Abstract:
A therapy utilizing inhaled anesthetic agents (such as desflurane, sevoflurane, isoflurane, or xenon) for the sedation of patients outside of the immediate perioperative space such as in the medical intensive care unit (MICU) and the surgical intensive care unit (SICU). The therapy includes controlled delivery of volatile anesthetic agents to patients undergoing ventilatory support on an ICU ventilator over extended periods of time. A system which provides for the delivery of anesthetic agents includes an anesthetic agent vaporizer element, an anesthetic agent reflector, and a plug-in cassette which contains both a cartridge housing liquid phase volatile anesthetic agent and an anesthetic vapor scrubbing medium.

Description:
BACKGROUND 
       [0001]    1. Field of the Disclosure 
         [0002]    This disclosure relates generally to devices for administering anesthesia to patients and, more particularly, to the long term delivery of inhaled anesthetic agents to intubated patients undergoing ventilatory support outside of the perioperative environment; and devices for the delivery of said agents. 
         [0003]    2. Description of the Related Art 
         [0004]    Anesthesia using volatile anesthetic agents is commonplace. The properties of inhaled anesthetics are well known in the literature for their favorable properties, including rapid onset and recovery, controllability and a favorable safety profile. Such favorable properties more closely approximate an ‘ideal’ agent than common intravenously-delivered (IV) agents typically used in the intensive care unit (ICU) environment for patient sedation. To the present day, the use of volatile anesthetic agents has been (largely) limited to operating rooms (OR&#39;s) for procedures where the clinical objectives include rendering and maintaining unconsciousness, analgesia and amnesia. 
         [0005]    A therapy of the present disclosure involves the use of these agents outside of the OR. Specifically, this disclosure includes a therapy in which the above clinical objectives are carried out outside the perioperative environment; such as in the ICU. The therapy of the present disclosure involves the use of primarily sub-maximum allowable concentration (sub-MAC) dosages of these agents over periods which can extend to multiple days in contrast to OR-based use of these agents in which patients are typically sedated at higher MAC concentrations and for shorter periods of time (hours). Additionally, the optimal volatile anesthetic agent for the therapy is described, and one embodiment of a hardware configuration suitable for implementation of this therapy is described in detail. 
         [0006]    The origins of inhaled anesthetic agents can be traced back to the late 18 th  century when British clergyman, philosopher, and educator, Joseph Priestly, first identified the gas nitrous oxide. Priestly&#39;s prodigy, Humphrey Davy, later recognized the analgesic effects of nitrous oxide in the early 19 th  century. Early attempts at using anesthetic agents for pain-free surgery included the use of Diethyl Ether, Chloroform, and Nitrous Oxide and the origins of the practice of anesthesiology is typically traced to the 1840&#39;s when the first operation using such agents for the excision of a neck mass was performed at Massachusetts General Hospital by Dr. William Morton (although others claim to have used the agents earlier). The somewhat concurrent rise of the use of injectable local anesthetics traces back to the 1850&#39;s when morphine was first injected for treatment of painful neuralgias. The use of other injectable and IV agents followed including cocaine in the 1880&#39;s, and procaine in the early 1900&#39;s. The science of both inhaled and IV anesthetic compounds has, of course, progressed and today common inhaled agents include sevoflurane, isoflurane, and desflurane. Common IV anesthetics have progressed from chloral hydrate, to short acting barbiturates such as thiopental, to the common medications used today such as propofol, midazolam and dexmedatomidine. 
