VENTILATION DEVICE DESIGNED TO IDENTIFY FUNCTIONAL IMPAIRMENT OF ITS O2 SENSOR ASSEMBLY

Abstract

A ventilation device for artificial ventilation, having: —a ventilation gas source; —a ventilation conducting assembly for conducting inspiratory ventilation gas from the ventilation gas source to a patient-side, proximal ventilation-gas outlet opening and for conducting expiratory ventilation gas away from a proximal ventilation-gas inlet opening; —a pressure-changing assembly for changing the pressure of the ventilation gas flowing in the ventilation conducting assembly; —a control device, which is designed to control the operation of the ventilation gas source and/or the operation of the pressure-changing assembly; —an evaluation device for processing sensor signals; and —an O2 sensor assembly for determining an O2 concentration value representing the oxygen concentration of the ventilation gas flowing in the ventilation conducting assembly, wherein the O2 sensor assembly outputs O2 sensor signals, which contain information regarding the O2 concentration value, to the evaluation device, and wherein the evaluation device is designed to determine, on the basis of the O2 sensor signals, an O2 change value representing a change in the O2 concentration value and, if the O2 change value satisfies a predefined condition, to infer degradation of the O2 sensor assembly and to output a signal.

Claims

1-21. (canceled)

22. A ventilation device for artificial respiration comprising: a respiratory gas source, a ventilation line arrangement in order to lead inspiratory respiratory gas from the respiratory gas source to a patient-side proximal respiratory gas outlet aperture and in order to lead expiratory respiratory gas away from a proximal respiratory gas inlet aperture, a pressure modification arrangement for modifying the pressure of the respiratory gas flowing in the ventilation line arrangement, a control device designed to control the operation of the respiratory gas source and/or of the pressure modification arrangement, an evaluation device for processing sensor signals, and an O.sub.2 sensor assembly for ascertaining an O.sub.2 content value which represents an oxygen content of the respiratory gas flowing in the ventilation line arrangement, where the O.sub.2 sensor assembly outputs to the evaluation device O.sub.2 sensor signals which contain information about the O.sub.2 content value, where the evaluation device is designed to determine on the basis of the O.sub.2 sensor signals an O.sub.2 change value which represents a change in the O.sub.2 content value and when the O.sub.2 change value satisfies a predetermined condition to deduce a degradation of the O.sub.2 sensor assembly and output a signal.

23. The ventilation device according to claim 22, wherein the evaluation device is designed to output the signal only if the predetermined condition is satisfied a predetermined plurality of times within a predetermined plurality of breaths.

24. The ventilation device according to claim 22, wherein the evaluation device is designed to ascertain, as the O.sub.2 change value, an O.sub.2 difference value which represents a quantitative difference between a characteristic expiratory O.sub.2 content value of the expiratory respiratory gas and a characteristic inspiratory O.sub.2 content value of the inspiratory respiratory gas, where the predetermined condition is that the O.sub.2 difference value is related in a predetermined relative relationship to an O.sub.2 difference limit.

25. The ventilation device according to claim 24, wherein the evaluation device is designed to take into account, when performing the comparisons of O.sub.2 difference values with the O.sub.2 difference limit, at least one atmospheric state value which represents a state of the ambient atmosphere of the ventilation device.

26. The ventilation device according to claim 25, wherein the evaluation device is either designed to ascertain the O.sub.2 difference limit as a function of the at least one atmospheric state value, or is designed to ascertain from the O.sub.2 difference value and the at least one atmospheric state value an atmosphere-based O.sub.2 difference value.

27. The ventilation device according to claim 25, wherein the at least one atmospheric state value represents the atmospheric pressure, that the O.sub.2 difference value represents an O.sub.2 partial pressure difference value between an O.sub.2 partial pressure in the expiratory respiratory gas and an O.sub.2 partial pressure in the inspiratory respiratory gas.

28. The ventilation device according to claim 24, wherein the characteristic inspiratory O.sub.2 content value derives from an inspiratory phase segment which is located at a first temporal distance from the beginning of an inspiratory phase and a second temporal distance from the end of the inspiratory phase, and/or that the characteristic expiratory O.sub.2 content value derives from an expiratory phase segment which is located at a third temporal distance from the beginning of an expiratory phase and at a fourth temporal distance from the end of the expiratory phase.

29. The ventilation device according to claim 28, wherein the evaluation device is designed to identify the inspiratory phase segment and/or the expiratory phase segment and to ascertain therein a characteristic inspiratory O.sub.2 content value and/or a characteristic expiratory O.sub.2 content value, respectively.

30. The ventilation device according to claim 22, wherein the ventilation device additionally exhibits a CO.sub.2 sensor assembly which is designed to ascertain a CO.sub.2 content value representing a carbon dioxide content of the respiratory gas flowing in the ventilation line arrangement, where the CO.sub.2 sensor assembly outputs to the evaluation device CO.sub.2 sensor signals which contain information about the CO.sub.2 content value.

31. The ventilation device according to claim 30, wherein the evaluation device is designed to ascertain from the CO.sub.2 sensor signals a CO.sub.2 gradient value representing a temporal change in the CO.sub.2 content value in the expiratory respiratory gas, and that the evaluation device is further designed to ascertain, as the O.sub.2 change value, an O.sub.2 gradient value representing a temporal change in the O.sub.2 content value in the inspiratory respiratory gas, where the predetermined condition is that a ratio of the CO.sub.2 gradient value to the O.sub.2 gradient value is related in a predetermined relative relationship to a change ratio limit.

32. The ventilation device according to claim 30, wherein the evaluation device is designed to ascertain, on the basis of a plurality of CO.sub.2 sensor signals of a breath, the quantitatively largest temporal change in the CO.sub.2 content value towards decreasing values as the CO.sub.2 gradient value.

