Abstract:
A closed circuit is provided in which tube connected to the patient&#39;s mouth is connected directly or via a connection tube to a heat-moisture exchanger which, in turn, is connected to the feed and the discharge, respectively, of the respirator. A moisture-removing device is arranged in the feed or discharge.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an artificial respiration system for artificial respiration of a patient while metering an anesthetic, with recirculation of the artificial respiration gases and the anesthetic. 
     BACKGROUND OF THE INVENTION 
     An artificial respiration system of this type is disclosed in U.S. Pat. No. 4,232,667. 
     In view of the harmful effects, both on the environment outside an operating theatre and on the staff present in such an operating theatre, when an artificial respiration system is used in combination with anaesthesia by a gas and laughing gas, the aim is increasingly for closed circulation of the respiration gases. That is to say, CO 2  is (partially) removed in a controlled manner in the respirator from the gases exhaled by the patient and the gases returned to the patient with the admixture of oxygen and/or anesthetic and/or laughing gas. 
     The system described in U.S. Pat. No. 5,482,031 has two circuits. A first open circuit is arranged between the heat-moisture exchanger (HME) and the respirator and there is a second circuit between the heat-moisture exchanger and the tube which is inserted into the patient. An air humidifier is present in the latter circuit, so that very moist air circulates in at least the second circuit. It is essential that such a patient inhales air with high moisture. After all, humidification no longer takes place in the nasal and oral cavities because of the short-circuiting with the tube by means of which the air is introduced. Outgoing air is at a relatively high temperature and is therefore able to have a high moisture content. This air cools on moving to the HME. After all, the temperature in an operating theatre is approximately 20° C. Downstream of the HME, the air cools even further. This moisture, which condenses on the walls of the tubing, can easily collect into droplets and a number of droplets can form a water lock, as a result of which the functioning of the equipment is restricted or completely terminated. It is clear that this could constitute a life-threatening situation. Moreover, the CO 2  determination is disturbed as a result, so that there is no longer accurate control of the functioning of the patient. Therefore, a variety of constructions have been proposed in the prior art for preventing such a formation of a water lock, but the effect of all these constructions is that the positioning of the various lines remains critical. 
     A system with recirculation of gases is described in U.S. Pat. No. 4,232,667, cited above. The abovementioned disadvantages are partially eliminated by this means, that is to say large quantities of anesthetic are no longer released into the operating theatre and the use of anesthetic can be restricted, as a result of which the costs decrease. A reduction by a factor of 5-20 with such a system in the case of operations lasting a long time is mentioned as an example. With the system according to U.S. Pat. No. 4,232,667, condensation will doubtless be produced, as a result of which liquid will be formed in the lines connected to the patient. The figures that the lines from the respirator all run downwards, so that such moisture moves back towards the patient. 
     This system does not operate satisfactorily. After all, in practice tubing will always be set up so that it is movable to some degree with respect to the patient, that is to say the lowest point of the tubing will not be the patient, but will be located at a point between the respirator and the mouth of the patient. Consequently, if there is condensation a water lock can be produced, with all the associated consequences. 
     Furthermore, it is a fact that although it will in general be possible for the lungs to cope with water supplied to the patient, if there is ingress of water into the lungs a reaction occurs which is not beneficial for the stability of the patient during the operation. The system according to said U.S. Patent is comparable with an active humidifying system. 
     British Patent 2 267 661 discloses a heat-moisture exchanger (HME), as well as a method for testing the latter. Comparison tests are proposed with a construction that simulates the patient and respirator in which, in one case, an HME is incorporated and, in the other case, such a heat-moisture exchanger is not present. This testing bears great similarity to the method specified in ISO 9360. In order to preclude the influence of moisture in the respirator. a separate driving device is present. In no way is it the intention to use the test set-ups proposed in the British Patent on patients. The system described in the British Patent is comparable with a passive humidifying system, the artificial respiration tubes remaining completely dry. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to provide an artificial respiration system wherein, without risk to the patient, the various lines can be positioned in a simple manner without taking account of the possibility of blockages as a result of a water lock. 
     This aim is realised with an artificial respiration system as described the hereafter. 
     The invention is based on the insight of, in contrast to the prior art, in which the air is always kept as moist as possible and blockage of the lines in critical locations has to be prevented by special measures, keeping the air in the first circuit as dry as possible and dispensing with the second circuit as far as possible. This is possible by positioning the heat-moisture exchanger close to the mouth of the patient and fitting the tube to be inserted in the patient&#39;s pharynx directly (if necessary via a connector) to the heat-moisture exchanger. As a result of this there is no risk of blockage caused by a water lock in the second part. Moisture-removing means are placed in the first circuit, by means of which any moisture that passes through the heat-moisture exchanger if this is not operating 100% is captured and removed. 
     The system described above is used in a particularly advantageous manner in a closed system, that is to say a system in which air exhaled by the patient is fed back via the HME to the equipment, CO 2  being removed in the system and fresh oxygen with anesthetic and, if appropriate, laughing gas being supplied. A further variant of a closed system of this type is that in which a large quantity of air with anesthetic and further necessary additives is circulated and air is tapped off at a specific point in the system and metered via an HME to the patient. Air exhaled by the patient is fed back to the system. A system of this type is known under the name ‘PhysioFlex’. 
     Furthermore, it is possible with this construction with recycling of the exhaled air to determine the percentage of CO 2  in said exhaled air close to the heat-moisture exchanger accurately. and with a reduced risk of disturbances. Such measurements are no longer unstable because of condensed moisture which can be present. Consequently, control of oxygen metering can be effected more accurately. In some cases the air flow is monitored by measuring the pressure drop over a certain section of the lines. Disturbances can arise if moisture is present in the lines, in the measurement tubes or, respectively, the measurement probe. Such disturbances are circumvented by the construction according to the invention. 
