GB2133714A - Nitrous oxide inhalation tranquilization apparatus - Google Patents

Abstract

The apparatus includes a mixture flow rate control valve means 27 comprising linked nitrous oxide and oxygen flow rate adjustment valves (45,26), for manual adjustment of the flow rate of a nitrous oxide/oxygen mixture to be inhaled by a patient. The mixture ratio is pre-fixed at a sufficiently low level (e.g. 20%) that there is no danger of a high concentration of nitrous oxide being administered by accident. A changeover valve (24) permits a flow of pure oxygen to be passed to the patient on completion of medical treatment, to effect rapid recovery from tranquilization. <IMAGE>

Description

SPECIFICATION Nitrous oxide inhalation tranquilization apparatus The present invention relates to a nitrous oxide inhalation tranquilization apparatus.

In certain types of medical treatment, such as dental surgery, it may be desirable that the patient remain conscious during the treatment while being free from mental distress caused by anxiety. Many patients have a strong fear of losing consciousness, and in such cases, where it is unnecessary to utilize general anasthesia, tranquilization of the patient can be effected, to relieve anxiety over the treatment. If necessary, a local anasthetic can be employed to eliminate physical pain, in addition to the tranquilization.

Methods of tranquilization can be broadly divided into those which are based upon the general environment of the medical treatment, and those which are based upon administering some form of drug. With the former method of tranquilization, the patient is set into a state of relaxation by such factors as the attitude and behaviour of the medical staff, the clean and attractive condition of the room in which medical treatment is performed, confidence in the capabilities of the medical staff, etc. That is to say, with such a method of tranquilization, the mental state of the patient is soothed by a combination of various factors in the treatment environment.

In the case of drug administration tranquilization methods, various types of anasthetic agents which are also utilizable for general anasthesia may be employed. The anasthetic agent may be inhaled as a gas, or directly injected into the patient's bloodstream, to thereby act on the central nervous system, and so bring about a condition of freedom from anxiety.

Various types of anasthetic may be used to produce such a state of tranquilization, however inhalation of an anasthetic gas containing a low concentration of nitrous oxide gas is a widely used method, and the apparatus of the present invention is directed towards the latter method.

In general, prior art types of nitrous oxide inhalation tranquilization apparatus are operated as follows. First, nitrogen is flushed from the patients body, by inhalation of pure oxygen for a period of 2 minutes. The concentration of nitrous oxide is then made equal to 10%, and this is inhaled for 2 minutes, then the nitrous oxide concentration is increased to 20%, and this is inhaled for approximately the same time interval. Finally the concentration of nitrous oxide is raised to 30%, and is held at that concentration, to thereby provide the desired tranquilization effect. However the final steady-state concentration of nitrous oxide in the anasthetic gas may be adjusted to a level other than 30% by the medical staff, depending upon the reaction of the patient to the anasthetic.As described hereinafter, some individual patients may enter a state of excitation in response to even such a low concentration of nitrous oxide as 30%, and therefore it is essential for the medical staff to monitor the reaction of the patient to the anasthetic while it is being administered.

When using a nitrous oxide inhalation tranquilization apparatus according to the present invention, the tranquilization process is initiated by administer.

ing nitrous oxide at a fixed concentration to the patient. This concentration is preferably in the range 20 to 25%. It has been found that little variation will occur in the effects of the anasthetic for variations in concentration within that range. The medical treatment can be started at that time, with the tranquilization condition being maintained. When the medical treatment has been completed, then a valve is actuated whereby a flow of 100% oxygen is supplied to the patient for several minutes, to effect recovery of the patient from the anasthetic. This method of recovery is identical to that used in prior art methods of tranquilization.

If a nitrous oxide concentration of the order of 30% is administered, then although this is a relatively low concentration, it is possible that in certain individual cases the patient may enter a state of excitation. As stated above, this can be avoided if the medical staff note the reactions of the patient to the anasthetic, and adjust the concentration of nitrous oxide accord- ingly. However with some types of medical treatment such as dental treatment, it will be difficult for the staff to devote time to administering the anasthetic and monitoring its effects upon the patient. In such cases it is therefore desirable to provide some means whereby it becomes unnecessary for the medical staff to be concerned with whether the patient has become too deeply anesthesized, so that they can concentrate upon the medical treatment itself.

