GB2631548A

GB2631548A - Apparatus and methods for medical gas delivery

Info

Publication number: GB2631548A
Application number: GB2310511.7A
Authority: GB
Inventors: William Higenbottam Timothy; Mornington-West Allen
Original assignee: Camcon Robotics Ltd
Current assignee: Camcon Robotics Ltd
Priority date: 2023-07-07
Filing date: 2023-07-07
Publication date: 2025-01-08
Also published as: WO2025012625A1; CN121398873A

Abstract

For regulating the flow rate of a pulse of oxygen to a patient 60 based upon their inspiratory flow rate and controlling its duration based on their oxygen saturation, an apparatus 100 for providing medical gas to a patient 60 for inhalation comprises a patient inspiratory flow rate sensor 24, a patient oxygen saturation sensor 50, a mass flow controller and associated valve (MFC/v) 26 operable to regulate a flow rate and control a duration of pulses of oxygen provided to the patient, a controller 22 which is configured to repeatedly and adjustably initiate the opening of the valve 26 to release a pulse of oxygen to a mask 30 for inhalation by the patient; to regulate the flow rate of the pulse of oxygen through the mass flow controller based on the inspiratory flow rate; and to control, based on the oxygen saturation, the duration of the pulse of oxygen by closing the valve.

Description

APPARATUS AND METHODS FOR MEDICAL GAS DELIVERY

Field of the Invention

The invention relates to apparatus and methods for medical gas delivery. Merely 5 by way of example, the invention may be used for delivery of oxygen for spontaneously-breathing acute respiratory failure patients in critical care units or at home. Associated apparatus and methods are also provided.

Background to the Invention

Chronic respiratory failure is common, with approximately 212 million people in the world with chronic obstructive pulmonary disease (COPD). Interstitial Lung Disease (ILD) is also a common cause of chronic respiratory failure. The major cause of death during COVID-19 was through acute respiratory failure due to pneumonia. Patients with acute or chronic respiratory failure have been treated for 138 years, by delivering a flow of oxygen (02) to the face via a face mask or cannula, for example, or later by inserting a tube into the throat to allow invasive mechanical ventilation (IMV).

Current methods to sustain life by delivering oxygen during acute respiratory failure with pneumonia were demonstrated to be inadequate during the recent waves of the COVID-19 epidemic. This viral infection is characterised by a severe pneumonia and caused many admissions to critical care units (CCUs) around the world. The pneumonias are caused by bacteria or viruses that reduce the lungs capacity to oxygenate the body. This is a result of injury to the alveolar wails through which oxygen and carbon dioxide are exchanged. There is a high mortality in pneumonias when there is severe limitation to gas exchange, and in particular when oxygen levels fall below a saturation of 90%. Between 2019 and 2023 in the USA there were 3.9 million deaths from pneumonia infections involving predominately SARSCoV-2, influenza viruses and bacteria.

A clinician is recommended to contra a flow of oxygen to a patient based on an oximeter recording of % 02 saturation of arterial oxygen (Sp02), which may be referred to as "arterial oxygen saturation", or simply "oxygen saturation". It is important to control the flow of oxygen to the patient and to monitor the Sp02 level, to ensure that the Sp02 level remains within a safe range. For example, the flow of oxygen may be controlled by the clinicians to try to maintain the Sp02 between 93% and 100% (a range which may be referred to as 'normoxia'). The relatively small target range for Sp02 is a result of the oxygen-haemoglobin dissociation curve, which has a sigmoid shape. Below a saturation of 90%, the partial pressure of 02 quickly falls to levels as low as 60 kPa, at which death can occur. For patients with severe chronic lung disease, the Sp02 level is low, which can result in oximeter measurements being less precise.

More serious is the variation of oxygen concentrations in the trachea with each selected flow rate, which can be as large as 60% dependent on the patient's breathing pattern or their inspiratory flow rate.

Clinical decision making, and control of the flow of oxygen to the patient, is therefore a significant challenge and can result in stress and "burnout' of critical care unit staff around the world.

Invasive mechanical ventilation (IMV) can be used to deliver a relatively precise dose of oxygen to the lungs of the patient. IMV provides a delivery of oxygen to the lungs in a closed system, and so the dose directly delivered to the lungs of the patient can be more easily measured and controlled. The closed delivery of oxygen is achieved by directly connecting the flow of oxygen to the lungs by a tube in the trachea. However, this requires the patient to be heavily sedated or anaesthetised.

Moreover, the pressure needed to pump the oxygen into the lungs can cause injury to the lung alveoli, despite advances in ventilators to limit the pressure rise in the lungs.

Alternatively, oxygen can be delivered to the patient in a non-invasive manner via a cannula or face mask. Oxygen is added to the air being inhaled by the patient via a face mask or nasal cannula, from a fixed hospital oxygen supply or cylinder 'The clinicians adjust the flow of oxygen to the face mask or nasal cannula to increase the fractional inspired oxygen (Fi02). However, whilst the amount of oxygen supplied to the mask can be controlled (for example, by a clinician adjusting a rotameter), it is difficult to measure and control the amount of oxygen delivered to the lungs of the patient, since ambient air will also be drawn into the lungs as the patient breathes in. Oxygen flow rates above 1 to 2 Umin are associated with a large variability of the elevated Fi02 that can be as high as 60%. Therefore, whilst the flow of oxygen to the mask can be manually controlled by the clinician, the precision of the dose of oxygen delivered to the lungs is relatively low and difficult to control. It is therefore challenging for the clinician to judge the flow rate of oxygen that should be provided to the cannula or mask lo maintain the SpO2within the target range.

