Abstract:
Embodiments of the present invention described and shown in the specification and drawings include a respiratory system for automatically and variably controlling the supply of a pressurized breathing gas to a patient via a breathing circuit that is in fluid communication with the lungs of the patient. The respiratory system has a demand valve in fluid communication with a source of pressurized gas. The demand valve is switchable between a open position, in which an inhalation conduit of the breathing circuit is in fluid communication with the source of pressurized gas, and a closed position, in which the inhalation conduit is not in fluid communication with the source of pressurized gas. The demand valve is proportionally moveable between the open and closed positions in response to a pressure within a reference chamber which is in fluid communication with the trachea of the patient. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that is will not be used to interpret or limit the scope or meaning of the claims. 37 C.F.R. § 1.72(b).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of respiratory assist devices for respiratory care of a patient and, more particularly, to a respiratory system that automatically and variably on demand regulates the supply of a pressurized gas source to provide the appropriate pressure assist to the patient. 
     2. Background 
     Mechanical ventilatory support is widely accepted as an effective form of therapy and means for treating patients with respiratory failure. Ventilation is the process of delivering oxygen to and washing carbon dioxide from the alveoli in the lungs. When receiving ventilatory support, the patient becomes part of a complex interactive system which is expected to provide adequate ventilation and promote gas exchange to aid in the stabilization and recovery of the patient. 
     In those instances which a patient requires mechanical ventilation due to respiratory failure, a wide variety of mechanical ventilators are available. Most modern ventilators, i.e., respiratory systems, allow the clinician to select and use several modes of inhalation either individually or in combination via the ventilator setting controls that are common to the ventilators. These modes can be defined in three broad categories: spontaneous, assisted, or controlled. During spontaneous ventilation without other modes of ventilation, the patient breathes at his own pace, but other interventions may affect other parameters of ventilation including the tidal volume and the baseline pressure, above ambient, within the system. In assisted ventilation, the patient initiates the inhalation by lowering the baseline pressure by varying degrees, and then the ventilator “assists” the patient by completing the breath by the application of positive pressure. During controlled ventilation, the patient is unable to breathe spontaneously or initiate a breath, and is therefore dependent on the ventilator for every breath. During spontaneous or assisted ventilation, the patient is required to “work” (to varying degrees) by using the respiratory muscles in order to breath. 
     The work of breathing (the work to initiate and sustain a breath) performed by a patient to inhale while intubated and attached to the ventilator may be divided into two major components: physiologic work of breathing (the work of breathing of the patient) and breathing apparatus imposed resistive work of breathing. The breathing apparatus is properly defined as the endotracheal tube, the breathing circuit including an inhalation conduit, an exhalation conduit and a “Y” piece, the gas regulator, such as a ventilator, and may include a humidifier. The work required to spontaneously inhale through the breathing apparatus is the imposed resistive work of breathing (WOB i ). The work of breathing can be measured and quantified in Joules/L of ventilation. In the past, techniques have been devised to supply ventilatory therapy to patients for the purpose of improving patient&#39;s efforts to breath by decreasing the work of breathing to sustain the breath. It is desirable to reduce the effort expended by the patient since a high work of breathing load can cause further damage to a weakened patient or be beyond the capacity or capability of small or disabled patients. 
     In conventional respiratory systems, pressure required to inflate the lungs of a patient connected to a life-support mechanical ventilator is typically measured within the breathing circuit from one of three conventional sites: the inspiratory conduit; the expiratory conduit; or the “Y” piece which is connected to both the inspiratory conduit and the expiratory conduit. Conventionally, at the onset of spontaneous inhalation in the pressure support ventilation (PSV) mode, for example, a pressure change is detected in the breathing circuit, the ventilator is triggered “on,” and lung inflation is assisted by positive pressure supplied by the ventilator. Pressure is then monitored/controlled from the pressure measuring site to ensure the preselected pressure is achieved, and with some ventilators, the rate of pressure rise over time is controlled based on preselected settings. At a specific pressure, for ventilatory support modes relying on pressure for cycling to the “off” phase, inhalation is terminated. 
