Virtual respiratory gas delivery systems and circuits

Abstract

A respiratory gas delivery system monitors gas flow over the course of a breath in real time and uses this parameter to simulate, in whole or part, the function of a reference respiratory gas delivery system, in particular structural features, particularly structural components of parts of the reference system, to overcome a structural limitation of the reference system.

1 . A respiratory gas delivery system adapted to deliver an inspiratory gas of variable composition comprising:
 A. a gas delivery apparatus operatively connected to a processor;   B. a flow sensor adapted to monitor in real time the rate of inspiration of a gas;   wherein, for a plurality of respective inspiratory cycles [i]1 to [i]n and a plurality of time points [t]1 to [t]n over the course of a respective inspiratory cycle [i], the processor is configured to:   (a) use output from the flow sensor to monitor the cumulative volume of gas inspired in the respective inspiratory cycle at any given time point [t]1 to [t] n ;   (b) execute an algorithm to determine a desired composition of the inspired gas based on whether or not at least one threshold cumulative volume of a desired gas composition has been inspired in the respective inspiratory cycle, the desired composition including a composition selected from a first composition selected for delivery for a first portion of an inspiratory cycle and at least one alternate nth composition selected for delivery during the course of the inspiratory cycle; and   (c) generate a control signal effective to signal the gas delivery apparatus to deliver the first composition in the first part of an inspiratory cycle and then the nth composition during the course of the inspiratory cycle based on whether or not the at least one threshold cumulative volume has been reached.   
     
     
         2 . A respiratory gas delivery system as claimed in  claim 1 , wherein the composition corresponding to a first portion of an inspiratory cycle is determined using at least one first criterion and wherein the at least one alternate composition is determined using at least one different criterion. 
     
     
         3 . A respiratory gas delivery system as claimed in any one of  claims 1  and  2 , wherein the at least one cumulative volume is set to be less than a subject's tidal volume minus anatomic dead space volume such that the entire volume of the composition corresponding to a first portion of an inspiratory cycle is destined to enter a subject's alveolar space. 
     
     
         4 . A respiratory gas delivery system according to any one of  claims 1 ,  2  and  3 , wherein the alternate composition is a neutral gas. 
     
     
         5 . A respiratory gas delivery system according to any one of  claims 1 ,  2  and  3 , wherein the alternate composition is a percentage composition of a constituent gas as low as 0%, wherein the constituent gas is of a type determined by a user to warrant conservation by reducing delivery to the anatomical dead space. 
     
     
         6 . A system as claimed in  claim 1 , wherein the processor is configured to simulate gas delivery from at least a virtual first gas reservoir and a virtual second gas reservoir, wherein:
 (a) the first gas reservoir and the second gas reservoir contain a gas of specifiable or specified composition;   (b) at least the first gas reservoir is assumed to contain a gas corresponding to a first portion of an inspiratory cycle, the processor configured to send a control signal to signal to the gas delivery apparatus to deliver a gas of a specified composition of the first gas reservoir for the first part of a respective inspiratory cycle [i], the first gas reservoir set to contain a volume of gas adapted to be depleted in each inspiratory cycle at a reservoir specific depletion rate which tracks the inspiratory flow rate measured by the flow sensor;   (c) the processor generates a control signal effective to signal the gas delivery apparatus to deliver a gas of composition substantially equal to the specifiable or specified composition of the at least second gas reservoir for a second part of a respective inspiratory cycle [i] when the first gas reservoir is depleted.   
     
     
         7 . A system as claimed in any one of the preceding claims, wherein the volume of the at least first gas reservoir is set based on an assumption that the first gas reservoir is continually filled with a gas of the specified composition at a specifiable or specified reservoir-specific fill rate which is less than the reservoir specific depletion rate. 
     
     
         8 . A system as claimed in any one of the preceding claims, wherein the volume of the at least first gas reservoir is set based on an assumption that the first gas reservoir is full at the start of an inspiratory cycle, the volume selected to be a volume that can be predicted to be depleted at a reservoir specific depletion rate which tracks the inspiratory flow rate measured by the flow sensor. 
     
     
         9 . A system as claimed in any one of the preceding claims, wherein the apparatus is configured to deliver a first gas of a first composition for a first part of each inspiratory cycle [i] and a second gas of a second composition for a second part of each inspiratory cycle [i]. 
     