The challenge to the clinicians to enable adequate oxygen to flow to acute respiratory failure patients led, during the first two waves of COVID-19, to patients being placed on IMV within 24 hours of admission to the CCUs. This led to a rise in mortality rates from 30% to 45%. It will be appreciated, therefore, that there is a need for improved apparatus and methods for medical gas delivery. For example, there is a need for improved apparatus and methods for more precise and effective delivery of oxygen for spontaneously-breathing patients.

In chronic lung disease oxygen is used long-term if patients are persistently hypoxic. The recommended flow rates of oxygen in such patients is between 2 and 3Lfmin. Use of oxygen in this way costs $2.8 Billion per annum in the USA. However there is growing evidence that patients fail to use it. Patients' inspiratory flow rates are 20 times this level when undertaking mild exercise. Such flow rates as high as IDOL/min are needed for exercise in order to maintain normal values of arterial oxygen saturation. Improvements for oxygen delivery are therefore required for home use of oxygen as well as CCUs.

The present invention solves or at least partially ameliorates one or more of the above issues.

Summary of the Invention

Aspects of the present invention are set out in the appended independent claims, while details of certain embodiments are set out in the appended dependent claims.

According to a first aspect of the invention there is provided apparatus for providing oxygen to a patient for inhalation, the apparatus comprising: a first sensor for generating a sensor output for determining an inspiratory flow rate of the patient; a second sensor for measuring an oxygen saturation of the patient; at least one mass flow controller and associated valve (MFC/v) operable to regulate a flow rate and control a duration of pulses of oxygen provided to the patient from the apparatus; and a controller configured to repeatedly and adjustably: initiate the opening of the valve to release a pulse of oxygen for inhalation by the patient; regulate the flow rate of the pulse of oxygen through the mass flow controller, based on the inspiratory flow rate; and control, based on the oxygen saturation, the duration of the pulse of oxygen by closing the valve.

Conventionally, flow rates of oxygen are determined by an adjustment made to a flow meter (a rotameter in the most hospitals) by a clinician. In contrast, the present apparatus (device) takes over the determination of flow rate of oxygen by controlling the flow using a mass flow controller and associated valve (MFC/v).

The expression "mass flow controller and associated valve" (MFC/v) as used herein should be interpreted broadly, to encompass a mass flow controller that is integrated with a valve as a single component within the apparatus, and also the possibility that the valve is separate from the mass flow controller within the apparatus but arranged and configured to operate in cooperation with the mass flow controller.

The valve opens and closes, so determining the duration of flow, whilst the mass flow controller (MFC) regulates the flow rate. So at the start of each breath the valve opens and remains open for such a time to provide a sufficient dose of oxygen to keep the Sp02 between 93% and 100%. More precise dosing of the oxygen is achieved by the matching of the flow rate of the delivered oxygen to the patient's inspiratory flow rate. This beneficially increases the entrainment of the oxygen into the lungs, and also enables more accurate dosing by lessening dilution with air. In contrast, when using a fixed flow rate of oxygen, the amount of entrained oxygen can vary by as much as 60%, because mismatches between the flow rate of the oxygen and the patient's inspiratory flow rate lead to increased fluctuations in the amount of oxygen entrained into the lungs per breath (and less oxygen being delivered into the lungs).

Advantageously, the present apparatus (device) is able to estimate the inspiratory flow rate based on a corresponding pressure measurement, and to deliver a pulse of oxygen at the start of each breath that flows at substantially the same flow rate as the patient's peak inspiratory flow rate. Delivery of the pulse of oxygen is controlled using an MFC/v. The duration of the pulse can be controlled based on a measurement of 402. By adjusting the pulse duration. the Sp02 can be more easily controlled to be within a target range (e.g. between 93% and 100%), compared to methods in which a fixed flow rate of oxygen is used. Moreover, the dose of oxygen delivered to the lungs with each breath can beneficially be estimated from the flow rate and duration of the oxygen pulse. The pressure and temperature are preferably measured by the MFC and so using the ideal gas law (or general gas law) it is possible to determine the delivered does of oxygen to the lungs in mL or mot. The delivery of pulses of oxygen whilst the patient is breathing in, rather than using a fixed flow rate of oxygen, also reduces wastage of oxygen due to a higher percentage of the delivered oxygen being entrained into the lungs and having no oxygen flowing during exhalation.

Advantageously, apparatus and methods of the present disclosure enable accurate prediction of the inspiratory flow rate based on pressure measurements. Control of the gas delivery based on the determined inspiratory flow rate and a measured arterial oxygen saturation (e.g. using a pulse oximeter) enables an arterial oxygen saturation to be more reliably and accurately controlled to be within a target range (e.g. between 93% and 100%). The actions described are performed automatically so that the apparatus may be provided as part of a portable device for supplementing oxygen delivery to spontaneously-breathing patients in acute or chronic respiratory failure at home or in a clinical environment.

Advantageously, apparatus and methods of the present disclosure bring together four synergistic elements: the estimation of the patient's inspiratory flow rate; the detection of the start of the inspiration; control of the delivery of the medical gas for efficient entrainment in the inspiratory flow; and control of the dose of gas based on the measured Sp02. Therefore, an automatic, more accurate and controllable dose of oxygen can be delivered to the patient, even when the gas is delivered via an open mask or nasal cannula.

The controller may be configured to regulate the flow rate of the oxygen released by the valve to be equal to the inspiratory flow rate of the patient.

The first sensor may be a pressure sensor, and the controller may be configured to determine the inspiratory flow rate based on a pressure measured by the pressure sensor.