33. The ventilation device according to claim 32, wherein the evaluation device is designed to use the point in time of the occurrence of the CO.sub.2 gradient value as the reference point in time and on the basis of a plurality of O.sub.2 sensor signals of a breath to use a change value of the O.sub.2 content value as the O.sub.2 gradient value which occurs at a predetermined temporal distance from the CO.sub.2 gradient value.

34. The ventilation device according to claim 33, wherein the predetermined temporal distance is chosen in such a way that the O.sub.2 gradient value lies in a segment of a changeover from an expiratory phase to an inspiratory phase with the oxygen contents increasing with time.

35. The ventilation device according to claim 34, wherein the CO.sub.2 gradient value lies in a segment of a changeover from an expiratory phase to an inspiratory phase, where the predetermined temporal distance lies in a range from 25 to 80 ms, preferably in a range from 35 to 65 ms, especially preferably in a range from 45 to 55 ms, such that the O.sub.2 gradient value preferably lies in an inspiratory process which immediately follows the expiratory process in which the CO.sub.2 gradient value lies.

36. The ventilation device according to claim 29, wherein the ventilation device additionally exhibits a CO.sub.2 sensor assembly which is designed to ascertain a CO.sub.2 content value representing a carbon dioxide content of the respiratory gas flowing in the ventilation line arrangement, where the CO.sub.2 sensor assembly outputs to the evaluation device CO.sub.2 sensor signals which contain information about the CO.sub.2 content value and wherein the evaluation device is designed to ascertain, on the basis of a plurality of CO.sub.2 sensor signals of a breath, the quantitatively largest temporal change in the CO.sub.2 content value towards decreasing values, where the evaluation device is further designed to identify the inspiratory phase segment and/or the expiratory phase segment as a function of the temporal determination of the occurrence of the quantitatively largest temporal change in the CO.sub.2 content value towards decreasing values.

37. The ventilation device according to claim 28, wherein the ventilation device additionally exhibits a CO.sub.2 sensor assembly which is designed to ascertain a CO.sub.2 content value representing a carbon dioxide content of the respiratory gas flowing in the ventilation line arrangement, where the CO.sub.2 sensor assembly outputs to the evaluation device CO.sub.2 sensor signals which contain information about the CO.sub.2 content value and wherein the evaluation device forms the characteristic inspiratory O.sub.2 content value as an average over a plurality of O.sub.2 sensor signals in the inspiratory phase segment and/or that the evaluation device forms the characteristic expiratory O.sub.2 content value as an average over a plurality of O.sub.2 sensor signals in the expiratory phase segment.

38. The ventilation device according to claim 32, wherein the evaluation device is designed to identify an examination time interval in a phase of a changeover from an expiratory process to an inspiratory process and to search only within the examination time interval for an occurrence of the quantitatively largest temporal change in the CO.sub.2 content value towards decreasing values.

39. The ventilation device according to claim 38, wherein the ventilation device additionally exhibits a CO.sub.2 sensor assembly which is designed to ascertain a CO.sub.2 content value representing a carbon dioxide content of the respiratory gas flowing in the ventilation line arrangement, where the CO.sub.2 sensor assembly outputs to the evaluation device CO.sub.2 sensor signals which contain information about the CO.sub.2 content value and wherein the evaluation device, in order to identify the examination time interval from a plurality of CO.sub.2 sensor signals as a triggering event, determines when the CO.sub.2 content value falls below a predetermined CO.sub.2 triggering limit, and starting from the ascertained triggering time event defines a time interval as a candidate interval.

40. The ventilation device according to claim 39, wherein the evaluation device is designed to use the candidate interval as the examination time interval, when the evaluation device ascertains in a validation procedure that during the candidate interval a CO.sub.2 content value level which during an expiratory phase which includes at least the beginning of the candidate interval, or which prevails at the beginning of the candidate interval, decreases by at least a predetermined validation magnitude and/or by at least a predetermined validation fraction.

41. The ventilation device according to claim 40, wherein the evaluation device is designed to perform the validation procedure iteratively as from the triggering event, in each case with a modified candidate interval.

42. The ventilation device according to claim 40, wherein the evaluation device is designed to ascertain the predetermined validation magnitude and/or the predetermined validation fraction on the basis of preceding CO.sub.2 sensor signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0078] The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail and illustrated in the accompanying drawings which forms a part hereof and wherein:

[0079] FIG. 1A rough schematic exploded view of a ventilation device according to the invention,

[0080] FIG. 2A rough schematic sectional view through the measuring cuvette 52 of FIG. 1 along the sectional plane II-II of FIG. 1 which is orthogonal to the flow path SB of the measuring cuvette 52,

[0081] FIG. 3A A rough schematic graphic depiction of a temporal course of a CO.sub.2 partial pressure of respiratory gas flowing in the measuring cuvette 52 of FIG. 1, as it is detected during a ventilation operation by the CO.sub.2 sensor assembly 54 in FIG. 1, with a normal measuring cuvette which is not humidity-loaded,

[0082] FIG. 3B A rough schematic graphic depiction of a temporal course of an O.sub.2 partial pressure of respiratory gas flowing in the measuring cuvette 52 of FIG. 1, as it is detected during a ventilation operation by the O.sub.2 sensor assembly 55 in FIG. 1, with a normal measuring cuvette which is not humidity-loaded,

[0083] FIG. 4A A rough schematic graphic depiction of a temporal course of a CO.sub.2 partial pressure of respiratory gas flowing in the measuring cuvette 52 of FIG. 1, as it is detected during a ventilation operation by the CO.sub.2 sensor assembly 54 in FIG. 1, with a measuring cuvette which is slightly humidity-loaded,

