CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. §119 of European Application EP 11 172 088.4 filed Jun. 30, 2011, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention pertains to a respiration system for feeding an anesthetic gas with a Y-piece, which is provided for connection to a patient, with a respiration circuit, in which are provided an inspiration branch and an expiration branch, which extend away from the Y-piece, whereby a branching is provided in the expiration branch, whereby a first supply line from the branching leads to an anesthetic gas discharge valve and whereby a second supply line from the branching leads to a reservoir. 
     BACKGROUND OF THE INVENTION 
     Such respiration systems known from the state of the art, for example, EP 0 894 506 B1, first have the advantage that the anesthetic gas is guided in a respiration circuit, so that a large part of the expensive anesthetic gas can be reused. Only CO 2  has to be removed from the circulating gas flow by means of an absorber, and the oxygen concentration has to be kept at a preset level. 
     By means of the manual respiration bag which is likewise contained in the circuit, it is possible for an anesthesiologist to manually adjust the pressure operating during the inspiration and the duration of the inspiration. 
     However, the following problem arises in such systems. During expiration, resistance or pressure opposing the expiration should not exceed a certain value, whereby, especially at the end of the expiration cycle, the pressure in the expiration branch may rise, for example, when the capacity of the reservoir is not sufficient. So that this resistance does not become unacceptably high, a so-called anesthetic gas discharge valve (hereinafter “NGF valve”), which opens in case of an excess pressure in the expiration branch of 1.2±0.1 mbar, is provided in the expiration branch. In the state of the art, this valve is usually designed, such that a valve body is pressed against a valve seat by means of a spring, whereby the prestress of the spring determines the threshold, at which the valve opens. 
     The flow in the expiration branch, especially at the beginning of expiration, is not constant, and also the pressure, which builds up during expiration at the NGF valve, fluctuates and has brief peaks or large pressure gradients in the range of 
                   Δ   ⁢           ⁢   p       Δ   ⁢           ⁢   t       =       5   ⁢           ⁢   mbar       0.5   ⁢           ⁢   sec         ,         
so that pressures briefly lying above the above-mentioned threshold at the NGF valve build up, which consequently bring about the situation that the gas in the supply line to the manual respiration bag has an inertia. On top of that, the NGF valve also briefly opens, although a pressure above the threshold has not built up for a longer period in the expiration branch. Thus, a so-called “rattling” of the NGF valve occurs, which leads to an unnecessary loss of anesthetic gas.
 
     SUMMARY OF THE INVENTION 
     Therefore, based on the above-described state of the art, an object of the present invention is to provide a respiration system, in which too high pressure opposing the expiration cannot build up in the expiration branch and which nevertheless keeps the losses of anesthetic gas to a minimum. 
     This object is accomplished by a respiration system for feeding an anesthetic gas having a density ρ Gas , with a Y-piece, which is provided for connection to a patient, with a respiration circuit, in which an inspiration branch and an expiration branch are provided, which extend away from the Y-piece, whereby a branching is provided in the expiration branch, whereby a first supply line from the branching leads to an NGF valve, whereby a second supply line from the branching leads to a reservoir, which has a mean cross-sectional area A supply line 2  and a length l supply line 2 , whereby the NGF valve has a valve body with a mass m valve , which is pressed, by the effect of gravity, towards the first supply line against a circular-ring-shaped valve seat having a diameter d valve , and whereby a prestressing means is provided, which exerts a prestressing force onto the valve body against the effect of gravity, so that the mass m valve  and the prestressing force determine a threshold pressure ΔP threshold , about which the pressure in the first supply line has to lie at least above that on the side of the anesthetic gas discharge valve facing away from the first supply line, so that this opens. 
     In conventional NGF valves known from the state of the art, the valve body is pressed against the valve seat by means of a prestressing means, such as a spring, whereby the spring force and the weight of the valve body acting in the same direction together determine the threshold pressure, at which the valve opens. Since the threshold pressure has to be very low, however, the weight and the spring force may also not be very high. 
     In the solution according to the present invention, a part of the gravity, which acts on the valve body and based on which the valve closes, is compensated by means of the prestressing means which acts against the direction of gravity. This makes it possible that the valve body may have a markedly greater mass and thus may be inactive in situations, so that only brief pressure peaks or preferably high pressure gradients of approximately 
                 Δ   ⁢           ⁢   p       Δ   ⁢           ⁢   t       =       5   ⁢           ⁢   mbar       0.5   ⁢           ⁢   sec             
and thus only a brief force action are not sufficient to raise the valve body from the valve seat. Therefore, the mass of the valve body as well as the density ρ gas  of the anesthetic gas used can be adapted to the remaining parts of the respiration system to achieve the inertia of the valve needed based on the design of the respiration system.
 
