Patent Publication Number: US-2023157574-A1

Title: End tidal carbon dioxide measurement during high flow oxygen therapy

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefits of U.S. Provisional Application No. 63/282,335 filed Nov. 23, 2021, entitled “End Tidal Carbon Dioxide Measurement During High Flow Oxygen Therapy,” which is incorporated herein by reference in its entirety. 
    
    
     INTRODUCTION 
     Medical ventilator systems have long been used to provide ventilatory and supplemental oxygen support to patients. These ventilators typically comprise a connection for pressurized gas (air, oxygen) that is delivered to the patient through a conduit or tubing. One example of such ventilation is known as high-flow oxygen therapy (HFOT). Where nasal cannulas are utilized for the patient interface, such therapy may also be known as high-flow nasal cannula (HFNC) oxygen therapy. During HFOT, a patient is provided a continuous, relatively high flow of breathing gases that include an increased concentration of oxygen. For example, some HFOT systems are capable of delivering flow rates of up to 100 liters per minute (lpm) with oxygen concentrations (e.g., fraction of inspired oxygen (FiO 2 ) levels) of up to or above 90% or 100%. 
     It is with respect to this general technical environment that aspects of the present technology disclosed herein have been contemplated. Furthermore, although a general environment is discussed, it should be understood that the examples described herein should not be limited to the general environment identified herein. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a general and/or simplified form that are further described below in the Detailed Description. 
     Among other things, aspects of the present disclosure include systems and methods for capturing capnography data during a temporary reduction in flow during high flow oxygen therapy. In an aspect, the technology relates to a method for capturing capnography data, for example during high-flow oxygen therapy (HFOT). The method includes delivering breathing gases at an operational flow rate and an operational oxygen concentration level, for example thereby delivering HFOT; initiating a temporary flow-reduction function for a set duration, wherein breathing gases are delivered at a temporary flow rate for the set duration, the temporary flow rate being less than the operational flow rate; capturing capnography data for exhaled air during the set duration; and upon expiration of the set duration, resuming delivery, for example HFOT delivery. 
     In an example, the temporary flow rate is at least 20 liters per minute (lpm) less than the operational flow rate. In another example, initiating the temporary flow-reduction function is performed by the ventilator. In a further example, initiating the temporary flow-reduction function is performed by a capnography monitor. In still another example, the capnography data is captured by the capnography monitor. The method may further comprise boosting an oxygen concentration level for at least 1 breath immediately prior to reducing the flow rate. In yet another example, during the set duration, the breathing gases are delivered at a temporary oxygen concentration level that is greater than the operational oxygen concentration level. In still yet another example, based on the captured capnography data, automatically adjusting the temporary flow rate or the set duration for a subsequent temporary flow-reduction function. 
     In another example, the set duration is at least 3 breaths. In still another example, the operational flow rate is at least 40 lpm. In yet another example, the method further includes calculating an end-tidal carbon dioxide value from the captured capnography data. The method may comprise generating capnography trend data from the capnography data and, based on the capnography trend data being outside a capnography threshold, activating a notification. 
     In another aspect, which may be provided independently, the technology relates to a system for capturing capnography data, for example during high-flow oxygen therapy (HFOT). The system includes a valve coupled to ventilation tubing, the valve configured to alter a flow of breathing gases delivered to a patient from a ventilator; and a capnography monitor communicatively coupled to the valve. The capnography monitor includes a processor; and memory storing instructions that, when executed by the processor, cause the capnography monitor to perform a set of operations. The operations include at least partially close the valve for a set duration to reduce a flow rate of breathing gases delivered to the patient; during the set duration, capturing capnography data for exhaled air; and upon expiration of the set duration, reopening the valve to allow the flow rate of breathing gases delivered to the patient to increase. 
     In an example, capturing capnography data comprises sampling the exhaled air at a sampling interface and drawing the air into the capnography monitor through a sampling line. In another example, the operations further comprise comparing the capnography data to a capnography threshold, and activating a notification when the capnography data exceeds the capnography threshold. In a further example, at least partially closing the valve reduces the flow rate of breathing gases by at least 20 liters per minute. In still another example, the set duration is at least two breaths. 
     The operations may include resuming, after the set duration, delivery of the breathing gases at the constant operational flow rate for a set interval time. The operations may include after expiration of the set interval time, again reducing the flow rate of the breathing gases to the temporary flow rate. The operations may include while the flow rate is again reduced, capturing second capnography data of exhaled air. The interval time may be at least 50 times greater than the set duration. The operations may further comprise boosting an oxygen concentration level for at least 1 breath immediately prior to reducing the flow rate. The operations may further comprise generating capnography trend data from the first capnography data and the second capnography data. The operations may further comprise, based on the capnography trend data being outside a capnography threshold, activating a notification. 
     In another aspect, which may be provided independently, the technology relates to a method for capturing capnography data. The method includes delivering breathing gases at a constant operational flow rate and an operational oxygen concentration level, wherein the operational flow rate is greater than 40 liters per minute (lpm); reducing, for a set duration, a flow rate of the breathing gases to a temporary flow rate, wherein the flow rate is at least 10 lpm less than the operational flow rate; while the flow rate is reduced, capturing first capnography data for exhaled air; resuming, after the set duration, delivery of the breathing gases at the constant operational flow rate for a set interval time; after expiration of the set interval time, again reducing the flow rate of the breathing gases to the temporary flow rate; and while the flow rate is again reduced, capturing second capnography data of exhaled air. 
     In an example, the interval time is at least 50 times greater than the set duration. In another example, the method further includes boosting an oxygen concentration level for at least 1 breath immediately prior to reducing the flow rate. In still another example, the method further includes generating capnography trend data from the first capnography data and the second capnography data. In yet another example, the method further includes based on the capnography trend data being outside a capnography threshold, activating a notification. 
