Abstract:
An indirect calorimeter for measuring the metabolic activity and related respiratory parameters of a subject includes a facial mask operative to be supported in contact with the subject so as to pass the inhalations and exhalations as the patient breathes. Both the inhaled and exhaled gasses pass through a tube which incorporates an ultrasonic pulse transit time flow meter adapted to generate electrical signals as a function of the instantaneous flow volume. A fluorescence quench oxygen sensor is supported in the flow tube and generates electrical signals as a function of the instantaneous oxygen content of the respiratory gasses. A computation unit receives output signals from the flow sensor and the oxygen sensor to calculate oxygen consumption and related parameters.

Description:
This application claims benefit of provisional applications Nos. 60/073,812 filed Feb. 5, 1998 and 60/104,983 filed Oct. 20, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a respiratory instrument for measuring metabolism and related respiratory parameters by indirect calorimetry. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. Nos. 5,038,792; 5,178,155; 5,179,958; and 5,836,300 all to the same inventor as the present application disclose systems for measuring metabolism and related respiratory parameters through indirect calorimetry. These instruments employ bidirectional flow meters which pass both the inhalations and the exhalations of a user breathing through the instrument and integrate the resulting instantaneous flow signals to determine total full flow volumes. The concentration of carbon dioxide generated by the user is determined by either passing the exhaled volume through a carbon dioxide scrubber before it passed through the flow meter so that the differences between the inhaled and exhaled volumes is essentially a measurement of the carbon dioxide contributed by the lungs or by the measurement of the instantaneous carbon dioxide content of the exhaled volume with a capnometer and integrating that signal with the exhaled flow volume. The oxygen consumption can then be calculated. 
     The scrubber used with certain of these systems was relatively bulky and required replenishment after extended usage. The capnometers used with the instruments to measure carbon dioxide concentration had to be highly precise and accordingly expensive because any error in measurement of the carbon dioxide content of the exhalation produces a substantially higher error in the resulting determination of the oxygen contents of the exhalation. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes these disadvantages of prior art indirect calorimeters by providing a respiratory calorimeter in which both the inhaled and exhaled flow volumes pass through a flow meter which provides an output representative of the instantaneous flow rate and the inhalations and exhalations also pass over an oxygen sensor in contact with the flow pathway which provides an output as a function of the instantaneous oxygen concentration in the flowing gas. These two signals are provided to a computer which integrates them to derive signals representative of the inhaled and exhaled oxygen volume. From these measurements the oxygen consumption, carbon dioxide production, respiratory quotient, caloric expenditure and related respiratory parameters are calculated and displayed. 
     The preferred embodiment of the invention utilizes an ultrasonic transit time flow meter and a fluorescence quench oxygen sensor. Both of these sensors operate upon the respiratory gasses as they pass through a flow tube with a substantially continuous, uninterrupted internal diameter so that the flow is substantially laminar. Previous indirect calorimeters, including those disclosed in the above-described U.S. patents, have employed flow measurement techniques that require protrusions in the flow path such as pressure differential transducers, hot wire transducers or the like. Great difficulties are encountered in maintaining a largely laminar flow in transducers of this type, resulting in inaccuracies in the flow measurement. The present invention preferably employs a volume flow meter which transmits ultrasonic pulses through the flow stream in a direction either parallel to the flow path or at least having a component parallel to the flow path. The transit time of the pulses is a function of the flow rate of the gas and because the interior diameter of the flow tube wall is substantially uninterrupted, laminar flow conditions are maintained providing a high uniformity of measurement. 
