Patent Publication Number: US-7721736-B2

Title: Self-contained micromechanical ventilator

Description:
This application is a continuation-in-part of application Ser. No. 10/228,166, filed Aug. 26, 2002 now U.S. Pat. No. 7,080,646 and a continuation-in-part of application Ser. No. 10/787,522, filed Feb. 26, 2004 now U.S. Pat. No. 7,320,321. 

   BACKGROUND OF THE INVENTION 
   Immediate medical care can save the lives of countless accident victims and military personnel. In the emergency medical services arena, there has long been an emphasis on the golden hour during which a patient must receive definitive medical attention. However, definitive medical attention is often limited, because of the lack of necessary equipment. While state of the art medical equipment can be found in medical facilities, such is not the case in emergency situations or military applications. This is particularly true in the area of ventilators. 
   Inspiration-only ventilators are known and widely used in hospital settings as they provide useful breathing circuits while minimizing the amount of oxygen utilized in treating the patient. 
   Current ventilators are generally designed for stationary, medical facilities. They are heavy, cumbersome and ill suited for portable applications. Most ventilators utilize medical grade air or highly flammable, compressed canisters of oxygen for its oxygen sources. These tanks air/oxygen are heavy, cumbersome, and unsuitable for transport. Prior-art ventilators also require large power sources, making them even less suitable for quick, on-site use. Lastly, most known ventilators require operation by trained personnel in treatment environments, where additional equipment and resources are easily available. 
   For example, U.S. Pat. No. 5,664,563 to Schroeder et al disclose a computer controlled pneumatic ventilator system that includes a double venturi drive and a disposable breathing circuit. The double venturi drive provides quicker completion of the exhalation phase leading to an overall improved breathing circuit. The disposable breathing circuit allows the ventilator to be utilized by multiple patients without risk of contamination. This device utilizes canistered oxygen sources. This device also would be rendered inoperable under the conditions anticipated by the present invention. 
   Therefore, there is a need for portable ventilators that overcome the disadvantages of the existing stationary ventilators. 
   The following portable ventilators address some of the needs discussed above. U.S. Pat. Nos. 6,152,135, 5,881,722 and 5,868,133 to DeVries et al. discloses a portable ventilator device that utilizes ambient air through a filter and a compressor system. The compressor operates continuously to provide air only during inspiration. The DeVries et al devices are utilized in hospital settings and are intended to provide a patient with mobility when using the ventilator. Since these devices are not directed to on-site emergency use, they provide closed loop control, sophisticated valve systems and circuitry that would render them inoperable under the types of emergency conditions anticipated by the present invention. 
   The references cited above recognize the need for portable ventilators that provide a consistent breathing circuit. As is the case with most portable ventilators, these devices provide breathing circuits including valve systems and an oxygen source. However, these devices lack the means by which they can be quickly facilitated in emergency situations where there are no stationary sources of power. Secondly, most of these devices depend on canister-style oxygen sources, which are cumbersome, and lessen the ability of the ventilators to be truly portable. Thirdly, the prior art ventilators do not provide breathing circuits that can be continuously used in the absence of stationary power sources. These and other drawbacks are overcome by the present invention as will be discussed, below. 
   SUMMARY OF THE INVENTION 
   It is therefore an objective of this invention to provide a portable ventilator that provides short-term ventilatory support. 
   It is another objective of the present invention to provide a portable ventilator that includes a pneumatic subsystem, a power subsystem and a sensor subsystem. 
   It is another objective of the present invention to provide a portable ventilator wherein the pneumatic subsystem includes two dual head compressor for increased air output. 
   It is another objective of the present invention to provide a portable ventilator wherein the pneumatic subsystem includes an accumulator. 
   It is another objective of the present invention to provide a portable ventilator that is a disposable one-use device having an indefinite shelf life. 
   It is also another objective of the present invention to provide a portable ventilator that includes a pneumatic subsystem, a power subsystem, a control subsystem and an alarm subsystem. 
   It is another objective of the present invention to provide a portable ventilator wherein the pneumatic subsystem includes one dual head compressor for increased air output and a means for relieving air manifold pressure with a single head compressor, thereby eliminating the need for an accumulator. 
   It is another objective of the present invention to provide a portable ventilator wherein the power subsystem includes a battery source and a jack that allows the ventilator to access an external power source, where the battery or the external power source is used to power the pneumatic, control and alarm subsystems. 
   It is another objective of the present invention to provide a portable ventilator wherein the power subsystem also includes a power conditioning circuit to eliminate fluctuating voltages to the control subsystem. 
   It is also another objective of the present invention to provide a portable ventilator wherein the control subsystem includes a timing circuit and a relay switch to control the on-off cycle of the dual-head and single head compressors. 
   It is also another objective of the present invention to provide a portable ventilator wherein the alarm subsystem is capable of visually indicating repairable, non-repairable and patient based problems as well as an audible alarm. 
   It is also another objective of the present invention to provide a portable ventilator system including a pneumatic subsystem having only one dual-head compressor, power subsystem, a control subsystem and an alarm subsystem, that is connected to a patient breathing circuit. 
   It is another objective of the present invention to provide a portable ventilator wherein the power subsystem also includes a current limit device to regulate current allowed to recharge battery source and a buck/boost that accepts a range of voltages from the power source and outputs a pre-set constant voltage to the vent to the control subsystem. 
   It is also another objective of the present invention to provide a portable ventilator wherein the control subsystem includes a timing circuit and a relay switch to control the on-off cycle of a dual-head compressor. 
   It is also another objective of the present invention to provide a portable ventilator wherein the alarm subsystem is includes a temperature sensor, a patient error LED, a device/system error LED and a battery monitor LED capable of visually indicating power, system and patient based problems as well as an audible alarm. 
   It is another objective of the present invention to provide a portable ventilator that is a disposable one-use device or a refurbished device having an indefinite shelf life. 
   These and other objectives have been described in the detailed description provided below. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of the portable ventilator, the pneumatic subsystem, the power subsystem and the sensor subsystem. 
       FIG. 2  is a schematic of the pneumatic subsystem shown in  FIG. 1 . 