     
         10 . A system as claimed in  claim 9 , wherein the apparatus is configured to simulate gas delivery from at least two gas reservoirs, wherein the first reservoir is exclusively depleted in a first part of a each inspiratory cycle [i], and the second reservoir is exclusively delivered in a second part of each inspiratory cycle [i]. 
     
     
         11 . A system as claimed in  claim 7 , wherein the fill rate of the first reservoir is less than the subject's total inspired volume minus the total volume of gas inspired into the anatomic dead space volume over a measurement interval. 
     
     
         12 . A system as claimed in  claim 11 , wherein the measurement interval is one minute. 
     
     
         13 . A system as claimed in any one of the preceding claims, wherein the composition of gas delivered in the second part of each inspiratory cycle [i] is neutral with respect to at least one gas X in the inspiratory gas. 
     
     
         14 . A respiratory gas delivery system adapted for use with a first breathing circuit having at least one gas conduit leading to a patient airway interface, characterized in that the respiratory gas delivery system virtualizes gas delivery characteristics of a reference respiratory gas delivery system that is adapted to be used in conjunction with a reference second breathing circuit, the gas delivery characteristics of the reference respiratory gas system dictated at least in part by at least one structural component of the reference second breathing circuit, the respiratory gas delivery system including:
 a) a flow sensor positioned for determining at least an inspiratory flow rate;   b) a gas delivery apparatus adapted to deliver a gas comprising a plurality of component gases through the patient airway interface, the gas delivery apparatus operatively connected to a computer, the computer configured to supplant at least one structural component of the reference second breathing circuit by using a mathematical model or algorithmic model of the at least one supplanted structural component to generate gas delivery characteristics that are functionally equivalent to those generated by the at least one supplanted structural component.   
     
     
         15 . A system as claim in  claim 14 , wherein the computer is configured to simulate structural features defined by at least one component of a sequential gas delivery circuit. 
     
     
         16 . A respiratory gas delivery system according to  claim 14 , wherein the reference breathing circuit is a re-breathing circuit which organizes the delivery of a first component gas of selected first composition and a second component gas comprising exhaled gas and wherein the computer is configured to signal the gas delivery apparatus to deliver a component gas of selected first composition and a component gas of second composition including a gas of composition which matches that of at least one component of an exhaled gas. 
     
     
         17 . A system as claimed in  claim 15 , wherein the components gases are formulated for delivery by the gas delivery apparatus by blending requisite constituent gases. 
     
     
         18 . A system as claimed in any one of  claims 15 ,  16  and  17 , wherein the component gas of second composition has a percentage composition of at least one gas X which substantially matches the percentage composition of a gas X in a subject's last exhaled end tidal gas. 
     
     
         19 . A system as claimed in  claim 18 , further comprising a gas analyzer operatively connected to the computer to provide input of a value corresponding to an amount of the at least a gas X in the subject's last exhaled end tidal gas. 
     
     
         20 . A respiratory gas delivery system according to any one of  claims 14  to  18 , wherein the reference breathing circuit is a sequential gas delivery circuit having a flow control system which alternately directs gas flow from: (a) a first circuit flow path organized to deliver a first gas component of the first gas composition; and (b) a second circuit flow path organized to deliver a second gas component of the second gas composition. 
     
     
         21 . A respiratory gas delivery system according to  claim 20 , wherein the at least one supplanted structural component of the reference breathing circuit comprises a component which arrests flow during the course of substantially each respective inspiratory cycle [i] from the first circuit flow path, and subsequently, in the same inspiratory cycle, initiates gas flow from the second circuit flow path and wherein flow from the first and second flow paths is substituted in the respiratory gas delivery system by controlling the gas delivery apparatus to alternately deliver a gas of first composition of a selected volume corresponding to that of the first gas component and a gas of second composition corresponding the second gas component. 
     
     
         22 . A respiratory gas system according to  claim 21 , wherein the reference respiratory gas delivery system is adapted to deliver a component gas of first composition from a first circuit flow path leading from a first gas reservoir, and when the first gas reservoir is depleted, a component gas of second composition from a second circuit flow path. 
     
     
         23 . A respiratory gas system according to  claim 22 , wherein the second flow path is a second gas reservoir, a gas output port operatively connected to a gas source or an ambient air inlet. 
     