[0084] FIG. 4B A rough schematic graphic depiction of a temporal course of an O.sub.2 partial pressure of respiratory gas flowing in the measuring cuvette 52 of FIG. 1, as it is detected during a ventilation operation by the O.sub.2 sensor assembly 55 in FIG. 1, with a measuring cuvette which is slightly humidity-loaded,

[0085] FIG. 5A A rough schematic graphic depiction of a temporal course of a CO.sub.2 partial pressure of respiratory gas flowing in the measuring cuvette 52 of FIG. 1, as it is detected during a ventilation operation by the CO.sub.2 sensor assembly 54 in FIG. 1, with a measuring cuvette which is heavily humidity-loaded, and

[0086] FIG. 5B A rough schematic graphic depiction of a temporal course of a O.sub.2 partial pressure of respiratory gas flowing in the measuring cuvette 52 of FIG. 1, as it is detected during a ventilation operation by the O.sub.2 sensor assembly 55 in FIG. 1, with a measuring cuvette which is heavily humidity-loaded.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0087] Referring now to the drawings wherein the showings are for the purpose of illustrating preferred and alternative embodiments of the invention only and not for the purpose of limiting the same, in FIG. 1, an embodiment of a ventilation device according to the invention is labelled generally by 10. The ventilation device 10 comprises a respiratory gas source 12 in the form of a fan and a control device 14 for adjusting operational parameters of the respiratory gas source 12. The respiratory gas source 12 and the control device 14 are accommodated in the same housing 16. In this housing there are also situated valves which are known per se, such as an inspiratory valve and an expiratory valve. These, however, are not specifically depicted in FIG. 1.

[0088] The control device 14 comprises an evaluation device 15 for the processing of data-transmitting signals from sensor assemblies 54, 55, and also 48 which are elucidated in further detail below. For the storage of an operating program and for storage of data to be processed and of interim and final results of the data processing, the evaluation device 15 exhibits a data memory 15a which can be read and at least segment-wise written to by the evaluation device 15. The evaluation device 15 or parts thereof can be arranged in immediate proximity to at least one of the sensor assemblies 54, 55, and also 48. In the housings of sensor assemblies too, there can be accommodated specific components as intelligent sensor assemblies, such as microprocessors and memory units, for signal processing of the sensor signals of the respective sensor assembly.

[0089] The control device 14 of the ventilation device 10 exhibits an input/output device 18 comprising numerous switches, such as pushbutton switches and rotary switches, so as to be able to input data into the control device 14 as required. The fan of the respiratory gas source 12 can be modified in its conveying rate by the control device 14 in order to modify the quantity of respiratory gas which is conveyed by the respiratory gas source per unit of time. The respiratory gas source 12 is therefore, in the present embodiment example, also a pressure modification device 13 of the ventilation device 10.

[0090] To the respiratory gas source 12 there is connected a ventilation line arrangement 20, which in the present example comprises five flexible hoses. A first inspiratory ventilation hose 22 proceeds from a filter arranged between the respiratory gas source 12 and itself to a conditioning device 26, where the respiratory gas supplied by the respiratory gas source 12 is humidified to a predefined degree of humidity and as appropriate provided with aerosol medications. The filter 24 filters and cleans the ambient air supplied by the fan as the respiratory gas source 12.

[0091] A second inspiratory ventilation hose 28 leads from the conditioning device 26 to an inspiratory water trap 30. A third inspiratory ventilation hose 32 leads from the water trap 30 to a Y-connector 34 which connects the distal inspiratory line 36 and the distal expiratory line 38 into a combined proximal inspiratory-expiratory ventilation line 40.

[0092] From the Y-connector 34 back to the housing 16 there proceeds a first expiratory ventilation hose 42 to an expiratory water trap 44 and from there a second expiratory ventilation hose 46 to the housing 16, where the expiratory respiratory gas is released into the environment via a non-depicted expiratory valve.

[0093] On the proximal, i.e. patient-near, combined inspiratory-expiratory side of the Y-connector 34 there follows the Y-connector 34 immediately a flow sensor 48, here: a differential pressure flow sensor 48, which detects the inspiratory and expiratory flow of respiratory gas towards the patient and away from the patient. A line arrangement 50 transmits the gas pressure prevailing on both side of a flow obstruction in the flow sensor 48 to the control device 14, which from the transmitted gas pressures and in particular from the difference between the gas pressures calculates the quantity of inspiratory and expiratory respiratory gas flowing per unit of time.

[0094] In the direction away from the Y-connector 34 there follows towards the patient after the flow sensor 48 a measuring cuvette 52 both for non-dispersive infrared detection of a CO.sub.2 fraction in the respiratory gas and for detecting an O.sub.2 fraction in the respiratory gas through luminescence extinction. The CO.sub.2 and the O.sub.2 fractions both in the inspiratory respiratory gas and in the expiratory respiratory gas are thereby of interest, since the change in the CO.sub.2 fraction between inspiration and expiration is a measure of the metabolizing competence of the patient's lung. In FIG. 1 there can be recognized one of the side windows 53, through which infrared light can be shone into the measuring cuvette 52 and/or emitted from the latter respectively, depending on the orientation of a multichannel infrared gas sensor 54a coupled detachably with the measuring cuvette as a CO.sub.2 sensor assembly 54.

[0095] On the upper side of the CO.sub.2 sensor assembly 54, preferably in a common sensor housing, there is situated a first, active part 55a of an O.sub.2 sensor assembly 55, which in a manner known per se is designed to excite a luminophore-containing observation region 66 (s. FIG. 2) as a second, passive part 55b of the O.sub.2 sensor arrangement 55 at the upper side of the measuring cuvette 52 by means of a radiation source to luminesce and to observe the luminescence behavior with regard to duration and/or intensity by means of a luminescence sensor.