       FIG. 3  is a schematic of the power subsystem shown in  FIG. 1 . 
       FIG. 4  is a schematic of the sensor subsystem shown in  FIG. 1 . 
       FIG. 5  is a drawing of the portable ventilator shown in  FIG. 1 . 
       FIG. 6  is a schematic of the portable ventilator, the pneumatic subsystem, the power subsystem, the control subsystem and the alarm subsystem. 
       FIG. 6   a  is a drawing of the portable ventilator shown in  FIG. 6 . 
       FIG. 7  is a schematic of the pneumatic subsystem shown in  FIG. 6 . 
       FIG. 8  is a schematic of the power subsystem shown in  FIG. 6 . 
       FIG. 9  is a schematic of the control subsystem shown in  FIG. 6 . 
       FIG. 9   a  is a graph of the dual head compressor on-off cycle. 
       FIG. 9   b  is a graph of resistors and capacitor charging and discharging timing cycle. 
       FIG. 9   c  is a graph of the output of the timing circuit. 
       FIG. 9   d  is a graph of the higher power on-off cycle from the relay switch to the dual head compressor. 
       FIG. 9   e  is a graph of the higher power on-off cycle from the relay switch to the single head compressor. 
       FIG. 10  is a schematic of the alarm subsystem shown in  FIG. 6 . 
       FIG. 11  is a drawing of a preferred embodiment of the portable ventilator including the pneumatic subsystem, the power subsystem, the control subsystem and the alarm subsystem. 
       FIG. 11(   a ) is a drawing of the portable ventilator shown in  FIG. 11 . 
       FIG. 12  is a schematic of a pneumatic subsystem shown in  FIG. 11 . 
       FIG. 12(   a ) is a schematic of the patient circuit that is part of the pneumatic subsystem shown in  FIG. 12 . 
       FIG. 13  is a schematic of the power subsystem shown in  FIG. 11 . 
       FIG. 14  is a schematic of the control subsystem shown in  FIG. 11 . 
       FIG. 14(   a ) is a graph of the dual-head compressor on-off cycle. 
       FIG. 14(   b ) is a graph of resistors and capacitor charging and discharging timing cycle. 
       FIG. 14(   c ) is a graph of the output of the timing circuit. 
       FIG. 14(   d ) is a graph of the higher power on-off cycle from the relay switch to the dual head compressor. 
       FIG. 14(   e ) is a graph of the higher power on-off cycle from the relay switch to the single head compressor. 
       FIG. 15  is a schematic of the alarm subsystem shown in  FIG. 11 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   The present invention is a portable ventilator that provides short-term ventilatory support to one or more patients for the management of trauma or respiratory paralysis. As shown in  FIG. 1 , the portable ventilator V assures consistent tidal volume and respiratory rate and provides hands free operational capabilities. The portable ventilator V is a fully functional multi-mode device suited for field hospital or forward surgical units, where experienced personnel can utilize the multi-mode capabilities unique to this device. Portable ventilator V is also suitable for use by untrained personnel, and in particularly useful in resource-limited environments. Additionally, the portable ventilator V can be configured as a disposable one-use device that has an indefinite shelf life. 
   Also in  FIG. 1 , the portable ventilator V of the present invention includes a pneumatic subsystem N, a power subsystem P, and a sensor subsystem S. Each of these systems shall be described below. 
   The Pneumatic Subsystem: 
   As shown in  FIG. 2 , the pneumatic subsystem N includes two dual head air compressors  1   a  and  1   b  for increased air output. Ambient or NVC filtered air is drawn into the dual head compressors  1   a  and  1   b  and compressed. The compressed air exits  1   a  and  1   b  and enters into the accumulator tank  2 . An accumulator tank  2  is connected to each of the compressors  1   a  and  1   b  to act as a pneumatic holding area for the combined outputs (4 in total) of compressors  1   a  and  1   b . The accumulator tank  2  overcomes the inconsistent nature of the phasing of the pressure waves inherent with dual head air compressors and prevents compressors  1   a  and  1   b  outputs from canceling each other. The accumulator tank  2  is further connected to a connector system  3 . Since the compressors  1   a  and  1   b  function as constant-flow rates over a wide range of physiologic pressures, the connector system  3  provides constant, total airflow through the accumulator  2  to the user, for a necessary period of time. The periods of time are controlled through a timing circuit T that is part of a logic board B. 
   The Logic Board: 
   The logic board B includes timing circuit T and is connected to the power subsystem P. Logic board B controls power to compressors  1   a  and  1   b  in order to turn  1   a  and  1   b  on and off. Duration of the on-time of compressors  1   a  and  1   b  determines the amount of air that is delivered to the user. The logic board B utilizes analog logic and does not require microprocessor control. The logic board B is also connected to the sensor subsystem S. 
   The Sensor Subsystem: 
   As shown in  FIG. 3 , the portable ventilator V includes a sensor subsystem S that provides critical care monitoring and support critically ill patients in the emergency situations. The sensor subsystem S includes an airflow sensor  4  that detects loss of connection of the portable ventilator V from the patient&#39;s face mask or endotracheal tube. The sensor subsystem S also includes an airway pressure sensor  5 . The pressure sensor  5  provides the desirable function of detecting the end of a previous breath (inhaled) in the user, so that air delivery can be delayed until the completion of the previous breath. An airflow sensor  6  is used to detect the cessation of exhalation of the previous breath if the scheduled start time for the next breath is not completed. The sensor subsystem S may be located within the ventilator V or be exterior to ventilator V. 
   The Power Subsystem: 
   As shown in  FIG. 4 , the power subsystem P of the portable ventilator V include disposable or rechargeable batteries  7  that are capable of operating under high capacity, wide temperature ranges and are compatible with the pneumatic subsystem N and the sensor subsystem S. In a preferred embodiment, the portable ventilator V of the present invention utilizes conventional lead-acid rechargeable batteries  7 . The batteries  7  must provide at least 30 to 60 minutes of operating time. 
   The Portable Ventilator: 
   As shown in  FIG. 5 , the pneumatic subsystem N is connected to the sensor subsystem S and the power subsystem P and enclosed within housing  8  of the portable ventilator V. Housing  8  includes an rigid frame structure  8   a  that is made of either plastic or metals and capable of withstanding physical and mechanical pressures. Portable ventilator V includes an input port  8   b  which allows rechargeable batteries  7  to be powered using an external power source or an AC power source. Alternatively, batteries  7  may include disposable type batteries. 
   Housing  8  also a recessed control panel  8   c . Control panel  8   c  includes ports for providing air to the user through known means. The panel  8   c  also includes a switch for selecting desired air flow rates, an on/off switch and can include a switch for recharging the batteries  7 . The control panel  8   c  is recessed to prevent damage to any instrumentation positioned thereon. 
   The portable ventilator V of the present invention implements controlled ventilation and assists control ventilation to a patient. Example 1 below shows functionality and performance of two portable ventilators V described above. 
   EXAMPLE 1 
   The Sekos 2 and 3 ventilators were tested. All tidal volumes, respiratory rates and other parameters were within ±10% of the settings existing on the ventilator V. 
   
