METHOD FOR DETERMINING THE FUNCTIONAL RESIDUAL CAPACITY OF A PATIENT'S LUNG AND VENTILATOR FOR CARRYING OUT THE METHOD

Abstract

A method for determining the functional residual capacity of a patient's lung, includes supplying a first inspiratory breathing gas having a first proportion of a metabolically inert gas, supplying a second inspiratory breathing gas having a second proportion of the metabolically inert gas, determining any arising volume difference, which represents a difference in volume between a volume of inspiratory and of expiratory metabolically inert gas for a determination period, determining the functional residual capacity taking into account the volume difference and a proportion difference between a first proportion quantity and a second proportion quantity, which represent the first proportion and the second proportion of the metabolically inert gas, respectively, and determining a base difference, which represents a difference between a tidal volume of inspiratory metabolically inert gas and of expiratory metabolically inert gas.

Claims

1. A method for ascertaining a functional residual capacity of a lung of a patient, comprising the following steps: supplying a first inspiratory respiratory gas having a first proportion of a metabolically inert gas during a first temporal supply phase, following the first supply phase: supplying a second inspiratory respiratory gas, differing from the first and having a second proportion of the metabolically inert gas differing from the first, during a second temporal supply phase, ascertaining a difference in amount occurring during the second supply phase, which represents a difference amount for an ascertainment period between an amount of inspiratory metabolically inert gas and an amount of expiratory metabolically inert gas, the ascertainment period not ending after the second supply phase, ascertaining the functional residual capacity by taking into account the difference in amount and a difference in proportion between a first proportion quantity, which represents the first proportion of the metabolically inert gas in the first inspiratory working gas, and a second proportion quantity, which represents the second proportion of the metabolically inert gas in the second inspiratory working gas, and ascertaining a base difference, which represents a difference between a tidal amount of inspiratory metabolically inert gas and a tidal amount of expiratory metabolically inert gas in at least one of the first and the second supply phase; wherein the ascertainment of the functional residual capacity occurring on the basis of a corrected difference in amount and the difference in proportion, the corrected difference in amount being formed by taking into account the base difference when ascertaining the difference in amount.

2. The method as recited in claim 1, the base difference comprises at least one average value from a plurality of differences in tidal amounts between respectively a tidal amount of inspiratory metabolically inert gas and a tidal amount of expiratory metabolically inert gas for a plurality of breaths in at least one of the first and the second supply phase.

3. The method as recited in claim 2, wherein at least one of the base difference comprises an average value from a plurality of differences in tidal amounts between respectively a tidal amount of inspiratory metabolically inert gas and a tidal amount of expiratory metabolically inert gas for a plurality of breaths in a temporal start detection segment in the first supply phase, the start detection segment being closer to the start of the second supply phase than to the start of the first supply phase, and the base difference comprises an average value from a plurality of differences in tidal amounts between respectively a tidal amount of inspiratory metabolically inert gas and a tidal amount of expiratory metabolically inert gas for a plurality of breaths in a temporal end detection segment in the second supply phase, the end detection segment being closer to the end of the second supply phase than to its start.

4. The method as recited in claim 1, wherein at least one of the base difference comprises at least during a segment in the second supply phase and in the detection period a tidal base difference, which is determined for a breath depending on a proportion of the metabolically inert gas in the respiratory gas of the respective breath.

5. The method as recited in claim 1, wherein the corrected difference in amount corresponds to a sum of corrected differences in tidal amounts over a number of breaths in the ascertainment period, a corrected difference in the tidal amounts being formed for every breath from a difference of a difference in the tidal amounts of this breath and a base difference associated with the breath, the difference in the tidal amounts being formed for every breath by the difference between a tidal amount of inspiratory metabolically inert gas and a tidal amount of inspiratory metabolically inert gas of this breath.

