Abstract:
Methods and apparatus for enhancing reliability of, and monitoring the operation of airway valves to enhance patient safety. Pneumatic control line pressure (positive or negative) for actuation of the airway valve may be specified at a given magnitude or within a selected range and monitored continuously. Reduced or excessive pressure may be compensated by actuation of a pressure source or a bleed valve, and monitoring may be effected so as to warn the user of any deviation from the range, or deviations of selected magnitudes or frequencies or a combination thereof. The inspired volume of CO 2  may be monitored using air flow and CO 2  sensing, with detection of excessive CO 2  volume triggering a warning. Similarly, measured end-tidal or end-inspired CO 2  or other appropriate measures of CO 2  concentration may be employed as a warning trigger. Other driving energy sources for airway valves, and monitoring thereof, are also disclosed. In addition, alternative devices and approaches for initiation of re-breathing, and their monitoring, are disclosed.

Description:
This application claims the benefit of U.S. provisional patent application Serial No. 60/104,347, filed Oct. 15, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to breathing circuits affording a re-breathing capability and, more specifically, to reliability and safety enhancements to apparatus employed to divert an exhaled breath volume for re-breathing by a patient and to subsequently remove such volume from the breathing circuit after re-breathing. 
     2. State of the Art 
     A so-called “airway” valve having a re-breathing mode and installed in a ventilator or other breathing circuit (the term “ventilator” being used generically herein to encompass various types of breathing circuits) selectively controls the diversion of an exhaled breath volume from the primary passage of the circuit into a “deadspace” volume defined by a chamber or other vessel such as a loop of hose for subsequent re-breathing by the patient. The re-breathing of the CO 2 -laden exhaled breath volume initiates a change in respiratory CO 2  concentration which may be employed to estimate cardiac output in a non-invasive manner. A discussion of a partial re-breathing technique wherein an additional, fixed deadspace is intermittently and briefly introduced into the ventilator circuit is discussed in detail in Capek, J. and Roy, R., “Noninvasive Measurement of Cardiac Output Using Partial CO 2  Rebreathing,”  IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING , VOL.35, NO. 9, SEPTEMBER 1988, pp. 653-661, the disclosure of which is hereby incorporated in its entirety by this reference. 
     An airway valve employed to divert an exhaled air volume into the deadspace volume, to subsequently add the diverted volume into the ventilator circuit for re-breathing and then to remove it from the breathing circuit requires a high degree of reliability. Specifically, failure to remove the added volume after an appropriately brief period of time results in an increased volume of inspired CO 2 , with an attendant higher level of ventilation and arterial CO 2 . 
     An exemplary breathing circuit including a deadspace volume for partial re-breathing defined by a loop of hose is schematically illustrated in FIG. 1 of the drawings. Exemplary breathing circuit  500  includes a tubular airway  502  that communicates air flow to and from the lungs of a patient. Tubular airway  502  may be placed in communication with the trachea of the patient by known intubation processes, by connection to a breathing mask positioned over the nose and/or mouth of the patient, by a mouthpiece for the patient or via an endotracheal tube. A flow meter  504 , which is typically referred to as a pneumotachometer, and a carbon dioxide sensor  506 , which is typically referred to as a capnometer, are disposed between tubular airway  502  and a length of hose  508 , and are exposed to any air that flows through breathing circuit  500 . Suitable pneumotachometers are disclosed in U.S. Pat. Nos. 5,379,650 and 5,535,633, and a suitable capnometer is disclosed in U.S. Pat. No. 5,793,044. If desired, a combined air flow and carbon dioxide sensor such as that disclosed in U.S. Pat. No. 5,789,660 may be employed in lieu of discrete flow and gas sensors. Both ends of another length or loop of tubing  510 , which may be referenced as defining a deadspace or re-breathing volume  512 , communicate with hose  508 . Deadspace volume  512  may optionally include an expandable section  514 , which may be provided by the use of corrugated tubing for tubing loop  510 . A Y-piece  516 , disposed on hose  508  opposite flow meter  504  and carbon dioxide sensor  506 , facilitates the connection of an inspiratory hose  518  and an expiratory hose  520  to breathing circuit  500  and the flow communication of the inspiratory hose  518  and expiratory hose  520  with hose  508 . 