The oxygen may be delivered to the patient via a mask, and the pressure sensor may be configured for sensing a pressure inside the mask.

The controller may be configured to determine a dose of oxygen to be delivered to the patient by the pulse of oxygen based on the oxygen saturation, and the controller 25 may be configured to determine the pulse duration based on the oxygen saturation and based on the inspiratory flow rate.

In one example, the controller may be configured to control the dose of oxygen delivered to the patient for inhalation, to control the oxygen saturation towards a 3o target value.

In another example, the controller may be configured to control the dose of oxygen delivered to the patient for inhalation, to control the oxygen saturation towards a target range. For example, the target range may be 93% to 100%.

The controller may be configured to determine, based on the sensor output from the first sensor, a time at which the patient begins to inhale; and the controller may be configured to control the valve to start flow of the pulse of oxygen based on the time at which the patient begins to inhale.

In one example, the controller may be configured to control the valve to start flow of the pulse of oxygen to the patient as the patient begins to inhale.

In another example, the controller may be configured to control the valve to begin to release the pulse of oxygen within a predetermined time period following the start of a breath of the patient.

The apparatus may further comprise a display for displaying at least one of the measured oxygen saturation or the dose of oxygen delivered to the patient.

The mass flow controller may be configured to measure the temperature and pressure of the oxygen provided to the patient, the measurements of temperature and pressure being used to enable a predetermined dose of oxygen to be delivered independently of its supply pressure.

The oxygen may be provided to the patient from the valve via a mask, and the mask may be an open face mask. The apparatus may comprise the mask, and the mask may comprise: a first port through which the oxygen is delivered to the patient from the valve; and a second port for connection to the first sensor.

The apparatus may comprise a memory storing correspondence information that indicates a mapping between the sensor output of the first sensor and the inspiratory flow rate. The correspondence information may comprise an equation or lookup table.

The apparatus may be a portable device.

According to a second aspect of the invention there is provided a method of providing oxygen to a patient for inhalation using the apparatus according to the first aspect.

According to a third aspect of the invention there is provided a method of providing oxygen to a patient for inhalation, the method comprising: determining an inspiratory flow rate of the patient; measuring an oxygen saturation of the patient; controlling output of a pulse of oxygen for inhalation by the patient; controlling a flow rate of the pulse of oxygen based on the inspiratory flow rate; and controlling a duration of the pulse of oxygen based on the oxygen saturation.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example only with reference to the attached figures in which: Figure 1 shows a simplified schematic illustration of apparatus for delivering a medical gas to a patient; Figure 2 shows a modification of the apparatus of gure 1, in which a plurality of mass flow control valves are provided; Figure 3 shows an example of a mask for providing a medical gas to a patient; Figure 4a shows a graph of inspiratory flow rate and pressure against time; during breathing cycles of a patient; Figure 4b illustrates the sigmoidal relationship between the pressure and the inspiratory flow rate; Figure 5 shows a graph of pressure; flow rate of medical gas, and dose of medical gas during a breathing cycle of a patient.

Figure 6 shows a table of medical gas doses mapped to corresponding ranges of Sp02; and Figure 7 shows a flow diagram of a method of controling a delivery of medical gas to a patient.

In the figures, like elements are indicated by like reference numerals throughout.

Detailed Description of Preferred Embodiments

The present embodiments represent the best ways known to the Applicant of putting the invention into practice. However, they are not the only ways in which this can be achieved.

Apparatus and methods for delivering a medical gas to a patient will now be described with reference to Figures 1 to 3. Figure 1 shows a simplified schematic illustration of apparatus 100 for delivering a medical gas to a patient 60. As shown in the figure, the apparatus 100 comprises a gas dispensing unit 20, a supply of oxygen 40, a display 70, an Sp02 sensor 50, and a mask 30. A patient 60 and ambient air 42 are also illustrated, but it will be appreciated that these are not part of the apparatus 100.

Advantageously, the gas dispensing unit 20 is operable to output oxygen to the mask 30 as a transient pulse of gas, and at a flow rate that is determined based on the inspiratory flow rate of the patient. By adjusting the pulse duration the Sp02 can be automatically maintained between a target range (e.g. between 93% and 100%), compared to methods in which a fixed flow rate of oxygen is used. Moreover, the dose of oxygen delivered to the lungs of the patient 60 with each breath can be more automatically estimated based on the flow rate and the duration of the pulse generated by the gas dispensing unit 20, and can be output to the display 70 for review by a clinician. This enables the clinician to monitor the progress of the patient, and to make clinical decisions.

Gas Dispensing Unit The gas dispensing unit 20 comprises a controller 22, a pressure sensor 24 and a mass flow controller with an associated valve (MFC/v) 26. The valve may be integrated with the mass flow controller, as a single component within the apparatus. Alternatively, the valve may be separate from the mass flow controller within the apparatus, but arranged and configured to operate in cooperation with the mass flow controller. Advantageously, the gas dispensing unit 20 is operable to determine an inspiratory flow rate of a patient 60, and to output a medical gas to the patient 60 at a flow rate that is controlled based on the estimated inspiratory flow rate of the patient 60 and a measured Sp02 level of the patient.

The MFC/v 26 is connected to a supply of medical gas. In this example the MFC/v 26 is connected to a supply of oxygen 40. The supply of oxygen 40 could be provided, for example, as a cylinder or a fixed hospital supply valve. The oxygen may be provided to the MFC/v 26 at, for example, 4 bar pressure from a regulator. The MFC/v 26 is operable to perform mass flow control and includes an on/off flow valve; both the MEG and the valve can be controlled by the controller 22. Particularly advantageously, the MFC may operate based on the ideal gas law (by measuring temperature and pressure of the gas) to ensure that a precise dose of a gas is delivered independently of the supply pressure. Moreover, the MFC/v 26 provides particularly responsive control of the flow, and duration of the flow which can be rapidly turned on and off.

