SLANT FABRIC SPIROMETER DESIGN

Abstract: The present air tubes include a planar resistive element which is disposed in the hollow space of an air tube for use with a spirometer. The resistive element is formed of a fabric material and is disposed within the air tube at a slant angle.

Description: SLANT FABRIC SPIRO ETER DESIGN This application claims the benefit of U.S. Provisional Application No. 60/168,203, filed November 30, 1999 and entitled SLANT FABRIC PNEUMATACH DESIGN, the contents of which are expressly incorporated herein by reference. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the field of measuring air flow and air volume discharged from mammalian lungs during pulmonary functional testing (spirometry) . More specifically, the invention is directed toward resistive elements for use with spirometers, and to spirometers using such resistive elements . 2. Description of Related Art Spirometers are devices used to measure the volume and flow rate of gas exhaled and inhaled by a user or patient such as, for example, a human being. These measurements are important for physiological studies and for diagnostic analysis of the pulmonary performance of the spirometer user. The effects of various medicines used to treat patients with pulmonary or asthmatic problems can be analyzed, for example, by monitoring the volume and flow rate of gas exhaled before and after the administration of medication. A number of articles and papers have been published over the years dealing with spirometry. In his Manual of Pulmonary Function Testing, 4th Edition, published by the C. V. Mosby Company in 1986, Gregg Ruppel, starting at page 147, describes various pulmonary testing equipment. Beginning at page 154, Ruppel describes pneumotachometers as flow sensing devices which use various physical principles to produce an analog output that can be integrated for measurement of volumes and flows. Amongst the pneumotachometers or flow sensors described by Ruppel is a pressure differential type which has an air- resistive element creating a pressure drop that is proportional to the flow of the gas or air through the tube in which the resistive element is disposed. A pressure transducer converts the pressure reading into electrical signals that can be integrated by suitable instrumentation to give air flow and volume readings. In another publication entitled Pulmonary Function Testing Guidelines and Controversies by Grune and Stratton, Inc. published by Harcourt, Brace, Jovanovich, copyright 1982, in Chapter 9 starting at page 91, is an article entitled Pneumotachography by Dr. Arthur Dawson. Dr Dawson describes a Fleisch pneumotachograph which utilizes capillary air flow resulting from the air flowing through a resistant element made up of a bundle of parallel capillary tubes in order to maintain a linear relationship between flow and pressure difference. Dr. Dawson in another article entitled How To Make The Most Of Pneumotachography appearing in the publication entitled Respiratory Management, dated Jan. /Feb. 1987 at page 46, explains that in a Fleisch pneumotachograph the resistive elements comprise one or more layers of fine metal screen so that air flowing through the resistance element generates a small pressure gradient which is measured with a sensitive manometer connected to ports on the upstream and downstream sides of the resistance. While it appears that a pneumotachometer or flow sensor or mouthpiece containing an integrated bundle of metallized tubes to provide capillary air flow may provide accurate and reliable readings, one problem is that a flow sensor containing a resistive element of this nature can be costly. Because of the cost, it may not be economically sound to make a disposable or throw-away flow sensor containing a resistive element of that nature. Moreover, a sensor of this nature is typically used repeatedly and thus must be sterilized between uses. Sterilization can affect the calibration of the flow sensor to the instrumentation with which it is used, and therefore periodic recalibration may be necessary. A number of spirometers have been constructed with non-metallic, synthetic fabric resistive elements disposed at the air discharge end of the air tube, but these devices have suffered from either complex constructions or unreliable readings. Figure 1A shows a prior-art disposable flow sensor, which has been sold by Puritan-Bennett Corporation as an FS 200 flow sensor, comprising an elongated hollow plastic tube 10a and a circular air inlet opening 11a at one end. The plastic tube 10a is flared outwardly at 12a in the fashion of the bell of a trumpet at the air discharge end. The air discharge opening is covered with an air-resistive fabric 13a, and a radially extending pressure pickoff opening or port 14a extends through the wall of the plastic tube 10a to facilitate attachment of a transducer for the conversion of the pressure differential signal to an electrical signal. Since air flow through the fabric 13a typically is not uniform across the entire breadth of the fabric 13a, a non-linear relationship between air flow and air pressure may result. Figure IB illustrates a prior-art device that has been sold by Chesebrough Pond's, Inc. under the trademark name "Respiradyne . " The device comprises an elongated hollow tube 16a with a circular air inlet opening 17a at one end and a similar circular air discharge opening 18a