     
         24 . A respiratory gas system according to  claim 21 , wherein the gas delivery apparatus is controlled in part by modelling depletion of the first gas reservoir at a rate which tracks a subject's real time inspiratory rate, the first gas reservoir set to have a volume which is replenished in each inspiratory cycle and depleted in substantially each inspiratory cycle. 
     
     
         25 . A respiratory gas system according to  claim 24 , wherein delivery of first gas composition virtualizes filling the first gas reservoir at a flow rate which matches the fill rate of first gas reservoir of the reference second breathing circuit and emptying of the first gas reservoir at a rate of inspiration monitored in real time, wherein the respiratory gas system is adapted to supplant the first gas reservoir and at least one structural component involved in cyclically arresting flow from the first circuit flow path and initiation of flow from the second circuit flow path. 
     
     
         26 . A respiratory gas system according to  claim 25 , wherein the at least one structural component is a sequential gas delivery valve. 
     
     
         27 . A respiratory gas system according to  claim 26 , wherein the first and second circuit flow paths are optionally replaced with a common circuit flow path leading from the gas delivery apparatus to the patient airway interface and wherein the gas delivery apparatus is configured to first deliver via the common circuit flow path a gas of first composition and then a gas of second composition. 
     
     
         28 . A respiratory gas system according to  claim 27 , wherein the gas delivery apparatus is a gas blender and wherein the computer signals the gas delivery apparatus to alternately deliver, in a repeating cycle, a gas blend of a volume of the gas of first composition for the first portion of an inspiratory cycle and then a gas blend the gas of the second composition for the remainder of an inspiratory cycle. 
     
     
         29 . A respiratory gas system according to  claim 28 , further comprising a pressure transducer to track the beginning and end of each inspiratory cycle. 
     
     
         30 . A respiratory gas system according to  claim 14 , wherein the reference breathing circuit comprises a first gas reservoir and a second gas reservoir and wherein the first and second gas reservoirs are absent from the first breathing circuit and wherein the gas delivery apparatus is programmed to simulate gas delivery from the first gas reservoir using at least one of an algorithmic and mathematical model (e.g. of replenishment and/or depletion) of the first gas reservoir by delivering a gas of the first composition corresponding to a full or selected partial volume of the first gas reservoir and subsequently a gas of the second composition upon simulated depletion or partial depletion of the gas of first composition in the first gas reservoir. 
     
     
         31 . A respiratory gas system according to  claim 1 , wherein the computer simulates filling of an inspiratory gas reservoir at a rate of flow that is less that the subject's minute ventilation minus anatomic dead space ventilation. 
     
     
         32 . A respiratory gas system according to  claim 14 , including a gas analyzer wherein the computer simulates filling of an inspiratory gas reservoir at a rate of flow that is less than the subject's minute ventilation minus anatomic dead space ventilation and wherein the gas of second composition corresponds to that of subject's exhaled gas from a breath n−1 (the immediately previous expiratory cycle) as determined by the gas analyzer. 
     
     
         33 . A respiratory gas system according to  claim 14 , wherein the reference respiratory gas delivery system is adapted to be used in conjunction with a reference sequential gas delivery circuit including an inlet port into a first gas reservoir operatively connected to a gas blender for receiving a blended component gas of first composition from, and when the first reservoir is depleted, a component gas of second composition from a second gas reservoir containing a subject's end tidal exhaled gas, and wherein at least the first gas reservoir and second gas reservoir are absent from the first breathing circuit and wherein the gas delivery apparatus is programmed to simulate gas delivery from the first and second gas reservoirs using a mathematical model of replenishment and depletion of the first gas reservoir by delivering a gas of the first composition first and subsequently a gas of the second composition upon simulated depletion of the first gas reservoir. 
     
     
         34 . A respiratory gas system according to  claim 33 , wherein the computer simulates filling of an inspiratory gas reservoir at a rate of flow that is less that the subject's minute ventilation minus anatomic dead space ventilation. 
     
     
         35 . A respiratory gas system according to  claim 34 , including a gas analyzer wherein the computer simulates filling of an inspiratory gas reservoir at a rate of flow that is less that the subject's minute ventilation minus anatomic dead space ventilation and wherein the gas of second composition corresponds to that of subject's exhaled gas from a breath n−1. 
     
