fig1 ( prior art ) is a schematic diagram illustrating a conventional setup for multiple - breath inert gas wash - in / wash - out tests for determination of frc and ventilation distribution ( lci ) as known in the art . the setup includes a bias flow of a mixture containing a non - resident inert tracer gas for wash - in in the flowpast assembly 107 . a test subject 101 having the nose occluded with a nose clip 102 breathes through a mouthpiece 103 , a bacterial filter 104 , a respiratory flowmeter 105 and a non - rebreathing valve assembly 106 . the gas reservoir 108 is coupled to a flowpast assembly 107 via a gas line . flowmeter connection ( s ) 109 and a gas sample line 110 are also part of the setup . to perform a multiple - breath inert gas wash - in / wash - out test , the test subject 101 inspires the non - resident inert tracer gas from the flowpast assembly 107 through the non - rebreathing valve assembly 106 . the non - rebreathing valve assembly 106 is constructed by one - way valves allowing gas to flow in one direction only . because of the construction of the valve 106 , the test subject does not breathe the non - resident inert tracer gas back to the flowpast assembly 107 during exhalation . instead the test subject expires to the surrounding air . the test subject 101 may use a face mask instead of nose clip 102 and mouthpiece 103 . the analyser unit 111 consists of a measuring apparatus comprising flowmeter electronics and at least one gas analyser . a typical test consists of a period where the test subject inspires from the flowpast and exhales to the surrounding air a number of times until the concentration of the tracer gas is constant e . g . below a predetermined threshold fluctuation ( wash - in period ) followed by a period where the test subject is breathing fresh air ( wash - out period ). during the testing ( both during the wash - in and the wash - out period ) the concentration in the inhaled and / or exhaled air of the inert gas in the mixture is measured by a fast responding gas analyser . instead of gas concentration the gas analyser may equally well measure the partial pressure of the gas . the partial pressure can be obtained from the fractional concentration of dry gas or any other measure of gas concentration or pressure using appropriate conversion factors as known in the art . also the flow of the inhaled and / or exhaled air is measured by means of the flowmeter 105 . these measurements are made continuously . fig2 ( prior art ) outlines a curve from a conventional multibreath wash - in / wash - out test from where lci can be determined . the insoluble gas , sf 6 , has become the gas of choice for measurement of lci . the concentration of sf 6 is monitored and when the concentration is constant ( below a predetermined threshold fluctuation ) 201 , the first time period called wash - in 202 is over . hereafter the wash - out period 203 begins where the concentration of sf 6 is monitored until the concentration has reached 1 / 40 of the concentration in the beginning of the wash - out period 204 . the cumulative expired volume ( v ce ) required to clear the lungs of the gas down to 1 / 40 of its start concentration can then be used in combination with the functional residual capacity ( frc ) to determine the lci of the test subject . in the conventional mbw test the frc is calculated from the net volume of inert gas exhaled divided by the difference in end - tidal concentration at the start and end of the wash - out : as the net volume of inert gas exhaled ( numerator ) is obtained by integration of the product of respiratory flow and tracer gas concentration ( i . e . expired and re - inspired tracer gas volumes on a breath - by - breath basis ), accurate determination of the frc requires a rapid dynamic response and data acquisition rate of the gas analyser . proper alignment in time of the respiratory flow signal and tracer gas concentration prior to the calculation is also critical . this makes demands on the performance of the gas analyser and the calibration of the equipment . fig3 is a schematic diagram illustrating a setup for wash - in / wash - out tests using inert gas rebreathing for determination of frc and ventilation distribution ( lci ) as used in conjunction with the disclosed invention . a test subject 301 having the nose occluded with a nose clip 302 breathes through a mouthpiece 303 , a bacterial filter 304 , a respiratory flowmeter 305 and one port 306 of a rebreathing valve assembly 307 . a rebreathing bag 308 is connected to the valve assembly and evacuated and pre - filled with a gas mixture from a gas reservoir 309 via a gas line 310 . flowmeter connection ( s ) 311 and a gas sample line 312 are also part of the setup . to perform a rebreathing test the valve assembly 307 is switched ( e . g . automatically by controlling line 313 ) to allow the test subject 301 to inspire and rebreathe to and from the bag 308 for a certain amount of time until the valve assembly 307 is switched back again . the test subject 301 may use a face mask instead of nose clip 302 and mouthpiece 303 . the control system 314 of the measuring apparatus consists of flowmeter electronics 315 , at least one gas analyser 316 , a valve control unit 317 ( unless the valve assembly is manually driven ) and a gas control unit 318 ( unless the bag is prepared manually ). a control unit 319 is also included , comprising a computing / processing unit ( cpu ) 320 with control interfaces 321 , one or more program and data storage devices 322 and user interfaces for example comprising a display 323 and a keyboard , touch screen or similar input device 324 . a data input / output module 325 may also be included . the processing unit ( cpu ) can e . g . comprise processing means for determining lci of the lungs of a test subject using the obtained fractional concentration of the inert tracer gas measured by the gas analysers and the gas flow measured by the flowmeter and associated flowmeter electronics . also , the processing unit can e . g . comprise processing means for determining frc based on gas analysis alone and processing means for determining v ce required to clear the inert tracer gas concentration from the lungs below 1 / 40th of the starting concentration . prior to the rebreathing tests the rebreathing bag is filled with a known volume of an inert gas mixture . during the testing the test subject is breathing through the respiration valve , which allows switching from breathing air to rebreathing