Abstract:
A respiratory assist device ( 10 ) adapted for transport use. The device ( 10 ) including a venturi means ( 16 ), a means ( 36 ) to deliver gas to a patient&#39;s airways, and a Carbon Dioxide absorber ( 44 ) in a gas circuit ( 12 ) having at least a breathable gas mixture of fresh gas and Carbon Dioxide depleted gas travelling therethrough. The venturi means ( 16 ) is adapted to receive fresh gas from a fresh gas source ( 18 ) and entrain the fresh gas into the gas mixture in the circuit ( 12 ), thereby increasing the velocity of the gas mixture in the circuit ( 12 ). The delivery means ( 36 ) is downstream of the venturi means ( 16 ) and is also adapted to deliver the gas mixture from the circuit ( 12 ) to a patient&#39;s airways and return the patient&#39;s exhaled gas to the circuit ( 12 ). The Carbon Dioxide absorber ( 44 ) is downstream of the delivery means ( 36 ) and upstream of the venturi means ( 16 ). The absorber ( 44 ′) is adapted to remove Carbon Dioxide from the gas mixture in the circuit ( 12 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a respiratory assist device and a method of providing respiratory assistance, the device/method being adapted for transport use.  
         [0002]     The invention has been primarily developed for transport use, particularly transport of patients to hospital (ie. pre hospital use) and transport of patients between and within hospitals, including emergency transportation such as land and air ambulances, and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited in this particular field of use and is also suitable for use in other out of hospital environments such as remote locations (eg. mines, altitude, submariner) in which decompression illness may arise and environments that may generate acute inhalational lung injury through heat, smoke, and (in the presence of fires) inhalational toxins.  
         [0003]     The primary use of the invention is in providing non-invasive respiratory assistance via a face mask. However, the invention can also be adapted for use with intubated patients receiving either mandatory or assisted respiratory support. As used herein, ‘mandatory’ refer to patients in whom the device is delivering all the respiratory effort and ‘assisted’ refers to patients who are making some respiratory effort but that effort is being assisted by the device.  
       BACKGROUND OF THE INVENTION  
       [0004]     There are presently two main types of respiratory assist devices. The first type are devices that generate/deliver continuous positive airway pressure (CPAP), which are generally used in the home treatment of sufferers of sleep apnea and other sleep disordered breathing conditions. The main disadvantage of such machines that are used to deliver CPAP in a home, pre-hospital or emergency environment is they are not suitable for delivering breathable gas with a high concentration of oxygen, as is often required for patients with compromised breathing due to illness or trauma. Machines that deliver CPAP also generally require a 240 volt power supply, can be noisy and can be cumbersome in transport environments. Machines that are used to deliver CPAP in a hospital environment can deliver higher levels of oxygen but have high gas consumption. This means they typically require attachment to a fixed wall outlet and/or to large gas cylinders that cannot easily be used during transportation of patients, other than distances that are far less than the usual practice.  
         [0005]     The other main type of respiratory assist devices are mechanical ventilators, as often used in intensive care units of hospitals. The main disadvantage of such ventilators is that they require a tube to be placed into the patient&#39;s trachea to deliver gas directly to the patient&#39;s lungs. This process is known as intubation and can itself place the patient at risk due to airway trauma during intubation, acute cardio-respiratory effects of mandatory positive pressure ventilation, the requirement for an anaesthetic and continual sedation, and greater complexity of ventilatory support. In cases of extended use (for example more than 48 hours), ventilators can lead to the patient developing pneumonia. Another disadvantage of ventilators is that their gas consumption is too high for use with portable gas supplies (eg. gas cylinders) for treatment times in excess of about 20 minutes. They are also very complicated to operate, are expensive and are not suited to transport environments. Some such ventilators may also deliver CPAP with higher levels of oxygen, but have the same limitations as those devices outlined above.  
       OBJECT OF THE INVENTION  
       [0006]     It is the object of the present invention to substantially overcome or at least ameliorate one or more of the above noted prior art disadvantages.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, in a first aspect, the present invention provides a respiratory assist device adapted for transport use, the device including the following components in a gas circuit having at least a breathable gas mixture of fresh gas and Carbon Dioxide depleted gas travelling therethrough:  
         [0008]     a venturi means adapted to receive fresh gas from a fresh gas source and entrain the fresh gas into the gas mixture in the circuit, thereby increasing the velocity of the gas mixture in the circuit;  
         [0009]     means to deliver the gas mixture to the patient&#39;s airways, the delivery means being downstream of the venturi means and adapted to deliver the gas mixture from the circuit to a patient&#39;s airways and return the patient&#39;s exhaled gas to the circuit; and  
         [0010]     a Carbon Dioxide absorber downstream of the delivery means, and upstream of the venturi means, and adapted to remove Carbon Dioxide from the gas mixture in the circuit.  