Generally speaking, prior art types of nitrous oxide inhalation tranquilization apparatus are capable of producing a nitrous oxide concentration of up to 70%, i.e. the order of concentration which is provided by apparatus used to produce total anasthesia for major surgical operations. Thus a doctor who is not a specialized anesthetist will be concerned with the danger that the concentration of nitrous oxide inhaled by the patient may become excessively high, with dangerous or even fatal results. For these reasons, such prior art tranquilization apparatus is unsuitable in many applications.

In addition, with such prior art types of nitrous oxide inhalation tranquilization apparatus, operation is time-consuming and troublesome due to the fact that the concentration of nitrous oxide must be successively increased. Thus, such prior art apparatus presents severe disadvantages.

According to the present invention, there is provided a nitrous oxide inhalation tranquilization apparatus, comprising flow rate control valve means for supplying a flow of oxygen and a flow of nitrous oxide to be mixed in a manifold to produce an anasthetic gas from said manifold, and for varying the rate of flow of said anasthetic gas while holding the ratio of the volumetric rate of flow of said oxygen to that of said nitrous oxide at a fixed value, said flow rate control valve means comprising at least an oxygen flow rate adjustment valve for supplying a flow of oxygen at an adjustable rate and a nitrous oxide flow rate adjustment valve for supplying a flow of nitrous oxide at an adjustable rate, and means for linking the operation of said oxygen flow rate adjustment valve to that of said nitrous oxide flow rate adjustment valve such as to hold said ratio of the oxygen and nitrous oxide flow rates at said fixed value when said oxygen and nitrous oxide flow rates are adjusted.

Figure 1 is a diagram for illustrating various stages of anesthetization resulting from different concentrations of nitrous oxide in the anasthetic gas administered by inhalation; Figure 2 is a simple block diagram illustrating the basic features of an embodiment of a tranquilization apparatus according to the present invention; Figure 3 is a cross-sectional view of a flow rate control valve used in the embodiment of Figure 2; and Figure 4 is a block diagram for illustrating another embodiment of a flow rate control valve for use with the present invention, whereby two fixed flow rates of anasthetic gas can be selected.

Before describing embodiments of the present invention, the effects of differing concentrations of nitrous oxide in an oxygen/nitrous oxide anasthetic gas mixture will be described. Figure 1 is a diagram for illustrating the relationships between the concentration of nitrous oxide which is administered by inhalation and the anasthetic effect upon the patient.

As shown the anasthetic effects can be divided into four stages. In stage 1, which will be referred to as the "ineffective stage", the concentration of nitrous oxide is kept at a level of 15% or less of the total anasthetic gas flow rate. In this case no tranquilization effect is produced, irrespective of how long the gas is administered to the patient.

Stage 2 will be referred to as the "effective tranquilization stage". In this condition, although there is no change in the state of consciousness of the patient, the patient will feel cheerful and will experience no feelings of anxiety. This effective tranquilization stage is reached when a concentration of nitrous oxide in the range of approximately 20% to 30% is inhaled. However, the effect will vary from patient to patient, and in some cases the patient may enter an excitation stage when the concentration of nitrous oxide is made 30%. If a concentration of 20% is employed on the other hand, then there will be no danger of any patient entering this excitation stage, although in a few cases the patient may not enter the effective tranquilization stage. However this will not normally present any obstacle to the actual medical treatment.

Stage 3 will be referred to as the "excitation stage". In this state, the patient floats between conditions of consciousness and unconsciousness, will vomit in many cases, may become violent, or may experience a feeling of panic. The excitation stage is entered when a nitrous oxide concentration of the order to 40% is inhaled.

Stage 4 will be referred to as the "total anasthesia stage". In this condition there is a complete loss of consciousness. In general, when the patient is to be set into a state of general anasthesia, the excitation stage is passed through as quickly as possible, to enter the total anasthesia stage. If no additional anasthetic agent other than nitrous oxide is used, then it will be necessary to employ a nitrous oxide concentration of 80% or more in order to attain this total anasthesia stage.

As shown in Figure 1, if a fixed concentration of nitrous oxide is continuously inhaled, then a condition of saturation is reached eventually. If the concentration is made higherthan the minimum required to ultimately reach a particular saturation state, then that saturation state will be reached in a shorter time.

