Abstract:
Methods and apparatus for calibrating volume measurement in a plethysmographic chamber are described. The present invention involves the use of a calibration volume chamber of known volume coupled to a plethysmographic measurement chamber in a plethysmographic measurement system for determining body composition, wherein a computer system calibrates the measurement system prior to conducting a volume measurement of a test subject, by measuring the chamber volume before and after opening an electronically controlled valve that connects the controlled calibration volume to the plethysmographic chamber, and comparing the measured chamber volumes based on the known reference volume.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to apparatus and methods for providing repeatable measurements of volume within an enclosed chamber. More specifically, the present invention provides methods and apparatus for calibrating volume measurement in a plethysmographic measurement system. 
     BACKGROUND OF THE INVENTION 
     The assessment of body composition, including measurement of fat and fat-free mass, provides physicians with important information regarding physical status. Excess body fat has been associated with a variety of disease processes, such as cardiovascular disease, diabetes, hypertension, hyperlipidemia, kidney disease, and musculoskeletal disorders. Low levels of fat free mass have been found to be critically adverse to the health of certain at-risk populations, such as the elderly, infants, and those suffering from muscle wasting diseases. 
     Assessment of body composition has also been found to be useful in the context of evaluating and improving athletic performance. Generally, athletes require a high strength to weight ratio to achieve optimal athletic performance. Because body fat adds weight without a commensurate increase in strength, low body fat percentages have been emphasized within many athletic fields. However, too little body fat can result in deterioration of both health and athletic performance. Thus, accurate measurement of body composition has been found extremely useful in analysis of athletic performance. 
     A variety of methods are currently used in the assessment of body composition. One common method is a skinfold measurement, typically performed using calipers that compress the skin at certain points on the body. While non-invasive, this method suffers from poor accuracy on account of variations in fat patterning, misapplication of population specific prediction equations, improper site identification for compressing the skin, poor fold grasping, and the necessity for significant technician training to administer the test properly. 
     Another method employed is bioelectric impedance analysis (“BIA”). Bioelectric impedance measurements rely on the fact that the body contains intracellular and extracellular fluids that are capable of conducting electricity. By passing a high frequency electric current through the body, BIA determines body composition based on the bodies&#39; measured impedance in passing current, and the known impedance values for human tissue. However, the accuracy of this method is greatly affected by the state of hydration of the subject, and variations in temperature of both the subject and the surrounding environment. 
     The most common method currently used when precision body mass measurements are required is hydrostatic weighing. This method is based upon the application of Archimedes principle, and requires weighing of the subject on land, repeated weighing of the subject under water, and an estimation of air present in the lungs of the subject using gas dilution techniques. However, hydrodensitometry is time consuming, typically unpleasant for the subjects, requires significant subject participation such as repeated, complete exhalation of air from the subject&#39;s lungs, requires considerable technician training and, due to the necessary facilities for implementation, is unsuitable for clinical practice. Further, its application to populations who would particularly benefit from body-mass measurement, such as the obese, elderly, infants, or cardiac patents, is precluded by the above concerns. 
     One technique offering particular promise in performing body mass measurement is the use of plethysmography. Plethysmographic methods determine body composition through application of Boyle&#39;s law to the differentiation in volume between the volume of an empty measurement chamber, and the volume of the chamber with the subject to be measured inside. Examples of this technique are disclosed in U.S. Pat. No. 4,369,652 issued to Gundlach, U.S. Pat. No. 5,450,750 issued to Abler, U.S. Pat. No. 4,184,371 issued to Brachet, and U.S. Pat. No. 5,105,825 issued to Dempster. This procedure, in contrast to hydrodensitometry, generally does not cause anxiety or discomfort in the subject, and due to the ease and non-invasiveness of the technique, can readily be applied to populations for whom hydrodensitometry is impractical. 
     However, such plethysmographic systems require very exact volume measurements to yield valid body composition results. In particular, calibration of the measurement chamber equipment used to generate the volume measurements for body composition analysis is necessary for achieving accuracy, on account of very small differences in measured volume yielding large differences in computed body composition. Although some efforts have been made in the field of calibration for plethysmographic systems, these methods are typically complicated, inexact, and/or inconvenient for the medical technicians who conduct plethysmographic body composition measurements by requiring manual activation and implementation of the calibration. 