     Inaccurate pressure measurements on the airways and lungs result from using the aforementioned conventional pressure measuring sites. During spontaneous breathing with continuous positive airway pressure (CPAP), for example, significant underestimations of pressure result compared with measuring tracheal pressure at the distal end of an endotracheal tube (P T ). This is especially true when using narrow internal diameter endotracheal tubes and when peak spontaneous inspiratory flow demands are high. The narrower the tube and the greater the flow rate demand, the greater the discrepancy or inaccuracy between pressure measured at the “Y” piece and P T . 
     Due to inherently resistive components within the breathing circuit, in which the endotracheal tube is regarded as being the most significant resistor, the further from the trachea pressure is measured, the smaller the deviations in pressure that occur with CPAP. Small deviations during spontaneous inhalation may lead the clinician to erroneously conclude that the flow rate provided on demand by the ventilator is sufficient and the imposed resistive work of the breathing apparatus is minimal when, in fact, large deviations in pressure at the distal end of the endotracheal tube and a highly resistive workload imposed by the apparatus may be present. 
     Overestimations in tracheal airway pressure may result during mechanical inflation from the use of the conventional pressure measuring sites. Peak inflation pressure (PIP) generated during mechanical inflation varies directly with total resistance (imposed endotracheal tube resistance and physiologic airways resistance) and inversely with respiratory system compliance (C rs ). PIP measured at the “Y” piece reflects the series resistance of the endotracheal tube and physiologic airways as well as Crs. Peak pressure measured in the trachea reflects physiologic resistance and Cr, only. Therefore, PIP measured at the “Y” piece may be greater than in the trachea. The narrower the internal diameter of the endotracheal tube and the greater the mechanical inspiratory flow rate, the more PIP measured at the “Y” piece overestimates pressure at proximate the distal end of the endotracheal tube. This becomes especially critical when dealing with intubated pediatric patients. 
     The conventional pressure measuring sites also compromise the responsiveness of the conventional respiratory systems to patient inspiratory efforts. This produces delays in triggering the supply of inspiratory assist “on” and cycling “off,” predisposing the patient to patient-respiratory system dysynychrony, and can dramatically increase the effort or work to inhale. Significantly less imposed work results from pressure triggering and controlling the respiratory system “on” at the distal end of the endotracheal tube compared with the conventional method of pressure triggering and controlling or by using Flow-By™. Flow-By™ is a method introduced by the Mallingckrodt Nellcor Puritan-Bennett Company that uses flow sensitivity to trigger the ventilator “on” instead of pressure sensitivity. Whether pressure triggering from inside the ventilator or using Flow-By™, an initial pressure drop across the endotracheal tube must be generated by the patient to initiate flow. This effort results in significant increases in imposed resistive work of breathing. Conventional respiratory systems introduce other more complex factors contributing to WOB i : the work to trigger the ventilator “on” (i.e., to initiate flow), the relative flow or pressure target used during the post trigger phase, and breath termination or cycling “off” criteria. 
     Thus, the design of conventional respiratory systems results in respiratory support systems that are predisposed to increase WOB i  or have limited ability to decrease the WOB i . The conventional designs also fail to provide any reliable degree of automatic responsiveness. To compensate, modern conventional respiratory systems contain complicated control algorithms so that the ventilators can “approximate” what is actually occurring within the patient&#39;s lungs on a breath-by-breath basis. In effect, the computer controlled prior art respiratory systems are limited to the precise, and unyielding, nature of the mathematical algorithms which attempted to mimic cause and effect in the ventilator support provided to the patient. Ventilatory support should be tailored to each patient&#39;s existing pathophysiology and should both provide automatic and variable levels of pressure assist in response to patient inspiratory demand and should minimize WOB i . Such a respiratory system is unavailable in current ventilators. 
