ANAESTHESIA BREATHING CIRCUITS

Abstract: An anaesthesia breathing circuit of a circle, rebreathing type comprising an endotracheal tube (1) to be installed into the airway of the patient, two breathing hoses (3, 4) attached to the tube via a Y-piece (2) and continuing to a carbon dioxide absorber (5) where one-way valves (6, 7) are placed to lead the gases in the circuit in a circular manner through the absorber (5) and back to the patient. In the circuit there are possibilities of giving ventilation to the patient either manually or by using a mechanical ventilator (9) and for supply of fresh gases. A shunt line is arranged to have some of the expiratory gases to bypass the absorber (5) and in this line there is a shunt valve (11) for regulating the flow that bypasses the absorber (5).

Description: Anaesthesia breathing circuits. The present invention refers to pulmonary ventilation of patients under general anaesthesia using breathing circuits including carbon dioxide gas absorption and connected to anaesthesia machines. More exactly it refers to a shunt valve, which makes it possible to let breathing gases by pass the carbon dioxide absorber. General anaesthesia is used to reduce anxiety, ensure sleep and - above all - eliminate pain in connection with surgical in¬ terventions. It can be achieved by using drugs injected directly into the patients blood, and by anaesthetic gases supplied to the blood via the patient's airways and lungs. These methods can be used by themselves or, most frequently, combined. It is of fundamental importance to establish control of the patient's airway and ventilation in order to ensure both oxygenation and removal of carbon dioxide as well as supply of anaesthetic gases. The gas/blood exchange in the lungs takes place in their most peripheral parts, in the alveoli. It has been found that at induction of a general anaesthesia the alveoli have a tendency to collapse, to be atelectatic. As a result no air can enter the atelectatic alveoli and thereby no gas exchange can take place between gas and blood. Nevertheless blood flow to atelectatic alveoli will continue although no oxygenation can take place. The oxygenation of the passing blood is the limiting factor here. The other important factor of ventilation, the elimination of carbon dioxide, is also hindered in the atelectatic area but can be compensated for by an increase in non atelectatic areas of the lung. This is not possible for oxygenation. The reason for this is that haemoglo¬ bin, which is the main oxygen carrier in blood, can only be saturated to 100% and no hyperventilation can compensate for the loss in atelectatic alveoli. In light of these facts the aim, when giving artificial ventila¬ tion to a patient in general anaesthesia, is adequate oxygenation and balanced elimination of carbon dioxide as well as keeping the lung sufficiently expanded with each breath to counteract the formation of atelectasis, however, having in mind that too high intrapulmonary pressures may harm the pulmonary tissue. While doing this experience tells that elimination of carbon dioxide is considerably easier than to oxygenate. This constitutes a conflict, which often ends with a compromise of excepting a too low carbon dioxide pressure than that which would give blood its normal pH-value. The most common way of establishing a free airway for a patient in general anaesthesia is to perform an endotracheal intubation. This implies that a plastic tube with an expandable plastic cuff is passed down the patient's throat. This tube is connected to the breathing circuit of the anaesthetic machine. This is a set of rubber or plastic hoses and valves. The ventilation offered to the patients by the.breathing circuit is a partial rebreathing of exhaled gases. There are principally two types of breathing circuits used. In the carbon dioxide absorber circuit the exhaled breathing gases are directed to pass a container with carbon dioxide absorbing soda lime before it is, with a fresh gas supply, available for the next breath. The second breathing circuit uses no carbon dioxide absorption, the non absorber circuit. With the absorber circuit the endotracheal tube is connected to a Y-piece in its turn connected to two anaesthetic hoses leading to a set of valves directing the inspiratory gases into one and the expiratory gases into the other hose. An anaesthetic bag is collecting the exhaled gases and the movements of this bag indicates the patients spontaneous breathing. This bag can, when needed, be manually and