Patent Publication Number: US-2006000256-A1

Title: System for testing performance of medical gas or vapor analysis apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation of PCT International Application No. PCT/US2003/040836, filed Dec. 22, 2003, designating the United States of America, and published, in English, as PCT International Publication No. WO 2004/059317 A1 on Jul. 15, 2004, which application claims priority to U.S. Provisional Application No. 60/435,906, filed Dec. 20, 2002, the entire contents of each of which are hereby incorporated herein by this reference. 
    
    
     TECHNICAL FIELD  
      The present invention relates to methods and systems for accurately assessing the performance of gas analyzers, such as carbon dioxide monitors and sensors.  
     BACKGROUND ART  
      Clinical practice standards for delivering anesthesia to a patient require that the concentration of carbon dioxide (CO 2 ) expired by the patient be monitored during all anesthetic procedures in which the respiratory drive of the patient (i.e., the patient&#39;s ability to breathe on his or her own) may be impaired. Typically, gas analyzers, such as carbon dioxide monitors and sensors (which sensors are also referred to as “capnometers”), are used to monitor the concentration of CO 2  expired by an anesthetized patient.  
      In addition to monitoring CO 2  levels, many hospitals employ gas analyzers which are configured to monitor, in real time, the amount of anesthetic agents (e.g., gases, vapors, etc.) that an anesthetist is delivering to the patient.  
      Gas analyzers measure the amount (typically in terms of partial pressure) of a specific gas (CO 2  in the case of capnometers) that is in a respiratory sample. There are currently two major types of gas analyzers that have found widespread use: (1) the so-called “mainstream,” or “on-airway,” type, which is positioned along a breathing circuit that communicates with the airway (i.e., trachea, bronchi, and lungs) of a patient to measure the amount of a particular gas in respiration that passes through the breathing circuit; and (2) the “so-called” side stream type, or sampling system, which includes a sensor that is positioned somewhat remotely from the breathing circuit and communicates therewith by way of one or more sample tubes, through which small samples of gases that are inhaled and exhaled by a patient are diverted to the sensor for analysis. Side stream sampling systems are also typically configured to draw the samples from the breathing circuit and to remove moisture from, or dry, a sample prior to presenting it to the sensor at a known and controlled pressure and flow rate.  
      Sometimes, gas analyzers are tested by passing a calibration gas or calibration gas mixture, which includes one or more gases or other constituents (e.g., CO 2 , gaseous or vaporized anesthetic agents, etc.) of known concentration therethrough. The amount or amounts of each evaluated gas or other constituent is then compared with the known amount of that constituent in the calibration gas. While this technique is sometime effective for measuring the performance of a gas analyzer, it is not always reliable, as the rate at which the calibration gas or calibration gas mixture flows through the gas analyzer may cause the gas analyzer to provide unreliable results. Further, due to excessive flow and failure to terminate the flow of calibration gases when the test is complete, calibration gases are often wasted when this type of technique is employed.  
      Moreover, while the amounts of the constituents in calibration gases have conventionally been measured in terms of the percent, by volume, they constitute of a given volume of a precisely controlled calibration gas mixture (e.g., 5% CO 2 , 16% O 2 , balance N 2  being common), such percentages do not readily translate to the units of gas concentrations that are typically measured by gas analyzers. Specifically, most gas analyzers are designed to evaluate the partial pressure (e.g., mm Hg in U.S., kilopascals in Europe) of a particular gas in a sample.  
      Further, the monitors that are associated with most capnometers (i.e., CO 2  analyzers) are designed to evaluate monitored data and report end-tidal gas concentrations in partial pressures, which are typically defined in terms of millimeters of mercury (mm Hg). End-tidal CO 2 , which occurs near the end the expiratory phase of a subject&#39;s respiration, or breathing, is the highest CO 2  concentration observed during a breath. Conventional techniques for calibrating capnometers, however, involve metering of a calibration gas mixture from a tank at a constant flow rate. Thus, the signal produced by a capnometer does not simulate the ebbs and flows of breathing, and no end-tidal value is reported. As a result, one must know how to cause the monitor associated with the capnometer to evaluate signals from the capnometer in an “instantaneous concentration” mode. Many of the currently available monitors require that a recalibration sequence be initiated to continuously evaluate constant concentrations of an analyzed gas, which may be undesirably time-consuming.  