     In a further aspect, which may be provided independently, there is provided a capnography monitor configured to be communicatively coupled to a valve, the capnography monitor comprising:
         a processor; and       

     memory storing instructions that, when executed by the processor, cause the capnography monitor to perform a set of operations comprising:
         outputting a control signal to at least partially close the valve for a set duration to reduce a flow rate of breathing gases delivered to the patient;   during the set duration, capturing capnography data for exhaled air; and   upon expiration of the set duration, outputting a further control signal to reopen the valve to allow the flow rate of breathing gases delivered to the patient to increase.       

     In a further aspect, which may be provided independently, there is provided a computer program product comprising computer-readable instructions that are executable by a processor to perform a set of operations comprising:
         outputting a control signal to at least partially close a valve for a set duration to reduce a flow rate of breathing gases delivered to a patient;   during the set duration, capturing capnography data for exhaled air; and       

     upon expiration of the set duration, output a further control signal to reopen the valve to allow the flow rate of breathing gases delivered to the patient to increase. 
     Features of one aspect or example may be provided as feature of any other aspect or example in any appropriate combination. For example, any one of method, system, monitor or computer program product features may be provided as any one or more other of method, system, monitor or computer program product features. 
     It is to be understood that both the foregoing general description and the following Detailed Description are explanatory and are intended to provide further aspects and examples of the disclosure as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawing figures, which form a part of this application, are illustrative of aspects of systems and methods described below and are not meant to limit the scope of the disclosure in any manner, which scope shall be based on the claims. 
         FIG.  1    is a diagram illustrating an example of a medical ventilation system. 
         FIG.  2    depicts an example capnography monitoring system displaying an unattenuated capnography waveform. 
         FIG.  3    depicts example capnography waveforms. 
         FIG.  4    depicts an example capnography monitoring system displaying an attenuated capnography waveform. 
         FIG.  5    is a flowchart illustrating an example method for measuring capnography data during high-flow oxygen therapy. 
         FIGS.  6 A- 6 C  depict example plots of flow rate and oxygen concentration over time. 
     
    
    
     While examples of the disclosure are amenable to various modifications and alternative forms, specific aspects have been shown by way of example in the drawings and are described in detail below. The intention is not to limit the scope of the disclosure to the particular aspects described. On the contrary, the disclosure is intended to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure and the appended claims. 
     DETAILED DESCRIPTION 
     Medical ventilators are used to provide breathing gases to patients who are otherwise unable to breathe sufficiently. In modern medical facilities, pressurized air and oxygen sources are often available from wall outlets, tanks, or other sources of pressurized gases. Accordingly, ventilators may provide pressure regulating valves (or regulators) connected to centralized sources of pressurized air and pressurized oxygen. The regulating valves function to regulate flow so that respiratory gases having a desired concentration are supplied to the patient at desired pressures and flow rates. Further, as each patient may require a different ventilation strategy, modern ventilators may be customized for the particular needs of an individual patient. 
     As discussed briefly above, one mode of ventilation is a known as high-flow oxygen therapy (HFOT), where a patient is provided a relatively high flow of breathing gases that includes an increased concentration of oxygen. For example, some HFOT systems are capable of delivering flow rates of up to 80-100 liters per minute (lpm) with oxygen concentrations (e.g., fraction of inspired oxygen (FiO 2 ) levels) of up to or above 90% or 100%. While HFOT has proven to be particularly useful in treating certain conditions, high flow rates generated during HFOT can cause challenges for accurately measuring breath characteristics of the patient. 
     Capnography, which measures the amount of carbon dioxide (CO 2 ) in exhaled air from a patient, is one such area that becomes difficult in HFOT. In capnography, the carbon dioxide levels are from gases at or near the patient&#39;s mouth nose. As the patient breathes, the amount of carbon dioxide present is those gases changes. For instance, the amount of carbon dioxide increases as the patient exhales and decreases as the patient inhales because inhaled air generally contains little to no carbon dioxide and the exhaled air may contain about 4-6% carbon dioxide. The ability to accurately measure the amount of carbon dioxide, however, becomes more difficult as the flow of air being provided to the patient increases. For example, the high flow rate in HFOT effectively washes out the carbon dioxide that is exhaled from the patient, which significantly lowers the amount of carbon dioxide that can be measured. The resultant capnography signal or waveform is thus significantly attenuated. 
     The attenuated capnography signal or waveform may prevent accurate use of the capnography data and reduce its overall clinical value. When the capnography waveform is robust (e.g., unattenuated), significant clinical determinations can be made from the capnography signal. For example, the frequency of the waveform may be calculated to determine a respiratory rate of the patient. The shape of the waveform may also indicate different conditions of the patient, such as apnea, hypoventilation, hyperventilation, rebreathing of carbon dioxide, partial airway obstructions, asthma, emphysema, and/or chronic obstructive pulmonary disease (COPD), among others. 
     The peak value of the waveform during an exhalation phase, known as the end tidal CO2 (etCO 2 ) value, also has clinical uses. For instance, a trend of the etCO 2  value over time may indicate improvement or decline in the pulmonary health of a patient. The etCO 2  value itself may also be indicative of potential issues, as a normal healthy patient may have an etCO 2  value between about 35-45 mmHg. When the capnography waveform, however, is attenuated due to washout from the high flow of gases in HFOT, the above uses of the capnography waveform and the etCO 2  value are diminished and/or the accuracy of such uses is reduced. 
     Among other things, to alleviate the above challenges resulting from carbon dioxide washout from HFOT, the present technology temporarily alters the HFOT delivery to allow for capnography measurements to occur. For example, the present technology may temporarily reduce the flow rate of gases being provided to the patient to allow for capnography measurements to be taken over one or more breaths. During the temporary reduction of flow rate, an increase in oxygen concentration may also be provided to help maintain blood oxygen levels of the patient. In other examples, the oxygen concentration levels may be adjusted prior to the temporary reduction in flow to raise the blood oxygen levels of the patient before the reduction in flow. The duration of the temporary reduction in flow and/or the amount of reduction in flow may be based on the type of use desired for the capnography data measured during the temporary reduction in flow. The duration and amount may also be iteratively adjusted between the temporary reductions based on the captured capnography data. For instance, capnography data may be captured during a first temporary reduction in flow. Based on the characteristics of that captured capnography data, the duration and/or reduction amount during a second temporary reduction in flow may be adjusted. 
