Abstract
A system for controlling a ventilation variable of a ventilation device for tracking a target curve includes at least one sensor to detect instantaneous values of the ventilation variable; an actuator to adjust the ventilation variable of the ventilation device; a memory for storing the target curve for the ventilation variable for an inspiration and/or expiration phase; and a controller having a proportional term and an integral term and that calculates a control difference between the target curve and the instantaneous values, and that controls the actuator accordingly. The controller adjusts the gain factor of the integral term: (i) increasing it when the integral of the control difference for the inspiration and/or expiration phase exceeds a predetermined integral threshold value, or (ii) reducing it when a number of zero crossings of the control difference during the inspiration and/or expiration phase exceeds a predetermined zero-crossing threshold value.
Claims
1. A system for controlling a ventilation variable of a ventilation device for tracking a target curve, comprising: at least one sensor which is adapted to detect instantaneous values for the ventilation variable; an actuator which is adapted to adjust the ventilation variable of the ventilation device; a memory in which the target curve for the ventilation variable is stored, the target curve being stored for an inspiration phase and/or for an expiration phase; and a controller which has at least a proportional term and an integral term and which is adapted to determine a control difference from the target curve and the instantaneous values for the ventilation variable, and which is adapted to control the actuator; wherein the controller is adapted to adjust a gain factor of the integral term according to at least one of the following features: (i) increasing the gain factor of the integral term when the integral of the control difference for the inspiration phase and/or the expiration phase exceeds a predetermined integral threshold value; (ii) reducing the gain factor of the integral term when a number of zero crossings of the control difference during the inspiration phase and/or during the expiration phase exceeds a predetermined zero-crossing threshold value.
2. The system according to claim 1, wherein the ventilation variable is one of ventilation pressure and ventilation volume.
3. The system according to claim 1, wherein the actuator comprises a controlled blower and/or a controlled valve.
4. The system according to claim 1, wherein the controller is a proportional integral controller.
5. The system according to claim 1, wherein the controller is adapted to leave a gain factor of the proportional term unchanged when the gain factor of the integral term is adjusted.
6. (canceled)
7. The system according to claim 1, wherein the controller is adapted to set the gain factor of the integral term to an initial value.
8. The system according to claim 7, wherein the initial value is dependent on at least one patient parameter, such as age or weight.
9. The system according to claim 7, wherein the controller is adapted to set the gain factor of the integral term to the initial value once at the start of operation, or is adapted to set the gain factor of the integral term to the initial value in predetermined intervals.
10. The system according to claim 1, wherein the controller is adapted to adjust the gain factor of the integral term for tracking the target curve for the current inspiration phase or the current expiration phase.
11. The system according to claim 1, wherein the controller is adapted to set the integral of the control difference to zero at a zero crossing of the control difference.
12. The system according to claim 1, wherein a plurality of integral threshold values are provided and wherein the controller is adapted to increase the gain factor of the integral term a plurality of times when the integral of the control difference exceeds several of the plurality of integral threshold values.
13. The system according to claim 1, wherein a plurality of zero-crossing threshold values are provided, and wherein the controller is adapted to reduce the gain factor of the integral term a plurality of times when the number of zero crossings exceeds several of the plurality of zero-crossing threshold values.
14. The system according to claim 1, wherein the controller is adapted to increase the gain factor of the integral term in feature with a first delta value, and/or wherein the controller is adapted to reduce the gain factor of the integral term in feature by a second delta value.
15. (canceled)
16. The system according to claim 1, wherein the controller is further adapted to adjust the gain factor of the integral term according to the following feature: (ii) reducing the gain factor of the integral term when the instantaneous value for the ventilation variable exceeds the target curve by a safety threshold value.
17. The system according to claim 16, wherein the safety threshold value is between 1.5 mbar and 3 mbar.
18. A ventilation device, comprising a system for controlling a ventilation variable according to claim 1.
19. The ventilation device according to claim 18, further comprising: a connecting member, in particular a single-lumen or double-lumen connecting hose, and a patient application piece, in particular a tube or a respiratory mask, wherein the patient application piece is connected to a patient-side end portion of the connecting member.