at the other end. The air discharge opening 18a is covered with an air- resistive fabric 20a in the form of an inverted converging cone or tent. The non-uniformity of air flow across the breadth of the air-resistive fabric 20a may produce a non-linear relationship between air flow and air pressure. U.S. Patent No. 4,905,709 to Bieganski et al. discloses yet another spirometry device, wherein the resistive member is disposed near a sealed end of the air tube. As shown in Figures 2 and 3, the device comprises a hollow elongated tube 22a with an air inlet opening 23a, a radially extending pressure pickoff port 24a, and an outlet end that is sealed with an imperforate plug 25a having a base section 26a and a proportionally tapered cone section 27a. Three rectangular air outlet openings 30a are formed through the side wall of the tube 22a. The openings 30a are spaced around the periphery of the tube 22a, and the plug 25a is located with respect to the openings 30a so that the tapered cone section 27a substantially extends over the length of the openings 30a. The air outlet of tube 22a is thus formed as a uniformly diverging annular opening for the air flow. A layer of fabric 31a covers the openings and is attached to the tube 22a using adhesive along its edges. To the extent this device purports to produce linear results, it suffers from a complex construction. Other prior-art spirometers have been constructed with the resistive element disposed between opposing, unobstructed ends of the air tube. The particular placement of the resistive element within the air tube will of course affect the performance of the overall spirometer. The resistive element is typically placed in a normal or perpendicular configuration relative to the interior wall of the air tube, at exact, predetermined distances from the two opposing ends of the air tube. A typical resistive element comprises a disk-shaped member with a large aperture through the center thereof. Other resistive elements may include disk- shaped members formed of a mesh material with no large apertures formed therein. Still other prior-art devices may include hinged windows formed in the disk shaped member, such as disclosed in U.S. Patent No. 5,743,270 to Gazzara et al . The hinged windows are adapted for opening and shutting to various extents or degrees, depending upon the air flow rate. Prior art resistive elements comprising windows have been somewhat effective for low air flow rates, or for high air flow rates, but have not provided fully effective resistance-versus-pressure responses at both high and low air flow rates. The above prior-art resistive elements often have not exhibited linear resistance-versus-flow-rate responses or have suffered from complex and cumbersome constructions. More particularly, resistive elements configured to exhibit good resistance at high air flow rates (at approximately atmospheric pressure) often do not perform adequately at low flow rates and, on the other hand, resistive elements configured to perform well at low flow rates often do not provide ideal resistance at high flow rates. Complex resistive elements can increase the cost of the spirometer and/or negatively impact the reliability of the spirometer. Moreover, prior-art resistive elements which can provide somewhat linear results may suffer from not being able to repeatedly provide consistent results through multiple uses of the resistive element by the same patient. It would be advantageous to provide spirometers and spirometer components which exhibit linear characteristics and which can be economically, conveniently and effectively produced and used. SUMMARY OF THE INVENTION New air tubes and resistive elements for use in spirometers and spirometers including such air tubes and resistive elements have been discovered. The present air tubes and resistive elements are disposable so that after use by a patient, they are removed from the spirometer and disposed. The air tubes are almost completely biodegradable. As used herein, the term "biodegradable" means that the component or material is decomposable into more environmentally acceptable components, such as carbon dioxide, water, methane and the like, by natural biological processes, such as microbial action, for example, if exposed to typical landfill conditions, in no more than five years, preferably no more than three years, and still more preferably no more than one year. Having the air tubes biodegradable provides substantial advantages. First, when the air tubes are disposed of, the burden on the environment of such disposal is reduced relative to, for example, a non- biodegradable air tube, such as those made out of conventional plastics or metals. In addition, because the air tubes are biodegradable, they can be made of materials which are inexpensive and plentiful (readily available) . The resistive elements of the present invention preferably comprise disposable fabrics or screens. Thus, the present air tubes and resistive elements are relatively inexpensive, and easy and straightforward to produce, requiring little or no sophisticated production equipment. Since the present air tubes and resistive elements can be made economically, replacing a used air tube with a new air tube can be achieved without substantial economic impact . Spirometers employing the present air tubes provide cost effective, reliable and reproducible, measurements of the