      It is also often difficult to consistently maintain the precise gas proportions of calibration gas mixtures for use with gas analyzers that are used in evaluating the amount of anesthesia present in a sample. This difficulty is caused, at least in part, by the condensation of anesthesia gases at relatively low pressures. In order to provide an anesthesia calibration gas mixture having accurate sample concentrations, the anesthesia gases must be stored at very low pressures. This means that only small amounts of anesthesia calibration gases may be stored in cylinders of conventional sizes, which results in the availability of undesirably small samples of undesirably large storage tanks.  
      Another challenge of maintaining anesthesia calibration gas mixtures is their typically short shelf lives.  
      In addition, calibration gases, including those configured for use with carbon dioxide analyzers and anesthesia analyzers, are often delivered at excessive flow rates, which may result in wastage thereof.  
      In view of the foregoing, there is a need for a system and method by which a gas analyzer may be tested or calibrated accurately, relatively quickly and conveniently, and without wasting a calibration gas mixture.  
     DISCLOSURE OF INVENTION  
      The present invention includes a system for testing or calibrating a gas analyzer, such as a capnometer, an anesthesia analyzer, or the like, as well as testing and calibration methods. Despite being useful for both testing and calibration, systems that incorporate teachings of the present invention are referred to herein as “test systems” for the sake of simplicity.  
      A test system according to the present invention may be used with both main stream and side stream gas analyzers. Calibration gas mixtures with known amounts of one or more gases (or vapors) may be used to evaluate the accuracy of both types of gas analyzers. The frequency response, a measure of how quickly a gas analyzer detects a change in the amount of one or more gases in a sample (e.g., a respiratory sample, a calibration gas mixture, etc.), of both types of gas analyzers may also be evaluated. In addition, the ability of a side stream gas analyzer to draw a sample from a breathing circuit may also be evaluated by use of a test system of the present invention.  
      An exemplary embodiment of test system that incorporates teachings of the present invention includes at least one tank within which a calibration gas mixture is held. A calibration gas line may be in communication with each tank to facilitate the removal of a calibration gas mixture therefrom. A pressure sensor and a pressure regulator communicate with tank, as does a flow control valve, and each of these elements may be positioned along the calibration gas line. The pressure regulator and flow control valve are located and configured to control flow of the calibration gas mixture from the tank. On an opposite side of the flow control valve, each calibration gas line communicates with a low pressure tube, or “low flow tube,” which ventilates to ambient, or “room,” air. A flow restriction system, which may include one flow restriction line or a series of flow restriction lines, may communicate with the low-pressure tube, downstream from the flow control valve. A valve and, optionally, a flow restrictor may be positioned along each flow restriction line. Further downstream, the test system includes a sample tube that communicates with the low pressure tube. If the test system includes a flow restriction system, communication between the sample tube and the low pressure tube may occur through the flow restriction system. A diversion valve is positioned along the sample tube. The diversion valve is configured to control the flow of ambient, or room, air into the sample tube. Thus, by operation of the diversion valve, the calibration gas mixture may be diluted with or replaced with ambient air. The sample tube includes a connector, or adapter, which is configured to connect a gas analyzer to be tested, which is also referred to herein as a “unit under test,” to the test system. Optionally, a flow meter of a known type may be positioned between the diversion valve and the connector.  
      In addition, the test system may include one or more processing elements (e.g., processors, computers, etc.) that are configured to communicate with the pressure regulator, flow control valve, and diversion valve thereof. The at least one processing element may be configured to control the flow of a calibration gas mixture from tank, as well as to automatically shut off the flow of the calibration gas mixture once testing has been completed or after a predetermined period of time, thereby preventing accidental emptying of the calibration gas mixture from its respective tank. Communication between the tank and the low pressure tube may also be terminated when the pressure sensor indicates to the at least one processing element that the calibration gas mixture is no longer flowing, which may prevent loss of calibration gas as a new tank is placed in communication with the calibration gas line.  