6. The method as recited in claim 1, wherein at least one of the first proportion quantity comprises or is an average value, formed over a plurality of breaths in the first supply phase, of the first proportion of the metabolically inert gas in the first inspiratory or expiratory working gas, and the second proportion quantity comprises or is an average value, formed over a plurality of breaths in the second supply phase, of the second proportion of the metabolically inert gas in the second inspiratory or expiratory working gas.

7. The method as recited in claim 6, wherein at least one of the plurality of breaths, over which the first proportion quantity is ascertained as an average value, is closer to the start of at least one of the ascertainment period and of the second supply phase than to the start of the first supply phase, and the plurality of breaths, over which the second proportion quantity is ascertained as an average value, is closer to at least one of the end of the ascertainment period and of the second supply phase than to the start of the ascertainment period or the second supply phase.

8. The method as recited in claim 1, wherein the ascertainment of the functional residual capacity occurs on the basis of a quotient of the corrected difference in amount and the difference in proportion.

9. The method as recited in claim 1, carried out at a ventilator, wherein at the end of a plurality of breaths during the second supply phase at the end of an expiration phase, a respiratory pressure in at least one the airway of of the patient and in a proximal area of a ventilation line is the PEEP.

10. The method as recited in claim 1, further comprising a sensorial detection both of an inspiratory respiratory gas flow as well as of an expiratory respiratory gas flow.

11. The method as recited in claim 1, carried out by a ventilator during an artificial respiration of a patient.

12. A ventilator, which is designed both for the at least partial artificial respiration of living patients as well as for carrying out the method as recited in one of the preceding claims, the ventilator comprising: a first respiratory gas source, which provides a first inspiratory respiratory gas component having a first fraction of a metabolically inert gas, a second respiratory gas source, which provides a second inspiratory respiratory gas component having a second fraction of the metabolically inert gas differing from the first fraction, a variably settable mixing device for forming an inspiratory respiratory gas having a variable proportion of metabolically inert gas from at least one of the first and the second inspiratory respiratory gas component, a ventilation line system for conveying the inspiratory respiratory gas to a patient-side respiratory gas outlet and for conveying expiratory respiratory gas away from a patient-side respiratory gas inlet, a control valve system, comprising an inspiration valve and an expiration valve, a pressure changing device for changing at least the inspiratory respiratory gas in the ventilation line system, a flow sensor system for detecting at least the inspiratory respiratory gas flow, a gas component sensor system for the indirect or direct detection of the proportion of the metabolically inert gas in the inspiratory and in the expiratory respiratory gas, a control device, which is designed to control the control valve system and the pressure changing device and which is connected in signal-transmitting fashion to the flow sensor system and to the gas component sensor system for transmitting respective detection signals to the control device.

13. The ventilator as recited in claim 12, wherein the gas component sensor system comprises at least one of the following sensors: an oxygen sensor for detecting an oxygen content in the inspiratory and in the expiratory respiratory gas, and a carbon dioxide sensor for detecting a carbon dioxide content in the inspiratory and in the expiratory respiratory gas.

14. The ventilator as recited in claim 12, wherein the gas component sensor system is situated in a main flow section of the ventilation line system for detecting the proportion of the metabolically inert gas in the inspiratory and in the expiratory respiratory gas, through which both the inspiratory respiratory gas fed to the patient as well as the expiratory respiratory gas flowing away from the patient flow.

15. The ventilator as recited in claim 14, wherein the main flow section conducts at least 95 vol % of the inspiratory and of the expiratory respiratory gas.

16. The ventilator as recited in claim 12, wherein the control device is designed for controlling the mixing device so as to change the proportion of metabolically inert gas in the inspiratory respiratory gas by controlling the mixing device.

17. The ventilator as recited in claim 13, wherein the gas component sensor system is situated in a main flow section of the ventilation line system for detecting the proportion of the metabolically inert gas in the inspiratory and in the expiratory respiratory gas, through which both the inspiratory respiratory gas fed to the patient as well as the expiratory respiratory gas flowing away from the patient flow.

