PROCESS FOR CONTROLLING A GAS CONCENTRATION IN A VENTILATION SYSTEM AND CORRESPONDING VENTILATION SYSTEM

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 of German Application 10 2024 131 496.6, filed Oct. 29, 2024, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a process for controlling a gas concentration in a ventilation system and a corresponding ventilation system.

BACKGROUND

It is known to control a gas concentration, particularly a patient-related gas concentration, in ventilation systems, including anesthesia ventilation systems (anesthesia systems for short), using closed-loop control rather than simply open-loop control, in order to improve compliance with desired target values (setpoints) for the gas concentration. Controlling the gas concentration requires feedback of measured values of the measured gas concentration in order to determine a control deviation and reduce it by appropriately controlling the actuator.

DE 10 2019 002 273 A1 discloses a control system for controlling fresh gas dosing for an anesthesia machine as an example of a ventilation system. The anesthesia machine has a patient gas measuring unit which is arranged in a breathing gas circuit of the anesthesia machine and which is configured to determine an end-tidal anesthetic concentration in the breathing gas circuit and to output a corresponding measurement signal at regular intervals. The anesthesia machine also has a fresh gas control unit that is set up to control a volume flow of continuously supplied fresh gas over time according to a predetermined volume flow curve. If the current end-tidal anesthetic concentration leaves a permissible range, the predetermined volume flow curve is adjusted in response to this in order to change the fresh gas flow so that the end-tidal anesthetic concentration returns to the target value.

In general ventilation systems or in the anesthesia machine according to DE 10 2019 002 273 A1, situations may arise in which the measured gas concentration is not available in a form that allows reliable control. For example, zeroing or calibrating a corresponding gas concentration sensor (patient gas measuring unit) or a pressure measurement maneuver to check for leaks in a measuring line of the gas concentration sensor may result in no suitable measured values for the gas concentration being available for a limited period of time. In such cases, the control unit regulating the gas concentration would have to be switched to safe operation with high gas consumption, which would result in high consumption of the corresponding gases, e.g., oxygen and anesthetics.

One way of dealing with situations in which the measured gas concentration is not available in a form that allows reliable control is known from the ZEUS anesthesia machine manufactured by Drägerwerk AG & Co. KGaA. Here, a second patient gas module is provided that is redundant for the first patient gas module used for control. However, providing a redundant additional patient gas module increases the cost of the anesthesia machine.

SUMMARY

It is therefore an object of the present invention to provide an alternative process for controlling a gas concentration in a ventilation system and a corresponding ventilation system which do not have the aforementioned disadvantages or have them only to a reduced extent.

These and other objects are attained by features of the process and features of the ventilation system according to the invention. The claims, the description, and the figures provide advantageous embodiments of the invention.

According to the invention, a process for controlling a gas concentration in a ventilation system is provided, wherein the process comprises the steps of: determining a first threshold (limit value) for a manipulated variable (controlled variable) depending on a set target value (an acquired setpoint value) and limiting the manipulated variable to the first threshold in a first control mode; determining that the set target value has been changed to a new set target value; determining a new first threshold for the manipulated variable depending on the new set target value; switching from the first control mode to a second control mode when it has been determined that the set target value has been changed to the new set target value; determining a second threshold for the manipulated variable and limiting the manipulated variable to the second threshold in the second control mode, wherein an extreme value of the second threshold is different from the first threshold; determining a permissible phase duration for the second control mode; switching from the second control mode to a third control mode when a duration of the second control mode has reached the permissible phase duration; and limiting the manipulated variable to the new first threshold in the third control mode.

According to the invention, the control is thus divided into three control modes depending on the gas concentration target value adjustment, in each of which thresholds for the manipulated variable of the control are determined. In other words, the control process according to the invention comprises a manipulated variable threshold (limitation) adapted to the respective state of the ventilation system. This principle can be applied to both inspiratory and expiratory target control values. The first control mode corresponds to a steady state with a set target value and a corresponding first threshold for the manipulated variable. The second control mode corresponds to a transient state with a new set target value and a corresponding new first threshold for the manipulated variable. In this second mode, a transition from the set target value to the new set target value is to be achieved. The third control mode again corresponds to a steady state with the new set target value and the new first threshold for the manipulated variable. The thresholds can be selected so that, after a target value adjustment, increased dynamics in the manipulated variable adjustment are permitted for a limited time (namely for the permissible phase duration) in order to quickly adjust to the desired target value and thus quickly reach the corresponding gas concentration.