     
       
         
             
             
             
           
             
                 
             
             
               PERFORMANCE 
                 
                 
             
             
               PARAMETER 
               SEKOS 2 
               SEKOS 3 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               APPROX. WEIGHT (lb0 
               12 
               &lt;6 
             
             
               APPROX. SIZE (in.) 
               10.75 W × 9.75 D × 7 H 
               5.7 W × 11.5 D × 3.5 H 
             
             
               PHYSICAL VOLUME (in 3 ) 
               733 
               230 
             
             
               BATTERY TYPE/SIZE 
               3.4 Ah lead acid 
               1.3 Ah lead acid 
             
             
               OPERATING LIFE (h) 
               1.5-3 
               0.3-1 
             
             
               COMPRESSORS 
               2 
               2 
             
             
               CONTROLLABLE I:E RATIO 
               No 
               No 
             
             
               RESP. RATE 
               6-30 
               10 OR 20 ONLY 
             
             
               ADJUSTMENT (bpm) 
             
             
               TIDAL VOLUME (ml) 
               200-1200 
               300, 900, OR 1200 
             
             
               MAX MINUTE VOLUME 
               20 (NOT YET TESTED) 
               20 (NOT YET TESTED) 
             
             
               (L/m) 
             
             
               INSPIRATORY FLOW 
               No 
               No 
             
             
               MEASUREMENT 
             
             
               EXPIRATORY FLOW 
               Yes 
               Yes 
             
             
               MEASUREMENT 
             
             
                 
             
          
         
       
     
   
   The portable ventilators tested above, have been shown to be superior in performance to traditional “ambu-bags”. These and other portable ventilators having the features discussed above are within the scope of this invention. 
   The present invention includes a preferred embodiment as shown in  FIG. 6 . The portable ventilator V 2 , as shown in  FIG. 6  includes a pneumatic subsystem N 2 , a power subsystem P 2 , a control subsystem C 2  and an alarm subsystem A 2 . 
   The portable ventilator V 2  as shown in  FIG. 6(   a ) includes a hard shell housing  100  having an exterior surface  100   a  and an interior surface  100   b.    
   The Pneumatic Subsystem N 2 : 
   As shown in  FIG. 7 , the pneumatic subsystem N 2  includes at least one dual head air compressor  101  for increased air output and a single head compressor  102  for closing a flutter valve  103 . The pneumatic subsystem N 2  is responsible for the inhalation and exhalation cycles of the portable ventilator V 2 . During the inhalation cycle, ambient air, a, is drawn into the dual head compressor  101  through the air input port  104 . Ambient air a may also be passed through an NBC filter NBC, to remove contaminants, before passing through air input port  104 . Alternatively, a small adapter (not shown) may be connected to the air input port  104  to allow the ventilator V 2  to operate by drawing air, a, from a purified source (not pictured). Upon entering the portable ventilator V 2 , ambient air a is divided into two air flow paths by y-shaped medical grade tubing  105 . The tubing  105  may also be pre-manufactured plastic or metal. As is understood by one of ordinary skill in the art, tubing  105  includes all necessary fittings and attachments. Additionally, tubing  105  may be an integral part of an interior portion  100   b  of the hard shell housing  100 , shown in  FIG. 6   a . Ambient air a enters the dual head compressor  101 , from tubing  105 , through dual-head compressor input ports  101   a  and  101   b . Dual head compressor  101  compresses ambient air a. It is important to note that combination of using a dual head compressor  101  with a single head compressor  102  is critical to the portable ventilator V 2  of the preferred embodiment of this invention as disclosed in  FIGS. 6 through 10 . It is also important to note that multiple single head compressors in place of the dual head compressor  101 , as disclosed in the preferred embodiment of  FIGS. 6 through 10 , are outside the scope of this present invention. This is because dual-head compressors provide for increased efficiency and smaller size. This factor is essential to the proper design and function of the portable ventilator V 2 . 
   EXAMPLE 2 
   For an equivalent tidal volume output:
     Dual Head Compressor: weight—14.2 oz, size—28.9 cubic inches.   2 Single Head Compressors: weight—20.4 oz, size—32.0 cubic inches.   