18. The ventilator as recited in claim 17, wherein the main flow section conducts at least 95 vol % of the inspiratory and of the expiratory respiratory gas.

19. The ventilator as recited in claim 13, wherein the control device is designed for controlling the mixing device so as to change the proportion of metabolically inert gas in the inspiratory respiratory gas by controlling the mixing device.

20. The ventilator as recited in claim 14, wherein the control device is designed for controlling the mixing device so as to change the proportion of metabolically inert gas in the inspiratory respiratory gas by controlling the mixing device.

Description

[0126] The present invention is explained in greater detail below with reference to the attached drawings. The figures show:

[0127] FIG. 1 a rough schematic view of a ventilator according to the invention,

[0128] FIG. 2 a graphic representation of the characteristic curve of a difference in tidal amounts, of a proportion of nitrogen in the inspiratory respiratory gas and of a difference in base amount during a second supply phase, including a representation of the end of a preceding first supply phase and a subsequent third supply phase, and

[0129] FIG. 3 a representation of respiratorily relevant breath or lung volumes.

[0130] In FIG. 1, a specific embodiment of a ventilator according to the invention is generally denoted by reference numeral 10. Ventilator 10 comprises a first respiratory gas source 12 in the form of a suction port opening toward surroundings U of ventilator 10. An output-variable blower 13, which is controllable by a control device 14, allows for ambient air to be aspirated as the first respiratory gas component A1. Blower 13 and control device 14 are accommodated in the same housing 16. This housing also accommodates valves known per se, such as an inspiration valve 19in and an expiration valve 19ex. Control device 14 additionally comprises a time measuring device 19a.

[0131] Furthermore, a second respiratory gas source 15 is connected to housing 16 in a flow-connecting manner. The second respiratory gas source 15 may be a pressurized gas cylinder, for instance with pressurized pure oxygen stored therein as a second respiratory gas component A2.

[0132] The first respiratory gas component A1 aspirated at the first respiratory gas source 12 and the second respiratory gas component A2 supplied by the second respiratory gas source 15 are conducted to a mixing valve 17, which mixes, as a function of its position, preferably steplessly, the two respiratory gas components into an inspiratory respiratory gas having an arbitrary mixture ratio from 100 vol % of the first respiratory gas component A1 and 0 vol % of the second respiratory gas component A2 to 0 vol % of the first respiratory gas component A1 and 100 vol % of the second respiratory gas component A2. The mixing valve 17 and thus the mixture ratio of the inspiratory respiratory gas is likewise controllable or adjustable by control device 14.

[0133] In the illustrated example, N.sub.2 is used as the metabolically inert gas. Since the first respiratory gas component A1 has an N.sub.2 fraction of approximately 71 vol % and the second respiratory gas component has an N.sub.2 fraction of approximately 0 vol %, the inspiratory respiratory gas mixed by mixing valve 17 may have an N.sub.2 proportion of between 0 and 71 vol %. Such an inspiratory respiratory gas is breathable by any land-dwelling creature of this planet that would be a candidate for artificial respiration. The change of the mixture ratio of the respiratory gas components may preferably be switched temporally within one breath, particularly preferably with an expiration from a first, earlier mixture ratio to a second, later mixture ratio.

[0134] The control device 14 of ventilator 10 has an input/output device 18, which comprises numerous switches such as push-button switches and rotary switches, in order to be able, if necessary, to input data into control device 14. Blower 13 of first respiratory gas source 12 may be changed in its conveying capacity by the control device, in order to change the amount of respiratory gas that is conveyed per unit of time. Blower 13 is therefore in the present exemplary embodiment a pressure changing device 13a of ventilator 10.

[0135] A ventilation line system 20, comprising five flexible hoses in the present example, is connected to the line leading away from blower 13 and toward patient P with the inspiration valve 19in situated in between. A first inspiratory ventilation hose 22 runs from a filter 24 situated between it and inspiration valve 19in to an optional conditioning device 26, where the respiratory gas supplied by the respiratory gas source 12 is humidified to a specified degree of humidity and, if indicated, is provided with aerosol medications. Filter 24 filters and cleans the ambient air supplied by blower 13.