     The moisture-removing means can be installed at any location in the system. Examples are the discharge from the patient, in the CO 2  measurement line, in the feed to the patient and combined with the CO 2  removal device. Furthermore, it is possible to install moisture-removing means at various locations. If a CO 2 -removing device is used, the preference is for installation of the drying means downstream of the CO 2 -removing means in the respirator. By this means the moisture which is produced during the removal of CO 2  if, for example, a calcium hydroxide or barium hydride system is used, can also be removed. 
     The moisture-removing means can be any construction disclosed in the prior art or any conceivable construction, such as means operating by absorption (for example using silica gel) or means operating by condensation. The actual means used are dependent on the application concerned. The moisture-removing means are preferably integrated in the respirator. 
     With the construction described above it is now possible to arrange the feed and discharge coaxially with respect to one another. After all, it is no longer necessary to check the various tubes and to run these in a specific manner because there is no risk of water locks. By fitting the tubes inside one another it is possible to use simple, standardised coupling means, both to the respirator and to the heat-moisture exchanger. In this way it is possible to leave the respirator behind in the operating theatre after the operation and to connect the patient to a different respirator via the free ends of the tubes which remain connected, with the heat-moisture exchanger, to the patient. These tubes can, for example, be coupled to a respirator located in the intensive care unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail below with reference to illustrative embodiments shown in the drawing. In the drawing: 
     FIG. 1 shows, highly diagrammatically, a first embodiment of the artificial respiration system according to the invention; and 
     FIG. 2 shows a second embodiment thereof. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 the artificial respiration system according to the invention is indicated in its entirety by  1 . The system consists of a respirator  2 , a coaxial tube  3 , which is connected to a heat -moisture exchanger  6 , and a tube  12 , which is connected to the heat-moisture exchanger and is introduced inside the patient via a cuff  13 . 
     Respirator  2  is provided with an inlet  7  for air originating from tube section  4 . This air is passed from inlet  7  through chamber  15 , where CO 2  is removed. The condition of the patient and his/her lung function is determined by an active suction CO 2  measurement probe  17  which is fitted at the end of a line  16  which opens into the heat-moisture exchanger. CO 2  measurement is stable in this way. Oxygen, anesthetic and laughing gas and further additives as desired are fed in a controlled manner via line  9  to the discharge from chamber  15 . This control can be realised in any way known from the prior art. A ventilator  18  then follows and a drying chamber  14 , which operates by means of absorption, is connected in series downstream thereof. The drying chamber is filled with silica gel. The outlet  8  thereof opens into line section  5  of coaxial tube  3 . Coaxial tube  3  opens into the inlet  11  of the heat-moisture exchanger  6 . 
     FIG. 2 shows a variant of the above construction which is indicated in its entirety by  21 . This variant consists of a respirator  22  which is essentially identical to the construction described above. Corresponding components are provided with the same reference numerals. There is now no question of a coaxial tube but of two separate lines, the discharge from the patient being indicated by  24  and the feed to the patient being indicated by  25 . The lines open, respectively, into inlet  27  and outlet  28  of system  22 . The supply of oxygen and anesthetic is indicated by  29 . Tubes  24  and  25  open into Y-piece  23 , which is connected to the inlet  31  of the heat-moisture exchanger (or HME).  34  indicates drying equipment operating by means of condensation. 
     In FIG. 2 other locations of the moisture-removing means  34 ′ are shown in dotted lines. 
     The heat-moisture exchanger can be any heat-moisture exchanger known from the prior art. 
     With both types of equipment described above, moisture present downstream of the heat-moisture exchanger, that is to say in line  4  and line  24 , respectively, which moisture content can also rise as a result of reaction which takes place in chamber  15 , is captured and removed in chamber  14  and chamber  34 , respectively. In this context the amount of moisture which must be removed is always such that no moisture condenses even at the lowest temperature (outside the dryer) which occurs in the system. 
     It has been found that the patient suffers no disadvantage whatsoever from such essentially dry feed and discharge. Moreover, there is no risk of blockage of lines  3  and  24 ,  25 , respectively, by the production of water locks and CO 2  measurement is not disturbed. 
     For determination of the capacity of the equipment for the removal of water  14 ,  34  it is, of course, necessary to take into account the quantity of moisture which passes through the heat-moisture exchanger and the quantity of moisture which may be liberated in chamber  15 . In particular, the quality of the heat-moisture exchanger is important. Under operating conditions the following values can be taken as an example: 
     Air to be inhaled preferably contains more than 30 mg H 2 O per litre air. 
     Water loss through a heat-moisture exchanger is less than 10 mg H 2 O per litre air. Depending on the body weight and other factors, between 2 litres (new born babies) and 10 litres (large adults) air per minute is used for artificial respiration of a patient. During artificial respiration approximately 0.5 litre fresh gas per minute is supplied and approximately 14 ml H 2 O per hour is produced in the system. 
     An adult with a normal metabolism produces 15 mol CO 2  per 24 hours (CO 2  is converted 1:1 to H 2 O in chamber  15 ). 
     The operating theatre temperature is usually approximately 20° C. 
     Based on the above average values, a breath volume per minute of 8/min and an operating time of 1 hour, the total quantity of water which has to be removed by chamber  14  or  34 , respectively, is 14 ml per operation. For the above, complete removal of CO 2  with so-called ‘soda lime’ has been assumed. If less CO 2  is removed, the quantity of water liberated during this operation will, of course, fall and the system will be adapted accordingly. 
     Although the invention has been described above with reference to a preferred embodiment, it will be clear that numerous variants of the circuit feed and discharge are possible and that it is possible to position the moisture-removing means at any other location in such a circuit. Such variants are all considered to fall within the scope of the appended claims.