20. The ventilation device according to claim 19, wherein the sensor is arranged at a ventilation device-side end portion of the connecting member or wherein the sensor is arranged in the region of the connection between the connecting member and the patient application piece or wherein a first sensor is arranged at a ventilation device-side end portion of the connecting member and a second sensor is arranged in the region of the connection between the connecting member and the patient application piece.
21. A method for controlling a ventilation variable of a ventilation device for tracking a target curve, comprising the steps of: acquiring instantaneous values for the ventilation variable; determining a control difference from the instantaneous values for the ventilation variable and the target curve for the ventilation variable, the target curve being given for an inspiration phase and/or for an expiration phase; determining a control variable for an actuator, wherein determining the control variable is performed according to a control function having at least a proportional term and an integral term; adjusting the integral term of the control function according to at least one of the following features: (i) increasing the gain factor of the integral term when the integral of the control difference for the inspiration phase and/or the expiration phase exceeds a predetermined integral threshold value; (ii) reducing the gain factor of the integral term when a number of zero crossings of the control difference during the inspiration phase and/or during the expiration phase exceeds a predetermined zero-crossing threshold value; and controlling the actuator according to the determined control variable for adjusting the ventilation variable of the ventilation device.
22. A computer program containing program instructions which, when executed on a data processing system, perform a method according to claim 21.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] Further exemplary embodiments of the invention will be described below with reference to the accompanying drawings, wherein:
[0054] FIG. 1 shows a ventilation device for mechanically ventilating patients according to an exemplary embodiment of the invention, the ventilation device being provided with a system for controlling a ventilation variable according to an exemplary embodiment of the invention;
[0055] FIG. 2 shows a system for controlling a ventilation variable of a ventilation device according to an exemplary embodiment of the invention in a block diagram;
[0056] FIG. 3 illustrates the operation of the system of FIG. 2 by way of an exemplary target curve for the ventilation variable and an exemplary course of the instantaneous values for the ventilation variable;
[0057] FIG. 4 illustrates the operation of the system of FIG. 2 by way of an exemplary target curve for the ventilation variable and a further exemplary course of the instantaneous values for the ventilation variable;
[0058] FIG. 5 illustrates the operation of the system of FIG. 2 by way of an exemplary target curve for the ventilation variable and a further exemplary course of the instantaneous values for the ventilation variable;
[0059] FIG. 6 illustrates the operation of the system of FIG. 2 over several ventilation cycles.
DETAILED DESCRIPTION OF THE INVENTION
[0060] FIG. 1 illustrates a ventilation device 100 for mechanically ventilating patients in accordance with an exemplary embodiment of the invention. The ventilation device 100 has a frame 102 movable on rolls, which is provided with a control unit 110 and a monitor 120 as well as a first line 130 for ventilation air supplied by the device and a second line 140 for exhaled air. The first line 130 for supplied ventilation air includes a first ventilation air hose 132 extending from the control unit 110 and leading to a humidification unit 134, where the ventilation air is passed through a water reservoir. From the humidification unit 134, a second ventilation air hose 136 leads to a T-piece 138. From the T-piece, the ventilation air provided by the ventilation machine 100 during an inspiration phase of the ventilation cycle passes to the patient via a patient application piece 150. The air exhaled by the patient during an expiration phase of the ventilation cycle also passes back toward the ventilation device 100 via the patient application piece 150. For this purpose, another breathing air hose 142 branches off from the T-piece 138, which belongs to the second line 140 for exhaled air. The breathing air hose 142 leads to an expiration valve 160 through which the exhaled air is released into the environment of the ventilation device.
[0061] Embedded within the ventilation device 100 is a system for controlling a ventilation variable according to an exemplary embodiment of the invention. The individual components of this system for controlling a ventilation variable will be described below with reference to FIG. 2, and reference will be made again to FIG. 1 with respect to an exemplary positioning of the components.
[0062] FIG. 2 shows a system 2 for controlling a ventilation variable of a ventilation device according to an exemplary embodiment of the invention in a block diagram.