     The preferred embodiment of the invention directly measures the oxygen concentration in the inhaled and exhaled gasses passing through the flow tube by a technique which does not introduce any protuberances into the flow area and which may be positioned to measure the oxygen content in the same area in which flow is measured. Thus, unlike previous systems which require some linear separation between the point of flow measurement and the point of gas analysis, and accordingly would result in inaccuracies were the two to be integrated, the present system does not create any phase lag between the oxygen measurement and the flow measurement which would otherwise result in inaccuracies and the need for signal processing to correct for the displacement of the measurements. The preferred embodiment of the invention employs a fluorescence quench technique for oxygen measurement which utilizes a fluoresceable chemical disposed on the interior diameter of the flow wall in the area of ultrasonic pulse transmission. This fluorescent coating may be formed on the tube wall directly or supported on the end of a fiberoptic probe terminating in alignment with the interior diameter of the tube. This coating is subjected to exciting radiation from the exterior of the tube and the resulting fluorescence may be measured from the exterior. The fluorescence is quenched by oxygen passing over the coating and the percentage of oxygen in the flow tube can be instantaneously measured by the intensity of the fluorescence. 
     The flow tube is preferably formed as a disposable insert which may be inserted into a permanent, reusable structure which includes the ultrasonic transmitter and receiver and the fluorescence oxygen sensor. The fluorescent coating may be covered on the tube side with a microbial filter formed as part of the disposable insert. This filter prevents the fluorescent coating from being bacterially contaminated. The disposable insert is utilized to avoid the spread of disease from user to user in situations in which the indirect calorimeter is used by a succession of persons. The insert is preferably produced of an inexpensive material such as plastic. 
     In the preferred embodiment, the disposable insert is supported by a disposable breathing mask that covers the nose and the mouth of the user, allowing normal breathing over the measurement time. Most prior art devices have employed mouthpieces; however, it has been determined that in certain applications the mouthpiece can induce a mild form of hyperventilation which increases the user&#39;s energy consumption and results in erroneous metabolic readings. In one embodiment of the present invention, the metabolic measurement components are integrated with and are contained within the mask with no requirement for external connections. When the mask is attached to the user&#39;s head by straps, adhesive, or the like, it allows a full range of user movement during the measurement. Thus, it can be used during normal exercise to allow determination of the effect of that activity on respiratory parameters and may also be used to measure resting energy expenditure. The increased user comfort resulting from the elimination of connections between the mask and associated apparatus allows measurements to be made over longer periods of time and minimizes the labored breathing often associated with conventional respiratory masks which affects accurate measurement of energy expenditure. 
     The mask also preferably incorporates a nasal spreader on its interior surface which adhesively attaches to the nares of the user&#39;s nose and pulls them outwardly to enlarge the nose flow area and minimize the energy expenditure in breathing, which is often increased with conventional masks. 
     In an alternative form of the invention the computation unit and display and controls are supported in a separate desktop or hand held unit and connected to the sensors within the mask by highly flexible cables or wireless transmission such as infrared or RF. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and applications of the present invention will be made apparent by the following detailed description of preferred embodiments of the invention. The description makes reference to the accompany drawings in which: 
     FIG. 1 is a perspective view in exploded form of a first embodiment of the invention; 
     FIG. 2 is a cross-sectional view through the flow tube of FIG. 1; and 
     FIG. 3 is a perspective view of a second embodiment employing a desktop computation and display unit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a preferred embodiment of the invention includes a disposable section, generally indicated at  10 , and a nondisposable section shown exploded into parts generally indicated at  12   a  and  12   b . The disposable section  10  is made of low cost materials and is intended to be replaced when the calorimeter is employed by serial users to avoid hygiene problems such as transfer of bacterial infections. The disposable section  10  may be retained by a user for reuse at a later date or may be discarded. If the calorimeter is repeatedly used by a single user, the section  10  may not need to be discarded between uses. The section  10  broadly consists of a mask  14  and a U-shaped breathing tube generally indicated at  16 . The mask is adapted to be retained over a user&#39;s face so as to cover the user&#39;s nose and mouth. The mask  14  has a resilient edge section  18  which engages the user&#39;s face in an airtight manner. The mask may be supported against the user&#39;s face by the user holding the outer side, but preferably the mask has straps  20  which connect to its edges and pass around the rear of the user&#39;s head. Alternatively, the mask could be retained by a pressure sensitive coating formed on the edge seal  18 . 