[0136] A second inspiratory ventilation hose 28 leads from the optional conditioning device 26 to an inspiratory water trap 30. A third inspiratory ventilation hose 32 leads from water trap 30 to a Y connector 34, which connects the distal inspiration line 36 and the distal expiration line 38 to a combined proximal inspiratory-expiratory ventilation line 40.

[0137] A first expiratory ventilation hose 42 runs from the Y connector 34 back to housing 16 to an expiratory water trap 44 and from there a second expiratory ventilation hose 46 runs to housing 16, where the expiratory respiratory gas is discharged into the surroundings via expiration valve 19ex.

[0138] On the combined inspiratory-expiratory side of Y connector 34 near the patient, the Y connector 34 is directly followed by a flow sensor 48, here: a differential pressure flow sensor 48, which detects the inspiratory and the expiratory flows of respiratory gas toward patient P and away from patient P. A line system 50 transmits the gas pressure prevailing on both sides of a variable flow obstruction, known per se, in flow sensor 48 to control device 14, which calculates from the transmitted gas pressures and in particular from the difference of the gas pressures the amount of inspiratory and expiratory respiratory gas flowing per unit of time.

[0139] In the direction away from Y connector 34 and toward patient P, flow sensor 48 is followed by a measuring cuvette 52 both for the non-dispersive infrared detection of a predetermined volumic gas proportion in the respiratory gas, here carbon dioxide (CO.sub.2) by way of example, as well as for the luminescence-based detection of the volumic gas proportion of oxygen (O.sub.2). The CO.sub.2 proportions and the O.sub.2 proportions are of interest both in the inspiratory respiratory gas as well as in the expiratory respiratory gas, since the change of the CO.sub.2 proportion and of the O.sub.2 proportion between the inspiration and the expiration is a measure of the metabolic ability of the patient's lung. FIG. 1 shows one of the lateral windows 53, through which infrared light may be radiated into the measuring cuvette 52 or may radiate out of the latter, depending on the orientation of a combined CO.sub.2—O.sub.2 gas sensor 54 that is releasably coupled to the measuring cuvette.

[0140] Gas sensor 54 may be coupled to measuring cuvette 52 in such a way that the gas sensor 54 is able both to radiate infrared light through the measuring cuvette 52 as well as to excite a luminophore-containing measuring surface of the measuring cuvette 52 to radiate.

[0141] From the intensity of the infrared light, more precisely from its spectral intensity, it is possible to infer, in a manner known per se, the amount or the proportion of a predetermined gas in the respiratory gas flowing through the measuring cuvette 52. The predetermined gas, here: CO.sub.2, absorbs infrared light of a defined wavelength. The intensity of the infrared light of this wavelength following the passage depends essentially on the absorption of the infrared light of this wave length by the predetermined gas. A comparison of the intensity of the infrared light of the defined wavelength with a wavelength of the infrared light, which does not belong to an absorption spectrum of an expected gas proportion in the respiratory gas, provides information about the proportion of the predetermined gas in the respiratory gas.

[0142] From the radiation response of the luminophore-containing measuring surface of measuring cuvette 52 to the above-described excitation by gas sensor 54, which is detected by gas sensor 54, it is possible to ascertain the volumic O.sub.2 proportion in the respiratory gas by taking into account an intensity difference and/or a phase difference between the preferably modulated excitation radiation and the excited radiation. 02 acts as quencher substance for the luminophore of the measuring surface and decisively influences the response radiation with respect to intensity and/or phase shift.

[0143] Gas sensor 54 is therefore connected to the control device 14 of ventilator 10 via a data line 56 and transmits the described intensity information via the data line 56 to control device 14.