     
         36 . A respiratory gas delivery system adapted to be operatively connected to a first breathing circuit and virtualize, at least one structural feature of a reference second breathing circuit, the respiratory gas delivery system including:
 a) at least one measurement device operatively connected to the first breathing circuit;   b) a gas delivery apparatus operatively connected to a computer for controlling the gas delivery apparatus;   
       the computer configured to: (1) obtain input from the measurement device; (2) execute at least one of an algorithmic and mathematical model of the reference second breathing circuit; and (3) generate a control signal that signals the gas delivery apparatus to output at least one gas composition to the first breathing circuit such that the gas delivery characteristics of the respiratory gas delivery system simulate the at least one structural feature of the reference second breathing circuit. 
     
     
         37 . A respiratory gas system according to any one of  claims 1  and  36 , wherein the reference respiratory gas delivery system is adapted to deliver a component gas of first composition from a first gas reservoir, and when the first reservoir is depleted, a component gas of second composition from an alternate flow path, and wherein at least the first gas reservoir and alternate flow path are absent from the first breathing circuit and wherein the gas delivery apparatus is programmed to simulate gas delivery from the first gas reservoir and alternate flow path using at least one of a mathematical model and algorithmic model of depletion of the first gas reservoir by delivering a gas of the first composition first and subsequently a gas of the second composition upon simulated depletion of the first gas reservoir. 
     
     
         38 . A respiratory gas delivery system according to  claim 37 , wherein the measurement device is a flow sensor positioned to monitor the inspiratory flow rate through the first breathing circuit and wherein the model depletes the first gas reservoir at the inspiratory flow rate. 
     
     
         39 . A system as claimed in any one of  claims 1  to  13 , wherein the gas delivery apparatus includes a gas output port operatively connected to a breathing circuit including a gas delivery portion comprising a single gas conduit leading to a patient airway interface. 
     
     
         40 . A system as claimed in any one of  claims 1  to  13 , wherein the gas delivery apparatus is operatively connected to a breathing circuit in which the gas delivery portion consists essentially of a gas conduit leading operatively connected to a patient airway interface. 
     
     
         41 . A respiratory gas delivery system adapted for use with a first breathing circuit including a patient airway interface, characterized in that the respiratory gas delivery system virtualizes at least one structural component of a reference, second breathing circuit, the respiratory gas delivery system including:
 a) at least one device adapted for selecting a juncture during an inspiratory cycle for switching between a first gas composition and at least one alternate, nth gas composition,   b) a gas delivery apparatus for delivering a gas comprising a plurality of component gases into the patient airway interface, the gas delivery apparatus operatively connected to a computer;   wherein the computer is configured to supplant the at least structural component, by using at least one of an algorithmic and a mathematical model of the at least one structural component to generate gas delivery characteristics that simulate the functions of the at least one structural component.   
     
     
         42 . A system according to  claim 41 , where the juncture demarcates a point at which inspired gas has already filled the alveoli and begins to fill the anatomical dead space. 
     
     
         43 . A system according to  claim 42 , wherein the juncture is identified by monitoring at least one parameter in real time. 
     
     
         44 . A system according to  claim 43 , wherein the parameter is selected from at least one of volume, pressure and gas concentration. 
     
     
         45 . A system according to  claim 44 , wherein the parameter is a gas flow rate or volume. 
     
     
         46 . A system according to any one of  claims 41  to  45 , wherein the device is a flow sensor, positioned in relation to the first breathing circuit, for at least determining the volume gas inhaled via the patient airway interface. 
     
     
         47 . A system according to any one of  claims 41  to  46 , wherein the at least one structural component comprises or consists of at least one set of structural parts adapted to direct gas flow from a first circuit flow path to at least one alternate, nth circuit flow path during the course of a given inspiratory cycle. 
     
     
         48 . A system according to any one of  claims 41  to  47 , wherein the first circuit flow path is adapted to provide a gas of a first gas composition and the at least one alternate flow path is adapted to provide gas of at least one alternate gas composition. 
     
     
         49 . A system according to any one of  claims 41  to  48 , wherein the first circuit flow path is operatively connected to a first gas source and the respiratory gas delivery system simulates at least one gas flow characteristic of the first gas source selected from a maximum volume, a maximum rate of flow and the % composition or partial pressure of at least one gas in the second gas composition. 
     
     
         50 . A system according to any one of  claims 41  to  49 , wherein the at least one alternate nth circuit flow path of the reference breathing circuit is a second gas source and the respiratory gas delivery system simulates at least one gas flow characteristic of at least one second gas source selected from a maximum volume, a maximum rate of flow and the % composition or partial pressure of at least one gas in the second gas composition. 
     
