Method to determine indices of ventilation inhomogeneity e.g. lung clearance index (LCI) of a paediatric test subject

Abstract

Methods and systems to determine the lung clearance index (LCI) or other indices of ventilation inhomogeneity of lungs of a paediatric subject are provided. Inert tracer gas is washed-in and the wash-out is conducted by inhaling atmospheric air from the surroundings and exhaling to a confined space until the end-tidal tracer gas concentration has fallen below a predetermined fraction of the starting concentration. The LCI is calculated as the ratio between the cumulative expired volume (V.sub.CE) required to clear the inert tracer gas concentration from the lungs below a predetermined fraction of the starting concentration and the functional residual capacity (FRC) determined by dividing the net volume of inert tracer gas exhaled with the difference in end-tidal fractional concentration of the inert tracer gas at the start and end of the wash-out period; where V.sub.CE is determined by measuring the total volume in the collection bag after completed wash-out period.

Claims

1. A method to determine a lung clearance index (LCI) of a paediatric test subject, the method comprising: wash-in of an inert tracer gas until a constant concentration of the inert tracer gas in lungs of a paediatric test subject is reached, the wash-in is conducted using open circuit wash-in comprising a non-rebreathing valve assembly constructed by one-way valves, where the paediatric test subject inhales the inert tracer gas from an inhalation bag to wash-in the inert tracer gas while expiring into room air; measuring an end-tidal fractional concentration of the inert tracer gas at a beginning of a wash-out period; wash-out of the inert tracer gas from the lungs of the paediatric test subject during a wash-out period until an end-tidal tracer gas concentration has fallen below a predetermined fraction of the concentration in the beginning of the wash-out period, wherein the wash-out is conducted by inhaling atmospheric air from surroundings and exhaling to a collection bag collecting aspirate; measuring an end-tidal fractional concentration of the inert tracer gas at an end of the wash-out period; measuring the concentration of the inert tracer gas in the collection bag after the wash-out period; and measuring a volume of the collection bag after the wash-out period to obtain a total cumulative expired volume (VCE); wherein the lung clearance index (LCI) is determined by: determining a net volume of inert tracer gas exhaled by the paediatric test subject based on the concentration of the inert tracer gas in the collection bag after completed wash-out period and the total cumulative expired volume (VCE); determining a functional residual capacity (FRC) by dividing the net volume of inert tracer gas exhaled by the paediatric test subject with a difference in end-tidal fractional concentrations of the inert tracer gas at the start and at the end of the wash-out period; and calculation of the LCI as a ratio between VCE and FRC, wherein during the method there is no determination of flow of air inhaled and/or exhaled by the paediatric test subject.

2. The method according to claim 1, wherein the inert tracer gas is a gas with a negligible solubility in blood and tissue.

3. The method according to claim 1, wherein the inert tracer gas is SF6.

4. The method according to claim 1, wherein the inert tracer gas is comprised in an oxygen enriched gas mixture.

5. The method according to claim 1, wherein determination of the gas concentration is performed by a mass spectrometer or at least one sensitive gas analyser.

6. The method according to claim 5, wherein the determination of the gas concentration is based on photoacoustic spectroscopy (PAS).

7. A system adapted to determine a lung clearance index (LCI) of lungs of a paediatric test subject, the system comprising; a valve assembly setup comprising: an inhaler device for the paediatric test subject to inhale an inert tracer gas mixture comprising an inert tracer gas in an open circuit wash-in, the inhaler device comprising a non-rebreathing valve assembly constructed by one-way valves, where the paediatric test subject inhales the inert tracer gas from an inhalation bag to wash-in the inert tracer gas while expiring into room air; a collection bag for the paediatric test subject to exhale to for collection of an exhaled volume of gas by the paediatric test subject during a wash-out period, the volume comprising the inert tracer gas; at least one gas analyser for obtaining a fractional concentration at an end of each breath of the inert tracer gas inhaled and exhaled by the paediatric test subject until the concentration has reached a predetermined fraction of the concentration in a beginning of the wash-out period; processing means for determining the LCI of the lungs of the paediatric test subject by: using a net volume of inert tracer gas exhaled by the paediatric test subject during the wash-out period, the net volume of inert tracer gas exhaled is obtained by measuring the concentration of the inert tracer gas in the collection bag after completed wash-out period and measuring a total cumulative expired volume (VCE) in the collection bag after completed wash-out period, where the total cumulative expired volume (VCE) is the volume required to clear an inert tracer gas concentration from the lungs of the paediatric test subject below the predetermined fraction of the concentration in the beginning of the wash-out period; determining a functional residual capacity (FRC) by dividing the net volume of inert tracer gas exhaled by the paediatric test subject during the wash-out period with a difference in end-tidal fractional concentration of the inert tracer gas at a start and an end of the wash-out period; where LCI is determined as a ratio between VCE and FRC, wherein the system does not comprise a flow meter that determines flow of air inhaled and/or exhaled by the paediatric test subject.