[0063] The system 2 has a memory 8 in which a target curve 16 for a ventilation variable is stored. In the exemplary embodiment of FIG. 2, the ventilation variable is the ventilation pressure provided by the ventilation device. Accordingly, the target curve 16 is a sequence of ventilation pressure values over time. In particular, the target curve 16 may comprise a sequence of ventilation pressure values over an inspiration phase and/or over an expiration phase of a ventilation cycle. The target curve 16 represents a desired course of the ventilation pressure over time. The target curve 16 is output from the memory 8 as a sequence of nominal or target pressure values. In particular, the target curve 16 is output as a signal 22 from the memory 8, the signal 22 representing an image of the target curve 16.
[0064] The system 2 comprises a sensor 4 that is designed to detect the ventilation variable. In the exemplary embodiment of FIG. 2, the sensor 4 is a pressure sensor. The sensor 4 senses the ventilation pressure 28 and outputs instantaneous values 20 for the ventilation pressure. The ventilation pressure 28 is represented as a physical quantity by a dashed line in FIG. 2, whereas the instantaneous values 20 for the ventilation pressure are a signal representation of the physical quantity and are represented by a solid line in FIG. 2.
[0065] The signal 22 representing the target curve 16 and the instantaneous values 20 of the ventilation pressure are supplied to a subtractor 14 which forms a control difference 24 from the target curve 16 and the instantaneous values 20 for the ventilation pressure. In doing so, the subtractor 14 in each case subtracts an instantaneous value of the ventilation pressure from a current or instantaneous target value of the target curve. Repeated subtraction results in a time curve of the control difference 24 and thus a time curve of the deviation between the target curve 16 and the instantaneous values 20 of the ventilation pressure.
[0066] The subtractor 14 is part of a controller 10 of the system 2. In addition to the subtractor 14, the controller 10 has a control function 18 which can also be referred to as control algorithm 18. The control difference 24 is provided to the control function 18 which is connected to the subtractor 14. A time curve or history of the control difference 24 is provided to the control function 18 over time. The control function 18 uses the time curve of the control difference 24 to generate a control variable 26 and to cause an actuator 6 by means of the control variable 26 to act on the ventilation pressure 28
[0067] The controller 10 has a proportional term and an integral term. In the exemplary embodiment of FIG. 2, the controller 10 has only a proportional term and an integral term. Thus, the controller 10 is a PI controller in the exemplary embodiment of FIG. 2. The controller 10 may operate according to the following formula:
[00001]
[0068] wherein u(t) designates the control variable 26 and e(t) designates the control difference 24. The linear term before the + sign is the proportional term of the controller 10, wherein K.sub.P designates the gain factor of the proportional term. The integral term after the + sign is the integral term of the controller 10, wherein K.sub.I designates the gain factor of the integral term.
[0069] It is emphasized that the controller 10 can also operate according to a different formula. However, the controller 10 always has a proportional term that takes into account the current or instantaneous control difference, and an integral term that takes into account the control difference over a certain or determinable time horizon in the past. Thus, the accumulated control difference over this time horizon is considered in the control. The time horizon may comprise a certain period of time or a period of time since a certain event, e.g. the period of time since the beginning of the current ventilation cycle or since the beginning of the current inspiration phase or expiration phase.
[0070] The actuator 6 adjusts the ventilation pressure 28 based on the control variable 26. The control variable 26 may indicate to the actuator 6 to increase or decrease the ventilation pressure 28 or keep it the same. Furthermore, the control variable 26 may indicate to the actuator 6 how much the ventilation pressure is to be adjusted.
[0071] In the exemplary embodiment of FIG. 2, the actuator 6 is a source of compressed air. For this purpose, the actuator 6 may comprise, for example, a controlled compressed air pump or a controlled blower or a controlled fan. It is also possible that the actuator 6 comprises a compressed air reservoir and a controlled valve. In either case, the actuator 6 is capable of outputting compressed air toward the patient being ventilated in controlled manner.