   Dual-head compressors draw in outside air and increase pressure within, to allow for the proper tidal volumes to be pushed through a small amount of space. Using the ideal gas law PV=nRT, where (P)=pressure, (V)=volume, (n)=number of molecules, (R)=gas law constant, and (T)=temperature, the values nRT must remain constant when dual head compressor  101  is operational. Thus, as necessitated by the proper operation of ventilator V 2 , obtaining particular volumes (V) of air from the environment into a small, fixed volume of the ventilator V 2 , requires that the pressure (P) of the air a must be increased to keep nRT the same. The increased pressure of air a forces the air a through the ventilator V 2  into the lungs of the patient H. This is due to the tendencies of fluids, here the compressed air a, to flow from the area of greater pressure of the ventilator V 2  to the area of lower pressure of the lungs of the patient H, thereby filling them. 
   As shown in  FIG. 7 , compressed air a exits the compressor  101  through compressor output ports  101   c  and  101   d  and into the air manifold  106 . Air manifold  106  is manufactured from plastic or metal. Air manifold  106  may also be an integral part of the interior portion  100   b . As is understood by one of ordinary skill in the art, air manifold  106  includes all necessary fittings and attachments. A pressure sensor  107  is connected to the air manifold  106  to monitor the pressure of air a delivered to the patient H. The pressure sensor  107  gauges the air pressure of compressed air a within air manifold  106 . When air a exceeds a known threshold, the dual head compressor  101  is stopped and the single head compressor  102  is started, and air is no longer delivered to the patient H, as discussed below. As shown in  FIG. 7 , the air manifold  106  is also connected to the flutter valve  103 . Flutter valve  103  allows compressed air a to enter through input port  103   a  and be delivered to the patient H through bi-directional port  103   b . When compressed air a is being delivered to the patient H through bi-directional port  103   b , exhale port  103   c  remains closed. When the patient H exhales however, the input port  103   a  is closed off, and exhale port  103   c  is open to allow exhaled air to be removed from the portable ventilator V 2 . The exhalation cycle is described below. Compressed air a, that is delivered to the patient H, passes through medical grade tubing  108 , flutter valve  103  and further through medical grade tubing  109  that is connected to the patient H through valve port  110 . It is important to note that tubing  108  is integral to air manifold  106 , and is shown in  FIG. 7  as a separate element for descriptive purposes. Medical grade tubings  108  and  109  may also be pre-manufactured plastic or metal. As is understood by one of ordinary skill in the art, tubings  108  and  109  include all necessary fittings and attachments. Tubings  108  and  109  may be integral to interior portion  100   b . A standard medical grade, patient endotracheal tube (not shown) or tubing to a respiratory mask (not shown) is connected between the portable ventilator V 2  and the patient H at patient valve port  110 . 
   During the exhalation cycle, exhaled air a e  is returned from the patient H through the patient valve port  110 , tubing  109  and the bi-directional port  103   b . The single head compressor  102  causes flutter valve  103  to close input port  103   a , thereby directing the exhaled air a e  into exhaust port  103   c . Exhaled air a e  passes from exhaust port  103   c  into medical grade tubing  111 . Tubing  111  may be premanufactured plastic or metal and may be integral to interior portion  100   b . As is understood by one of ordinary skill in the art, tubing  111  includes all necessary fittings and attachments. Tubing  111  includes a t-junction  111   a  that directs the exhaled air a e  into a second pressure sensor  112 . Second pressure sensor  112  verifies whether patient H is exhaling. In an alternate embodiment, t-junction  111   a  and pressure sensor  112  can be replaced with an in-line flow sensor (not shown). The exhaled air a e  is directed to a patient exhale port  115 , positioned on the ventilator housing  100 . Prior to reaching the exhale port  115 , the exhaled air a e  is directed through an in-line capnography chamber  113 . The capnography chamber  113  is used to detect the presence of exhaled CO 2  in exhaled air a e . The exhaled air a e  travels from the capnography chamber  113  through medical grade tubing  114 . Tubing  114  may be premanufactured plastic or metal and may be integral to interior portion  100   b . As is understood by one of ordinary skill in the art, tubing  114  includes all necessary fittings and attachments. An additional colorometric or chemical capnography sensor CS may be connected externally to portable ventilator V 2  at exhale port  115 , to further monitor ventilation efficiency. As shown in  FIG. 7 , the single head compressor  102 , is connected to the flutter valve  103  and the air manifold  106  through medical grade tubing  116 . It is important to note that tubing  116  is integral to air manifold  106 , and is shown in  FIG. 7  as a separate element for descriptive purposes. Tubing  116  may be premanufactured plastic or metal and may be integral to interior portion  100   b . As is understood by one of ordinary skill in the art, tubing  116  includes all necessary fittings and attachments. The single head compressor  102  operates only when the dual-head compressor  101  is not running. The single-head compressor  102  is used in this manner to ensure that the flutter valve input port  103   a  remains fully closed and the exhaust port  103   c  to be fully open in the exhalation cycle. This alternating operation of the dual head compressor  101  and the single head compressor  102  allows for dead volumes of air located in air manifold  106  to be evacuated through tubing  116 , medical grade tubing  117  and exhaust port  118 , between the inhalation cycles. Tubing  117  may be premanufactured plastic or metal and may be integral to interior portion  100   b . As is understood by one of ordinary skill in the art, tubing  117  includes all necessary fittings and attachments. It is important to note that the single head compressor  102  functions to mechanically close flutter valve  103 . This mechanism is preferred over electronically controlled valves, as they lead to pressure losses. This mechanism is preferred over other venting systems and pressure relief valves to reduce loss of inspiration air and pressure gradients. Secondly, use of the single head compressor  102  forcibly pulls air a out of air manifold  106 , thereby allowing for the next inhalation cycle to begin unimpeded by dead air within air manifold  106 . Thirdly, the single head compressor  102  provides a brief instance of negative pressure during the closure of input port  103   a  that assists the patient H to exhale. In addition, the operation of this dual head compressor  101  and the single head compressor  102  precludes the use of the accumulator  2 , as discussed in the embodiments of  FIG. 1 , above. In an alternate embodiment, single head compressor  102 , tubing  117  and exhaust port  118  can be used to relieve pressure and/or heat buildup within the portable ventilator V 2 . Exhaust port  118  also protects the portable ventilator V 2  from contamination in extreme environmental hazards, as well as contamination from water, dust, mud, etc. 
   It is important to note that the exhaust port  118  is positioned away from exhaust port  115  so as not to alter capnography measurements obtained from capnography sensors  113  and CS. 
   The Power Subsystem P 2 : 
   The power subsystem P 2 , as shown in  FIG. 8 , is discussed below. The power subsystem P 2  provides power to the portable ventilator V 2 . The power subsystem P 2  includes a battery source  201  and a power jack  202  that accepts an external power source EP. A 12-14 volt rechargeable battery is preferred as the battery source  201 . However, replaceable batteries may also be utilized. Power jack  202  is connected to electronic circuit  203  which is further connected to the battery source  201 . The electronic circuit  203  accepts power from the external power source EP through the power jack  202  to regulate voltage necessary to recharge battery source  201  and/or bypass battery source  201 . When an external power source EP is connected to the power jack  202 , the by-pass from the electronic circuit  203  allows the portable ventilator V 2  to operate if battery  201  is missing, inoperational or recharging. Power is directed from either the battery  201  or the electronic circuit  203  into a power switch  204 . When the power is turned on, it is directed from the power switch  204  to a voltage regulator circuit  205  that provides power for the subsystems within the ventilator V 2 . 
   The power subsystem P 2  utilizes the voltage regulator circuit  205  to eliminate fluctuating voltages to the control subsystem C 2 . For components in the control and alarm subsystems C 2  and A 2 , respectively, that require a lower voltage, a second voltage regulator circuit  206  is utilized. Additionally, the power subsystem P 2  provides driving voltage through the control subsystem C 2  to the dual head compressor  101  and the single head compressor  102  of the pneumatic subsystem N 2 . 
   The Control Subsystem C 2 : 
   As discussed under the pneumatic subsystem N 2  above, the on-off cycle between dual head compressor  101  and single head compressor  102  is critical to the operation of the preferred embodiment as shown in  FIG. 6 . As shown in  FIG. 9 , the control subsystem C 2  includes a timing circuit  401  that is used to control a mechanical relay switch  402  that in turn determines the on/off cycle between dual head compressor  101  and the single head compressor  102 . The relay is configured as an electronically controlled single-pole double-throw switch  402 . In a preferred embodiment, timing circuit  401  is a “555” circuit. The relay switch  402  is in turn connected to the single head compressor  102  of the pneumatic subsystem N 2  through a relay switch bar  402   a  and a first connector position  402   b . Relay switch  402  and relay switch bar  402   a  are preferably mechanical. The relay switch  402  is also connected to the dual head compressor  101  through the switch bar  402   a  and second connector position  402   c . The timing circuit  401  is connected to a relay control  402   d , that is used to move the relay switch bar  402   a  between first connector position  402   b  and second connector position  402   c , based upon a breath-timing cycle generated by the timing circuit. The breath-timing cycle is discussed below. The timing circuit  401  is also connected to a capacitor  403 , a first resistor  404  and a second resistor  405 . Second resistor  405  is in turn connected to the power subsystem P 2  The connection between the power subsystem P 2  and the pneumatic subsystem N 2  is not shown in  FIG. 9 . 
   The breath-timing cycle is defined by the respiratory rate and the tidal volume, the values for which have been selected in accordance with American Medical Association guidelines. 
   As shown in  FIG. 9   a, t   1  represents the desired on time of compressor  101 , correlating to the inhalation time, and t 2  represents the desired off time of compressor  101 , correlating to the exhalation time. The sum of the inhalation and exhalation times (t 1 +t 2 ) is one complete breath-timing cycle. 
   The respiratory rate is the number of complete breath-timing cycles per minute. The tidal volume is determined by the amount of air delivered during the inspiration phase in one breath-timing cycle. Tidal volume is the product of the flow rate of the compressor  101  by the on time t 1  of compressor  101 . 
   Therefore:
 
