Abstract:
A breathing circuit of the closed circuit type has improved means for removing water vapor to prevent condensation within the circuit. A dryer is placed in the breathing circuit, downstream of the CO 2  absorber, for removing water vapor from the breathing gases, including that entrained in the breathing gases during passage through the CO 2  absorber. The dryer may utilize a thermoelectric cooling element or a water vapor permeable membrane.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a breathing circuit having an improved means for removing water vapor from the circuit. Condensation of the water vapor within the breathing circuit, and its attendant problems, is thus lessened or eliminated. The invention is particularly suited for use in breathing circuits characterized as being of the closed type. 
     During breathing, a volume of breathing gas, termed the tidal volume, is inhaled into the lungs during inspiration and exhaled during expiration. Tidal volumes typically range from 400-1000 milliliters (ml), depending on the size of the subject&#39;s lungs. When in the lungs, the breathing gases become moistened and water vapor is discharged with the breathing gases when they are expired. About 35 milligrams (mg) of water are discharged by a person with each breath, assuming a tidal volume of about 1000 milliliters. 
     A mechanical ventilator may be used to supply and remove breathing gases to/from the subject. This may be done to assist or replace the natural breathing action of the subject, in connection with the supply of an anaesthetic agent to the subject, or for other reasons. A typical mechanical ventilator has an inspiration limb for supplying breathing gases to the subject and an expiration limb for receiving breathing gases from the subject. The inspiration and expiration limbs are each connected to arms of a Y-connector. A patient limb extends from a third arm of the Y-connector to an intubation tube or face mask for the subject. 
     A common type of mechanical ventilator recirculates the expired breathing gases of the subject in the expiration limb through a CO 2  absorber back to the inspiration limb for rebreating by the subject. Such a closed breathing circuit prevents loss of anaesthetic agents to the ambient air. Such breathing circuits are often operated in a “low flow” mode in which, at least in principle, the amount of fresh, dry breathing gases added to the breathing circuit is only that necessary to replace the gases consumed by the subject. 
     However, the CO 2  absorber in such a circuit acts as a moisture reservoir so that additional moisture, for example, an additional 15 mg of water per breath becomes entrained in the breathing gases circulating in the closed breathing circuit. 
     While it is preferable that the subject breath moist, warm breathing gases, the presence of such gases in the breathing circuit does have disadvantages. When the warm, moist breathing gases expired by the subject, which are at body temperature, pass through the breathing circuit, which is at room temperature, the water vapor in the breathing gases condenses on components of the breathing circuit. As the breathing of the subject continues, the condensed water accumulates. The accumulated water may interfere with the operation of valves, sensors, or other components of the breathing circuit or form a medium for microbiological growth within the circuit. Such accumulations therefore present a problem in closed circuit breathing systems. 
     Various solutions have been proposed to remedy this problem. Water traps may be inserted in the breathing circuit in an effort to prevent water from reaching critical components. Or, all, or the portions, of the breathing circuit particularly effected by moisture accumulation, may be heated to prevent condensation of the water vapor. This may be carried out for example by resistance heaters, such as wires that are wrapped around the tubing of the limbs, and around valves, etc. 
     However, while heating can delay the onset of condensation and prevent condensation in critical parts of the circuit, it is difficult or impossible to fully prevent precipitation of water vapor out of the breathing gases. 
     Many breathing circuits incorporate a humidity and moisture exchanger (HME) in the patient limb in which heat and moisture from exhaled breathing gases are exchanged to the breathing gases to be inhaled by the subject. The primary purpose of such an exchanger is to provide for patient comfort by preheating the inhaled breathing gases and to ensure that the patient does not inhale dry breathing gas. However, in the usual case, not all moisture in the exhaled breathing gases is transferred to the inhaled breathing gases. A small amount, which can be characterized as “leakage” remains in the exhaled gases and circulates in the breathing circuit. Thus, the reduction in the moisture level of the exhaled gases entering the breathing circuit, as a result of humidifying the inhaled gases, may also delay, but usually will not fully prevent, condensation and moisture accumulation in other portions of the breathing circuit. 