[0096] The CO.sub.2 sensor assembly 54 and the active part 55a of the O.sub.2 sensor assembly 55 are couplable to the measuring cuvette 52 in such a way that the infrared gas sensor 54b can shine infrared light through the measuring cuvette 52 and that the active part 55a of the O.sub.2 sensor assembly 55 can excite and observe the observation region 66 of the measuring cuvette 52. From the intensity of the infrared light, more precisely from its spectral intensity, it is possible to deduce in a manner known per se the quantity and/or fraction respectively of CO.sub.2 in the respiratory gas flowing through the measuring cuvette 52, that is, its CO.sub.2 content. CO.sub.2 absorbs infrared light of a defined wavelength band. After passing through the measuring cuvette 52, the intensity of the infrared light at this wavelength depends essentially on the absorption of the infrared light of this wavelength by CO.sub.2. 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 any absorption spectrum of a gas fraction to be expected in the respiratory gas, provides information about the fraction of CO.sub.2 in the respiratory gas. The CO.sub.2 sensor assembly 54 is therefore connected via a data cable 56a with the control device 14 of the ventilation device 10 and transmits via the data cable 56a the described intensity data to the control device 14 and/or to the evaluation device 15. As ‘intensity information’ there is deemed to be any information unambiguously related to the intensity of the infrared radiation, such as for instance a CO.sub.2 content value ascertained from an infrared radiation intensity. In the case of the aforementioned ‘intelligent’ sensor assemblies, a first evaluation of the infrared signal can already take place, as described, in the evaluation device or part evaluation device in immediate proximity to the sensor assembly.

[0097] The active part 55a of the O.sub.2 sensor assembly 55 excites the luminophore-containing observation region 66 of the measuring cuvette 52 to luminesce, where the luminescence is extinguished as a function of the quantity of oxygen reaching the excited luminophore. From the observed duration and intensity of the luminescence, therefore, it is possible to deduce the quantity or the fraction of O.sub.2 in the respiratory gas, i.e. its O.sub.2 content. The active part 55a of the O.sub.2 sensor assembly 55 is, therefore, connected via a data cable 56b with the control device 14 and the evaluation device 15 and transmits via the data cable 56b data obtained from the observation of the luminescence behavior of the observation region 66 to the control device 14 and the evaluation device 15.

[0098] There follows after the measuring cuvette 52 in the direction towards the patient a further hose section 58, at which an endotracheal tube 60 is arranged as a ventilation interface to the patient. A proximal aperture 62 of the endotracheal tube 60 is both a respiratory gas outlet aperture, through which inspiratory respiratory gas is introduced through the endotracheal tube 60 into the patient, and a respiratory gas inlet aperture, through which expiratory respiratory gas is fed from the patient back into the endotracheal tube 60.

[0099] The difficulty underlying the present application is described below, using FIG. 2 as an example. FIG. 2 shows a section along the sectional plane II-II shown in FIG. 1 through the measuring cuvette 52 of FIG. 1, where the sectional plane II-II is oriented orthogonally to the drawing plane of FIG. 1 and hence also orthogonally to the flow path SB, along which a measurement chamber 64 has respiratory gas flowing through it bidirectionally in the ventilation operation.

[0100] The measurement chamber 64 is surrounded all around the flow path SB by the measuring cuvette 52, which for this purpose exhibits a preferably integrally injection-molded cuvette structure 52a with a bottom segment 52a1 and two frame struts 52a2 and 52a3 which are parallel to one another and to the flow path SB. Laterally, between the bottom segment 52a1 and each frame strut 52a2 and 52a3 there is in each case inset a side wall 52b or 52c respectively. On the side opposite the bottom segment 52a1 there is inset a ceiling wall 52c between the frame struts 52a2 and 52a3.

[0101] The sectional plane II-II proceeds through the windows 53, each of which is covered towards the measurement chamber 64 by a windowpane 53a and 53b transparent to infrared light. The windowpanes 53a and 53b can be made of a foil or of a dimensionally stable rigid material. The optical axis of the O.sub.2 sensor assembly 54 lies in the sectional plane II-II and is indicated by the reference symbol OA-CO.sub.2.

[0102] The upper side of the measuring cuvette 52 exhibits the observation region 66 already mentioned previously, at which when observed from outside the recess 66a is first noticeable, which penetrates through the ceiling wall 52c. Through the recess 66a, the luminophore of a luminophore layer 68 is excited to luminesce by the non-depicted radiation source. Through the recess 66a, besides, the luminescence behavior of the luminophore layer 68 after its excitation is detected by an appropriately designed luminescence sensor.

[0103] The luminophore layer 68 is carried by an oxygen-permeable membrane 70, namely only on the side of the membrane 70 facing away from the measurement chamber 64.

[0104] Such an oxygen-permeable membrane 70 can, for example, be made of polyvinylidene fluoride, PVDF for short. Towards the recess 66a the luminophore layer 68 is made of an oxygen-impermeable covering layer 72, for example out of biaxially oriented polypropylene.

[0105] In principle, this combined arrangement of a CO.sub.2 sensor assembly 54 and an O.sub.2 sensor assembly 55 including the laminate 74 belonging to the passive part 55b of the O.sub.2 sensor assembly 55, consisting of the membrane 70, the luminophore layer 68, and the covering layer 72, functions well. It can, however, be the case that humidity carried along and/or body fluids, such as saliva or mucus, carried along by the respiratory gas flowing in the measurement chamber 64 precipitate on the side 70a of the membrane 70 facing towards the measurement chamber 64 and there form a liquid film covering the membrane 70 completely or in part. Due to the porosity of the membrane 70, liquid precipitating on the side 70a can also migrate into the membrane 70, thus modifying the oxygen permeability of the membrane 70.