     The two ends of tubing loop  510  defining deadspace volume  512  are connected to a two-mode airway valve  550 , the two modes being a normal operating mode and a re-breathing mode. During normal breathing, airway valve  550  is maintained in the normal operating mode to prevent inhaled and exhaled air from flowing through deadspace volume  512 . Airway valve  550  may be selectively actuated to shift from the normal operating mode to the re-breathing mode to divert a volume of a patient&#39;s exhaled breath into deadspace volume  512 , the breath volume being subsequently removed from deadspace volume  512  for re-breathing by the patient. Subsequent to re-breathing, airway valve  550  is shifted back to the normal operating mode so that the re-breathed air volume is expired through hose  508  and expiratory hose  520 . During inhalation, gas flows into inspiratory hose  518  from the atmosphere or a ventilator (not shown). Processing unit  522  (preferably included within a patient monitor and hereinafter referred to as a “monitor processing unit”) processes air flow and carbon dioxide input signals from flow meter  504  and sensor  506  (or preliminary processing units associated therewith as known in the art), and preferably directly or indirectly controls operation of airway valve  550  to shift same between the normal operating mode and the re-breathing mode. 
     (Airway valves such as valve  550  illustrated in FIG. 1 may be controlled pneumatically via a control line (tubing) which actuates the valve employing an actuation energy source comprising either a positive air pressure (i.e., a pressure greater than the internal breathing circuit pressure) or a negative air pressure (i.e., a partial vacuum lower than internal breathing circuit pressure).) Thus, there is always a risk of a leak, tubing disconnect, pump failure, power loss or, however unlikely, a valve component jam or failure. Accordingly, it would be desirable to provide enhanced assurance that the expired breath volume added to the circuit from the deadspace volume is removed from the circuit by appropriate switching of the airway valve, by reversion of the airway valve to a normal operating mode upon partial or total failure of the actuation energy source or delivery system, and by alerting the clinician to any problems with the actuation or control of the airway valve. 
     It would also be desirable to afford enhanced reliability to a variety of apparatus which may be employed to provide a deadspace volume or otherwise cause re-breathing of a patient&#39;s CO 2 -laden exhalations. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention includes methods and apparatus for enhancing reliability of, and monitoring, the operation of various apparatus for providing a breathing circuit with a re-breathing capability. As used herein, the term “breathing circuit” includes and encompasses any apparatus through which a patient or other subject may breath, such as, without limitation, ventilator breathing circuits, masks, mouthpieces, and endotracheal tubes. 
     In one aspect of the invention, fluid control line pressure (positive or negative) for actuation of a pneumatic airway valve for diverting an exhalation into a tubing loop or other receptacle or element defining or providing a deadspace volume may be specified as a selected pressure or within a selected range and monitored. 
     In a positive pressure pneumatic system, pressure reduced below a selected threshold may be compensated by actuation of a pump or a vessel containing compressed air, while pressure elevated above a selected threshold may be compensated by a bleed valve open to the ambient environment. 
     In a negative pressure pneumatic system, pressure elevated above a selected threshold may be compensated by actuation of a vacuum pump or opening of a valve connected to a vacuum line, while pressure reduced below a selected threshold may be compensated by opening an inlet valve to the ambient environment. 
     Monitoring of control line pressure may be effected on an intermittent (periodic sampling) or continuous basis and a controller, or processor such as a patient monitor processor linked to the controller, programmed so as to warn the user of any deviation from a selected pressure, a selected pressure range, or pressure deviations of selected magnitudes or frequencies or a combination thereof. 