[0144] In the direction toward patient P, measuring cuvette 52 is followed by a further hose section 58, on which an endotrachealtubus 60 is attached as the respiratory interface to patient P. A proximal opening 62 of endotrachealtubus 60 is both a respiratory gas outlet opening, through which inspiratory respiratory gas is fed through endotrachealtubus 60 into patient P, as well as a respiratory gas inlet opening, through which expiratory respiratory gas is conducted out of the patient and back into endotrachealtubus 60.

[0145] The entire ventilation line system is a main flow line, without branching of a bypass flow line. The proximal single strand section of the Y connector 34, flow sensor 48, measuring cuvette 52 and hose section 58 form a main flow section 64 situated outside of the body of patient P, through which both inspiratory as well as expiratory respiratory gases flow.

[0146] Control device 14 is designed to control blower 13 and mixing device 17 according to the method described at the outset, in order to ascertain from the detection values, which are detected by gas sensor 54 and flow sensor 48, a functional residual capacity FRC of the lung of patient P.

[0147] For this purpose, initially, in a first supply phase 70, a first inspiratory respiratory gas is fed to patient P, which is formed from a mixture of the two respiratory gas components A1 and A2, so that the inspiratory respiratory gas has a higher oxygen content than first respiratory gas component A1, that is, the ambient air, by itself.

[0148] The end of this first supply phase 70 is indicated in the diagram of FIG. 2 by an arrow.

[0149] FIG. 2 shows three diagrams and has two scales for this purpose. The left ordinate scale in FIG. 2 refers to differences in tidal amounts in milliliters and applies to the difference in tidal amounts .sup.iZpΔv(n).sub.N.sub.2.sup.tid, abbreviated in FIG. 2 as Δv(x), represented by a solid line and labeled by reference numeral 72, as it is determined for each breath by equation 1 from the values detected by gas sensor 54. It also applies to the tidal base difference .sup.iZpB.sup.tid, abbreviated in FIG. 2 as B(x), represented by a dotted line and labeled by reference numeral 74, as it is ascertained in accordance with equation 6.

[0150] The right ordinate scale in FIG. 2 refers to the proportion .sub.in.sup.iZpA.sub.O.sub.2, abbreviated in FIG. 2 as A, of oxygen in the inspiratory respiratory gas in percentage by volume. The graph of the oxygen proportion in the inspiratory respiratory gas is shown in FIG. 2 by a dashed line and labeled by reference numeral 76.

[0151] The abscissa of the representation of FIG. 2 indicates the breaths x. The abscissa has two scales. one of which starts at zero and increments by the value 1 for each breath in the considered period. The other scale increments likewise by the value 1, but starts to count in each supply phase anew with the value 1.

[0152] Since in the first supply phase 70, due to the greater admixture of the second respiratory gas component A2, this first inspiratory respiratory gas contains a smaller amount or a smaller proportional amount of nitrogen as the metabolically inert gas than the second inspiratory respiratory gas, the first supply phase 70 corresponds to a wash-out phase described in the introduction of the specification.

[0153] In the process, the composition of the first inspiratory respiratory gas and of the first expiratory respiratory gas formed from it is detected tidally, that is, for every breath. From the flow information obtained from flow sensor 48 as respiratory gas volume flowing inspiratorily and expiratorily per unit of time, and from the volume proportions of oxygen and carbon dioxide both of the inspiratory respiratory gas as well as of the expiratory respiratory gas obtained from gas sensor 54, it is possible to obtain for every breath both the inspiratorily administered amounts as well as the expiratorily discharged amounts of oxygen, carbon dioxide and nitrogen on the simplifying, but sufficiently accurate assumption that the inspiratory and the expiratory respiratory gas contains no further components of a significant amount beyond oxygen, carbon dioxide and nitrogen.

[0154] Thus, it is possible to ascertain the differences in tidal amounts according to equation 1 directly from the detection results available to control device 14. Together with the differences in tidal amounts, it is also possible for control device 14 to ascertain their moving arithmetic average according to equation 2. In the same way, the average proportion of nitrogen in the first inspiratory respiratory gas is ascertained in accordance with equation 8 or 8a. The ascertained values are stored in a data storage unit of control device 14.

