# Patent

## Title
Minute volume as a surrogate for EtCO2 in automatic ventilation

## Abstract

A method for automatically controlling ventilation of a patient includes receiving a target expiratory CO2 concentration, measuring an actual expiratory CO2, and comparing the actual expiratory CO2 concentration to the target expiratory CO2. A ventilation rate for the patient is then calculated based on the comparison of the actual expiratory CO2 concentration and the target expiratory CO2 in order to maintain the actual expiratory CO2 within a predetermined range of the target expiratory CO2. The patient is then automatically ventilated based on the calculated ventilation rate.

## Background
BACKGROUND
Current ventilation systems require control by a clinician, who inputs control values for the ventilator, including ventilation rate and gas values. For example, during surgery and in an intensive care unit a patient is ventilated mechanically by a mechanical ventilator that is controlled by a clinician. In such applications, end tidal CO2 (EtCO2) is often measured to evaluate ventilation adequacy and to supervise patient status. Changes in EtCO2 can be an indication of metabolic and/or hemodynamic changes in a patient, and thus EtCO2 is a valuable monitoring parameter to clinicians.
Currently available systems that require clinician control are prone to user error. Thus, it is desirable to create an automated ventilation system that eliminates the requirement of clinician control. If administered, properly, automatic ventilation control can eliminate user error and provide a safer ventilation control to a patient. However, choosing the right control variables and effectuating the automatic control algorithms is challenging, because the human respiratory system is a complicated system with many variables that must be accounted for.
SUMMARY
In one embodiment, a method for automatically controlling ventilation of a patient includes receiving a target expiratory CO2 concentration, measuring an actual expiratory CO2, and comparing the actual expiratory CO2 concentration to the target expiratory CO2. A ventilation rate for the patient is then calculated based on the comparison of the actual expiratory CO2 concentration and the target expiratory CO2 in order to maintain the actual expiratory CO2 within a predetermined range of the target expiratory CO2. The patient is then automatically ventilated based on the calculated ventilation rate.
Another embodiment of a method for automatically controlling ventilation of a patient includes receiving a target EtCO2 for the patient, receiving an alveolar minute volume value for the patient, measuring an expiratory CO2 in a gas expired from the patient, and then calculating an actual EtCO2. The actual EtCO2 is compared to the target EtCO2. If the actual EtCO2 is not within a predetermined range of the target EtCO2, then the alveolar minute volume value is adjusted and the patient is automatically ventilated using the adjusted alveolar minute volume value. A change in patient status is then indicated.
An embodiment for a system for automatically ventilating a patient includes a ventilator, a gas analyzer, a controller, and a display. The controller automatically controls the ventilator to ventilate the patient. The controller receives a target EtCO2 for the patient and an initial alveolar minute volume value for the patient. The controller also receives the actual EtCO2 from a gas analyzer. The controller then compares the actual EtCO2 to the target EtCO2. If the actual EtCO2 is not within a predetermined range of the target EtCO2, then the controller calculates an adjusted alveolar minute volume value and automatically ventilating the patient based on the adjusted alveolar minute volume value.



BRIEF DESCRIPTION OF THE DRAWINGS
The drawings illustrate the best mode presently contemplated of carrying out the disclosure. In the drawings:
FIG. 1 depicts one embodiment of an automatic ventilation system.
FIG. 2 depicts a graph comparing measured EtCO2 changes where MValv is held constant to measured changes in the MValv value when EtCO2 is automatically maintained at a constant level according to the methods and systems disclosed herein.
FIG. 3 depicts one embodiment of a method of automatically ventilating a patient.
FIG. 4 depicts another embodiment of a method of automatically ventilating a patient.
FIG. 5 depicts an exemplary output display presenting an MValv value and a target EtCO2 value for a patient over time.