     SUMMARY 
     In accordance with the purposes of this invention, as embodied and broadly described herein, this invention, in one aspect, relates to a respiratory system that supplies a pressurized breathing gas from a pressurized gas source to a patient via a breathing circuit in fluid communication with the lungs of a patient. The breathing circuit includes an endotracheal tube, an inhalation conduit in fluid communication with the proximal end of the endotracheal tube, and an exhalation conduit in fluid communication with the proximal end of the endotracheal tube. The respiratory system includes a demand valve in selective fluid communication with the pressurized gas source and the inhalation conduit and a tracheal pressure conduit in fluid communication with the distal end of the endotracheal tube and a reference camber. 
     The demand valve is movable between a first position, in which the demand valve is opened so that the inhalation conduit is placed in fluid communication with the pressurized gas source, and a second position, in which the demand valve is closed so that the inhalation conduit is not in fluid communication with the pressurized gas source. The respiratory system further includes a means for proportionally opening the demand valve in response to pressure changes within the reference chamber so that WOB i  is automatically nullified by providing levels of inspiratory pressure assist that are automatic and proportional on demand of the patient. 
    
    
     DETAILED DESCRIPTION OF THE FIGURES OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principals of the invention. 
     FIG. 1 is a schematic diagram of an exemplified structure of the present invention. 
     FIG. 2 shows the relationship between the patient inspiratory flow demands, the automatic and variable levels of inspiratory pressure assist applied by the respiratory system of the present invention to minimize inspiratory imposed resistive work of breathing (WOB i ), and the pressure proximate the distal end of an endotracheal tube in communication with the lungs of the patient. 
     FIG. 3 shows the relationship between inspiratory assist pressures measured at the “Y” piece (P Y ) of the breathing circuit proximate the proximal end of the endotracheal tube and proximate the distal end of the endotracheal tube (P T ) and peak spontaneous inspiratory flow rate demands. 
     FIG. 4 shows the relationship between work of breathing provided by the respiratory system to assist inhalation (WOB v ), inspiratory imposed resistive work of breathing (WOB i ), and peak spontaneous inspiratory flow rate demands. 
     FIG. 5A shows the relationship between P Y , P T , and percent occlusion of the endotracheal tube. 
     FIG. 5B shows the relationship between WOB v , WOB i , and percent occlusion of the endotracheal tube. 
     FIG. 6 is a schematic diagram of an exemplified structure of the present invention including a mandatory breath support ventilation subsystem, the mandatory breath support ventilation subsystem shown in a first position, in which gas from a pressurized gas source in supplied to an inhalation conduit upon patient demand for a predetermined expiratory time interval. 
     FIG. 7 is a schematic diagram of an exemplified structure of the present invention including a mandatory breath support ventilation subsystem, the mandatory breath support ventilation subsystem shown in a second position, in which gas within a reference chamber is vented to atmosphere and gas from the pressurized gas source is supplied to an inhalation conduit for a predetermined inspiratory time interval. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is more particularly described in the following examples that are intended to be illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. As used in the specification and in the claims, the singular form “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. 
     As depicted in FIG. 1, the respiratory system  10  of the present invention preferably comprises a pressurized gas source  20 , a breathing circuit  24 , a tracheal pressure conduit  60 , and a demand valve  70 . The breathing circuit  24  includes an endotracheal tube  30 , an inhalation conduit  40 , and an exhalation conduit  50 . The endotracheal tube  30  has a distal end  32  and a proximal end  34 . In use, the distal end  32  is placed within the trachea of the patient to be in fluid communication with the patient&#39;s lungs. The inhalation conduit  40  is in fluid communication with the proximal end  34  of the endotracheal tube  30 . In like fashion, the exhalation conduit  50  is in fluid communication with the proximal end  34  of the endotracheal tube  30 . The exhalation conduit  50  includes an exhalation valve  52  that is in selective fluid communication with the atmosphere. To connect the respective inhalation and exhalation conduits  40 ,  50  to the proximal end  34  of the endotracheal tube  30 , a “Y”-piece, for example, may be used. The tracheal pressure conduit  60  is in fluid communication with the distal end  32  of the endotracheal tube  30  and a reference chamber  72 . 