[0106] If such precipitation takes place, respiratory gas and oxygen carried along by the respiratory gas reach the luminophore layer 68 only to a reduced extent, such that the luminescence extinction caused by the oxygen no longer describes correctly the true oxygen content of the respiratory gas.

[0107] Since the detection of the carbon dioxide in the respiratory gas relies on radiation absorption and not on molecular contact, the result of the carbon dioxide detection by the CO.sub.2 sensor assembly 54 is, to the greatest extent possible, unaffected by liquid precipitating on the windowpanes 53a and 53b and/or can within certain limits be compensated for by using a reference radiation with a wavelength which is absorbed neither by the respiratory gas nor by the precipitated humidity.

[0108] The mode of operation of the ventilation device 10 for identifying an impairment of the O.sub.2 sensor assembly 55, in particular of the luminophore layer 68, is elucidated below by reference to FIGS. 3A to 4B.

[0109] The data processing operations for determining an impairment of the O.sub.2 sensor assembly 55 take place during the ongoing ventilation of a patient, but preferably are performed on CO.sub.2 sensor signals and O.sub.2 sensor signals saved in the data memory 15a and/or content values ascertained from these sensor signals respectively, such that within a plurality of data values available in a time interval it is possible to jump forward and back in time. Since during the determination of an impairment of the O.sub.2 sensor assembly 55, the finite data memory 15a is written to with current sensor signals or content values formed therefrom, a resource-conserving determination of an impairment with the lowest possible demands on computing power and memory space is desirable in order to be able to perform the determination on the basis of available data before these are overwritten by more recent data. When overwriting data, the oldest stored data are always overwritten by the current data to be saved.

[0110] FIG. 3A shows in rough schematic form, in steps with a step width of 0.05 minutes, a temporal course 76 ascertained by the CO.sub.2 sensor assembly 54 of the CO.sub.2 partial pressure as a CO.sub.2 content value under normal, in particular not liquid-loaded, operational conditions. Since ambient air supplied as respiratory gas contains almost no CO.sub.2, the expiratory processes can be well distinguished from the inspiratory processes which exhibit a CO.sub.2 partial pressure of below 5 mbar.

[0111] FIG. 3B shows in rough schematic form, to the same time scale as FIG. 3A, a temporal course 78, detected by the O.sub.2 sensor assembly 55 during the detection of the CO.sub.2 partial pressure, of an O.sub.2 partial pressure in the respiratory gas, i.e. in ambient air, as an O.sub.2 content value. Since during one breath only a part of the oxygen present in the ambient air is converted into CO.sub.2 by the metabolism of the patient, the expiratory respiratory gas also still contains oxygen, but with a lower content. The temporal course 78 of the O.sub.2 partial pressure, therefore, behaves qualitatively inversely to the temporal course 76 of the CO.sub.2 partial pressure, in as much as the O.sub.2 partial pressure takes on quantitatively higher values during an inspiratory process than during an expiratory process.

[0112] Since an inspiratory process begins by the respiratory gas source 12 beginning to supply to the patient inspiratory respiratory gas against the quantitatively decreasing flow of expiratory respiratory gas and since an expiratory process begins by respiratory gas which is under pressure beginning to flow out of the patient's lung away from the patient against the quantitatively decreasing flow of inspiratory respiratory gas, inspiratory and expiratory phases can overlap in time.

[0113] By reference to the second peak of the temporal course of the CO.sub.2 partial pressure shown in FIG. 3A, the segments which such a temporal course exhibits during expiration are elucidated below.

[0114] In the changeover from inspiration to expiration, the flow direction of the respiratory gas necessarily changes in the ventilation line arrangement 20 and hence in the measuring cuvette 52, such that at the beginning of an expiration, starting from a flow velocity of approximately 0 m/s, the flow velocity of the expiratory respiratory gas and hence the volume and mass flow of expiratory respiratory gas increase, whereby the quantity of carbon dioxide carried along by the expiratory respiratory gas per unit of time into the measurement chamber 64 of the measuring cuvette 52 also increases. This increase in carbon dioxide content forms a first transient expiratory segment ET1.

[0115] To this first transient expiratory segment ET1 there adjoins an expiratory plateau segment EP, in which the change in the CO.sub.2 partial pressure as the CO.sub.2 content value chosen by way of an example is considerably smaller than in the first transient expiratory segment ET1. To the expiratory plateau segment EP there adjoins towards the end of the expiratory process a second transient expiratory segment ET2, in which the flow velocity and hence the mass and volume flow of expiratory respiratory gas decrease, in particular decrease down to 0 m/s. With the decreasing flow of expiratory respiratory gas, the quantity of carbon dioxide flowing per unit of time past the CO.sub.2 sensor assembly 54 also necessarily decreases.

[0116] Likewise, the inspiratory processes exhibit at the beginning and at the end transient inspiratory segments IT1 and IT2, and between the transient segments an inspiratory plateau segment IP. These segments are depicted at the third peak in FIG. 3B. The time fractions of an inspiration phase which fall on the individual segments IT1, IP, and IT2 can differ from the time fractions of the respective segments ET1, EP, and ET2 of an expiration phase and normally are different because of the different fluid-mechanical processes and their causes.

[0117] Since the ventilation device 10 preferably uses both the gradient criterion and the difference criterion, the ventilation device 10 begins by determining a characteristic CO.sub.2 gradient value, since the latter preferably serves as a reference point for further determinations of values. It should, however, be pointed out that as elucidated in the introduction to the description, the further values described below can also be determined without previously determining a characteristic CO.sub.2 gradient value.