In order to provide a suitable degree of tranquilization, the effective tranquilization stage shown in Figure 1 should be produced, i.e. no adverse reaction should be developed by the patient's organism and normal consciousness should be retained, while the mood of the patient should become cheerful.

The procedure for applying a nitrous oxide inhalation tranquilization apparatus according to the present invention are as follows. First, an inhalation mask is set on the patient (the mask being of course set on the nose, in the case of dental treatment). The concentration of nitrous oxide to be inhaled is held at a fixed value, of the order of 20% of the total anasthetic gas flow, from the start of the tranquilization procedure until the end of the medical treatment, with the anasthetic being continuously administered during the medical treatment so long as no adverse reaction by the patient is noted. As described herein after, extensive tests on various subjects have established that there is a negligible possibility of any such adverse reaction.During the medical treatment, it is unnecessary for the medical staff to adjust or monitor the operation of the tranquilization apparatus, and the treatment can begin soon after commencement of administering the anasthetic gas, at the discretion of the medical staff. When the medical treatment has been completed, a 100% flow of oxygen is supplied to the patient for approximately 2 minutes, to effect recovery by the patient from the effects of the anasthetic gas. This terminates the tranquilization procedure.

This method of tranquilization has been tested on approximately 200 individuals, including hospital patients, students, etc. Upon completion of the tranquilization treatment, each patient was asked for their reaction, which were classified as being one of the following: 1. Improvement of mood was experienced.

2. No change in mood occurred.

3. A worsening of mood was experienced.

If the patient experienced reaction 1 above, then this indicates that the tranquilization was effective. If reaction 2 was experienced, then this does not necessarily indicate that the tranquilization method is a failure, since it is sufficient that the tranquilization treatment should not result in a worsening in the patient's mood or produce discomfort. In other words, the establishment of the state of tranquilization is not in itself an essential part of the medical treatment, and it is much more important to ensure that no danger to the patient's health or any worsening of mood can occur as a result of the tranquilization procedure, even if no monitoring or adjustment of the tranquilization apparatus is performed during the medical treatment.

As a result of this survey, it was found that 87% of those tested experienced an improvement in mood, i.e. experienced reaction 1 above. One person experienced drowsiness. This only occurred to a slight degree, but can be classified as reaction 3 above.

The remaining 13% of these tested experienced no change in mood, i.e. reaction 2 above occurred. As a result of this and other tests, it has been found that it is not necessary to use a concentration of nitrous oxide as high as 30%, in order to attain the desired tranquilization effect. In addition, it has been found unnecessary to effect inhalation of 100% oxygen initially, in order to flush out nitrogen from the patient's body, as is done with prior art methods of nitrous oxide inhalation tranquilization. Moreover, it has been found unnecessary to subsequently increase the concentration of nitrous oxide in a successive manner, to establish the tranquilization state. It has been found instead that if an anasthetic gas mixture containing a fixed concentration (e.g.

20%) of nitrous oxide is inhaled continuously from the start of the tranquilization procedure, then the nitrogen in the patient's body becomes replaced by nitrous oxide and oxygen. When a condition of saturation is reached, then the proportions of nitrous oxide and oxygen in the patients body will have become 20% and 80%, respectively.

Thus, by using a tranquilization apparatus according to the present invention, the following advantages are obtained: 1. It is unnecessary to spend time in removing nitrogen from the patients body by inhalation of 100% oxygen, so that the time required before medical treatment can begin will be shortened.

2. It is not necessary to operate the apparatus such as to successively increase the concentration of nitrous oxide.

3. Due to the low concentration of nitrous oxide which is utilized, the procedure is completely safe.

If for example a concentration of 30% nitrous oxide is inhaled, then the patient may enter the excitation stage, and may become violent. With a nitrous oxide concentration of 20%, the tranquilization effect may only be weak, in the case of some individuals, however the medical staff need not be concerned with whether a favourable reaction of the patients organism occurs with the anasthesia, so that the nitrous oxide inhalation tranquilization method according to the present invention can be used with complete confidence as to its safety.

During the medical treatment, it may be necessary to use a local anasthetic agent, as an analgesic. In such a case, even if the degree of tranquilization produced in the patient is weak, this will present no obstacle to carrying out the medical treatment.