         [0007]    To this day, the use of inhaled agents is generally limited to the operating room (OR), (although uses in MRI and Labor and Delivery settings are envisioned), and patients who have undergone surgical procedures are typically switched from inhaled agents in the OR to IV medications when moved (post-surgery) to the intensive care unit (ICU). All IV medications currently in use for sedation suffer from drawbacks including undesirable variability in patient wakeup time following discontinuation, and difficulty in clinical control of depth of sedation. Modern inhaled anesthetics such as desflurane, by contrast, are widely regarded as having rapid onset and recovery, and good controllability. Therefore, it is the intent of the therapy and system of this disclosure to extend the useful range of inhaled anesthetics beyond the operating room environment into the ICU environment for intubated, mechanically ventilated patient populations. Further, it is the intent of the present disclosure to extend the duration of therapy from hours, as is current practice in the OR, to potentially (multiple) days of therapy. 
         [0008]    In order to achieve these goals, a hardware system will be described which integrates into existing technologies being used in the ICU environment for patient respiratory support. Specifically, the therapy of the present disclosure integrates with typical ICU ventilators, and as such provides inhaled agents to the patient concomitant with ventilator gas flow. The integration of the technology is not ventilator specific, meaning that it will integrate with any modern ICU ventilator. Further the technology is not mode specific and will work independent of ventilator mode and settings, ensuring broad functionality of the technology with modern ICU ventilators. 
         [0009]    Prior art can be broadly classified as falling into three categories, first are patents related to technologies for anesthetic vapor delivery in an OR setting, second are patents on volatile anesthetic scavenging/reclamation as applied to the OR setting, and third are patents related to anesthetic reflector technologies for the on-airway conservation of anesthetic vapor for use outside the OR setting. 
         [0010]    It is an object of the present disclosure to provide an anesthesia therapy and device which can be implemented using modern, microprocessor-based ICU ventilators. This objective being, in general, at odds with the plurality of art which seeks to provide such therapy in an OR setting. The difference in clinical setting may require different sedative regimens, as well as technology with notably different pneumatic and functional characteristics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a perspective view of the vaporizer-based anesthesia delivery system of the present disclosure; 
           [0012]      FIG. 2  is a partially exploded perspective view of a package containing a refill cassette and a reflector for use in the vaporizer-based anesthesia delivery system of  FIG. 1 ; 
           [0013]      FIG. 3  is a perspective view of a reflector, partially broken away, for use as part of the vaporizer-based anesthesia delivery system of  FIG. 1 ; 
           [0014]      FIG. 4  is an exploded perspective view of a vaporizer and refill cassette of the anesthesia delivery system of  FIG. 1 ; 
           [0015]      FIG. 5  is a cross-sectional view of the refill cassette of the anesthesia delivery system of  FIG. 1 , taken along lines  5 - 5  of  FIG. 4 , the refill cartridge containing a bottle of an anesthetic agent to be vaporized and a surrounding scrubbing agent for filtering exhaled gases; 
           [0016]      FIG. 6  is a perspective view, partially broken away, of the bottle of anesthetic agent provided in the refill cassette; 
           [0017]      FIG. 7  is a cross-sectional view of the bottle of anesthetic agent, taken along lines  7 - 7  of  FIG. 6 ; 
           [0018]      FIG. 8  is a schematic diagram of the components of the vaporizer of the anesthesia delivery system of  FIG. 1 ; 
           [0019]      FIG. 9  is a perspective view of an alternate embodiment of the vaporizer-based anesthetic delivery system of the present disclosure; 
           [0020]      FIG. 10  is a schematic diagram of a cassette of the alternate embodiment of the vaporizer-based anesthetic delivery system of  FIG. 9 ; and 
           [0021]      FIG. 11  is a schematic view of a lower portion of the cassette of  FIG. 10 , with  FIG. 11   a  illustrating a gas flow path through the lower portion of the cassette in a first state, and  FIG. 11   b  illustrating a gas flow path through lower portion of the cassette in a second state. 
       
    
    
     SUMMARY OF DISCLOSURE 
     The Therapy 
       [0022]    The therapy of the present disclosure involves the delivery of volatile anesthetic agent (preferably using, but not limited to, desflurane) for patients undergoing mechanical ventilatory support using an ICU ventilator, in an ICU or similar setting. The therapy generally includes sub-MAC dosages of the anesthetic agents over short to prolonged periods of up to several days. The use of such agents, which will decrease or replace the use of sedative IV medications in these patients, is intended to provide therapeutic benefit and/or cost advantages over IV medications commonly in use in the ICU. 