[0155] If the moving average of the differential value according to equation 2 for the first supply phase 70 lies below a predetermined threshold value according to amount or is equal to the same, then control device 14 ends the first supply phase by adjusting the mixing valve as mixing device 17 and feeds a second inspiratory respiratory gas to patient P, whose nitrogen component is changed with respect to the first inspiratory respiratory gas, that is, increased in the present example. The second supply phase 78 thus starts, as may be seen in FIG. 2 by the left value 1 in the lower abscissa scale. The second supply phase 78 continues for about 130 breaths.

[0156] The amount of respiratory gas component A2 that is admixed to respiratory gas component A1 is lower in the second supply phase 78 than in the first supply phase 70. Due to the adjustment of mixing valve 17, the oxygen proportion in the respiratory gas falls abruptly from approximately 57 vol % to approximately 38 vol %. The oxygen proportion, however, is still higher than in pure ambient air.

[0157] The ascertainment period over which the FRC is ascertained begins with the second supply phase 78. It is not necessary for the FRC to be ascertained in real time during the second supply phase, but rather it is only necessary that the differences in tidal amounts used for ascertaining the FRC originate from the ascertainment period.

[0158] From the start of the second supply phase, a difference in tidal amounts is therefore calculated for every breath in accordance with equation 1. If a sufficient number of breaths for averaging have already occurred, the moving average of the differences in tidal amounts is also formed in accordance with equation 2. Again, if the moving differential value according to equation 2 falls to or below a threshold value predetermined for this purpose, the ascertainment period ends.

[0159] The characteristic curve of the difference in tidal amounts over the observed period is indicated by graph 72. Since, due to dead volumes in the lung of the patient, there is still respiratory gas of the first supply phase with a lower nitrogen proportion in the patient's lung, patient P initially exhales second expiratory respiratory gas beginning with the start of the second supply phase 78, which has a higher volumic proportion of nitrogen than the second inspiratory respiratory gas. The difference in tidal amounts for the breaths at the start of the second supply phase 78 is therefore positive and deviates significantly, on the one hand, from the value 0, which is reached when the expiratory and the inspiratory respiratory gas have a nitrogen proportion of identical magnitude. The difference in tidal amounts 72, on the other hand, also deviates significantly from base difference 74 toward the end of first supply phase 70. So that nitrogen is washed out of the patient's lung in the second supply phase 78, the difference in tidal amounts 72 falls with increasing distance from the start of the second supply phase 78, until it levels off around a constant offset value starting approximately with the 50th breath of the second supply phase 78. Starting approximately with this 50th breath of the second supply phase 78, the difference in tidal amounts 72 no longer changes substantially, but is henceforth essentially only influenced by interference effects of ventilator 10, such as leakages and the like.

[0160] As already mentioned above, the tidal base difference 74 is ascertained in accordance with the above equation 6. Starting from the value of the tidal base difference 74 toward the end of the first supply phase 70, it initially rises sharply, then ever more slightly, until it essentially converges to the offset value of the difference in tidal amounts 72.

[0161] Following the end of the ascertainment period, the proportion of nitrogen in the second inspiratory respiratory gas is ascertained in accordance with equation 8 or 8a. When working with equation 8a, the constant p should be chosen to be greater than 100 and the constant q should be chosen to be no greater than 50, so that the detection values used for the application of equation 8a originate from the range, for example the end detection range 81 in the second supply phase 78, in which the difference in tidal amounts 72 has an essentially constant value or a value that oscillates around a constant value.

[0162] The FRC is then preferably calculated from equation 11b, in order to obtain the FRC with high accuracy. Alternatively, however, equation 11a could be used as well.