pulmonary performance of the user, with reduced risk of contamination. In short, the present disposable, biodegradable spirometer air tubes and resistive elements are inexpensive and easy to produce to acceptably precise specifications (for reproducible performance) , are effective and reliable in use, and are conveniently and effectively disposed of in an environmentally acceptable or safe manner to reduce the risks of contamination caused by spirometer use. In one broad aspect, the present invention is directed to air tubes and planar resistive elements, which are disposed non-perpendicularly to the direction of air flow travel through the air tubes, each of the present air tubes comprise a tubular portion which defines an open inlet, an open, preferably opposing, outlet and a hollow space therebetween. The tubular portion is sized and adapted to be removably coupled to the housing of a spirometer. The air tube is disposable, i.e., can be removed or decoupled from the spirometer housing and disposed of without disposing of the housing. Substantially all of the tubular portion is preferably biodegradable. The open inlet is sized and adapted to be received in the mouth of the user of the spirometer. Thus, this open inlet and the area of the tubular portion near the open inlet act as a mouthpiece for the spirometer so that the user or patient using the spirometer can exhale into the air tube directly through the open inlet. No separate and/or specially configured (relatively expensive) mouthpiece/filter is needed when using the present air tubes. Each resistive element is sized to cause a pressure difference or differential as air flows across the resistive element, and is adapted for providing an alinear flow-versus-pressure response. This response is subsequently linearized with software. The resistive element has an approximately linear pressure response over a range of flow rates from zero liters per second to about 15 liters per second. According to another aspect of the present invention, an air tube is formed of a first tube, a second tube, and a collar tube. The first tube has a proximal end, a distal end, and a first diameter. The second tube, similarly, has a proximal end, a distal end, and a second diameter that is approximately equal to the first diameter. A resistive element contacts the proximal end of the first tube and the distal end of the second tube, and has a substantially planar surface that is oriented non- perpendicularly to the direction of travel of air through the air tube. A collar tube fits over both the proximal end of the first tube and the distal end of the second tube. The collar tube has an inner diameter that is approximately equal to the first diameter, and has an outer diameter that is larger than the first diameter. A through port is formed in the second tube. The through port opens directly into a hollow space defined by the tube assembly and is spaced from the resistive element. The through port provides communication between the hollow space of the tubular assembly and a pressure sensing assembly of a spirometer. The pressure sensing assembly of the spirometer compares a pressure from the hollow space with an atmospheric pressure. The tubular portions and resistive elements of the present air tubes preferably comprise biodegradable and disposable materials. Preferred biodegradable materials of construction include cardboard, paper, biodegradable polymeric materials and the like and mixtures thereof. In one particularly useful embodiment, the tubular portion is made of cardboard or paper or mixtures thereof, more preferably produced by methods analogous to those conventionally used to produce tubes around which are wound bathroom tissue. Such production methods often include forming a cardboard or paper tube over a mandrel or a like implement and then cutting the resulting tube to the desired length. In the event that the tubular portion is made from a biodegradable polymeric material such tubes can be formed by conventional polymer molding techniques. The resistive element is placed relative to the tubular portion so that the pressure difference for any given rate of flow of air across the resistive element is substantially the same from air tube to air tube. The orientation of the resistive element is not normal to the longitudinal axis of the tubular portion. The resistive element can be placed in the tubular portion by adhering (for example, using biodegradable adhesives) the resistive element to the interior wall of the tubular portion or by joining two separate segments of the tubular portion together with the resistive element therebetween. Other methods or techniques for placing the resistive elements in the tubular portions may be employed. Preferably, the resistive elements of the present air tubes designed for use in the same spirometer are structured and configured essentially the same, so that little or no recalibration of or other adjustment to the spirometer is needed because one air tube is replaced by another air tube. Although many of the features of the present invention are described separately, more than one or all of such features can be used in various combinations, provided that such features are not mutually inconsistent, and all of such combinations are within the scope of the present invention. These and other aspects and advantages of the present invention are set forth in the