Hitherto, anasthesia apparatus of the type used for general anasthesia have been used to perform nitrous oxide inhalation tranquilization. With such an apparatus, the rates of flow of nitrous oxide and oxygen are measured by individual flow rate meters, and the flow rates of each are set by manipulation of flow rate adjustment valves, to obtain the desired concentration of nitrous oxide and the desired total flow rate of the output anasthetic gas. In recent years, a type of tranquilization apparatus has been introduced in which a single mixture ratio adjustment knob is provided, whereby the concentration of nitrous oxide can be set. When this has been done, the total flow rate of the anasthetic gas can be adjusted by means of a total flow rate adjustment knob, while the operator monitors the flow rates of nitrous oxide and oxygen, as indicated by the flow meters.However the design of such apparatus has been strongly influenced by prior art types of apparatus intended for general anasthesia, so that it is possible to set the concentration of nitrous oxide to as high as 70%. Due to this, there is a danger that an excessively high concentration of nitrous oxide may be administered, in the event of erroneous operation of the apparatus.

Figure 2 is a diagram for illustrating the configuration of an embodiment of a nitrous oxide inhalation tranquilization apparatus according to the present invention, with JRS standard gas pressure/air pressure symbols being used. There are two gas flow paths, i.e. for oxygen (02) and nitrous oxide (N2O), which enter through gas connector inlets 10 and 28 respectively. The function of each section of the apparatus will be described in accordance with the gas flow. The oxygen which flows into connector inlet 10 can be admitted or shut off by operation of main valve 14, which is a high pressure control valve that is normally closed, and is opened manually when tranquilization operation is to be started. With main valve 14 opened, oxygen flows through oxygen line 16 and so applies the gas pressure of the oxygen through a pilot line 17 to control a nitrous oxide valve 30.Valve 30 is of pressure-controlled type, i.e. if the oxygen pressure is insufficient then valve 30 will be held closed, thereby ensuring safe operation at all times. If the oxygen pressure applied through pilot line 16 is sufficiently high, then nitrous oxide valve 30 is opened, to admit nitrous oxide into line 32 from connector inlet 28. A oxygen throttle valve 18 reduces the primary oxygen pressure (which is approximately 4 kg/cm2) to a secondary pressure (of 0.6 kg/cm2). A nitrous oxide throttle valve 34 performs a similar function to reduce the nitrous oxide pressure from a primary pressure (approximately 4 kg/cm2) to a secondary pressure of (0.83 kg/cm2). The oxygen line 20, leading from oxygen throttle valve 18 is coupled to an oxygen flow rate adjustment valve 26, which comprises part of a flow rate control valve 27.

Oxygen line 21 which branches from oxygen line 20 is connected to an oxygen port 37 of a changeover valve 24. Valve 24 is a 3-port, 2-position directional control valve, which is set into either of two positions by manual operation.

For the condition shown in Figure 2, valve 24 is set in the operating position thereof whereby nitrous oxide from throttle valve 36 is passed to the output from valve 24. A nitrous oxide line 36 is connected to input port 38 of changeover valve 24, and a nitrous oxide line 44 leads from exit nitrous oxide port 42 of changeover valve 24, to a nitrous oxide flow rate adjustment valve 45 of flow rate control valve 27.

The oxygen flow rate adjustment valve 26 and nitrous oxide flow rate adjustment valve 45 are mutually coupled such as to act in conjunction to hold the mixture ratio of the anasthetic gas produced by the apparatus at a constant value, i.e. the ratio of the volumetric flow rates of the oxygen and nitrous oxide passing through these valves. The oxygen flow path passes from oxygen flow rate adjustment valve 26 through oxygen flow rate meter 46, to a manifold 47. The nitrous oxide flow path similarly passes from nitrous oxide flow rate adjustment valve 45 though a nitrous oxide flow rate meter 48, to manifold 47, where the nitrous oxide is mixed with the oxygen. The gas mixture flowing from manifold 26 passes through a reserve bag 52, and a reserve flow prevention valve 54, to then reach the patient.

The reserve bag 52 serves as a reservoir to temporarily store anasthetic gas which is output from the apparatus. Generally, such a reserve bag consists of an elastic (e.g. rubber) bag. The reserve flow prevention valve 54 is a one-way valve, which serves to prevent the patient's breath from flowing back onto the tranquilization apparatus. Numeral 50 denotes an air inhalation valve, which is also a one-way valve. If the gas which is output from manifold 47 is at an insufficient flow rate to meet the patient's respiration needs,then a negative pressure is produced in manifold 47, whereby valve 50 is opened and air is thereby y supplied to the patient.