       FIG.  1    is a diagram illustrating an example of a medical ventilation system  100 . The medical ventilation system  100  includes a medical ventilator  102 . The medical ventilator  102  is connected to a human patient  150 . The ventilator  102  may provide positive pressure ventilation to the patient  150 . For instance, the ventilator  102  may provide HFOT to the patient  150 . 
     The ventilator  102  delivers breathing gases to the patient  150  via ventilation tubing  122 . In the example depicted, a single limb circuit is depicted. The single limb of the ventilation tubing  122  that delivers breathing gases to the patient may also be referred to as an inspiratory limb. The inspiratory limb extends from an inspiratory port  103  of the medical ventilator  102 . The inspiratory limb extends to a patient interface  180  which couples the patient to the pneumatic system via an invasive (e.g., endotracheal tube) or a non-invasive (e.g., nasal cannula, nasal mask) patient interface  180 . In the example depicted, the patient interface  180  is a nasal cannula. As such, when the ventilator  102  delivers HFOT to the patient  150 , such ventilation may be referred to as HFNC oxygen therapy. In other examples, a dual limb circuit may be implemented where an expiratory limb extends from the patient interface  180  and returns to an expiratory port of the ventilator  102 . 
     The breathing gases delivered to the patient  150  may be generated from a combination of oxygen and environmental or room air (e.g., ambient air outside of the ventilator  102 ). The oxygen may be received from an external oxygen source  108 . The oxygen source  108  may include pressurized oxygen from a tank or a wall port, such as oxygen ports that are available in many medical facilities. The ambient air may be received from a pressurized air source  107 . The pressurized air source  107  may include pressurized air from a tank or a wall port, such as pressurized air ports that are available in many medical facilities. Valves may be included on the ventilator  102  and/or at the pressurized air source  107  and/or the oxygen source  108  to control the flow of air and oxygen into the ventilator  102 . Alternatively or additionally, the medical ventilator  102  may include a compressor  104  or blower that pressurizes the air and/or oxygen. In such examples, a pressurized air source  107  may be omitted, and the compressor  104  may draw ambient air in through a vent or ambient air inlet of the ventilator  102 . The compressor  104  may be controlled to generate a desired pressure or flow of gases. 
     An oxygen mixer  106  may also be included in the ventilator  102  to mix the oxygen from the oxygen source  108  and the ambient air to achieve a set oxygen concentration level (e.g., FiO 2  level). The oxygen mixer may include a chamber and/or a series of controllable valves to control the flow of oxygen and/or ambient air to achieve a set oxygen concentration level (e.g., FiO 2  level). The mixture of oxygen and ambient air results in breathing gases that are delivered to the patient  150 . 
     In some examples, the inspiratory port  103  may include a valve, which may be a proportional valve such as a proportional solenoid valve (PSOL). The rate of flow (e.g., the liters per minute) of breathing gases delivered to the patient may then be controlled by operating or adjusting at least one of the pressurized air source  107  (or associated valves), the oxygen source  108  (or associated valves), the compressor  104 , valves of the oxygen mixer  106 , and/or the valve of the inspiratory port  103 . 
     In some examples, a humidification system  170  may also be incorporated to humidify the breathing gases that are delivered to the patient  105 . The humidification system  170  may be coupled to the inspiratory limb of the ventilation tubing such that humidification (e.g., water vapor) is added to the breathing gases as they pass through the humidification system  170 . 
     A coupling  146  may also be incorporated into the ventilation tubing  122  where the inspiratory limb is connected to tubing of the patient interface  180 . For instance, the coupling  146  may be a point where the inspiratory limb from the ventilator  102  and/or humidifier  170  connects or meets the tubing from the patient interface  180 . The portion of tubing on the patient side of the coupling  146  may be an integrated or detachable portion of the patient interface  180  that connects to the nasal cannula (or other type of patient interface  180 ). 
     In some examples, the coupling  146  may also include a valve, which may be a proportional valve, such as a scissor valve or PSOL. In other examples, the valve may be located in another portion of the ventilation tubing  122 , such as at or near the inspiratory port  103 . The valve may be controlled to initiate the temporary flow-reduction function described herein. 
     The ventilator  102  may also include a display  118  to display data and/or settings regarding ventilation being delivered to the patient  150 . The ventilator  102  may also include one or more processors  120  to execute instructions to the control the ventilator  102  and/or other devices and components of the medical ventilation system  100 . The ventilator  102  may also include memory  114  that stores instructions that, when executed by the processor  120 , cause the ventilator to perform the operations discussed herein. For instance, the memory  114  includes non-transitory, computer-readable storage media that stores software that is executed by the processor  120  and which controls the operation of the ventilator  102 . In an example, the memory  114  includes one or more solid-state storage devices such as flash memory chips. In an alternative example, the memory  114  may be mass storage connected to the processor  120  through a mass storage controller (not shown) and a communications bus (not shown). Although the description of computer-readable media contained herein refers to a solid-state storage, it should be appreciated by those skilled in the art that computer-readable storage media can be any available media that can be accessed by the processor  120 . That is, computer-readable storage media includes non-transitory, 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. For example, computer-readable storage media includes RAM, ROM, EPROM, EEPROM, flash memory or other solid state memory technology, CD-ROM, DVD, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the computer. Communication between components of the system  100  or between the system  100  and other therapeutic equipment and/or remote monitoring systems may be conducted over a distributed network, as described further herein, via wired or wireless means. 