following detailed description and claims, particularly when considered in conjunction with the accompanying drawings in which like parts bear like reference numerals. The present invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying illustrative drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1-3 illustrate prior-art spirometer air tubes; Figures 4, 4A and 4B are side, front and rear views of a spirometer in accordance with a presently preferred embodiment; Figures 5, 5A and 5B are exploded, assembly and cross-sectional views of the air tube of the presently preferred embodiment; Figure 6 is a cross-sectional view of the air tube of the presently preferred embodiment; and Figure 7 is a somewhat schematic illustration showing a spirometer in accordance with the present invention. DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS Referring more particularly to the drawings, Figures 4, 4A and 4B illustrate a spirometer 10 in accordance with the present invention, including a disposable, biodegradable air tube 12, a housing 14 and control electronics 16. The spirometer 10 is what is commonly known as a differential pressure spirometer and, in general, operates in a manner similar to the spirometer disclosed in U.S. Pat. No. 5,137,026 to Waterson et al. The air tube 12 is described with reference to Figures 5 and 6. The air tube 12 includes a first tubular segment 18, a second tubular segment 20, and a collar tube 21. The air tube 12 includes an open inlet 46 and an open outlet 48. The area surrounding the open inlet 46 is sized and adapted to be fitted into a human being's mouth. This mouthpiece area is employed by the patient using spirometer 10 by placing the area 46 into the mouth and exhaling into hollow space 30 of the air tube 12. The air tube 12 is preferably substantially biodegradable, while the resistive element is preferably disposable. The tubular segments 18, 20, and 21 are made of biodegradable cardboard or heavy paper, for example, in a manner similar to how cardboard tubes are conventionally made, such as for use with bathroom tissue and the like products. These segments 18, 20, and 21 are preferably coated with a thin glossy layer. A resistive element 22 fits between the first tubular segment 18 and the second tubular segment 20. The resistive element 22 preferably comprises a mesh having a relatively high ratio of open area to total surface area. This construction may be achieved, for example, by decreasing the diameter of the threads or members used in the mesh. As presently embodied, the resistive element 22 comprises a polyester fabric and, more preferably, comprises PES 53/40 polyester (food grade) mesh made by Saaditech. The resistive element 22 material may, alternatively, comprise any polyester, nylon, or screen. In accordance with one aspect of the present invention, the material should be somewhat resistant to moisture. With reference in particular to Figures 5, 5A and 5B, the first tubular segment 18 and the second tubular segment 20 are preferably formed to have diagonal, complementary rims that fit together. The two rims can be formed, for example, by slant cutting a single tube in half, using, for example, a band saw, to form the first tubular member 18 and the second tubular member 20. In the presently preferred embodiment, the two rims angle up and away from the sensing leg 76, as shown in Figures 5A and 5B. The two rims of the first tubular segment 18 and the second tubular segment 20 are preferably formed to accommodate a fabric and to hold the fabric in a planar orientation, which is non-perpendicular to the longitudinal axis of the air tube 12. In accordance with this construction, the resistive element 22 comprises an oval or elliptical perimeter. In accordance with the present invention, when the resistive element 22 is sandwiched between the rim of the first tubular member 18 and the rim of the second tubular member 20, the plane of the resistive element 22 is oriented at an angle which is not normal to the longitudinal tube axis or direction of air flow through the air tube 12. As presently embodied, the plane of the resistive element 22 forms an acute angle with the longitudinal axis of the air tube 12 and, preferably, forms an angle that is less than about 70 degrees with the longitudinal axis of the air tube 12. More preferably, the plane of the resistive element 22 forms an angle less than about 50 degrees with the longitudinal axis of the air tube 12. In the presently preferred embodiment, the resistive element 22 forms an angle Al of from about 36.5 degrees to about 39.5 degrees with the longitudinal axis of the air tube 12 and, more preferably, forms an angle of about 38 degrees with the longitudinal axis of the air tube 12. The resistive element 22 is first secured to the first tubular segment 18, and then the second tubular segment 20 is secured to the resistive element 22. Alternatively, the resistive element 22 may first be secured to the second tubular segment 20 and the first tubular segment 18 then secured to the resistive element 22. In the presently preferred embodiment, glue is used to secure the resistive element 22 to the first tubular segment 18 and the second tubular segment 20. The glue is preferably applied to the rims of both the first tubular segment 18 and the second tubular segment 20, and preferably comprises, for example, rubber cement or 3M® Scotch Grip® 4224-NF