[0072] The actuator 6 is adapted to adjust the ventilation pressure 28. For a given application of compressed air by the actuator 6, very different results in the ventilation pressure 28 may arise. The reason for this is the ventilation tract 12 of the patient being ventilated, which varies from person to person. In particular, the ventilation tract 12 of each patient has a certain resistance with respect to applied compressed air and a certain expansion capacity of the ventilation tract 12 when compressed air is applied. Because of the difference in resistance and expansion capacity of the ventilation tract 12, a patient's response to the compressed air applied by the actuator 6 can vary greatly. These relationships are illustrated in FIG. 2 by showing the ventilation pressure 28 as a dashed line connected to the actuator 6 and extending through the ventilation tract 12. The ventilation pressure 28 is influenced by the compressed air output from the actuator 6 and by the characteristics of the ventilation tract 12. The resulting ventilation pressure 28 is measured by the sensor 4, as illustrated by the connection of said dashed line to the sensor 4.
[0073] In the ventilation device 100 of FIG. 1, the memory 8 and the controller 10 may be arranged in the control unit 110. The actuator 6 may also be arranged in the control unit 110. The sensor 4 may be disposed in the ventilation device-side end portion of the first line 130 or in the region of the T-piece 138 or at another suitable location between the control unit 110 and the patient application piece 150. The position of the sensor 4 may be selected depending on requirements for distance between the sensor 4 and the patient and/or depending on requirements for signal transmission between the sensor 4 and the control unit 110. Also, the position of the sensor 4 may depend on whether the control of the ventilation pressure takes place only for the inspiration phase or only for the expiration phase or for the inspiration phase and the expiration phase.
[0074] The controller 10 may be implemented in hardware or implemented as software to be executed on a processor, or implemented as a combination of hardware and software. In addition to the control of the actuator 6 described above, the controller 10 of the exemplary embodiment of FIG. 2 has a further functionality affecting the control of the ventilation pressure 28. The controller 10 is adapted to adjust its integral term based on the control difference 24. Thus, the controller 10 is adapted to adjust itself based on the control difference 24. The controller 10 can thereby change the influence on the control of the actuator 6 and the influence on the ventilation pressure 28 based on the control difference. Thus, the control variable 26 is dependent not only on the history of the control difference 24, but also on the adjustment of the control algorithm 18 within the controller 10 based on the history of the control difference 24. In this way, the controller 10 can be very flexible in responding to the circumstances of the ventilation at hand. In particular, by adjusting its own control algorithm 18, the controller 10 can achieve an implicit adjustment of the control to the ventilation tract 12 of the patient to be ventilated.
[0075] In the exemplary embodiment of FIG. 2, the controller 10 is adapted to increase the gain factor K.sub.I of the integral term when the integral of the control difference 24 exceeds a predetermined integral threshold value. Furthermore, the controller 10 is adapted to reduce the gain factor K.sub.I of the integral term when a number of zero crossings of the control difference 24 exceeds a predetermined zero-crossing threshold value. Still further, the controller 10 is adapted to reduce the gain factor K.sub.I of the integral term when the instantaneous value 20 for the ventilation variable exceeds the target curve 16 by a safety threshold value. In the exemplary embodiment of FIG. 2, all three of the aforementioned criteria for adjusting the gain factor K.sub.I of the integral term are implemented. However, it is also possible that only one of the three criteria mentioned or a subset of the criteria mentioned is implemented for adjusting the gain factor K.sub.I of the integral term. The three criteria mentioned will be explained in more detail below with reference to FIGS. 3 to 5.
[0076] In the exemplary embodiment of FIG. 2, the controller 10 considers each inspiration phase or expiration phase in isolation for adjusting the gain factor K.sub.I of the integral term. In other words, the integral of the control difference 24 or the number of zero crossings of the control difference 24 is set to zero at the end of an inspiration phase or at the end of an expiration phase. Furthermore, the integral of the control difference 24 is also set to zero within an inspiration phase or within an expiration phase at a zero crossing of the control difference 24. However, it is also possible that the controller 10 determines the integral of the control difference 24 or the number of zero crossings of the control difference 24 over a longer period of time and adjusts the gain factor K.sub.I of the integral term based on this determination.
[0077] FIG. 3 shows an exemplary course of the instantaneous values 20 of a ventilation variable for a sudden increase of the target curve 16 of the ventilation variable as well as an associated integral 30 of the control difference. FIG. 3 illustrates the operation of the system 2 of FIG. 2 for an exemplary target curve 16 and an exemplary course of the instantaneous values 20 of the ventilation variable.