     For example, Dempster, U.S. Pat. No. 5,108,825, discloses the use of a calibration volume structure that is manually placed in a plethysmographic reference chamber. However, this process is slow, cumbersome, and requires active participation by the medical technician to calibrate the system. 
     Ganshorn, U.S. Pat. No. 5,626,005, discloses a method of calibration for a plethysmographic chamber for measuring the volume of a subject&#39;s thorax-lung system. The method disclosed by Ganshorn involves the use of a harmonically oscillating piston pump that generates pressure fluctuations that simulates a test subject&#39;s breathing, and relies on these pressure fluctuations to calibrate a chamber pressure gauge based on the simulated breathing. However, this method is unnecessarily complex and not generally applicable to calibration of plethysmographic chambers used in the measurement of body composition. 
     Therefore, it would be desirable to provide a computer assisted calibration system for a whole body plethysmographic measurement chamber that provides accurate calibration of the measurement system. 
     It would further be desirable to provide a computer assisted calibration system for a whole body plethysmographic measurement chamber that does not require active, manual participation of medical technician to conduct the calibration. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a computer assisted calibration system for a whole body plethysmographic measurement chamber that provides accurate calibration of the measurement system. 
     It is another object of the present invention to provide a computer assisted calibration system for a whole body plethysmographic measurement chamber that does not require active, manual participation of medical technician to conduct the calibration. 
     These and other objects of the present invention are accomplished by proving computer assisted methods and apparatus for calibration of a plethysmographic measurement system using a calibration volume chamber. 
     The present invention generally consists of a calibration volume chamber of known, fixed volume coupled to a plethysmographic measurement chamber in a plethysmographic measurement system, wherein a computer system is used to calibrate the measurement system prior to conducting a volume measurement of a test subject, by measuring the chamber volume before and after opening (or alternatively, before and after closing) an electronically controlled valve that connects the controlled calibration volume to the plethysmographic chamber, and comparing the measured chamber volumes based on the known reference volume. 
     In one embodiment of the present invention, the actuation assembly for opening and closing the valve in response to a signal from the computer system is a cam and motor assembly coupled to a shaft that is mounted to the valve by means of a ball joint. 
     In a second embodiment of the present invention, the actuation assembly for opening and closing the valve in response to a signal from the computer system is a solenoid coupled to a shaft that is mounted to the valve by means of a ball joint. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
     FIG. 1 is a representational view of an adult-sized plethysmographic chamber and control system in which embodiments of the present invention operate; 
     FIG. 2 is a flow chart describing the calibration sequence of one embodiment of the present invention; 
     FIG. 3 is a flow chart describing the calibration sequence of a second embodiment of the present invention. 
     FIG. 4 is a cross-sectional view of one embodiment of the calibration volume chamber and valve actuation assembly of the present invention; 
     FIG. 5A is a detailed cross sectional view of one embodiment of the valve and valve actuation assembly of the present invention, with the valve in the open position; 
     FIG. 5B is a detailed cross sectional view of one embodiment of the valve and valve actuation assembly of the present invention, with the valve in the closed position; 
     FIG. 6 is a representational view of the infant sized plethysmographic chamber in which the present inventions operate; 
     FIG. 7 is a cross sectional view of a second embodiment of the calibration volume and valve actuation assembly of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, a representational view of an adult-sized plethysmographic chamber in which embodiments of the present inventions operate are described. 
     Plethysmographic measurement system  20  comprises measurement chamber  22 , chamber door  24 , plethysmographic measurement components  26 , and computer  30 . The operation of plethysmographic measurement components  26  is controlled by computer  30 , which computer is operated by the medical technician performing the plethysmographic measurement. (As used herein, the term “medical technician” refers to any individual conducting the plethysmographic measurements of the test subject.) 