     The ventilator  102  may also include various input and/or output (I/O) devices  110 . The I/O devices  110  may include a touchscreen, which may be incorporated into the display  118 . The I/O devices may also include speakers for emitting audible signals, such as alarms or other notifications. A keyboard, buttons, a mouse or pointer, or other types of input devices may also be incorporated to allow for input from a user. For instance, the flow and oxygen concentration levels of the delivered breathing gases may be set by a medical professional using the I/O devices  110 . 
     The medical ventilation system  100  may also include a capnography monitor  130 . The capnography monitor  130  may receive sampled air via a sampling line  126  that is connected to a sampling interface  124 , which may be incorporated into the patient interface  180 . For instance, a sampling of air captured at the sampling interface  124  is drawn into the capnography monitor  130  where the air is analyzed for carbon dioxide content. The air may be drawn into the capnography monitor  130  via a vacuum system incorporated into the monitor  130 . While not shown in  FIG.  1   , the sampling interface  124  may also include portions that extend over the patient&#39;s mouth, such as an oral scoop, that captures exhaled air from the mouth of the patient  105 . In other examples, the carbon dioxide measurement may be made at the sampling interface  124 , and an electrical signal representing the measured carbon dioxide amount may be communicated to the capnography monitor  130 . Different types of sampling interfaces and capnography devices, such as sidestream, mainstream technology may be implemented. 
     While the sampling interface  124  is depicted as being at, near, and/or integrated into the patient interface  180  in  FIG.  1   , the sampling interface  124  may be incorporated in other positions of the breathing circuit or ventilation tubing  122 . For instance, the sampling interface  124  may be integrated at or near the coupling  146 . In examples where an expiratory limb is also incorporated, the sampling interface  124  may be incorporated into the expiratory limb. 
     Once the capnography monitor  130  receives the gas sample from the sampling line  126 , a carbon dioxide sensor  136  of the capnography monitor  130  measures the carbon dioxide content of the sampled gas. The carbon dioxide sensor  136  may be a variety of sensors suitable for measuring carbon dioxide, such as a sensor that utilizes molecular correlation spectrography. The carbon dioxide measurement may be made as a partial pressure of carbon dioxide in the sample gas. As a result, the carbon dioxide measurement may have units of pressure, such as millimeters of mercury (mmHg). 
     The measurement operations, calculations, and control operations of the capnography monitor  130  may be performed by or with computing components integrated into the capnography monitor  130 . For example, the capnography monitor  130  may include one or more processors  134 , I/O devices  138 , memory  140 , and a display  132 . The processor(s)  134  may execute instructions to the control the capnography monitor  130  and/or other devices and components of the medical ventilation system  100 . The memory  140  may store instructions that, when executed by the processor  134 , cause the ventilator to perform the operations discussed herein. For instance, the memory  114  includes non-transitory, computer-readable storage media that stores software that is executed by the processor  134  and which controls the operation of the capnography monitor  130 . The memory  140  may be the same type of memory as the memory  114  of the ventilator  102 . The I/O devices  138  and the display  132  may also include the same or similar types of I/O devices  110  and display  118 , respectively, of the ventilator  102 . For instance, the capnography monitor  130  may include an on/off button, a battery/power indicator, a medium priority alarm, a high priority alarm, an ambient light sensor, a back button, a home button, a menu button, an alarm-silence button, directional buttons, and/or an enter button, among other or alternative features. Accordingly, a medical professional is able to interact with, and change the settings of, the capnography monitor  130 . 
     The capnography monitor  130  may control the initiation of the temporary flow-reduction function discussed herein. For example, the capnography monitor  130  may send a signal to the ventilator  102  to cause the ventilator  102  to reduce the flow and make other related adjustments during the duration of the temporary flow-reduction function. In other examples, the capnography monitor  130  is able to control the valve that is located in the coupling  146  or in another position of the ventilation tubing  122 . Accordingly, the capnography monitor  130  may be communicatively coupled to the valve. The capnography monitor  130  may then initiate the temporary flow-reduction function by at least partially closing the valve to reduce the flow of breathing gases reaching the patient. The capnography monitor  130  may then cease the temporary flow-reduction function by re-opening the valve. 
     While the capnography monitor  130  is depicted as being separate from the ventilator  102  in the example system  100 , in other examples the capnography monitor  130  may be integrated into the ventilator  102 . For example, one or more functionalities and/or components of the capnography monitor  130  may be incorporated into the ventilator  102 . Accordingly, the ventilator itself may measure the carbon dioxide amounts and/or output the capnography data. 
       FIG.  2    depicts an example capnography monitoring system  130  displaying an unattenuated capnography waveform  162 . The capnography waveform  162  is referred to herein as an “unattenuated” waveform because the waveform is generated from an acceptable breath sample from the patient that is not washed out by high flow rates from a ventilator. For instance, the unattenuated capnography waveform  162  may be captured when the ventilator is delivering a lower flow, or no flow, or gas to the patient. 
     In the example depicted, one side of the capnography monitor  130  includes an input port  144  for receiving the sampling line  126 . The front of the capnography monitor  130  includes the display  132 , and a row of buttons  142  are provided below the display  132 . The display  132  may display capnography data among other types of data, such as pulse oximetry (e.g., SpO2) data. In  FIG.  2   , a capnography plot  160  including a capnography waveform  162  is displayed. The y-axis of the capnography plot  160  represents a carbon dioxide level, and in the example depicted, has units of mmHg. The x-axis of the capnography plot  160  represents time. 
     In addition, an etCO 2  value  164  and a respiratory rate value  166  are displayed. The etCO 2  value  164  and the respiratory rate value  166  may both be calculated or determined by the capnography monitor  130  from the capnography waveform  162 . Additional information may also be monitored and/or displayed by the capnography monitor  130 , such as pulse oximetry (e.g., SpO 2 ) data. 