8. The system according to claim 7, wherein the inhalation bag is prefilled with the inert tracer gas mixture comprising the inert tracer gas.

9. The system according to claim 7, wherein the inert tracer gas is a gas with a negligible solubility in blood and tissue.

10. The system according to claim 7, wherein the inert tracer gas is SF6.

11. The system according to claim 7, wherein the inert tracer gas is comprised in an oxygen enriched gas mixture.

12. The system according to claim 7, wherein the at least one gas analyser is a mass spectrometer or a sensitive gas analyser.

13. The system according to claim 12, wherein the at least one gas analyser is based on PAS.

14. The system according to claim 7 comprising a cylinder for containing the inert tracer gas mixture and a dilution device to dilute the inert tracer gas mixture with air or oxygen.

15. The system according to claim 7, wherein the at least one gas analyser for obtaining the fractional concentration of the inert tracer gas inhaled and exhaled by the test subject comprises means for obtaining the partial pressure of the inert tracer gas.

16. A non-transitory computer-readable medium having stored therein instructions for causing a processing unit to execute the processes of: determining when a constant concentration of an inert tracer gas in lungs of a paediatric test subject has been reached during wash-in of the inert tracer gas; measuring an end-tidal fractional concentration of the inert tracer gas at a beginning of a wash-out period, wherein the inert tracer gas is directed to a collection bag during the wash-out period; determining when an end-tidal tracer gas concentration has fallen below a predetermined fraction of the concentration in the beginning of the wash-out period; measuring an end-tidal fractional concentration of the inert tracer gas at an end of the wash-out period; measuring the concentration of the inert tracer gas in the collection bag after the wash-out period; and measuring a volume of the collection bag after the wash-out period to obtain a total cumulative expired volume (VCE); wherein the lung clearance index (LCI) is determined by: determining a net volume of inert tracer gas exhaled by the paediatric test subject based on the concentration of the inert tracer gas in the collection bag after completed wash-out period and the total cumulative expired volume (VCE); determining a functional residual capacity (FRC) by dividing the net volume of inert tracer gas exhaled by the paediatric test subject with a difference in end-tidal fractional concentrations of the inert tracer gas at the start and at the end of the wash-out period; and calculation of the LCI as a ratio between VCE and FRC, wherein the non-transitory computer-readable medium does not have instructions stored therein for causing a processing unit to execute the process of determining flow of air inhaled and/or exhaled by the paediatric test subject.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, preferred embodiments of the invention will be described referring to the figures, where;

(2) FIG. 1 (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.

(3) FIG. 2 (prior art) illustrates a curve from a conventional multi-breath wash-in/wash-out.

(4) FIG. 3 is a schematic diagram illustrating one setup before starting the test for determination of FRC and ventilation distribution (LCI) as used in conjunction with the disclosed invention.

(5) FIG. 4 is a schematic diagram illustrating an embodiment of the wash-in phase where the paediatric subject breathes in from the wash-in bag and out to room air.

(6) FIG. 5 is a schematic diagram illustrating an embodiment of the washout phase where the paediatric subject breathes in from room air and out into the confined space (here shown as a collection bag).

(7) FIG. 6 is a schematic diagram illustrating an embodiment of the setup after the test is completed.

(8) FIG. 7 shows the results from a model lung.

(9) FIG. 8 shows the control system of the system.