     Measurement components  26  can include such devices as an oscillating diaphragm or speaker, pressure transducers, their respective control systems, and other components necessary to conduct plethysmographic measurements. Further information regarding such plethysmographic measurement components, and the techniques used to derive volume and body composition measurements using them, are described in detail in Dempster, U.S. Pat. No. 5,105,825, assigned to Life Measurement Instruments, the specification of which is hereby incorporated by reference in its entirety. The algorithms used in conducting plethysmographic measurements are likewise well known to one of ordinary skill in the art, and therefore are not disclosed herein. 
     Housed within measurement chamber  22  is a calibration volume chamber  36 , including an opening  38 , a valve  40  for sealing and unsealing said opening, and valve actuation assembly  42  for opening and closing said valve in response to commands from computer  30 . 
     When the medical technician initiates a measurement sequence, computer  30  is used to calibrate plethysmographic measurement system  20  prior to measuring the body composition of the test subject. The actual programming of computer  30  to conduct calibration of the plethysmographic measurement system is done in accordance with conventional programming techniques suitable for performing basic calculations and supplying control signals to the measurement components and actuation assembly of the present invention. These techniques are well known to one of ordinary skill in the art, and as such are not disclosed herein. 
     The calibration of the plethysmographic system can be, but need not be, performed without technician intervention. In a preferred embodiment, the calibration of the plethysmographic measurement system is conducted transparently to the medical technician, such that the calibration occurs automatically upon the technician initiating a plethysmographic measurement sequence. 
     As illustrated by flow chart  43  in FIG. 2, in a first embodiment of the present invention, computer  30  directs plethysmographic system  20  to first measure the volume of measurement chamber  22  when valve  40  is in the open position (i.e. with calibration reference volume  36  open to measurement chamber  22 ). 
     Specifically, in step  43 A, computer  30  first activates the measurement components. In step  43 B, computer  30  determines whether the valve is in the proper (open) state. If not, computer  30  sends a signal to actuation assembly  42  to open valve  40 . In step  43 C, computer  30  directs measurement components  26  to measure the combined volume of the measurement chamber and calibration volume chamber. In step  43 D, computer  30  stores the values generated from the measurement in  43 C. In step  43 E, computer  30  sends an electrical signal to valve actuation assembly  42  to close valve  40 , thereby reducing the net chamber volume. In step  43 F, computer  30  directs measurement components  26  to measure the volume of measurement chamber  22 . In step  43 G, computer  30  stores the values generated by the measurement of step  43 F. In step  43 H, the measured volumes are then compared based on the known volume of calibration volume chamber  36 . Based on the above comparison, computer  30  finalizes calibration of measurement system  20 , and indicates to the technician that measurement of the test subject can begin. 
     The algorithms used to calibrate the plethysmographic measurement system based on the calibration processes of the present invention are known to those of skill in the art, and as such are not described herein. 
     In an alternative embodiment of the present invention, illustrated in the flow chart of FIG. 3, computer  30  can direct plethysmographic system  20  to first measure the volume of measurement chamber  22  when valve  40  is in the closed position. 
     Specifically, in step  45 A, computer  30  first activates the measurement components. In step  45 B, computer  30  determines whether the valve is in the proper (closed) state. If not, computer  30  sends a signal to actuation assembly  42  to close valve  40 . In step  45 C, computer  30  directs measurement components  26  to measure the volume of measurement chamber  22 . In step  45 D, computer  30  stores the values generated from the measurement in  45 C. In step  45 E, computer  30  sends an electrical signal to valve actuation assembly  42  to open valve  40 , thereby increasing the net chamber volume. In step  45 F, computer  30  directs measurement components  26  to measure the combined volume of the measurement chamber and calibration volume chamber. In step  45 G, computer  30  stores the values generated in step  45 F. In step  45 H, the measured volumes are then compared based on the known volume of calibration volume chamber  36 . Based on the above comparison, computer  30  finalizes calibration of measurement system  20 , and indicates to the technician that measurement of the test subject can begin. This calibration process results in calibration based on what is, in net effect, a negative volume measurement. 
     One of ordinary skill in the art would recognize that the are not limited to single measurements. Rather, multiple measurements of chamber volume with valve  40  open and closed can be used in accord with the present invention, with the system being calibrated based on the multiple data points generated by the measurements. 