The gas concentration can be, for example, an anesthetic gas concentration and/or oxygen concentration.

The ventilation system may, for example, comprise a ventilator or anesthesia machine and one or more ventilation tubes (lines) connected thereto. The one or more ventilation tubes may comprise an inspiratory ventilation tube, an expiratory ventilation tube, and/or a Y-piece.

The process according to the invention may be configured in particular to control the gas concentration in an inspiratory line of a ventilator or anesthesia machine and/or in the inspiratory ventilation tube and/or in an expiratory line of the ventilator or anesthesia machine and/or in the expiratory ventilation tube and/or in or on the Y-piece. The control may be performed on a per-breath basis or on a non-per-breath basis. The control may be performed with an inspiratory oxygen concentration (FiO2) as the manipulated variable and/or with an expiratory oxygen concentration (etO2) as the manipulated variable and/or with an inspiratory anesthetic gas concentration (FiAA) as the manipulated variable and/or with an expiratory anesthetic gas concentration (etAA) as the manipulated variable and/or with an alveolar concentration of the anesthetic agent (MAC—minimum alveolar concentration) as the manipulated variable.

In the context of the invention, control refers to control by means of a closed control loop.

The target value for the gas concentration can, for example, be set by a user of the ventilation system and thus be obtained (acquired by a control unit of the ventilation system) as a set target value. In addition or alternatively, the set target value for the gas concentration can, for example, be automatically determined and set by the control unit of the ventilation system, for example, depending on an operating mode of the ventilation system, and thus be obtained as a set target value. The information about the target value for the gas concentration can be obtained in real time or not in real time.

If the target value for the gas concentration can be set by the user of the ventilation system, the ventilation system—as a component of the anesthesia machine or ventilator or separately from them—preferably also has a user interface, wherein the user interface can be configured as an element of a graphical user interface.

The first and any other thresholds may, within the scope of the invention, be a lower threshold and/or an upper threshold, i.e., a minimum value and/or a maximum value for the permissible set target value. The first threshold may therefore be a first lower threshold and/or a first upper threshold. It is preferred that, if the gas concentration being controlled is the anesthetic gas concentration, a first lower threshold and a first upper threshold are determined as the first thresholds. It is preferred that, if the gas concentration being controlled is the oxygen concentration, at least a first lower threshold is determined as the first threshold.

The manipulated variable for the gas concentration can be, for example, an opening state of a valve for supplying the corresponding gas or gas mixture, a delivery quantity of anesthetic agent, and/or a mixture of fresh gas provided by the anesthesia machine.

The second threshold (and the new second threshold) can be variable as a function of time or constant over time. The second threshold can be variable in sections over time and constant in sections over time.

Preferably, the process according to the invention is performed continuously and repeatedly.

The anesthesia machine or ventilator of the ventilation system may include a fresh gas mixer for mixing gases, such as oxygen and air, as well as any other gases, such as nitrous oxide, and for supplying this gas mixture as fresh gas. If the ventilation system comprises an anesthesia machine, the anesthesia machine may comprise an anesthetic dispenser device for dosing a volatile anesthetic as an anesthetic gas. The ventilation system may comprise a breathing system. The breathing system is understood to be an assembly of the anesthesia machine or the ventilator, which comprises the components relevant for ventilating a patient. The breathing system thus serves to receive the fresh gas and/or the anesthetic gas and to ventilate the patient with these gases, possibly mixed with other gases, via the inspiratory gas connection. An anesthetic gas dispenser is a device for supplying anesthetic gas. This can be, for example, an anesthetic gas vaporizer or an anesthetic gas evaporator. The anesthetic gas dispenser can be configured as an electronic anesthetic gas dispenser. The anesthetic dispenser and/or the anesthesia machine may have one or more control units for controlling some or all of the components of the anesthesia system. The or each control unit may be set up to receive and process signals from some or all of the components of the anesthesia system. The or each control unit may be implemented in whole or in part as a hardware circuit, which may comprise, for example, gate arrays, commercially available semiconductors such as logic chips, transistors, or other discrete components. The or each control unit may also be implemented in programmable hardware components such as field-programmable gate arrays, programmable array logic, programmable logic components, or the like. The or each control unit may also be implemented in software for execution by various types of processors and may comprise, for example, one or more physical or logical modules of computer instructions that may be organized, for example, as an object, procedure, or function. The or each control unit may, for example, be configured as a computer, processor, microprocessor, (field) programmable logic array ((F)PLA=(Field) Programmable Logic Array), (field) programmable gate array ((F)PGA=(Field) Programmable Gate Array), digital signal processor hardware (DSP hardware; DSP=Digital Signal Processor), application-specific integrated circuit (ASIC=Application Specific Integrated Circuit), and/or field-programmable logic array (FPGA=Field Programmable Gate Array).