     It is also contemplated that hydraulic, electrical, magnetic, mechanical and light or other radiation sources may be employed as driving energy sources to actuate an airway valve or other apparatus for providing a deadspace and monitored in appropriate ways to provide enhanced reliability according to the invention. 
     Furthermore, hardware and software “watchdogs” may be incorporated into the controller for the valve or the monitor processing unit with which such controller is associated in order to preclude a software error from inadvertently causing an airway valve or other apparatus initiating re-breathing to maintain the breathing circuit in the re-breathing state. 
     In another aspect of the invention, the volume or level of CO 2  inspired by the patient may be monitored using a sensor which measures both air flow and CO 2 , or individual air flow and CO 2  sensors. Detection of excessive inspired CO 2  volume triggers a warning. Similarly, end-tidal or end-inspired CO 2  concentration or other appropriate measures of CO 2  measured with a CO 2  sensor may be employed as a warning trigger. 
     In still another aspect of the invention, monitoring of the correct operation of an airway valve may be effected by using the patient monitor to analyze CO 2  or other gas waveforms (such as, for example, O 2  or N 2 ), either alone or optionally in combination with air pressure or flow waveforms (or both) already being processed for other purposes to ascertain whether the response expected for a particular airway valve mode (normal operating or re-breathing) is actually being produced. If the response is not as expected, an alarm or other alert may be generated to alert the operator. Thus, the system is “self-checking”, in that the monitor is able to ascertain whether the airway valve actually did shift from one mode to another, responsive to the applied actuation pressure, and if a leak, blockage or mechanical failure has occurred in the airway valve actuation system or perhaps the valve itself. In this embodiment, no actual monitoring of control line pressure would be required, although it is contemplated that such monitoring in combination with waveform analysis might be effected for redundancy. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of a conventional breathing circuit including a pneumetically operated re-breathing valve; 
     FIG. 2 is a semi-schematic drawing of breathing circuit components including a pressure-actuated airway valve for re-breathing and an associated flow and CO 2  sensor arrangement modified in accordance with the present invention; and 
     FIG. 3 is a drawing of three sets of exemplary flow and CO 2  waveforms during the inspiratory period and a portion of the expiratory period illustrating a leak, re-breathing and normal operation of the airway valve. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 2 of the drawings, an exemplary Pneumatically operated airway valve  550  is depicted in association with a combined air flow and CO 2  sensor  560 , such as that described in previously referenced U.S. Pat. No. 5,789,660. One particularly preferred airway valve is disclosed in various embodiments in U.S. patent application Ser. No. 09/173,517 filed Oct. 15, 1998, assigned to the assignee of the present invention and incorporated herein by this reference. The airway valve as disclosed in the foregoing patent application is spring-biased to a normal mode, requiring application of a positive pneumatic pressure to shift to a re-breathing mode and divert an exhalation into a tubing loop. Of course, the present invention is not limited to the valve as disclosed in the foregoing patent application. Two pressure signals from the flow sensor portion of combined sensor  560  providing a differential indicative of air flow rate between airway valve  550  and the patient are detected by transducers in flow processing unit  562 , which outputs a flow signal to patient monitor processing unit  522 . A CO 2  processing unit  564  processes the output from the CO 2  sensor portion of combined sensor  560  and outputs a CO 2  concentration signal to patient monitor processing unit  522 . 
     Airway valve  550  may be selectively actuated by valve control system  100 , which comprises control line tubing  102  connected to a pressure source  104 . Pressure source  104  may comprise either a positive or negative pressure source as known in the art, but preferably comprises a positive pressure source  104  such as a vessel containing compressed air or, more preferably, a pump to provide same at a suitable magnitude on demand. Such a pressure source  104  in the form of compressed air is commonly made available as needed through lines extending to a plurality of locations via pneumatic taps distributed throughout a health care installation such as a hospital or convalescent care facility. However, line pressure is usually high enough so that a regulator is required, making a pump preferable. 