 t   1   =TV/f   (1)
         where TV=tidal volume, f=flow rate of compressor  101 ;
 
 t   1   +t   2 =60 seconds/ RR   (2)
   where RR=respiratory rate, the number of breaths per minute;
 
 t   2 =60 /RR−t   1 =60 /RR−TV/f.   (3)
       

   The values for t 1  and t 2  are thus determined by using the AMA&#39;s respiratory rate and tidal volume guidelines, as well as the flow rate of compressor  101 . Diode  406  is used to allow the possibility that t 1  less than t 2 . 
   As would be understood by one of ordinary skill in the art, the capacitor  403 , first resistor  404  and second resistor  405  form a charging and discharging timing circuit. In the present invention, as shown in  FIG. 9   b , the charge cycle duration is selected to be equal to the desired inhalation time t 1 . The discharge timing cycle is selected to be equal to the determined exhalation time t 2 . Thus:
 
 t   1 =0.693( r   1   +r   2 ) c   1  and  (4)
 
 t   2 =0.693( r   2 ) c   1 ;  (5)
         where r 1  is the value of the first resistor  404 , r 2  is the value of the second resistor  405  and c 1  is the value of the capacitor  403 .       

   Because the output of the charging and discharging circuit is indeterminate with respect to an on or off state of compressor  101 , timing circuit  401  is utilized to establish a clear demarcation of on and off states, as shown in  FIG. 9   c , triggered by the output of the charging and discharging circuit. 
   It is important to note that timing circuit  401  is not powerful enough to operate compressors  101  and  102  directly. Therefore, the relay  402  is used where the output of timing circuit  401 , as shown in  FIG. 9   c , is the control input to the relay  402 . A resistor  407  is used to prevent an electrical short, when the output of timing circuit  401  is on. 
   As shown in  FIG. 9   d , the output of the charging and discharging circuit from timing circuit  401  controls the relay  402  such that the on-cycle of circuit  401  causes the relay  402  to create a pathway to deliver a high power on-cycle to dual head compressor  101 . 
   As shown in  FIG. 9   e , the off-cycle of timing circuit  401  causes the relay  402  to create a pathway to single head compressor  102 . The on-cycle of compressor  101  and off cycle of compressor  102  make up the on-off cycle discussed above. 
   It is also important to note that the timing characteristics, as shown in  FIGS. 9   c  and  9   d , must correspond to the desired timing characteristics in  FIG. 9   a  for the proper operation of portable ventilator V 2 . 
   The Alarm Subsystem A 2 : 
   As shown in  FIG. 10 , the alarm subsystem A 2  includes a light alarm suppression switch  501  connected to a repairable LED indicator  502 , a non-repairable LED indicator  503  and a patient problem LED indicator  504 . The LED indicators  502 ,  503  and  504  are configured to indicate repairable problems, non-repairable problems, and patient based problems, respectively, within the portable ventilator V 2 . The LED indicators  502 ,  503  and  504  are positioned on the outer surface  100   a  of hard shell  100  of portable ventilator V 2 . The alarm suppression switch  501  is used to disengage LED alarms  502 ,  503  and  504  when necessary. An audible alarm suppression switch  505  connected to an audible alarm switch  506 . The audible alarm switch  506  is positioned on the outer surface  100   a  of hard shell  100 . The audible alarm suppression switch  505  is used to disengage audible alarm  506  when necessary. 
   A low voltage detect circuit  507  is connected to the battery  201  and the power switch  205  of the power subsystem P 2  to indicate when voltage is too low. Low voltage detect circuit  507  is also connected to the light alarm suppression switch  501  and repairable LED indicator  502  to denote a repairable problem to the user U. The low voltage detect circuit  507  is also connected to the audible alarm suppression switch  505  and the audible alarm to indicate a sound-based alarm to the user U. 
   A missing pulse/device/component failure detect circuit  508  is connected to the control subsystem C 2 . The missing pulse/device/component failure detect circuit  508  is also is also connected to the light alarm suppression switch  501  and non-repairable LED indicator  503  to denote a non-repairable problem to the user U, ie portable ventilator V 2  must be replaced. The missing pulse/device/component failure detect circuit  508  is also connected to the audible alarm suppression switch  505  and the audible alarm to indicate a sound-based alarm to the user U. 
   Carbon dioxide detect circuit  509  is connected to a carbon dioxide event counter  510  and a carbon dioxide event trigger  511 . The circuit  509 , counter  510  and trigger  511  is connected to the capnography sensor  113  of the pneumatic subsystem N 2  to indicate insignificant carbon dioxide concentrations in exhaled air a e . The carbon dioxide event trigger  511  is further connected to the light alarm suppression switch  501  and patient problem LED indicator  502  to denote a improper connection or patient distress to the user U. The circuit  509 , counter  510  and trigger  511  are also connected to the audible alarm suppression switch  505  and the audible alarm to indicate a sound-based alarm to the user U. 
   An exhale airflow detect circuit  512  is connected to an exhale event counter  513  and an exhale event trigger  514 . The exhale circuit  512 , event counter  513  and event trigger  514  is connected to the pressure sensor  112  of the pneumatic subsystem N 2 . The exhale event trigger  514  is further connected to the light alarm suppression switch  501  and patient problem LED indicator  502  to denote a improper connection or patient distress to the user U. The exhale circuit  512 , event counter  513  and event trigger  514  are also connected to the audible alarm suppression switch  505  and the audible alarm to indicate a sound-based alarm to the user U. 