     Five points (A-E) have been superimposed on the capnography waveform  162  to assist in explaining the various portions of the capnography waveform  162 . Phase 1 is defined between points A-B and may be referred to as the inspiratory baseline. During inhalation, little to no carbon dioxide is present at the nose and mouth of the patient. Exhalation begins near point B with air leaving the trachea, posterior pharynx, mouth and nose. The gas which is exhaled from these large conducting airways contains a negligible concentration CO 2 . Phase 2 is defined between points B-C and may be referred to as the expiratory upstroke. During this ascending phase, CO 2  rich air from the alveoli begins to reach the upper airway and mix with the dead space air. Phase 3 is defined between points C-D and may be referred to as the alveolar plateau. During Phase 3, the carbon dioxide concentration remains relatively constant as primarily alveolar gas is exhaled. The alveolar plateau may be relatively flat with a slight trend upwards towards the end of the alveolar plateau. The etCO 2  value may be the maximum value of the capnography waveform during Phase 3 or during an exhalation phase of the breath. Phase 4 is defined between points D-E and may be referred to as the start of inhalation. During the start of inhalation, the oxygen fills the airway and CO 2  levels quickly drop back to baseline. 
     The capnography waveform  162  depicted in  FIG.  2    is for a normal patient or a normal capnography waveform. The characteristics of a normal capnography waveform generally includes a somewhat square waveform. For instance, Phase 2 and Phase 4 should be fairly steep and Phase 3 should be fairly flat with some increase during the end of the phase. The respiratory rate value of 18 breaths and the etCO 2  value of 38 are also considered to be within a normal or acceptable range. 
       FIG.  3    depicts example capnography waveforms that may be considered abnormal. As discussed above, the shape and characteristics of a capnography waveform may indicate clinical conditions of the patient. The capnography waveforms  162  depicted in  FIG.  3    are indicative of different clinical conditions of the patient. 
     The capnography waveform  162  in plot (A) is indicative of apnea where the etCO 2  value is trending downward for a series of breaths and then no breath occurs for a period of time such as 10 seconds. Causes of apnea may occur due to cardiac arrest, respiratory arrest, equipment failure, a displaced airway adjunct, or obstructive sleep apnea (OSA), among other things. 
     The capnography waveform  162  in plot (B) is indicative of hypoventilation. The capnography waveform  162  is characterized by a rapid increase at the start of exhalation, smooth and possibly prolonged uploaded during exhalation, and an abrupt descent to a baseline during inhalation. In addition, the etCO 2  value from the capnography waveform  162  in plot (B) may be above 45 mmHg, which also may indicate hypoventilation. Causes of hypoventilation may include a decrease in respiratory rate, a decrease in tidal volume, chest compressions, obesity hypoventilation syndrome (OHS), or use of central nervous system (CNS) depressant drugs, among others. 
     The capnography waveform  162  in plot (C) is indicative of hyperventilation which results in low carbon dioxide level resulting from excessive elimination through rapid or deep breathing or from metabolic acidosis. The shape of the capnography waveform  162  has a rapid increase at the beginning of exhalation, gradual, smooth and possibly shortened or peaked upslope during exhalation, and an abrupt descent to baseline during inhalation. The etCO2 value of the capnography waveform  162  in plot (B) may be below 35 mmHg, which also may indicate hyperventilation. Causes of hyperventilation may include an anxiety/panic disorder, excessive exercise, an increase in respiratory rate, or an increase in tidal volume, among others. 
     The capnography waveform  162  in plot (D) is indicative of asthma, emphysema, or COPD. The shape or morphology of the capnography waveform  162  is abnormal with a marked Phase 2 to Phase 3 curve with a shark fin appearance and an abrupt descent during Phase 4 back to baseline during inhalation. The shark fin shape is seen in more severe bronchospasm. 
       FIG.  4    depicts the example capnography monitoring system  130  displaying an attenuated capnography waveform  163 . The attenuated capnography waveform  163  displayed in  FIG.  4    results from the exhaled breath of the patient being washed out by the high flow of breathing gases delivered from the ventilator. For example, such an attenuated capnography waveform  163  may be captured when the ventilator is delivering a high flow of gas to the patient, such as 60-80 lpm of breathing gas. As can be seen, the average and maximum amplitude of the attenuated capnography waveform  163  is significantly less than the unattenuated capnography waveform  162 . The lower, or attenuated, amplitude is due to the washout of carbon dioxide, exhaled from the patient, due to the high flow of breathing gases being delivered to the patient. 
     The significant attenuation of the attenuated capnography waveform  163  reduces the clinical usefulness of the capnography waveform  163 . For instance, the calculated etCO 2  value  164  is now calculated at 11 mmHg, which would normally be indicative of hyperventilation. But, that low value in this example is due to the carbon dioxide washout rather than a condition of the patient. In addition, the shape of the attenuated capnography waveform  163  is substantially suppressed, which further reduces the possibility of, or the confidence in, matching the shape of the attenuated capnography waveform  163  to one of the waveform shapes known to be indicative of a patient condition, such as the waveforms discussed above in  FIG.  3   . 
     Because HFOT involves a constant flow that does not change, or substantially change, during a breath or from breath to breath, the attenuated capnography waveform  163  of  FIG.  4    is all that is able to be captured during HFOT therapy. The present technology alleviates the attenuated capnography waveform problem, by temporarily pausing or reducing the flow of breathing gases during HFOT to allow for an unattenuated capnography waveform to be captured during the temporary pause or reduction in flow. 
       FIG.  5    is a flowchart illustrating an example method  500  for measuring capnography data during high-flow oxygen therapy (HFOT). The example method  500  includes operations that may be implemented or performed by the systems and devices disclosed herein. For example, ventilator  102  (or components thereof) and/or capnography monitor  130  (or components thereof) depicted in  FIG.  1    may perform the operations described in the methods. In addition, instructions for performing the operations of the methods disclosed herein may be stored in the memories of the ventilator  102  and/or capnography monitor  130 . 