Preferably, the process further comprises the steps of: determining, in the second control mode, that the new set target value has been changed to a further new set target value; determining a further new first threshold for the manipulated variable depending on the further new set target value as a new first threshold; determining a new second threshold for the manipulated variable and limiting the manipulated variable to the new second threshold in the second control mode if it has been determined in the second control mode that the new set target value has been changed to a further new set target value; and/or determining a new permissible phase duration as the permissible phase duration for the second control mode if it has been determined in the second control mode that the new set target value has been changed to a further new set target value.

In this way, the process can be extended so that changes to the set target value occurring in the second control mode (namely, a change of the new set target value to a further new set target value) can be responded to by corresponding adjustment of the threshold (namely by determining a new first threshold and a new second threshold) and/or by adjusting the phase duration for the second control mode, so that the process can be flexibly adapted to the behavior of the ventilation system.

An example of a change in the set target value in the second control mode is when the user of the ventilation system sets a new target gas concentration.

The new permissible phase duration may, for example, be shorter or longer than the permissible phase duration. In some cases, however, the new permissible phase duration may also be exactly the same length as the permissible phase duration.

Preferably, the first threshold and, optionally, the new first threshold and, optionally, the further new first threshold are determined according to a first calculation rule, whereby the second threshold and, optionally, the new second threshold are determined according to a second calculation rule that differs from the first calculation rule.

The first calculation rule may, for example, have as parameters the set target value, a measured or calculated respiratory minute volume, and a measured or calculated fresh gas volume flow. In other words, the first calculation rule may be dependent on the set target value, the measured or calculated respiratory minute volume, and the measured or calculated fresh gas volume flow. Optionally, the first calculation rule may also include a safety factor and/or a safety constant in order to take into account possible tolerances, for example, dosing tolerances of the gas-dispensing actuator or, for example, measurement accuracies of sensors. The safety factor and/or safety constant may take into account further and/or other properties of the system, for example, patient-side physiological uptake when controlling an end-tidal gas concentration.

Preferably, the second threshold and the optional new second threshold are each configured as a predetermined function of time.

In this way, the dynamic behavior of the control in the second control mode can be specifically adjusted by selecting a suitable predetermined function of time.

Preferably, the respective predetermined function has a linear range and/or a non-linear range. Most preferably, the respective predetermined function has a linear range and a non-linear range that follows it (the linear range) in time.

In this way, a second threshold with a large absolute value can initially be permitted in the second control mode in order to bring about a rapid adjustment of the gas concentration. Later in the second control mode, the second threshold can be approximated to the new first threshold in accordance with a predetermined curve in order to bring about a new steady state.

Preferably, the non-linear range flattens out and approaches the new first threshold or the further new first threshold.

This allows a smooth transition from the second control mode to the third control mode.

It is particularly preferred that the non-linear range is configured as an exponential curve of the second threshold (or the new second threshold).

The steps of the process do not have to be performed in the order specified in the claims. Some or all of the steps of the process can be performed in parallel or quasi-parallel.

The invention also provides a ventilation system, the ventilation system comprising a control unit which is configured to carry out some or all of the steps of the aforementioned process.

All features disclosed in connection with the process are also considered to be disclosed in connection with the ventilation system and vice versa.

These and other features and advantages of the invention are also apparent from the following description of the figures. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic flow chart of an embodiment of a process according to the invention;

FIG. 2 is a schematic view showing an example of a ventilation system according to the invention;

FIG. 3a is a schematic diagram of set target values over time;

FIG. 3b is a schematic diagram of certain thresholds over time based on the target values according to FIG. 3a;

FIG. 4a is a schematic diagram of set target values over time; and

FIG. 4b is a schematic diagram of certain thresholds over time based on the target values according to FIG. 4a.