     Pressure source  104  may feed control line tubing  102  through a valve  106 , which may comprise a conventional spring-loaded, two-position solenoid vent valve normally in an open state to vent control line tubing  102  to ambient air pressure. If a pump is employed as pressure source  104 , two events must occur in order to shift airway valve  550  to the re-breathing mode: first, the pump must be actuated (for example, electrically, by closure of a switch) and second, the solenoid valve  106  must be electrically actuated to create a closed system between pressure source  104  and airway valve  550 . It is preferred that the solenoid valve be closed before the pump is actuated, although this is not required. 
     In lieu of a solenoid vent valve, control line valve  106  may comprise a three-way control line valve, as known in the art. In a first position, three-way control line valve  106  communicates pressure source  104  with control line tubing  102  to initiate a pressure increase, while in a second position three-way control line valve  106  closes off control line tubing  102  from pressure source  104  to maintain control line pressure, and in a third position, three-way control line valve  106  communicates control line tubing  102  with a bleed orifice  108  open to a lower pressure environment and usually the ambient environment, to reduce control line pressure. 
     In yet another alternative arrangement, two, two-way control line valves may be employed at  106 , one to open and close communication between pressure source  104  and control line tubing  102 , the other to open and close communication between control line tubing  102  and bleed orifice  108 . 
     With any of the foregoing arrangements, a pressure sensor  110  is placed on control line tubing  102  between a control line valve (such as  106 ) closest to airway valve  550  and airway valve  550  itself to monitor line pressure acting on airway valve  550 . Pressure sensor  110  is electrically linked with a combined power and signal cable as known in the art to a controller  112  preferably integral with patient monitor processing unit  522 , and sends substantially continuous signals thereto indicative of control line pressure in control line tubing  102 . Optionally, one or more pressure-responsive switches triggered at a selected threshold pressure may be employed as a pressure sensor or sensors to provide a signal or signals, and as used herein, the term “pressure sensor” includes pressure switches. If desired, pressure sensor  110  may comprise a disposable sensor fabricated with disposable control line tubing  102  and a disposable control line valve  106  to provide single use capability, the processing circuitry for pressure sensor  110  being included within a patient monitor incorporating patient monitor processing unit  522 . At least control line valve  106  (or optionally two, two-way valves arranged as described above), which may be electrically (including magnetically), pneumatically or hydraulically actuated, is also linked to controller  112 . Optionally, if pressure source  104  comprises a pump associated directly with breathing circuit  500 , controller  112  may be employed to actuate the pump for initiation of each re-breathing cycle or, if air pressure in a reservoir vessel associated therewith falls below a selected threshold, to cause the pump to replenish reservoir pressure. 
     Controller  112  preferably comprises a commercially available controller suitably programmed to respond during a re-breathing cycle, after it has closed control line solenoid vent valve  106 , to open the valve  106  again to cause airway valve  550  to return to its normal operating mode. Similarly, if a three-way valve  106  is employed, controller  112  shifts control line valve  106  to its first position to elevate control line pressure and place airway valve  550  in its re-breathing mode and then to its second position to maintain control line pressure, responsive to sensed control line pressures above or below a selected threshold pressure (or below a selected lower threshold pressure and above a selected upper threshold pressure defining a pressure range, as desired) to shift control line valve  106  to either its first or third position so as to maintain control line pressure at the desired level or within the desired range. Controller  112  may be actuated responsive to a signal from patient monitor processing unit  522  to initiate a re-breathing cycle in breathing circuit  500  by switching control line valve  106  to elevate control line pressure from ambient to elevated, which may comprise pressure source pressure or something less if a suitable regulator  114  as known in the art is interposed between pressure source  104  and control line valve  106 . If, in fact, a regulator  114  is employed or a pump is employed as a pressure source  104 , only the above-referenced two-way solenoid vent valve need be employed to alternatively close communication between the regulated (or initiated) pressure and airway valve  550  through control line tubing  102  or to open communication between control line tubing  102  and a bleed orifice  108 , which may be integral with the solenoid vent valve. In either case, responsive to the elevated control line pressure, airway valve  550  shifts to its re-breathing mode, diverting exhaled breath into deadspace  512 . Similarly, a circuit re-breathing cycle may be terminated by controller  112  responsive to a signal from patient monitor processing unit  522  by switching control line valve  106  to bleed line pressure back to ambient, at which point airway valve  550  returns to its normal operating mode and the re-breathed volume is expired by the patient through expiratory hose  520 . As an alternative to re-breathing cycle initiation and termination by patient monitor processing unit  522 , controller  112  may itself be programmed to periodically initiate and terminate re-breathing cycles at appropriate intervals by shifting airway valve  550  between its two modes. It is also contemplated that controller  112  and patient monitor processing unit  522  may be designed and fabricated as a single, integral unit. 