     
         51 . A system according to any one of  claims 41  to  50 , second gas source is a second gas reservoir adapted to receive a subject's exhaled gas. 
     
     
         52 . A system according to any one of  claims 41  to  51 , the reference breathing circuit is adapted to deliver the subjects last expired gas from the immediately preceding breath first. 
     
     
         53 . A respiratory gas delivery system adapted to deliver a plurality of constituent gases comprising:
 A. a gas delivery apparatus operatively connected to a processor,   B. a measurement device, optionally a flow sensor, adapted to monitor in real time a parameter correlated with a selected juncture in the inspiratory cycle, optionally the rate of inspiration of a gas; and optionally   C. an input device configured for obtaining input of at least of one subject specific parameter corresponding to or sufficient to determine a subject's minute ventilation;   
       wherein the processor is configured to:
 (a) generate a control signal effective to signal the apparatus to output a first component gas of a first composition for a first portion of a plurality of respective inspiratory cycles [i] 1  to [i] n , optionally using input of the subject specific parameter; 
 (b) use output from the measurement device (optionally a flow sensor) to monitor in real time a selected juncture in the inspiratory cycle, optionally the rate of inspiration of the first gas during a first portion of a respective inspiratory cycle [i]; 
 (c) generate a control signal effective to signal the apparatus to deliver a second component gas of a second composition corresponding to that of a neutral gas in a second portion of a respective inspiratory cycle [i]; 
 (d) use output from the measurement device, optionally the flow sensor to signal the apparatus to deliver the first gas at a volume output or at a rate which is less than the subject's minute ventilation such that the apparatus is adapted to satisfy the subject's inspiratory requirement in cycles [i] 1  to [i] n  by the first gas in a first portion of a respective inspiratory cycle [i] and by the second gas in a second portion of an inspiratory cycle [i]. 
 
     
     
         54 . A system as claimed in  claim 53 , wherein the entire volume of first gas delivered over the course of inspiratory cycles [i] 1  to [i] n  is less than the subject's minute volume minus anatomic dead space volume and wherein the entire volume of the first gas is organized to enter alveolar space of the subject. 
     
     
         55 . A system as claimed in any of the preceding claims, wherein the gas delivery apparatus is configured for use with a breathing circuit including a gas delivery portion, the gas delivery portion consisting essentially of a conduit operatively connected a patient airway interface, the gas delivery portion operatively connected to at least one measurement device selected from a flow sensor, a gas analyzer and a pressure transducer. 
     
     
         56 . A system as claimed in claim any of the preceding claims, wherein the processor is configured to simulate gas delivery from a first gas reservoir characterized in that:
 (a) it is a reservoir for the first component gas;   (b) it is depleted in a first portion of a respective respiratory cycle at a rate which corresponds to the rate of inspiration of the first component gas as measured by the flow sensor;   (c) at least one of the parameter selected from parameters (i) and (ii) pertains to the first gas reservoir, as follows:
 (i) it is filled at a rate which is less than the patient's minute ventilation; (ii) it has a maximum volume capacity which is set to be less than the subject's tidal volume per breath minus the subject's anatomic dead space volume; 
 wherein the processor is configured to use at least one of an algorithmic and a mathematical model of replenishment (e.g. filling) and depletion of the first gas reservoir by delivering a gas of the first gas composition first and subsequently a gas of the second gas composition upon simulated depletion of the first gas reservoir. 
   
     
     
         57 . A respiratory gas delivery system adapted for use with a first breathing circuit having at least one gas conduit leading to a patient airway interface, characterized in that the respiratory gas delivery system virtualizes gas delivery characteristics of a reference respiratory gas delivery system that is adapted to be used in conjunction with a reference breathing circuit, the gas delivery characteristics of the reference respiratory gas system dictated at least in part by structural features, for example components such as tubing, valves and gas reservoirs, of the reference breathing circuit, the respiratory gas delivery system including:
 a) a flow sensor, optionally positioned in or proximal to the patient airway interface;   b) a gas delivery apparatus adapted to deliver a gas comprising a plurality of component or constituent gases into the patient airway interface, optionally into the gas conduit, the gas delivery apparatus controlled by an on-board computer, for example a microprocessor, or an external computer; and optionally   c) a gas analyzer, wherein the gas analyzer is optionally positioned in or proximal to the patient airway interface;   wherein the computer is programmed to supplant structural features of the reference breathing circuit by using a mathematical model of the supplanted structural features to generate gas delivery characteristics that are functionally equivalent to those generated by the supplanted structural features, the supplanted structural features optionally defined by components of a sequential gas delivery circuit.   
     