     Due to the additional amount of water inserted in the breathing gases by the CO 2  absorber, the amount of breathing gas water vapor is increased in the portions of the breathing circuit between the absorber and the subject, exacerbating the problems of moisture condensation on sensors, traps, and the like in this portion of the circuit. 
     The problem is particularly acute when a ventilator is operated in a low-flow manner since the breathing gases are continually recirculating and little fresh, dry gas is being added. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is thus directed to a breathing circuit having an improved means for removing water vapor from the breathing gases in the breathing circuit. Condensation within the breathing circuit and its attendant problems is thereby lessened or eliminated, including that occurring under low flow conditions. 
     Briefly, the breathing circuit of the present invention incorporates a dryer downstream of the CO 2  absorber for removing water vapor from the breathing gases. The dryer may incorporate means, such as a thermoelectric element, for cooling breathing gases passing through the absorber so that the water vapor condenses out of the breathing gases. A fan may be used in conjunction with the thermoelectric element to improve its performance. The dryer may include a heat transfer means for reheating the cooled, drier breathing gases with incoming gases from the CO 2  absorber. 
     Or, the breathing gases may be passed along a water permeable membrane such as a water permeable tube to remove the water vapor from the breathing gases. The tube may be jacketed so that dry air can be supplied to the other side of the tube to improve the removal of the water vapor. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the drawings in which: 
     FIG. 1 is a general, somewhat schematic view of a breathing circuit of the present invention; 
     FIG. 2 is a schematic, cross sectional view of a dryer incorporated in the breathing circuit of the present invention; 
     FIG. 2A shows a modification of the dryer shown in FIG. 2; 
     FIG. 3 is a schematic, cross sectional view of a further modification of the dryer; and 
     FIG. 4 is a schematic, cross sectional view of an alternative embodiment of a dryer suitable for incorporation in a breathing circuit of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a ventilation system  10  for mechanically ventilating subject  12 . Ventilation system  10  includes closed breathing circuit  14  which can be charged with breathing gases from source  15  via conduit  17 . Breathing circuit  14  has inspiration limb  16  having inspiration check valve  18 . Inspiration limb  16  is connected to the inlet of Y-piece connector  20 . Patient limb  22 , also connected to Y-connector  20 , supplies breathing gases to subject  12  during inspiration and receives breathing gases from the subject during expiration. 
     Expiration limb  24  is connected to the output of Y-connector  20  to receive the exhaled breathed gases and includes expiration check valve  26 . Expiration limb  24  is connected to the inlet of carbon dioxide (CO 2 ) absorber  28 , the outlet of which is customarily connected to inspiration limb  16  to complete the closed breathing circuit. CO 2  absorber  28  may contain soda lime or other suitable CO 2  absorbent. Anaesthesia machine  30  may be connected to inspiration limb  16  via conduit  32  to supply and maintain an anaesthetic agent in the breathing gases in circuit  14 . Various flow sensors and pressure sensors, not shown, may also be connected in the breathing circuit. 
     Bellows assembly  34  is used to separate the breathing gases in breathing circuit  14  from driving gases that supply the energy necessary to provide the breathing gases to subject  12  during inspiration. Bellows assembly  34  includes expandable, pleated bellows  36 . Bellows  36  is connected to expiration limb  24  by pipe  38 . Bellows  36  is contained in housing  40 . In a typical ventilation system, bellows  36  expands upwardly and contracts downwardly in housing  40 . 
     Bellows assembly  34  is operated by ventilator driver  42  which is coupled to housing  40  by supply line  44 . Driver  42  is connected to a driving gas supply  46 . Ventilation driver  42  includes a gas flow control valve which may be operated by a waveform generator that provides desired gas flow in supply line  44 . Gas so supplied to housing  40  compresses bellows  36  downwardly, forcing the breathing gases in the bellows and in the downstream portions of breathing circuit  14  through CO 2  absorber  28 , inspiration limb  16 , Y-piece connector  20 , and patient limb  22  to subject  12 . As noted above, water retained in CO 2  absorber  28  is entrained in the breathing gases as they pass through CO 2  absorber  28 . The volume of breathing gases delivered to subject  12  is determined by the amount of driving gas supplied to housing  40 . 