The MFC/v 26 is operable to output a flow of oxygen to a mask 30, to provide a precise and controllable dose of oxygen per breath to the patient 60, under the control of the controller 22. It will be appreciated that whilst in the present example the medical gas delivered to the patient is oxygen, this need not necessarily be the case. The apparatus could alternatively be used to deliver any other suitable gas or gases to the patient 60.

In the example of Figure 1 the apparatus 100 also comprises a display 70 connected to the gas dispensing unit 20. The display 70 can be used to display various measurements and parameters to clinicians. For example, the display 70 can be used to display an indication of the inspiratory flow rate, Sp02, heart rate, breathing rate, inspiratory pressure or electrocardiogram (ECG) trace of the patient, or any other suitable parameter. The display may also output the dose of oxygen (e.g. per breath, or per any suitable time interval) needed to maintain normoxia, based on the Sp02 sensor 50 measurements and the dose of oxygen dispensed by the gas dispensing unit 20.

The controller 22 is configured to control the overall operation of the gas dispensing unit 20. The controller 22 operates in accordance with software stored in a memory 29 of the gas dispensing unit 20. However, whilst in the example of Figure 1 the controller 22 is illustrated as part of the gas dispensing unit 20, this need not necessarily be the case. Alternatively, for example, the controller 22 may be provided as part of separate apparatus configured to control the gas dispensing unit 20 (e.g. remotely), for example via a network connection. Software for the controller 22 may be executed on any suitable programmable apparatus. The programmable apparatus typically includes at least a Central Processing Unit (CPU) for executing the software stored in a Read Only Memory (ROM) or a Random Access Memory (RAM).

The apparatus 100 may also include one or more user input devices using which the user can interact with the gas dispensing unit 20 or the display 70. The user input devices may comprise, for example, a mouse, keyboard, hand-held controller (e.g. incorporating a joystick and/or various control buttons), or a touchscreen interface integral with the display screen 70.

Advantageously, the controller 22 is configured to control the flow rate of the gas dispensed by the MFC/v(s) 26 based on sensor readings from an SpO2 sensor 50 and a pressure sensor 24. In this example, the pressure sensor 24 is illustrated as part of the gas dispensing unit 20, but this need not necessarily be the case. Alternatively, the pressure sensor 24 could be provided externally to the gas dispensing unit 20, and configured to output sensor readings to the controller 22 (e.g. via any suitable wired or wireless connection to the gas dispensing unit 20).

The SpO2 sensor 50 may be an oximeter operating with either red or green light to measure arterial oxygen saturation. The SpO2 sensor 50 may be wrist-mounted and may also comprise other physiological sensors that allow, for example, blood pressure, ECG and/or respiratory rate to be recorded and displayed on the apparatus.

The controller 22 may receive the sensor readings from the Sp02 sensor 50 via any suitable wired or wireless connection. For example, the controller may be configured to receive measurements from the Sp02 sensor 50 using BluetoothTM Low Energy (BLE) communication, or any other suitable radio link (e.g. infrared). As will be described in more detail later, the inspiratory flow rate of the patient can advantageously be determined based on the measurements from the pressure sensor 24, which measure a pressure adjacent to the patient's nose or mouth (e.g. inside the mask 30). The controller is configured to determine a flow rate and pulse duration of the oxygen delivered to the patient via the mask 30 based on the determined inspiratory flow rate and the measured Sp02 sensor 50.

Advantageously, the gas dispensing unit 20 is operable to output the oxygen to the mask 30 at a flow rate that substantially matches the inspiratory flow rate of the patient 60, increasing the entrainment of oxygen into the lungs, and increasing the precision of the delivered dose. Moreover, since the oxygen is delivered to the patient 60 as a pulse, the dose can be accurately controlled by controlling the duration of the pulse, and delivery of the pulse can be timed to coincide with the beginning of inhalation of a breath. This results in improved distribution of the gas within the lungs. Control of the dose of oxygen as a pulse of gas to maintain the Sp02 within the target range also reduces the amount of wasted oxygen (e.g. compared to apparatus that delivers oxygen at a constant flow rate, even when the patient is exhaling).

In the example of Figure 1 the controller controls a single MFC/v 26, to control the flow of oxygen that is output to the mask 30. Alternatively, a plurality of MFC/vs 26 may be provided. Merely by way of example, Figure 2 shows a modification of the apparatus of Figure 1, in which the gas dispensing unit 20 comprises three MFC/vs 26a, 26b, 26c. In this example, it will be appreciated that the total flow rate of oxygen delivered to the mask 30 is the sum of the flow rates output by each of the MFC/vs 26a, 26b, 26c. A plurality of MFC/vs 26 may be used, for example, when a single MFC/v 26 is unable to output a sufficiently high flow rate of oxygen, or when the precision of the gas delivery by an MFC/v is improved at lower flow rates (in which case a plurality of MFC/vs operating at lower flow rates could potentially deliver gas at a more precise flow rate than a single MFC/v operating at a higher flow rate). The gas dispensing unit 20 may be configured, for example, to output up to 210 L/m in to the mask 30 (e.g. by each MFC/v 26 outputting 70 L/min) to meet the oxygen dose demands during respiratory failure or exercise. It will be appreciated that the maximum flow rate that need be provided by the gas dispensing unit 20 will depend on the particular use case and the type of dispensed gas.