   An inspiration pressure detect circuit  515  is connected to an inspiration event counter  516  and inspiration event trigger  517  to generate an alarm response when the ambient air, a, pressure is too high or too low. The inspiration circuit  515  is connected to the pressure sensor  107  of the pneumatic subsystem N 2 . The inspiration event trigger  517  is further connected to the light alarm suppression switch  501  and patient problem LED indicator  502  to denote a improper connection or patient distress to the user U. The inspiration pressure detect circuit  515 , inspiration event counter  516  and inspiration event trigger  517  are also connected to the audible alarm suppression switch  505  and the audible alarm to indicate a sound-based alarm to the user U. This inspiration pressure detect circuit  515  can also cause the relay control switch  402   d  to immediately switch from operating the dual head compressor  101  to operating the single head compressor  102  when a preset pressure threshold is exceeded, to prevent harm to patient H. 
   A preferred embodiment of the present invention is shown in  FIG. 11 . The portable ventilator V 3  as shown in  FIG. 11  includes a pneumatic subsystem N 3 , a power subsystem P 3  a control subsystem C 3 , an alarm subsystem A 3  that is connected to a patient breathing circuit PB. The preferred embodiment of as shown in  FIG. 11  includes one dual-head compressor. 
   The portable ventilator V 3  as shown in  FIG. 11(   a ) includes a hard shell housing  1000  having an exterior surface  1000   a  and an interior surface  1000   b.    
   The Pneumatic Subsystem N 3 : 
   As shown in  FIG. 12 , the pneumatic subsystem N 3  includes at least one dual head air compressor  1001  for increased air output. The pneumatic subsystem N 3  is responsible for executing the inhalation and exhalation cycles of the portable ventilator V 3 . During the inhalation cycle, ambient air, a, is drawn into the dual head compressor  1001  through the air input port  1002   a  through an optional NBC filter NBC, to remove contaminants, and through an intake connector  1002   b . Intake connector  1002   b  is designed with nubs around the port opening to prevent any accidental blocking of the inlet port. Additionally, purified air a p , obtained from an optional purified source (not shown), is delivered via purified air port  1002   c  and flow tube  1002   d . Upon entering the portable ventilator V 3 , ambient air a enters the manifold  1002   e  and divided into two air flows  1002   f  and  1002   g  by y-shaped medical grade tubing m. If utilized, ambient air a is mixed with the purified air a p , in the manifold  1002   e . The tubing m may be pre-manufactured plastic or metal and incorporates all necessary fittings and attachments. Additionally, tubing m may be an integral part of an interior portion  1000   b  of the hard shell housing  1000 , shown in  FIG. 11   a . Air a/a p  enters the dual head compressor  1001 , from air flow paths  1002 ( f ) and  1002 ( g ) through dual-head compressor input ports  1001   a  and  1001   b . Dual head compressor  1001  compresses air a/a p  (hereinafter referred to as a c ). Dual-head compressors draws in outside air and increase pressure within, to allow for the proper tidal volume to be pushed through a small amount of space. Using the ideal gas law PV=nRT, where (P)=pressure, (V)=volume, (n)=number of molecules, (R)=gas law constant, and (T)=temperature, the values nRT must remain constant when dual head compressor  1001  is operational. Thus, as necessitated by the proper operation of ventilator V 3 , obtaining particular volumes (V) of air a c  from the environment into a small, fixed volume of the ventilator V 3 , requires that the pressure (P) of the air must be increased to keep nRT the same. The increased pressure of air a c  forces the air a c  through the ventilator V 3  into the lungs of the patient H. This is due to the tendencies of fluids, here the compressed air a c , to flow from the area of greater pressure of the ventilator V 3  to the area of lower pressure of the lungs of the patient H, thereby filling them. 
   As shown in  FIG. 12 , compressed air a c  exits the compressor  1001  through compressor output ports  1001   c  and  1001   d  and into the air manifold  1003 . Air manifold  1003  is manufactured from plastic or metal. Air manifold  1003  may also be an integral part of the interior portion  1000   b . As is understood by one of ordinary skill in the art, air manifold  1003  includes all necessary fittings and attachments. A pressure sensor  1004  is connected to the air manifold  1003  to monitor the pressure of air a c  delivered to the patient H. The pressure sensor  1004  gauges the air pressure of compressed air a c  within air manifold  1003 . When air a c  exceeds a preset threshold, the dual head compressor  1001  air is no longer delivered to the patient H, as discussed below. The air manifold  1003  is connected to a flow path  1005  and an output port  1006  though which the compressed air a c  flows to the patient breathing circuit PB, as shown in  FIG. 12  and  FIG. 12(   a ). As shown in  FIG. 12(   a ), the compressed air a c  enters respiratory tubing  1007  and a breathing valve (flutter valve)  1008  through breathing valve input  1008   a  which then provides compressed air a c  a pathway for entering a breathing valve bi-directional port  1009 . It is important to note that the airflow prior to the bi-directional port  1009  is only oriented in the direction of the patient circuit PB. The compressed air a c  then flows through respiratory tubing  1010  though an inline End-tidal CO2 (EtCO 2 ) detector  1012 , respiratory tubing  1013  and is delivered to the patient H through bi-directional port  1014 . When compressed air a c  is being delivered to the patient H through bi-directional port  1014 , exhale port  1015  remains closed. When the patient H exhales, however, the breathing valve input port  1008   a  is closed off, and exhale port  1015  is opened to allow exhaled air a c  to be removed from the portable ventilator V 3 . The EtCO 2  detector  1012  is used to detect the presence of exhaled CO 2  in exhaled air a c . A second pressure sensor  1016 , through manifold  1017 , a pressure port  1017   a  and tubing  1018  is used to monitor both the patient&#39;s H inhale and exhale pressures. The exhalation cycle is described below. As is understood by one of ordinary skill in the art, tubings m,  1005 ,  1007 ,  1010 ,  1013 , and  1017  are premanufactured plastic or metal, may be integral to interior portion  1000   b  and include all necessary fittings and attachments. A standard medical grade, patient endotracheal tube (not shown) or tubing to a respiratory mask (not shown) or LMA (not shown) is connected between the portable ventilator V 3  and the patient H at bi-directional port  1014 . 