DETAILED DESCRIPTION OF THE DRAWINGS
The present inventor has recognized that in automatic ventilation the expiratory CO2 concentration, such as EtCO2, can be used as a control parameter. This means that the controller may adjust ventilation automatically to maintain EtCO2 at a given target value. However, the inventor has recognized that automatically maintaining EtCO2 at a given target value via an automated ventilation system could be problematic because the system controller may not be capable of determining the cause of changes in EtCO2, as it cannot differentiate between all of the potential reasons that may lead to EtCO2 changes. Furthermore, since the controller would keep EtCO2 at a constant value by adjusting other ventilation parameters, a clinician would not see changes in a patient's EtCO2 value, and thus valuable information about a patient's physiological status may be lost. Adequate information must be provided to a clinician indicating a patient's metabolic and hemodynamic states. The present inventor has realized that ventilation rate can be used along side target expiratory CO2 concentration to indicate changes in a patient hemodynamic and metabolic status. In the present invention, expiratory CO2 concentration is kept constant with automatic control. In a preferred embodiment, EtCO2 is used as the parameter indicating expiratory CO2 concentration.
Automatically ventilating a patient by using EtCO2 as a control parameter differs from currently available or practiced ventilation techniques and systems. Traditionally, clinicians control ventilation by setting a ventilation rate on the ventilator which is kept constant throughout the ventilation process unless the clinician manually adjusts the ventilation rate. For example, minute volume (MV), including minute alveolar volume (MValv), is kept constant at a value set by a clinician. The clinician may change the input value, for example, upon seeing, a need to compensate for a change in a patient's EtCO2 value, which is allowed to fluctuate and is monitored as an output value indicating patient status. For example, in currently available ventilation control methods EtCO2 may vary as much as +/−10% from an optimum or anticipated value before a clinician manually adjusts ventilation parameters to compensate for an EtCO2 change. Thus, in current ventilation systems, ventilation rate is set by a user and changes in expiratory CO2 concentration, such as the EtCO2 value, indicate a change in the hemodynamic and/or metabolic, status of the patient.
The present inventor recognized that the challenges posed by deficiency of information when expiratory CO2 concentration is kept constant can be overcome by reporting changes in ventilation rate, such as changes in MValv, to indicate a change in patient hemodynamic and/or metabolic, status. By the present invention, patient hemodynamic and metabolic monitoring during ventilation where ventilation rate is automatically adjusted to minimize variation in measured patient expiratory CO2 concentration is conducted by adjusting the ventilation rate to maintain an approximately constant expiratory CO2 concentration and reporting the change in the ventilation rate to indicate a change in the patient hemodynamic and/or metabolic status. For example, in automatic ventilation control according to the present invention, a target EtCO2 value may be set by a clinician and changes in MValv may be reported to indicate a change in patient hemodynamic and/or metabolic status. In one embodiment of such a system, the target EtCO2 value automatically maintained by a ventilation control system is shown together in the same view as the calculated MValv value so that a clinician can monitor a patient's status. Because MValv has always been held constant in current and prior ventilation systems, it is not obvious that MValv can be presented as a patient parameter.
As shown in FIG. 1, an automatic ventilation system 1 comprises a ventilator 3 connected to a patient 9 through a breathing circuit 6. The breathing circuit may include an endotracheal tube or a mask to connect the breathing circuit 6 to the patient 9. A gas analyzer 7 is connected to the breathing circuit 6 such that it can analyze gas expired by the patient 9. The gas analyzer 7 may be, for example, a time capnogram. Time-based capnography, such as EtCO2 monitoring, is widely used in ventilation applications, including anesthesia and ICU applications. Time capnographs make use of main-stream sensors or side stream sensors. Volumetric capnography which provides direct measurement of VCO2, requires specialized equipment that is not widely available. Accordingly, one benefit of the presently disclosed method and system is that patient CO2 monitoring is provided using the widely available time-based capnography equipment, as MValv estimation does not require any extra hardware than that available on currently available ventilation systems. Volumetric capnography, on the other hand, requires a separate device that is often not available in many ventilation applications, such as in the operating room or in the ICU. Accordingly, the gas analyzer 7 indirectly reflects the production of CO2 by the patient's tissues and the patient's circulatory transport of CO2 to the lungs.
The ventilator 3 is controlled by controller 5, which may be any type of controller capable of automatically controlling the ventilator 3. For example, the controller may be control software integrated into the control module in an anesthesia machine. Alternatively, the controller may be control software stored and executed on a separate computing device, such as a laptop, used in conjunction with an anesthesia machine or other ventilator. It is to be recognized that the controller 5 may be any combination of software and hardware implemented to perform the methods disclosed here in, and may include one or more processors that are communicatively connected so as to cooperate in providing a control function. The controller 5 may further comprise a microprocessor and other circuitry that retrieves and executes software front a storage system. Examples of processors include general purpose central processing units, application specific processors, and logic devices, as well as any other type of processing device, combinations of processing devices, or variations thereof. The storage system can comprise any storage media readable by processing system, and capable of storing software for the execution of control algorithms. The storage system can include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information, such as computer readable instructions, data structures, program modules, or other data.