      The one or more processing elements of the test system may also be configured to communicate with and receive signals from the unit under test and the flow meter, if any.  
      Additionally, the test system may include a barometer that communicates with at least one processing element that also communicates with the device under test. This arrangement facilitates the accurate calculation of partial pressures that correspond to the concentration of one or more gases or vapors included in the calibration gas mixture.  
      In another example of a test system that incorporates teachings of the present invention, the flow control valve comprises a three- or more-way valve with at least two inlets and one outlet. In addition to controlling communication between the tank and the low pressure tube, the flow control valve of this embodiment controls communication between an air pump and the low pressure tube. The one or more processing elements may communicate with and control operation of one or both of the flow control valve and the air pump such that the calibration gas mixture may be delivered to the remainder of the test system in such a way as to mimic a subject&#39;s (e.g., a patient&#39;s) breathing.  
      The present invention also includes methods for testing and calibrating gas analyzers by assembling or otherwise placing the same in communication with a test system that incorporates teachings of the present invention and operating the test system in accordance with a desired test or calibration protocol, which are also within the scope of the present invention. Examples of test methods include methods for testing the accuracy of a gas analyzer, testing the responsiveness of a gas analyzer to changes in the amounts of a gas or vapor that are present in an evaluated sample, and testing the ability of the gas analyzer to respond to changes in the airway pressure of a subject.  
      Other features and advantages of the present invention will become apparent to those of ordinary skill in the art through a consideration of the ensuing description, the accompanying drawing, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Various aspects of exemplary embodiment of test systems according to the present invention are shown in the drawings, in which:  
       FIG. 1  is a schematic representation of an exemplary embodiment of test system; and  
       FIG. 2  schematically depicts another exemplary embodiment of test system, including an air pump that mimics a subject&#39;s breathing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  depicts an exemplary embodiment of test system  10  for gas analyzers. Test system  10  comprises a “smart tank” and is configured to test or calibrate a device under test  100 , such as a capnometer, other gas analyzer, or anesthesia analyzer. As depicted, test system  10  includes a tank  12  and various conduits, sensors, regulators, valves, and flow restrictors to provide a complete system for verifying that device under test  100  is functioning correctly or for calibrating device under test  100 . Additionally, one or more processing elements  50  may control operation of one or more of the other elements of test system  10  and, if processing elements  50  control operation of more than one other element of test system  10 , synchronize operation of the elements.  
      Test system  10  employs a tank  12  of a known type (e.g., a conventional cylinder-type tank) which contains a precision blended calibration gas mixture for use in testing or calibrating device under test  100 . The pressure within tank  12  may be measured electronically, such as by the depicted pressure sensor  14 , which communicates with tank  12  and comprises a pressure sensor of a known type.  
      A pressure regulator  16  of a known type is positioned downstream from pressure sensor  14  and may regulate the pressure of the calibration gas mixture downwardly, as desired (e.g., to less than about 1 psig). It is currently preferred that pressure regulator  16  be set to deliver the calibration gas mixture at a pressure that exceeds the ambient (e.g., atmospheric) pressure in the environment in which test system  10  is being used. Pressure regulator  16  may also be operated in a manner that will not cause the calibration gas mixture to be delivered to unit under test  100  at a pressure which is substantially different from the ambient pressure. Pressure sensor  14  and pressure regulator  16  may both be in communication with a processing element  50  of test system. Pressure sensor  14  may communicate pressure signals to processing element  50 . Processing element  50  may then, under control of programming thereof and based on the pressure signals that have been received from pressure sensor  14 , control operation of pressure regulator  16 .  