The controller 22 is configured to control the (or each) MFC/v 26 to output the pulse of oxygen to coincide with the beginning of a breath of the patient (which can be determined based on the measurements from the pressure sensor 24). The controller 22 may take into account latency in the operation of the electronics and the MFC/v 26 (e.g. 30 ms) when timing the delivery of the pulse to coincide with inhalation. The inspiratory flow rate is determined based on the measurements from the pressure sensor 24. The controller 22 is configured to control the MFC/v 26 to output oxygen to the mask 30 at a flow rate that substantially matches the inspiratory flow rate of the patient 60, beneficially increasing the amount of oxygen that is entrained into the lungs of the patient 60. As will be described in more detail later, the duration of the pulse is advantageously determined based on the sensor reading from the Sp02 sensor. The duration of the pulse can be controlled, for example, to maintain (or restore) normoxia with Sp02 between 93% to 100%.

Mask and Sensors A mask 30 is provided for delivering the oxygen output by the MFC/v 26 to the patient 60. In the present example the mask 30 is an 'open' mask, such as a so-called 'Hans Rudolph' type mask. It will be appreciated that an 'open' mask is a mask that provides a relatively low resistance to airflow as the patient breathes in and out. However, the mask 30 need not necessarily be a Hans Rudolph mask. An alternative open mask could be used, provided it generates a small resistance to flow.

Alternatively, for example, a nasal cannula could be used instead of the mask 30 to deliver the medical gas to the patient 60. Ideally the use of an open mask or nasal canula enables the patient to talk or drink whilst receiving the oxygen.

Whilst some ambient air 42 may be drawn into the lungs of the patient 60 during inspiration when an open mask 30 is used, the use of the controlled pulse of oxygen delivered to the mask 30 at a flow rate matching the inspiratory flow rate mitigates against this effect, and advantageously enables a more precise and controllable dose of oxygen to be delivered to the patient. Therefore, the apparatus 100 is able to provide a precise dose of the medical gas (similar to a closed system, such as IMV), whilst enabling the use of an open mask 30 or nasal cannula having a relatively low resistance.

An example of an open mask 30 that could be used to deliver the medical gas to the patient 60 is illustrated in Figure 3, which shows a view of the front of the mask 30. As shown in the figure, in this example the mask 30 is a Hans Rudolph type mask.

The mask has a generally central opening 36 that is relatively large, allowing a flow of ambient air 42 to be drawn into the mask during inspiration, provided that the oxygen flow rate is lower that the patient's inspiratory flow rate, and a flow of exhaled air out of the mask 30 when the patient breathes out. However, even when an open mask 30 is used as illustrated in Figure 3, the patient's start of breath (the 'commencement of inspiration') can be determined by measuring a drop in pressure measured close to the face using the pressure sensor 24. In the present example the pressure sensor 24 is connected, via first tubing 35, to a first port 34 provided in the main body 31 of the mask 30, enabling the pressure inside the mask to be measured 30. A second port 32 is provided for delivery of the medical gas into the mask 30 from the MFC/v 26 via second tubing 33. A diffuser (not shown in the figure) may be provided at the second port 32, to reduce buffeting of the gas against the patient's face caused by high flow rates of the medical gas into the mask 30.

The diameter of the first and second tubing 33, 35 could be, for example, between 6 and 8 mm. However, it will be appreciated that any other suitable diameter tubing could alternatively be used. Also masks with channels in the construct can carry the oxygen into the mask and the pressure sensors could similarly record the pressure changes.

As will be described in more detail later, the magnitude of the pressure inside the mask 30 over time is measured using the pressure sensor 24 and is used by the controller 22 to estimate inspiratory flow rate.

Control of Medical Gas Delivery Methods of controlling the delivery of the medical gas to the patient 60 (e.g. using the apparatus illustrated in Figures 1 to 3) will now be described in more detail, with reference to Figures 4 to 7.

Estimation of Flow Rate In order to efficiently entrain oxygen into the lungs of the patient 60, the flow rate of oxygen delivered by the gas dispensing unit 20 is advantageously controlled based on the patient's inspiratory flow rate. By matching the flow rate of the delivered medical gas to the inspiratory flow rate, the fall in pressure around the nose and mouth caused by the start of the breath more effectively entrains the oxygen into the lungs, with reduced dilution by ambient air 42. This helps to ensure that the oxygen needed to restore normoxia in respiratory failure is efficiently and reliably delivered to the patient 60. In contrast, delivery of oxygen flowing at a flow rate that is mismatched with the inspiratory flow rate of the patient means that the Fi02 of the inhaled air reaching the alveoli will be lowered by entrainment of additional ambient air 42 via the open mask 30.

The mean peak inspiratory flow rate of the patient 60 may, for example, exceed 100 At this level of work, the patient 60 may achieve a minute ventilation (MV) rate as high as 30 Such a ventilation rate however corresponds to much higher inspiratory flow rates. The MV is equal to the tidal volume multiplied by the breathing frequency. The tidal volume at rest in a normoxic person is on average 500 ml, for a man. The MV increases in proportion to the oxygen needed. In chronic lung disease, as the patient becomes hypoxic (when the Sp02 falls below 93% or lower), the MV increases exponentially. This is also seen at rest when an acute lung illness affects the gas exchange inside the lungs. It will be appreciated, therefore, that the inspiratory flow rate is highly variable but is driven by the oxygen need. Advantageously, the apparatus is able to mitigate against this variability by determining the flow rate based on the measurements from the pressure sensor 24, to control the output of medical gas to match the inspiratory flow rate. In contrast, apparatus using a fixed flow rate cannot perform breath-by-breath adjustments of the flow rate, resulting in less efficient entrainment of the medical gas into the lungs.