DESCRIPTION OF PREFERRED EMBODIMENTS

(10) FIG. 1 (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 flow past 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 flow past assembly 107 via a gas line. Flowmeter connection(s) 109 and a gas sample line 110 are also part of the setup.

(11) 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 flow past 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 flow past 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.

(12) A typical test consists of a period where the test subject inspires from the flow past 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.

(13) FIG. 2 (prior art) outlines a curve from a conventional multibreath wash-in/wash-out test from where LCI can be determined. The insoluble gas, SF.sub.6, has become the gas of choice for measurement of LCI. The concentration of SF.sub.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.sub.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.sub.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:

(14) $\mathrm{FRC}=\frac{\mathrm{Net}\mathrm{volume}\mathrm{of}\mathrm{inert}\mathrm{gas}\mathrm{exhaled}}{C_{\mathrm{ET},\mathrm{start}}-C_{\mathrm{ET},\mathrm{end}}}$

(15) 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.

(16) FIG. 3 is a schematic diagram illustrating one embodiment of the setup for determination of FRC and ventilation distribution (LCI) as used in conjunction with the disclosed invention. The setup in FIG. 3 shows the setup before the test is started. A breathing (inhalation) bag 305 is connected to the valve assembly 303 and is evacuated and pre-filled via a filling/evacuation line 307 with a gas mixture comprising the inert tracer gas. The control system is connected to the one way valve assembly via a gas sample line 309. The control system comprises a measuring apparatus comprising processing means and at least one gas analyser.

(17) FIG. 4 is shows an embodiment of the setup during wash-in. A test subject 301 having a facemask on or having the nose occluded with a nose clip breathes through the facemask or a mouthpiece, a bacterial filter and one port of the breathing valve assembly 303. The valve assembly 303 is a one way valve assembly comprising pneumatic valves that can open and close. A breathing (inhalation) bag pre-filled with inert tracer gas 305 is connected to the valve assembly and evacuated. The test subject 301 inhales the gas mixture comprising the inert tracer gas from the breathing bag 305 and exhales to the surroundings (see the arrows in the figure). The entry to the confined space (collection bag or washout bag) 313 via a filling/evacuation line 311 is closed by a valve during wash-in. During the wash-in the inert tracer gas concentration is measured to determine when the wash-in is complete (when the concentration is constant—below a predetermined threshold value regarding the fluctuation of the concentration). The pressure in the airways is also determined to determine the breathing phase of the test subject. The pressure is measured close to the mouth of the test subject (close to the end of the arrow of 303 in FIG. 4). The reason why the breathing phase of the test subject is important to know is that when V.sub.CE is calculated, dead space of the instruments are accounted for, which is why one need to know how many breaths (numbers) the test subject is having during the washout. The measure of the pressure in the airways to determine the breathing phase of the test subject is also important in order to be able to shift the valves in the assembly at the correct times during the test.

(18) To perform the wash-in part of the test the valve assembly 303 is switched (e.g. automatically) to allow the test subject 301 to inspire from the bag 305 and exhale to the surroundings for a certain amount of time until the valve assembly 303 is switched to the next setup. The test subject 301 preferably uses a face mask instead of nose clip and mouthpiece. A typical test consists of a period where the test subject is inhaling the gas mixture from the breathing bag 305 and exhales to the surroundings (see the arrows in the figure) as explained above followed by a period where the test subject is inhaling fresh air from the surroundings and exhaling to the confined space (collection or washout bag) (see FIG. 5). 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.

(19) The control system 817 (shown on FIG. 8) of the system comprises at least one gas analyser 801, a pressure monitoring unit 819 (unless the valve assembly is manually driven), a valve control unit 803 (unless the valve assembly is manually driven) and also a gas control unit 805 unless the inhalation bag 305 filled with the inert tracer gas is prepared manually. A control unit 807 can also be included, comprising a computing/processing unit (CPU) 809 with control interfaces, one or more program and data storage devices 811 and user interfaces for example comprising a display 813 and a keyboard 815, touch screen or similar input device. A data input/output module may also be included.

