Abstract:
An anesthetic delivery system has a delivery unit containing a carbon dioxide retaining element and a reversible action anesthetic absorber/desorber for releasably retaining therein at least a portion of a charge of anesthetic agent. An externally accessible first internal flow section in the delivery unit directs gas through the delivery unit first through the anesthetic absorber/desorber and then through the carbon dioxide retaining element, sequentially. An externally accessible second internal flow section directs gas through the delivery unit via the anesthetic absorber/desorber and bypassing the carbon dioxide retaining element.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an anesthetic delivery system and in particular to a system adapted to re-use anesthetic that remains unabsorbed by a patient from a previously inhaled anesthetic dose.  
         [0003]     2. Description of the Prior Art  
         [0004]     It is know from U.S. Pat. No. 4,015,599 to provide an anesthetic delivery system having a delivery unit that houses a carbon dioxide absorber and a reversible action anesthetic adsorption filter arranged in series and in gaseous communication with a gas flow passage that provides a flow path for gas through the unit via the carbon dioxide absorber and the anesthetic adsorption filter. A charge of a gas-forming anesthetic is also provided as part of the system, pre-loaded into the anesthetic adsorption filter.  
         [0005]     In use, the unit of the known anesthetic delivery system is disposed in gas flow connection with a tubing circuit of a so-called “closed” inhalation anesthesia system. The unit is intended to be used in a manner such that exhaled breathing gas within the tubing circuit passes first through the carbon dioxide absorber and then through the adsorption filter to collect. This anesthetic gas is then supplied into the tubing circuit for inhalation by the patient, together with fresh breathing gas that is added after the unit to compensate for the gas that was consumed.  
       SUMMARY OF THE INVENTION  
       [0006]     An object of the present invention is to provide an anesthetic delivery system adapted for use in a so-called “open” inhalation anesthesia system and which also allows the re-use of exhaled anesthetic.  
         [0007]     This object is achieved in accordance with the present invention by an anesthetic delivery system having a delivery unit with two internal gas flow passages, an exhalation gas flow passage that conducts gas through the adsorption filter only, and an inhalation gas flow passage that conducts gas through first the adsorption filter and then through the carbon dioxide absorber. Thus any unused anesthetic in exhalation gas is retained by the filter and is returned (“reflected”), essentially free of carbon dioxide, for re-inhalation by a patient while permitting the majority of exhaled carbon dioxide to pass through the unit. In this manner the lifetime of the anesthetic charge may be extended without increasing its size and the amount of carbon dioxide absorber material may be reduced compared to the known system, thereby enabling a reduction in material costs and size of the delivery unit.  
         [0008]     Usefully a bypass gas flow passage may be included within the unit and configured to provide a flow path for an amount, preferably a variable amount, of gas from the inhalation passage to bypass the anesthetic filter. In this manner take up of anesthetic may be controlled by controlling the gas flow through the anesthetic adsorption filter.  
         [0009]     A variable flow restriction may be provided within either of the bypass gas flow passage and the inhalation gas flow passage to regulate the flow of gas in the inhalation line through the filter and thereby variably control the concentration of anesthetic in the gas that passes out of the delivery unit. Usefully the variable flow restriction may be adapted to automatically regulate the flow of gas dependent on a sensed concentration of anesthetic in the gas. Preferably a material, such as silicone rubber, that reacts to change its physical dimensions in response to an exposure to anesthetic, is employed in the variable flow restriction. In this way sensing of the anesthetic concentration and the dependent control of the flow restriction may be carried out within the delivery unit without the need of additional electronic sensor or control arrangements.  
         [0010]     The above object also in achieved in accordance with the present invention by an inhalation anesthetic system having a mechanical breathing aid which may be a ventilator or respirator of a stationary system or which may be, for example, a compressible bag or bottled gas supply, connectable to the airways of a patient by a gas flow circuit having a common gas flow section in which inhalation gas from the breathing aid can flow towards the patient and in which exhalation gas from the patient can flow towards the breathing aid. A delivery unit of the anesthetic delivery system is provided in fluid communication with the flow circuit, preferably the common gas flow section, so that inhalation gas can flow through the unit to receive a dose of the anesthetic agent held by the absorption filter and so that the exhalation gas can flow through the unit to deposit unused anesthetic agent in the absorption filter together with a small amount of the carbon dioxide present in the exhalation gas. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows schematically a first embodiment of an anesthetic delivery system according to the present invention;  
         [0012]      FIG. 2  is a schematic representation of an inhalation anesthetic system according to the present invention.  