         [0011]     In a non-invasive form, the delivery means is preferably in the form of a face mask. In an invasive form, the delivery means is preferably in the form of a tracheal tube.  
         [0012]     The venturi means is preferably adapted to receive the fresh gas, which is most preferably Oxygen or Oxygen enriched, from a bottled or piped supply, desirably via a blender and an adjustable gas flow meter. In a preferred form, the venturi means is a jet delivery nozzle.  
         [0013]     The Carbon Dioxide absorber desirably also includes a water trap.  
         [0014]     The device preferably also includes a first unidirectional flow device in gas communication with the circuit between the venturi means and the delivery means, the first unidirectional flow device adapted to only permit gas flow from the venturi means towards the delivery means. The first unidirectional flow device desirably includes a fan or other device adapted to increase the velocity of the breathable gas mixture in the circuit.  
         [0015]     The device preferably also includes a second unidirectional flow device in gas communication with the circuit between the delivery means and the Carbon Dioxide absorber, the second unidirectional flow device adapted to only permit gas flow from the delivery means towards the Carbon Dioxide absorber.  
         [0016]     The device preferably also includes a gas reservoir in gas communication with the circuit between the Carbon Dioxide absorber and the venturi means. The gas reservoir is desirably in the form of a compliant bag, most desirably with a capacity of 500-2000 mls. The interior of the bag is preferably in gas communication with the circuit.  
         [0017]     In the non-invasive form, the exterior of the bag is preferably adapted, upon sufficient inflation, to block a supplemental gas flow chamber from communicating with the circuit and, in the absence of sufficient inflation, to allow the supplemental gas flow chamber to communicate with the circuit. The supplemental gas flow chamber is preferably connected to the circuit via at least one third unidirectional flow device adapted to only permit gas flow from the supplemental gas flow chamber towards the circuit. The third uni-directional flow device is most preferably a pair of flap valves. The supplemental gas flow chamber is preferably supplied with gas from the blender, most preferably via a pressure reducing valve, an adjustable pressure regulating valve and an over pressure relief valve.  
         [0018]     In the invasive form, the exterior of the bag is located in a sealed chamber adapted for pressurisation by a ventilator. In one arrangement, the ventilator is gas powered. In another arrangement, the ventilator is electrically powered. The ventilator is actuated to pressurise the chamber and thereby cause the volume of the gas in the bag to be added to the circuit.  
         [0019]     The device preferably also includes one or more of: a circuit pressure monitor and associated alarm; an Oxygen analyser; a Carbon Dioxide analyser; a safety valve adapted to open the circuit to atmosphere upon sensing that the fresh gas supply is exhausted; and a device adapted to servo control the fresh gas flow delivery.  
         [0020]     In a second aspect, the present invention provides a method of providing respiratory assistance adapted for a transport environment, the method including performing the following steps in a gas circuit having at least a breathable gas mixture of fresh gas and Carbon Dioxide depleted gas travelling therethrough:  
         [0021]     entraining fresh gas into the circuit, thereby increasing the velocity of the gas mixture in the circuit;  
         [0022]     delivering the gas mixture, downstream of the fresh gas entrainment, from the circuit to a patient&#39;s airways and returning the patient&#39;s exhaled gas to the circuit; and  
         [0023]     absorbing Carbon Dioxide from the circuit downstream of the patient&#39;s airways and upstream of the entrainment.  
         [0024]     The method preferably also includes the step of receiving the fresh gas for entrainment, which is most preferably Oxygen or Oxygen enriched, from a bottled or piped supply, desirably after passing it through a gas blender that incorporates air and an adjustable gas flow meter. The entrainment is preferably performed with a venturi means, most preferably a jet delivery nozzle.  
         [0025]     The method preferably also includes the step of absorbing water from the circuit downstream of the patient&#39;s airways and upstream of the entrainment, most preferably during the Carbon Dioxide absorption.  
         [0026]     The method preferably also includes the step of allowing only unidirectional gas flow in the circuit from the venturi means towards the patient&#39;s airways. The method preferably also includes the step of increasing the velocity of the breathable gas mixture in the circuit, most preferably with a fan or the like.  
