there is generally indicated at 10 in fig1 in simplified schematic form , a ventilation system according to the present invention and comprising a ventilator apparatus 12 which provides a source of pressurized breathing gas and other cpap system control elements , and having connected thereto a gas delivery conduit 14 for delivery of a suitable breathing gas mixture via conduit branch 16 to a patient 18 . the breathing gas mixture may be supplied to the patient 18 via any suitable appliance , for example a tight fitting tracheal tube or a tracheal tube with a cuff that forms a tracheal seal in the airway of the patient . another option is a tight fitting mask applied to the patient &# 39 ; s face . these or other conventional breathing gas supply appliances may be utilized . the appliance shown in fig1 is an endotracheal tube holder 20 that is strapped to the face of patient 18 to retain and support an endotracheal tube 22 within the airway of patient 18 in the conventional manner . also forming a part of ventilation apparatus 10 as an element of cpap pressure control is an adjustable pressure relief valve 24 which is connected to conduit 14 at junction 26 and which operates to establish the cpap pressure level by opening in response to pressure excursions exceeding the selected cpap level . such pressure excursions occur during spontaneous patient expiration . typically , valve 24 may be a mushroom type valve which is controlled by a control pressure from the ventilator 12 . alternatively , valve 24 may be any valve with low flow resistance which is programmed to open and close at a predetermined pressure . an exhaust valve 28 is also connected to conduit 14 at junction 26 to open conduit 14 to the ambient atmosphere upon actuation thereof by an aprv pressure release control 30 . if desired valve 28 may release pressure to some level above ambient , but lower than the cpap pressure level . also , valve 28 may release expiratory gas into a spirometer or other device or container but the pressure must still be lower than cpap . of course , the cross - sectional flow areas and gas flow path lengths of conduits 14 , and 16 , and of valves 24 and 28 , are such that in the overall system design the desired control of pressure levels , as below described , may be readily established through suitable programming and / or adjustment of the ventilator 12 , control unit 30 , and valves 24 and 28 . the apparatus 10 is operable to provide patient 18 with aprv as follows . a continuous supply of breathing gas is provided from ventilator system 12 via the conduits 14 and 16 to patient 18 . the gas pressure fills the conduits of apparatus 10 and therefore also reaches valves 24 and 28 . valve 24 is set to a preselected cpap pressure level to preclude cpap pressure excursions . the elements of the cpap system , namely ventilator 12 and connected valve 24 , are well known in the art and further detailed description thereof is believed to be unnecessary for an understanding of the present invention . it will suffice to note that the cpap system elements function in the conventional manner to maintain a selected level of cpap ( continuous positive airway pressure ) that restores normal or establishes optimal patient frc ( thus maintaining increased lung volume ) and enhances lung compliance while permitting spontaneous inspiration and expiration by the patient at a substantially constant elevated airway pressure . at selected intervals , valve 28 is opened to permit system pressure ( and therefore , patient airway pressure ) to drop a controlled increment , and for a selected interval of time , below the cpap pressure level . the pressure drop may go fully to ambient pressure or may reach a threshhold level between cpap and ambient pressure . the frequency of these pressure drops , as well as the rate , magnitude and duration of pressure drop , are controlled to provide periodic cycles of assisted exhalation for improved co 2 excretion and other benefits without resorting to airway pressure excursions above frc . the mechanics , apparatus and method of the invention are described in more detail hereinbelow ; however , for an initial better understanding of the invention , it is helpful to consider a comparison between the physiological effects attained thereby and the effects of conventional ventilatory techniques as shown in fig2 and 3 . referring first to fig2 conventional positive pressure ventilation generally is provided by imposing a transient increase in patient airway pressure over a base pressure to induce inspiration , thus causing inspired gas to enter the lungs . during the expiratory phase of the ventilation cycle the airway pressure is returned to the base pressure . the base pressure in all conventional ventilatory therapies corresponds to the patient &# 39 ; s frc . under assist - control ( a - c ) ventilation the inspiration phase is triggered by a patient induced drop in airway pressure , whereas under controlled ventilation ( cv ) the start of inspiration is time - cycled . in either case , patient airway pressure rises from ambient pressure ( the base pressure ) 32 to a maximum , thus providing assisted inspiration , and subsequently airway pressure returns to ambient to provide assisted expiration and therby complete a respiration cycle as shown by airway pressure traces 34 . during the cv and a - c ventilation cycles , lung volume follows the traces depicted at 36 as lung volume is directly proportional to airway pressure . positive pressure application may be controlled by pressue ( pressure - limited ) or pressure cycled ventilation , volume ( volume limited or volume cycled ventilation ), time ( time cycled or controlled ventilation ), or flow ( flow limited , pressure assisted ventilation ). the expiratory phase may be controlled by either time or pressure . if , in the above modes of ventilation , the expiratory phase of each cycle is augmented by including an increase in expiratory airway pressure ( i . e . positive end - expiratory pressure or peep ), the pressure and volume traces would be elevated as indicated in fig2 at 38 and 40 . some ventilation techniques , referred to as intermittent mandatory ventilation or imv , will allow a patient to breathe spontaneously between mechanically induced positive pressure breaths . the resulting pressure and volume traces show spontaneous ventilation at 42 and 44 , respectively , in fig2 with a cycle of mechanically induced respiration shown at 46 and 48 . if such ventilatory technique is combined with cpap , the spontaneous and induced portions of the airway pressure trace would appear as at 50 and 52 , respectively , whereas the spontaneous and induced portions of the lung volume trace would appear as at 54 and 56 . referring now to fig3 other conventional ventilatory techniques include high frequency , low - tidal volume systems ( e . g . infant ventilators ) which produce above - ambient airway pressure and lung volume traces as at 58 and 60 . pressure and volume traces for other conventional modes of assisted ventilation include reverse inspiratory - expiratory ratio ventilation ( 62 , 64 ) and cpap ( 66 , 68 ). by contrast to the above , fig4 illustrates simultaneously generated traces of airway pressure and lung volume for a patient under aprv ventilatory therapy . a pressure trace 70 illustrates cpap pressure component 72 maintained at a selected base level above ambient pressure 74 to provide optimal frc and enhanced lung mechanics . at predetermined intervals , airway pressure is released ( by opening of valve 28 ) to provide as rapid as possible an airway pressure decrease as shown at 76 . the pressure decrease may proceed fully to ambient pressure or may be truncated by a positive pressure limit or threshhold as shown at 78 . the patient being treated with aprv will undergo lung volume variation as indicated by trace 80 , with spontaneous breathing producing cyclic volume increases and decreases 82 above the cpap imposed frc volume 84 , and the pressure release below cpap pressure producing periodic decreases from , and subsequent return to the imposed frc volume 84 as shown at 86 . the present invention , as described , permits augmentation of alveolar ventilation as an adjunct to cpap therapy by periodic reduction of airway pressure below the cpap level . during aprv , cpap may be applied to a pressure level determined to be desirable , based on a variety of physiologic measurements , calculations and observations . these may include , inter alia , observation of respiratory effort , respiratory rate , depth of respiration , arterial oxygen tension , calculated right - to - left intra - pulmonary shunting of blood and lung - thorax compliance . if it is determined that mechanically assisted ventilation is desirable , rather than applying intermittent increases in airway pressure above the pressure of optimal frc , followed by airway pressure reduction to the frc level , aprv is initiated by allowing cyclical release of the cpap to a lower airway pressure no less than ambient atmospheric pressure . thus , the assisted ventilation occurs at pressure and volume levels which are less than , as opposed to greater than , the cpap pressure and the optimal frc , respectively . this assisted exhalation enchances co 2 excretion by causing an airway pressure reduction 78 and corresponding lung volume reduction 86 below frc . as a result , co 2 laden gas leaves the lungs . the degree of ventilatory assistance provided by aprv will be determined by the frequency and duration of pressure release 76 , the level of cpap , the pressure release level , the