         [0023]    Therapeutic benefits may include a reduction in ICU and hospital length of stay (LOS), an increased predictability and reduction in patient wake-up time, a reduction of incidents of self-extubation, and a reduction in morbidity and mortality. 
         [0024]    The therapy includes the delivery of physician controlled desflurane vapor to the breathing circuit of patients undergoing ventilatory support. The physician will select the end tidal concentration of the agent to be delivered, this concentration being reflective of patient alveolar (deep lung) concentration. It is intended that the patients&#39; end tidal concentration will be held at this clinician selected level until intervention such as a concentration settings change or until patient extubation is performed by a clinician. 
         [0025]    The therapy of the present disclosure may be applied to the use of all modern volatile anesthetic agents including isoflurane, sevoflurane and desflurane. Isoflurane, the least expensive agent of the three and a reasonable candidate for use with the invention described here, might be obviated by its high blood and tissue solubility The use of isoflurane (an older agent) in the OR environment was associated with a longer wake-up time and return to cognitive function compared to newer agents such as desflurane. Sevoflurane, also a reasonable candidate for use with the therapy of the present disclosure, can produce a buildup of inorganic fluorides in the body of patients ostensibly due to metabolism of the agent. While the buildup of these fluorides has not been proven clinically to be dangerous in such patients, the levels seen after multiple day usage of this agent are worrisome to clinicians and, as a result, sevoflurane is a less attractive agent for this invention. Sevoflurane fat and muscle solubility coefficients are as high as isoflurane and this may lead to prolonged elimination. Finally, desflurane is the lowest soluble agent in blood, fat and muscle; also desflurane presents the lowest degree of biodegradation (0.02% compared to isoflurane 0.2% and sevoflurane 5%). In summary, desflurane does not possess the attendant drawbacks noted for isoflurane and sevoflurane and is, thus, currently seen as the agent of choice for the therapy of the present disclosure with two caveats. The first caveat being that since desflurane boils at room temperature due to its high vapor pressure, it presents unique technical challenges relative to its controlled delivery in a clinical environment. The second caveat being that desflurane has not (as of the writing of this patent) been proven to be free of the buildup of dangerous metabolites or the production of significant side-effects when used on a multi-day basis. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    In order to integrate with a typical ICU ventilator, the hardware platform must provide many of the functions of a modern anesthesia workstation including a method for the controlled delivery of volatile anesthetic vapor to the patient airway, a method for clearing of exhaled carbon dioxide (CO 2 ), a method of producing adequate airway humidification, and a method of sequestering of exhaust anesthetic vapor to prevent pollution of the clinical environment. In addition, in order to be compatible with modern ICU ventilators, the therapy must be relatively pneumatically transparent to the ventilator. Pneumatic transparency implies that the therapy will not introduce a large amount of compressible volume to the patient circuit (in order to prevent lowered effective ventilation i.e. subsequent reductions in the volume of fresh gas entering the patients lungs). Further, the therapy must not introduce significant additional resistance to gas flow in the patient circuit as flow resistance can produce trapping of end exhalation lung gas leading to inadvertent positive end expiratory pressure (inadvertent PEEP). In the preferred embodiment of the present disclosure, the ICU ventilator functions as it normally would, providing fresh gas flow from the inspiratory circuit, and exhausting exhaled, CO 2  laden gas to the exhaust port and is not functionally impaired or compromised by the presence of the additional components of the system. 