     The demand valve  70  is in selective fluid communication with the pressurized gas source  20  and the inhalation conduit  40 . The demand valve  70  is proportionally movable between a first position during inhalation, in which the demand valve  70  is opened so that the inhalation conduit  40  is placed in fluid communication with the pressurized gas source  20 , and a second position during exhalation, in which the demand valve is closed so that the inhalation conduit  40  is not in fluid communication with the pressurized gas source  20 . The respiratory system  10  further includes a means for proportionally opening the demand valve  70  in response to pressure changes within the reference chamber  72 . 
     The opening means opens and closes the demand valve  70  in proportion to a preselected baseline pressure. Thus, during inhalation, a drop in fluid pressure proximate the distal end  32  of the endotracheal tube  30  is communicated to the reference chamber  72  via the tracheal pressure conduit  60  so that the demand valve  70  opens proportionally upon a pressure drop in the reference chamber  72  relative to the baseline pressure. During exhalation, a rise in fluid pressure proximate the distal end  32  of the endotracheal tube  30  is similarly communicated to the reference chamber  72  via the tracheal pressure conduit  60  so that the demand valve  70  closes when pressure within the reference chamber  72  is equal to or greater than the baseline pressure. 
     Such an opening means, demand valve  70 , and reference chamber  72  may comprise portions of a pressure regulator  74  whose control system detects a drop/rise in pressure with respect to the baseline pressure. Such pressure regulators  74  are known to one skilled in the art and is of a type that is exemplified by a Bird #4715 Baseline Compensator Pressure Regulator. The demand valve  70  of the pressure regulator  74  may be adjustable so that the setting of the baseline pressure is operator-selectable. This allows the operator of the respiratory system to manually select the desired level of CPAP (as the baseline pressure) supplied by the respiratory system  10 . The pressure regulator  74  may also include a spring  73  operatively connected to the demand valve  70  for assisting the opening of the demand valve  70 . 
     The inhalation conduit  40  of the respiratory system  10  may also comprise a gas reservoir  42  and a venturi tube  44 . The venturi tube  44  has an inlet  46 , and outlet  47 , and an augmentation valve  49 . Such a venturi tube  44  is known to one skilled in the art and is of a type that is exemplified by Bird 6549, 2027, 2029 Venturi with One-Way Valves. The inlet  46  of the venturi tube  44  is in fluid communication with the demand valve  70  and the outlet  47  of the venturi tube  44  is in fluid communication with the proximal end  34  of the endotracheal tube  30 . The augmentation valve  49  is in fluid communication with the gas reservoir  42 . The gas reservoir  42  has the same relative percentage mixture of gases as the pressurized gas source  20 . The augmentation valve  49  is movable between a closed position during exhalation, in which the venturi tube  44  is not in fluid communication with the gas reservoir  42 , and an opened position during inhalation, in which the venturi tube  44  is placed in fluid communication with the gas reservoir  42 . In use, when gas from the pressurized gas source  20  enters the inlet  46  of the venturi tube  44  when the demand valve  70  is opened, i.e., during inhalation, the augmentation valve  49  allows gas of the same relative composition to be drawn from the gas reservoir  42  to augment the flow of gas that, in turn, exits the outlet  47  of the venturi tube  44 . Thus, the venturi tube  44  acts as a flow amplifier to increase the flow of gas supplied via the inhalation conduit  40  to the proximal end  34  of the endotracheal tube  30 . 
     The exhalation valve  52  of the exhalation conduit  50  may also have an exhalation valve control conduit  54  that is in fluid communication with the inhalation conduit  40  intermediate the inlet  46  of the venturi tube  44  and the demand valve  70 . Such an exhalation valve  52  having a port for communication to an exhalation valve control conduit  54  is known in the art and is of a type that is exemplified by a Bird #2757 Exhalation Valve. During exhalation, the pressure within the inhalation  40  conduit drops because the demand valve  70  is closed, which therefore causes the pressure in the exhalation valve control conduit  54  to drop. The exhalation valve  52  is responsive to pressure in the exhalation valve control conduit  54  so that, during exhalation, the exhalation valve  52  opens in proportion to the pressure drop in the exhalation valve control conduit  54  relative to the pressure of gas within the exhalation conduit  50 . When opened, the exhalation valve  52  allows fluid within the exhalation conduit  50  to vent to the atmosphere. Conversely, during inhalation, the pressure within the inhalation conduit  40 , and thus the pressure within the exhalation valve control conduit  54 , increases, which closes the exhalation valve  52  when pressure in the exhalation valve control conduit  54  is equal to or greater than pressure in the exhalation conduit  50 . 
