimise false positive sound generation due to impulse-like flow patterns, such as might occur during explosive as opposed to smooth expiratory flow through the device. In order further to improve the signal to noise ratio of the device, the dominant frequency of the sound generator is set to within the greatest area of human aural sensitivity, preferably between 800Hz and 2000Hz and conveniently at approximately 1000Hz. It is possible to minimise erroneous sound generation due to large fluctuations in the flow, such as might occur during explosive as opposed to smooth expiratory flow through the device, simply by displacing the sound generator with respect to the principal fluid flow path through the body. This is best done by placing the sound generator downstream of a transverse barrier extending across the flow path. The signal to noise ratio of the device can be improved yet further if the body is adapted to form a resonance chamber for the sound generator. To this end, the body may be shaped in the form of a substantially bulbous resonance chamber. In a form of the invention that is suitable for manufacture in injection moulded plastics, the device may comprise a plurality of body shells that are interengageable to define the body, the outlet being formed in one body shell and another of the body shells being provided with an occluder for the outlet, the shells being movable relatively to one another for the occluder to occlude the outlet to a greater or lesser extent, depending on the relative positions of the shells thereby to vary the size of the vent outlet and the relatively movable shells being at least partially lockable to one another to set the vent outlet at a predetermined variation of the vent outlet size. In this form of the invention, the body may comprise inlet and outlet body shells that are interengageable with one another in a plurality of positions in which the shells are rotated relatively to one another, the outlet shell being constituted by an external wall that encloses a resonance chamber formed with the vent opening and the inlet shell being constituted by a mouthpiece that extends into a cylindrical occluder that is adapted for location within the outlet shell and the upper edge of which is spirally shaped, the vent opening in the outlet shell extending axially long the length of the outlet shell and the occluder being adapted, when the shells are interengaged, to extend into the outlet shell to occlude the vent to a greater or lesser extent depending on the rotation of the shells relatively to one another. The inlet and outlet body shells may conveniently be formed with complementary engagement formations by means of which the two shells can be clipped together, the engagement formations being constituted by an inwardly directed bead formed on an engagement end of an external wall of the outlet shell and an outwardly directed undercut groove formed on a flange extending peripherally about the exterior of the outlet shell, the bead and the engagement end of the inlet shell and the groove and the flange being dimensioned for the inlet and outlet shells to clip together with the bead engaging within the undercut groove. The outlet may be defined by inwardly curved portions of the outer wall of the outlet shell, the flange on the inlet shell being formed with an engagement ring formed with outwardly directed notches that are shaped complementally to the base ends of the walls defining the vent outlet, the base ends of the curved walls being interengageable with the notches in the engagement ring, thereby to lock the inlet shell to the outlet shell in a predetermined rotational position. The outwardly directed notches may conveniently be adapted to define relatively small increments of rotation. In this form of the invention, the occluder may be provided with a stop formation that is positioned to catch against the curved walls at the maximum rotational positions of the inlet shell relatively to the outlet shell. While the description that follows is directed to a flow monitoring device that is intended for use as a peak expiratory flow monitor, the device can be adapted for the bidirectional passage of fluids if the sound generator is adapted to generate sounds in either direction of flow across the sound generator. A bi-directional reed would be an example of such a sound generator. Brief description of the drawings In the drawings: Figure 1 an exploded perspective view of the monitoring device according to the invention, seen from the outlet end of the device; Figure 2 is an exploded perspective view of the device of figure 1, seen from the inlet end of the device; Figure 3 is a diagrammatic section on a line 3-3 in figure 2; Figure 4 is an isometric view of the reed assembly of the monitoring device according to the invention; Figure 5 is a diagrammatic section on a line 5-5 in figure 4; Figure 6 is a diagrammatic section on a line 6-6 in figure 5; Figure 7 is a side elevation (partly in section) of the device according to the invention; and Figure 8 is an end elevation (partly in section) of the device according to the invention. Description of embodiments of the invention The monitoring device of the invention is not a flow meter in the sense that it does not measure absolute flow through the device. Instead, it is a device intended to monitor the peak expiratory flow rate (PEFR) of a subject. This is done by monitoring the achievement or otherwise of a predetermined threshold PEFR by the subject. In this sense, the monitor of this invention is a threshold monitor in that it monitors the subject's achieved PEFR as compared to a diagnostic threshold. The predetermined PEFR is a diagnostic threshold that will depend on the treatment protocol prescribed for the subject after measurement of the subject's anticipated or predicted PEFR. Referring to figure 3, the monitor 10 of the invention can be seen to comprise a pair of interengageable shells 12, 14 and a sound generator constituted by a reed assembly 16. The shell 12 is essentially an inlet shell and the shell 14 is an outlet shell that are joined together in the manner described below with reference to figures 7 and 8. In practice, the subject breathes in as far as possible and then blows out or expires through the cylindrical tube constituting the mouthpiece 18 at the inlet end of the inlet shell 12. The subject is normally instructed not to cough nor to allow explosive expiratory flow, such as spitting, coughing or interrupted flow resulting from tongue obstructions of the mouthpiece, but as will be seen below, the monitoring device 10 of the invention is designed to minimise the possible inaccuracies arising from such explosive expiratory flow. The outlet shell 14 is constituted by an external wall 24 that encloses a resonance chamber 46 formed with a vent opening 26 that extends axially long the length of the outlet shell 14. A central housing 22 for the reed assembly 16 extends axially within the resonance chamber 46. The shells 12, 14 are intended for interconnection with one another (by means of interengaging clip formations which will be described in further detail below and the reed assembly 16 is intended for location within the horn-shaped housing 22. The mouthpiece 18 extends into a cylindrical occluder 20, the upper edge of which is spirally cut. When the shells 12, 14 are joined, the occluder 20 extends into the outlet shell 14 to occlude the vent 26 to a greater or lesser extent depending on the location of the upper edge of the occluder 20 relatively to the vent 26, which depends, in turn, on the relative degree of rotation of the shells 12, 14 relatively to one another. When the subject blows through the mouthpiece 18 and the outlet shell 14, a portion of the subject's expired flow will escape through the vent outlet 26 and a portion thereof will be directed through the reed assembly 16. By rotating the inlet shell 12 relatively to the outlet shell 14, the spiral occluder 20 occludes the vent opening 26 to a greater or lesser extent thereby determining the extent to which expired flow will be directed through the reed assembly 16. If sufficient flow is directed through the reed assembly 16, it will generate sound. Referring to figures 4, 5 and 6, the reed assembly can be seen to consists of a pair of baffle plates 28, 30 each occluding opposite halves of the ends of a tubular housing 32. A reed bed 34 extends axially between the baffle plates 28, 30, the reed bed 34 being formed with an opening 36 within which a resiliently deformable reed 38 is set by the attachment of the fixed end 40 of the reed 38 into the reed bed 34. The reed assembly 16 is non-linear (a linear whistle being one that will produce, for example, double the sound intensity for a doubling in the flow rate) , and the length, shape and mass of the reed is chosen so as to produce a sound within the frequency range that human hearing is most sensitive. The reed 38 is adapted to produce sound with a pitch of approximately 1000Hz. While the reed 38 is non-linear, the device 10 can, in effect, be "linearised" by proper design of the spiral occluder. More importantly, the reed assembly 16 provides a clear and rapid onset of sound when its flow threshold is exceeded. in this regard it can be viewed as an analogue-to-digital device which is either "on" or "off" with nothing in between. To this end, the reed assembly 16 is manufactured to exacting tolerances to ensure minimum variation from unit to unit so that each reed 38 is activated at the same flow threshold. This eliminates the need to calibrate every monitoring device 10. In addition, the reed 38 is shaped to ensure that the dynamic and static flow thresholds of the reed assembly 16 are close to one another to minimise erroneous sound generation due to large fluctuations in the flow, such as might occur during explosive as opposed to smooth expiratory flow through the device 10. Referring to figure 3, the reed assembly 16 is pressed into the housing 22 extending down the interior of the outlet shell 14, the outer shape of the reed assembly 16 and the inner shape of the housing 22 being complementally frusto-conical. The inlet end of the reed assembly 16 is open to the interior of the inlet end 42 of the housing 22 and the outlet end of the reed assembly opens outwardly to atmosphere within the horn-shaped mouth 48 of the housing 22. The horn-shaped mouth 48 enhances the audibility of the device 10 as does the outlet shell 14 which forms a resonance cavity 46 around the housing 22. The inlet end 42 of the housing 22 is formed with a transversely extending barrier 44 (which can be seen more clearly in figure 2) . The barrier 44 acts as a low pass filter and assists in preventing the reed 38 from being activated by the subject using explosive or impulse-like expiratory flow techniques in order to obtain an overly optimistic result of a test. The inlet and outlet shells 12, 14 are formed with complementary engagement formations by means of which the two parts 12, 14 can be clipped together. As can be seen from figures 3 and 7, the engagement formations are constituted by an inwardly directed bead 54 formed on an engagement end 52 of the external wall 24 of the outlet shell 14 and an outwardly directed undercut groove 56 formed on a flange 58 extending peripherally about the exterior of the tubular mouthpiece 18. The bead 54 and the engagement end 52 of the inlet shell 14 as well as the groove 56 and the flange 58 are dimensioned for the inlet and outlet shells 12, 14 to clip together with the bead 54 engaging within the undercut groove 56. Referring now to figures 1, 2 and 8, it can be seen that the edges of the vent aperture 26 are defined by inwardly curved portions 50 of the outer wall 24 of the outlet shell 14. The vent walls 50 are curved and aerodynamically shaped to minimise noise resulting from air escaping from the vent 26. The flange 58 is formed with a notched engagement ring 60, the outwardly directed notches of which are shaped complementally to the base ends 62 of the curved in walls 50 defining the vent 26. During interengagement of the inlet and outlet shells 12, 14, the practitioner will be required to align the base ends 62 of the curved walls 50.1, 50.2 with the notches in the engagement ring 60, thereby to lock the inlet shell 12 to the outlet shell 14 in a predetermined rotational position. The combination of the groove 56 and flange 58 engagement of the body shells 12, 14 with the notched engagement ring 60 and the base ends 62 of the walls 50, results in positive locking of the body shells 12, 14 that the subject will alter only with difficulty. By locking the inlet shell 12 to the outlet shell 14 in a predetermined rotational position, the spiral occluder 20 is locked in a particular position relatively to the vent 26, thereby to determine the effective vent opening. As has been stated above, the vent opening will be determined by the treatment protocol. The advantage of the notched engagement ring 60 is that it permits the adjustment of the monitoring device 10 of the invention in small increments. The occluder 20 is provided with a stop formation 64 that is positioned to catch against the curved walls 50 at the maximum rotational positions of the inlet shell 12 relatively to the outlet shell 14. At one end, the stop 64 will catch inside the curved wall 50.1 in the maximum "closed" position of the monitoring device 10, in which position the spirally cut upper surface 20.1 of the occluder is as close to the upper extremity of the vent 26 as rotation of the inlet shell 12 relatively to the outlet shell 14 will allow. At the other rotational extremity, the stop 64 will catch inside the curved wall 50.2 in the maximum "open" position of the monitoring device 10, in which position the spirally cut upper surface 20.1 of the occluder is as close to the lower extremity of the vent 26 as relative rotation of the shells 12, 14 will allow. In use, the clinician will determine the subject's diagnostic threshold whereupon the clinician will assemble the device 10 or instruct the subject in the assembly of the device 10, by engagement of the inlet and outlet shells 10, 12, to one another with the occluder 20 located in the position determined by the subject's treatment protocol. To assist in such assembly, appropriate markings may be made on one or more of the occluder 20, the flange 58 and the outlet shell 14. For instance, matching markings can be moulded into the outer wall of the occluder 20 and on one or both of the curved vent walls 50. Alternatively, such markings can be moulded into the outer surface of the wall 24 of the outlet shell 14 and the facing surface of the flange 58. The appropriate markings will than have to be matched up in accordance with the treatment protocol. The markings may take the form of flow rate values. The subject is required merely to implement the home monitoring regime - the subject will be told when flow should be measured and how the monitoring device is to be used. The dominant frequency of the reed 38 is well within the most sensitive auditory range of human hearing and it has a relatively high signal to noise ratio as a result of the round surfaces of the device, the large resonance chamber and the horn-shaped housing for the reed assembly 16. To improve the signal to noise ratio even further, the subject will be encouraged to turn the vent opening downwardly away from the ear during use. The outer wall 24 of the outlet shell 14 can be formed with grip formations to facilitate such a grip and also to minimise the risk of the subject accidentally occluding the vent 26 by hand. If required, the device 10 can be predesigned for use by children or adults. In the former case, the size of the vent 26 can be predetermined to permit peak flows of up to 4501.min "1 , while in the latter case, the size of the vent can be predetermined to permit peak flows of up to 8001.min *1 . The invention thus also provides a method for monitoring the peak exhalation flow rate of a subject using a device of the invention, which method is characterised in that the subject exhales into the inlet of a device in which the aperture of the vent outlet has been pre- set to correspond with the predicted value of the peak exhalation flow rate which the subject should achieve, for example by the clinician setting the relative rotational positions of the two inlet and outlet shells; and the subject monitors whether the signal generating device is actuated, indicating that the subject has achieved the predicted peak exhalation flow rate, or whether the signal generating device has not been actuated, indicating that the subject has failed to achieve the predicted peak exhalation flow rate and that remedial action, for example to take medication or to visit the clinician, is required.

Claims: CLAIMS 1. A flow monitoring device comprising a signal generator that is adapted to generate a signal in dependence on the achievement of a predetermined volume flow rate of fluid across the signal generator, the signal generator being located in a fluid flow passage extending between an inlet and a vent outlet formed in a body, the size of the vent outlet being variable to vary the fluid flow resistance characteristics of the passage and the variation of the vent outlet being settable at a predetermined variation of the vent outlet size. 2. A flow monitoring device according to claim 1 in which the size of the vent outlet is incrementally variable and settable. 3. A flow monitoring device according to either of claims 1 or 2 in which the sound generator is constituted by a reed located in an outlet formed in the body, the reed being threshold activated in that it is adapted to generate sound in dependence on the achievement of a predetermined volume flow rate of fluid across the reed and the flow rate of the fluid being predeterminable by the setting of the vent outlet. 4. A flow monitoring device according to claim 3 in which the reed and vent outlets are separate outlets formed in the body. 5. A flow monitoring device according to either of claims 3 or 4 in which the reed outlet is shaped to expand outwardly to atmosphere in a horn-like shape. A flow monitoring device according to any one of claims 3 to 5 in which the static and dynamic flow thresholds of the reed are close to one another in value. 7. A flow monitoring device according to any one of the preceding claims in which the dominant frequency of the sound generator is between 800Hz and 2000Hz 8. A flow monitoring device according to claim 7 in which the dominant frequency of the sound generator is approximately 1000Hz. 9. A flow monitoring device according to any one of the preceding claims in which the sound generator is displaced with respect to the principal fluid flow path through the body. 10. A flow monitoring device according to any one of the preceding claims in which the body is adapted to form a resonance chamber for the sound generator. 11. A flow monitoring device according to claim 12, the body of which is shaped in the form of a substantially bulbous resonance chamber. 12. A flow monitoring device according to any one of the preceding claims in which the body comprises a plurality of body shells that are interengageable to define the body, the outlet being formed in one body shell and another of the body shells being provided with an occluder for the outlet, the shells being movable relatively to one another for the occluder to occlude the outlet to a greater or lesser extent, depending on the relative positions of the shells thereby to vary the size of the vent outlet and the relatively movable shells being at least partially lockable to one another to set the vent outlet at a predetermined variation of the vent outlet size. 13. A flow monitoring device according to claim 12 comprising inlet and outlet body shells that are interengageable with one another in a plurality of positions in which the shells are rotated relatively to one another, the outlet shell being constituted by an external wall that encloses a resonance chamber formed with the vent opening and the inlet shell being constituted by a mouthpiece that extends into a cylindrical occluder that is adapted for location within the outlet shell and the upper edge of which is spirally shaped, the vent opening in the outlet shell extending axially long the length of the outlet shell and the occluder being adapted, when the shells are interengaged, to extend into the outlet shell to occlude the vent to a greater or lesser extent depending on the rotation of the shells relatively to one another. 14. A flow monitoring device according to claim 13 in which the inlet and outlet body shells are formed with complementary engagement formations by means of which the two shells can be clipped together, the engagement formations being constituted by an inwardly directed bead formed on an engagement end of an external wall of the outlet shell and an outwardly directed undercut groove formed on a flange extending peripherally about the exterior of the outlet shell, the bead and the engagement end of the inlet shell and the groove and the flange being dimensioned for the inlet and outlet shells to clip together with the bead engaging within the undercut groove. 15. A flow monitoring device according to claim 14 in which the outlet is defined by inwardly curved portions of the outer wall of the outlet shell, the flange on the inlet shell being formed with an engagement ring formed with outwardly directed notches that are shaped complementally to the base ends of the walls defining the vent outlet, the base ends of the curved walls being interengageable with the notches in the engagement ring, thereby to lock the inlet shell to the outlet shell in a predetermined rotational position. 16. A flow monitoring device according to claim 15 in which the outwardly directed notches are adapted to define relative small increments of rotation. 17. A flow monitoring device according to either of claims 15 or 16 in which the occluder is provided with a stop formation that is positioned to catch against the curved walls at the maximum rotational positions of the inlet shell relatively to the outlet shell. 18. A flow monitoring device according to any one of the preceding claims that is adapted for the bidirectional passage of fluids, the sound generator being adapted to generate sounds in either direction of flow across the sound generator. 19. A flow monitoring device substantially as described in this specification with reference to the accompanying drawings. 20. A method for monitoring the peak exhalation flow rate of a subject using a device as claimed in any one of the preceding claims, characterised in that the subject exhales into the inlet of a device in which the aperture of the vent outlet has been pre-set to correspond with the predicted value of the peak exhalation flow rate which the subject should achieve; and the subject monitors where the signal generating device is actuated, indicating that the subject has achieved the predicted peak exhalation flow rate, or whether the signal generating device has not been actuated, indicating that the subject has failed to achieve the predicted peak exhalation flow rate and that remedial action is required.

Inventors: FOURIE PIETER ROUSSEAU, FOURIE, PIETER, ROUSSEAU
Applicants: DUMMETT THOMAS IAN PETER, FOURIE PIETER ROUSSEAU, HARWILL IND PTY LTD, DUMMETT, THOMAS, IAN, PETER, FOURIE, PIETER, ROUSSEAU, HARWILL INDUSTRIES (PTY.) LIMITED
Application year: 1996
Main classification: A61B5
IPC codes: A61B5, G01F1