         [0013]      FIG. 3  shows schematically a second embodiment of an anesthetic delivery system according to the present invention.  
         [0014]      FIG. 4  shows part of a third embodiment of an anesthetic delivery system according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     Considering now the anesthetic delivery system of  FIG. 1 , a delivery unit  2  is, in the present embodiment, formed with ports  6   a ,  6   b  for providing gas communication between internal and external the unit  2 . Gas flow directions through the unit  2 , when in use are shown, in  FIG. 1  by the arrows.  
         [0016]     The port  6   a  provides for gas communication between external the unit  2  and a common flow passage  8   a  internal the unit  2  whilst the port  6   b  provides for gas communication between external the unit  2  and a common flow passage  8   b  internal the unit  2 . These common flow passages  8   a ,  8   b  form part of both an inhalation gas flow passage (for gas flowing within the unit  2  from port  6   a , towards the port  6   b ) and an exhalation gas flow passage (for gas flowing within the unit  2  from port  6   b , towards the port  6   a ).  
         [0017]     A reversible action anesthetic (adsorption/desorption) filter  10  formed of a suitable sorption material for anesthetic agent, such as zeolites of crystalline aluminum silicates which may be pellets or supported on a carrier; an activated carbon filter such as formed from carbon-impregnated material, carbon fiber cloth, or granulated or microporous carbon material; or other microporous material, is arranged in direct gas communication with the common flow passage  8   a . In the present embodiment this anesthetic sorption filter  10  is formed into two regions. A first region  10   a  is provided initially free of the anesthetic agent and a second region  10   b  is pre-loaded with an anesthetic agent to be delivered to a patient. Optionally and as illustrated in the present embodiment, a first removable sealing membrane  4   a , such as may be formed from an impermeable plastics material, is located between the first region  10   a  and the second region  10   b  to act as a barrier for the transport of the pre-loaded anesthetic agent into the second region  10   b . A second removable sealing membrane  4   b  is located to seal the second region  10   b  against escape of anesthetic agent from the filter  10 . The two membranes  4   a ,  4   b  are removable from the filter  10  by pulling on externally accessible tab sections and are intended to be removed immediately before use of the unit  2 . In this manner the pre-loaded delivery unit  2  may be stored for extended periods without loss of anesthetic from the second region  10   b  of the filter  10 .  
         [0018]     The filter  10  is located within the delivery unit  2  with the anesthetic free region  10   a  relatively closer to the port  6   a  and in fluid communication with the common flow passage  8   a . Pre-loading may be achieved in a number of ways, for example by passing an anesthetic containing gas, in this embodiment preferably in a direction from the port  6   b  to the port  6   a , through the unit  2  before any removable sealing membrane  4   a ,  4   b  is in place and until a required amount of anesthetic agent has been retained by the anesthetic filter  10 . This can be monitored by monitoring the anesthetic concentration in gas exiting the unit  2  through the port  6   a . Pre-loading of the filter  10  may alternatively be carried out by passing an anesthetic containing gas through it before it is placed within the delivery unit  2 .  
         [0019]     A flow channel  12  is provided within the unit  2  for fluid communication between the second region  10   b  of the anesthetic filter  10  and a carbon dioxide absorber  14 . A one-way valve  16  is disposed relative to the carbon dioxide absorber  14  to prevent gas flow into the absorber  14  from the port  6   b.    
         [0020]     A bypass gas flow passage  18  is connected through an opening  20  with the common flow passage  8   a  at a location between the port  6   a  of the unit  2  and the anesthetic filter  10 .  
         [0021]     A one-way valve  22  is provided to permit gas flow along the bypass gas flow passage  18  in a direction from the common flow passage  8   a  only. The bypass gas flow passage  18  is arranged to provide a flow path for gas from the port  6   a  to the port  6   b , avoiding the anesthetic sorption filter  10  and in the present embodiment terminates at an opening  24  in the flow channel  12 . A variable flow restriction  26  is provided in communication with the bypass gas flow passage  18  and is movable to vary the resistance to gas flow within the bypass gas flow passage  18 .  
         [0022]     A flow passage  28  within the delivery unit  2  communicates with the common gas flow passage  8   b ; with the second region  10   b  of the anesthetic sorption filter  10  through the opening  24  in the flow channel  12  and forms part of an exhalation gas flow passage. The flow passage  28  is here shown to be provided with a one-way valve  30  to ensure gas flow through the passage  28  in one direction only—from the port  6   b  towards the anesthetic filter  10 , avoiding the carbon dioxide absorber  14 .  