   During the exhalation cycle, exhaled air a e  is returned from the patient H through the bi-directional port  1014 , tubing  1013  and the bi-directional port  1009 . The dual-head compressor  1001  is powered off to allow valve  1008  to close input port  1008   a , thereby directing the exhaled air a e  into exhaust port  1015 . Exhaled air a e  passes from the exhaust port  1015  and into the atmosphere. 
   The Power Subsystem P 3 : 
   The power subsystem P 3 , as shown in  FIG. 13 , is discussed below. The power subsystem P 3  provides power to the portable ventilator V 3 . The power subsystem P 3  includes a battery source  2001  and a power jack  2002  that accepts an external power source EP. A 12-14 volt rechargeable battery is preferred as the battery source  2001 . However, replaceable batteries may also be utilized. Power jack  2002  is connected to a current limit device  2003  which is further connected to the battery source  2001 . The current limit device  2003  accepts power from the external power source EP through the power jack  2002  to regulate current allowed to recharge battery source  2001 . When an external power source EP is connected to the power jack  2002 , the by-pass from the current limit device  2003  allows the portable ventilator V 3  to operate if battery  2001  is missing, inoperational or recharging. Power is directed from either the battery  2001  or the current limit device  2003  into a power switch  2004 . When the power is turned on, it is directed from the power switch  2004  to a buck/boost circuit  2005 . The buck/boost circuit accepts a range of voltages from the power source and outputs a pre-set constant voltage. Additionally, the power switch is also connected to a voltage regulator circuit  2006  that provides a lower voltage for the subsystems within the ventilator V 3 , specifically, the control and alarm subsystems C 3  and A 3 , respectively. 
   The power subsystem P 3  utilizes the voltage regulator circuit  2006  to eliminate fluctuating voltages to the control subsystem C 3  and alarm subsystem A 3 . Additionally, the power subsystem P 3  provides driving voltage through the control subsystem C 3  to the dual head compressor  1001  of the pneumatic subsystem N 3 . 
   The Control Subsystem C 3 : 
   As discussed under the pneumatic subsystem N 3  above, the on-off cycle (during inhalation and exhalation) of the dual head compressor  1001  is critical to the operation of the preferred embodiment as shown in  FIG. 11 . As shown in  FIG. 14 , the control subsystem C 3  includes a timing circuit  4001  that is used to control a mechanical relay switch  4002  that in turn determines the on/off cycle of the dual head compressor  1001 . The relay is configured as an electronically controlled single-pole double-throw switch  4002 . In a preferred embodiment, timing circuit  4001  is a “555” circuit. The relay switch  4002  includes a relay switch bar  4002   a  and a first connector position  4002   b . Relay switch  4002  and relay switch bar  4002   a  are preferably mechanical. The relay switch  4002  is also connected to the dual head compressor  1001  through the switch bar  4002   a  and second connector position  4002   c . The timing circuit  4001  is connected to a relay control  4002   d , through a logic circuit  4003  that is used to move the relay switch bar  4002   a  between first connector position  4002   b  and second connector position  4002   c , based upon a breath-timing cycle generated by the timing circuit. The breath-timing cycle is discussed below. The logic circuit  4003  is also connected to a secondary breathing cycle switch  4004  and an alternative breathing cycle circuit  4005 . Dependent upon need, a user may utilize switch  4004  to select a secondary breathing-timing cycle of breathing cycle circuit  4005 . The timing circuit  4001  is also connected to a capacitor  4006 , a first resistor  4007  and a second resistor  4009 . Second resistor  4009  is in turn connected to the power subsystem P 3 . The connection between the power subsystem P 3  and the pneumatic subsystem N 3  is not shown in  FIG. 14 . 
   The relay switch  4002  is in turn connected to current sensors  4011  and  4012  for determining the patient H&#39;s breathing state during the inhalation-exhalation cycle (where inhalation and exhalation cycle each have a corresponding on-off cycle) of the dual-head compressor  1001 . When relay switch bar  4002   a  is connected to the first connector position  4002   b , current sensor  4011  is utilized to verify the position of switch bar  4002   a . In this position, the ventilator should be in the exhale-cycle. Current sensor  4011  is also connected to alarm subsystem A 3 , to alert a user to alarm conditions should threshold current values not be met. When relay switch bar  4002   a  is connected to the second connector position  4002   c , current sensor  4012  is utilized to determine whether there is current to the dual-head compressor  1001  during the inhalation-cycle. Current sensor  4012  is also connected to alarm subsystem A 3  to alert a user to alarm conditions should threshold current values not be met Current sensor  4012 , dual-head compressor  1001 , and alarm subsystem A 3  are also connected to a safety cutoff circuit  4013 . The safety cut off circuit  4013  can cut off power to the dual-head compressor  1001  should it receive a signal from the alarm subsystem A 3  that the patient or ventilator is in an alarm state. In at least one alarm state, the duel-head compressor  1001  is kept powered off to prevent any potential harm to patient H and/or to conserve enough battery  2001  energy to alarm the user that the patient H or ventilator V 3  needs immediate attention. 
   The breath-timing cycle is defined by the respiratory rate and the tidal volume. 
   As shown in  FIG. 11   a, t   1  represents the desired on time of compressor  1001 , correlating to the inhalation time, and t 2  represents the desired off time of compressor  1001 , correlating to the exhalation time. The sum of the inhalation and exhalation times (t 1 +t 2 ) is one complete breath-timing cycle. 
   The respiratory rate is the number of complete breath-timing cycles per minute. The tidal volume is determined by the amount of air delivered during the inspiration phase in one breath-timing cycle. Tidal volume is the product of the flow rate of the compressor  1001  by the on time t 1  of compressor  1001 . 
   Therefore:
 