         [0027]     The method preferably also includes the step of allowing only unidirectional gas flow in the circuit from the patient&#39;s airways towards the Carbon Dioxide absorber.  
         [0028]     The method preferably also includes the step of providing supplemental gas flow to the circuit upon sensing that the gas flow in the circuit is not sufficient to meet the patient&#39;s respiratory needs.  
         [0029]     In a third aspect, the present invention provides a respiratory assist device, the device comprising the following components in a gas circuit having at least a breathable gas mixture of fresh gas and Carbon Dioxide depleted gas travelling therethrough:  
         [0030]     a venturi for receiving fresh gas from a fresh gas source and entraining the fresh gas into the gas mixture in the circuit, thereby increasing the velocity of the gas mixture in the circuit;  
         [0031]     an airway gas delivery assembly downstream of the venturi for delivering the gas mixture from the circuit to a patient&#39;s airways and returning the patient&#39;s exhaled gas to the circuit; and  
         [0032]     a Carbon Dioxide absorber downstream of the mask, and upstream of the venturi, for removing Carbon Dioxide from the breathable gas mixture in the circuit.  
         [0033]     In a fourth aspect, the present invention provides a method of providing respiratory assistance in a gas circuit having at least a breathable gas mixture of fresh gas and Carbon Dioxide depleted gas travelling therethrough, the method comprising the following steps:  
         [0034]     entraining fresh gas into the circuit to increase the velocity of the gas mixture in the circuit;  
         [0035]     delivering the gas mixture, downstream of the fresh gas entrainment, from the circuit to a patient&#39;s airways and returning the patient&#39;s exhaled gas to the circuit; and  
         [0036]     absorbing Carbon Dioxide from the circuit downstream of the patient&#39;s airways and upstream of the entrainment.  
         [0037]     In a fifth aspect, the present invention provides a respiratory assist device comprising:  
         [0038]     a gas circuit having at least a breathable gas mixture of fresh gas and Carbon Dioxide depleted gas travelling therethrough;  
         [0039]     a venturi for receiving fresh gas from a fresh gas source and entraining the fresh gas into a gas mixture in the circuit, thereby increasing the velocity of the gas mixture in the circuit;  
         [0040]     an airway gas delivery assembly downstream of the venturi for delivering the gas mixture from the circuit to a patient&#39;s airways and returning the patient&#39;s exhaled gas to the circuit; and  
         [0041]     a Carbon Dioxide absorber downstream of the mask, and upstream of the venturi, for removing Carbon Dioxide from the breathable gas mixture in the circuit.  
         [0042]     In a sixth aspect, the present invention provides a method of providing respiratory assistance in a gas circuit, the method comprising the following steps:  
         [0043]     entraining fresh gas into the circuit to increase the velocity of a breathable gas mixture in the circuit, the gas mixture having the fresh gas and also Carbon Dioxide depleted gas travelling therein;  
         [0044]     delivering the gas mixture, downstream of the fresh gas entrainment, from the circuit to a patient&#39;s airways and returning the patient&#39;s exhaled gas to the circuit; and  
         [0045]     absorbing Carbon Dioxide from the circuit downstream of the patient&#39;s airways and upstream of the entrainment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0046]     Preferred embodiments of the invention will now be described, by way of examples only, with reference to the accompanying drawings in which:  
         [0047]      FIG. 1  is a schematic diagram of a first embodiment of a non-invasive respiratory assist device according to the invention;  
         [0048]      FIG. 2  is a schematic view of a jet delivery nozzle used in the device shown in  FIG. 1 ;  
         [0049]      FIG. 3   a  is a schematic side view of a face mask used in the device shown in  FIG. 1 ;  
         [0050]      FIG. 3   b  is a partial schematic underside view of the face mask shown in  FIG. 3   a;    
         [0051]      FIG. 3   c  is a partial schematic cross sectional side view of the face mask shown in  FIG. 3   a;  and  
         [0052]      FIG. 4  is a schematic diagram of a second embodiment of an invasive respiratory assist device according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0053]     Referring firstly to  FIG. 1 , there is shown a schematic diagram of a first embodiment of a non-invasive respiratory assist device  10  adapted for transport use, particularly emergency patient transportation. The device  10  includes a number of components, as will be described in detail below, connected in a (substantially closed) gas circuit  12 . The circuit  12  is divided into four main circuit sections ( 12   a,b,c  and  d ), which are preferably formed from hospital grade, semi-opaque or transparent, plastic tubing of approximately 22-25 mm in diameter.  