A user interface 11 is provided to allow an operator 17 to interact with and/or monitor the controller 5 and the ventilator 3. The user interface is connected to the ventilator 3 and/or the controller 5 via connection 12. Connection 12 may be any wired or wireless connection between the ventilator 3 and the user interface 11. For example, the connection 12 may utilize Wi-Fi, Bluetooth, or wireless medical telemetry service (WMTS) technology, or any other wireless technology available. In other embodiments, the connection 12 may be a physical wired data connection. The user interface 11 may have a display 13 and an input device 15. The user interface 11, display 13, and input device 15 may be any device or devices that allow an operator 17 to interface with the ventilator 1 and controller 5 to oversee and control the automatic ventilation of a patient. The user interface 11 may be integrated into an anesthesia cart or provided separate from an anesthesia cart. For example, the user interface 11 may be integrated into an anesthesia cart as a touch screen that acts as a display 13 to display monitoring data from the patient 9 and output data from the automatic ventilation system 1, and also to allow an operator 17 to input control commands to the system. In other embodiments, the user interface 11 may include a mouse, a keyboard, a voice input device, any touch input device for receiving a gesture from a user, a motion input device for detecting non-touch gestures and other motions by a user, and other comparable input devices and associated processing elements capable of receiving user input from a user. Output devices such as a video display or graphical display can display an interface further associated with embodiments of the system and method as disclosed herein. Speakers, printers, haptic devices and other types of output devices may also be included in the user interface 11.
Expired CO2 concentration can be related to the expired volume over time to yield CO2 elimination rate, which is an important measure of patient metabolism and hemodynamic status. For example, sudden changes in CO2 elimination during lung or heart surgery often imply important changes in cardiorespiratory function. For example, changes in a patient's EtCO2 value or MValv value would be used by clinicians to provide indication of a change in cardiorespiratory function. Similarly, when one of these parameters (EtCO2 or MValv) is maintained constant, changes in the other parameter may be used to detect lung conditions such as a pulmonary embolism, hemodynamic conditions such as a drop in cardiac output, or metabolic conditions such as metabolic hyperactivity.
The automatic ventilation system 1 includes controller 5 configured to keep the patient's expiratory CO2 concentration at a target value by controlling the ventilation rate. In the present disclosure, the CO2 concentration measurement is exemplified as EtCO2 and ventilation rate is exemplified as alveolar minute volume (MValv). However, it is contemplated that other parameters may be used to gauge patient ventilation rate, including minute volume. It is further contemplated that other parameters indicating expiratory CO2 concentrations may be used other than EtCO2.
Both MValv and EtCO2 measurements track carbon dioxide production (VCO2). The MValv value is an effective patient monitoring value because it reflects changes in patient status very well. MValv is the net effect of all of the ventilation rate settings and variables, including tidal volume, respiration rate, and dead space. Thus, MValv is a sensitive variable. Further, the inventor has recognized that MValv has more fluctuation and sensitivity than EtCO2 for the same patient situation, and thus may provide more immediate detection of changes in patient metabolic or hemodynamic status. MValv has high sensitivity because it is primarily a controlled variable.
MValv may be defined as MValv=MV−DS×RR wherein MV is minute volume. DS is estimated dead space for the patient, and RR is respiration rate. VCO2 can be depicted as a product of both MValv and EtCO2 where VCO2=EtCO2×MV. Thus, carbon dioxide production trends can be reported and visualized using MV values in place of EtCO2 values.
The technical feasibility of using MValv as a patient parameter and as a variable for controlling EtCO2, and thus VCO2, was tested and the results of the feasibility testing are exemplified in FIG. 2. FIG. 2 depicts a graph 20 comparing measured EtCO2 changes where MValv is held constant (line 24), as is done in current ventilation systems, and measurement of changes in the MValv value when EtCO2 is automatically maintained at a constant level (line 22) according to the methods and systems disclosed herein. In FIG. 2, the x axis 26 depicts time, while the y axis 27 and 28 depicts either the MValv value 27 corresponding to line 22 or the EtCO2 value 28 corresponding to line 24. Tests were performed with a physical patient simulator which was ventilated with an anesthesia machine and automatic ventilation control. The patient simulator CO2 production was changed to simulate a change in patient metabolism. FIG. 2 depicts the results, which show that a change in the MValv value effectuated by the automatic controller was reflected in the EtCO2 value (line 22). More specifically, a 20% change in MValv value (line 22) corresponded to approximately a 20% change in the EtCO2 value (line 24).
FIG. 3 depicts one embodiment of a method of automatic ventilation control 35. A target expiratory CO2 concentration is received at step 36. For example, the target expiratory CO2 concentration value may be set by an operator, such as a clinician. At step 38, CO2 levels are measured in a gas expired by a patient to determine an actual expiratory CO2 level. At step 40, the actual expiratory CO2 concentration is compared to the target CO2 concentration. Then, a ventilation rate may calculated at step 42 based on the comparison between the actual expiratory CO2 concentration and the target expiratory CO2 concentration. For example, if the actual expiratory CO2 concentration equals the target expiratory CO2 concentration, then the ventilation rate may be maintained at a current ventilation rate. Alternatively, the ventilation rate may be increased if the actual expiratory CO2 concentration is higher than the target expiratory CO2 concentration. At step 44, the system automatically ventilates the patient at the ventilation rate determined or calculated at step 42. Then, the ventilation rate calculated at step 42 is displayed on a display at step 46, along with the target expiratory CO2 concentration.