     Further, one of ordinary skill in the art would recognize that the calibration methods disclosed herein could be conducted after plethysmographic measurement has been performed on the subject to be measured, because the methods of calibration disclosed herein are conducted by numerical calculations on measurement values. Thus, in such an embodiment, when the medical technician initiates the measurement sequence, measurements are first taken of the test subject in measurement chamber  22 . The data generated in conducting this plethysmographic measurement of the test subject is stored on computer  30 , after which the calibration methodology described above is conducted. Finally, the results of the calibration are applied to the measurements taken of the test subject to arrive at an accurate volume measurement for the subject. 
     Referring now to FIG. 4, a cross-sectional view of a first embodiment of the calibration volume chamber and valve actuation assembly of the present invention is described. Calibration volume chamber  36  is a roughly cylindrical chamber with a known, stable internal volume. Although any shape can be used for reference volume chamber  36 , it is preferred that the internal volume of reference volume chamber  36  be comparable to the volumes expected to be measured by the plethysmographic measurement system  20  in order to provide for more accurate calibration of the measurement system. 
     At one end of calibration chamber  36  is opening  38  that allows air to pass between calibration chamber  36  and plethysmographic chamber  22 . 
     Mounted about the circumference of opening  38  is valve mount collar  42 . Valve  40  is housed within valve mount collar  44 . Valve  40  is coupled to valve actuation assembly  42 , which opens and closes valve  40  in response to a signal from computer  30 . At the end of valve mount collar  44  distal from said opening  38  is valve opening  46 . When valve  40  is in the closed position, valve  40  creates a seal about valve opening  46  that completely seals off reference volume chamber  36  from plethysmographic chamber  22 . 
     Referring now to FIG. 5A, a detailed cross sectional view of the valve and valve actuation assembly of the present invention, in which valve  40  is in the open position, is described. 
     In this embodiment, valve actuation assembly  42  includes cam  50 , cam follower  52 , cam shaft  54 , stamping  56  (which is further comprised of follower stamping  62  and spring stamping  64 ), cam spring  58 , valve ball joint  60 , and valve assembly mounting plate  66 . 
     Valve  40  is coupled to a proximal end of cam shaft  52  by ball joint  60 . Cam shaft  52  is further coupled to stamping  56  at the end of cam shaft  52  distal from valve  40 . 
     Mounted around cam shaft  54  is cam spring  58 , which is coupled at one end to spring stamping  64 , and coupled at the opposite end to valve assembly mounting plate  66 . Cam spring  58  generates an extension force against stamping  56 . 
     Follower  52  is coupled to roller stamping  64 . The force generated by cam spring  58  pushes against follower  52  by means of its coupling to spring stamping  62 . This force ensures that follower  52  maintains solid contact with cam  50 . 
     To open valve  40 , cam motor  68  rotates cam  50  into an extended position, which exerts force on follower  52 , thereby pushing on stamping  56  and compressing spring  58 . This force applied to stamping  56  causes cam shaft  54  to move in the direction towards opening  38 , thereby opening valve  40 . 
     Referring now to FIG. 5B, a detailed cross sectional view of the valve and valve actuation assembly of the present invention, in which valve  40  is in the closed position, is described. 
     To close valve  40 , cam motor rotates cam  50  into a retracted position, which allows cam spring  58  to push on stamping  56 , and move cam shaft  54  until the edge of valve  40  makes contact with valve mount housing  44 , thereby sealing off reference chamber  36  from plethysmography chamber  22 . 
     In a preferred embodiment, a seal  70  is mounted about the circumference of valve  40 , such that when valve  40  is in the closed position, seal  70  is compressed by valve  40  against valve mount housing  40 , creating an air tight seal. Further, because ball joint  60  allows valve  40  to rotate with respect to cam shaft  54 , valve  40  forms a repeatable, air tight seal against valve mount  40 . 
     Referring now to FIG. 6, a representational view of an infant sized plethysmographic system in which embodiments of the present invention operate is described. 