      A flow control valve  18  is positioned so as to control the flow of the calibration gas mixture from tank  12 . Flow control valve  18  may comprise any suitable valve of known type and may be manually, mechanically, or electronically actuated. By way of example only, flow control valve  18  may communicate with a processing element  50  of test system  10 . Thus, operation of flow control valve  18  may be under control of programming of processing element  50 .  
      After passing through flow control valve  18 , the calibration gas mixture flows into a low pressure tube  20 . Once the calibration gas mixture enters low pressure tube  20 , the pressure to which the calibration gas mixture is subjected is decreased to substantially ambient (e.g., atmospheric) pressure.  
      Optionally, from low pressure tube  20 , the calibration gas mixture may flow into a flow restriction system  30 . As shown, flow restriction system  30  includes three flow restriction lines  31 ,  34 ,  37 , although flow restriction systems with only a single flow restriction line or other numbers of flow restriction lines are also within the scope of the present invention. A valve  33 ,  36 ,  39  is respectively associated with each flow restriction line  31 ,  34 ,  37  to control the flow of the calibration gas mixture therethrough. Valve  33 ,  36 ,  39  may communicate with and be under control of programming of a processing element  50 , as known in the art. Valve  33 ,  36 ,  39  may be configured to function between full-opened and full-closed positions in a plurality of intermediate positions (e.g., continuously or incrementally), which facilitates selection in an amount of resistance along flow restriction line  31 ,  34 ,  37  that simulates a desired level of occlusion in a sample line through which gases are conveyed to a side stream-type unit under test  100 . Alternatively, valve  33 ,  36 ,  39  may be configured to either completely open or to completely close the respective flow paths through restriction line  31 ,  34 ,  37 . If a valve  33 ,  36 ,  39  is configured to only open completely or close completely, a flow restrictor  32 ,  35 ,  38  may be positioned along (within) each flow restriction line  31 ,  34 ,  37 , with flow restrictors  32 ,  35 , and  38  restricting the flow of the calibration gas mixture to differing degrees to simulate various levels occlusion in a sample line through which gases are conveyed to a side stream-type unit under test  100 .  
      Test system  10  also includes a sample tube  22 , which communicates with low pressure tube  20 . If test system  10  also includes a flow restriction system  30 , sample tube  22  may be positioned downstream from flow restriction system  30  and communicate indirectly with low pressure tube  20  through flow restriction system  30 .  
      A diversion valve  28  is positioned along sample tube  22 . Diversion valve  28 , which is at least a three-way valve including at least two inlets and a single outlet, is configured to select from gases with an upstream portion  22   u  of sample tube  22 , external ambient air, or a combination thereof and to permit flow of the selected gases to a downstream portion  22   d  of sample tube  22 . If diversion valve  28  is intended to permit the calibration gas mixture and ambient air to simultaneously flow into downstream portion  22   d  of sample tube  22 , it may be configured to have a variety (e.g., incremental or continuous) of inlet positions (i.e., both inlets of diversion valve  28  may be partially open at the same time). Alternatively, diversion valve  28  may be configured to be selectively disposed in one of only two inlet positions (e.g., from upstream portion  22   u  of sample tube  22  or from the environment in which test system  10  is located). Of course, positioning of either type of diversion valve  28  may be controlled manually or automatically, under control of suitable programming of a processing element  50  in communication therewith.  
      In use, diversion valve  28  may be switched between the two sources (i.e., upstream portion  22   u  of sample tube  22  and the environment in which test system  10  is located) at a frequency or combination of frequencies that simulates a variety of breath rates. Such switching may, by way of example only, be effected under control of processing element  50 . By switching diversion valve  28 , the frequency response of unit under test  100  may be evaluated (e.g., by a processing element  50  of test system  10 ).  
      Optionally, a side stream or main stream flow meter  26  of a known type (e.g., differential flow, spinning vane, hot wire anemometers ultrasonic Doppler, vortex shedding, time of flight, etc.) may be positioned along sample tube  22 , downstream from diversion valve  28 . Flowmeter  26  may be configured to measure the rate at which gases (e.g., the calibration gas mixture, ambient air, or a combination thereof) flow through sample tube  22  and into unit under test  100 .  