intermittently compressed to achieve ventilation. The inspiratory gases will pass the absorber and, after receiving the fresh gases, be infused into the patients lungs. The bag can be exchanged for a ventilator to give mechanical artificial ventilation to the patient. The expiration is a passive procedure driven by the pressure left in the patient's lungs from the inspiration. In summary this breathing circuit creates a partial rebreathing of expired gases cleared from carbon dioxide by passing the absorber. This circuit has had an increased popularity in recent years because its economic low fresh gas flow. The breathing provided by the second breathing circuit is a partial rebreathing without carbon dioxide absorption. This circuit corresponds to a D in Mapleson" s (a British physicist and anaesthetic analyst) classification that he performed in the late 1950th. An important practical improvement by the Canadian anaesthetist J. Bain was the beginning of a renewed popularity of this circuit during the 1970th. The circuit has only one hose from the endotracheal tube to the anaesthetic machine. The fresh gases are supplied to the hose near the connection with the tube and they are led there through a thin plastic tube placed coaxially inside the hose. This coaxial placement of the fresh gas supply tube is Dr Bain' s . contribution and makes the circuit small and flexible. In the other end of the hose there is an anaesthetic bag or a ventilator and an excess valve. Each inspiration provides gases partly from the fresh gas supply and partly from the fresh gases accumulated in the hose during the expiratory pause and from the expiratory gases of the previous breath. The difference between this and previous system is that the rebreathing of part of last breath will contain carbon dioxide. The size of this rebreathing is regulated with the fresh gas flow. Hypothetically we can imagine that no fresh gas flow was provided, which would mean that the rebreathing would be 100%. For every increase of the fresh gas flow the less is the rebreathing. In practice Dr Bain recommends a fresh gas flow of 70 mL/kg of body weight/min combined with a tidal volume, the size of each breath, of 10 mL/kg and a respiratory rate of 12 breaths per min. This will lead to a normal carbon dioxide pressure in arterial blood leading to a normal pH. This would mean a fresh gas flow of nearly 5 L/kg/min for a patient of 70 kg. One drawback of this circuit is that expensive inhalational anaesthetic gases will be wasted. The absorber circuit on the other hand allows a fresh gas flow of 1 to 2 L/min compared to 5 L/min with the Bain circuit, but tends to lead to a subnormal carbon dioxide pressure. With the Bain circuit it is possible to counteract the formation of atelectasis by using slightly supernormal tidal volumes and an intentional rebreathing of carbon dioxide leading to a normal pressure. In recent years a third circuit has become available, the so called Mentell circuit. This is a combination of an absorber circuit and a Bain circuit. The construction of the Mentell circuit demonstrates an ambition to find a solution to the problem of using low fresh gas flows and still be able to use carbon dioxide rebreathing and supernormal tidal volumes and thereby achieve the benefits of both circuits. Even with this circuit there are some difficulties with the ability to regulate rebreathing and keep an economic and low fresh gas flow. The object of the present invention is to solve these problems. This object is achieved by a shunt valve described by the patent claims for anaesthesia breathing circuits used in connection with anaesthetic machines. The invention is given a closer description below and in connection with the following figures: FIG. 1 depicts schematically an anaesthetic breathing circuit (an absorber circuit) according to the invention and for regulation of the carbon dioxide concentration and the supply of fresh gases to the circuit used for general anaesthesia to patients. FIG. 2 is a cut through the shunt valve included in FIG. 1. The cut is laid along the line II - II seen in FIG. 3. FIG. 3 is a cut along the line III - III seen in FIG. 2, and FIG. 4 is a profile view of the valve body of the valve seen in FIG. 2 and 3. The breathing circuit in FIG. 1 in accordance with the invention is connected to an anaesthetic machine. It comprises an endotra¬ cheal tube 1, which is meant to be put into the patient's airway, a Y-piece 2 (and a device for analysing the gases to and from the patient), from which