Method and apparatus for measuring a parameter of a gas in isolation from gas pressure fluctuations

Abstract: A method and apparatus for measuring a para­ meter of a test gas in isolation from gas pressure fluctuations. A pump draws the test gas through an air capacitor into a sensor, storing the gas in the capaci­ tor. A valve closes a fluid flow path which admits the test gas to the air capacitor and opens a fluid flow path between the air capacitor and a constant pressure alter­ nate gas source, isolating the air capacitor and the sensor from pressure fluctuations in the test gas. The air capacitor prevents any mixing of the test gas with the alternate gas until measurement of the unknown parameter has been accomplished.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to the measurement of parameters of gases and more particularly to a method and apparatus for measuring an unknown parameter of a gas in isolation from fluctuations in the pressure of the gas. Description of the Related Art There are numerous instances in which an unknown parameter of a gas must be measured. For example, in the medical field it is often necessary to measure such parameters as oxygen and carbon dioxide concentrations in a gas mixture in either an inspiration or an expiration line of a patient ventilator. See e.g. US-A-4 202 352 which discloses an apparatus for measurement of expired gas in infants. Fluctuations in the pressure of the gas during the measurement process can affect the accuracy of the measurements or even render performance of the measurements impossible. Therefore, it is highly desirable to keep the pressure of the gas relatively constant while the measurements are being performed. If the gas is not in motion, it is relatively easy to keep the pressure constant so as to carry out the necessary measurements. Even if the gas is in motion, if the pressure of the moving gas remains relatively constant, the concentration measurements can be performed satisfactorily. However, if the gas flow rate and pressure are not uniform, as is the case in an airway of a patient respirator, accurate parameter measurements become difficult or impossible. Recent advances in medical diagnostic and therapeutic procedures have created a need for a way to continuously monitor parameters of the gases being inhaled or exhaled by a patient on a respirator. Various methods of measuring such parameters have been proposed, but none of them has adequately solved the problem of making accurate measurements under the conditions of fluctuating pressure and flow rate which are encountered in respirator airways. Therefore, there remains a need for a way to measure parameters of a gas such as a gas in a patient airway in isolation from fluctuations in the pressure and flow rate of the gas. SUMMARY OF THE INVENTION According to one aspect of the invention, there is a system for measuring a parameter of a test gas, the system including a source of the test gas, a sensor in fluid communication with the source of the test gas operative to measure the parameter of the test gas, and pump means in fluid communication with the sensor to draw the test gas from the source of the test gas into the sensor, means for isolating the sensor from any fluctuations in the pressure of the test gas, an air capacitor upstream from the sensor in fluid communication therewith and operative to store a quantity of gas; a source of an alternate gas having an essentially constant pressure; fluid flow control means having a first fluid flow path connectable between the air capacitor and the source of the test gas, and a second fluid flow path connectable between the air capacitor and the source of alternate gas, the fluid flow control means being operative to open the first fluid flow path for the test gas into the air capacitor, to thereafter close said first flow path and to open the second fluid flow path between the air capacitor and the alternate source and to thereby isolate the air capacitor and the sensor from any fluctuations in the pressure of the test gas; indicator means in electrical communication with the sensor for providing an output signal representing the measure of the parameter of the test gas; characterized by a timing control means in electrical communication with the fluid flow control means and the indicator means for opening the first fluid flow path for a predetermined period of time until the air capacitor and the sensor are filled with the test gas, closing the first fluid flow path and opening the second fluid flow path, and for controlling the indicator means to commence providing the output signal a predetermined period of time after the second fluid flow path is opened when the pressure of the test gas is held constant, whereby the output signal is provided only when the pressure of the test gas in the sensor is held constant. According to another aspect of the invention there is provided a method of measuring a parameter of a test gas in isolation from gas pressure fluctuations comprising the steps of opening a first fluid flow path for the test gas into an air capacitor; drawing the test gas through the air capacitor and into a sensor for measurement of the parameter of the test gas, thereby filling the air capacitor