patient &# 39 ; s lung - thorax compliance and the flow resistance of the airway pressure release valve 28 . as opposed to conventional mechanical ventilation , aprv will allow spontaneous inhalation to occur whenever the patient attempts to breathe . exhalation will occur passively following any normal spontaneous breath and additionally at predetermined intervals when airway pressure is released . accordingly , the enhancement of co 2 excretion and alveolar ventilation occurring during airway pressure release is achieved independent of patient effort . although subsequent reinstitution of positive pressure following a pressure release may appear at first glance to be similar to conventional mechanical inspiration , there are considerable significant differences which may be illustrated by analysis of the above - described pressure and volume tracings . during aprv , the base line lung volume , or frc is determined by the imposed cpap level . during spontaneous inspiration 72 , lung volume increases above frc and during spontaneous exhalation it decreases , returning to the imposed frc level 84 . the entire cycle occurs with little alteration in airway pressure . during airway pressure release 76 , lung volume decreases ( 86 ), and following reapplication of cpap , increases back to frc 84 . with all forms of conventional mechanical ventilation , lung volume is increased above frc , whether it be normal or artificially maintained frc , during inspiration . this holds true for both mechanically induced and spontaneous breaths . lung volume subsequently decreases to frc during exhalation . thus , no prior mode of mechanical ventilation augments alveolar ventilation and causes excretion of co 2 by decreasing lung volume below frc . this is a crucial difference because frc is not merely an arbitrary lung volume parameter , it is the lung volume which minimizes work of breathing , and oxygen at ion efficiency as explained above . to recapitulate briefly , a combination of pressurized gas containing elements are provided to the airway pressure release circuit 10 to maintain a nearly constant continuous positive airway pressure such that spontaneous breathing causes neither significant increase nor decrease in airway pressure . the positive pressure gas maintains an increased frc over the frc volume which would result during breathing of gas at atmospheric pressure . preferably this is an optimal ( i . e . minimal work of breathing ) frc . this increase in frc is responsible for previously reported benefits of peep and cpap . to accompany the cpap breathing assistance , positive airway pressure is briefly released at predetermined intervals to allow a transient decrease of airway pressure ( and therefore of lung volume ) to promote excretion of carbon dioxide from the lungs . the pressure release is of sufficiently limited duration that frc is re - established before any detrimental effect from loss of frc may occur . the disclosed unique ventilation therapy realizes all the beneficial effects of conventional mechanical ventilation without the disadvantages of conventional mechanical ventilators , conventional mechanical ventilator circuits , and conventional cpap systems and circuits . as will be be appreciated from the foregoing description , conventional mechanical ventilation techniques function with a control philosophy nearly the opposite of the aprv control philosophy , in that conventional techniques cause active lung expansion to a volume above desired frc and passive exhalation to frc . by contrast , arpv causes an increase in frc , but assists ventilation by means of airway pressure release thus allowing intermittent passive decrease in lung volume below frc . of course the invention may be practiced in many ways , using a variety of valve configurations . for example , valves 24 and 28 may be combined in a single valve structure having the functional qualities specified above . ventilator 12 may provide a continuous or intermittent flow of pressurized gas to produce the desired constant increase in airway pressure . furthermore , it will be understood that system 10 typically will include many features to fulfill other requirements of conventional ventilation systems . for example , the system typically would include variable breathing gas mix , humidification capability , low flow resistance circuitry , and disconnect , apnea , o 2 rate , and temperature alarms , in addition to the illustrated aprv pressure release control element . these and other features of the invention , as well as various alternative and modified embodiments thereof , would readily occur to those skilled in the art once apprised of may invention . accordingly , it is intended that the invention be construed as broadly as permitted by the scope of the claims appended hereto .