two anaesthetic hoses 3 and 4 lead and where the expiratory branch leads to a carbon dioxide absorber 5. The breathing gases in the hoses are forced by a set of one-way valves 6 and 7 to follow in the direction of the arrows 8. Artificial ventilation can be performed by a ventilator 9, either comprising a manually operated anaesthetic bag or a mechanical ventilator. In addition there is a fresh gas inflow, which is not indicated in the figure, but placed in-between the absorber and valve 7. So far it constitutes a conventional circular absorber breathing circuit earlier described and the function of which the present invention aims to improve. A shunt line 10 is introduced between the hoses 3 and 4 parallel to the absorber 5 and a shunt valve 11 is interposed in the shunt line 10. The one way valves 6 and 7 as well as the ventilator 9 are placed in-between the Y-piece 2 (with the analysing device) and the connection of the shunt line 10 to the hoses 3 and 4. The gas analysing device (not shown in the figure) is in a conventional way connected to the gases in the breathing circuit and registers both during inspiration and expiration and continuously in a suitable way presents the values to the person responsible for the anaesthetic. With the adjustable shunt valve 11 it is possible to direct a suitable flow of expiratory gas to bypass the absorber 5. This adjustable flow of gas containing carbon dioxide can in an easy and superviseable way compensate for the loss of carbon dioxide caused by the increased tidal volumes required to keep the alveoli of the lungs from collapsing into atelectasis. In this manner it is possible to achieve the aim of optimal oxygenation of the blood and at the same time keep the carbon dioxide pressure normal and thereby maintaining a normal pH of the blood. Figures 2 and 3 depict in a schematic way cuts of a preferred embodiment of the shunt valve 11. It comprises a valve house block 12 with two parallel gas passages 13 and 14 where passage 13 constitutes the continuing passage from hose 4 of the inspiratory gases and passage 14 correspondingly the continuing passage from hose 3 of the expiratory gases. Between these passages there is placed a valve chamber 15 which contains a turnable valve body 16. A duct 17 connects the valve chamber 15 with passage 14 and a duct 18 connects the valve chamber with passage 13. These ducts 17 and 18 are both open into the valve chamber 15 but at different sites. The valve body 16 having a circular cross-section, as best seen in Fig. 4, has a form that corresponds to the valve chamber 15, which implies that it can be rotated while it continuously maintains an air tight seal against the valve chamber wall. The valve body 16 is at one end provided with a knob 19, which is situated outside the valve house block 12 when the valve body 16 is placed within the valve chamber 15 and allows a rotation of the valve body 16 and thereby a regulation of the gas flow through the shunt 11. At the opposite end of the valve body 16, as seen from the valve knob 19, a cylindrical end tap 20 is provided, which is part of the assembly to hold the valve body 16 tightly in the valve chamber 15, as illustrated in FIG. 3. The valve body 16 has a section in the form of a frustum of a cone adjacent to the tap 20 and between that section and the knob 19 a cylindrical section 22 is provided. The tap 20, the conical section 21 and the cylindrical section 22 are concentric. In the conical section 21 a groove 23 is formed, which extends in a radial plane and which at one end is elongated V-shaped and turns into a portion of parallel walls. The bottom of the groove 23 has a slight spiral form such that the groove gets deeper from the top of the V up to the position where the walls becomes parallel, and from there on the depth is constant. The groove 23 has its largest cross-sectional area where the walls become parallel and this area is at least as large as the cross-section of the ducts 17 or 18, which in turn have a smaller cross- sectional area than the passages 13 or 14. The groove 23 does not extend all the way around the circumference of the conical section 21 but leaves a full segment 24, which is left in order to make it possible to totally shut off the shunt between the ducts 17 and 18. The shape of the groove 23 can of course be different from the one described here. However, fundamental is that the groove has a shape that makes it possible to open the shunt from zero to fully open in a gradual and easy adjustable way. The breathing