non-wicking filler glue. As presently embodied, an outer diameter of the first tubular segment 18 is equal to an outer diameter of the second tubular segment 20, and an inner diameter of the collar tube 21 is approximately equal to the outer diameters of the first and second tubular segments 18 and 20. The collar tube 21 is adapted to fit over both the first tubular segment 18 and the second tubular segment 20. Although adhesives are preferably used for securing the resistive element 22 between the first tubular segment 18 and the second tubular segment 20, the close, frictional fit of the collar tube 21 over the first tubular segment 18 and the second tubular segment 20 may be sufficient, alone, in a modified embodiment to secure the resistive element 22 between the first tubular segment 18 and the second tubular segment 20. The distal end 23 of the collar tube 21 is flush with the distal end 25 of the first tubular segment 18, when the collar tube 21 is properly secured over both the first tubular segment 18 and the second tubular segment 20. Additionally, a notch 27, which preferably comprises a punched out semicircle in the distal end 23 of the collar tube 21, is preferably lined up with a port 24 of the second tubular segment. The port 24 of the second tubular segment 20 preferably comprises a punched out circle in the second tubular segment 20. The notch 27 and/or the port 24 may be formed in the collar tube 21 and/or the second tubular segment 20 either before or after assembly of the three pieces 18, 20, and 21. After assembly of the three elements 18, 20, and 21, the port 24 opens directly into a hollow space 30 (Figure 6) of the air tube 12. Figure 6 illustrates the air tube 12 in an assembled state. Although a three piece configuration of the air tube 12 is presently preferred, these three pieces 18, 20, and 21 be replaced by a single tube, for example, and/or the resistive element 22 may be secured to an oval or elliptical ring (not shown) , which is inserted within the single tube. A human patient blowing into an end of the air tube 12 at about atmospheric pressure generates an air flow through the resistive element 22 which, typically, may comprise an air flow rate of between zero and about 16 liters per second. The resistance provided by the resistive element 22 should, ideally, be approximately linear among these various air flow rates. The alinear flow versus-pressure response of the resistive element 22 is subsequently linearized by software, as presently embodied. A purpose of the resistive element 22 is to create a back pressure (in the case of exhalation) and a vacuum (in the case of inspiration) that is read via the port 24. A prior-art perpendicularly disposed resistive element may in certain instances create too much back pressure in the exhalation case. The slanted orientation of the resistive element 22, in accordance with the present invention, exposes more of the porous fabric surface area and, hence, serves to reduce the back pressure. In the presently preferred embodiment, it is an objective to generate no more than 8.2 inches of water pressure at 14 liters per second of flow through the air tube 12. This is an American Thoracic Society (ATS) requirement. Another objective, which is peculiar to the specific hardware and software used in a particular embodiment, is to keep the back pressure up to about 0.012 inches at 100 milliliters per second to overcome hysterysis affects within the pressure transducer 80. Other transducers, for example, may not present this issue. A third objective, which is also peculiar to the particular transducer, is to limit the inhale vacuum to about 10 inches of water or less at an air flow rate of -14 liters per second. Generally speaking, the resistive element 22 provides a very good, approximately linear flow-rate- versus-resistance response for flow rates between zero and about 16 liters per second. One important element of the resistive element 22 of the present invention is the resistance supplied at low flow rates, since, typically, unhealthy patients are unable to generate high flow rates. The same resistive element also functions well at high flow rates. The resistive element 22 thus provides good resistance at various flow rates, regardless of whether the patient is exhaling or inhaling. Turning back to Figure 4, when it is desired to use air tube 12, it is unpackaged and is coupled to housing 14. In particular, the air tube 12 is coupled to the housing tube 51. The housing tube 51 includes a tab 52, which is adapted to fit within the notch 27 (Figure 5A) of the air tube 12. Before the air tube 12 is placed into the housing tube 51, the notch 27 is aligned with the port 24 (Figure 5A) and, as presently embodied, is manually aligned by the user just before insertion into the housing tube 51. When the notch 27 is aligned with the port 24, the port 24 will align with the pressure sensing leg 76, as shown in Figure 7. More particularly, a suction cup shape 77 of the pressure sensing leg 76 fits around the port 24 for an airtight fit. The suction cup shaped fitting 77 preferably comprises silicone rubber or vinyl, and is adapted to provide a good fit around the port 24, to thereby attenuate any leakage of air at this interface. Consequently, breath from the patient is not introduced into the pressure sensing leg 76 and contamination of the pressure sensing leg 76 is avoided. In one