         [0027]      FIG. 1  shows the hardware system  10  for the preferred embodiment at a systems level. The hardware system  10  is intended to be functional with a modern ICU ventilator  12  and is also intended to function to hold end tidal anesthetic agent concentrations at the set level regardless of ventilator setting or mode. The hardware configuration includes three primary elements: a vapor delivery device, also referred to herein as a vaporizer  14 , a disposable or reusable anesthetic cassette  16 , and an anesthetic reflector  18 . 
         [0028]    The vapor delivery device  14  serves the purpose of vaporizing liquid phase volatile anesthetic agents and delivering these agents, in vapor phase, and in a controlled fashion, to the patient airway. 
         [0029]    The disposable or reusable anesthetic cassette  16  includes two primary elements, a cartridge  20  containing liquid phase volatile anesthetic agent LA and a scrubber agent S responsible for removing waste anesthetic gas from the exhausted breathing path. 
         [0030]    Finally, the anesthetic reflector  18  is a device which is placed between the patient endotracheal tube  22  and a connector joining an inspiratory limb  24  and an expiratory limb  26  of a two-limb patient circuit to the endotracheal tube  22 , referred to herein as a patient circuit WYE  28  and functions to conserve the volume of anesthetic required for therapy thereby reducing the drug consumption and monetary cost of the therapy. 
         [0031]    The concentration of the anesthetic agent, such as desflurane, will be monitored and controlled by the system of the present disclosure using an anesthetic agent sensor AAS internal to the vaporizer element  14  and this sampled concentration will be used by a closed loop controller C (also internal to the vaporizer  14  and in electronic communication with the anesthetic agent sensor AAS) to maintain the anesthetic agent concentration at the clinician selected level. 
         [0032]    As shown in  FIG. 2 , the disposables for the hardware system  10 , including the anesthetic reflector  18  and the cartridge  20 , may be supplied in a disposable or reusable package  30  to clinicians based on therapy duration (e.g. the contents of one package  30  provides 8 hours of therapy). 
       The Reflector 
       [0033]    The reflector  18  shown in  FIG. 3  includes a (preferably transparent) plastic, tubular outer shell  32 , which is dimensionally constructed to interface with both the endotracheal tube  22  and the patient circuit WYE  28  by press fit. The reflector  18  houses an adsorbent media  34 . Adsorbents are high surface area materials (1000 m 2 /g) that collect vapors out of an enriched gas stream and have been observed to subsequently release vapors into a lean gas stream. In addition, the reflector  18  optionally includes at least one filter medium  36  to prevent the free movement of adsorbent media  34  into the patient airway. The reflector also optionally includes heat and moisture exchanger (HME) media  38  for preservation of patient airway humidification. Such HME media is typically hygroscopic or hydrophobic in nature. The reflector  18  preferably has an enlarged-diameter central portion  62  in which the adsorbent media  34 , the filter media  36 , and the HME media  38  are provided. 
         [0034]    The reflector  18  is designed to have a relatively low airflow resistance (approximately 1-2 cmH 2 O drop at 60 LPM flow) and relatively low internal volume (&lt;100 ml) so as to ensure adequate fresh gas ventilation of the patient ensuring that the re-breathing of CO 2 -laden gas is minimized. The reflector  18  is designed with specific adsorbent material(s) which work well with desflurane (i.e. produce good capture and release efficiencies and kinetics). Finally, the reflector  18  is designed to be effective for at least the therapeutic lifespan of the anesthetic cassette  16  and is designed to be relatively low cost in construction as it is a disposable element of the therapy. 
       The Cassette 
       [0035]    The cassette  16  is a disposable and/or reusable element of the system  10  designed both to provide volatile anesthetic agent in liquid form to the vaporizer element and to scrub exhausted breathing gas of anesthetic agent. In order to achieve this functionality, the anesthetic agent cartridge  20  (described in more detail below) and aggressive scrubber media are co-located and pneumatically isolated from one another in the same anesthetic cassette  16 .  FIG. 4  shows the basic configuration of the cassette  16 , which is designed to easily drop into a cavity  40  in the vaporizer  14  and removably plug into the vaporizer  14  for rapid change out once the cassette  16  is depleted. The cassette  16  is designed such that the anesthetic agent LA and the scrubber S have an equivalent therapeutic lifespan ensuring that both elements are somewhat equally consumed at the end of the cartridge therapeutic life of the cartridge  20 . 