with the test gas; closing the first fluid flow path; opening a second fluid flow path between a constant pressure alternate gas source and the air capacitor and thereby isolating the sensor from any fluctuations in the pressure of the test gas; allowing pressure within the sensor to stabilize for a predetermined period of time after the second fluid flow path is closed before measuring the parameter of the test gas, and measuring the parameter of the test gas under constant pressure conditions before any substantial mixing of the alternate gas with the test gas stored in the air capacitor occurs. The present invention provides a method and apparatus for measuring a parameter of a test gas in isolation from fluctuations in the pressure and flow rate of the gas by drawing the gas into an air capacitor and then admitting an alternative gas at a constant pressure to an inlet of the capacitor while the measurement is being performed. Briefly and in general terms, apparatus according to the invention includes a sensor to measure the unknown parameter of the test gas, a pump downstream from the sensor for drawing the gas into the sensor, an air capacitor upstream from the sensor for storing a quantity of the gas, a constant pressure alternate gas source, and fluid flow control means. According to the invention, the fluid flow control means opens a first fluid flow path for the test gas into the air capacitor for a sufficient time to enable the pump to draw the gas through the air capacitor into the sensor and thereby to fill the air capacitor with the test gas. Thereafter, the first flow path is closed and a second flow path between the air capacitor and the alternate gas source is opened, thereby isolating the air capacitor and the sensor from any fluctuations in the pressure of the test gas. The air capacitor prevents any substantial mixing of the alternate gas with the stored test gas for a sufficient time to enable the sensor to measure the unknown parameter of the test gas under constant pressure conditions. The pressure isolation provided by the air capacitor permits the test gas to flow continuously through the sensor during the measurement process while preventing pressure fluctuations from interfering with accuracy of the measurement. In a preferred embodiment of the invention, the fluid flow control means comprises a valve, such as a solenoid valve, having a first inlet in fluid communication with a source of the test gas, a second inlet in fluid communication with the alternate gas source and an outlet in fluid communication with the air capacitor. The valve opens the first flow path by connecting the outlet to the first inlet and opens the second flow path by connecting the outlet to the second inlet. The fluid flow control means preferably also includes timing control means such as a microprocessor or the like which selectively applies power to the solenoid valve to control the flow of gases into the air capacitor. The microprocessor also controls an output display or the like to ensure that the sensor output is provided only during the time that the pressure of the test gas in the sensor is being held constant. In a preferred embodiment, the test gas comprises an inspired gas in an airway of a patient respirator. The alternate gas source preferably comprises the ambient atmosphere. The air capacitor preferably comprises an elongated chamber characterized by a length-to-diameter ratio much larger than unity; for example, a chamber having a length of about 75 centimeters and a diameter of about 0.5 centimeters has been found to give satisfactory results. The air capacitor can be fabricated, for example, by forming an elongated channel which establishes a fluid flow path between two cavities in a block of material, or can simply comprise a similar length of tubing having a similar diameter, and having appropriate adapters at each end. According to the invention, a method of measuring an unknown parameter of a test gas in isolation from gas pressure fluctuations is provided that utilizes apparatus of the kind described above. The method comprises the steps set out in claim 11. From the above, it may be seen that the present invention provides a system and method that enhances the accuracy and reliability of measurment of gas parameters in a patient's airway. These benefits are provided by a relatively simple and robust apparatus that does not interfere with the basic function of the respirator. Other aspects and advantages of the present invention will become apparent from the following detailed description, which taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a fluid flow diagram of apparatus according to the invention for measuring a parameter of a gas in isolation from pressure fluctuations; FIG. 2 is a perspective view of an air capacitor block located upstream of the sensor of the apparatus of Fig. 1; FIG. 3 is a top plan schematic view of the air capacitor FIG. 2; FIG. 4 is a top plan view of a pressure damping air capacitor block to be connected between the sensor and pump of FIG. 1; and FIG. 5 is a sectional elevational view of an assembly of the pressure damping block and the air capacitor, taken along line 5-5 of FIG. 3, and line 5-5 of FIG. 4. DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawings, which are included for purposes of illustration, the invention is embodied in a novel method and apparatus for measuring a parameter of a gas in isolation from fluctuations in the pressure of the gas. The monitoring of parameters such as oxygen and carbon dioxide concentrations in inspired or expired gases in a patient's airway has become of great importance with advances in medical technology, but the accuracy of the measurement of such parameters has been adversely affected by pressure fluctuations in the airway. In accordance with the invention, a test gas is drawn into a sensor through an air capacitor which stores the test gas. Thereafter, a constant pressure alternate gas source is placed in fluid communication with the air capacitor, isolating the air capacitor and the sensor from any fluctuations in the pressure of the main body of the test gas while an unknown parameter of the test gas is being measured. The invention thus provides a simple and effective means for accurately measuring unknown parameters of the test gas notwithstanding fluctuations in the pressure of the main body of the test gas. More particularly, as is shown schematically in FIGURE 1, an apparatus according to the invention for measuring an unknown parameter of a test gas in isolation from gas pressure fluctuations comprises a sensor 11 operative to measure the unknown parameter, a pump 13 downstream from the sensor 11, an air capacitor 15 upstream from the sensor 11, a constant pressure alternate gas source generally designated 17, and fluid flow control means generally designated 19. The pump 13 is in fluid communication with the sensor 11 through fluid flow lines 75, 73 cavities 63 and 65 and channel 67. The pump 13 draws the test gas into the sensor 11 for measurement of the unknown parameter. The gas ultimately flows out of the sensor 11 into the pump 13 and is then discarded, for example, through an exhaust vent 23. The air capacitor 15 in a currently preferred embodiment comprises a block 16 shown in Figures 2 and 3 having an enclosed channel 18 which is in fluid communication with the sensor 11 as shown by a fluid flow line 25 and stores a quantity of gas. The air capacitor channel 18 preferably comprises an elongated channel or chamber having a length-to-diameter ratio much larger than unity. For example, a channel having a length of about 75 centimeters and a diameter of about 0.5 centimeters has been found to be satisfactory. The air capacitor 15 is typically sealed by a cover 20 shown in Figure 5 to form the enclosed channel. Alternatively, the air capacitor may be formed by an appropriate length and diameter tubing which can be connected and function in a manner similar to the block form of the air capacitor. The fluid flow control means 19 opens a first fluid flow path for the test gas into the air capacitor 15 for a sufficient time to enable the pump 13 to draw the test gas through the air capacitor 15 into the sensor 11 and thereby to fill the air capacitor 15 with said gas. Then the flow control means 19 closes the first fluid flow path and opens a second fluid flow path between the air capacitor 15 and the alternate gas source 17 and thereby isolates the sensor 11 from any fluctuations in the pressure of the test gas. The air capacitor 15 prevents any substantial mixing of the alternate gas with the stored test gas for a sufficient time to enable the sensor 11 to measure the parameter of the test gas under constant pressure conditions without degrading the quality of the measurement. The pressure isolation provided by the air capacitor 15 permits the test gas to flow continuously through the sensor 11 during the measurement process while preventing pressure fluctuations from interfering with the accuracy of the measurement. The fluid flow control means 19 preferably comprises a valve such as a solenoid valve 27 having a first inlet 29 in fluid communication with a source such as an airway 3 1 of the test gas as indicated by a fluid flow line 33, a second inlet 35 in fluid communication with the alternate gas source 17 as indicated by a fluid flow line 37, and an outlet 39 in fluid communication with the air capacitor 15 as indicated by a fluid flow line 41. The valve 27 opens the first fluid flow path by connecting the outlet 39 to the first inlet 29 as indicated by a first valve connection 43 and opens the second flow path by connecting the outlet 39 to the second inlet 35 as indicated by a second valve connection 45. The fluid flow control means 19 preferably also includes timing control means 47 such as a microprocessor or the like which selectively applies power to the solenoid valve 27 as indicated by a control line 49 to control the flow of gases into the air capacitor 15. The timing control means 47 causes the valve 27 to keep the first fluid flow path open long enough to fill the sensor 11 and the air capacitor 15 with the test gas, then causes the valve 27 to close the first fluid flow path and to open the second fluid flow path to isolate the air capacitor 15 and the sensor 11 from any pressure fluctuations in the source of the test gas. The timing control means 47 also controls an output indicator 51 or the like as indicated by