[0118] In experiments conducted thus far, the value of the quantitatively largest possible slope of the temporal course of the CO.sub.2 partial pressure (or generally, of a CO.sub.2 content value) in the falling flank of an expiratory phase has proved to be the most informative characteristic CO.sub.2 gradient value. First of all, the evaluation device 15 determines as effectively as possible and at little computational cost a candidate interval KI in which the falling flank of the CO.sub.2 partial pressure could lie in an expiratory phase. For example, for the second peak in FIG. 3A this can be the second transient segment ET2 indicated there.

[0119] One of several possible forms of the determination of a candidate interval KI, which has proved reliable in experiments conducted thus far, comprises first of all determining that point in time at which the CO.sub.2 content value has decreased to a predetermined level which reliably no longer belongs to the expiratory plateau segment EP, for example at which the CO.sub.2 partial pressure has fallen below a CO.sub.2 triggering limit 79 of 20 mbar or even of 10 mbar. In FIG. 3A, the CO.sub.2 triggering limit 79 of 10 mbar is used, which in the first expiratory process is fallen below at the point 80. It lies at approximately 0.204 minutes.

[0120] Starting from the triggering event thus ascertained of falling below the predefined CO.sub.2 triggering limit 79, the candidate interval KI is defined from the starting point in time t80, i.e. in the example 0.204 minutes, which begins a predetermined first time interval before the triggering time point t80 and ends a predetermined second time interval after the triggering time point t80. The predetermined first and second time intervals are chosen, based on experience gained thus far in ventilation operations, in such a way that the falling flank lies in the candidate interval KI in at least 85%, preferably in at least 95% of the expiratory phases.

[0121] The evaluation device 15 subsequently verifies whether the candidate interval KI is a valid examination time interval ZI, by checking whether the CO.sub.2 content value, starting from the CO.sub.2 content value prevailing at the beginning of the candidate interval KI, decreases within the candidate interval KI by at least a predetermined validation magnitude VB of 23 mbar. Since this is the case in the depicted candidate interval KI, the candidate interval KI is validated as an examination time interval ZI. From now on it is used as examination time interval ZI. As elucidated in the introduction to the description, other validation criteria can additionally or alternatively be utilized for validating the candidate interval KI.

[0122] Within the examination time interval ZI thus defined, the evaluation device 15 determines, for example through iterative forming of differences between two CO.sub.2 partial pressure values lying at a distance of Δt and through division of the difference thus formed by the temporal distance Δt, a sequence of CO.sub.2 partial pressure gradient values and selects therefrom the quantitatively largest. In the depicted example it lies at point 82.

[0123] Subsequently the point in time t82 at which the quantitatively largest CO.sub.2 partial pressure gradient value occurs, is used as a reference point in time. By adding a predetermined positive time interval to the reference point in time t82, the later point in time t84 is obtained which lies in the rising flank of the second peak of the graph 78.

[0124] The magnitude of the slope of the graph 78 as the O.sub.2 gradient value is ascertained at the point in time t84 thus obtained. This is the slope of the graph 78 at the point 84. For applying the gradient criterion, the point 84 does not have to be a key point on the graph 78. It suffices that this point 84 lies the predetermined time interval away from the reference point in time t82 in the rising flank of the temporal course of the O.sub.2 content values. The predetermined time interval is chosen in such a way that in at least 85%, preferably at least 95%, especially preferably in 100% of the inspiratory processes it lies in the rising flank of the temporal course of the O.sub.2 content value, which is already possible due to the fact that by virtue of the mechanical ventilation of the patient, inspiratory processes follow expiratory processes directly and predictably.

[0125] The evaluation device 15 then forms a ratio of the magnitudes of the two obtained gradient values: CO.sub.2 gradient value and O.sub.2 gradient value, as a change ratio of the magnitudes of the gradient values, and compares the value of the change ratio thus determined with a predetermined change ratio limit stored in the data memory 15a. When the ratio of the magnitudes, with the CO.sub.2 gradient value in the numerator and the O.sub.2 gradient value in the denominator, is larger than the change ratio limit previously determined in the laboratory, which for example can equal 16.6, the evaluation device 15 outputs a signal which indicates that the O.sub.2 sensor assembly 55 and with it the obtained O.sub.2 sensor signals are degraded. This is not the case here in FIGS. 3A and 3B.

[0126] Additionally to the gradient criterion described previously, the evaluation device 15 uses the difference criterion described above in the introduction to the description. To this end, in FIG. 3B the difference is formed between a characteristic inspiratory O.sub.2 content value, i.e. here an O.sub.2 partial pressure, and a characteristic expiratory O.sub.2 content value (partial pressure). For this purpose there can be determined, for example, starting from the already determined reference point in time t82, an inspiratory time interval II-O.sub.2 lying at a predetermined temporal distance in respect of its start and its end relative to the reference point in time t82, and an expiratory time interval EI-O.sub.2 likewise lying at a predetermined temporal distance in respect of its start and its end relative to the reference point in time t82. From the O.sub.2 content values lying with the intervals II-O.sub.2 and EI-O.sub.2 there can respectively be ascertained an average O.sub.2 content value as the characteristic inspiratory O.sub.2 content value and the characteristic expiratory O.sub.2 content value. The characteristic inspiratory O.sub.2 content value is preferably ascertained in the inspiratory process which immediately precedes the expiratory process in which the characteristic CO.sub.2 gradient value is ascertained.

[0127] In the example depicted in FIG. 3B, there is obtained for the characteristic inspiratory O.sub.2 content value in the predetermined inspiratory time interval II-O.sub.2 an averaged inspiratory O.sub.2 content value of about 182 mbar indicated by the reference symbol 90. For the characteristic expiratory O.sub.2 content value in the predetermined expiratory time interval EI-O.sub.2, there is obtained an averaged expiratory O.sub.2 content value of about 166.3 mbar indicated by the reference symbol 92. The O.sub.2 difference value consequently equals about 15.7 mbar. For the atmosphere with an ambient pressure of 1013 mbar present in the example, the atmosphere-based O.sub.2 difference value is about 0.015. The evaluation device 15 compares this atmosphere-based O.sub.2 difference value with an O.sub.2 difference limit which was determined in the laboratory in advance. The latter can, for example, be 0.01. If the atmosphere-based O.sub.2 difference value falls below the O.sub.2 difference limit, then the difference criterion for a signal output is satisfied for the atmosphere-based O.sub.2 difference value. This is not the case here.