circuit in accordance with this present invention presumes that the breathing of the anaesthetised patient runs through this circuit. The expiratory gas leaves the patient through the tube 1, passes the Y-piece 2 (and is sampled by the gas analyser) and continues through hose 3 and passes one-way- valve 6 and runs through passage 14 in the shunt 11 and reaches the carbon dioxide absorber 5. Cleaned from carbon dioxide the gases are transformed into inspiratory gases and continue through hose 4 and maybe receiving an addition of shunted gases from passage 13 to the shunt 11. The inspiratory gases also receive an addition of fresh gases here (not depicted in the figures) and run along to the one-way-valve 7 over to the Y-piece 2 and down into the tube and into the lungs of the patient. At the Y-piece 2 sampling for gas analysis is performed continuously both of the inspiratory and expiratory gases. Based on the results from these analysis combined with clinical observation of the patient the medically qualified person responsible for the anaesthetic can decide about the fresh gas flow and concentration of anaesthetic gases, the respiratory rate, tidal volume etc. The tidal volume and respiratory rate are regulated either by changing the manual ventilation or by adjusting the mechanical ventilator 9. In order to avoid and counteract the formation of atelectasis in the lungs and thereby sub optimal oxygenation of the blood during anaesthesia a slightly larger tidal volume is chosen. This increase in tidal volume plus the normal respiratory rate results in a hyperventilation in its turn resulting in a decrease in carbon dioxide pressure in blood and also an increase in blood pH compared to normal. To remedy this the shunt 11 can be regulated with knob 19 and turn the valve body 16 and get an opening of the shunt valve 11 resulting in a shunt flow of expiratory gases directly from hose 3 to the inspiratory gases of hose 4. The shunt flow is determined by how much the shunt 11 is opened as well as by the resistance to gas flow in the absorber 5. The amount of carbon dioxide from the shunted gas that results from this manoeuvre is seen on the values from the gas analyser for the inspiratory gases and the result on the carbon dioxide level in the patients is seen on the values for the expiratory gases. By regulating the shunt 11 with knob 19 the desired level of carbon dioxide pressure can be achieved. Those skilled in the art realise that besides differences in flow resistance it does not matter which type of carbon dioxide absorber or anaesthetic machine one uses. The shunt depicted and described in this application is shown to function reliably and is easy to regulate. Even in other respects the invention can be designed differently but this is included in the scope of the attached patent claims.

Claims: CLAIMS 1. A valve (11) for regulation of flow per time unit through e.g. a shunt line (10) comprised in a circular type of breathing circuit intended for use in connection to anaesthesia apparatu¬ ses, by which shunt line (10) a part of the expiratory gases from the patient pass by a carbon dioxide absorber (5), c h a r a c ¬ t e r i z e d in that it comprises a valve house block (12) comprising a valve chamber (15), in which a valve body (16) of a circular cross-section is turnably provided, ducts (17,18) connecting the valve chamber (15) to hoses (3,4) for inspiratory and expiratory gases, respectively, said valve body (16) having a groove (23) extended in a radial plane and over a part of the circumference of the valve body (16), the groove (23) further being placed opposite to the duct (17,18) openings leading into the valve chamber (15) and having a cross-sectional area which increases from one end to the other. 2. A valve according to claim 1, c h a r a c t e r i z e d in that said groove (23) is of a continuously increasing cross- sectional area from one end up to about half the length of the groove (23), whereupon it is of a constant cross-sectional area. 3. A valve according to claim 1 or 2, c h a r a c t e r i ¬ z e d in that said valve house block (12) further comprises passages (13,14) therethrough connecting the respective hoses (3,4) to the carbon dioxide absorber (5), whereby said ducts (17,18) exit in the respective passage (13,14).

Inventors: ENGLESSON SOEREN, JACOB PETER, ENGLESSON, SOEREN, JACOB, PETER
Applicants: ENGLESSON SOEREN, JACOB & ALLARD AB, JACOB PETER, ENGLESSON, SOEREN, JACOB, PETER
Application year: 1996
Main classification: A61M16
IPC codes: A61M16