preferred embodiment, the pressure sensing leg 76 and suction cup shape 77 are glued onto the air tube 12, and the pressure sensing leg 76 snugly fits into a larger diameter leg which is coupled to the differential pressure transducer 80. The glue may comprise, for example, rubber cement or 3M® Scotch Grip ® 4224-NF wicking or non- wicking filler glue. After the notch 27 of the air tube 12 is placed within the housing tube 51 and, more particularly, placed over the alignment tab 52, the distal end 23 of the collar tube 21 should be flush with the distal end of the housing tube 51. At this point, spirometer 10 is ready for use. Note that air tube 12 is longer than housing tube 51 and, when properly coupled to the housing tube, extends beyond one end of the housing tube. The relatively long air tube 12 reduces the risk of air exhaled from the spirometer user coming into effective contact with and contaminating the housing. Figure 7 illustrates the general operation of a spirometer 10 in accordance with the present invention after the air tube 12 is properly located and positioned relative to the pressure sensing leg 76. This general description is applicable using any spirometer, such as spirometer 10, in accordance with the present invention. Through port 24 (Figure 5A) communicates with pressure sensing leg 76. As a further protection against contamination, pressure sensing leg 76 may be equipped with a filter, although this is not required. The pressure sensing leg 76 communicates with the differential pressure transducer 80, which may be, for example, a transducer sold by Motorola under the trademark MPX 2020D. The pressure transducer 80 generates an electrical signal on a pair of output wires 82 and 84, which signal is proportional to the differential pressure between pressure sensing leg 76 and a sensed atmosphere pressure. This signal is amplified by a differential amplifier stage 86 and fed into an analog-to-digital converter 88 which converts the amplifier output into digital signals. The output from converter 88 is fed to a microprocessor 90, which is part of control electronics 16. The microprocessor 90 uses an algorithm stored in a ROM 92 to perform several calculations on the signal from converter 88, and to display the results, e.g., volume and flow rate, on display 94, for example, a conventional monitor or liquid crystal display module. Microprocessor 90 is powered by a power source 91, and switch 96 can be activated to initiate the operation of the spirometer 10 through microprocessor 90. The results during each measurement may be stored in a RAM 98 for future reference. An input/output port 100 may also be provided to allow for changing the programming of the microprocessor 90. Furthermore, the microprocessor 90 may be programmed so that on command it may download the results accumulated in RAM 98 through input/output port 100 to a printer or a computer. The above-referenced U.S. Pat. No. 5,137,026 to Waterson et al., the contents of which are incorporated by reference herein, provides further details regarding the operation of a spirometer. In any event, when a patient has concluded one treatment or diagnostic exercise using the spirometer 10, the biodegradable air tube 12 is removed from the housing tube and is disposed of in an environmentally safe manner. As shown in Figures 4, 4A and 4B, the housing 14 is structured to be gripped in one hand of the user. For example, the shaft 102 of housing 14 is configured for easy hand gripping. The embodiment shown in Figures 4, 4A and 4B includes control electronics 16 located within the hand held housing 14. Communication with external computers or printers can occur through cable 106 which can be connected to the converter using a jack 105, such as a conventional RJ-11 quick connect jack, on housing 14. As presently preferred, communication can also occur through an additional infrared data association (IRDA) link, which is conventional, and operable between the housing 14 and the external computer or printer. Converter 88, amplifier stage 86 and pressure transducer 80 can be powered through cable 106 from microprocessor 90 and power source 91. Alternatively, the electronics in the housing 14 can be independently powered by a battery pack, such as a conventional rechargeable nickel-cadmium battery. If such a battery pack is used, the housing 14 includes a port through which the battery pack can be charged. The embodiment shown in Figures 4, 4A and 4B is useful as a completely new spirometer, or the air tube 12 and housing 14 can be used to retrofit an existing spirometer. For example, an existing spirometer includes a permanent breathing tube, pressure sensing leg, a pressure transducer, an amplifier and an analog-to-digital converter, and is connected to a dedicated control system, which functions in a manner substantially similar to control electronics 16. Simply by replacing the existing hand held unit with housing 14 and the components coupled to or disposed in the housing, a retrofitted spirometer is produced which has many of the advantages of the present invention. Although an exemplary embodiment of the invention has been shown and described, many other changes, modifications and substitutions, in addition to those set forth in the above paragraphs, may be made by one having ordinary skill in the art without necessarily departing from the spirit and scope of this invention.