     To provide a means for controlling the resistance of the exhalation valve  52  to opening, the exhalation valve control conduit  54  may have a selectable restrictor valve  56  which controls the resistance of the exhalation valve  52  to proportionally open. The restrictor valve  56  is moveable from an open position, in which fluid flow into the exhalation valve control conduit  54  is unrestricted, towards a closed position, in which the exhalation valve control conduit  54  is obstructed so that fluid flow into the exhalation valve control conduit  54  is restricted. As one skilled in the art will appreciate, if the exhalation valve control conduit  54  is unrestricted, the exhalation valve  52  must overcome the resistance of the pressure within the exhalation valve control conduit  52  in order to proportionally open into fluid communication with the atmosphere. As the exhalation valve control conduit  54  is restricted by actuating the restrictor valve  56 , the flow of gas into the exhalation valve control conduit  54  decreases, which decreases the resistance of the exhalation valve  52  to opening. Thereby, as one skilled in the art will appreciate, when the resistance of the exhalation valve  52  decreases, the exhalation valve  56  may proportionally open at a lower pressure within the exhalation conduit  50  relative to the pressure required within the exhalation conduit  50  to open the exhalation valve  52  if the exhalation valve control conduit  54  is unrestricted. 
     The respiratory system  10  may also comprise a pressure sensor  66  having an indicator to display pressure. The pressure sensor  66  is in fluid communication with the tracheal pressure conduit  60  so that pressure within the tracheal pressure conduit  60  and the reference chamber  72  may be displayed. The displayed pressure allows the operator to determine if the respiratory system  10  is maintaining pressure at the distal end  32  of the endotracheal tube  30  as the proximate level of the baseline pressure (i.e., the desired level of CPAP) set in the selectable demand valve  70 . 
     In operation, the respiratory system  10  preferably operates on the basis of closed-loop feedback control. Pressurized gas flow on spontaneous inspiratory demand of the patient is directed from the pressurized gas source  20  via the demand valve  70  through the inhalation conduit  40  and endotracheal tube  30  to the lungs of the patient. The exhalation valve  52  is pressurized closed during inhalation and functions as a threshold resistor for maintaining CPAP during exhalation. Tracheal pressure at the distal end  32  off the endotracheal tube  30  is communicated back to the reference chamber  72  via the tracheal pressure conduit  60  and is used for triggering the respiratory system  10  “ON” by opening the demand valve  70 , controlling the inspiratory assist pressure by proportionally controlling the demand valve  70 , and cycling “OFF” the respiratory system  10  by closing the demand valve  70 . 
     As shown in FIG. 2, the inspiratory assist pressure is automatic and variable on demand, i.e., the greater the demand-flow, the greater the inspiratory assist pressure and the ventilator work of breathing (WOB V ) to minimize the inspiratory imposed resistive work of breathing (WOB i ). In the illustrated examples “A,” “B,” and “C,” peak inspiratory flow rate demands (V) increase from example “A” to example “C.” The respiratory system  10  responds by automatically providing increasing inspiratory assist pressures to match the increasing peak inspiratory flow rate demands. Breathing circuit pressure measured at the “Y” piece (P Y ), adjacent the proximal end  34  of the endotracheal tube  30  is increased in response to the increased inspiratory flow rate demands. The pulmonary airway pressure, which is reflected by the tracheal pressure (P T ) measured at the distal end  32  of the endotracheal tube  30 , is not increased. Thus, in operation, the greater the inspiratory flow rate demand, the greater the inspiratory assist pressure supplied by the respiratory system  10  to minimize the inspiratory imposed resistive work of breathing (WOB i ), and vice versa. 