      As another alternative, flowmeter  26  may comprise a combined gas/flow sensor of a known type, such as the NICO® CO 2 /flow sensors available from Respironics, Inc. of Murraysville, Pa. Alternatively, a separate analyzer (e.g., a gas analyzer, anesthetic agent analyzer, etc.) may be included along sample tube  22 . Inclusion of such an analyzer in test system  10  may be useful for providing a user of test system  10  with information about whether or not the calibration gas mixture being used with test system  10  includes appropriate constituents and constituent amounts for evaluating or calibrating a particular unit under test  100 . For example, if unit under test  100  is a carbon dioxide analyzer, but the concentration of carbon dioxide in the calibration gas mixture is unacceptably high or low (or nonexistent), the analyzer (e.g., an analyzer of flowmeter  26 ) may indicate the possibility that a calibration gas mixture which is inappropriate for evaluation of unit under test  100  may be used in test system  10 . Such a determination may be made by a processing element  50  in communication with the analyzer, which processing element  50  may then indicate the possibility of an inappropriate calibration gas mixture to a user of test system  10  or require the user to check and replace the calibration gas mixture.  
      A connector  24 , or adapter, is positioned at a downstream end  23   d  of sample tube  22  to facilitate connection of a unit under test  100 , such as a side stream type gas analyzer, to downstream end  23   d  and, thus, to facilitate communication between sample tube  22  and unit under test  100 . Connector  24  may be configured to generate signals indicative of whether or not a unit under test  100  has been properly assembled therewith and to transmit such signals to a processing element  50  of test system  10 .  
      Test system  10  may also include a barometric pressure sensor, or barometer  29 , of a known type. Barometer  29  may communicate measurements of the ambient barometric pressure of the environment within which test system  10  is located to a processing element  50  of test system  10 . Alternatively, such information may be manually obtained by a user of test system  10 . A barometric pressure measurement obtained with barometer  29  is used, as known in the art, to convert the volume percentages of the calibration gas mixture within tank  12  to partial pressure measurements. The partial pressure of each gas of the calibration gas mixture within tank  12  may then be displayed to the user for comparison with one or more corresponding partial pressure values obtained with unit under test  100 .  
      With continued reference to  FIG. 1 , an example of the use of test system  10  is described. Prior to use of test system  10 , or at any other time test system  10  is not in use, flow control valve  18  should be in a closed position, preventing a calibration gas mixture within tank  12  from flowing or leaking therefrom. If gas flow is detected by flowmeter  26  when test system  10  is not being used (e.g., when a unit under test  100  is not assembled with connector  24  of test system  10 ), programming (e.g., computer logic) of processing element  50 , which communicates with flowmeter  26  and flow control valve  18 , may cause flow control valve  18  to completely close. Such programming will greatly decrease the amount of costly calibration gas mixtures that are wasted.  
      Processing element  50  may likewise be programmed to control one or both of pressure regulator  16  and valve  18  in such a way as to control the amount of calibration gas mixture that is released from tank  12  into the remainder of test system  10  and, thus, to optimize the efficiency with which the calibration gas mixture is used.  
      When test system  10  is to be used, a gas analyzer to be tested, or a unit under test  100 , is secured to test system  10  by way of adapter  24 . A tank  12  including a desired calibration gas mixture may also be assembled with the remainder of test system  10 . Thereafter, flow control valve  18  is opened, permitting the calibration gas mixture to flow into low pressure tube  20 , where the pressure of the calibration gas mixture is reduced substantially to atmospheric or ambient pressure. The calibration gas mixture may then be drawn or forced into sample tube  22 , where it flows into unit under test  100 . When it operates, unit under test  100  provides the user of test system with data regarding the amount (e.g., partial pressure) of one or more substances in the calibration gas mixture, which may then be compared, manually or automatically (e.g., by a processing element  50 ), with the known amount of each substance in the calibration gas mixture.  