Claims: CLAIMS 1. An air tube assembly for use in a spirometer, comprising: a first tube having a first proximal end, a first distal end, and a first longitudinal tube axis extending between the first proximal end and the first distal end, the first distal end comprising a distal rim that does not lie entirely within a normal plane, wherein the normal plane is defined as a plane that is substantially perpendicular to the first longitudinal tube axis; a second tube having a second proximal end, a second distal end, and a second longitudinal tube axis extending between the second proximal end and the second distal end, the second proximal end comprising a proximal rim that does not lie entirely within the normal plane; and a coupler contacting both the first tube and the second tube to thereby join the first tube to the second tube. 2. The air tube as set forth in Claim 1, wherein the distal rim lies in a first plane and the proximal rim lies in a second plane. 3. The air tube assembly as set forth in Claim 1, and further comprising a resistive element disposed within the air tube assembly and contacting at least one of the first tube and the second tube. 4. The air tube assembly as set forth in Claim 3, wherein the resistive element contacts at least one of the proximal rim and the distal rim. 5. The air tube assembly as set forth in Claim 4, wherein the resistive element contacts both the proximal rim and the distal rim. 6. The air tube assembly as set forth in Claim 5, wherein the resistive element contacts the entire proximal rim and the resistive element contacts the entire distal rim. 7. The air tube assembly as set forth in Claim 6, wherein the resistive element is glued to both the proximal rim and the distal rim. 8. The air tube assembly as set forth in Claim 6, wherein a perimeter of the resistive element is substantially equal to a perimeter of the proximal rim, and a perimeter of the proximal rim is substantially equal to a perimeter of the distal rim, and wherein the resistive element is glued to both the proximal rim and the distal rim. 9. The air tube assembly as set forth in Claim 8, wherein the resistive element comprises a polyester or nylon mesh. 10. The air tube assembly as set forth in Claim 8, wherein the resistive element comprises a PES 53/40 mesh. 11. The air tube as set forth in Claim 1, wherein the distal rim lies in a first plane and the proximal rim lies in a second plane, and wherein the first plane is substantially parallel to the second plane. 12. The air tube assembly as set forth in Claim 11, and further comprising a resistive element contacting the proximal rim and the distal rim. 13. The air tube assembly as set forth in Claim 12, wherein the resistive element is sandwiched between the proximal rim and the distal rim. 14. The air tube assembly as set forth in Claim 13, wherein the resistive element is glued to both the proximal rim and the distal rim. 15. The air tube assembly as set forth in Claim 14, wherein a perimeter of the resistive element is substantially equal to a perimeter of the proximal rim, and a perimeter of the proximal rim is substantially equal to a perimeter of the distal rim. 16. The air tube assembly as set forth in Claim 14, wherein the resistive element comprises a polyester or nylon mesh. 17. The air tube assembly as set forth in Claim 16, wherein the resistive element comprises a PES 53/40 mesh. 18. The air tube assembly as set forth in Claim 17, wherein the coupler comprises a third tube adapted to fit around the first distal end and the second proximal end. 19. The air tube assembly as set forth in Claim 18, wherein the first tube comprises a pressure port disposed at a point in a wall of the first tube, wherein when the normal plane is defined to intersect the point a second point can be defined opposite the first point and also lying in the normal plane, and wherein the distal plane ramps up and away from the point so that a length of the first tube measured through the point and along a direction parallel to the first longitudinal tube axis, is less than another length of the first tube measured through the second point and along the direction parallel to the first longitudinal tube axis. 20. The air tube assembly as set forth in Claim 11, wherein the first tube comprises a pressure port disposed at a point in a wall of the first tube, wherein when the normal plane is defined to intersect the point a second point can be defined opposite the first point and also lying in the normal plane, and wherein the distal plane ramps up and away from the point so that a length of the first tube measured through the point and along a direction parallel to the first longitudinal tube axis, is less than another length of the first tube measured through the second point and along the direction parallel to the first longitudinal tube axis. 21. The air tube assembly as set forth in Claim 20, and further comprising a resistive element lying in a plane which is substantially parallel to the distal plane. 