(20) FIG. 5 is a schematic diagram illustrating an embodiment of the washout phase where the paediatric subject breathes in from room air (see arrows in FIG. 5) and out into the confined space (collection bag) 313. The tracer gas (e.g. SF.sub.6) concentration is monitored at the end of each breath until the concentration has reached a predetermined concentration (e.g. 1/40) of the concentration in the beginning of the wash-out period and the expired gas is collected and measured to allow LCI calculation. The filling/evacuation line 307 is closed during washout by one of the valves in the valve assembly, whereas the filling/evacuation line 311 is open.

(21) Since the test setup according to the invention does not need a flowmeter to determine the flow, it solves the issue of too large a dead space in the conventional MBW setup when it comes to testing paediatric subjects especially newborns and infants where the extra volume induced by the flowmeter will result in a very large dead space volume compared to the physiologic dead space.

(22) FIG. 6 is a schematic diagram illustrating an embodiment of the setup after the test is completed where the confined space (washout bag) is full of the inert tracer gas exhaled by the test subject. The control system is connected to the one way valve assembly via a gas sample line 315. The control system comprises a measuring apparatus comprising processing means and at least one gas analyser. The expired gas in the bag is measured to allow LCI calculation.

(23) 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:

(24) $\mathrm{LCI}=\frac{V_{\mathrm{CE}}}{\mathrm{FRC}}$

(25) The concentration of the inert tracer gas is monitored and when the concentration is constant (below a predetermined threshold value regarding the fluctuation of the concentration), the first time period called wash-in is over. Hereafter the wash-out period begins where the concentration of the inert tracer gas is monitored at the end of each breath until the concentration has reached a predetermined (e.g. 1/40) of the concentration in the beginning of the wash-out period. It is not necessary to continuous monitor the instantaneous concentration during the whole breathing cycle since the net volume of inert tracer gas exhaled is determined by measuring the concentration of the inert tracer gas in the collection bag after completed wash-out and measuring the total volume in the collection bag after completed wash-out.

(26) The net volume of inert tracer gas exhaled and the difference in end-tidal fractional concentration of the inert tracer gas at the start (in the below equation SF6.sub.init) and end (in the below equation SF6.sub.end) of the wash-out is used for accurate determination of the functional residual capacity (FRC) which is calculated according to the equation below:

(27) $\mathrm{FRC}=\frac{\mathrm{SF}{6}_{\exp }}{\mathrm{SF}{6}_{\mathrm{init}}-\mathrm{SF}{6}_{\mathrm{end}}}.Math.V_{\mathrm{bag}}\left(\mathrm{ATPS}\right)$

(28) Where V.sub.bag is calculated as below:
V.sub.bag=T.sub.empty.Math.Fill.sub.flow  (ATPS)
And where VCE=V.sub.bag (ATPS)

(29) Where T.sub.empty: Time to empty bag in seconds Fill.sub.flow: Calibrated filling flow (=emptying flow) in l/s SF6.sub.exp: Mean gas concentration of inert tracer gas in the collection bag SF6.sub.init: Gas concentration of inert tracer gas at the start of washout SF6.sub.end: End tidal gas concentration of inert tracer gas at the end of washout

(30) The multiple-breath wash-out (MBW) is used for determination of the cumulative expired volume (V.sub.CE) required to clear the inert tracer gas from the lungs. V.sub.CE is determined by measuring the total volume in the collection bag after completed wash-out.

(31) In the interest of brevity dead spaces on each side of the valve are not accounted for, but these can easily be incorporated.

(32) In the below, are shown the equations when dead space are accounted for.

(33) Formula for V.sub.bag:
V.sub.bag=T.sub.empty.Math.(Fill.sub.flow+0.120/60)+(T.sub.stop−T.sub.start).Math.0.5.Math.0.120/60  (ATPS)

(34) Where T.sub.empty: Time to empty bag in seconds Fill.sub.flow: Calibrated filling flow (=emptying flow) in l/s 0.120/60: The gas sample flow is assume to be 120 ml/min and is evacuating the bag at the same time as the evacuation pump. (T.sub.stop−T.sub.start): Washout time in seconds. During expiration in the washout period, the gas sample flow is reducing the amount of air going to the bag. It is assumed that T.sub.insp=T.sub.exp, so only during 0.5 of time the gas sample flow is added.