         [0054]     The tubing section  12   a  is preferably about 500-750 mm long and has a mixture of fresh and carbon dioxide depleted gas flowing therethrough as indicated by arrow  14 , preferably at a rate of about 80 L/min and at a pressure of 5-15 mm Hg.  
         [0055]     A venturi means, in the form of a jet delivery nozzle  16 , receives fresh gas (eg. Oxygen) from a supply  18 . The jet delivery device  16  is in gas communication with the circuit section  12   a  and will be described in more detail below (with reference to  FIG. 2 ), suffice to presently say that the gas leaving the nozzle  16  causes, by virtue of entrainment due to a venturi effect, an acceleration, and thus a velocity increase, of the gas mixture  14  flowing in circuit section  12   a.  The gas leaves the nozzle  16  at about 4-8 L/min.  
         [0056]     During transportation, the gas supply  18  is normally in the portable form of gas cylinders or bottles. After transportation, the gas can be supplied from cylinders or can be in the fixed form of a piped supply, as is present in most hospitals. The cylinder and piped supplies are connected to a gas blender  20  by lines  22  and  24  respectively. Suitable blenders and lines will be well known to persons skilled in the art and will not be described in further detail herein. The blender  20  has output lines  26  and  28  through which may pass only cylinder supplied gas, only pipe supplied gas or, if desired, a mixture of both. The length of the lines  22  and  24  will not influence the functioning of the device  10  and are determined by the transport environment in which the device  10  will be operated.  
         [0057]     The line  26  connects the blender  20  to an adjustable gas flow meter  30 , which provides an indication of the gas supply flowrate. Such meters are also well known to persons skilled in the art and will not be described in further detail herein. The gas leaving the meter  30  is supplied to the jet delivery nozzle  16  by an output line  32 . The line  32  is one commonly used in association with such meters and is preferably not more than 50 cm in length.  
         [0058]     The gas mixture  14  in the circuit section  12   a  is communicated to a uni-directional flow device in the form of a one-way valve  34 . The valve  34  also has a fan therein which further assists in increasing, or at least maintaining, the velocity of the gas mixture  14  passing from the circuit section  12   a  to the circuit section  12   b.  Numerous types of suitable one way valves and fans would also be well known to a person skilled in the art.  
         [0059]     The circuit section  12   b  supplies the mixture  14  of fresh and carbon dioxide depleted gas to a face mask  36  placed over the patient&#39;s airways. The mask  36  shall be described in more detail below.(with reference to  FIG. 3 ), suffice to presently say that it is able to deliver the gas mixture  14  from the circuit section  12   b  to the patient&#39;s airways and return only the patient&#39;s exhaled gas  38  to the circuit section  12   b.  Any gas flowing through circuit section  12   b  which is surplus to the patient&#39;s requirements flows directly through circuit section  12   b  also as indicated by arrow  14 .  
         [0060]     The mixture of gas streams  14  and  38  in the circuit section  12   b  then pass through an overpressure relief valve  41  and another one-way valve  42  into circuit section  12   c.  The mixture of gas streams  14  and  38  leaving the one-way valve  42  pass through circuit section  12   c  and through a carbon dioxide absorber and water trap  44 . The gas flowing through circuit section  12   d,  down stream of the carbon dioxide absorber  44 , is thus a mixture of fresh and carbon dioxide depleted gas, as again indicated by the arrow  14 .  
         [0061]     A gas reservoir, indicated generally by the reference numeral  45 , is connected in gas communication with the circuit section  12   d.  The reservoir  45  is supplied with fresh gas, as indicated by arrows  43 , from the output  28  of the blender  20 , via a pressure reducing valve  46 , a connecting line  47 , an adjustable pressure regulating valve  48 , a connecting line  49  and an overpressure release valve  50 .  
         [0062]     The reservoir  45  includes a compliant bag  52 , a supplementary gas flow chamber  54  and chamber dividers  55 . The interior of the bag  52  is in gas communication with the circuit section  12   d  at opening  56 . The exterior of the bag  52 , when sufficiently inflated, seals against the dividers  55  which blocks the gas supplied by the blender  20  to the supplementary gas flow chamber  54  from communicating with the circuit section  12   d.  When the patient&#39;s respiratory demands have drawn enough gas from the circuit to sufficiently deflate the bag  52  away from the dividers  55 , then gas from the supplementary gas flow chamber  54  flows past the bag  52  and through one way flap valves  58 , and into the circuit section  12   d,  as indicated by the arrows  59 . This supplementary gas flow ensures that the patient&#39;s respiratory needs are always met. When the patient&#39;s respiratory needs are being satisfied by the circuit  12 , then the excess gas flowing through the circuit  12  reinflates the bag  52  into sealing engagement with the dividers  55  to stop further gas communication from the supplementary gas flow chamber  54  to the circuit  12 .  