The flow rate of the oxygen output from the MFC/v may be controlled to be between, for example, 1 Um in to 200 Um in or higher, in order to match the inspiratory flow rate of a patient 60 in acute respiratory failure with pneumonia, or in chronic respiratory failure during exercise.

Figure 4a shows a graph of inspiratory flow rate and pressure against time, during breathing cycles of a patient. As shown in the figure, there is a strong correspondence between the flow rate and the pressure measured inside the mask 30 (or measured, for example, adjacent to the nose for the case of a nasal cannula) using the pressure sensor 24. As illustrated in Figure 4b the relationship between the inspiratory flow rate and the measured pressure is a generally sigmoidal relationship.

The pressure sensor 24 may comprise a pressure transducer, or any other suitable apparatus for measuring pressure. The sensor 24 outputs a waveform that closely matches the shape of the plot of the flow rate during inspiration and expiration. Therefore, it is possible to estimate the flow rate based on the pressure measured using the sensor 24. For example the flow rate can be mapped to the measured pressure using an experimentally derived mathematical relationship (e.g. polynomial relationship) derived from measurements of both inspiratory pressures and inspiratory flow, for example during an exercise test. During the experimental measurements a turbine flow meter can be used to directly measure the flow rate.

It will be appreciated that the magnitude of the pressure measurement will depend on the resistance of the path to ambient atmosphere. For example, for the mask illustrated in Figure 3, the magnitude of the pressure measurement will depend on the size of the central aperture 36. However, this effect can be accounted for by calibrating the gas dispensing unit 20 (the mask 30 used to deliver gas to the patient 60 can be the same type of mask 30 used when experimentally deriving the mapping between the flow rate and the pressure). The important factor is that there is a strong correlation between the measured pressure and the inspiratory flow rate. It will be appreciated, therefore, that the controller 22 is operable to estimate the inspiratory flow rate of the patient 60 based on measurements from the pressure sensor 24.

The mapping between the measured pressure and the inspiratory flow rate is stored in the memory 29 of the gas dispensing unit 20, and used by the controller 22 to determine the inspiratory flow rate of the patient 60 based on the measured pressure. The controller 22 can then perform control of the MFC/v 26 to match the flow rate of the medical gas delivered to the patient 60 with the determined inspiratory flow rate. Whilst the mapping between the measured pressure and the inspiratory flow rate could be in the form of an equation stored in the memory 29 (e.g. corresponding to the sigmoidal function illustrated in Figure 4b), this need not necessarily be the case. For example, the mapping could alternatively be in the form of a lookup table.

Oxygen Pulse Control and Duration Figure 5 shows a graph of pressure measured using the pressure sensor 24, flow rate of medical gas output by the MFC/v 26, and the corresponding dose of medical gas delivered during a breathing cycle of a patient.

The solid line in the graph of Figure 5 illustrates the pressure measurements from the pressure sensor 24. The diagonally shaded region 51 of the graph corresponds to a time period during which the patient 60 is exhaling, and the dotted region 52 of the graph corresponds to a time period during which the patient 60 is inhaling.

The dotted line shows the flow rate of the oxygen delivered to the mask 30 by the MFC/v 26. As shown in Figure 5, the flow rate of the medical gas has a generally square-wave' type profile, as the flow rate is matched to the inspiratory rate of the patient 60, which is relatively constant for a period of time. Advantageously, the pulse of gas delivered to the patient 60 is controlled to be delivered during the beginning of inhalation, resulting in the gas being preferentially delivered to the healthier, well ventilated, parts of the lungs. This beneficially results in improved oxygenation and gas exchange, without unnecessarily increasing the intrathoracic pressure. Moreover, in contrast to apparatus that deliver the medical gas at a fixed flow rate, the gas is output to the mask 30 by the MFC/v 26 only during inhalation, reducing wastage of the gas that would occur if the gas was also delivered whilst the patient 60 is exhaling. The durations of the pulses are determined, per breath, based on measurements of Sp02 obtained using the Sp02 sensor 50, and taking into account the flow rate (since the delivered dose depends on both the flow rate and the duration of the pulse). Iterative control can be performed to increase or reduce the dose of oxygen delivered per breath, to maintain the value of Sp02 between the target range.

The dashed line shows the integrated dose per breath. In the graph shown in Figure 5, the illustrated integrated dose is reset to zero between doses for clarity. The dose of oxygen per breath may be, for example, between 0 and 150 ml (the dose per breath is approximately 40 ml in the example illustrated in Figure 5), but is controlled by the controller 22 depending on the Sp02 measurement from the Sp02 sensor 50. Alternatively, rather than the minimum dose to be output by the MFC/v 26 being zero, a non-zero minimum dose per breath could be used (100 ml). The non-zero minimum dose could be used as a precaution to ensure that the Sp02 does not drop too low, for example in the event of a sensor failure.