DESCRIPTION OF PREFERRED EMBODIMENTS

According to the invention, a process 100 for controlling a gas concentration in a ventilation system 200 is provided. An embodiment of a process 100 according to the invention is shown as a schematic flowchart in FIG. 1.

The invention also provides a corresponding ventilation system 200. An embodiment of a ventilation system 200 according to the invention is shown schematically in FIG. 2.

The process 100 comprises at least steps S1 to S8.

• • Step S1 is determining a first threshold G1 for a manipulated variable S depending on a set target value C and limiting the manipulated variable S to the first threshold G1 in a first control mode.
  • Step S2 is determining that the set target value C has been changed to a new set target value Cnew, e.g., because a user sets the new set target value Cnew.
  • Step S3 is to switch from the first control mode to a second control mode when it has been determined that the set target value C has been changed to the new set target value Cnew.
  • Step S4 is to determine a new first threshold G1new for the manipulated variable S depending on the new set target value Cnew.
  • Step S5 is to determine a second threshold G2a, G2b, G2c for the manipulated variable S and to limit the manipulated variable S to the second threshold G2b, G2c, G2d in the second control mode. An extreme value G2amin, G2bmin, G2cmin of the second threshold G2a, G2b, G2c is different from the first threshold G1. For example, an extreme value of the upper threshold may be greater than the first threshold G1. For example, an extreme value of a lower threshold G2amin, G2bmin, G2cmin may be smaller than the first threshold G1.
  • Step S6 is the determination of a permissible phase duration Tphas for the second control mode. The permissible phase duration TPhas can be divided into different sections and, for example, comprise a first section (bolus phase Tbol) and a second section (transient phase Tphas).
  • Step S7 is to switch from the second control mode to a third control mode when a duration of the second control mode has reached the permissible phase duration Tphas.
  • Step S8 is to limit the manipulated variable S to the new first threshold G1new in the third control mode.

In all embodiments, it is possible that the first threshold G1 is determined according to a first calculation rule, whereby the second thresholds G2a, G2b, G2c, G2d are determined according to a second calculation rule that differs from the first calculation rule.

The first threshold G1 can, for example, be a first upper threshold G1max, and the first calculation rule can be:

G ⁢ 1 = G ⁢ 1 ⁢ max = C + C · K ⁢ 1 · A ⁢ M ⁢ V V ⁢ F ⁢ G

The first threshold G1 can, for example, be a first lower threshold G1min, and the first calculation rule can be:

G ⁢ 1 = G ⁢ 1 ⁢ min = C - C · K ⁢ 2 · A ⁢ M ⁢ V V ⁢ F ⁢ G

AMV stands for airway minute volume (unit of volume per time) and VFG stands for fresh gas flow rate (unit of volume per time). K1 and K2 are predetermined constants used to influence the behavior of the control system. The two examples for the first calculation rules may contain additional and/or different terms. The new first threshold G1new can be calculated according to the same calculation rule or according to the same calculation rules.

In all embodiments, it is possible that the second thresholds G2a, G2b, G2c, G2d, and an optional new second threshold G2d* are each configured as a predetermined function of time, wherein the respective predetermined function may each have a linear range and/or a non-linear range, wherein the respective non-linear range flattens out at the new first threshold G1new (in the case of the second threshold G2a, G2b, G2c, G2d) or the further new first threshold G1new* (in the case of the new second threshold G2d*).

Examples of possible courses of the second threshold G2a, G2b, G2c, G2d as a function of time are shown in FIGS. 3a and 3b. FIG. 3a shows a schematic diagram of set target values C, Cnew over time t. FIG. 3b shows a schematic diagram of the correspondingly determined thresholds G2a, G2b, G2c over time t based on the set target values according to FIG. 3a.

As shown in FIG. 3a, in the example shown, the target value C is set as the target value in the time range t<t1. At time t1, a new set target value Cnew is set, for example by a user of the ventilation system.

FIG. 3b shows that, correspondingly, in the time range t<t1, the first control mode is present, in which a first threshold G1 is determined, which acts to limit the manipulated variable S. Setting a new set target value Cnew initiates the second control mode, which in particular comprises determining a new first threshold G1new, which is to be reached after the permissible phase duration Tphas has elapsed, and which further comprises determining the second thresholds G2a, G2b, G2c. Three alternatives for possible time courses of the second threshold G2a, G2b, G2c are shown.