         [0036]    The anesthetic agent LA is volatile, and is housed in an aerosol or pressurized cartridge  20  which is capped by a valve  42  which is normally sealed and opens when engaged with a receiving mechanism located in the vaporizer  14 . The scrubber agent S is packed around the cartridge  20 . The cassette  16  is provided with an exhausted ventilator gas inlet port  44  and a vent outlet port  46 . 
         [0037]    Exhaled gas is drawn from the patient, through the endotracheal tube  22 , through the reflector  18  (with at least some of the exhaled anesthesia agent being caught by the adsorbent  34  within the reflector  18 ), through the patient circuit WYE  28  and the expiratory limb  26  of the patient circuit, and to the ventilator  12 . An exhausted ventilator gas tube  48  feeds exhausted air from the ventilator  12  to the exhausted ventilator gas inlet port  44 . A gas-tight inlet aperture  50  may be provided in wall  52  of the cavity  40  of the vaporizer  14  to facilitate connection between the exhausted ventilator gas tube  48  and the exhausted gas inlet port  44 . Exhausted ventilator gas circulates through the cassette  16  and past the scrubber agent S, before being vented through the vent outlet port  46 . At least one vent outlet aperture  54  may also be provided in wall  52  of the cavity  40  of the vaporizer  14 , so that scrubbed gas may be vented from the vent outlet port  46  to the surrounding atmosphere in the clinical environment. As the anesthetic agent has been scrubbed from the gas prior to venting, medical professionals and other patients in the clinical environment will not be exposed to unacceptable levels of anesthetic vapor in the air. 
         [0038]    Since this system  10  is intended for use in an ICU (and possibly OR) clinical environment, and further since the system  10  is designed to (preferably) impose little if any additional workload on the clinicians over existing technologies, such as IV delivery of medicaments, the cassette  16  itself is thus designed to be rapidly changed out (of the overall system) with little effort and time required of the clinicians. This can be achieved by self locking (electronic or mechanical) or user latched mechanisms which engage the cassette  16  with the vaporizer  14  in a liquid and air tight coupling. 
         [0039]    The cassette  16  in one embodiment may be electronically identified as being a valid (non-counterfeit) cassette  16  by the use of an RFID tag affixed to the cassette which is in turn read by the vaporizer system during cassette engagement (plug-in). Using such an RFID (or similar electromagnetic or optical identification) modality, only valid cassettes would be allowed by the system  10  for use in therapy and counterfeit cassettes  16  would generate a system halt. This measure being taken to prevent the use of counterfeit cassettes  16  with the system and further to ensure that only approved volatile anesthetic agent can be used for the therapy. 
       The Vaporizer 
       [0040]    The vaporizer  14  contains internal hardware which takes liquid phase anesthetic agent LA from the cassette  16 , converts it to vapor phase anesthetic agent, and delivers it, in controlled fashion, to the patient airway through a tube  55  that leads from a vaporized anesthetic outlet port  56  on the vaporizer  14  to a vaporized anesthetic inlet port  58  provided on a side of the reflector  18  opposite the patient circuit WYE. As shown in  FIG. 3 , the vaporized anesthetic inlet port  58  may be provided on the outer shell  32  of the reflector between an endotracheal tube interface connection  60  of the reflector  18  (by which the reflector  18  connects to the endotracheal tube  22 ) and an enlarged-diameter central portion  62  of the reflector containing the adsorbent  34 , the filter medium  36  and the HME media  38 . 