a control line 53. The sensor 11 provides a parameter measurement signal to the output indicator 51 as indicated by a control line 55, and the timing control means 47 ensures that an output value is provided only during the time that the pressure of the test gas in the sensor 11 is being held constant. Preferably, the output value is delayed after the second fluid flow path has been opened just long enough for the pressure in the air capacitor 15 and the sensor 11 to stabilize. In applications requiring frequent monitoring of an unknown parameter of the test gas, the timing control means 47 switches back and forth between the two fluid flow paths, the first path being kept open long enough to ensure that the air capacitor 15 and the sensor 11 are filled with the test gas and the second path being kept open long enough to ensure that pressures have stabilized before measurements are taken. The alternate gas source 17 preferably comprises access to the ambient atmosphere, although other sources may be used. The second inlet 35 of the valve 27 is in fluid communication with the ambient atmosphere through the fluid flow line 37 and a vent 57. A flow constrictor 59 in the flow line 37 is preferably used to maintain constant pressure from the alternate gas source 17. In a preferred embodiment the test gas comprises inspired gas in the airway 31 of a patient respirator (not shown). As shown in FIGS. 4 and 5, in an alternative embodiment, in order to damp out pressure fluctuations generated by the pump, the portion of the line 21 between the sensor and the pump may include a dual air capacitor block 61 which includes first and second cavities 63 and 65 and an elongated constrictor channel 67 establishing a resistor fluid flow path between the cavities 63 and 65, the channel 67 also having a length-to-diameter ratio much larger than unity. The cavities 63 and 65 and the constrictor channel 67 are formed, for example, in an upper surface of the block 61. The block 61 may be covered by the bottom of the air capacitor block 16 and solvent bonded together to form a gas-tight seal with the adjacent cavities 63 and 65 and the channel 67 to constrain any gas flowing between the cavities 63 and 65 to flow through the channel 67. Fluid communication for a gas to enter and exit the cavities 63 and 65 is provided by means of orifices 73 and 75, respectively, extending through the block 16 and the cover 20. The block 16 and the cover 20 may be fabricated from various materials. Acrylic plastic or the like has been found to give satisfactory results. A method according to the invention of measuring an unknown parameter of a test gas in isolation from gas pressure fluctuations by means of apparatus of the kind described above includes the steps as defined in claim 11. From the foregoing it will be appreciated that the method and apparatus of the invention provide a way to accurately measure an unknown parameter of a gas which is subject to pressure fluctuations by storing the gas in an air capacitor and then isolating the air capacitor from the pressure fluctuations for a sufficient time to permit measurement of the unknown parameter. The invention is advantageously used, for example, to accomplish the accurate measurement of such parameters as oxygen and carbon dioxide concentration in inspired or expired gases in an airway of a patient respirator. Although certain specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated, and various modifications and changes can be made without departing from the scope of the invention. Within the scope of the appended claims, therefore, the invention may be practiced otherwise than as specifically described and illustrated.

Claims: A system for measuring a parameter of a test gas, the system including a source (31) of the test gas, a sensor (11) in fluid communication with the source (31) of the test gas operative to measure the parameter of the test gas, and pump means (13) in fluid communication with the sensor (11) to draw the test gas from the source (31) of the test gas into the sensor (11), means for isolating the sensor from any fluctuations in the pressure of the test gas, an air capacitor (15) upstream from the sensor in fluid communication therewith and operative to store a quantity of gas; a source of an alternate gas (17) having an essentially constant pressure; fluid flow control means (19) having a first fluid flow path (29, 43) connectable between said air capacitor (15) and said source of the test gas, and a second fluid flow path (35, 45) connectable between said air capacitor (15) and said source of alternate gas (17), the fluid flow control means (19) being operative to open the first fluid flow path (29, 43) for the test gas into the air capacitor (15), to thereafter close said first flow path (29, 43) and to open the second fluid flow path (35,45) between the air capacitor (15) and the alternate source (17) and to thereby isolate the air capacitor (15) and the sensor (11) from any fluctuations in the pressure of the test gas; indicator means (51) in electrical communication with said sensor (11) for providing an output signal representing the measure of the parameter of the test gas; characterized by: a timing control means (47) in electrical