     During expiration, the driving gas in housing  40  is allowed to exit the housing, permitting bellows  36  to expand upwardly and receive the exhaled gases as subject  12  breaths out. The exhaled gases are provided to bellows  36  via expiration limb  24  and expiration check valve  26 . 
     On the next breath for subject  12 , bellows  36  is again compressed by the driving gas to provide breathing gases to the subject. The CO 2  in the breathing gases previously exhaled by the subject is removed by CO 2  absorber  28  and the breathing gases pass through inspiration limb  16  for delivery to subject  12 . The breathing gases subsequently exhaled by the subject are again received in expiration limb  24  and bellows  36 . 
     Breathing circuit may include various sensors, such as flow sensors qualitative gas sensors, and pressure sensors, that monitor the operation of the breathing circuit. Conventionally, the breathing circuit may also contain water traps (not shown) at locations, such as at sensors, known to collect water. Patient limb  22  will typically include breathing gas sampling tubes for the sensors, a bacterial filter, and other elements, collectively shown as  52 . 
     Patient limb  22  also includes humidity and moisture exchanger (HME)  50  for exchanging heat and moisture from the exhaled breathing gases to the inhaled breathing gases. To this end, heat and moisture exchanger  50  may include a porous element  51  impregnated with a hygroscopic agent. 
     In accordance with the present invention, and as shown in FIG. 1, dryer  100  is inserted in breathing circuit  14 , downstream of CO 2  absorber  28  for removing moisture from the breathing gases circulating in the breathing circuit. In the embodiment shown in FIG. 2, dryer  100  has housing  102  formed of plastic or other suitable material. An inlet connector  104  receives conduit  106  leading from CO 2  absorber  28  for receiving warm, moist, exhaled breathing gases which have been scrubbed of CO 2  in the absorber. The air passes through a first chamber  108  of housing  102  in conduit  109  to second chamber  110 . Second chamber  110  contains thermoelectric element  112  which may be a Peltier or similar element. In such an element, a current through the element creates a cold surface of the element and a warm surface of the element. In dryer  100 , element  112  is arranged so that the cold surface  112   a  is exposed in second chamber  110 . One or both surfaces of thermoelectric element  112  may be finned to improve heat transfer. If desired, a fan  114  may move air from vents  116  across the warm surface  112   b  of thermoelectric element  112  to remove heat from that surface to further improve the performance of the thermoelectric element. Element  112  and fan  114  may be provided with electric current in conductors  118 , respectively. Thermoelectric element  112  may be provided with a temperature sensor for control or safety purposes. 
     The warm, moist breathing gases from conduit  106  pass over the cold surface of thermoelectric element  112  and are cooled, condensing water vapor out of the breathing gases. The condensed water can be removed from chamber  110  at drain  120 . 
     The breathing gases then pass from second chamber  110  back through first chamber  108  in conduit  113  and to connector  122  for conduit  124 . Conduit  124  returns the dried breathing gases to the breathing circuit. In the return passage through first chamber  108  in conduit  113 , the breathing gases are reheated by incoming breathing gases from conduit  109 . Heat exchange fins  111  and  115  may be provided on conduits  109  and  113 , respectively, to improve the heat transfer. Re-heating the air discharged from dryer  100  assists in providing comfort to subject  12  as the breathing gases are respired while providing a measure of cooling to the breathing gases being supplied to dryer  100  in chamber  110 . If further reheating of the breathing gases is desired, the discharge from fan  114  may be provided to heat exchanger  126  to transfer the heat drawn off thermoelectric element  112  to the breathing gases in conduit  124 , as shown schematically in FIG.  2 A. 
     In some cases, it may be possible to eliminate thermoelectric element  112  and employ the heat exchange between the outgoing and incoming breathing gases to cool the latter to remove moisture. 
     To facilitate cleaning of dryer  100 , thermoelectric element  112  and the associated components may be removed from housing  102 , as shown by the dotted line in FIG. 2 for cleaning in a manner similar to that of other mechanical or electric components of the breathing circuit  10  or anaesthesia machine  30 . Housing  102  and the associated parts may be autoclaved. 