A detailed description will now be given of the special features of the present invention, and more specifically, of the flow rate control valve 27 and the changeover valve 24. These valves serve to hold the mixture ratio of oxygen and nitrous oxide constant when the flow rate is changed, and to implement changeover between a supply of the oxygen and nitrous oxide mixture to supplying a flow of pure oxygen, with the total flow rate after such a changeover being held identical to that prior to the changeover. The changeover valve 24 is a 3-port 2-position directional control valve. For the condition of the apparatus shown in Figure 2, the internal flow path through valve 24 leads from nitrous oxide port 38 to the exist port 42.This changeover valve 24 is provided with changeover operating members 22 and 40, for manually performing changeover be- tween the two operating positions of the valve. If changeover member 22 is depressed, with the valve in the condition shown in Figure 2, then the internal flow path through the valve will be changed to lead from the oxygen port 37 to the exit port 42. When this occurs, then the nitrous oxide port 38 becomes blocked, and the oxygen which enters through oxygen port 37 will flow out from the exit port 42.

Thus, after changeover from the condition of valve 24 shown in Figure 2 is implemented, nitrous oxide flow rate adjustment valve 45 provides a flow of oxygen at an identical rate to the flow of nitrous oxide which was output therefrom prior to the changeover. In this way, no change in the total flow rate is produced by such a changeover, and 100% pure oxygen will now be output from manifold 47. A subsequent actuation of changeover member 40 will result in a return to the condition in which nitrous oxide flows from port 38 to be output from exist nitrous oxide port 42.Thus, successive actuators of changeover members 22 and 40, to change these between the two positions described above, will result in operation of the apparatus alternating between a tranquilization operating state and a restoration operating state (i.e. to effect recovery of the patient from the effects of the anasthetic gas), respectively.

Figure 3 shows the configuration of flow rate control valve 27 shown in Figure 2. This is made up of an oxygen flow rate adjustment valve 26 and a nitrous oxide flow rate adjustment valve 45, each being of similar configuration. The oxygen flow rate adjustment valve 26 is made up of an oxygen orifice 56 and an oxygen needle 58, while nitrous oxide flow rate adjustment vale 45 is made up of a nitrous oxide orifice 64 and a nitrous oxide needle 66.An advancement screw portion 60 is provided on the shaft of oxygen needle 58, which engages in a threaded aperture of the apparatus, so that rotation of the shaft results in movement of oxygen needle 58 back or forward along the direction of the axis of needle 58. Similarly, nitrous oxide needle 66 is provided with an advancement screw portion 68, having an identical screw pitch to that of advancement screw portion 60. A gear-wheel 61 is provided on the end of oxygen needle 58, and similarly a gear-wheel 70 is mounted on the end of nitrous oxide needle 66, these gear-wheels being provided with mutually different numbers of teeth, in this embodiment.

These gearwheels mesh with a drive pinion 72, which can be manually rotated to adjust the anasthetic gas flow rate by means of a flow rate adjustment knob 62.

Since in this embodiment the threaded portions 60 and 68 are of identical screw pitch and needle valves 58 and 66 are of identical shape, a as are the orifices 56 and 64, the mixture ratio of oxygen and nitrous oxide is determined as the inverse of the gear ratio of the oxygen gear-wheel 61 and the nitrous oxide gear-wheel 70, since this gear ratio determines the relative rates of advancement of needles 58 and 66 when pinion 72 is rotated. If the concentration of nitrous oxide in the gas which is produced during tranquilization operation is to be set at 20%, then for this embodiment, the gear ratio of gear-wheel 61 to that of gear-wheel 70 must be 1:4. in this way, the degree of opening of oxygen flow rate adjustment valve 256 will always be 4times that of nitrous oxide flow rate adjustment valve 45. Thus, when the total flow rate of the anasthetic gas is altered by adjustment of flow rate control valve 27 (i.e. by rotation of knob 62), the mixture ratio will remain fixed. More generally, the fixed mixture ratio provided bythe degrees of opening of needles 58 and 66 is determined by the pitch of each of the screw portions 60 and 68 of these needle valves, together with the numbers of teeth on gear-wheels 61 and 70, i.e. the relationships between these quantities determine the ratio of the oxygen and nitrous oxide flow rates.