communication with said fluid flow control means (19) and said indicator means (51) for opening said first fluid flow path (29, 43) for a predetermined period of time until said air capacitor (15) and said sensor (11) are filled with the test gas, closing said first fluid flow path (29, 43) and opening said second fluid flow path (35, 45), and for controlling said indicator means (51) to commence providing said output signal a predetermined period of time after said second fluid flow path (35, 45) is opened when said pressure of the test gas is held constant, whereby said output signal is provided only when the pressure of the test gas in the sensor is held constant. The system according to Claim 1, wherein there is further provided means (61) for damping fluctuations in pressure in said test gas at said sensor due to said pump means (13) downstream from said sensor, said means for damping (61) including a first chamber (63) connected for fluid communication with said pump means (13), a second chamber (65) connected for fluid communication with said sensor (11), and an elongated constrictor channel (67) for establishing a resistor fluid flow path connected for fluid communication between said first and second chambers (63, 65). The system according to Claim 1 or Claim 2, wherein the test gas is inspiration gas provided to a patient, and the fluid flow control means (19) comprises a valve (27) having a first inlet (29) in fluid communication with said source (31) of the test gas through a first flow path (43), a second inlet (35) in fluid communication with the alternate gas source (17) through a second flow path (45) and an outlet (39) in fluid communication with the air capacitor (15), the valve being operative to open the first flow path (43) by connecting the outlet (39) to the first inlet (29) and to open the second flow path (45) by connecting the outlet (39) to the second inlet (35). The system according to any preceding claim, wherein the alternate gas source (17) comprises the ambient atmosphere. The system according to any preceding claim, wherein the test gas is inspiration gas provided to a patient, and the source (31) of the test gas comprises an airway of a patient respirator. The system according to any preceding claim, wherein the air capacitor (15) comprises an elongated chamber (18) characterized by a length-to-diameter ratio larger than unitY. The system according to Claim 6, wherein the chamber (18) has a length of approximately 75 centimeters and a diameter of approximately 0.5 centimeters. The system according to any preceding claim, wherein the air capacitor (15) comprises a block of material defining an elongated channel (18) establishing a fluid flow path having a length-to-diameter ratio larger than unity. The system according to Claim 2 or according to any of Claims 3 to 8 when dependant on Claim 2, wherein said first and second chambers (63, 65) and said constrictor channel (67) of said damping means (61) are formed in a top surface of a first block member (61) covered by a bottom surface of a second block member (16), and said air capacitor (15) is formed in a top surface of said second block member (16) covered by a third block member (20). The system according to Claim 9, wherein said second block member (16) includes a first pair of orifices (73, 75) therethrough in fluid communication with said first and second chambers (63, 65), said third block member includes a second pair of orifices (73, 75) therethrough, said first and second pairs of orifices being aligned to provide a first flow path (73) to said first chamber (63) and a second flow path (75) to said second chamber (65), and said third block member includes a third pair of orifices (73, 75) therethrough to provide a third and fourth flow path (73, 75) to said air capacitor (15). A method of measuring a parameter of a test gas in isolation from gas pressure fluctuations characterized by the steps of opening a first fluid flow path (29, 43) for the test gas into an air capacitor (15); drawing the test gas through the air capacitor (15) and into a sensor (11) for measurement of the parameter of the test gas, thereby filling the air capacitor (15) with the test gas; closing the first fluid flow path (29, 43); opening a second fluid flow path (35, 45) between a constant pressure alternate gas source and the air capacitor (15) and thereby isolating the sensor (11) from any fluctuations in the pressure of the test gas; allowing pressure within said sensor (11) to stabilize for a predetermined period of time after said second fluid flow path (35, 45) is closed before measuring the parameter of the test gas, and measuring the parameter of the test gas under constant pressure conditions before any substantial mixing of the alternate gas with the test gas stored in the air capacitor (15) occurs. The method according to Claim 11, wherein the step of opening a first fluid flow path (29, 43) comprises opening a fluid flow path (33, 29, 43) between an airway (31) of a patient respirator and the air capacitor (15).

Inventors: GOULDING PETER, GOULDING, PETER
Applicants: PURITAN BENNETT CORP, PURITAN-BENNETT CORPORATION
Application year: 1990
Main classification: G01N1
IPC codes: G01N1, G01N33, A61B5, G01N27