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Description  |
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This invention concerns human ventilatory capacity measurement instruments
and more particularly such instruments of the kind which measure the
maximum or peak flow rate of a single forced expiration.
According to the present invention an instrument of the kind in question
comprises: a hollow cylindrical body; a rod mounted substantially
coaxially within said body; a piston slidably mounted on said rod; spring
means connected between said piston and said body, in the annular space
between said body and rod, to apply a progressively increasing bias
against movement of said piston from one end to the other end of said
body; a slot formed in and extending along said cylinder; an indicator
member mounted in said slot for movement therealong towards said body
other end by engagement with said piston, but separate movement towards
said body one end; and a scale on said body alongside said slot.
In use of the proposed device a forced expiration is applied to the body
one end to impel the piston against the spring bias. As the piston is
impelled it moves the indicator along the slot and also exposes more of
the slot to vent the expired air from the cylinder in progressively
increasing manner. Thus, the piston travels to the point at which the
venting of the body balances the expired air input and then subsequently
returns under spring action while the indicator member remains to mark the
point of maximum travel relative to the scale.
For a fuller understanding of the invention one embodiment thereof will now
be described with reference to the accompanying drawings, in which:
FIGS. 1a and 1b respectively illustrate the body of the embodiment in end
and side elevations;
FIGS. 2a, 2b and 2c respectively illustrate an end member for the body in
one end, side, and opposite end elevations;
FIGS. 3a, 3b and 3c similarly illustrate another end member for the body;
FIGS. 4 to 8 respectively illustrate the rod, piston, spring, a spring
connecting member, and the indicator member of the embodiment;
FIGS. 9 and 10 illustrate an alternative indicator member,
AND FIG. 11 is a side view illustrating the assembled device of the present
invention.
Referring to the drawings, the body of FIG. 1 comprises a right circular,
hollow, cylindrical body 10 having two diametrally-opposed flats 11 formed
axially along its outer surface. The body outer surface is also formed
with an axially-directed groove 12 therein, the groove having a flat base
13 and a shoulder 14 along only one side thereof, and being located
approximately centrally between the two flats 11 in the circumferential
direction around the body. Lastly, the body is formed with an
axially-directed slot 15 extending between points adjacent the ends of the
groove 12, a notch 16 in one end of the groove 12, and a scale 17 located
on the groove base 13 alongside the slot 15.
The first end member, illustrated by FIG. 2, comprises a circular disc 20
with flats corresponding to those of the body, and having shorter and
longer hollow circular cylindrical bosses 21 and 22 extending coaxially
from respectively opposite sides thereof. The diameter of the disc 20 is
equal to the external diameter of the body, and the external diameter of
the shorter boss 21 is equal to the internal diameter of the body. The
longer boss 22 is hollowed in convergently tapered form towards the disc
20, and is formed with a plug 23 extending coaxially partway therealong
from the disc. The plug 23 is hollowed to communicate, through the disc
20, with the hollow of the shorter boss 21, part of the former hollowing
consisting of a tapped coaxial bore 24 in the end wall of the plug remote
from the disc 20. The disc 20 and the adjoining end wall of the longer
boss 22 bordering the annular gap between the radially outer wall and plug
of the latter is formed with six axially-directed apertures 25
therethrough, these apertures being uniformly spaced in a circumferential
sense around the end member. Lastly, a key member 26 is formed on the
outer surface of the shorter boss 21 to extend radially outwardly along
the adjoining face of the disc 20, this key member being complementary
with the notch 16 of the body 10.
The second end member, illustrated by FIG. 3, also comprises a circular
disc 30 having opposed shorter and longer circular cylindrical bosses 31
and 32 extending coaxially from respectively opposite sides thereof. The
external diameters of the disc 30 and shorter boss 31 respectively equal
the external and internal diameters of the body 10, and the longer boss 32
is of smaller diameter than the shorter boss 31. The disc 30 and shorter
boss 31 are formed with a plurality of axially-directed bores 33
therethrough, in a uniform annular array extending circumferentially
therearound at locations radially beyond the longer boss 32, and the disc
and both bosses are coaxially bored at 34 to a diameter slightly less than
that of bore 24 in the first end member. Lastly, the longer boss is
externally knurled to facilitate manual gripping thereof.
The rod of FIG. 4 is of circular cylindrical form denoted at 40 having
equal diameter with the bore 24 of the first end member, and is threaded
over a short length 41 at one end thereof in corresponding manner with
such bore. The rod is also formed with a diametral bore 42 adjacent said
length 41.
The piston of FIG. 5 is denoted at 50 in the form of a circular disc 51
having a boss 52 extending coaxially from one side thereof. The boss 52 is
of reduced external diameter over an end portion 53 thereof remote from
the disc, which portion is threaded. Also, the disc 51 and boss 52 are
coaxially bored at 54, an end portion 55 of this bore remote from the disc
51 being of a reduced diameter equal to that of the rod 40.
The spring of FIG. 6 is a tension spring 60 of helical form having one end
portion 61 which terminates diametrally.
The spring connecting member of FIG. 7 is a sleeve 70.
The indicator member of FIG. 8 comprises a stud 80 having an end plate 81
serving as a pointer, a stem 82 projecting perpendicularly from the plate
and an enlarged end portion 83 for the stem. The stud is also associated
with a spring 84 in the form of two washers 85 integrally connected by a
strip 86 therebetween, the washers snapping over the stem end portion 83
with the strip 86 bent to spring the washers into mutually separated
positions on the stem.
Assembly of the embodiment of FIGS. 1 to 8 comprises pressfitting of the
non-threaded end of the rod in the second end member bore 34 with the rod
extending from the boss 31. The open circular end of the spring 60 is
screwed on to the threaded portion 53 of the piston 50 and the sleeve is
located over the end of the spring engaged around the piston portion 53 to
assist in securing the spring. The piston and spring assembly is then slid
on the rod 40, piston disc 51 first, and the diametral end 61 of the
spring 60 engaged in the rod bore 42. Then the boss 21 of the first end
member is engaged in the notched end of the body 10 with the key member 26
received in the relevant notch 16, and the second end member is engaged in
the other end of the body to allow threaded engagement of the rod portion
41 in the tapped bore of such member. Before this last engagement the
indicator member stud 80 is passed through the body slot 15 with the plate
81 outermost, and the associated spring 84 snapped on the stud from within
the body to spring load the stud on the cylinder.
In use of the assembled instrument a disposable mouthpiece (not shown), of
the kind employed with existing peak ventilatory flow measuring
instruments, is engaged in the tapered hollow of the first end member boss
22, the indicator member is set to zero on the scale 17 by abutting the
same against the piston, and the patient effects a forced expiration into
the mouthpiece. As discussed generally hereinbefore, the expired air
passes into the body through the first end member apertures 25 to impel
the piston along its rod against the bias of the spring 60 until the
increasing venting effect of the slot 15 balances the air input. On
termination of the expiration, the piston returns under spring action but
leaves the indicator member stud positioned to mark the peak air flow
relative to the scale.
The structure of the invention as exemplified by this embodiment is
advantageous in its compactness, and simplicity and economy of manufacture
compared with existing instruments. The compactness stems from the
generally cylindrical nature of the instrument which is easily gripped and
applied to the mouth, and from the fact that all moving parts are confined
within the bounds of the cylindrical shape. As to simplicity and economy
of manufacture: it will be seen that all parts with the exception of the
helical spring 60 can be made by moulding from plastics material with the
requirements for machining and like operations being reduced to a minimum.
In addition, it is to be noted that the relevant embodiment can be readily
dissassembled for cleaning.
Also, it is preferred that the slot be of uniform width and that the spring
be of substantially constant rate, so that, in the result, the piston
movement has a curvilinear relationship with expiration flow rate.
Accordingly, the scale is of corresponding curvilinear form and it is
possible to accommodate a range of flow rates which take account of child
and adult use of a single instrument. This has been confirmed during
development of the invention with an instrument of the form just described
having overall dimensions of about 140mm length and 45mm diameter, a body
of about 110mm length and 40mm internal diameter, a slot of about 2mm
width, a piston weight of about 3gm, a spring with a rate of about
40gm/cm, and an indicator movable in response to an acceleration greater
than 3g, which prototype provided results comparable with existing
instruments over a range from 20 to 800 liters/minute expiration rate.
However, the characteristics of the instrument can be modified readily by
changing the spring 60, which is easily accessible, and changing the
scale.
A further advantage in manufacture can arise from the fact that the piston
50 need not be a precise fit on the rod 40 and within the cylinder 10, but
can be free to wobble slightly by virtue of the relatively short bore
portion 55. This eases manufacturing tolerances and assembly, and any
resultant leakage of expired air past the piston during use of the
instrument is equivalent simply to a prolongation of the slot 15 in the
reducing direction of the scale 16. Indeed, no significant error is found
to arise in use of the feature. However, available moulding techniques
should allow the provision of sufficiently accurate tolerances that little
leakage past the piston occurs, and the piston can accordingly have a
uniform bore in place of 54,55 if preferred.
While the invention has been more particularly described with reference to
the embodiment of FIGS. 1-8, it is not intended to be limited thereby, but
is capable of modification and variation within the scope of the appendant
claims. Indeed the invention is currently being further developed for the
purposes of large scale production. In this development one end member is
integrally moulded with the cylinder, and both of the end members are
apertured to form a more pronounced spoked wheel form to enhance air flow
therethrough while still affording a rigid construction. Also, entry of
the spring-loaded indicator member during assembly is facilitated by
opening the associated slot at one end of the cylinder, this opening being
closed by a key equivalent to 26 in FIG. 2 on the separable end member. In
this connection, a simplified integral sprung indicator member is now
preferred, such member having a general form as illustrated by FIGS. 9 and
10. The member is formed from a metal blank denoted at 90 in FIG. 9 which
blank has end portions 91 and 92 of greater width than the associated
slot, such portions being connected by a strip 93 narrower than the
relevant slot. This blank 90 is formed to a generally spiral shape as
shown in FIG. 10 to serve as a spring. In use, the strip 93 is engaged in
the cylinder slot 15 denoted in broken outline with one end portion 91
located within the cylinder to engage the piston disc 51 also denoted in
broken outline, and the other end portion 92 located in the groove 12
outside the cylinder. It is to be noted that the spiral shape is
orientated relative to the piston so that movement of the latter in the
direction denoted by arrow due to expiration by a user will urge the
piston against portion 91 to open the spiral shape and so facilitate
movement of the member, and the same action occurs when subsequently
pushing the portion 92 in the opposite direction to reset the member. The
portion 91 will, of course, need to be formed to a laterally curved shape
for accommodation in the cylinder, and the portion 92 is conveniently
formed to incorporate a marker and to have an unflattened shape to
facilitate pushing, such formations being denoted by broken fold lines.
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