[0078] In FIG. 3A, the target curve 16 for the ventilation pressure 28 and the course of the instantaneous values 20 for the ventilation pressure 28 are plotted over time. In the illustrative example of FIG. 3A, the target curve 16 makes a positive pressure jump at the time to. The positive pressure jump at the time to is suitable for illustrating the behavior of the controller 10 at the beginning of an inspiration phase. It is apparent to those skilled in the art that the target curve 16 at the beginning of the inspiration phase may have a different increase than the hard jump shown in FIG. 3A.
[0079] The controller 10 responds to the pressure jump of the target curve 16 at the time to and acts to track the ventilation pressure 28 so as to follow the target curve 16. FIG. 3A shows an example of a so-called step response of the controller 10. As illustrated in FIG. 3A, the instantaneous values 20 for the ventilation pressure 28 increase only slowly in the example of FIG. 3A, as compared to the step increase of the target curve 16. The controller 10 can track the ventilation pressure 28 so as to follow the target curve 16 only slowly. In FIG. 3A, the integral 30 of the control difference 24, i.e. the integral 30 of the difference between target curve 16 and instantaneous values 20 of the ventilation pressure 28 is illustrated as a hatched area. By the time t1, when the instantaneous value 20 of the ventilation pressure reaches the target curve 16 for the first time, the integral 30 of the control difference 24 has reached a comparatively high value.
[0080] In FIG. 3B, the integral 30 of the control difference 24, which is illustrated as a hatched area in FIG. 3A, is plotted as a curve versus time. At the time to, the integral 30 is zero, and at the time t1, the integral 30 is reset to zero. In operation, the integral 30 of the control difference 24 is compared to an integral threshold value 32. Between t0 and t1, there is a time t* at which the integral 30 of the control difference 24 exceeds the integral threshold value 32.
[0081] As a result of exceeding the integral threshold value 32, the controller 10 increases the gain factor K.sub.I of the integral term. In this way, the controller 10 responds more strongly to a cumulative control difference starting from the time t*. This, in turn, enables faster tracking of the ventilation pressure 28 along the target curve 16 for the future.
[0082] FIG. 4 shows a further exemplary course of the instantaneous values 20 of a ventilation variable for a sudden increase in the target curve 16 of the ventilation variable, as well as an associated course of the control difference. FIG. 4 illustrates the operation of the system 2 of FIG. 2 for an exemplary target curve 16 and a further exemplary course of the instantaneous values 20 of the ventilation variable.
[0083] In FIG. 4A, the target curve 16 for the ventilation pressure 28 and the course of the instantaneous values 20 for the ventilation pressure 28 are plotted over time. In the illustrative example of FIG. 4A, the target curve 16 makes a positive pressure jump at the time to. Again, the positive pressure jump at the time to is suitable for illustrating the behavior of the controller 10 at the beginning of an inspiration phase. It is apparent to those skilled in the art that the target curve 16 at the beginning of the inspiration phase may have a different increase than the hard jump shown in FIG. 4A.
[0084] The controller 10 responds to the pressure jump of the target curve 16 at the time to and acts to track the ventilation pressure 28 so as to follow the target curve 16. FIG. 4A shows another example of a so-called step response of the controller 10. In contrast to the situation illustrated in FIG. 3A, the controller 10 in the situation of FIG. 4A is able to track the ventilation pressure 28 so as to follow the target curve 16 much faster. The instantaneous value 20 of the ventilation pressure 28 reaches the target curve 16 already shortly after the time to. However, the increase of the ventilation pressure 28 is so fast that the ventilation pressure 28 overshoots the target curve 16. The course of the instantaneous values 20 of the ventilation pressure 28 crosses the target curve very steeply and overshoots the target curve 16 significantly. The controller 10 reacts to the overshooting of the target curve 16 and acts to reduce the ventilation pressure 28. As a result, the course of the instantaneous values 20 of the ventilation pressure 28 overshoots the target curve 16 downward. The attempt of the controller 10 to control the ventilation pressure 28 so as to follow the target curve 16 results in a rather high-frequency oscillation of the course of the instantaneous values 20 of the ventilation pressure 28 around the target curve 16.