[0128] Consequently, the evaluation device 15 assesses the O.sub.2 sensor assembly 55 as not impaired and therefore does not output a signal indicating an impairment of the O.sub.2 sensor assembly 55.

[0129] FIGS. 4A and 4B show a temporal course of the CO.sub.2 partial pressure and of the O.sub.2 partial pressure during ventilation of the same patient with the same ventilation parameters. However, in the meantime a little liquid has precipitated on the walls bordering the measurement chamber 64, consequently also on the windows 53 and on the membrane 70 of the O.sub.2 sensor assembly 55.

[0130] The evaluation device 15 performs on the sensor signals and on the content values ascertained therefrom the same operations as were elucidated previously in connection with FIGS. 3A and 3B. Equal and significance-equivalent values and regions as in FIGS. 3A and 3B are indicated in FIGS. 4A and 4B by the same reference symbols, to which an apostrophe is added merely for the purpose of distinguishing the wet state of the O.sub.2 sensor assembly 55 from the previously discussed dry state. Consequently, the procedure described previously by reference to FIGS. 3A and 3B for determining an impairment of the O.sub.2 sensor assembly 55, also applies to the data and data courses of FIGS. 4A and 4B.

[0131] It is important here that the falling flank of the temporal course 76′ of the CO.sub.2 partial pressures in the liquid-loaded measuring cuvette 52 exhibits an approximately identical slope as previously in the dry measuring cuvette 52. The characteristic CO.sub.2 gradient value at the point in time t82′, ascertained at the point 82′ as the quantitatively largest ‘wet’ CO.sub.2 gradient value in the examination time interval ZI′, is quantitatively approximately equal to the ‘dry’ CO.sub.2 gradient value previously ascertained at point 82.

[0132] In contrast, the course of the rising flank of the O.sub.2 gradient values is flatter, such that the ‘wet’ O.sub.2 gradient value, ascertained at the point in time t84′ as a function of the reference point in time t82′, as previously, exhibits a quantitatively smaller value.

[0133] The change ratio, ascertained with the data shown in FIGS. 4A and 4B as the ratio of the magnitudes of the CO.sub.2 gradient value in the numerator and the O.sub.2 gradient value in the denominator, becomes quantitatively larger and now exceeds the predetermined change ratio limit of 16.6. The evaluation device 15 therefore deduces a degradation of the O.sub.2 sensor assembly 55 and outputs a corresponding signal.

[0134] Alternatively to an immediate output of the signal, the evaluation device can also output the signal, for example, only if within five consecutive breaths for each breath at least one criterion out of the gradient criterion and difference criterion is satisfied.

[0135] In the example depicted in FIG. 4B, there is obtained for the characteristic inspiratory O.sub.2 content value in the predetermined inspiratory time interval II-O.sub.2 an averaged inspiratory O.sub.2 content value, determined as described above and indicated by the reference symbol 90′, of about 181.5 mbar. As the characteristic expiratory O.sub.2 content value 92′, determined as described above, there is obtained an averaged expiratory O.sub.2 content value of about 166.7 mbar. The O.sub.2 difference value consequently equals about 14.8 mbar. The atmosphere-based O.sub.2 difference value in the atmosphere present in the example, with an ambient pressure of 1013 mbar, equals about 0.014. The evaluation device compares this atmosphere-based O.sub.2 difference value with the aforementioned O.sub.2 difference limit of 0.01.

[0136] The gradient criterion is satisfied, but the difference criterion is not. This can be down to the relatively low quantity of precipitated humidity in the example of FIGS. 4A and 4B. To determine an impairment of the O.sub.2 sensor assembly 55, however, satisfying one of the indicated criteria is already sufficient.

[0137] FIGS. 5A and 5B show, in analogy with FIGS. 4A and 4B, CO.sub.2 and O.sub.2 content values respectively as a function of time, with an O.sub.2 sensor assembly 55 which is more heavily contaminated with liquid than is the case in the example of FIGS. 4A and 4B. The diagrams in FIGS. 5A and 5B are based on detection of respiratory gas in a measuring cuvette 52 whose inner surface is covered with a layer of mucus. This also applies to the surface 70a of the membrane 70 facing towards the measurement chamber 64. Equal and significance-equivalent values and regions as in FIGS. 3A to 4B are indicated by the same reference symbols, but identified by a double apostrophe. Consequently, the liquid loading of the O.sub.2 sensor assembly 55 increases with increasing number of apostrophes.

[0138] Once again, the data operations elucidated by reference to FIGS. 3A and 3B are performed on the CO.sub.2 and O.sub.2 content values, in order to assess whether or not the O.sub.2 sensor assembly 55 is impaired.

[0139] Based on the example of FIGS. 5A and 5B, the difference criterion shall first of all be elucidated by reference to an example. As FIG. 5B shows, due to the heavier liquid loading than in the case of FIG. 4B, the level of the O.sub.2 partial pressure in the inspiratory plateau segment has decreased from about between 181.5 and 182 mbar to about 167 mbar. Likewise, the level of the O.sub.2 partial pressure in the expiratory plateau segment has decreased from about 167 mbar to about 151 mbar.