     Alarm  116  may be incorporated into ventilator circuit  500  in association with patient monitor processing unit  522 , which may be suitably programmed to actuate alarm  116  if pressure in control line valve  106  falls below or increases above a selected threshold or thresholds (when a pressure range rather than a single pressure is selected) during a re-breathing mode of airway valve  550  or increases above atmospheric pressure during a normal operating mode of airway valve  550 . In the first instance, low (sub-threshold) control line pressure may signal a leak in control line tubing  102  or in its connections to control line valve  106  or airway valve  550 , or a failure of pressure source  104  or in actuation of control line valve  106  to its first position. Similarly, high (supra-threshold) control line pressure may signal failure in pressure regulator  114  (if employed) or failure of control line valve  106  to operate properly when termination of a re-breathing cycle is attempted. Likewise, when control line pressure is supposed to reside at atmospheric pressure, an elevated pressure may signal failure of control line valve  106  to vent control line tubing  102  to ambient, or may signal that bleed orifice  108  is blocked or that there is a kink in the control line tubing  102 . Patient monitor processing unit  522  may optionally be programmed to respond to deviations from selected threshold pressures only beyond a certain magnitude so as to avoid false alarms triggered by normal pressure sensor tolerances and transient responses initiating false readings. Similarly, patient monitor processing unit  522  may be optionally programmed to respond to repeated variations from selected pressure thresholds which are not significant enough to immediately compromise operation of airway valve  550  but which may be indicative of a pinhole leak in control line tubing  102 , a leak in control line valve  106 , a leak in the actuating mechanism of airway valve  550  or a loose or leaky connection between two components in the pressurized system. 
     Since operation of the controller  112 , whether separate from or integral with patient monitor processing unit  522 , is software initiated and guided, there is a possibility, however, remote, that a software error occurring while airway valve  550  is in a re-breathing mode “locks up” the monitor. Without any protection from such an error, the monitor will keep airway valve  550  in the re-breathing mode, to the detriment of the patient. To avoid this situation, both the pump used as a pressure source and the solenoid vent valve have hardware “watchdogs” in the form of circuitry that requires receipt of frequent updating by the system software to function. Thus, in order to either actuate the pump or close the solenoid vent valve, the monitor must continually write to the hardware addresses which actuate these devices. Therefore, if a software or hardware error occurs and a device hardware address is no longer written to, the monitor will automatically switch to the normal operating mode. 
     More specifically, and in the context of the preferred embodiment, the pump used as pressure source  104  is controlled by the software. The pump control line is AC-coupled so that the software must pulse the line about every 10 milliseconds (ms) or the pump will turn off. In addition, the control line solenoid valve  106  is AC-coupled such that the software must pulse that control line about every 10 ms to keep the airway valve  550  in re-breathing mode. 
     Further, the software control employs two redundant time bases to keep track of the time the airway valve  550  is in re-breathing mode: a 10 ms interrupt time and a real-time clock. In addition, the software task which controls the airway valve  550  includes a watch-dog that, if not written to in the preceding 200 ms, will reset the patient monitor processing unit  522  within one (1) second. 