         [0023]     An exemplary “open” inhalation anesthetic system  32  is shown in  FIG. 2 . A mechanical breathing aid  34 , such as a ventilator, is shown in use in gas communication with the airways of a patient  36 . The system  32  is provided with a common gas line  38  for the delivery to and recovery from the airways of a patient  36  of anesthetic containing gases. Separate inhalation  40  and exhalation  42  gas lines are provided to connect the breathing aid  34  with the common gas line  38 .  
         [0024]     The anesthetic delivery unit  2  of  FIG. 1  is shown here as being series connected to the common gas line  38  so that gas passing both to and from the patient will pass through the unit  2 . The unit  2  is oriented within the common gas line  38  so that inhalation gas from the breathing aid  34  will enter the unit  2  through the port  6   a  and exhalation gas from the airways of the patient  36  will enter the unit  2  through the port  6   b . To facilitate this orientation a visible indication, such as an arrow  44  showing the intended direction of gas flow through the unit  2  towards the patient  36 , may be provided on an external surface of the unit  2 .  
         [0025]     In use the delivery unit  2  is intended to receive inhalation gas for inhalation by a patient  36  through the port  6   a  and into the common gas flow passage  8   a . This inhalation gas may then be divided to flow partly through the anesthetic filter  10  and partly through the bypass gas flow passage  18  to avoid the filter  10 . The gas flowing through the filter  10  picks up anesthetic agent together with carbon dioxide that may be present within the filter  10  and flows towards the carbon dioxide absorber  14 . It will be appreciated that by moving the flow restriction  26  to alter the resistance to flow it presents then the amount of inhalation gas flowing through the absorption filter  10  can be varied and the concentration of anesthetic in the inhalation gas that exits the unit  2  through the port  6   b  controlled.  
         [0026]     In the present example this anesthetic containing inhalation gas is recombined with the inhalation gas from the bypass gas flow passage  18  in the flow channel  12  before it passes through the carbon dioxide absorber  14 . Carbon dioxide that was picked up by the inhalation gas as it passed through the anesthetic filter  10  will be captured by the carbon dioxide absorber  14 . The essentially carbon dioxide free inhalation gas then flows through the one-way valve  16 , along the common flow passage  8   b  and out of the delivery unit  2  through the port  6   b  carrying with it a dose of anesthetic for inhalation by the patient  36 .  
         [0027]     Exhalation gas from the patient  36  will typically contain carbon dioxide and an amount of unused anesthetic. In use the delivery unit  2  is intended to receive this exhalation gas through the port  6   b  and in to the common flow passage  8   b . The combination of one-way valves  16 ,  22 ,  30  ensures that exhalation gas flows only through the exhalation flow passage  28 , via the gas flow channel  12 , and into the anesthetic filter  10 , avoiding the carbon dioxide absorber  14 . As the exhalation gas passes through the filter  10  any unused anesthetic in the gas will be retained together with a small amount of the carbon dioxide that will also be present in the exhalation gas. The substantially anesthetic free exhalation gas then flows into the common flow passage  8   a  and out of the unit  2  through the port  6   a . In this manner the effectiveness of the delivery unit  2  in delivering anesthetic doses is prolonged since the anesthetic charge that was initially loaded into the sorption filter  10  is partially restored with unused anesthetic present in the exhalation gas that the delivery unit  2  “reflects” back to the patient&#39;s airways  36 .  
         [0028]     A second embodiment of an anesthetic delivery system is shown in  FIG. 3 . A delivery unit  46  is configured with gas flow paths substantially similar to those illustrated in  FIG. 1  and are again shown by arrows in the present figure. For ease of understanding items of the unit  46  of  FIG. 3  that are substantially similar to items of the unit  2  of  FIG. 1  are identified with corresponding reference numerals.  
         [0029]     As described with respect to  FIG. 1 , a port  6   a  is provided in the unit  46  and delimits one end of a common flow passage  8   a . An anesthetic filter  48  is located with a first side in gas communication with the common flow passage  8   a  and with a second, opposing, side for gas communication with a removable carbon dioxide filter  50  by means of a flow channel  12 . The carbon dioxide filter  50 , when inserted into the unit  46  (broken line construction of  FIG. 2 ) through the co-operating access slot  52 , is also located in gas communication with a second common flow passage  8   b  that is delimited at one end by a port  6   b  in the unit  46 . A one-way valve  16  is disposed to prevent gas flow from the common flow passage  8   b  into the carbon dioxide filter  50 .  