[0085] FIG. 4B shows the control difference 24, i.e. the difference between target curve 16 and instantaneous values 20 of the ventilation pressure 28, versus time. Furthermore, FIG. 4B shows a tolerance range 40 for the control difference 24 around the value 0. In the exemplary embodiment of FIG. 4, a zero crossing of the control difference 24 is defined as starting from or crossing the value zero and then leaving the tolerance range 40. Based on this definition, the exemplary control difference 24 of FIG. 4 involves six zero crossings 41, 42, 43, 44, 45, and 46.
[0086] In the exemplary embodiment of FIG. 4, a zero-crossing threshold value of three is provided. At the time t*, as illustrated in FIG. 4A, the control difference 42 passes through the fourth zero crossing 44 and has thus exceeded the zero-crossing threshold value of three. It is also possible that the concept of exceeding the zero-crossing threshold value includes reaching the zero-crossing threshold value. In this case, the time t* would already be at the time of the third zero crossing 43.
[0087] As a consequence of exceeding the zero-crossing threshold value, the controller 10 reduces the gain factor K.sub.I of the integral term. In this manner, the controller 10 responds less strongly to a cumulative control difference starting from the time t*. This, in turn, allows slower tracking of the ventilation pressure 28 along the target curve 16 for the future, and thus less overshooting and oscillation around the target curve 16.
[0088] FIG. 5 shows a further exemplary course of the instantaneous values 20 of a ventilation variable for a sudden increase in the target curve 16 of the ventilation variable as well as an associated course of the control difference. FIG. 5 illustrates the operation of the system 2 of FIG. 2 for an exemplary target curve 16 and a further exemplary course of the instantaneous values 20 of the ventilation variable.
[0089] FIG. 5 is identical to FIG. 4 except that a safety threshold value 48 is provided in addition to the target curve 16 and in addition to the course of the instantaneous values 20 of the ventilation variable. In the exemplary embodiment of FIG. 5, the safety threshold value 48 is defined as an incremental value with respect to the target curve 16. The safety threshold value may also be defined as an absolute value or in any other suitable manner. The controller 10 is adapted to reduce the gain factor K.sub.I of the integral term when the course of the instantaneous values 20 exceeds the safety threshold value 48, i.e. when the course of the instantaneous values 20 exceeds the target curve 16 by more than the safety threshold value 48. In this way, overshooting of the instantaneous values 20 of the ventilation pressure above the target curve 16 can be reduced and reaching of potentially dangerous values for the ventilation pressure can be rendered more difficult or prevented.
[0090] In the exemplary course of the instantaneous values 20 of FIG. 5, the safety threshold value 48 is exceeded twice. Consequently, the gain factor K.sub.I of the integral term is reduced twice as a result of the safety threshold value 48 being exceeded and once as a result of the zero-crossing threshold value being exceeded. Thus, a total of three reductions of the gain factor K.sub.I of the integral term take place.
[0091] FIG. 6 shows a further exemplary course of the instantaneous values 20 of a ventilation variable for a further exemplary target curve 16 of the ventilation variable. FIG. 6 shows the course of the instantaneous values 20 as well as the target curve 16 for a plurality of ventilation cycles and shows the associated adjustment of the gain factor of the integral term of the controller. FIG. 6 illustrates the operation of the system 2 of FIG. 2 for another exemplary target curve 16 and another exemplary course of the instantaneous values 20 of the ventilation variable.
[0092] FIG. 6A shows five ventilation cycles 51, 52, 53, 54, and 55, each of which has an inspiration phase and an expiration phase. The target curve 16 represents a good approximation to a target curve as it is used in the actual ventilation of a patient. In the inspiration phase, the target ventilation pressure first rises sharply and then remains at a plateau. In the expiration phase, the target ventilation pressure first drops sharply and then remains at the so-called positive end-expiratory pressure (PEEP). The positive end-expiratory pressure remains applied to the patient to prevent collapse of the lungs or individual alveoli of the lungs.