When the Sp02 sensor has been connected to the patient 60 and delivery of the gas to the patient 60 first begins, the gas dispensing unit 20 may be configured to deliver an initial first pulse of oxygen using a predetermined dose of, for example, 100 mL (or any other suitable predetermined first dose). The predetermined dose could be used for each pulse until the Sp02 measurements obtained from the Sp02 sensor 50 stabilise (e.g. over a period of approximately three minutes, depending on the response time of the particular Sp02 sensor used). This is because there is a delay between the oxygen being delivered to the patient 60 and the corresponding increase in oxygen saturation being detected by the Sp02 sensor 50, that is caused by the time taken for the oxygen to reach the blood of the patient 60, as well as latency introduced by the response time of the sensor 50. After the Sp02 sensor 50 reading has stabilised, the pulse duration can be adjusted to control the Sp02 towards the target range. For example, if the measured Sp02 is higher than the upper limit of the target range (e.g. the Sp02 is greater than 100%), the pulse duration can be decreased, to lower the dose of oxygen delivered to the patient 60. In contrast, if the Sp02 is lower than the lower limit of the target range (e.g. the Sp02 is lower than 93%) then the pulse duration can be increased. Advantageously, therefore, by virtue of the use of pulses of oxygen at a known flow rate, the apparatus 100 is able to accurately control the dose delivered to the patient 60, to control the Sp02 towards the target range. Moreover, the automatic control of the pulse duration that can be achieved using the controller 22, based on the measured Sp02 and the determined inspiratory flow rate, reduces the burden on the clinicians who would otherwise manually control the flow rate of the medical gas to the patient.

During the initial part of inhalation, the inhaled air tends to travel to healthier regions of the lung that are most easily expanded. The maximum flow rate of the medical gas output by the gas dispensing unit 20 is controlled to be shortly after the start of the breath, enabling the oxygen to be carried into the regions of the lungs that are well ventilated. Advantageously, therefore, by delivering the pulse of oxygen at the beginning of a breath, the oxygen is beneficially be delivered to the heathier parts of the patient's lungs. The trigger for the pulse of oxygen to be output by the MFC/v 26 to the mask 30 is the detection of the drop in pressure at the start of each breath (which can be detected by the controller 22 based on the sensor readings from the pressure sensor 24). The valve (e.g. pin valve) associated with the MFC/v 26 will open to begin the release of the gas by the MFC/v 26, at a flow rate that is controlled by the controller 22 based on the estimated inspiratory flow rate of the patient 60 (e.g. to match the inspiratory flow rate). The measurements of Sp02 output by the Sp02 sensor 50 then provide feedback for increasing or decreasing the dose (per breath) of the oxygen, by lengthening or shortening the pulse duration, respectively.

Any suitable feedback control method could be used to adjust the pulse duration based on the Sp02 measurements. For example, the apparatus 100 may be configured to sequentially decrease the pulse duration by a fixed value (e.g. 50 ms) for each breath whilst the SpO2 is higher than the target Sp02 range. Similarly, the apparatus 100 may be configured to iteratively increase the pulse duration by a fixed value (e.g. 50 ms) for each breath whilst the Sp02 is lower than the target Sp02 range. Alternatively, a proportional-integral-derivative (PID) controller could be used to adjust the pulse duration based on the measured SpO2, to drive the SpO2 towards the target range.

In a further alternative, the Sp02 level could be mapped to the pulse duration (dose) using a formula or lookup table stored in the memory 29. Figure 6 shows an example of a lookup table that could be used. As illustrated in the figure, the table provides a mapping between each value of the SpO2 and a corresponding dose to be delivered per breath (which corresponds to a particular pulse duration, depending on the flow rate).

Figure 7 shows a flow diagram of a method of controlling a delivery of medical gas (e.g. oxygen) to a patient, e.g. using the apparatus of Figure 1 or Figure 2. The method is performed repeatedly, with each pulse of medical gas able to be adjusted on a breath-by-breath basis.

In step S701 the Sp02 of the patient 60 is measured using the Sp02 sensor 50. The Sp02 sensor reading is output to the controller 22. In step S702 the controller determines the dose of medical gas to be delivered to the patient. As described above, the dose to be delivered to the patient can be determined using any suitable feedback control method based on the Sp02 measurement.

In step S703 the pressure in the mask 30 (or adjacent to the nose of the patient 60 when a nasal cannula is used) is measured using the pressure sensor 24. In step S704 the controller 22 determines the inspiratory flow rate based on the pressure 10 measurement, using correspondence information stored in the memory 29.

In step S705, the controller 22 determines the pulse duration for the pulse of medical gas to be output by the MFC/v(s) 26, based on the dose determined in step S702 and the flow rate determined in step S704.

In step S706 the controller 22 performs control of the MFC/v(s) 26 to deliver the pulse of gas to the patient 60 via the mask 30, using the pulse duration determined in step S705 and the flow rate determined in step S704 (the flow rate of the gas output by the MFC/v(s) 26 is controlled to match the determined inspiratory flow rate). As illustrated in Figure 5, the pulse of gas is delivered to the patient 60 during the beginning of inhalation.

Advantageously, therefore, the apparatus 100 provides a controllable dose of the medical gas to the patient 60, that is efficiently entrained into the patient's lungs by virtue of the flow rate of the medical gas being matched to the inspiratory flow rate of the patient 60. The medical gas is more effectively entrained into the lungs compared to constant-flow systems, and the dose can be more easily and accurately controlled. By virtue of the use of pulses of gas, less of the medical gas is wasted, the delivery of the gas can be timed with the start of inhalation (e.g. at the beginning of inhalation, or within a predetermined time period following the start of inhalation), and the overall dose can be adjusted per breath. Moreover, the known and controllable dose of the medical gas can be delivered to the patient 60 even when an 'open' type of mask 30 is used.

It will be appreciated that steps S701 and S702 need not necessarily be performed 5 before steps S703 and S704. Alternatively, steps S703 and S704 could be performed before steps S701 and S702, or could be performed substantially simultaneously.

Modifications and Alternatives Detailed embodiments and some possible alternatives have been described above.