         [0030]     The common flow passage  8   b  also provides for gas communication between the port  6   b  and a flow passage  28  that is arranged to communicate with the anesthetic filter  48  via an opening  24  in the common flow passage  12 . A one-way valve  30  ensures that gas can only flow in the flow passage  28  in a direction from the port  6   b.    
         [0031]     Different from the embodiment of  FIG. 1 , a bacteria filter  54  is located, optionally removably located, in the flow passage  28  to prevent contamination of the anesthetic filter  48  by bacteria that may be present in exhalation gas flowing into the unit  46  through the port  6   b.    
         [0032]     Also different from the embodiment of  FIG. 1 , the delivery unit  46  of  FIG. 3  contains a housing  56  in which is held a charge  58  of anesthetic agent within a frangible container  60 . The housing  56  is provided with an opening  62  through which the charge  58  of anesthetic agent may flow to load the anesthetic filter  48  prior to use. The housing  56  is here also provided with internal walls  64 , shaped to funnel the flow of anesthetic agent towards the opening  62 . The housing  56  is further provided with an inwardly deformable wall section  66  that is accessible from external the delivery unit  46 . In use, an external force may be applied to this wall section  66  to cause its deformation and a consequent transmission of the force to the frangible container  60 . This results in the container  60  breaking to release the charge  58 . A removable rigid cover  68  is preferably provided to overlay the deformable wall section  66  to prevent accidental breakage of the container  60 . The housing  56  and the anesthetic filter  48  may be formed as a single unit, so as to be removable, within the delivery unit  46 .  
         [0033]     A bypass gas flow passage  18  is connected for fluid communication with the common flow passage  8   a  by an opening  20  and with the carbon dioxide filter  50  through the opening  24  in the flow passage  12 . Similar to the embodiment of  FIG. 1 , a one-way valve  22  is provided to ensure that gas is able to flow through the bypass gas flow passage  18  only in the direction from the common flow passage  8   a , towards the carbon dioxide filter  50 . A vane  70  is provided within the passage  18  and is rotatable to present a variable resistance to gas flow from the common flow passage  8   a  and thereby control the amount of gas bypassing the anesthetic filter  48 . The vane  70  is coupled to an anesthetic concentration sensor  72  via a linkage  74 . The rotational position of the vane  70  is automatically variable to change the flow resistance it presents dependent on the concentration of anesthetic that is sensed by the sensor  72 . In the present exemplary embodiment the concentration sensor  72  is formed of a silicone rubber block, a material that varies its physical dimensions in response to exposure to anesthetic, configured such that, in co-operation with the linkage  74 , it will exert a force on the vane  70  tending to cause the vane  70  to rotate and present a reducing resistance with increasing anesthetic concentration at the sensor  72 .  
         [0034]     A part of third embodiment of an anesthetic delivery system according to the present invention is shown in  FIG. 4  and shows an anesthetic absorption filter arrangement that may be employed as an alternative to those of  FIG. 1  and  FIG. 2 . In this third embodiment a membrane  76  replaces part of an external wall  78  of an anesthetic delivery unit  80 . The membrane  76  is of a type well known in the art of, for example implantable insulin pumps or of drug administration in ventilators, and is formed of a material that re-seals when a puncturing syringe needle is withdrawn. The membrane  76  of the present embodiment partially overlays and is presented here as being in intimate contact with an outer surface  82  of an anesthetic sorption filter  84 . A charge  86  of anesthetic agent is provided in a syringe  88  for injection through the membrane  76  and into an anesthetic receiving portion  84   b  of the filter  84  to load at least part of the filter  84  with anesthetic agent for delivery to a patient. In this manner a region  84   a  of the anesthetic filter  84 , which corresponds to that region  10   a  of the filter  10  of the embodiment shown in  FIG. 1 , may be provided that is initially substantially anesthetic free.  
         [0035]     It will be appreciated that by using the combination of re-sealable membrane  76  and syringe  88  the sorption filter  84  may be optionally re-loaded during use. Moreover, this combination enables the filter  84  to be loaded immediately before use, which facilitates the storage of the anesthetic delivery system. Additionally the filter  84  may be loaded with an anesthetic agent of choice so that only a single construction type of delivery unit  80  needs to be manufactured.  
         [0036]     Although modifications and changes may be suggested by those skilled in the art, it is the invention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.