         [0041]    The internal elements of the vaporizer  14  are represented schematically in  FIG. 8  and include a vaporizing technology  64 , a microcontroller  66  and supporting electronics, a pump  68 , pressure and/or flow sensors  70 , and at least one volatile anesthetic sensor element  72 . Suitable vaporizing technology  64  may include, but is not limited to, the use of wicks for evaporative production of anesthetic vapor, the use of heated sumps with bypass gas flow which entrains anesthetic vapor, and the use of valves to precisely deliver liquid anesthetic to a gas stream wherein it is vaporized. The vaporizer  14  draws gas out of the patient airway (distal to the endotracheal tube  22  and proximal to the reflector  18 ) and records the airway concentration using the anesthetic sensor element  72 . The gas is then preferably returned through a multi-lumen umbilical to the patient airway. The concentration of anesthetic agent, as measured by the anesthetic sensor element  72 , is then used as the feedback signal for a closed loop feedback system, including the microcontroller  66 , which is employed to hold the end tidal level of anesthetic agent at a clinician-selected level. Closed loop anesthetic agent delivery is realized by a controlled flow of vapor phase agent out of the vaporizer  14  through the multi-lumen umbilical and into the patient circuit. The vaporizer microcontroller  66  provides control for the entire system and as such, monitors and controls system function and can provide user alarm functionality. The vaporizer  14  is preferably provided with a user interface  74 , including a display  76  and an input modality  78 , such as a touch-sensitive screen or push buttons. 
       The Cartridge 
       [0042]    The cartridge  20  of the cassette  16  may include a pressurize-able metallic or plastic housing similar to an aerosol paint can in which the internal volatile anesthetic agent is pressurized by a gas propellant and liquid anesthetic agent is allowed to exit via a spring activated valve. The internal anesthetic agent may be separated from the gas propellant by a flexible medium which maintains hermetic isolation between the agent and the propellant. It is further understood that the pressurized propellant may serve to prevent such high vapor pressure agents as desflurane from boiling (inasmuch as desflurane at atmospheric pressure boils at room temperature). 
         [0043]    An alternate embodiment of the cartridge  20  is shown in  FIGS. 6 and 7 , in which a rubber banding places consistent pressure on an internal bladder  80  which houses liquid volatile anesthetic agent LA thus allowing for nearly consistent anesthetic liquid outlet pressure during use of the cartridge  20 . The bladder  80  may use an external rubber tube  82  surrounding the bladder  80  to exert pressure onto the bladder walls forcing the liquid anesthetic agent LA out of the bladder  80 . In order to ensure that the anesthetic agent LA inside the cartridge can be used without ambient agent leakage, the interface between the cartridge  20  and the vaporizer  14  must be sealed. 
       Anesthetic Scrubbing Media 
       [0044]    The anesthetic scrubbing media S may include an absorbent and/or adsorbent and/or chemically reactive media which is co-located with, yet hermetically isolated from the internal contents of the cartridge  20 . The scrubbing media S and cassette  20  being designed to allow exhaled gas to pass through the scrubbing media S facilitating the scrubbing of the anesthetic agent from this waste exhaled gas. It is further understood that the scrubbing media S might be disposable or might consist of materials such as zeolites (e.g., silacalite—a hydrophobic zeolite which has been shown to capture volatile anesthetic agents effectively) which may allow for the reuse of both the zeolite material as well as the recycling of the captured anesthetic agent after a post-processing stage. 
         [0045]    The anesthetic scrubbing agent is either in pelletized form or is sufficiently loosely packed so as not to significantly increase resistance to exhaled gas flow. 
         [0046]    Additional embodiments of this anesthetic scrubbing element S incorporate the use of active elements which scrub or capture the anesthetic given the input of external energy to the process. Such elements might include a miniature condenser based on (for instance) liquid oxygen or liquid nitrogen cooled devices oriented to cool the gas stream allowing for condensation of the anesthetic vapor into a collection reservoir for either external pharmaceutical post-processing for eventual reuse or for immediate reintroduction to the patient airway. A further embodiment of this anesthetic scrubbing element incorporates the use of pressure based devices such as miniature compressors which seek to condense out the anesthetic vapor by pressurizing the exhalation gas stream. 