22. The air tube assembly as set forth in Claim 21, wherein the resistive element comprises a polyester or nylon mesh. 23. The air tube assembly as set forth in Claim 22, wherein the resistive element comprises a PES 53/40 mesh. 24. The air tube assembly as set forth in Claim 20, wherein the resistive element is adapted to repeatedly generate a resistances of about 8.2 inches of water pressure each time air is passed through the resistive element at a flow rate of about 14 liters per second. 25. An air tube assembly for use in a spirometer, comprising a proximal end, a distal end, and a longitudinal tube axis extending between the proximal end and the distal end, the air tube comprising a substantially planar resistive element disposed within the air tube, wherein the substantially planar resistive element does not lie entirely within a normal plane that is defined as a plane substantially perpendicular to the longitudinal tube axis . 26. The air tube assembly as set forth in Claim 25, wherein the resistive element lies in a plane. 27. The air tube assembly as set forth in Claim 26, wherein the air tube comprises a pressure port disposed at a point in a wall of the air tube, wherein when the normal plane is defined to intersect the point a second point can be defined opposite the first point and also lying in the normal plane, and the plane of the resistive element ramps up and away from the point so that a length of the air tube measured through the point and along a direction parallel to the longitudinal tube axis, is less than another length of the air tube measured through the second point and along the direction parallel to the longitudinal tube axis. 28. The air tube assembly as set forth in Claim 27, wherein the resistive element comprises a polyester or nylon mesh. 29. The air tube assembly as set forth in Claim 28, wherein the resistive element comprises a PES 53/40 mesh. 30. A resistive element for use in an air tube of a spirometer, the resistive element comprising a base material sized and configured to be placed at a slant angle within the air tube, wherein the slant angle is not perpendicular to a longitudinal axis of the air tube, the base material having a porosity that is suitable to cause a resistance to air flow, when the resistive element is secured within the air tube, that is approximately linear among various air flow rates between zero and about 15 liters per second at about atmospheric pressure, wherein the resistive element has a planar configuration and an oval or elliptical perimeter to facilitate placement of the resistive element in the air tube at a slant angle. 31. The resistive element for use in a spirometer as recited in claim 30, wherein the resistive element when secured within the air tube at the slant angle, is adapted to repeatedly generate a resistances of about 8.2 inches of water pressure each time air is passed through the resistive element at a flow rate of about 14 liters per second. 32. The resistive element for use in a spirometer as recited in claim 20, 24, 30 or 31, wherein the resistive element when secured within the air tube, is adapted to repeatedly generate a resistances of about 0.012 inches of water pressure each time air is passed through the resistive element at a flow rate of about 100 milliliters per second. 33. The resistive element for use in a spirometer as recited in claim 32, wherein the resistive element when secured within the air tube, is adapted to repeatedly generate a resistances (inhale vacuum) of about 10 inches of water pressure each time air is drawn through the resistive element at a flow rate of about -14 liters per second. 34. The resistive element for use in a spirometer as recited in claim 20, 24, 30 or 31, wherein the resistive element when secured within the air tube, is adapted to repeatedly generate a resistances (inhale vacuums) of about 10 inches of water pressure each time air is drawn through the resistive element at a flow rate of about - 14 liters per second. 35. The air tube as set forth in Claim 11, 13 or 17, wherein the first plane forms an angle less than about 50 degrees with the first longitudinal tube axis. 36. The air tube as set forth in Claim 11, 13 or 17, wherein the first plane forms an angle of about 38 degrees with the first longitudinal tube axis. 37. The air tube assembly as set forth in Claim 21 or 23, wherein the distal plane forms an angle less than about 50 degrees with the first longitudinal tube axis. 38. The air tube assembly as set forth in Claim 21 or 23, wherein the distal plane forms an angle of about 38 degrees with the first longitudinal tube axis. 39. The air tube assembly as set forth in Claim 26, wherein the plane forms an angle less than about 50 degrees with the longitudinal tube axis. 40. The air tube assembly as set forth in Claim 26, wherein the plane forms an angle of about 38 degrees with the longitudinal tube axis.

Inventors: HAMILTON DOUGLAS L, LICHTER PATRICK A, HAMILTON, DOUGLAS, L., LICHTER, PATRICK, A.
Applicants: HAMILTON DOUGLAS L, LICHTER PATRICK A, QRS DIAGNOSTIC LLC, HAMILTON, DOUGLAS, L., LICHTER, PATRICK, A., QRS DIAGNOSTIC, LLC.
Application year: 2000
Main classification: G01F
IPC codes: A61B5, G01F1