     The mask proper is preferably formed of a rigid plastic but the section  22  at the top of the mask which is intended to surround the user&#39;s nose, is preferably formed of a more resilient material. Pressure sensitive adhesive pads  24  are formed on the interior surfaces of the nose section  22  and allow the user to press the outer surfaces of the nose section together so as to engage the outer surfaces of the user&#39;s nares with the pressure sensitive pads  24 . When the pressure on the outer surface of the nose section  22  is released, the sections will spring outwardly and will pull the nares away from the nose so as to enable easy breathing through the nose into the mask. 
     The U-shaped breathing tube  16  connects to the interior of the mask  14 . The tube then extends from the lower forward section of the mask and extends laterally as at  26  to the right of the user in a generally horizontal plane. At the extreme right it forms a  180  degree bight  28  and extends to the left of the user in an elongated measurement section  30 . The far end of the tube  16  is opened at  32  so that as the user inhales while wearing the mask  14  air is drawn into the tube  16  through the end  32  and as the user exhales air is expelled through the end  32 . The straight section  30  of the tube has three windows or openings, one,  34 , formed at its lower side adjacent to the bight  28 , the second,  36 , formed on its upper side adjacent to the opening  32  and a third,  38 , formed on the side of the tube in the middle of the section  30 . 
     The nondisposable portion of the calorimeter consists of the interlocking upper section  12   a  and lower section  12   b . The upper section  12   a  is formed about a semni-cylindrical section of tube  40 . The inner diameter  42  of the tube section  40  matches the outer diameter of the disposable tube section  30  and the section  40  is slightly shorter than the straight line tube section  30 . Similarly, the nondisposable section  12   b  is formed of a semi-cylindrical tube half  44  having an inner diameter matching the outer diameter of the tube section  30  and having a slightly shorter length. 
     The tube section  40  is formed with two rearward facing tubular supports  46  and  48 , spaced along its length. These supports removably engage bosses  50  and  52  which are formed integrally with the face mask  14  and project forwardly from its upper sides. The lower tube section  44  is then locked to the upper tube section  40  so as to surround the breathing tube section  30 . Cam sections  54  and  56  formed at the forward end of the tube section  40  engage latches  58  and  60  formed on the lower tube half and a similar cam (not shown) projecting from the rear of the tube  40  engages a latch  62  formed at the rear of the lower tube section  44  adjacent its free edge. 
     An ultrasonic transceiver  64  which is housed in a ring  66  formed in the lower tube section  44  projects into the window  34  of the tube section  30 . An anti-microbial filter  68  covers the surface of the transducer  64 . Similarly, an ultrasonic transducer  70  supported within a section  72  formed on the upper tube  40 , and protected by a cover  74 , projects into the window  36  adjacent the outlet and inlet end of the tube  30 . An anti-microbial filter (not shown) may protect the surface of the transducer. The lower tubing section  44  is integrally formed with a housing  76  which contains the microprocessor which receives the signals from the transducers and sensors and controls their operation, and computes the oxygen consumption and other respiratory factors measured by the device. The unit  76  includes a display  78  and control switches  80 . In certain embodiments of the invention a digital keypad may be included on the unit  76 . 
     The computation unit determines oxygen consumption by solving the equation VO 2 =V 1 ×(F 1 O 2 )−V E ×(F E O 2 ) where VO 2  is the consumed oxygen, V 1  is the inhaled volume, V E  is the exhaled volume, F 1 O 2  is the fraction of oxygen in the inhalation, and F E O 2  is the fraction of volume in the exhalation. The system integrates the instantaneous flow volumes with the instantaneous oxygen levels over an entire breathing cycle, which is typically three to ten minutes. The system calculates carbon dioxide production in accordance with the following equation: 
     
       
           V   CO     2     [V   E −( V   E   ·F   E O 2 )]−[ V   1 −( V   1   ·F   1 O 2 )] 
       
     
     Other respiratory parameters such as RQ, REE, etc. may be calculated in the manner disclosed in my previous issued patents. 