     The relationship between inspiratory assist pressures measured at the “Y” piece (P Y ) and at the distal end  32  of the endotracheal tube  30  (P T ) and peak spontaneous inspiratory flow rate demands is illustrated in FIG.  3 . The respiratory system  10  automatically and directly varies inspiratory assist pressure levels in response to flow rate demands. Pulmonary airway pressure, as reflected by P T , varies inversely with flow rate demands and are maintained a fairly constant levels at low to moderate flow rate demands and demonstrate a slight decreasing trend at higher flow rate demands. As noted previously, breathing circuit pressures, as reflected by P Y , are substantially greater than P T  especially under high flow rate demands. 
     Referring now to FIG. 4, the relationships between work of breathing provided by the respiratory system  10  to assist inhalation (WOB v ), inspiratory imposed resistive work of breathing (WOB i ), and peak spontaneous inspiratory flow rate demands are shown. WOB v  varies automatically and directly with flow rate demands. Breathing apparatus imposed resistive workloads, as reflected by WOB i , are nullified at low and moderate flow rate demands and demonstrate a slight increasing trend at higher flow rate demands. 
     The respiratory system  10  also automatically and variably adjusts for partial endotracheal tube occlusion. Partial endotracheal tube occlusion by secretions significantly increases imposed resistive work across the endotracheal tube  30  and poses a significant threat to intubated patients, especially patients presenting with copious amount of secretions. Substantial deposits of secretions narrow the internal diameter of the endotracheal tube which results in significant increases in resistance and work. In conventional respiratory systems, this results in inappropriately low levels of pressure assist being provided which elevates the imposed work of breathing and predisposes the patient to respiratory muscle fatigue. Referring to FIGS. 5A and 5B, because P T  is measured directly and the triggering site of the respiratory system  10  is at the distal end  32  of the endotracheal tube  30  which is disposed within the patient&#39;s trachea, the respiratory system  10  provides automatic and variable levels of pressure assist to overcome the breathing apparatus impedance, including the increased impedance of the partially occluded endotracheal tube, and to satisfy inspiratory flow rate demands of the patient. With partial endotracheal tube occlusion, the level of pressure assist increases proportionally, nullifying the increased imposed resistive work of breathing (WOB i ). The greater the impedance and flow rate demand to inhale, the greater the pressure assist and ventilator work of breathing (WOB v ) to minimize the imposed work of breathing (WOB i ), and vice versa. 
     Referring to FIGS. 6 and 7, the respiratory system may include a mandatory breath support ventilation subsystem  100  for providing mechanical ventilatory support to apneic subjects or those requiring periodic breaths to augment spontaneous breathing. The respiratory system  10 , via the mandatory breath support ventilation subsystem  100 , may provide conventional mechanical ventilation (CMV) and intermittent mandatory ventilation (IMV). 
     The mandatory breath support ventilation subsystem  100  permits the respiratory system to function as a time cycled mechanical ventilator. With this type of ventilator, tidal volume is the product of inspiratory flow rate and inspiratory time, i.e., 
     
       
         Tidal Volume (ml)=Inspiratory Flow Rate (ml/sec)×Inspiratory Time (sec).  
       
     
     During mechanical inhalation with the respiratory system  10  of the present invention, gas flow rate is substantially constant while inspiratory time is variable. Thus, by regulating inspiratory time, tidal volume may be regulated. Following mechanical inhalation, the respiratory system  10  provides ventilator support in the manner described previously, automatically and variably supplying levels of inspiratory assist pressure to minimize WOB i . “The mandatory breath support ventilation subsystem  100  allows gas from the pressurized gas source  20  to pass into the inhalation conduit  40  to the patient for a predetermined inspiratory time interval. The mandatory breath ventilation subsystem  100  comprises a mandatory breath ventilation subsystem source of pressurized gas  102 , a first normally-open cartridge valve  110 , a second normally-open cartridge valve  120 , a normally-closed cartridge valve  130 , a fluid conduit  140 , an exhalation fluid line  150 , an inhalation fluid line  160 , and a back-pressure conduit  170 .” 