       Alternate Embodiment 
       [0047]    An alternate preferred embodiment of the present disclosure shares many of the elements of the preferred embodiment, however, notably absent from this embodiment is the reflector element. Further, the construct of the plug-in cassette is somewhat different with another stage being added to this element.  FIG. 9  shows the alternate preferred embodiment from a systems level and as seen during therapy. This alternate preferred embodiment has four patient circuits which plug into the vaporizer body. These patient circuits include the inhale and exhale circuits of the ventilator, and two circuits which extend from the vaporizer body to the patient. In addition, there continues to exist an exhaust scrubbing line which plugs into the back of the cassette as in the first preferred embodiment, described above. However as described below, this exhaust scrubbing capability is optional. Finally, as with the first preferred embodiment, this alternate preferred embodiment is highly pneumatically transparent to the ventilator, thus imposing little additional resistance to gas flow and only a small amount of additional compressible volume to the patient circuit. 
       The Cassette 
       [0048]    The cassette  116  (as illustrated in  FIGS. 10 and 11 ) of the alternate preferred embodiment includes an upper level and a lower level, designed for distinct functional roles. The upper level of the cassette is intended to perform the same functions as the cassette  16  described in the first embodiment, described above, and serves to both supply volatile anesthetic agent to the vaporizer, as well as to scrub ventilator exhaust gas of residual volatile anesthetic agent before releasing exhaust gas into the clinical environment. The lower level of the cassette includes two pneumatically isolated adsorbent beds which, when the cassette  116  is connected to the vaporizer, allows for the switching of gas flow between the two. 
         [0049]    For example, during a period of ventilation the gas flow paths will be configured with the inhale path traversing one of the two adsorbent beds, while the exhale path traverses the other. During this period, exhaled volatile anesthetic agent will accumulate in the adsorbent bed of the exhaled gas path resulting from exhaled gas laden with anesthetic agent. At some point, “Breakthrough” of anesthetic agent will occur in this exhaled gas path. Breakthrough is defined as a sudden and significant increase in volatile anesthetic agent being evident at the adsorbent bed on the ventilator side of the exhaled path. When breakthrough is sensed (preferably by an anesthetic sensor located within the body of the vaporizer element) the adsorbent material on the exhaled path is assumed to be functionally saturated with anesthetic agent. Therefore, at this time the vaporizer will switch internal valve states placing the two adsorbent beds in the others&#39; former pneumatic path. This switching allows the now saturated adsorbent bed which was previously in the exhale path, to be placed in the inhale path. When this occurs, adsorbed anesthetic agent will be available for release from this saturated adsorbent bed into the inhaled gas stream of the patient. This process the repeats itself during the course of therapy, always utilizing breakthrough as a trigger to switch pneumatic paths. 
         [0050]    Such a pneumatic implementation will lead to relatively high efficiencies of agent conservation, as the noted pneumatic pathway changes always occur when exhausted agent levels are first noted to be rising significantly from a low baseline level. This configuration might also obviate the need for the scrubber element located in the upper portion of the cassette given that the technique provides acceptably low agent release into the clinical environment. 
       The Vaporizer 
       [0051]    The vaporizer in the second preferred embodiment performs the same functions as the vaporizer described in the first preferred embodiment, with two notable differences. First the vaporizer now includes valves (and optionally manifolds) allowing for the switching of the pneumatic pathways between the two adsorbent beds as described earlier. Second, an anesthetic sensor to sense anesthetic breakthrough is additionally preferably provided. 
         [0052]    While various embodiments have been described herein, it is understood that the appended claims are not intended to be limited thereto, and may include variations that are still within the literal or equivalent scope of the claims.