     FIG. 3 shows an alternative embodiment of the dryer as  100 A. In FIG. 3, elements similar to those found in FIG. 2 are identified with similar or analogous reference numerals. Housing  102 A of dryer  100 A forms chamber  128 . Chamber  128  contains thermoelectric element  112   c.  Thermoelectric element  112   c  is oriented so that the cold surface of the element is adjacent incoming breathing gases from breathing circuit  14  in conduit  106 . FIG. 3 shows a vertical orientation for thermoelectric element  112   c.  The breathing gases are cooled in their passage past the cold surface  112   a.  The moisture condensed out of the breathing gases by the cooling exits chamber  128  through drain  130 . The dried breathing gases are returned to the breathing circuit along the warm surface  112   b  of thermoelectric element  112   c  for discharge into conduit  124 . Depending on the exact configuration of chamber  128 , a greater, lesser, or no amount of heat transfer will occur between the warm breathing gases received in conductor  106  and the dried breathing gases exiting via conduit  124 . 
     The passages in dryer  100 / 100 A are preferably sized to minimize any increase in resistance to breathing gas flow in breathing circuit  14  as a result of the movement of the breathing gases through the dryer. 
     The rehumidification provided by exchanger  50  in patient limb  22  occurs out of the inspiration limb expiration limb portions of the breathing circuit so that the problem of moisture condensation in these portions of the breathing circuit is not exacerbated. Also, the heat exchange carried out in heat and moisture exchanger  50  provides some reduction in the temperature of the exhaled breathing gases provided to exhalation limb  24 , and ultimately to dryer  100 / 100 A, thereby facilitating the cooling and moisture condensation carried out in dryer  100 / 100 A. 
     FIG. 4 shows an alternative approach to avoiding the accumulation of moisture in breathing circuit  14 . In dryer  100   b  shown in FIG. 4, the breathing gases from CO 2  absorber  28  in conduit  106  are passed through a moisture permeable element, such as tube  140 . The moisture permeable tube may be formed of the water vapor permeable material made and sold by the Perma Pure, Inc. of New Jersey, USA under the trademark “Nafion.” The water vapor in the breathing gases passes through the wall of tubing  140  and is thus removed from the breathing gases. It is deemed preferable to provide turbulent, rather than laminar, flow in tubing  140  to improve the removal of water vapor from the breathing gases. Ridges, several of which are shown as  148 , may be provided on the inside of tube  140  for this purpose. 
     To enhance the removal of moisture, tube  140  may be surrounded along all or a portion of its length by chamber  142 . Chamber  142  can be formed as a jacket to surround tube  140 . Air is supplied to jacket  142 , as for example from a hospital medical air supply, at inlet  144  and removed from the jacket at outlet  146 . The air so removed may be discharged to the ambient environment. The air in jacket  142  carries off the moisture passing through the walls of tube  140 . 
     The volume of drying air provided to jacket  142  may be generally the same as the volume of breathing gases passing through tubing  140 . However, to reduce the consumption of drying air provided to jacket  142 , the supply of drying air can be synchronized with the breathing cycle of subject  12 . The supply of air would be provided only during the portion of the breathing cycle in which moist breathing gases in breathing circuit  12  are moving through tube  140 . In the configuration shown in FIG. 1, this would be during the inspiration phase of the breathing cycle. 
     Dryer  100   b  can be a disposable component of the breathing circuit or can be sterilized as by autoclaving, for reuse. 
     In designing a breathing circuit including a dyer, it is currently deemed preferable to size the dryer to remove the water vapor released by CO 2  absorber  28 , as well as any moisture not transferred to the inhaled breathing gases by heat and moisture exchanger  50 , i.e. the leakage moisture from heat and moisture exchanger  50 . As noted above, approximately 15 mg/liter of moisture is released by CO 2  absorber  28 . The moisture leakage from heat and moisture exchanger  50  is approximately 7 mg/liter. Heat and moisture exchanger  50  is sized to return the moisture in the exhaled breathing gases to the inhaled breathing gases, except for the moisture leakage amount. In dryer  100   b,  the water vapor is removed in the drying air in gaseous form. In dryers  100 / 100 A, the water is removed as a liquid, which form may be more convenient for disposal. 
     It is recognized that other equivalents, alternatives, and modifications aside from those expressly stated, are possible and within the scope of the appended claims.