     
         58 . A respiratory gas delivery system according to  claim 57 , wherein the reference breathing circuit is a re-breathing circuit which organizes the delivery of a first component gas of selected first composition and a second component gas comprising exhaled gas and wherein the computer is programmed to deliver, for example by blending requisite constituent gases, a component gas of selected first composition and a component gas of second composition which matches that of an exhaled gas, optionally the subject's last exhaled end tidal gas. 
     
     
         59 . A respiratory gas delivery system according to  claim 57  or  58 , wherein the reference breathing circuit is a sequential gas delivery circuit having a flow control system which alternately directs gas flow from: (a) a first circuit flow path organized to deliver a first gas component, optionally of selected volume; and (b) a second circuit flow path organized to deliver a second gas component; optionally, one or more structural components of the reference breathing circuit arresting flow from the first circuit flow path, for example a first valve, and subsequently initiating gas flow from the second circuit flow path, for example via a second valve, optionally in each inspiratory cycle; and wherein flow from the first and second flow paths is substituted in the respiratory gas delivery system by controlling the gas delivery apparatus to alternately deliver (e.g. by controlling flow of a blended gas or blending gases under computer control), optionally in a repeating cycle, optionally within each inspiratory cycle, a gas of first composition of the selected volume corresponding to the first gas component and a gas of second composition corresponding the second gas component. 
     
     
         60 . A respiratory gas system according to  claim 59 , wherein the reference respiratory gas delivery system is adapted to deliver a component gas of first composition from a first circuit flow path leading from a first gas reservoir, and when the first gas reservoir is depleted, a component gas of second composition from a second circuit flow path, optionally an ambient air port or second gas reservoir and wherein the gas delivery apparatus is controlled in part by modelling filling and depletion of the first gas reservoir (the delivered composition and volume virtualizes filling and emptying of the first gas reservoir) to supplant the first gas reservoir, and wherein the first and second circuit flow path are optionally replaced with a common circuit flow path leading from the gas delivery apparatus to the patient airway interface which delivers a gas of first composition and a gas of second composition, optionally the gas delivery apparatus alternately, optionally in a repeating cycle, blending a volume of the gas of first composition and blending the gas of second composition which is available for the remainder of the breath (optionally, a pressure transducer repeatedly tracks the beginning and end of each inspiratory cycle). 
     
     
         61 . A respiratory gas system according to  claim 57 , wherein the reference breathing circuit comprises a first and a second gas reservoir and wherein the first and second gas reservoirs are absent from the first breathing circuit and wherein the gas delivery apparatus is programmed to simulate gas delivery from the first gas reservoir using a mathematical model of filling and depletion of the first gas reservoir by delivering a gas of the first composition corresponding to a full or selected partial volume of the first gas reservoir and subsequently a gas of the second composition upon simulated depletion of the gas of first composition in the first gas reservoir. 
     
     
         62 . A respiratory gas system according to  claim 57 , wherein the computer simulates filling of an inspiratory gas reservoir at a rate of flow that is less that the subject's minute ventilation minus anatomic dead space ventilation. 
     
     
         63 . A respiratory gas system according to any of preceding claims, including a gas analyzer wherein the computer simulates filling of an inspiratory gas reservoir at a rate of flow that is optionally less that the subject's minute ventilation minus anatomic dead space ventilation and wherein the gas of second composition corresponds to that of subject's exhaled gas from a breath n−1 as determined by the gas analyzer. 
     
     
         64 . A respiratory gas system according to any of the preceding claims, wherein the reference respiratory gas delivery system is adapted to be used in conjunction with a reference sequential gas delivery circuit including an inlet port into a first gas reservoir operatively connected to a gas blender for receiving a blended component gas of first composition from, and when the first reservoir is depleted, a component gas of second composition from a second gas reservoir containing a subject's end tidal exhaled gas, and wherein at least the first gas reservoir and second gas reservoir are absent from the first breathing circuit and wherein the gas delivery apparatus is programmed to simulate gas delivery from the first and second gas reservoirs using a mathematical model of filling and depletion of the first gas reservoir by delivering a gas of the first composition first and subsequently a gas of the second composition upon simulated depletion of the first gas reservoir. 
     