Thus it will be apparent that an identical effect to that described above can be achieved by making gearwheels 61 and 70 identical, and making the pitch of screw portion 60 greater than that of screw portion 68.

Rather than using gear-wheels and screws, it is equally possible to use a lever and fulcrum arrange ment, with the distances from the fulcrum to the ends of the needle valves being established such as to provide the desired ratio of flows of oxygen and nitrous oxide.

It can thus be seen that the proportion of nitrous oxide concentration to oxygen concentration can be set to any arbitrary valve by suitably selecting a gear ratio, a lever ratio, or a screw pitch ratio. By utilizing changeover valve 24 and flow rate control valve 27, coupled to the two gas lines to provide a fixed mixture ratio irrespective of changes in the total flow rate of the gas output from the apparatus, changeover can easily be carried out between administration of a tranquilizing gas and restoration of the patient from the effects of the gas, and in addition the total rate of flow of inhaled gas can be adjusted in a simple manner, thereby ensuring ease of operating the apparatus.

The rate of gas flow required for inhalation by an adult patient is 10 liters/minute, while a child requires a rate of 5 liters/minute. Thus, rather than providing means for continuously varying the total flow rate, it is possible in practice to provide only means for implementing a two-stage change in flow rate, i.e. to provide a flow rate of either 10 liters/ minute or 5 liters/minute. This will enable simplification of the flow rate adjustment operation, since only a 2-position changeover between the adult flow rate and child flow rate will be required. An example of a configuration for this purpose is shown in Figure 4, which illustrates a suitable arrangementforflow rate control valve 27 shown in the example of Figure 2.

The flow rate control valve shown in Figure 4 is made up of a 3-port 2-position unidirectional control valve for providing an oxygen flow (i.e. changeover valve 76), a flow rate adjustment valve 80 to provide a fixed flow rate of 8 liters/minute, and a flow rate adjustment valve 82 for providing a fixed flow rate of 4 liters/minute. Similarly, the nitrous oxide gas flow is provided by a changeover valve 84, a fixed flow rate adjustment valve 86 for providing a fixed flow rate of 2 liters/minute, and a flow rate adjustment valve 88 for providing a fixed flow rate of 1 liter/minute.

The changeover valves 76 and 83 are linked together, as indicated in Figure 4. When the oxygen line 20 leading into changeover valve 76 becomes coupled to the input of flow rate adjustment valve 82, then nitrous oxide supply line 44 becomes coupled to an input port of flow rate adjustment valve 88. If changeover member 78 is depressed, with the valves in the conditions described above, then oxygen supply line 20 will become coupled to the input of flow rate adjustment valve 80, while nitrous oxide supply line 44 will become coupled to the input of flow rate adjustment valve 86. In this condition, if changeover member 84 is now depressed, then the original connection arrangements of changeover valves 76 and 83, i.e. as shown in Figure 4, will be restored.

Thus, by utilizing a changeover arrangement whereby the apparatus can be switched to supply either an adult flow rate of anasthetic gas (e.g. 10 liters/minute) or a child flow rate (e.g. 5 liters/ minute). Since the nitrous oxide concentration is held constant for either flow rate, operation of the apparatus is considerably simpler than in the case of continuous flow rate adjustment means being util ized.

As described in the above, the present invention provides a nitrous oxide inhalation tranquilization apparatus which by comparison with prior art types of such apparatus is free from the danger of supplying an excessively high concentration of nitrous oxide, and provides greater simplicity of changeover operation between two different types of output gas flow i.e. one for producing tranquilization in a patient and one for implementing recovery of the patient from the effects of the tranquilizing gas. In addition, the different flow rates required for inhalation by adults and by children, respectively, can be set in a very simple manner. There is therefore less concern over the possibility of accidents occuring with an apparatus according to the present invention than is the case with prior art types of such apparatus.Medical personnel, who have been hitherto relsuctant to use such nitrous oxide concerning its operation, can utilize the apparatus according to the present invention with complete assurance, and without the need to spend time in operating or monitoring the apparatus while medical treatment is in progress. Thus, the invention marks a significant advance in tranquilization technology, and will be extremely effective for enabling patients to undergo medical treatment with greater freedom from anxiety.