[0140] The ratio of inspiratory to expiratory mean O.sub.2 partial pressure in the plateau segments of the O.sub.2 partial pressure curves 78′ and 78″ of the liquid-loaded O.sub.2 sensor assembly 55 lies in both cases at about 1.1. This applies likewise to the O.sub.2 partial pressure curve 78 of the not liquid-loaded O.sub.2 sensor assembly 55 of FIG. 3B.

[0141] By comparing FIGS. 4B and 5B it becomes clear that with increasing liquid loading of the O.sub.2 sensor assembly 55, a state could be reached in which the rising flank of an O.sub.2 partial pressure curve can turn out so short in the changeover segment during the transition from an expiratory phase to an inspiratory phase that an informative O.sub.2 gradient value at this flank can no longer be reliably determined. Then the O.sub.2 gradient criterion could risk failing.

[0142] In the present case, the gradient criterion is also satisfied in the liquid-loaded O.sub.2 sensor assembly 55 which leads to the content values of FIGS. 5A and 5B.

[0143] Additionally at the O.sub.2 partial pressure curve 78″ of FIG. 5B too, as already elucidated previously by reference to FIGS. 3B and 4B, there is defined an inspiratory phase segment II-O2″ for ascertaining the characteristic inspiratory O.sub.2 content value 90″ and an expiratory phase segment EI-O2″ for ascertaining the characteristic expiratory O.sub.2 content value 92″.

[0144] The ventilation frequency in FIGS. 5A and 5B is somewhat lower than in the preceding FIGS. 3A to 4B, which is why the evaluation device 15 has adjusted the temporal distances of the beginning and the end of the respiratory part-phase segments II-O2″ and EI-O2″ from the reference point in time t82″ through a minor increase. It is, however, equally possible to define the beginning and the end of the respiratory part-phase segments II-O2″ and EI-O2″ such that the respiratory part-phase segments II-O2″ and EI-O2″ lie in the plateau segments for all reasonably to be expected breathing frequencies during the operation of the ventilation device 10. The respiratory part-phase segments II-O2″ and EI-O2″ can turn out, for this purpose, shorter than depicted.

[0145] In the first inspiratory phase in FIG. 5B, in which the inspiratory phase segment II-O2″ is indicated, there is obtained an O.sub.2 content value arithmetically averaged over the inspiratory phase segment II-O2″ as the characteristic inspiratory content value 90″ of 156.8 mbar. In the following expiratory phase segment EI-O2″, there is obtained an O.sub.2 content value arithmetically averaged over the expiratory phase segment EI-O2″ as the characteristic expiratory content value 92″ of about 150.4 mbar. From this there is obtained an O.sub.2 difference value of 6.4 mbar, that is to say, an atmosphere-based O.sub.2 difference value of 0.006. The atmospheric pressure of 1013 mbar is unchanged.

[0146] The evaluation device 15 compares the ascertained atmosphere-based O.sub.2 difference value of 0.006 with the O.sub.2 difference limit of 0.01 already quoted above and determines that the ascertained atmosphere-based O.sub.2 difference value is smaller than the predetermined O.sub.2 difference limit. The difference criterion is consequently satisfied in addition to the gradient criterion. The evaluation device 15 outputs, in the case of the ventilation situation of FIGS. 5A and 5B too, a signal which indicates a degradation of the O.sub.2 sensor assembly 55.

[0147] It should be added that the inspiratory phase segments and expiratory phase segments identified above can also be utilized for ascertaining characteristic inspiratory CO.sub.2 content values and characteristic expiratory CO.sub.2 content values. These characteristic CO.sub.2 content values too, can be values arithmetically averaged over their associated respiratory part-phase segment. A CO.sub.2 difference value can be calculated on the basis of at least one characteristic expiratory CO.sub.2 content value and at least one characteristic inspiratory CO.sub.2 content value, for instance as the magnitude of the difference between the characteristic content values. The aforementioned validation magnitude and/or the validation fraction can be ascertained on the basis of the CO.sub.2 difference value thus obtained. For example, the validation magnitude can be a predetermined fraction of the CO.sub.2 difference value. The validation fraction in percent can be determinable as a function of the CO.sub.2 difference value, for instance by storing a table or a characteristic diagram of validation fractions as a function of the CO.sub.2 difference value.

[0148] Even though the respiratory part-phase segments can be the same for determining characteristic content values of O.sub.2 and CO.sub.2, it should not be ruled out that different respiratory part-phase segments are determined for each respiratory gas component: O.sub.2 and CO.sub.2, for instance due to different predetermined temporal distances of the beginning and the end of a respiratory part-phase segment from a reference point in time, such as the point in time of the occurrence of the quantitatively largest temporal change in the CO.sub.2 content value. The reference point in time preferably lies in a changeover segment in the transition from an expiratory phase to an inspiratory phase.

[0149] What has been said in the present application regarding the determination of characteristic inspiratory and expiratory O.sub.2 content values, also applies mutatis mutan-dis to characteristic inspiratory and expiratory CO.sub.2 content values with the stipulation that O.sub.2 should be replaced by CO.sub.2.

[0150] It is clear that the verification procedures which the evaluation device performs on the data transmitted by the sensor signals, were elucidated above merely by way of example on the basis of graphic representations of these data. The verification procedures are, however, performed on numerical values and/or numerical value-pairs respectively or generally on linked numerical values.

[0151] Any value which stands in an unambiguous functional relationship with a content value addressed in the present application is equivalent to the content value.

[0152] The present invention allows a degradation of the O.sub.2 sensor assembly 55 to be recognized promptly without additional sensors.

[0153] While considerable emphasis has been placed on the preferred embodiments of the invention illustrated and described herein, it will be appreciated that other embodiments, and equivalences thereof, can be made and that many changes can be made in the preferred embodiments without departing from the principles of the invention. Furthermore, the embodiments described above can be combined to form yet other embodiments of the invention of this application. Accordingly, it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.