     At operation  502 , settings for the HFOT are received. Among other things, the settings may include a flow rate (e.g., liters per minute) and an oxygen concentration level (e.g., an FiO2 level). The settings may be received from a medical professional via interactions with the ventilator  102 . The flow rate to be delivered for the HFOT may be referred to as the operational flow rate, and the oxygen concentration level to be delivered for the HFOT may be referred to as the operational oxygen concentration level. In some examples, default setting values may be stored such that the HFOT may be initiated without requiring additional settings to be received from the medical professional. The medical professional, however, may adjust those default settings values. 
     At operation  504 , settings for a temporary flow-reduction function are received. The settings may be received by a medical professional via interactions with the ventilator  102  and/or the capnography monitor  130 . The settings may include how frequently the temporary flow-reduction function should be activated (e.g., a time interval between activations), a duration of the temporary pause function (e.g., a time duration or a number of breaths duration), the flow rate of breathing gases during the temporary pause function (e.g., a flow setting or a percentage reduction in flow), the oxygen concentration (e.g., FiO2 level) during the temporary flow-reduction function, and/or a pre-function oxygen boost setting. The flow rate for the temporary flow-reduction function may be referred to as the temporary flow rate, and the oxygen concentration level for the temporary flow-reduction function may be referred to as the temporary oxygen concentration level. 
     The pre-function oxygen boost setting may be a binary setting for turning a pre-function oxygen setting on or off. The pre-function oxygen boost setting controls whether the oxygen concentration level is boosted prior to the temporary flow-reduction function being initiated. The pre-function oxygen boost setting may also include underlying settings, such as a percent or value increase in oxygen concentration during the boost and a duration of the oxygen boost. The duration of the boost setting may be based on the set duration of the temporary flow-reduction function. For instance, the duration of the boost may be at least 50% of the duration of the temporary flow-reduction function. In some examples, the boost duration may be between 1-3 breaths. 
     In some examples, certain settings may be automatically adjusted or set by the ventilator  102  and/or capnography monitor  130  based on other settings received by the medical professional. As an example, a temporary oxygen concentration level may be set based on a duration of the temporary flow-reduction function and/or a temporary flow setting. For instance, as the duration increases and/or the flow decreases, less oxygen is being delivered to the patient during the temporary flow-reduction function. Accordingly, a temporary oxygen concentration level may be automatically increased for longer durations or lower temporary flow settings. For similar reasons, maximums may be provided for some of the settings that do not allow a medical physician to adjust the setting beyond the maximum. For instance, the duration of the temporary flow-reduction function may have a maximum value to prevent under-delivery of breathing gases. 
     In some examples, default setting values may be stored such that the temporary flow reduction may be initiated without requiring additional settings to be received from the medical professional. The medical professional, however, may adjust those default settings values. 
     At operation  506 , the HFOT is delivered to the patient according to the settings received in operation  502 . For instance, a constant flow of breathing gases is delivered at the operational flow rate and the operational oxygen concentration level. As example, breathing gases may be delivered at constant operational flow rate of greater than 40 lpm may be delivered over a plurality of breaths, such as at least 10 or 100 breaths. 
     A temporary flow-reduction function is initiated at operation  508  according to the settings received in operation  504 . The temporary flow-reduction function may be initiated after an interval setting, such as interval settings received in operation  504 . In other examples, the temporary flow-reduction function may be initiated in response to receiving a selection (such as a button or user interface element) to activate the temporary flow-reduction function. Initiating the flow-reduction function includes reducing the flow rate of the delivered breathing gases and implementing other settings where utilized, such as changing oxygen concentration levels. For instance, initiating the temporary flow-reduction function may include reducing the flow rate of breathing gases to the temporary flow rate for a set duration and, in some examples, increasing the oxygen concentration level to the temporary oxygen concentration level. Reducing the flow rate of the delivered breathing gases may include reducing the flow rate by at least 10, 20, or 30 lpm. 
     Initiating the temporary flow-reduction function may be performed by the ventilator  102  and/or the capnography monitor  130 . For example, where the settings for the temporary flow-reduction function are received at the ventilator  102  and the capnography functions are also incorporated within the ventilator  102 , the ventilator  102  may initiate the temporary flow-reduction function directly. In other examples, where the settings for the temporary flow-reduction function are received at the capnography monitor  130 , the capnography monitor  130  send a signal to the ventilator  102  to initiate and/or cease the temporary flow-reduction function. 
     In yet other examples, where the settings for the temporary flow-reduction function are received at the capnography monitor  130 , the capnography monitor  130  may control initiation and cessation of the temporary flow-reduction function directly by controlling a valve coupled to the ventilation tubing  122  (such as at the coupling  146 ). For example, the capnography monitor  130 , after the set interval or upon receipt of an activation selection, at least partially closes the valve to initiate the temporary flow-reduction function. After a set duration from initiation, the capnography monitor  130  reopens the valve to cease the temporary flow-reduction function. 
     At operation  510 , capnography data is captured during the temporary flow-reduction function (e.g., while the flow rate is reduced). The capnography data may be captured by sampling the air around the nose and/or mouth of the patient, as discussed above. The sampling of the capnography data may be performed by the capnography monitor  130  and/or the ventilator  102  when such capabilities are incorporated into the ventilator  102 . For instance, capturing the capnography data may include drawing air from a sampling interface  124  through a sampling line  126  and analyzing the sampled air with a carbon dioxide sensor  136 . The captured capnography data may include a measurement of the partial pressure of carbon dioxide present in the sampled air over a period of time. Capturing the capnography data in operation  510  may also include generating capnography waveforms, calculating etCO2 values, calculating respiratory rates, and/or determining other values or patient conditions from the capnography data. The capnography data and/or generated data may also be displayed in operation  510 . In some examples, the capnography data is captured only during the temporary flow-reduction function. For instance, the capnography data is not captured during delivery of breathing gases at the constant operational flow rate. In other examples, the capnography data is continuously captured during the HFOT delivery and the temporary flow-reduction function. 
     At operation  512 , the after a set duration, the temporary flow-reduction function is ceased and the HFOT delivery is resumed according to the settings received in operation  502 . The set duration may be an amount of time or a number of breaths. In examples, where the duration is a number of breaths, the passage of the breaths may be determined from the capnography data, such as the capnography waveform, itself. For instance, as discussed above, the exhalation and inhalation phases of a breath may be identified from the capnography waveform. Thus, the occurrence of a number of breaths may be determined from, or based on, the capnography data. Resuming the HFOT delivery (and/or ceasing the temporary flow-reduction function) may be controlled by the ventilator  102  and/or the capnography monitor  130  in the same or similar manner as described above for initiating the temporary flow-reduction function in operation  508 . 
     At operation  514 , the settings for the temporary flow-reduction function may be adjusted. The adjustments to the settings may be automatically made based on a machine analysis of the capnography data captured in operation  510 . The ventilator  102  and/or capnography monitor  130  may analyze the capnography waveform generated from the capnography data captured in operation  510  to determine the fidelity of the capnography data. As an example, the shape of each phase of the waveform may be analyzed to determine whether the shape of the respective phase matches what is expected or considered to be a high-fidelity waveform. For instance, in some examples, Phase 2 and Phase 4 should be fairly steep and Phase 3 should be fairly flat with some increase during the phase. If the captured capnography data does not produce such a capnography waveform, then the temporary flow rate may be automatically adjusted to be lower in a subsequent temporary flow-reduction function to further reduce the attenuation of the capnography waveform due to washout from the high flow breathing gases. 
     Alternatively or additionally, the adjustments to the settings may be made in response receiving adjustment input at the ventilator  102  and/or the capnography monitor  130 . For instance, upon reviewing the capnography data captured in operation  510 , a medical professional may determine that the temporary flow rate, duration, or any other setting may need to be adjusted. The medical professional may then provide adjustment input into the ventilator  102  and/or the capnography monitor  130  to adjust the settings of the temporary flow-reduction function. The ventilator  102  and/or the capnography monitor  130  then adjusts the settings based on the received input. 
     In examples where the capnography monitor  130  controls a valve to initiate and cease the temporary flow-reduction function, the adjustment to settings may be an adjustment to a valve position settings when the temporary flow-reduction function is initiated. For example, by closing the valve further (e.g., a more closed position), the delivered flow rate is further reduced. Thus, the respective setting may not need to be a temporary flow rate setting as the actual flow rate value is not needed. Rather the respective setting may be a valve position setting, such as percentage of closure. 
     At operation  516 , a trend of capnography data is generated. As an example, a trend of the capnography data may include generating a trend for the etCO2 values calculated from the capnography data captured during the temporary flow-reduction functions. For instance, an etCO2 value may be calculated for each breath during a temporary flow-reduction function. Multiple temporary flow-reduction functions may be performed during ventilation of a patient. The calculated etCO2 values from the multiple temporary flow-reduction function may be compared to determine or generate a trend of the etCO2 values. For example, an etCO2 value from a first temporary flow-reduction function (e.g., an average etCO2 value) may be compared to an etCO2 value from a second temporary flow-reduction function. In some examples, the trend may be displayed as plot showing the trend of the etCO2 values. Alternatively or additionally, a number or indicator (color, arrow, etc.) may be displayed to indicate the trend of the etCO2 values (e.g., upward, downward, and/or magnitude of trend). 
     At operation  518 , the capnography data (or data generated therefrom) is compared to one or more capnography thresholds to determine whether the capnography data is within acceptable limits. For instance, the raw capnography data, the capnography waveforms, the etCO2 values, the respiratory rates, and/or the trends generating operation  516  may be compared to one or more capnography thresholds. If the capnography data is not outside of the capnography threshold(s) (e.g., within the limits), method  500  flows back to operation  508  where another temporary flow-reduction function is initiated after an interval period (or in response to a selection to initiate the temporary flow-reduction function). Method  500  then repeats from operation  508 . 
     If, at operation  518 , the capnography data is outside the capnography threshold(s), a notification is activated or delivered. The notification may be an alarm to indicate a clinical condition of the patient and/or to alert a medical professional that assistance or review may be needed. The notification may be displayed on a display of the ventilator  102  and/or the capnography monitor  130 . The notification may also or alternatively be sounded on a speaker of the ventilator  102  and/or the capnography monitor  130 . One or more indicators or lights on the ventilator  102  and/or the capnography monitor  130  may also or alternatively be illuminated. The notification may also be communicated or delivered to another device, such as a central monitoring station and/or a device of a medical professional to alert the medical professional. After operation  520  is performed, method  500  flows back to operation  508  where another temporary flow-reduction function is initiated after an interval period (or in response to a selection to initiate the temporary flow-reduction function). Method  500  then repeats from operation  508 . 
       FIGS.  6 A- 6 C  depict example plots  600 A-C of flow rate  602  and oxygen concentration  604  over time. In the plots  600 A-C, time is represented on the x-axis and the magnitude of the flow rate and the oxygen concentration are represented on the y-axis. Each of the plots include periods of HFOT delivery and two temporary flow-reduction functions. 
       FIG.  6 A  depicts an example where the settings of the temporary flow-reduction function are adjusted after a first temporary flow-reduction to increase the duration of the temporary flow-reduction function. As can be seen from the plot  600 A, at the beginning of the HFOT, the delivered flow rate  602  is at an operating flow rate, and the oxygen concentration level is at an operating oxygen concentration level. The operating flow rate may be between 40-80 lpm in some examples. For purposes of this example, the operating flow rate is 60 lpm and the operating oxygen concentration level is 65%. 
     When a first temporary flow-reduction function is initiated, the delivered flow rate  602  drops to the temporary flow rate setting, which is in this example is 20 lpm. In other examples, the temporary flow rate may be at least 10 lpm, 20 lpm, or 30 lpm less than the operating flow rate. The temporary flow rate may also be less than 80%, 60%, 50%, 40%, 30%, or 20% of the operating flow rate. As discussed above, a lower temporary flow rate reduces the attenuation of the resultant capnography waveform. In the example depicted in  FIG.  6 A , the oxygen concentration level remains unchanged during the first temporary flow-reduction function. 