the inert gas mixture from the bag and switching back again . a typical test consists of a period where the test subject is breathing to and from the bag ( rebreathing period ) followed by a period where the test subject is breathing fresh air ( wash - out period ). during the testing ( both during the rebreathing and the wash - out period ) the concentration in the inhaled and / or exhaled air of the inert gas in the mixture is measured by a fast responding gas analyser 316 . instead of gas concentration the gas analyser may equally well measure the partial pressure of the gas . the partial pressure can be obtained from the fractional concentration of dry gas or any other measure of gas concentration or pressure using appropriate conversion factors as known in the art . also the flow of the inhaled and / or exhaled air is measured by means of the flowmeter 315 . these measurements are made continuously . fig4 is an example comparing the wash - in curve of an inert tracer gas by conventional multibreath wash - in 401 ( dotted line ) with the wash - in curve of an inert tracer gas by rebreathing wash - in 402 ( solid line ) as used in conjunction with the disclosed invention , respectively . the simulation is based on the single compartment lung model and the example is performed using the following input values : frc = 3 . 0 l , deadspace v d = 0 . 2 l , and bag volume v rb = tidal volume v t = 0 . 8 l . it can be seen that the rebreathing wash - in method 402 reaches equilibration 404 much faster , 5 - 10 times ( ratio between wash - in breaths n mbw / n rb = 6 in this example ), than the conventional multibreath wash - in 401 , 403 , and since the conventional open - circuit wash - in phase lasts longer than the subsequent wash - out phase this means a reduction of total test time by typically more than 50 %. fig5 is a typical example of a test sequence as used in conjunction with one embodiment of the disclosed invention comprising two pulmonary gas exchange techniques for determination of lci . lci represents the number of lung volume turnovers ( i . e . frcs ) that the subject must breathe to clear the lungs from the tracer gas ( by convention , to an end - tidal concentration of 1 / 40th of the starting concentration over three subsequent breaths ). disregarding the correction for external dead space the equation is : inert gas rebreathing manoeuvre 501 is used for rapid wash - in of the inert tracer gas followed by a subsequent multiple - breath wash - out period 502 . the concentration of the inert tracer gas is monitored and when the concentration is constant 503 ( below a predetermined threshold value regarding the fluctuation of the concentration ), the first time period called wash - in 501 , is over . hereafter the wash - out period 502 begins where the concentration of the inert tracer gas is monitored until the concentration has reached 1 / 40 of the concentration in the beginning of the wash - out period 504 . the wash - in period is used for accurate determination of the functional residual capacity ( frc ) which is calculated by inert gas dilution alone according to the equation below : v rb = initial rebreathing bag volume c rb , i = initial fractional concentration of insoluble gas in the rebreathing bag c eq , i = equilibrium fractional concentration of insoluble gas obtained after mixing in the interest of brevity dead spaces on each side of the valve are not accounted for , but these can easily be incorporated . the multiple - breath wash - out ( mbw ) is used for determination of the cumulative expired volume ( v ce ) required to clear the inert tracer gas from the lungs . v ce is determined by integrating the part of the wash - out flow curve which has a sign corresponding to expiration ( e . g . all positive flow signals ) over time . by integrating flow ( l / s ) over time ( s ), a volume ( l ) is obtained . the gas dilution technique by inert gas rebreathing is more robust than the traditional wash - out technique for determination of frc , because it is independent of the critical time alignment between gas analyser and flowmeter signals . further , it relaxes the requirements to rise time of the gas analyser because only end - tidal concentrations are needed in determining the gas dilution , whereas in the open - circuit method a short rise time and accurate time alignment prior to integrating the product of flow and gas concentration signals are important in order to obtain accurate values of the flux of sf 6 in the rapid transitions during the beginning of expiration ( phase ii of the breath ) and inspiration . it should be noted that the above - mentioned means of implementation illustrate rather than limit the invention , and that those skilled in the art will be able to suggest many alternative means of implementation without departing from the scope of the appended claims . rather , the words used in the specification are & lt ; words of description rather than limitation , and it is understood that various changes may be made without departing from the scope of the invention . the word ‘ comprising ’ does not exclude the presence of other elements or steps than those listed in a claim . the invention can be implemented by means of hardware and software comprising several distinct elements , and by means of a suitably programmed computer . in a device claim enumerating several means , several of these means can be implemented by one and the same item of hardware or software . the mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage .
the present invention discloses a method to determine the lung clearance index or other indices of ventilation inhomogeneity of the lungs by combining two pulmonary gas exchange techniques ; inert gas rebreathing is used for rapid wash - in of the inert tracer gas and for accurate determination of the functional residual capacity by gas analysis alone . this is followed by multiple - breath wash - out for determination of the cumulative expired volume required to clear the inert tracer gas from the lungs , and calculation of the lci as the ratio between v ce and frc . the advantages of the method are i ) significant reduction of required test time , ii ) significant reduction of consumed gas mixture for wash - in of tracer gas , iii ) potential for further reduction of the use of tracer gas , and iv ) more accurate determination of the frc by gas dilution alone . furthermore the present invention relates to a corresponding system and computer - readable medium .