(35) Formula for FRC: When wash-in is completed all dead spaces and lung volume are equilibrated to SF6.sub.init:
SF6_volume(0)=SF6.sub.init.Math.(FRC+V.sub.Inst.DS,0+V.sub.Inst.DS,1+V.sub.Inst.DS,2)  (ATPS)

(36) V.sub.Inst DS,0 is the wash-in valve dead space which is in the order of 12 ml such as between 10 14 ml. V.sub.Inst DS,1 is the mouth piece dead space which is in the order of 4 ml (+bacterial dead space) such as in the order of 3 to 5 ml and V.sub.Inst DS,2 is the washout valve dead space which is in the order of 12 ml such as between 10 14 ml. When wash-out is completed the lung volume and dead space 1 and 2 are filled with 1/40 of the SF6.sub.init and the rest is collected in the bag:
SF6_volume(1)=V.sub.bag.Math.SF6.sub.exp+0.025.Math.SF6.sub.init.Math.(FRC+V.sub.Inst.DS,1+V.sub.Inst.DS,2)  (ATPS)

(37) The above 2 volumes of SF.sub.6 are equal, i.e.:
SF6.sub.init.Math.(FRC+V.sub.Inst.DS,0+V.sub.Inst.DS,1+V.sub.Inst.DS,2)=V.sub.bag.Math.SF6.sub.exp+0.025.Math.SF6.sub.init.Math.(FRC+V.sub.Inst.DS,1+V.sub.Inst.DS,2)  (ATPS)

(38) Rearranging them gives the formula for FRC:

(39) $\mathrm{FRC}=\frac{\mathrm{SF}{6}_{\exp }}{\mathrm{SF}{6}_{\mathrm{init}}-0.975}.Math.V_{\mathrm{bag}}-\frac{1}{0.975}.Math.V_{\mathrm{Inst}.\mathrm{DS},0}-V_{\mathrm{Inst}.\mathrm{DS},1}-V_{\mathrm{Inst}.\mathrm{DS},2}\left(\mathrm{ATPS}\right)$

(40) Formula for VCE: V.sub.bag is the total expired volume during the washout and in order to correct for instrument dead space the term N.sub.breath.Math.V.sub.InstDS,1 is subtracted.
VCE=V.sub.bag−N.sub.breath.Math.V.sub.Inst.DS,1  (ATPS)

(41) LCI is then determined as:
LCI=VCE/FRC

(42) The gas dilution technique according to the present invention 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 conventional 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.sub.6 in the rapid transitions during the beginning of expiration (phase II of the breath) and inspiration.

(43) FIG. 7 shows the results from a model lung. A prototype paediatric MBW system was assessed in vitro using a lung model based on a mechanically driven 100 ml syringe and ventilation tank. A 100 ml calibrated syringe is driven by a linear motor, the syringe movement distance (equivalent to tidal volume) and rate are set by a computer. The model was set to deliver FRC's of 100 ml-250 ml, with corresponding tidal volumes of 30-90 mls and respiratory rates of 20-60 min.sup.−1. Improvements to hardware and wash-in protocol were made following initial testing. This offers improved accuracy at small volumes and fast respiratory rates, compared to previously described lung models which have used a portable ventilator. The lung tank consists of two separate compartments of known volume, filled with water to a set level to determine FRC. The lung tank is then heated and humidified to BTPS conditions by placing in an external tank containing hot water. Temperature is monitored and kept constant throughout at 36.5-37.3 degrees. The system was initially tested in an unheated lung model. Overall error of FRC measurement was 0.04%, and only one of 21 repeat FRC measurements was outside the +/−5% error range (see the below table). Testing was repeated in the lung model heated to BTPS conditions. Over 36 tests, overall error of FRC measurement was 0.0006%. The results are illustrated in FIG. 7 which shows the percentage error from the overall mean of each measurement at FRCs of 100 ml, 200 ml and 250 ml at BTPS conditions. The dotted lines indicate the 95% accuracy target range.

(44) TABLE-US-00001 95% CI Model FRC (ml) Measured FRC measured FRC Mean (SD) % (inc deadspace) (ml) (ml) error 102 99.2  97.0-101.5 −1.76 (1.8)  202 203.3 199.4-207.3 1.17 (2.34) 252 250.6 247.4-253.8 −0.18 (1.63) 

(45) 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 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.