     An oxygen concentration sensor  82  is supported within the housing  76  so that when the tube sections  40  and  44  are joined, the surface of the oxygen sensor, preferably covered with an anti-microbial filter  83 , is disposed within the window  38  so that its outer surface is substantially flush with the internal diameter of the tube section  30 . In alternate embodiments of the invention the fluorescent chemical, which is formed on the end of the oxygen concentration sensor  82  in the preferred embodiment, could be coated directly on the interior diameter of the tube section  30  and the fluorescence stimulating radiation and sensing of the resulting fluorescence intensity could be performed through a suitable window in the wall of the tube  30 . 
     In use, a subject dons the mask  14  and attaches the straps so that the subject&#39;s nose is disposed within the section  22  of the mask, the subject&#39;s mouth is covered, and the area surrounding the mouth and nose are sealed by contact of the section  18  with the subject&#39;s face. The subject then pinches the outer surface of the section  22  of the mask so that the adhesive pads  24  are brought into pressured contact with the two sides of the subject&#39;s nose. The resilient section  22  is released so that the nares are separated, allowing free breathing within the mask. 
     Either prior to donning the mask or subsequently, the nondisposable sections  12   a  and  12   b  are attached so as to surround the tube  30  and the connecting sections  46  and  48  are attached to the bosses  50  and  52  on the front surface of the mask  14 . 
     The user may then breathe in a normal manner so that the inhalations and exhalations are passed through the tube  16  and connect to the atmosphere at the tube end  32 . After the subject has breathed through the mask for a minute or two to stabilize the breathing, one of the buttons  80  is depressed to start the measuring cycle. In alternative embodiments of the invention, rather than manually depressing the button  80  to start the measuring cycle, the computation unit  76  could sense the flow of gasses through the tube  30  and automatically initiate the measurement cycle when the breathing reached a normal level. 
     The ultrasonic transducers  64  and  70  face each other and transmit and receive ultrasonic pulses along a path  90  illustrated in FIG. 2 or some alternative path which is either parallel to or has a substantial component in the direction of the flow. The gas flow acts to advance or retard the flow of the pulses so that the full transmit time of the pulses is a function of the flow rate. The system preferably employs an ultrasonic flow meter manufactured by NDD Medizintechnik AG, of Zurich, Switzerland, and disclosed in U.S. Pat. Nos. 3,738,169; 4,425,805; 5,419,326; and 5,645,071. 
     The oxygen concentration center  82  is preferably of the fluorescent quench type as disclosed in U.S. Pat. Nos. 3,725,658; 5,517,313 and 5,632,958. The preferred embodiment may employ a sensor manufactured by Sensors for Medicine and Science, Inc. of Germantown, Maryland. The computation unit includes a source (not shown) for directing exciting radiation to the fluorescent coating on the end of the oxygen sensor  82  from exterior of the tube  30  and sensing the resulting fluorescence intensity which is diminished as a function of the concentration of oxygen and gas flowing over its surface to produce a direct measurement of oxygen concentration. The exciting radiation and fluorescent signal may be carried to the sensor by an optical fiber (not shown). In practice, after a user&#39;s breathing has stabilized and a test cycle is initiated either automatically or through manual depressions of one of the buttons  80 , the flow rate and oxygen levels through the tube  30  are monitored by the sensors and provided to the computation unit. At the end of the cycle, which is preferably automatically timed, the measured quantity such as oxygen consumption will be shown on the display  78 . 
     FIG. 3 illustrates an alternative embodiment of the invention in which the computation and display unit,  76 , instead of being incorporated integrally with the nondisposable section which is secured to the master in use, is formed in a separate desktop unit  94 . The unit incorporates a display  96 , control switches  98 , and a keyboard  100 . It is connected to the section  12   a  by a flexible electrical cable  102 . This arrangement lowers the weight of the unit which must be supported on the mask  14  during testing and allows more convenient user control of the unit and observation of the display. The computation and control unit  76  of the first embodiment is replaced in the embodiment by a box  104  which includes a connector for the cable  102  and also supports the oxygen sensor  82  in the same manner as the embodiment illustrated in FIG.  1 . Otherwise, the system of FIG. 3 is identical to the system of FIG.  1  and similar numerals are used for similar sections.