In other embodiments, minute volume or tidal volume may be adjusted in volume controlled ventilation in response to a determination that the actual expiratory CO2 concentration is not equal to the target expiratory CO2 concentration. In a pressure controlled ventilation system, minute volume or inspired pressure may be adjusted instead of or in addition to ventilation rate.
FIG. 4 demonstrates another embodiment of a method for automatically ventilating a patient 35. At step 51, a target EtCO2 value is received. For example, the target EtCO2 value may be set by an operator. At step 53, an MValv value is received, which is the alveolar minute volume currently delivered to the patient. An actual EtCO2 is determined at step 57 for the patient, such as based on a measurement of CO2 in gas expired by the patient. For example, the gas analyzer measures expiratory CO2 in a gas expired from the patient and then calculates an actual EtCO2 based on the expiratory CO2 measurement. At step 59, the actual EtCO2 is compared to the target EtCO2 to determine whether the actual EtCO2 is approximately equal to the target EtCO2. For example, the system may determine whether the actual EtCO2 is within a 0.01% predetermined range of the target EtCO2. In another embodiment, the system may determine whether the actual EtCO2 is within a 0.5% predetermined range of the target EtCO2. Preferably, the system determines that the actual EtCO2 is not approximately equal to the target EtCO2 if the difference between the actual and target EtCO2 values is greater than 0.5%. However, it is contemplated that the system may be more sensitive and require the actual EtCO2 to be closer than 0.5% to the target EtCO2 in order to be considered approximately equal. In another embodiment, the system may require the actual EtCO2 to be precisely equal to the target EtCO2 within the measurement sensitivity of the system. If the actual EtCO2 is equal to or approximately equal to the target EtCO2 at step 59, then no change is made to the patient ventilation at that time and the monitoring and control method restarts at step 53 where the current MValv value is received for the current patient ventilation.
If the actual EtCO2 is not equal to or approximately equal to the target EtCO2, then the system continues to step 61 where an adjusted MValv value is calculated. The adjusted MValv value may be calculated based on the difference between the actual EtCO2 value and the target EtCO2 value. The MValv value is adjusted with the goal of bringing the actual EtCO2 value within the predetermined range of the target EtCO2 value. Then, at step 63, the patient ventilation is automatically adjusted to comport with the adjusted MValv value calculated at step 61. For example, the MValv value is translated into ventilation parameters for the patient, such as respiration rate or respiration volume, and then the patient ventilation is changed to reflect the adjusted ventilation parameters. Once the ventilation parameters are automatically adjusted at step 63, the adjusted MValv values are displayed at step 67, such as by display 12, and the control method returns to step 53 where the adjusted MValv value is received as the new current MValv value for the patient respiration.
At step 67, the adjusted MValv value and target EtCO2 may be displayed to an operator, such as a clinician. For example, the display, such as display 12, may provide the MValv value over time, and the display may be updated at step 67 to reflect the adjusted MValv value. FIG. 5 depicts one exemplary display presenting an MValv value and a target EtCO2 value for a patient over time. The upper portion of the exemplary display presents an MValv trend line 76 showing the MValv value over a time scale 74. The MValv magnitude scale 72 is shown on the left hand side. The current MValv value 80 is also shown. The lower portion of the exemplary display presents trend line 90 showing a target EtCO2 over time (scale 74). The EtCO2 magnitude scale 88 is shown in the lower left corner. The current EtCO2 target value setting 84 is also shown below the current MValv value 80. In other embodiments. MValv may be replaced with minute volume or any other ventilation rate parameter, in such embodiments, the display may provide the ventilation rate parameter over time and the current value for the ventilation rate parameter in the same manner as shown and described in the exemplary embodiment of FIG. 5.
In one embodiment, the control system may alert a clinician if the MValv value changes by more than a predetermined amount. The predetermined amount for the threshold MValv change may be adjustable, for example by an operator 17 setting the threshold value through input device 15 (FIG. 1). For example, the system may provide an alert or an alarm if the MValv value changes by more than 20% from a stable value.
The functional block diagrams, operational sequences, and flow diagrams provided in the Figures are representative of exemplary architectures, environments, and methodologies for performing novel aspects of the disclosure. While, for purposes of simplicity of explanation, the methodologies included herein may be in the form of a functional diagram, operational sequence, or flow diagram, and may be described as a series of acts, it is to be understood and appreciated that the methodologies are not limited by the order of acts, as some acts may, in accordance therewith, occur in a different order and/or concurrently with other acts from that shown and described herein. For example, those skilled in the art will understand and appreciate that a methodology can alternatively be represented as a series of interrelated states or events, such as in a state diagram. Moreover, not all acts illustrated in a methodology may be required for a novel implementation.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the an. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