[0163] Since the base difference is ascertainable at the end of the first supply phase 70, before the second supply phase 78 begins, and since the nitrogen proportion in the inspiratory respiratory gas of the second supply phase 78 is known at least as a setpoint value, by using the setpoint value for the nitrogen proportion of the second supply phase 78, it is possible to ascertain the FRC even in real time during an artificial respiration of patient P. For then all data required for calculating equation 11b are known at the time of every breath of the second supply phase.

[0164] Although the entire second supply phase 78 may be used for ascertaining the FRC, a shorter ascertainment period 79 suffices. Preferably, it suffices if the ascertainment period 79 begins together with the second supply phase 78 and if it ends in the range in which the difference in tidal amounts 72 and the tidal base difference do not differ according to amount by more than a predetermined small threshold value sw.

[0165] FIG. 2 shows the start of a third supply phase 80, with which again first respiratory gas having a higher oxygen proportion is fed to patient P. Consequently, nitrogen still present due to the dead spaces of the lung is washed out of the patient's lung, so that the expiratory respiratory gas has a higher proportion of nitrogen than the inspiratory respiratory gas of the same breath. The difference in tidal amounts is thus negative and diminishes in with increasing distance from the start of the third supply phase 80. Since in the first and third supply phases 70 and 80, respectively, the same first inspiratory respiratory gas is used, the same conditions set in with the continuance of the third supply phase 80 as toward the end of the first supply phase 70.

[0166] Alternatively, the FRC may also be calculated in accordance with equation 11a, instead of equation 11b, the base difference B being calculated for this purpose as the average value of the difference in tidal amounts Δv(x) over an end detection segment 81 in second supply phase 78 or, although less preferred due to the lower achievable accuracy of the FRC ascertainment, over a start detection segment 82 in first supply phase 70. The end detection segment 81 is closer to the end of the detection period 79 than to its start or to the simultaneous start of second supply phase 78. The start detection segment 82 is closer to the start of the second supply phase 78 than to the start of first supply phase 70.

[0167] FIG. 3, which originates from the source “Vihsadas” in en.wikipedia, explains the various partial volumes of a patient's lung in order to clarify what precisely is meant by a functional residual capacity according to the present invention.

[0168] In FIG. 3 on the left, a spirometric curve of a respiratory gas volume in a patient's lung is plotted over multiple breaths.

[0169] A total lung capacity TLC is the volume that is theoretically feedable into a lung starting from a completely collapsed lung of a patient to the maximally possible inhalation. This value is purely theoretical, since a completely collapsed lung would be lethal for patient P.

[0170] In a functioning lung of a patient, a residual volume RV therefore always remains, which the patient P is not able to drive out of his lung even with the greatest effort. The vital capacity VC of the lung of a patient is the volume, which the patient P is able to supply to his lung or remove from his lung between a state maximally exhaled with maximum effort and a state maximally inhaled with maximum effort.

[0171] In normal breathing, occurring essentially without effort, the tidal volume TV is fed to the lung of the patient P and is again removed from the latter. If patient P, starting from an effortlessly exhaled state, inhales maximally, then he supplies to his lung the so-called inspiration capacity IC of respiratory gas. If, starting from an effortlessly inhaled state, he inhales maximally by mobilizing his entire inspiratory force, then he fills his lung additionally with the inspiratory reserve volume, the IRV. If, starting from an effortlessly exhaled state, he exhales maximally by mobilizing his entire expiratory force, patient P thereby exhales the expiratory reserve volume ERV of his lung.

[0172] The sum of the residual volume RV and the vital capacity VC corresponds to the total lung capacity TLC just as the sum of the residual volume RV, the expiratory reserve volume ERV and the inspiratory capacity IC.

[0173] The difference between the total lung capacity TLC and the inspiratory capacity IC is the functional residual capacity of the lung of the patient. The latter also results from the sum of the residual volume and the expiratory reserve volume ERV. The functional residual capacity FRC is equally the total lung capacity TLC minus the tidal volume and further minus the inspiratory reserve volume IRV.