         [0063]     The bag  52  has a volume of about 2 litres and is produced from a highly compliant plastics material, for example Neoprene, so that changes in volume over the range of the patient&#39;s inhaled tidal volume (approximately 500 ml) do not substantially alter the tension in the bag wall and thus the internal pressure in the bag  52 .  
         [0064]     Referring to  FIG. 2 , the jet delivery nozzle  16  includes a connector  16   a  which is connected to the output line  32  of the meter  30 . The connector  16   a  allows the gas from the meter  30  to be communicated to a nozzle  16   b  within the circuit section  12   a,  as indicated by arrow  16   c.    
         [0065]     Referring to  FIGS. 3   a  to  3   c,  the mask  36  includes a mask shell  36   a  with an opening  36   b  for the patient&#39;s nose and mouth and a cradle  36   c  for connection with the circuit section  12   b.  The mask  36  also includes a diverter  36   d  within a port  36   e,  which connects the mask  36  to the circuit section  12   b.  When the patient inhales, the fresh gas flow  14  is communicated through the port  36   e,  aided by the diverter  36   d,  into the mask shell  36   a  and so to the patient&#39;s airways. When the patient exhales, exhaled gas  38  is drawn back through the port  36   e  and past the diverter  36   d,  into the circuit section  12   b.  The venturi effect of the excess fresh gas  14  flowing through the circuit section  12   b  assists in drawing the exhaled gas towards the ‘exhalation side’ of the diverter  36   d,  which minimises rebreathing.  
         [0066]     A typical use of the device  10  will now be described. The circuit  12  is connected to the mask  36 . The blender  20  is connected to the gas supply  18 , which during transportation will normally be a gas cylinder filled with Oxygen. The blender  20  is also connected to the flow meter  30  and the pressure regulating valve  48 . The output line  32  of the meter  30  is connected to the connector  16   a  of the jet delivery nozzle  16 .  
         [0067]     The required inspired oxygen concentration value is selected from the blender  20  and maximal flow rates are selected on the flow meter  30  and the mask  36  placed over the patient&#39;s face and sealingly secured in the manner well known to persons skilled in the art. At these flow rates, the circuit  12  will tend to overpressurise. Excess gas in the circuit  12  or in the bag  52  will be released through the overpressure release valve  41 . With the escape of this excess gas, there will be a washout effect of the circuit  12   b  so that the fresh gas  14  circulating within the circuit  12  will rapidly be replaced and be predominantly that gas delivered via the flow meter  30  and containing the prescribed oxygen concentration. At that point, the flow of the fresh gas  14  can be reduced to levels that are more economical for transport and that maintain the desired level of ventilatory assistance. Excess gas in the chamber  54  will be released through overpressure release valve  50 .  
         [0068]     The settings of the overpressure release valves  41  and  50  thus determine the treatment or working pressure of the device  10 .  
         [0069]     With alterations of oxygen concentration within the circuit, a similar wash out process would be required. The more frequent such washout processes are performed, the greater the fresh gas consumption and the lesser the efficacy of the device  10  in a transport environment. However, with transport times being mostly of less than 2 hours duration (for interhospital) and less than 1 hour for primary transfers to hospitals, and with the need for frequent alterations of inspired oxygen concentration being uncommon, such wastage is minimal and made up for by the overall gas conservation properties of the device  10 .  
         [0070]     The advantages of the respiratory assist device described above are as follows. Firstly, as the device operates with a (substantially closed) gas circuit then its gas consumption is minimised thereby allowing it to operate for relatively long periods of time (eg. 1-1.5 hours when fed from a C-sized portable gas cylinder). Secondly, the device can be produced from small, light weight, generally plastic, components making it suitable for use in transport environments where weight and space must be minimised. The plastic components can also be easily sterilised between uses. Thirdly, the positive pressurisation of the gas supplied at the patient&#39;s face mask reduces patient respiratory effort, which is particularly desirable for a patient suffering respiratory trauma. Fourthly, the external face mask avoids the disadvantages associated with intubation. Fifthly, patent comfort is improved. Finally, the device allows verbal communication from the patient.  