      Optionally, flow restriction system  30 , if any, may be used to determine whether or not unit under test  100  responds as designed in the presence of a change in patient airway pressure and provides some sort of alarm (e.g., audible, visual, etc.) in the presence of an occluded sample line. Valves  33 ,  36 ,  39  on restriction lines  31 ,  34 ,  37  may be actuated such that their corresponding flow restrictions  32 ,  35 ,  38  simulate various levels of sample line occlusion. Ideally, unit under test  100  will hold a constant flow and will continue to measure the correct gas concentration during all levels of occlusion that do not trigger an alarm condition.  
      If unit under test  100  does not measure the amount or amounts of one or more gases in the calibration gas mixture with a desired degree of accuracy, unit under test  100  may be set aside for calibration or discarded.  
      Turning now to  FIG. 2 , another exemplary embodiment of test system  10 ′ that incorporates teachings of the present invention is illustrated. Test system  10 ′ resembles test system  10  ( FIG. 1 ), but more amenable than test system  10  to being used in testing mainstream type gas sensors (e.g., capnometers, sensors for other types of gases, anesthesia sensors, etc.).  
      In particular, test system  10 ′ includes a three-way valve  42  in communication between (e.g., along a supply tube  19 ) flow control valve  18  and an upstream end  21   u  of low-pressure tube  20 . As shown, three-way valve  42  includes two inlets, one which receives gases from the outlet of flow control valve, the other inlet communicating with a relatively high pressure source  44 . The same calibration gas mixture that flows from tank  12  or a different gas mixture (e.g., air) may be forced into test system  10 ′ by way of relatively high pressure source  44 . The outlet of three-way valve  42  communicates with low-pressure tube  20 .  
      Test system  10 ′ also includes a connector  24 ′ at a downstream end  21  of low-pressure tube  20 . Connector  24 ′ is configured to facilitate assembly of a mainstream, or on-airway, analyzer  100 ′ of known type (e.g., a capnometer, another gas analyzer, an anesthetic agent analyzer, etc.) to low pressure tube  20  of test system  10 ′.  
      Relatively high pressure source  44  of test system  10 ′ may comprise an air pump of a known type (e.g., an electric air pump under control of programming of a processing element  50  of test system  10 ′, a tank or other source of compressed air or gas, etc.).  
      Three-way valve  42  maybe electronically (e.g., under control of programming of a processing element  50 ), mechanically, or manually actuated, to completely or partially select from the two inlets thereof. Thus, three-way valve  42  facilitates control over introduction of one or both of the calibration gas mixture from tank  12  and pressurized gas or air from relatively high pressure source  44  into the remainder of test system  10 ′. Switching between relatively high pressure source  44  and tank  12  at various frequencies may simulate various breath rates and create a flow in low-pressure tube  20  that simulates a patient&#39;s breathing. The simulated breathing may then be observed within low-pressure tube  20  or sample tube  22 .  
      Test system  10 ′ may, by way of example only, be used in the same manner that has been described above with respect to test system  10 . When unit under test  100  is, for example, a side-stream analyzer, three-way valve  42  may be positioned to accept the calibration gas mixture directly from tank  12 . If unit under test  100  is a mainstream type sensor, three-way valve  42  may be repeatedly switched to cause the calibration gas mixture from tank  12  and gases under pressure from relatively high pressure source  44  to flow through the remainder of test system  10 ′ in an alternating fashion and in a manner which simulates a patient&#39;s breathing.  
      Although the foregoing description contains many specifics, these should not be construed as limiting the scope of the present invention, but merely as providing illustrations of some of the presently preferred embodiments. Similarly, other embodiments of the invention may be devised which do not depart from the spirit or scope of the present invention. Features from different embodiments may be employed in combination. The scope of the invention is, therefore, indicated and limited only by the appended claims and their legal equivalents, rather than by the foregoing description. All additions, deletions and modifications to the invention as disclosed herein which fall within the meaning and scope of the claims are to be embraced thereby.