     The first normally-open cartridge valve  110  is connected to the tracheal pressure conduit  60  and in its normally open position allows fluid to be communicated from the distal end  32  of the endotracheal tube  30  to the reference chamber  72 . When pressurized closed, the first normally-open cartridge valve  110  closes the tracheal pressure conduit  60  to prevent communication of fluid from the distal end  32  of the endotracheal tube  30  to the reference chamber  72 . The second normally-open cartridge valve  120  is connected to the source of pressurized gas  102 . As one skilled in art will appreciate, the source of pressurized gas  102  may be a separate pressurized gas source or it may be the same pressurized gas source  20  that is in communication with the demand valve. 
     The normally-closed cartridge valve  130  is connected to the tracheal pressure conduit  60  between the first normally-open cartridge valve  110  and the reference chamber  72 . The normally-closed cartridge valve  130  has an outlet port  132  in selective fluid communication with the atmosphere. When pressurized open, the normally-closed cartridge valve  130  allows the reference chamber  72  to be in fluid communication with the atmosphere through the outlet port  132 . The first and second normally-open cartridge valves  110 ,  120  and the normally-closed cartridge valve  130  are pressure actuated valves that are well known in the art and are exemplified by Bird #6830 and #6668 normally-open and normally-closed cartridge valves. 
     The fluid conduit  140  is connected to the first normally-open cartridge valve  110 , the second normally-open cartridge valve  120 , and the normally-closed cartridge valve  130 . The fluid conduit  140  acts as a central line to conduit fluid from the source of pressurized gas  102  to various components of the mandatory breath ventilation subsystem  100 . The fluid conduit  140  also has a fluid conduit line resistor  142  in selective fluid communication with the atmosphere. The fluid conduit line resistor  142  allows fluid in the fluid conduit  140  to be selectively vented to the atmosphere. 
     The exhalation fluid line  150  is connected to the second normally-open cartridge valve  120 . The exhalation fluid line  150  has a first one-way valve  152  and an expiratory resistor  154 . The first one-way valve  152  is positioned intermediate the second normally-open cartridge  120  and the expiratory resistor  154 , i.e., upstream of the expiratory resistor  154 , and is oriented to allow fluid flow within the exhalation fluid line  150  downstream toward the expiratory resistor  154 . The expiratory resistor  154  is in selective fluid communication with the atmosphere. 
     The inhalation fluid line  160  is connected to the fluid conduit  140  and to the exhalation fluid line  150  intermediate the second normally-open cartridge valve  120  and the first one-way valve  152 . The inhalation fluid line  160  has a second one-way valve  162  and an inhalation resistor  164 . The second one-way valve  162  is positioned intermediate the fluid conduit  140  and the inhalation resistor  164 , i.e., upstream of the inhalation resistor  164 , and is oriented to allow fluid flow within the inhalation fluid line  160  downstream toward the inhalation resistor  164 . The inhalation resistor  164  is in selective fluid communication with the exhalation fluid line  150 . 
     The inhalation fluid line  160  and the exhalation fluid line  150  are interconnected by the back-pressure conduit  170 . The back-pressure conduit  170  is connected to the exhalation fluid line  150  intermediate the first one-way valve  152  and the exhalation resistor  154  and is connected to the inhalation fluid line  160  intermediate the second one-way valve  162  and the inhalation resistor  164 . 
     The mandatory breath support ventilation subsystem  100  is switchable from a first position, as shown in FIG. 6, in which gas from the pressurized gas source  20  is supplied to the inhalation conduit  40  upon demand of the patient for a predetermined expiratory time interval, to a second position, as shown in FIG. 7, in which the reference chamber  72  of the pressure regulator  74  is vented to the atmosphere through the outlet port  132  of the normally-closed cartridge valve  130 . In the second position, because the pressure of gas within the vented reference chamber  72  is less than the baseline pressure, gas from the pressurized gas source  20  is supplied to the inhalation conduit  40  for the predetermined inspiratory time interval. 