 t   1   =TV/f   (4)
         where TV=tidal volume, f=flow rate of compressor  1001 ;
 
 t   1   +t   2 =60 seconds/ RR   (5)
   where RR=respiratory rate, the number of breaths per minute;
 
 t   2 =60 /RR−t   1 =60 /RR−TV/f.   (6)
       

   The values for t 1  and t 2  are thus determined by using the desired respiratory rate and tidal volume as guidelines, as well as the flow rate of compressor  1001 . Diode  4008  is used to allow the possibility that t 1  is less than t 2 . 
   As would be understood by one of ordinary skill in the art, the capacitor  4006 , first resistor  4007  and second resistor  4009  form a charging and discharging timing circuit. In the present invention, as shown in  FIG. 14   b , the charge cycle duration is selected to be equal to the desired inhalation time t 1 . The discharge timing cycle is selected to be equal to the determined exhalation time t 2 . Thus:
 
 t   1 =0.693  r   1   c   1  and  (7)
 
 t   2 =0.693( r   2 ) c   1 ;  (8)
         where r 1  is the value of the first resistor  4007 , r 2  is the value of the second resistor  4009  and c 1  is the value of the capacitor  4006 .       

   Because the output of the charging and discharging circuit is indeterminate with respect to an on or off state of compressor  1001 , timing circuit  4001  is utilized to establish a clear demarcation of on and off states, as shown in  FIG. 14   c , triggered by the output of the charging and discharging circuit. The output of timing circuit  4001  is then an input to logic circuit  4003 . If the secondary breathing cycle switch  4004  is not selected by the user, the output of logic circuit  4003  is a square wave with the same timing of the output of timing circuit  4001 . 
   It is important to note that logic circuit  4003  output is not powerful enough to operate compressor  1001  directly. Therefore, the relay  4002  is used where the output of logic circuit  4003 , as shown in  FIG. 14   c , is the control input to the relay  4002 . A resistor  4010  is used to prevent an electrical short, when the output of timing circuit  4001  is on. 
   As shown in  FIG. 14   d , the output of the logic circuit  4003  controls the relay  4002  such that the on-cycle of circuit  4003  causes the relay  4002  to create a pathway to deliver a high power on-cycle current to dual head compressor  1001  for the inhalation state. 
   As shown in  FIG. 14   e , the off-state cycle of logic circuit  4003  causes the relay  4002  to create a pathway to force the dual-head compressor  1001  into an off-state, thus creating the timing of the exhalation cycle.  FIG. 14   e  represents the output of current sensor  4011 . 
   It is also important to note that the timing characteristics, as shown in  FIGS. 14   c  and  14   d , must correspond to the desired timing characteristics in  FIG. 14   a  for the proper operation of portable ventilator V 3 . 
   The Alarm Subsystem A 3 : 
   As shown in  FIG. 15 , the alarm subsystem A 3  includes a light suppression switch  5001  connected to a patient error LED indicator  5002 , a device/system error LED indicator  5003 , and battery monitor LEDs  5004 . The LED indicators  5002 ,  5003  and  5004  are configured to indicate repairable patient-based problems, non-repairable device problems, and battery power, respectively, within the portable ventilator V 3 . The LED indicators  5002 ,  5003  and  5004  are positioned on the outer surface  1000   a  of hard shell  1000  of portable ventilator V 3 . The light suppression switch  5001  is used to disengage LEDs  5002 ,  5003  and  5004  when necessary. An audible alarm suppression switch  5005  is connected to an audible alarm  5006 . The audible alarm  5006  is positioned on the inner surface  1000   b  of hard shell  1000 , but could also easily be on the outer surface  1000   a . The audible alarm suppression switch  5005  is used to disengage audible alarm  5006  when necessary. 
   A low voltage detect circuit  5007  is connected to the battery  2001  and the power switch  2004  of the power subsystem P 3  to indicate when voltage is too low. Low voltage detect circuit  5007  is also connected to the light suppression switch  5001 . The low voltage detect circuit  5007  is also connected to the audible alarm suppression switch  5005  and the audible alarm  5006  to indicate a sound-based alarm to the user. 
   A patient/system/battery failure detect circuit  5008  is connected to the control subsystem C 3 . The patient/system/battery failure detect circuit  5008  is also connected to the light suppression switch  5001 . The patient/device/battery failure detect circuit  5008  is also connected to the audible alarm suppression switch  5005  and the audible alarm  5006  to indicate a sound-based alarm to the user. 
   A temperature sensor T 3 , to determine ventilator V 3  temperature, is connected to a temperature circuit  5009  which is also connected to light suppression switch  5001 . The temperature circuit  5009  is also connected to the audible alarm suppression switch  5005  and the audible alarm  5006  to indicate a sound-based alarm to the user. The temperature circuit  5009  is used to determine if the ventilator V 3  is operating in a temperature range outside of the specified parameters. 
   An exhale pressure detect circuit  5010  is connected to at least one exhale event counter  5011  and an exhale event trigger  5012  per event counter. The exhale circuit  5010 , event counter  5011  and event trigger  5012  is connected to the pressure sensor  1016  of the pneumatic subsystem N 3 . The exhale event trigger  5012  is further connected to the light suppression switch  5001  and patient error LED indicator  5002  to denote an improper connection or patient distress to the user. The exhale circuit  5010 , event counter  5011  and event trigger  5012  are also connected to the audible alarm suppression switch  5005  and the audible alarm  5006  to indicate a sound-based alarm to the user. 
   An inspiration pressure detect circuit  5013  is connected to at least one inspiration event counter  5014  and inspiration event trigger  5015  per event counter. When the air a c  pressure is too high or too low, pressure detect circuit  5013  generates an alarm response. The inspiration circuit  5013  is connected to the pressure sensor  1004  of the pneumatic subsystem N 3 . The inspiration event trigger  5015  is further connected to the light suppression switch  5001  and patient error LED indicator  5002  to denote a improper connection or patient distress to the user. The inspiration pressure detect circuit  5013 , inspiration event counter  5014  and inspiration event trigger  5015  are also connected to the audible alarm suppression switch  5005  and the audible alarm  5006  to indicate a sound-based alarm to the user. This inspiration pressure detect circuit  5013  can also cause the relay control switch  4002   d  to immediately switch from operating the dual head compressor  1001  inhalation cycle to dual head compressor expiration cycle (off-state) through an alarm subsystem A 3  signal to safety cutoff circuit  4013 , when a preset pressure threshold is exceeded, to prevent harm to patient H.