[0093] In the exemplary embodiment of FIG. 6, the controller 10 operates to track the instantaneous values 20 of the ventilation pressure 28 so as to follow the target curve 16 in the inspiration phase as closely as possible. In the expiration phase, a passive release of the ventilation pressure 28 takes place first, i.e. the controller 10 does not support the reduction of the ventilation pressure 28 in an active manner. The course of the instantaneous values 20 of the ventilation pressure 28 decreases more slowly than the target curve 16. In the further course of the expiration phase, the controller 10 acts to keep the ventilation pressure 28 at the positive end-expiratory pressure.
[0094] FIG. 6B illustrates the gain factor of the integral term of the controller 10 over time. The gain factor is plotted in FIG. 6B as a quantity normalized to the initial value of the gain factor. In other words, FIG. 6B shows along the y axis the dimensionless value for the quotient of instantaneous gain factor of the integral term to initial value of the gain factor of the integral term, i.e. the dimensionless value for the quotient K.sub.I/K.sub.I,init.
[0095] In the inspiration phase of the first ventilation cycle 51, there is present a relatively high value for the integral of the control difference 24. This is evident from the relatively large area between the target curve 16 and instantaneous values 20 for the ventilation pressure 28. In the exemplary embodiment of FIG. 6, the gain factor of the integral term is increased twice due to this relatively high value for the integral of the control difference 24. This two-fold increase in the gain factor of the integral term corresponds to two integral threshold values being exceeded. That is, the exemplary embodiment of FIG. 6 has a plurality of integral threshold values, and when a respective one of the plurality of integral threshold values is exceeded, there is taking place an increase in the gain factor of the integral term. Each increase occurs by 0.3 on the normalized scale of FIG. 6B.
[0096] In the inspiration phase of the second ventilation cycle 52, the controller 10 is able to better track the instantaneous values 20 of the ventilation pressure 28 to follow the target curve 16. However, the integral of the control difference 24 also has a significant value in the inspiration phase of the second ventilation cycle 52. This value is above the lowest integral threshold value, so that in the inspiration phase of the second ventilation cycle 52 there is taking place a further increase of the gain factor of the integral term. This increase also occurs by 0.3 on the normalized scale of FIG. 6B.
[0097] In the expiration phase of the second ventilation cycle 52, the course of the instantaneous values 20 of the ventilation pressure 28 oscillates around the target curve 16, in particular around the positive end-expiratory pressure. There is a number of zero crossings occurring that is greater than the predetermined zero-crossing threshold value. In the exemplary embodiment of FIG. 6, a zero-crossing is defined similarly to FIG. 4B and FIG. 5B, namely as an overshoot over a predetermined tolerance range about the positive end-expiratory pressure. It is evident from FIG. 6A that the instantaneous values 20 of the ventilation pressure 28 oscillate comparatively strongly about the target curve 16 during the expiration phase of the second inspiration cycle 52. As a result, the predetermined zero-crossing threshold value is exceeded. In the exemplary embodiment of FIG. 6, the gain factor of the integral term is reduced due to the predetermined zero-crossing threshold value being exceeded. The reduction is by 0.15 on the normalized scale of FIG. 6B.
[0098] In the inspiration phase of the third ventilation cycle 53, the integral of the control difference is so small that no adjustment is made to the gain factor of the integral term. In the expiration phase of the third ventilation cycle 53, the course of the instantaneous values 20 of the ventilation pressure 28 again oscillates around the target curve 16 such that the number of zero crossings exceeds the predetermined zero-crossing threshold value. Again, the gain factor of the integral term is reduced by 0.15 on the normalized scale of FIG. 6B.
[0099] In the fourth ventilation cycle 54 and the fifth ventilation cycle 55, the controller 10 tracks the ventilation pressure to follow the target curve 16 so well that no further adjustment is made to the gain factor of the integral term. The controller has adapted itself so well to the conditions of the present ventilation situation over the course of the first through third ventilation cycles 51 to 53 that an effective tracking of the ventilation pressure 28 along the target curve is possible.
[0100] Although the invention has been described with reference to exemplary embodiments, it is apparent to one skilled in the art that various modifications may be made and equivalents used without departing from the scope of the invention. The invention is not intended to be limited by the specific embodiments described. Rather, it includes all embodiments covered by the appended claims.