         [0071]      FIG. 4  shows a schematic diagram of a second embodiment of an invasive respiratory assist device  70  adapted for transport use, particularly emergency patient transportation. The device  70  is similar to the device  10  and like components to that in the first embodiment are denoted with like reference numerals in the second embodiment.  
         [0072]     The device  70  is able to be used with intubated patients. In the device  70 , the interior of the chamber  54  is sealed with respect to the exterior of the bag  52  and does not contain the dividers  55  or valves  58 . Also, the chamber  54  is selectively fed pressurised gas from a ventilator  72 , which is supplied with pressurised gas from the output of the blender  20  and via the valve  48 . Further, the mask  36  is replaced with a tracheal tube (not shown), which are well known, and a typical Y-piece connector to connect the circuit to the tube.  
         [0073]     The use of multi purpose connectors, and the majority of common componentry, allows removal and fitting of the components that differ between device  10  and  70  in order to easily convert between the two depending on patient need and the circumstances of use.  
         [0074]     When the ventilator  72  is actuated, it pressurises the chamber  54 . This results in the bag  52  being squeezed and thus having its contents delivered into the circuit section  12   d.    
         [0075]     The ventilator  72  can be one of several known medical transport ventilators, which are commonly dependent upon a gas supply for power. On the one hand, when such a ventilator  72  is utilised, the device  70  conserves the gas usage from the supply  18 . On the other hand, the gas consumption of the ventilator  72  is wasted on squeezing the bag as that gas is not directly used to supply the patient. However, the device  70  can be used for intubated patients who only require partial assistance because of the limitation of existing transport ventilators to deliver assisted (as compared to full) ventilation. Intubated patients are otherwise currently re-sedated or given muscle paralysing drugs to be transported, and each of these actions have their own associated risks.  
         [0076]     The devices  10 ,  70  also provide the choice of switching during transport from non invasive ventilation to intubation, as opposed to the present situation of intubation or no ventilatory assistance.  
         [0077]     For intubated patients, some patients can require only continuous positive airway pressure (CPAP), only inspiratory assistance (eg. bi-level positive airway pressure), full ventilatory support (ie. mandatory) or a mixture of the latter two treatment modes.  
         [0078]     Those that require just CPAP breathe through an intubation tube that, in adults, is usually 8 to 9 mm in diameter. The maximal achievable inspiratory gas flow rate achievable/required is much lower than that for adult patients breathing spontaneously without an endotracheal tube. Accordingly, known ventilators can match the lower flow rates required by intubated patients but have difficulty in providing the rates of flow required to provide respiratory assistance to non-intubated patients. As the device  10  is able to cope with the flow requirements of non intubated patients, the modified form of device  70  is also able to do so for intubated patients.  
         [0079]     This is in contrast to known portable/transport ventilators which have difficulty in matching the gas flow requirements for those patients who are on bi-level or a mixture of assist and mandatory ventilation. For such patients the current safest means of ventilation is to “convert” them to mandatory only mode of ventilation. This requires sedation and/or paralysing of the patient, which each carry their own risk.  
         [0080]     The devices  10 ,  70  achieve the required flow rates noted above due to the jet delivery nozzle  16 , the available reservoir of gas in the bag  52  and the gas conserving benefits of the carbon dioxide absorber  44 . They are thus able to improve the efficiency with which existing transport ventilators can function in an assist or assist/mandatory modes.  
         [0081]     In another embodiment (not shown), the ventilator  72  used in the device  70  is of the type currently utilised in home bi-level ventilatory support devices. These devices are powered by mains or battery and don&#39;t deplete the fresh gas supply  18 . Instead, they draw in atmospheric air, which is then used to squeeze the bag  52 . The added advantage of this arrangement is the fresh gas supply  18  is conserved and the rate of gas flow it achieves within the circuit  12  is maximised.  
         [0082]     Although the invention has been described with reference to specific embodiments, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.  
         [0083]     For example, in another embodiment of the invention (not shown), the device is simplified by omitting the gas reservoir. However, this would result in increased fresh gas consumption and, during inspiration, the device may not be able to sustain the desired pressure level, which could lead to patient discomfort.  
         [0084]     In other embodiments of the invention (not shown), the device also includes one or more of: a circuit pressure monitor and an associated alarm; an oxygen level analyser; a carbon dioxide analyser; a safety valve adapted to open the circuit to atmosphere upon sensing that the fresh gas supply is exhausted; and a device adapted to automatically control the fresh gas flow delivery and in response to detecting pressure changes in the circuit.