     The adaptation is especially done depending on the length of the second supply line to the reservoir and on the cross section of this supply line as well as on the driving pressure of the patient, such that the pressure gradients in the supply line to the reservoir are not sufficient at the NGF valve to move the valve body thereof from its closed position, in which it lies against the valve seat. Consequently, the NGF valve opens only if a pressure, whose difference to the ambient pressure lies above the threshold pressure ΔP threshold , builds up in the expiration branch for a longer period, so that the loss of anesthetic gas is minimized. 
     Preferably, the mass m valve  of the valve body is selected in such a way that 
               m   valve     &gt;         2     ·     π   2     ·     l     supplyline   ⁢           ⁢   2     2     ·       V   .     intersection     ·     ρ   gas   2     ·     d   valve   3     ·     (       Δ   ⁢           ⁢   P     -     Δ   ⁢           ⁢     P   threshold         )           8   ·     A     supplyline   ⁢           ⁢   2     2     ·   Δ     ⁢           ⁢       P   2     ·         ρ   gas       Δ   ⁢           ⁢   P                     
applies, whereby the parameter {dot over (V)} intersection  is selected corresponding to {dot over (V)} intersection ≧10 L/min and ΔP=5 mbar is assumed for the driving pressure ΔP during the expiration of the patient. ΔP threshold =1.1 mbar is selected for the threshold pressure, and the density ρ gas  of the anesthetic gas lies between 1.12 kg/m 3  and 2.19 kg/m 3 . A breathing gas temperature of 20° C. is used as the basis here. When the density of the breathing gas is established, which shall be used in the system, the corresponding value for the density in Equation (1) can be used to thus adapt the system accordingly, whereby this density value lies within the interval defined above.
 
     Thus, a specification is given by Equation (1) for dimensioning the valve body depending on the other relevant dimensions within the respiration system. When this specification is complied with, it is reliably guaranteed that the NGF valve does not already open at brief high pressure gradients in the range of 
                 Δ   ⁢           ⁢   p       Δ   ⁢           ⁢   t       =       5   ⁢           ⁢   mbar       0.5   ⁢           ⁢   sec             
based on the adequate inertia of the valve body, which pressure gradients result from the flow resistance in the second line to the reservoir or manual respiration bag and the inertia of the gas present therein. Rather, the NGF valve opens only if the pressure in the expiration branch lies about the threshold pressure above the ambient pressure for a longer time.
 
     Such pressure gradients appear cyclically during the respiration operation due to a continual change between inspiration and expiration of the patient, whereby the inertia of the gas in the supply line to the reservoir leads to pressure gradients building up precisely in the supply line to the NGF valve. The respiration rate and thus the frequency of such peaks lies within the range of 6 to 15 breaths per minute in a typical patient. 
     Equation (1) is based on the following marginal conditions and considerations. 
     It is assumed that during expiration based on the driving pressure ΔP of the patient, anesthetic gas flows along the expiration branch, in particular both along the first supply line to the NGF valve and along the second supply line to the reservoir or to the manual respiration bag. For the inertia I pneu  of the gas in the second supply line, 
                       I   pneu     =         Δ   ⁢           ⁢   P       (       ⅆ     ⅆ   t       ⁢     V   .       )       =         ρ   gas     ·     I     supplyline   ⁢           ⁢   2           A     supplyline   ⁢           ⁢   2             ,           (   2   )               
applies, whereby a supply line with an essentially circular cross section is assumed. Moreover, for the inertia of the NGF valve,
 
                       I   NGF     =         2   ·     m   valvebody           π   2     ·     d   valve   3         ·         2   ·     ρ   gas         Δ   ⁢           ⁢   P             ,           (   3   )               
applies, whereby a circular valve seat having a diameter d valve  is assumed. The valve seat is that surface area, on which the valve body lies against the valve housing. Thus, the volume flow can be calculated as a function of the time t through the second supply line to the reservoir via the equation
 
                         V   .       supplyline   ⁢           ⁢   2       =         t   ·   Δ     ⁢           ⁢   P       I   pneu         ,           (   4   )               
while for the time curve of the volume flow through the NGF valve
 
                         V   .     NGF     =         1   2     ·     t   2       ⁢         Δ   ⁢           ⁢   P     -     Δ   ⁢           ⁢     P   threshold           I   pneu           ,           (   5   )               
applies. This means that the volume flow {dot over (V)} supply line 2  through the second supply line to the reservoir or manual respiration bag increases linearly with time, while the volume flow {dot over (V)} NGF  through the NGF valve has a square course.
 
     This arises due to the fact that volume flow {dot over (V)} supply line 2  into the reservoir at constant driving pressure ΔP is proportional to time t, while volume flow {dot over (V)} NGF  through the NGF valve is proportional to the size of the gap between valve body and valve seat. This is in turn increased by the movement of the valve body, whereby the acceleration of the valve body is proportional to the force that is exerted on it. This is in turn proportional to the driving pressure ΔP. When this is constant, there is a constant acceleration and the size of the gap is proportional to the time squared t 2 . 
     Now, for the dimensioning of the respiration system, it is required that the volume flow {dot over (V)} NGF  through the NGF valve shall exceed the volume flow through the supply line only after a relatively long time and not already at the above-defined high pressure gradients. The NGF valve shall thus open only very slowly, which means that the intersection {dot over (V)} intersection  between the two volume flows {dot over (V)} supply line 2  and {dot over (V)} NGF  has to lie first at a point in time t intersection , which is greater than the duration of the pressure gradients occurring, or at a relatively high volume flow. 
     When Equations (4) and (5) are compared to determine the intersection {dot over (V)} intersection  and Equations (2) and (3) are used for the inertias, at first a relationship arises for t intersection . Thus, the relationship 
                         V   .     intersection     =         4   ·     A     supplyline   ⁢           ⁢   2       ·     m   valve     ·   Δ     ⁢           ⁢     P   2     ⁢         2   ·     ρ   gas         Δ   ⁢           ⁢   P               π   2     ·     l     supplyline   ⁢           ⁢   2     2     ·     ρ   gas   2     ·     d   valve   3     ·     (       Δ   ⁢           ⁢   P     -     Δ   ⁢           ⁢     P   threshold         )           ,           (   6   )               
can be established by using the intersection time t intersection  in Equation (4) together with Equation (2).
 
     The condition for the mass of the valve body M valve  according to Equation (1) in turn arises from this by transposing. In this case, the relationship {dot over (V)} intersection ≧10 L/min, and preferably {dot over (V)} intersection ≧60 L/min should apply for {dot over (V)} intersection . 
     If {dot over (V)} intersection ≧10 L/min is selected, a significant effect is already produced especially in the area of pediatrics. A value of {dot over (V)} intersection ≧60 L/min is adjusted to an adult and corresponds approximately to the peak volume flow that an adult can achieve. 
     While previously circular cross sections of the supply lines were used as the starting point, it is clear that variations with noncircular cross sections are also comprised by the present invention. 
     In a preferred embodiment, the NGF valve is designed in such a way that it has a valve housing, in which the valve seat is provided, whereby the valve seat runs in a seat plane running at right angles to the direction of gravity and points in a direction opposite the direction of gravity, and whereby the valve body is arranged in such a way that it lies on the valve seat under the effect of gravity. 
     It is further preferred, when a screw spring is provided as prestressing means, which lies with a first end against the valve body, which presses on the valve body against the effect of gravity and which is arranged on the side of the valve body pointing in the direction of gravity. 
     To be able to adjust the prestressing by means of the spring and thus the threshold pressure ΔP in a simple manner, it is further preferred that an adjusting screw is axially adjustably mounted in the valve housing, whereby the second end of the screw spring is supported against the adjusting screw. 
     The present invention is explained below based on a drawing showing only one preferred exemplary embodiment. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic view of the exemplary embodiment of a respiration system according to the present invention; and 
         FIG. 2  is a sectional view of the anesthetic gas discharge valve used in the respiration system of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings in particular, the exemplary embodiment of a respiration system shown in  FIG. 1  comprises a respiration circuit (shown only schematically here) with a Y-piece  1 , from which a connection line  3  originates, via which a patient can be connected to the respiration system. The respiration circuit further comprises an inspiration branch  5  originating from the Y-piece  1 , in which a first nonreturn valve  7 , a first measuring means  8  for measuring the volume flow in the inspiration branch  5  as well as a ventilation and gas dispensing device  9  are provided. 
     The nonreturn valve  7  is designed here such that only a flow from the ventilation and gas dispensing device  9  towards the Y-piece  1  can take place in the inspiration branch  5 , while the first nonreturn valve  7  closes in case of a flow running in the reverse direction. A respiration drive, with which a flow can be brought about to the Y-piece  1  and thus to the patient, as well as a fresh gas dispensing means are present in the ventilation and gas dispensing device  9 . 
     Moreover, an expiration branch  11  originates from the Y-piece  1 , in which a second measuring means  12 , a second nonreturn valve  13  as well as a PEEP (PEEP corresponds to “positive end expiratory pressure”) valve  15  are provided. The second nonreturn valve  13  is arranged in such a way that a flow from the Y-piece  1  to the PEEP valve  15  is made possible, while the second nonreturn valve  13  closes in case of a flow in the reverse direction. 
     The functionality of the PEEP valve  15  is such that the PEEP valve  15  is closed during the inspiration phase, so that gas, which shall be fed from the ventilation and gas dispensing means  9  actually to the patient, cannot flow away via the expiration branch  11 . In the expiration phase, the PEEP valve  15  is set in such a way that it maintains a positive pressure in the section between the PEEP valve  15  and the Y-piece  1 . 
     Furthermore, the expiration branch  11  has a CO 2  absorber  17 , whereby, viewed from the direction of the Y-piece  1 , the expiration branch  11  and the inspiration branch  5  are connected to each other behind the CO 2  absorber  17 . 
     Moreover, a branching  19 , from which a first supply line  21  originates, at whose end an anesthetic gas discharge valve (hereinafter “NGF valve”)  23  is arranged, is provided in the expiration branch  11 . The output side of the NGF valve  23  is in turn connected to an anesthetic gas discharge system  25 , not shown in detail. 
     Furthermore, a second supply line  27 , which may have a comparatively long length l supply line 2  of more than 1 m and extends to a manual respiration bag  29 , which forms the reservoir in the present exemplary embodiment, originates from the branching  19 . Due to this long length l supply line2 , the gas therein may also have a considerable inertia. The second supply line  27  has a cross-sectional area A supply line 2 . If the cross section of the second supply line  27  varies over its length, the inertias of the breathing gas would have to be calculated separately in sections each with a constant cross section (see above), whereby a relationship deviating from Equation (1) would arise. However, it is obvious to the person skilled in the art how the correspondingly transposed equation must appear. 
     With regard to the branching  19 , it should be noted that both supply lines  21 ,  27  originate from one point in the expiration branch  11  in the present exemplary embodiment of the branching  19 . However, it is conceivable as well that the branching  19  is split, so that the first and second supply lines  21 ,  27  originate from the expiration branch  11 , spaced apart from one another. 
       FIG. 2  shows the NGF valve  23  in detail, and it can be seen that this NGF valve  23  has a housing  31  with a circular valve seat  33  arranged therein, which extends in a seat plane  35  which runs at right angles to the direction of gravity G, and has a diameter d valve . Moreover, a mobile valve body  37 , which has a mass m valve  and lies on the valve seat  33  because of its weight and thus closes the valve  23 , is provided in the housing  31 . Finally, an adjusting screw  39  is arranged in the housing  31 , whereby a screw spring  41  is arranged between the end of the adjusting screw  39  located in the housing interior and the valve body  37 . The first end  43  of the screw spring is supported against the valve body  37 , and the second end  45  lies against the adjusting screw  39 . Thus, a part of the weight of the valve body  37  is compensated by the prestressing means in the form of the screw spring  41 , such that the weight of the valve body  37 , on the one hand, and the prestressing of the valve body  37 , on the other hand, determine at what threshold pressure ΔP threshold  the NGF valve  23  opens, provided that this threshold pressure ΔP threshold  is exceeded by the pressure difference between the side of the NGF valve  23  that is facing the supply line  21  and the side that points to the anesthetic gas discharge system  25 . Thus, this threshold pressure ΔP threshold  can be adjusted by adjusting the adjusting screw  39 . 
     Finally, in this exemplary embodiment described above, the mass m valve  of the valve body  37  is determined according to the equation 
                     m   valve     &gt;         2     ·     π   2     ·     l     supplyline   ⁢           ⁢   2     2     ·       V   .     intersection     ·     ρ   gas   2     ·     d   valve   3     ·     (       Δ   ⁢           ⁢   P     -     Δ   ⁢           ⁢     P   threshold         )           8   ·     A     supplyline   ⁢           ⁢   2     2     ·   Δ     ⁢           ⁢       P   2     ·         ρ   gas       Δ   ⁢           ⁢   P                       (   1   )               
already explained above, to embody the NGF valve  23  as being sufficiently inert, so that brief high pressure gradients do not already bring about an opening of the NGF valve  23 .
 
     The parameters contained in this equation describe, on the one hand, the dimensions of parts of the respiration system. Thus, the length of the second line  27  l supply line 2 , the diameter of the valve seat  31  d valve , the threshold pressure ΔP threshold  adjusted at the NGF valve  23  (usually approximately 1.1 bar) and the cross-sectional area A supply line 2  of the second supply line  27  appear in Equation (1). When the cross-sectional shape of the second supply line varies along the length thereof, Equation (1), as was already explained, has to be transposed accordingly in an obvious manner. 
     On the other hand, other parameters, which concern the respirated patient or the breathing gas used and are likewise readily known, appear. These [parameters] are the density of the breathing gas ρ gas , which may lie between 1.12 kg/m 3  and 2.19 kg/m 3 , so that a comparatively close range is preset here, the driving pressure ΔP, which is generated by the patient breathing out and is 5 mbar, and the intersection volume flow {dot over (V)} intersection , for which {dot over (V)} intersection ≧10 L/min and preferably {dot over (V)} intersection ≧60 L/min applies. The density of the breathing gas is determined by the composition of air, oxygen, nitrous oxide and volatile anesthetics in the gas mixture, whereby a breathing gas temperature of 20° C. is used as a basis. 
     Thus, the valve body can easily be determined by means of Equation (1), even if the anesthetic gas and thus its density are not determined accurately. 
     The respiration system described above operates as follows. In the inspiration phase, the respiration drive provided in the ventilation and gas dispensing means  9  is turned on, so that the first nonreturn valve  7  opens and the PEEP valve  15  is closed. Consequently, breathing gas, whose concentration is controlled by the ventilation and gas dispensing means  9 , reaches the patient via the Y-piece  1  and the connection line  3 , whereby a part of the breathing gas comes from the manual respiration bag  29 . 
     During expiration, the first nonreturn valve  7  is closed because of the driving pressure ΔP which is generated by the patient and the second nonreturn valve  13  is opened, so that the expired gas flows along the expiration branch  11  through the PEEP valve  15  to the branching  19 , whereby the PEEP valve  15  guarantees that the pressure, for example determined by the second measuring means  12 , does not fall below a preset threshold. 
     The gas flows from the branching  19  along second supply line  27  into the manual respiration bag  29  until a pressure builds up in the expiration branch  11 , which, compared to the ambient pressure, lies above the threshold pressure ΔP threshold  of the NGF valve  23 . Only then can the NGF valve  23  open, whereby this cannot happen already beforehand due to brief pressure fluctuations or pressure gradients in the range of 
                   Δ   ⁢           ⁢   p       Δ   ⁢           ⁢   t       =       5   ⁢           ⁢   mbar       0.5   ⁢           ⁢   sec         ,         
since the opening behavior of the NGF valve  23  is sufficiently inactive (inert) because of the comparatively high mass m valve  determined according to Equation (1).
 
     In the next inspiration phase, gas again flows from the manual respiration bag  29  through the CO 2  absorber  17  back into the inspiration branch  5  and is possibly changed in its composition by means of the ventilation and gas dispensing means  9  and then fed to the patient via the Y-piece  1 . 
     As emerges from the above, the respiration system according to the present invention is embodied by the NGF valve  23 , whose valve body  37  is dimensioned corresponding to the conditions in the system, in such a way that the NGF valve  23  only opens if a pressure, whose difference to ambient pressure lies above the threshold pressure ΔP threshold , builds up in the expiration branch for a longer period, so that the loss of anesthetic gas is minimized. 
     While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
A respiration system feeds an anesthetic gas having, a density r gas, with a Y-piece for connection to a patient, with a respiration circuit having an inspiration branch and an expiration branch, which extend away from the Y-piece. A first supply line from a branch in the expiration branch leads to an anesthetic gas discharge valve and a second supply line from the branch leads to a reservoir. A too high pressure, opposing expiration, cannot build up in the expiration branch and losses of anesthetic gas are kept to a minimum. A prestressing device exerts a prestressing force onto the valve body of the anesthetic gas discharge valve against the effect of gravity. The mass m valve and the prestressing force determine a threshold pressure in the anesthetic gas discharge line that results in an opening of the anesthetic gas discharge valve.