     The patient monitor processing unit  522  monitors the control line tubing  102  to make sure that the pressure in the control line tubing  102  decreases at the end of re-breathing and that the line pressure increases at the start of re-breathing. The pressure source (pump)  104  has several seconds (preferably five) to put the airway valve  550  into re-breathing mode before flagging the failure and triggering an alarm. 
     In addition to the foregoing watchdog system, it is desirable for enhanced reliability that two independent time sources, or clocks  600   a  and  600   b , are employed by the patient monitor processing unit  522  to measure the re-breathing cycles or periods. Should the elapsed time signals of the two time sources  600   a  and  600   b  disagree, the airway valve  550  will be reverted back to its normal operating mode, and an alarm triggered. 
     It should be noted at this point that airway valve  550  may be actuated by a driving energy source other than pneumatic, for example, hydraulic, electrical, magnetic, mechanical or even light or other electromotive source. If hydraulic valve actuation is employed, pressure in the hydraulic line would be monitored. If electrical or electrically-induced magnetic actuation of airway valve  550  is employed, current or voltage (or both) in the electrical cable employed to power the airway valve  550  would be monitored. If airway valve  550  is mechanically actuated, as by a slidable wire-in-sleeve cable similar to a lawn mower throttle cable, cable position can be monitored with contact or proximity switches responsive to contact by, or proximity to, a telltale on the wire. In any instance, an alarm may be triggered if a valve of at least one parameter associated with a driving energy source is detected and continued as abnormal. 
     Again referring to FIG. 2 of the drawings, in addition to or in lieu of control line pressure monitoring, breathing circuit  500  and specifically patient monitor processing unit  522  may be configured and programmed to monitor the inspired volume of CO 2  by processing the output signals from combined flow and CO 2  sensor  560  to detect a failure of airway valve  550  to revert to its normal operating mode. A greater than expected inspired CO 2  volume would be indicate that the re-breathing valve&#39;s control system has failed, or that the airway valve  550  itself has failed or jammed in the re-breathing mode, triggering alarm  116 . Similarly, a threshold value of end-tidal or end-inspired CO 2  concentration measured using a CO 2  sensor or the output signal from the CO 2  sensor portion of combined sensor  560  may be employed as a trigger for alarm  116 . 
     As another approach to airway valve monitoring, the flow and CO 2  waveforms generated responsive to the flow sensor and CO 2  sensor employed with the breathing circuit and conventionally employed to monitor patient condition and responses may also be employed to detect problems with operation of the airway valve  550  and its associated actuation system. Referring now to FIG. 3 of the drawings, exemplary flow and CO 2  waveforms are plotted one above the other against elapsed time during an exemplary inspiratory period and a portion of a subsequent expiratory period for three airway valve system conditions, “Leak”, “Rebreathing” and “Normal”. As can readily be seen, the flow waveforms do not vary significantly during the three conditions shown, while the Leak CO 2  waveform differs significantly in shape and in indicated Rebreathed Volume from either the Normal or Rebreathing waveforms. Therefore, if a valve actuation system leak has occurred when actuation pressure is applied through the control line to an airway valve to shift to the re-breathing mode, analysis of the CO 2  waveform by the patient monitor processing unit  522  may be used to detect a partial or complete failure of the valve to shift modes, or an initial shift followed by a return to the normal operating mode as the leak bleeds off actuation pressure. An alarm may then be initiated, and additional actuation pressure automatically applied to the valve to compensate for the leak. Similarly, and by way of example only, a kink in the control line precluding application of actuation pressure to the valve will result in a Normal waveform during a time period when a Rebreathing waveform is expected. Likewise, a control line kink trapping actuation pressure at the valve after one re-breathing cycle has been initiated and completed and actuation pressure is sought to be released will result in a second Rebreathing waveform when a Normal waveform is expected. The foregoing waveform recognition technique may be generalized to smart waveform analysis of various types, so that pattern recognition, syntactic waveform analysis, neural networks, adaptive filters, etc., may be employed to determine the status of the airway valve. Waveform slope, encompassed area, peak height and/or location as well as other waveform features may be used to characterize the waveform. Since waveform analysis is already being performed for other purposes by the patient monitor processing unit, it may be readily additionally programmed to determine breathing mode as well as the existence of valve problems. “Sample” waveforms exemplary of various desirable and undesirable conditions associated with operation of the airway valve and the breathing circuit in different modes may be provided in the memory of the patient monitor processing unit for use as a reference against which to compare the actual waveforms being produced. Further, while analysis of the CO 2  waveform is convenient, the invention is not so limited. Instead, O 2 , N 2 , or any other gas which exhibits a change in concentration as a result of addition or removal of deadspace volume or CO 2  may be monitored for purposes of the present invention. In addition, while telltale waveform changes are emphasized in gas waveforms, flow or pressure waveforms, or both, may be employed in combination with gas waveforms. 