     The first temporary flow-reduction function has a first duration (D1). The duration of the temporary flow-reduction function may be between 1-10 or 2-7 breaths in some examples. The duration may also be based on the temporary flow rate. For example, for a temporary flow rate between 20-40 lpm (or lower), a duration of 2-3 breaths may be appropriate. For a higher temporary flow rate between 50-60 lpm, a longer duration of 5-6 breaths may be appropriate because the capnography data is more attenuated with the higher flow rate. 
     At the end of the duration (D1), the temporary flow-reduction function ceases and the HFOT delivery resumes. When the HFOT delivery resumes, the flow rate increases from the temporary flow rate to the operational flow rate. The HFOT delivery resumes for a set interval period (I). 
     Based on the capnography data captured during the first temporary flow-reduction period, the duration setting for the temporary flow-reduction function is adjusted. More specifically, the duration is increased from D1 to D2. In the depicted example, the duration is increased by two breaths. For instance, additional breaths may have been desired to better understand the characteristics of the capnography data and/or the capnography waveform. After a set interval time (I), a second temporary flow-reduction function is initiated for the adjusted or second duration D2. 
     As should be appreciated, the plots  600 A-C may not be drawn to scale. For instance, while the duration (D1) is depicted as being roughly ⅓ of interval time, in many examples the interval time (e.g., the time where HFOT is being delivered) may be significantly greater than the duration of the temporary flow-reduction function. As an example, in some implementations the interval period may be 10 minutes or greater, whereas the duration of the temporary flow-reduction function may be a few breaths (e.g., 5-20 seconds). In some examples, the internal between temporary flow-reduction functions may be at least 20, 50, 100, 150, or 200 times greater than the duration of the temporary flow-reduction function. 
       FIG.  6 B  depicts an example where the settings of the temporary flow-reduction function are adjusted after a first temporary flow-reduction period to lower the temporary flow rate of the temporary flow-reduction function. In addition, in  FIG.  6 B , the temporary oxygen concentration is higher than the operational oxygen concentration. Accordingly, when the temporary flow-reduction function is initiated, the oxygen concentration level  604  increases and the delivered flow rate  602  decreases. In some examples, the temporary oxygen concentration may be 10-30% higher than the operational oxygen concentration. For instance, where the operational oxygen concentration is and FiO2 level of 60%, the temporary oxygen concentration level may be between 70-90%. 
     In  FIG.  6 B , the temporary flow rate setting is lowered after the first temporary flow-reduction function. Thus, upon initiating the second temporary flow-reduction function, the delivered flow rate  602  is dropped below delivered flow rate during the first temporary flow-reduction function. 
       FIG.  6 C  depicts an example where a pre-function oxygen boost is implemented. Prior to the temporary flow-reduction function being implemented, the oxygen concentration level is increased or a boosted. The boost of oxygen may be for a few breaths (e.g., 1-5 breaths) immediately prior to the initiation of the temporary flow-reduction function. The pre-function oxygen boost may also occur prior to every temporary flow-reduction function, such as the first and the second temporary flow-reduction function depicted in  FIG.  6 B . In some examples, the boosted oxygen level may be at least 10-30% higher than the operational oxygen concentration. For instance, where the operational oxygen concentration is and FiO2 level of 60%, the boosted oxygen concentration level may be between 70-90%. When the temporary flow-reduction function is initiated, the delivered oxygen concentration level drops to the temporary oxygen level, which in the example depicted, is the same as the operational oxygen concentration level. 
     A person of skill in the art will understand that the technology described in the context of a medical ventilator for human patients could be adapted for use with other systems such as ventilators for non-human patients or general gas transport systems. Additionally, a person of ordinary skill in the art will understand that the temporary flow-reduction function may be implemented in a variety of breathing circuit setups. 
     Those skilled in the art will recognize that the methods and systems of the present disclosure may be implemented in many manners and as such are not to be limited by the foregoing aspects and examples. In other words, functional elements being performed by a single component or multiple components, in various combinations of hardware and software or firmware, and individual functions, can be distributed among software applications at either the client or server level or both. In this regard, any number of the features of the different aspects described herein may be combined into single or multiple aspects, and alternate aspects having fewer than or more than all of the features herein described are possible. 
     Functionality may also be, in whole or in part, distributed among multiple components, in manners now known or to become known. Thus, a myriad of software/hardware/firmware combinations are possible in achieving the functions, features, interfaces and preferences described herein. Moreover, the scope of the present disclosure covers conventionally known manners for carrying out the described features and functions and interfaces, and those variations and modifications that may be made to the hardware or software firmware components described herein as would be understood by those skilled in the art now and hereafter. In addition, some aspects of the present disclosure are described above with reference to block diagrams and/or operational illustrations of systems and methods according to aspects of this disclosure. The functions, operations, and/or acts noted in the blocks may occur out of the order that is shown in any respective flowchart. For example, two blocks shown in succession may in fact be executed or performed substantially concurrently or in reverse order, depending on the functionality and implementation involved. 
     Further, as used herein and in the claims, the phrase “at least one of element A, element B, or element C” is intended to convey any of: element A, element B, element C, elements A and B, elements A and C, elements B and C, and elements A, B, and C. In addition, one having skill in the art will understand the degree to which terms such as “about” or “substantially” convey in light of the measurement techniques utilized herein or used by those having skill in the medical ventilation art. To the extent such terms may not be clearly defined or understood by one having skill in the art, the term “about” shall mean plus or minus ten percent. 
     Numerous other changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the disclosure and as defined in the appended claims. While various aspects have been described for purposes of this disclosure, various changes and modifications may be made which are well within the scope of the disclosure. Numerous other changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the disclosure and as defined in the claims.