     
         65 . A respiratory gas delivery system according to any of the preceding claims, wherein the computer simulates filling of an inspiratory gas reservoir at a rate of flow that is less that the subject's minute ventilation minus anatomic dead space ventilation. 
     
     
         66 . A respiratory gas system according to any of the preceding claims, including a gas analyzer wherein the computer simulates filling of an inspiratory gas reservoir at a rate of flow that is less that the subject's minute ventilation minus anatomic dead space ventilation and wherein the gas of second composition corresponds to that of subject's exhaled gas from a breath n−1. 
     
     
         67 . A respiratory gas delivery system including or adapted for use with a first breathing circuit having at least one gas conduit leading to a patient airway interface, characterized in that the respiratory gas delivery system is adapted to virtualize, for example, simulate the function, for example selected gas delivery characteristics, of a reference respiratory gas delivery system which includes or is adapted for use with a second, reference breathing circuit, that is structurally different (e.g. less wasteful of gas and/or less complex (e.g. fewer parts or parts more easy to assemble, integrate or coordinate) and/or less bulky, and/or less expensive and/or less prone to failure or physical limitations), the respiratory gas delivery system including:
 a) a flow sensor, optionally positioned in or proximal to the patient airway interface;   b) a gas delivery apparatus adapted to deliver a gas comprising a plurality of component or constituent gases into the patient airway interface, optionally into the gas conduit (the gas delivery apparatus may include an on-board computer for controlling the gas delivery apparatus or may adapted to receive input from an external computer); and optionally   c) a gas analyzer, wherein the gas analyzer is optionally positioned in or proximal to the patient airway interface;   
       wherein control of the gas delivery apparatus simulates selected gas delivery characteristics of the reference gas respiratory gas delivery system that are defined at least in part by structural features, for example, structural parts of the reference breathing circuit and optionally define: (1) the source and/or order of delivery of one or more component gases; and/or (2) the composition and volume of the gas or a component or constituent of the gas made available for inspiration in a breath, series of breaths, breath segment or series of breath segments, or time period;
 and wherein the computer is programmed provide inputs to the gas delivery apparatus to:
 (1) control the gas delivery apparatus by executing an algorithm that employs as inputs data obtained from the flow sensor (and optionally the gas analyzer) and at least a mathematical model of at least a portion of the second, reference breathing circuit, including parameters that describe supplanted structural features e.g. structural parts of the second, reference breathing circuit, the supplanted features e.g. structural parts:
 a. defining at least in part the selected gas delivery characteristics; 
 b. absent in the first breathing circuit; and 
 
 (2) generate an output signal to the gas delivery apparatus that simulates the supplanted structural features of the second, reference breathing circuit, such that when the respiratory gas delivery system outputs gas to the first breathing circuit the selected gas delivery characteristics of the respiratory gas delivery system simulate portions of the reference respiratory gas delivery system defined by the supplanted parts of the second reference breathing circuit. 
 
 
     
     
         68 . A respiratory gas system according to  claim 67 , wherein the reference respiratory gas delivery system is adapted to deliver a component gas of first composition from a first gas reservoir, and when the first reservoir is depleted, a component gas of second composition from a second gas reservoir, and wherein at least the first gas reservoir and second gas reservoir are absent from the first breathing circuit and wherein the gas delivery apparatus is programmed to simulate gas delivery from the first and second gas reservoirs using a mathematical model of filling and depletion of the first gas reservoir by delivering a gas of the first composition first and subsequently a gas of the second composition upon simulated depletion of the first gas reservoir. 
     
     
         69 . A respiratory gas delivery system adapted for use with a first breathing circuit having at least one gas conduit leading to a patient airway interface, characterized in that the respiratory gas delivery system virtualizes structural components a reference breathing circuit, the respiratory gas delivery system including:
 a) a flow sensor, positioned for at least determining the volume gas inhaled via the patient airway interface;   b) a gas delivery apparatus including or controlled by a computer for delivering a gas comprising a plurality of component or constituent gases into the patient airway interface;   c) a gas analyzer for analyzing the gas concentration of one or more constituent gases inhaled and/or exhaled by the subject;   wherein the computer is programmed to supplant one or more structural components of the reference breathing circuit by using a mathematical model of the structural component(s) to be supplanted to generate gas delivery characteristics that supplant it/them.