As those skilled in the art will appreciate, a number of modifications and further alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will therefore be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

For example, whilst in certain examples above the gas delivered to the patient has been described as being oxygen, this need not necessarily be the case. The apparatus and methods could alternatively be used to deliver any other suitable gas to a patient.

Whilst the above examples have been described with reference to delivery of medical gas to a human patient, this need not necessarily be the case. Alternatively, the medical gas could be delivered to an animal, such as a dog, cat or horse (for example; under the supervision of a veterinarian).

Whilst in the above examples the controller 22 determines the inspiratory flow rate based on the measurements from the pressure sensor 24; this need riot necessarily be the case. Alternatively, a direct measurement of the inspiratory, flow rate could be used, for example by using a turbine flow meter fitted to the mask 30 (e.g in the central aperture 36 of Figure 3) to directly measure the inspiratory flow rate. The inspiratory flow rate could also be estimated using any other suitable indirect measurement, for example by measuring the expansion of the patient's chest, or by estimating the flow rate based on the breathing rate (e.g. breaths per minute) of the patient.

Whilst the above examples have been described with reference to controlling the pulse duration to drive the Sp02 to be within a target range, it will be appreciated that a target value of Sp02 could he used rather than a target range. For example, a target 402 value of 95% could be used. Advantageously, this means that adjustments to the pulse duration (and therefore dose) can also be made even when to the Sp02 is within an acceptable range, increasing the pulse duration as the Sp02 fails towards the lower limit of the acceptable range, and decreasing the pulse duration as the Sp02 rises towards the upper limit of the acceptable range (e.g. towards zero as the Sp02 approaches 100%).

Claims

CLAIMS1. Apparatus for providing oxygen to a patient for inhalation, the apparatus comprising: a first sensor for generating a sensor output for determining an inspiratory flow rate of the patient; a second sensor for measuring an oxygen saturation of the patient; at least one mass flow controller and associated valve operable to regulate a flow rate and control a duration of pulses of oxygen provided to the patient from the 10 apparatus; and a controller configured to repeatedly and adjustably: initiate the opening of the valve to release a pulse of oxygen for inhalation by the patient; regulate the flow rate of the pulse of oxygen through the mass flow controller, based on the inspiratory flow rate; and control, based on the oxygen saturation, the duration of the pulse of oxygen by closing the valve.
2. The apparatus according to claim 1, wherein the controller is configured to regulate the flow rate of the oxygen released by the valve to be equal to the inspiratory flow rate of the patient.
3. The apparatus according to claim 1 or claim 2, wherein the first sensor is a pressure sensor, and wherein the controller is configured to determine the inspiratory flow rate based on a pressure measured by the pressure sensor.
The apparatus according to claim 3, wherein the oxygen is delivered to the patient via a mask; and wherein the pressure sensor is configured for sensing a pressure inside the mask.
The apparatus according to any preceding claim, wherein the controller is configured to determine a dose of oxygen to be delivered to the patient by the pulse of oxygen based on the oxygen saturation; and wherein the controller is configured to determine the pulse duration based on the oxygen saturation and based on the inspiratory flow rate.
6. The apparatus according to claim 5, wherein the controller is configured to control the dose of oxygen delivered to the patient for inhalation, to control the oxygen saturation towards a target value.
7. The apparatus according to claim 5, wherein the controller is configured to control the dose of oxygen delivered to the patient for inhalation, to control the oxygen saturation towards a target range.
8. The apparatus according to claim 7, wherein the target range is 93% to 15 100%.
9. The apparatus according to any preceding claim, wherein the controller is configured to determine, based on the sensor output from the first sensor, a time at which the patient begins to inhale; and wherein the controller is configured to control the valve to start flow of the pulse of oxygen based on the time at which the patient begins to inhale.
10. The apparatus according to claim 9, wherein the controller is configured to control the valve to start flow of the pulse of oxygen to the patient as the patient begins to inhale.
11. That apparatus according to claim 9, wherein the controller is configured to control the valve to begin to release the pulse of oxygen within a predetermined time period following the start of a breath of the patient.
12. The apparatus according to any preceding claim, wherein the apparatus further comprises a display for displaying at least one of the measured oxygen saturation or the dose of oxygen delivered to the patient.
13. The apparatus according to any preceding claim, wherein the mass flow controller is configured to measure the temperature and pressure of the oxygen provided to the patient, the measurements of temperature and pressure being used to enable a predetermined dose of oxygen to be delivered independently of its supply pressure.
14. The apparatus according to any preceding claim, wherein the oxygen is provided to the patient from the valve via a mask; and wherein the mask is an open face mask.
15. The apparatus according to claim 14, wherein the apparatus comprises the mask, and wherein the mask comprises: a first port through which the oxygen is delivered to the patient from the valve; and a second port for connection to the first sensor.
16. The apparatus according to any preceding claim, wherein the apparatus comprises a memory storing correspondence information that indicates a mapping between the sensor output of the first sensor and the inspiratory flow rate.
17. The apparatus according to claim 16, wherein the correspondence information comprises an equation or lookup table.
18. The apparatus according to any preceding claim, wherein the apparatus is aportable device.
19. A method of providing oxygen to a patient for inhalation using the apparatus according to any preceding claim.
20. A method of providing oxygen to a patient for inhalation, the method comprising: determining an inspiratory flow rate of the patient; measuring an oxygen saturation of the patient; controlling output of a pulse of oxygen for inhalation by the patient; controlling a flow rate of the pulse of oxygen based on the inspiratory flow 1 o rate; and controlling a duration of the pulse of oxygen based on the oxygen saturation.