In the first example for the second threshold G2a, the phase duration Tphas of the second control mode is divided into two sections: a first section or a first phase (bolus phase) Tbol and a second section or a second phase (transient phase). In the first phase, the threshold G2a approaches the extreme value G2amin exponentially. In the second phase, the threshold G2a approaches the new first threshold G1new exponentially.

In the second example for the second threshold G2b, the phase duration Tphas of the second control mode is divided into two sections: a first phase (bolus phase) Tbol and a second phase (transient phase). In the first phase, the threshold G2b corresponds to a linear time curve, as it constantly corresponds to the extreme value G2bmin. In the second phase, the threshold G2b approaches the new first threshold G1new exponentially. This configuration of the second threshold G2b is preferred.

In the third example for the second threshold G2c, the threshold G2c corresponds to a linear time curve, as it constantly corresponds to the extreme value G2cmin.

The second calculation rule for the second thresholds G2a, G2b, G2c can, for example, be as follows when the set target value is reduced:

G ⁢ 2 = { G ⁢ 2 ⁢ bmin , t ⁢ 1 < t < t ⁢ 1 + Tbol G ⁢ 1 ⁢ new ⁢ ( 1 + exp ⁢ ( - K ⁢ 3 · t Ttra ) , t ⁢ 1 + Tbol < t < t ⁢ 1 + Tphas ,

where K3 is a constant for adjusting the curve of the exponential function.

The second calculation rule for the second threshold G2a, G2b, G2c can, for example, be as follows when the set target value is increased:

G ⁢ 2 = 
 { G ⁢ 2 ⁢ bmax , t ⁢ 1 < t < t ⁢ 1 + Tbol G ⁢ 1 ⁢ new ⁢ ( G ⁢ 2 ⁢ bmax - G ⁢ 1 ⁢ new ) · exp ⁢ ( - K ⁢ 4 · t Ttra ) , t ⁢ 1 + Tbol < t < t ⁢ 1 + Tphas

where K4 is a constant for adjusting the curve of the exponential function.

The process 100 may further optionally comprise steps S9 to S12.

Step S9 is determining, in the second control mode, that the new set target value Cnew has been changed to a further new set target value Cnew*, e.g., because the user sets a new gas concentration in the bolus phase or in the transient phase.

Step S10 is determining a further new first threshold G1new* for the manipulated variable S depending on the further new set target value Cnew* as the new first threshold G1new.

Step S11 is to determine a new second threshold G2d* for the manipulated variable S and to limit the manipulated variable S to the new second threshold G2d* in the second control mode if it has been determined in the second control mode that the new set target value Cnew has been changed to the further new set target value Cnew*. Alternatively, or in addition to step S11, the process may include step S12.

Step S12 is to determine a new permissible phase duration Tphas* (possibly comprising a new bolus phase Tbol* and a new transient phase Ttra*) as the permissible phase duration Tphas* for the second control mode if it has been determined in the second control mode that the new set target value Cnew has been changed to the further new set target value Cnew*.

FIGS. 4a and 4b show the behavior of the ventilation system with a corresponding modification of the aforementioned process 100 with the additional steps S9, S10, S11, and S12.

FIG. 4a shows a schematic diagram of set target values C, Cnew, Cnew* over time t. FIG. 4b shows a schematic diagram of the correspondingly determined thresholds G2d, G2d* over time t based on the target values according to FIG. 4a.

As shown in FIG. 4a, in the example shown, in the time range t<t1, the target value C is set as the target value. At time t1, a new set target value Cnew is set, for example by a user of the ventilation system 200. At time t3, a further new set target value Cnew* is set, for example by the user of the ventilation system 200.