     Plethysmographic system  80  comprises plethysmographic measurement chamber  82 , chamber door assembly  84 , plethysmographic measurement components  86 , manifold  88  and computer  90 . Calibration volume chamber  94  is coupled to measurement chamber  82  by manifold  88  (which also couples measurement components  86  to measurement chamber  82 ). Calibration chamber opening  96  allows air to pass from calibration volume chamber  94 , through manifold  88 , and into measurement chamber  82 . 
     Valve actuation assembly  100  is coupled to valve  101 , and seals and unseals opening  96  in response to commands from computer  90 . 
     As disclosed in connection with the previous embodiment, when the medical technician initiates a body composition measurement sequence for a test subject, computer  90  calibrates plethysmographic measurement system  80  prior to measuring the body composition of the test subject, without the necessity of technician intervention to conduct the calibration. 
     Specifically, as described above in connection with the flow chart illustrated in FIG. 2, computer  90  directs plethysmographic system  80  to first measure the volume of measurement chamber  82  when valve  101  is in the open position. Computer  90  then sends an electrical signal to valve actuation assembly  100  to close valve  101 , thereby reducing the net measurement chamber volume. The measured volumes are then compared to the expected volumes based on the known volume of calibration volume chamber  94 . Based on this comparison, computer  90  finalizes calibration of measurement system  80 , and indicates to the technician that measurement of the test subject can begin. 
     Similarly, the calibration system described above can calibrate measurement system  80  using the process illustrated in the flow chart of FIG.  3 . 
     Referring now to FIG. 7, a detailed cross-sectional view of the calibration volume and valve actuation assembly of the second embodiment of the present invention is described. As described above with respect to FIG. 4, measurement chamber  82  is coupled to calibration volume chamber  94  by manifold  88 , and calibration volume chamber opening  96  allows air to pass from calibration volume chamber  94 , through manifold  88 , and into measurement chamber  82 . 
     Valve actuation assembly  100  consists of solenoid  102 , solenoid mount  104 , inner manifold  106 , shaft  110 , and ball joint  112  coupled to valve  101 . Valve actuation assembly  100  is housed within inner manifold  106 , which is mounted across manifold  88  such that valve  101  can open and close calibration volume chamber opening  96 . 
     Solenoid  102  is coupled to inner manifold  106  by solenoid mount  104 . Solenoid  102  includes a plunger  116 , which is coupled to shaft  110 , such that the motion of shaft  110  tracks the motion of plunger  116 . Shaft  110  is further coupled to valve  101  by means of ball joint  112  at the end of shaft  110  that is distal to solenoid  102 . Valve  101  therefore opens and closes about calibration volume opening  96  in response to the motion of shaft  110 . 
     Particularly, when plunger  116  is extended, it exerts a force on shaft  110 , causing it to move in the direction of the force exerted by solenoid plunger  116 . Shaft  110  thereby pushes on valve  101  against calibration chamber opening  96 , sealing calibration volume chamber  94  from measurement chamber  82 . Further, because ball joint  112  allows valve  101  to rotate with respect to shaft  110 , valve  101  forms a repeatable, air tight seal against calibration volume chamber  94 . Alternatively, any other type of pivotal joint, such as a universal joint, can be used in place of ball joint  112 . 
     Similarly, when plunger  116  is retracted, it pulls shaft  110  away from the surface of calibration chamber opening  96 , thereby opening valve  101  and allowing air to pass from calibration volume chamber  94  to measurement chamber  82 . 
     One of ordinary skill in the art would recognize that the above disclosed embodiments for the valve actuator assemblies can be used interchangeably between infant and adult sized measurement chambers. 
     One of ordinary skill in the art would also recognize that alternative methods of controlling valves  40  and  101  can be used in accord with the present invention. For example, the use a pneumatic system that responds to a signal from a computer to open and close said valve is also contemplated by the present invention. Alternatively, a rotary motor coupled to ball screw, wherein the motor responds to a signal from a computer to open and close said valve, is also contemplated by the present invention. 
     Further, while preferred illustrative embodiments of the invention are described above, it will be apparent to one skilled in the art that various changes and modifications may be made therein without departing from the invention. The appended claims are intended to cover all such changes and modifications that fall within the true spirit and scope of the invention.