     Use of an airway valve configuration structured to return to a normal operating mode has been disclosed in co-pending U.S. patent applications Ser. No. 09/173,517, previously referenced herein, and Ser. No. 09/173,518, also filed on Oct. 15, 1998, assigned to the assignee of the present invention and the disclosure of which is also incorporated herein by reference. In each of the valves disclosed in these applications, a valve element internal to the re-breathing valve structure is spring-biased to a position occluding air flow through a re-breathing loop deadspace and directing air flow through a primary passage in the re-breathing valve to directly communicate the patient&#39;s respiration with an inspiratory hose such as hose  518  and an expiratory hose such as hose  520 . The spring bias must be overcome by application of positive control line pressure in excess of that internal to the ventilator circuit plus a magnitude sufficient to overcome the spring force on the valve element. In other words, the “default” mode of operation by their design for these re-breathing valves is the normal operating mode. However, as noted above, should control line pressure be unintentionally maintained above a threshold (for example, if the valve control line becomes kinked and prevents pressure bleed-off) so as to overcome the spring force acting on the valve element when the airway valve is to be returned from its re-breathing mode to its normal operating mode, the re-breathing mode will be maintained to the possible detriment of the patient. In addition, should control line pressure be unintentionally reduced during a re-breathing cycle, a variable volume may be re-breathed, resulting in an inaccurate value being presented or a wasted re-breathing cycle. Accordingly, the present invention may provide an additional operational margin and safety factor for even these superior airway valve designs. 
     While the foregoing embodiments of the invention have been described in the context of “passive” devices for introducing deadspace or initiating re-breathing, an active apparatus such as a driven bellows for expanding to accept an exhalation and contracting to expel the exhalation for re-breathing by the patient are contemplated as being within the scope of the present invention. Likewise, CO 2  may be injected into the ventilator circuit for re-breathing, tracheal gas insufflation (TGI) may be controlled to facilitate re-breathing, transient storage of CO 2  may be effected by other means such as the above-mentioned bellows, a piston and cylinder arrangement or other positively-variable volume, or CO 2  may be chemically stored and subsequently released. In all of these instances, performance of the attempted maneuver may be monitored directly through monitoring of an actuation, drive or power source associated with the apparatus employed (e.g., pneumatic, hydraulic, electric, etc.), as well as by a detectable change in position or mode of the apparatus itself. Alternatively, or in combination with monitoring of the apparatus, the results of the attempted maneuver can be monitored in terms of proper performance by appropriate analysis of system outputs such as gas concentration, air flow, air pressure, etc., commonly taken and processed for other purposes, as described above. 
     While the present invention has been disclosed in terms of certain preferred embodiments, those of ordinary skill in the art will understand and appreciate that it is not so limited. Specifically and without limitation, additions, deletions and modifications to the disclosed embodiment may be effected without departing from the scope of the invention as defined by the claims. Similarly, the presence of less than all of the features of the disclosed embodiment does not remove the invention from the scope of the claims.