     Referring to FIG. 6, in the first position, the first normally-open cartridge valve  110  is open so that the distal end  62  of the tracheal pressure conduit  60  is in fluid communication with the reference chamber  72  of the pressure regulator  74 , the second normally-open cartridge valve  120  is closed so that the source of pressurized gas  102  is not in fluid communication with the fluid conduit  140 , and the normally-closed cartridge valve  130  is closed so that the reference chamber  72  of the pressure regulator  74  is not in fluid communication with the atmosphere. In operation, during spontaneous inhalation by the patient, tracheal pressure is directed back through the first normally-open cartridge valve  110  via the tracheal pressure conduit  60  to the reference chamber  72 . The respiratory system  10  responds to changes in the pressure within the reference chamber  72  relative to the baseline pressure as described previously. The second normally-open cartridge valve  120  blocks off pressurized gas  102  from the source of pressurized gas and is allowed to gradually depressurize via fluid being vented from the exhalation fluid line  150  through the exhalation resistor  154  to the atmosphere over the predetermined expiratory time interval (T E ). The exhalation resister  154  governs the rate of depressurization of the second normally-open cartridge  120  and depressurizes the second normally-open cartridge  120  over the predetermined expiratory time interval (T E ). 
     Referring now to FIG. 7, in the second position, the first normally-open cartridge valve  110  is closed so that the distal end  62  of the tracheal pressure conduit  60  is not in fluid communication with the reference chamber  72  of the pressure regulator  74 , the second normally-open cartridge valve  120  is opened so that the source of pressurized gas  102  is in fluid communication with the fluid conduit  140 , and the normally-closed cartridge valve  130  is opened so that the reference chamber  72  of the pressure regulator  74  is in fluid communication with the atmosphere. In operation, when the exhalation fluid line  150  is depressurized at the end of the expiratory time interval T E , the second normally-open cartridge valve  120  is depressurized and is opened by the source of pressurized gas  102  which, via the fluid conduit  140 , simultaneously pressurizes closed the first normally-open cartridge valve  110  and opens the normally-closed cartridge valve  130 . Opening the normally-closed cartridge valve  130  permits the reference chamber  72  to depressurize as it vents to the atmosphere. Because the pressure of gas within the vented reference chamber  72  is less than the baseline pressure, gas from the pressurized gas source  20  is supplied to the inhalation conduit  40  and to the exhalation valve  52  via the exhalation valve control conduit  54  to pressurize the exhalation valve  52  closed. This results in the lungs of the patient being actively inflated (i.e., mechanical inhalation). While the lungs of the patient are being actively inflated, pressurized gas from the source of pressurized gas  102  is flowing through the inhalation fluid line  160  and the back-pressure conduit  170 . The gas flowing through the inhalation fluid line  160  flows through the first second-way valve  162  and the downstream inhalation resistor  164  that regulates the predetermined inspiratory time interval (T I ). The inhalation resister  164  governs the rate of repressurization of the second normally-open cartridge  120  and repressurizes the second normally-open cartridge  120  over the predetermined inspiratory time interval (T I ). When the second normally-open cartridge  120  is repressurized at the end of the inspiratory time interval (T I ), the mandatory breath ventilation subsystem  100  is placed back at the first position, as shown in FIG. 6, so that spontaneous breathing by the patient is once again permitted. 
     The inhalation and exhalation resistors  164 ,  154  of the mandatory breath support ventilation subsystem  100  may be adjustable so that the predetermined inspiratory time interval and the predetermined expiratory time interval are selectable by the operator. The resistors are well known in the art and are exemplified by Industrial Specialties #RR-16-025 and #RR-16-012 resistors. 
     The invention has been described herein in considerable detail, in order to comply with the Patent Statutes and to provide those skilled in the art with information needed to apply the novel principles, and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modification, both as to equipment details and operating procedures can be affected without departing from the scope of the invention itself. Further, it should be understood that, although the present invention has been described with reference to specific details of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims.