GB2185112A

GB2185112A - Respiratory flow rate measurement

Info

Publication number: GB2185112A
Application number: GB08528314A
Authority: GB
Inventors: Dr C M Lewis; Dr A E Bunn
Original assignee: Morgan Ltd P K
Current assignee: Morgan Ltd P K
Priority date: 1985-11-18
Filing date: 1985-11-18
Publication date: 1987-07-08
Also published as: GB2185112B; GB8528314D0

Abstract

Apparatus for measuring respiratory flow rate comprises a tube (10) for the passage of exhaled or inhaled gas, a flexible membrane (16) fitted in the tube, and pressure tappings (24,26) on respective sides of the membrane to detect pressure differences across the membrane, from which gas flow rate through the tube is determined. The membrane has a plurality of flaps (18) and contains apertures (19), 20. Each flap is secured at its arcuate edge to the tube, the inner extremities of the flaps deflecting resiliently with gas flow through the tube. A screen (22) prevents any detached flap from entering a user's mouth. <IMAGE>

Description

SPECIFICATION Apparatus for and method of measuring respiratory flow rate This invention relates to apparatus for, and a method of, measuring respiratory flow rate.

According to one aspect of the invention appar atusformeasuring respiratoryflowratecomprisesa tubeforthe passage of exhaled or inhaled gases, a membrane which has slits to divide the membrane into a plurality of flexible flaps which tend to occupy a normal position extending across the tube but which progressively separate with progressively increasing gas flow through the tube, meansforsensing the gas pressure in the tube on respective sides ofthe membrane, and means which are responsive to the pressure sensing meansforproviding an output representative of the respiratory flow rate throughthetube.

The membrane is conveniently made of a flexible synthetic plastics material and the outer periphery may be retained with respect to the wall of the tube, the inner area ofthe membrane being slit to define said flaps. In the preferred embodiment, the tube has a circular cross-section and the membrane is therefore also circular, the slits extending radially so that each flap is sector shaped. A hole may be formed in each flap, at or adjacentthe arcuate edge of the flap, and the radially innermost extremity of each flap may be shaped so that in the normal position of the flaps there is a central hole in the membrane. Preferably, there are eight flaps of identical shape and size, each secured around its arcuate edge atthetubewall and each subtending45#atthecentreofthemem- brane.

The tube may incorporate a screen which acts to prevent any detached membrane flap from entering the user's mouth, and in the preferred embodiment to be described the screen is a wire mesh screen disposed between the membrane and the end of the tube which serves as a mouthpiece.

The meansforsensing gas pressure conveniently iiicludetappings in the wall ofthetube on respective sides of, and close to, the membrane. The difference in pressure is representative ofthe respiratory flow rate, and the relationship between the pressure difference and flow rate may be established by an initial setting up or calibration procedure.

According to another aspect ofthe invention a method of measuring respiratory flow rate comprises passing exhaled or inhaled gases through a tube having a membrane which presents a resistance to flow which decreases with increasing flow rat-#s, sensing the pressure in the tube on respective sides ofthe membrane, and using the sensed pressure to determinethe respiratory flow rate.

Apparatus for measuring respiratory flow rate and forming the preferred embodiment ofthe invention will now be described by way of example, with refer encetotheaccompanying drawings, in which: Figure 1 is a sectional view through a tube ofthe apparatus, Figures2 and 3 are sectional views on the lines Il-Il and Ill-Ill respectively of Figure 1, Figure 4 is a diagrammatic view showing the separation of a membrane in the tube at small volumetrip flow rates, and Figure 5 shows the deflection of the membrane at higher gas flow rates.

Referring to Figure 1, the apparatus comprises a circulartube 10 one end 12 ofwhich serves as a mouthpiece so that inhaled air passes through the tube in the direction of the arrow 14. At an intermediate point along the length of the tube 10, a membrane 16 extends accross the cross-sectional area ofthetube 10. The membrane 16 is made of a silicon plastics material and is retained in the tube by a clamping ring 15.

The membrane 16 is divided by slits so as to form eight flaps 18 each of which is sector shaped and subtends an angle of45 atthe centre of the tube. The arcuate edge of each flap 18 is secured to the tube wall by the ring 15, and a hole 20 is disposed at the mid point ofthe arcuate length of each flap 18. The inner extremities of the flaps 18 are cut away to form a small central hole 19 in the membrane when the latter is in the normal undeflected position shown in Figure 1. Hence, the membrane presentstothe gas flowthe central hole 19, eight holes 20 and eight radial slits which enabletheflapsto deflect resili- entlyorelasticallywith gasflowthrough the tube 10.

To prevent the risk of detached flaps 18 being in haled by the user,thetube 10 has a wire mesh screen 22 positioned between the membrane 16 and the mouthpiece end 1 2 of the tube 10.

Figure 4 shows howtheflaps 18 separate by a small amount to allow comparatively low gasflow rates through the tube 10, and Figure 5 shows how the flaps 18 deflectto a largerextentto allowforthe passage ofhighergasflow rates. The highertheflow rate the lower the resistance presented by the mem branetothegasflow.

Gas pressure in the tube 10 is sensed on respective sides ofthe membrane 16 by respective pressure tappings 24 and 26. The difference in pressure from tappings 24 and 26 is representative ofthe gasflow rate, and the relationship between the pressure difference and gasflow rate is established by a setting up or calibration routine. Hence, the difference in pressure may be fed to a transducerfrom which may be obtained an output representative of inhalation gas flow rate through the tube 10. The relationship between pressure difference and volumetricflow rate is curvilinear and partially quadratic in character and is best described throughout its extent by a fifth degree polynomial expression. The transformation is effected by a microcomputer program.

The described apparatus has a'sensitivitya sensitivitydepen- dent on the gasflow rate and is suitablefor measuring both lowflow rates at resting ventilation levels, as well as high flow rates at peak exhalation. It will be appreciated that the apparatus could equally be used to measure gaseous flow rates on exhalation.

Thetube 10 may have a pin 28to locatethetube 10 in surrouna,ng structure.

Claims

1. Apparatus for measuring respiratory flow rate, comprising a tube for the passage of exhaled or inhaled gases, a membrane which has slits to divide the membrane into a plurality of flexible flaps which tend to occupy a normal position extending across the tube but which progressively separate with pro gressivelyincreasing gas flow through the tube, means for sensing the gas pressure in the tube on respective sides of the membrane, and means which are responsive to the pressure sensing means for providing an output representative of the respiratory flow rate through the tube.

2. Apparatus according to claim 1 wherein the membrane is made of a flexible synthetic plastics material.

3. Apparatus according to claim 1 or 2, wherein the outerperipheryofthe membrane is retained with respectto the wall ofthe tube, the inner area ofthe membrane being slitto define said flaps.

4. Apparatus according to any of the preceding claims, wherein the tube has a circular cross-section and the membrane is also circular, the slitsexten- ding radially so that each flap is sector shaped.

5. Apparatus according to claim 4, wherein a hole is formed in each flap, atoradjacentthe arcuate edge of the flap.

6. Apparatus according to claim 4 or 5, wherein the radially innermost extremity of each flap is shaped so that the normal position of the flaps there is a central hole in the membrane.

7. Apparatus according to any of claims 4 to 6, wherein there are eight flaps of identical shape and size, each subtending 45 at the centre ofthemem- brane.

8. Apparatus according to any of the preceding claims, wherein the tube incorporates a screen which acts to prevent any detached membrane flap from entering the user's mouth.

9. Apparaus according to claim 8, wherein the screen is a wire mesh screen disposed between the membrane and the end of the tube which serves as a mouthpiece.

10. Apparatus according to any ofthe preceding claims, wherein the meansforsensing gas pressure includetappings in the wall of the tube on respective sides of, and close to, the membrane, the difference in pressure being representative of the respiratory flow rate.

11. A method of measuring respiratory flow rate, comprising passing exhaled or inhaled gases through a tube having a membrane which presents a resistance to flow which decreases with increasing flow rates, sensing the pressure in the tube on respective sides ofthe membrane, and using the sensed pressure to determine the respiratory flow rate.

12. Amethod according to claim 11 and carried out by means of apparatus according to any of ciaims2to 10.

13. Apparatus for measuring respiratoryflow rate substantially as herein particularly described with reference to the accompanying drawings.

14. A method of measuring respiratoryflow rate, substantially as herein particularly described with reference to the accompanying drawings.