this invention will be further understood in view of the following detailed description of exemplary embodiments . fig1 shows an embodiment of the present invention . subject breathes on the apparatus through subject port 1 . the hypoxia breathing circuit is comprised of inspiratory limb 14 and expiratory limb 12 , said limbs connected by crossover limb 13 . inspiratory limb 14 transports substantially all gas for breathing to the subject . inspiratory limb 14 contains a one way valve 2 directed toward the subject . expiratory limb 12 contains a one way valve 3 directed away from the subject towards expiratory reservoir 6 . crossover limb 13 contains a one way crossover valve 4 directed toward the inspiratory limb . one way valve 4 opens at a first differential pressure , which is greater than the second differential pressure required to open the one way valve 2 inspiratory . port 8 is open to ambient air . ambient air enters the circuit through port 8 at a flow rate determined by variable resistance 7 and the pressure generated in inspiratory reservoir 9 by the pull of mass 10 on the bottom of the reservoir 9 . reservoir 9 is preferably a bellows . pressure generated by mass 10 is preferably less than opening pressure of crossover valve 4 . a co 2 removal device or means 5 removes co 2 from rebreathed gas . expired gas leaves the circuit via vent 11 , which may optionally contain a one - way valve 24 directed toward the exit . expiratory reservoir 6 preferably has high compliance and is large enough so that gas drawn from the expiratory side of the circuit comes from the reservoir 6 as it collapses and shrinks , and not from ambient air via vent 11 . the expiratory reservoir 6 may be made , for example , from a suitably thin polymeric material . the port 8 constitutes an oxygen inlet for the apparatus , or alternatively can be referred to as a means for introducing oxygen into the apparatus . the optional variable resistance 7 may also be referred to as a flow rate controller 7 controls the rate of entry of oxygen into the apparatus , and can also be referred to as a means for controlling the flow rate of entry of oxygen into the apparatus . the flow rate controller 7 may be , for example , a voltage sensitive orifice ( vso ). alternatively , any other suitable flow rate controller for controlling the rate of entry of oxygen into the apparatus or means for controlling the flow rate of entry of oxygen into the apparatus may be used . the co2 removal device or means 5 may be a commercially available co2 scrubber known in the art . the co2 removal device or means 5 may include a co2 removal material 5 a , such as soda lime , for absorbing co2 . other materials 5 a are also usable however , such as , for example , a zeolyte . alternatively , any other suitable co2 removal device or means 5 may be used . the function of the circuit is as follows . the alveolar ventilation of the subject may be determined , for example , using the method disclosed by preiss et . al . in u . s . patent application ser . no . 10 / 135 , 655 published as us patent publication no . 2002 - 0185129 or is estimated from known values based on physiological parameters such as sex , weight , height , etc . mass 10 causes constant negative pressure in inspiratory reservoir 9 , drawing ambient air into port 8 at a rate controlled by resistance 7 . resistance 7 is set so that the flow is equal to the desired fraction of the subject &# 39 ; s alveolar ventilation to achieve the desired hypoxic level . the subject inspires from inspiratory reservoir 9 . when reservoir 9 is depleted , if the subject is still inspiring , pressure in the inspiratory limb 14 will become further reduced until valve 4 opens , allowing the subject to breath previously exhaled gas . to prevent co 2 buildup , the co2 scrubber 5 is positioned in the crossover limb and removes co 2 from gas passing through crossover limb 13 for inspiration by the subject . upon expiration , one way valve 3 opens allowing expired gas to enter the expiratory reservoir 6 . if the expiratory reservoir is filled , further expiration vents via vent 11 . instead of measuring or estimating the subject &# 39 ; s alveolar ventilation , the method could include , for example , measuring or estimating the subject &# 39 ; s oxygen consumption . the sequential gas delivery ( sgd ) circuit can also referred to as a sequential gas delivery ( sgd ) device , or as a sequential gas delivery ( sgd ) means . alternatively , any other suitable sequential gas delivery ( sgd ) device or means may be used . it should be noted that numerous variations on the embodiment described above are possible . for example , inspiratory reservoir 9 and mass 10 could be replaced with a different passive method of entrainment . for example , mass 10 could be replaced by a constant spring mechanism that opens the reservoir with a constant force . alternately , self - inflating foam inside the reservoir could be used . any self inflating container capable of creating a constant negative pressure is suitable . another exemplary embodiment is shown in fig2 . in this circuit , scrubber 5 is positioned within the expiratory limb 12 and is positioned to receive substantially all of the expired gas before the gas enters the expiratory reservoir 6 . many types of flow resistances and flow controls to control the rate of entrainment of ambient air are known to those skilled in the art . many of the sequential gas delivery circuits described by fisher et . al . in canadian patent application 2 , 419 , 575 , which is incorporated herein by reference , are suitable for use with the present invention . for example sgd circuits described in fig3 b , 3c , 3d , 3e , 5b , 5c , 5a , and 6a of the &# 39 ; 575 application would be suitable , as long as a flow control means capable of setting the fresh gas flow rate into the inspiratory reservoir below the alveolar ventilation of the subject is provided . as an example , fig3 herein shows a further exemplary embodiment of a hypoxia apparatus using a sequential gas delivery circuit wherein instead of a crossover limb between inspiratory and expiratory limbs , there is a bypass limb 23 through which rebreathed gas is inspired . the co 2 scrubber 5 would preferably be on this limb , although it could also be on expiratory limb 12 . in this embodiment , the one way bypass valve , shown at 4 , opens at a first differential pressure , which is greater than the second differential pressure required to open the one way inspiratory valve 2 . referring to fig5 , the oxygen inlet 8 in any of the embodiments shown and described herein may be connected to a source of oxygen 24 . the oxygen source 24 provides a gas with a concentration of oxygen that may be greater than or less than the concentration of oxygen in ambient air , or may alternatively provide a gas with oxygen in the same concentration as ambient air . fig4 shows a further exemplary embodiment of the present invention . in fig4 , a preferably adjustable pump 21 capable of pumping a desired rate of gas ( eg . ambient air ) is connected to fresh gas port 8 , also referred to as the oxygen inlet 8 . with such an embodiment , the inspiratory reservoir 9 may be a simple bag . pump 21 speed may optionally be adjusted via controller 22 which may be further controlled by an optional oxygen saturation measurement device or means 20 , which would preferably be a pulse oximeter but could be any other suitable oxygen saturation measurement device or means . when used in this configuration , the oxygen saturation measurement device or means 20 would measure the subject &# 39 ; s oxygen saturation and send output relating to the measurements to the controller . the controller 22 would compare the saturation to the saturation required to achieve the desired hypoxic level . controller 22 would adjust the speed of the pump 21 up or down to provide the required fresh gas flow based on the comparison . thus , the pump 21 acts as a flow rate controller in embodiments wherein its speed is variable . it will be appreciated that the rate of entry of oxygen into the apparatus shown in fig4 is controlled by the pump 21 based on output from the oxygen saturation measurement device 20 and based on the target oxygen saturation selected for the subject . the pump 21 may be used to provide air to an inspiratory reservoir 9 in any of the embodiments described herein , such as , for example , the embodiments shown in fig2 and 3 . in any such embodiments , a self - inflating reservoir could be replaced by a reservoir similar to the reservoir 9 shown in fig4 . optionally , the controller 22 could be used in these embodiments also . as a further option , the oxygen saturation measurement device or means 20 could be used in these embodiments also . reference is made to fig6 . as yet another alternative , the oxygen saturation measurement device or means 20 and controller 22 could be used with any of the embodiments shown herein without a pump . the controller 22 could control the variable resistance 7 to control the rate of entry of oxygen into the apparatus , based on the output from the oxygen saturation measurement device or means 20 to the controller 22 and based on the target oxygen saturation for the subject . the oxygen inlet 8 in this embodiment could be connection either to ambient air , or to a source of oxygen , such as a pressurized tank . in the embodiment shown in fig8 , the reservoir 9 is preferably a bellows , however , other structures may be alternatively suitable . reference is made to fig7 , which shows the apparatus with the co2 removal device or means 5 on the inspiratory limb 14 . in this embodiment , all of the gas inspired by the subject passes through the co2 scrubber . reference is made to fig8 . it is optionally possible for the apparatus to include a plurality of inspiratory reservoirs 9 instead of just one , independent of the number of expiratory reservoirs 6 the apparatus has . separately , it is optionally possible for the apparatus to include a plurality of expiratory reservoirs 6 instead of just one , independent of the number of inspiratory reservoirs 9 the apparatus has . provided the detailed disclosure herein , those skilled in the art may envision how the present invention could be practiced using alternative embodiments and variations thereof . the foregoing detailed description should be regarded as illustrative rather than limiting .
an apparatus for inducing hypoxia in a subject is provided . the apparatus includes a breathing port , an inspiratory reservoir , means for introducing oxygen into the apparatus , means for controlling the flow rate of entry of oxygen into the apparatus at a rate below the subject &# 39 ; s metabolic requirements , an expiratory reservoir having a vent , sequential gas delivery means , and means for removing co 2 from the circuit . the sequential gas delivery means are for directing the gases such that upon expiration , the subject expires into the expiratory reservoir , and , upon inspiration , subject inspires first from the inspiratory reservoir , and , on any breath , once said inspiratory reservoir is depleted , gas for the balance of that inspiration is delivered from the expiratory reservoir