## Claims

1. A method for automatically controlling ventilation of a patient, the method comprising:
receiving a target EtCO2 for the patient at a controller associated with a ventilator;
receiving an alveolar minute volume value for the patient at the controller;
measuring with a gas analyzer expiratory CO2 in a gas expired from the patient;
calculating with the controller an actual EtCO2;
comparing with the controller the actual EtCO2 to the target EtCO2; and
if the actual EtCO2 is not within a predetermined range of the target EtCO2,
adjusting the alveolar minute volume value based on a difference between the actual EtCO2 and the target EtCO2;
operating the ventilator to automatically ventilate the patient based on the adjusted alveolar minute volume value; and
operating a display to indicate a change in patient status.

2. The method of claim 1 wherein the indicating the change in patient status comprises displaying the actual EtCO2 and the target EtCO2 on the display.

3. The method of claim 1 wherein the indicating the change in patient status comprises displaying the alveolar minute volume value and the adjusted alveolar minute volume value on the display.

4. The method of claim 3 further comprising storing the alveolar minute volume value over time; and
wherein the indicating the change in patient status comprises displaying the target EtCO2 and alveolar minute volume value over time together with respect to a time axis.

5. The method of claim 3 further comprising storing the alveolar minute volume value over time; and
wherein the indicating the change in patient status comprises displaying the actual EtCO2 and alveolar minute volume value over time together with respect to a time axis.

6. The method of claim 1 wherein the target EtCO2 for the patient is received from a user input device operated by a operator.

7. A system for automatically ventilating a patient, the system comprising:
a ventilator;
a gas analyzer;
a controller; and
a display
wherein the controller automatically controls the ventilator to ventilate the patient by
receiving a target EtCO2 for the patient;
receiving an initial alveolar minute volume value for the patient;
receiving from the gas analyzer an actual EtCO2 in a gas expired from the patient;
comparing the actual EtCO2 to the target EtCO2; and
determining that the actual EtCO2 is not within a predetermined range of the target EtCO2,
calculating an adjusted alveolar minute volume value based on a difference between the actual EtCO2 and the target EtCO2; and
automatically ventilating the patient based on the adjusted alveolar minute volume value.

8. The system of claim 7 wherein, if the actual EtCO2 is not within a predetermined range of the target EtCO2, the controller operates the display to indicate a change in patient status.

9. The system of claim 7 wherein the controller continually calculates the adjusted alveolar minute volume value as required to maintain the actual EtCO2 within the predetermined range of the target EtCO2.

10. The system of claim 7 wherein the controller stores the adjusted alveolar minute volume value to memory.

11. The system of claim 9 wherein the controller operates the display to display the adjusted alveolar minute volume value with respect to time.

12. The system of claim 9 wherein the controller operates the display to display the adjusted alveolar minute volume value and the target EtCO2 with respect to time.

13. The system of claim 7 wherein the target EtCO2 for the patient and the initial alveolar minute volume value for the patient are set by an operator via a user input device.