FIG. 4b shows that, accordingly, in the time range t<t1, the first control mode is present, in which a first threshold G1 is determined, which acts to limit the manipulated variable S. Setting the new set target value Cnew initiates the second control mode, which in particular comprises determining a new first threshold G1new, which is to be reached after the permissible phase duration Tphas has elapsed, and which further comprises determining the second threshold G2d. However, before the permissible phase duration Tphas is reached, a further new set target value Cnew* is set at time t3, so that the additional process steps S9-S12 are performed. First, a new second threshold G2d* with a new second extreme value G2dmax* is determined for the new second threshold G2d*. Furthermore, a new permissible phase duration Tphas* is determined. For example, in this context, a timer for the second control mode can be reset and the phase duration measured again, in which case the new permissible phase duration is determined as Tphas*. Alternatively, the timer can continue to run so that the new permissible phase duration can be determined as Tphas*+(t3−t1). In any case, as already described, the new permissible phase duration Tphas* can also comprise a new first phase (bolus phase) with the duration Tbol* and a new second phase (transient phase) with the duration Ttra*. The time course of the new second threshold G2d* can correspond qualitatively to the time course of the previously described second thresholds G2a, G2b, G2c, and G2, and thus also be configured to be linear and/or non-linear in time. It is preferred and shown that the new second threshold G2d* is constantly the extreme value G2dmax* in the first phase and approaches the further new first threshold G1new* exponentially in the second phase, flattening out. However, it is not necessary to divide the new permissible phase duration Tphas* into two sections (phases). Depending on the change in the set target value C and the time of the change during the (original) permissible phase duration Tphas, it can also be decided to omit one of the phases depending on the situation.

The procedure described above can be performed in part or in full by a control unit 210 of the ventilation system 200 shown in FIG. 2. Solid lines between components represent fluid connections such as pipes and/or hoses. Dotted lines between components represent signal connections such as data lines and/or wireless connections. Arrows on lines indicate a possible flow direction of gases or signals.

The ventilation system 200 shown in FIG. 2 can be configured as an anesthesia system as shown, and for this purpose includes the control unit 210, a fresh gas mixer 220 for mixing gases such as air and oxygen (O2) and providing this mixture as fresh gas FG, an anesthetic dispenser device 230 for receiving the fresh gas FG and supplying an anesthetic gas or supplying a mixture of fresh gas FG and anesthetic gas, a breathing system 240 for ventilating a patient P with the fresh gas FG and/or with oxygen and/or with the anesthetic gas, a gas concentration sensor 250 for measuring the gas concentration relevant for control and providing the measured gas concentration M to the control unit 210, an inspiratory ventilation tube 261, an expiratory ventilation tube 262, and a Y-piece 263.

The control unit 210 can receive (acquire) the target value for the gas concentration C or the new set target value for the gas concentration Cnew or the further new set target value for the gas concentration Cnew*. The control unit 210 provides an output to the fresh gas mixer 220 and/or the anesthetic dispenser 230, as an actuator. This output is in accordance with the aforementioned manipulated variable S within the respective thresholds G1, G1new, G1new*, G2a, G2b, G2c, G2d determined in accordance with the process.

All features and embodiments of the invention disclosed herein can be combined with each other as desired, provided that this does not concern alternatives or is contradictory.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

LIST OF REFERENCE CHARACTERS

• • 100 Process
  • 20 Ventilation system
  • 210 Control unit (controller)
  • 220 Fresh gas mixer
  • 230 Anesthetic dispenser device
  • 240 Breathing system
  • 250 Gas concentration sensor
  • 261 Inspiratory ventilation tube
  • 262 Expiratory ventilation tube
  • 263 Y-piece
  • AMV Airway minute volume
  • C Target value for gas concentration
  • Cnew New set target value for gas concentration
  • Cnew* Additional new set target value for the gas concentration
  • FG Fresh gas
  • G1 First threshold
  • G1max First upper threshold
  • G1max First lower threshold
  • G1new New first threshold
  • G1new* Additional new first threshold
  • G2a, G2b, G2c, G2d Second threshold
  • G2d* New second threshold
  • G2amin, G2bmin, G2cmin Extreme value of the second threshold
  • G2dmax* Extreme value of the new second threshold
  • K1, K2, K3, K4 Constant
  • M Measured gas concentration
  • P Patient
  • S Manipulated variable
  • S1, S2, . . . Steps of the process
  • t Time
  • t First point in time
  • t2 Second point in time
  • t3 Third point in time
  • t4 Fourth point in time
  • Tphas Permissible phase duration
  • Tbol Bolus phase
  • Ttra Transient phase
  • Tphas* New permissible phase duration
  • Tbol* New bolus phase
  • Ttra* New transient phase
  • UI User interface
  • VFG Fresh gas volume flow