Although the present invention has been described in the' above with reference to a specific embodiment, is should be noted that various changes and modifications to the embodiment may be envisaged, which fall within the scope claimed for the invention as set out in the appended claims. The above specification should therefore be interpreted in a descriptive and not in a limiting sense.

Claims (10)

1. A nitrous oxide inhalation tranquilization apparatus, comprising flow rate control valve means for supplying a flow of oxygen and a flow of nitrous oxide to be moxed in a manifold to produce an anasthetic gas from said manifold, and for varying the rate of flow of said anasthetic gas while holding the ratio of the volumetric rate of flow of said oxygen to that of said nitrous oxide at a fixed value, said flow rate control valve means comprising at least an oxygen flow rate adjustment valve for applying a flow of oxygen at an adjustable rate and a nitrous oxide flow rate adjustment valve for supplying a flow of nitrous oxide at an adjustable rate, and means for linking the operation of said oxygen flow rate adjustment valve to that of said nitrous oxide flow rate adjustment valve such as to hold said ratio of the oxygen and nitrous oxide flow rates at said fixed value when said oxygen and nitrous oxide flow rates are adjusted.

2. A nitrous oxide inhalation tranquilization apparatus according to claim 1, and further comprising changeover valve means operable for selectively supplying nitrous oxide and oxygen to be input to said nitrous oxide flow rate adjustment valve, for thereby selectively supplying said flow of nitrous oxide from said nitrous oxide flow rate adjustment valve to produce said anasthetic gas from said manifold and supplying a flow of oxygen from said nitrous oxide flow rate adjustment valve to thereby produce a flow of pure oxygen from said manifold.

3. A nitrous oxide inhalation tranquilization apparatus according to claim 1, in which said flow rate of nitrous oxide from said nitrous oxide flow rate adjustment valve is substantially equal to 20% of the total flow rate of anasthetic gas from said manifold.

4. A nitrous oxide inhalation tranquilization apparatus according to claim 1, in which said flow rate of nitrous oxide from said nitrous oxide flow rate adjustment valve has a value in the range of 20% to 25% of the total flow rate of anasthetic gas from said manifold.

5. A nitrous oxide inhalation tranquilization - apparatus according to claim 1, in which said oxygen flow rate adjustment valve and said nitrous oxide flow rate adjustment valve each comprise a tapered needle movable in the axial direction thereof within an aperture.

6. A nitrous oxide inhalation tranquilization apparatus according to claim 5, in which each of said tapered needles is mounted on a shaft having a screw portion formed thereon, each of said screw portions engaging with a corresponding threaded aperture, each of said shafts further having a gear-wheel fixedly mounted thereon, and further comprising a manually rotatable pinion which meshes with each of said gear-wheels, said gear wheels, screw portions and pinion constituting said means for linking the operation of said oxygen flow rate adjustment valve and said nitrous oxide flow rate adjustment valve.

7. A nitrous oxide inhalation tranquilization apparatus according to claim 6, in which said threaded portions of said tapered needle shafts have an identical screw pitch, and in which said gear wheels have mutually different numbers of teeth, with the gear ratio of said gear-wheels thereby determining the relationship between the rate of linear movement in said axial direction of said tapered needles when said pinion is rotated and hence determining the value of said ratio of the flow rates of oxygen and nitrous oxide.

8. A nitrous oxide inhalation tranquilization apparatus according to claim 6, in which said threaded portions of said tapered needle shafts have mutually different values of screw pitch, and in which said gear-wheels have mutually identical numbers of teeth, with the ratio of said screw pitches thereby determining the relationship between the rate of linear movement in said axial direction of said tapered needles when said pinion is rotated and hence determining the value of said ratio of the flow rates of oxygen and nitrous oxide.

9. A nitrous oxide inhalation tranquilization apparatus according to claim 1, further comprising changeover means for effecting a two-stage change in flow rate of said anasthetic gas while holding the ratio of the volumetric rate of flow of said oxygen to that of said nitrous oxide at said fixed value.

10. A nitrous oxide inhalation tranquilization apparatus substantially as hereinbefore described with reference to and as illustrated in figures 2 to 4 of the accompanying drawings