In order to supply such oxygen, the airplane carries an onboard source of oxygen (an oxygen cylinder or a chemical generator known as an"oxygen candle", or else an on-board generator for generating air under pressure that is highly enriched in oxygen), which source serves to feed one or more general distribution pipes. Each location that is to be occupied by a passenger can be provided with at least one inhalation device such as a face mask and/or a flexible bag to be put over the head, with oxygen or oxygen-enriched gas being inhaled through a mouthpiece. Under all circumstances, the inhalation device is connected to the general distribution pipe.

Masks are already known, in particular from patent document FR 83/20941, which use not only an economizer bag, but also a reinhalation bag. The purpose of adding this flexible reinhalation or"rebreathing"bag is to use cause the wearer to inhale a mixture having an increased content of carbon dioxide so as to increase pulmonary ventilation, thus making it possible to use a passenger mask at increased cabin altitude, higher than 12,700 meters (m), without pressurizing the oxygen. The reinhalation bag stores the breathed-out gas that is enriched with carbon dioxide and returns it to the mask during the following intake of breath. The economizer bag remains conventional concerning its characteristics, and oxygen flow rate remains unchanged up to an altitude of 12,000 m. It has been found that the concentration for CO2 needed to excite breathing rate sufficiently, also leads to physiological disorders.

An object of the invention is to provide a method of protecting passengers that enables the content of the

economizer bag to be taken before taking the content of the reinhalation bag, while also taking account of the ventilation behavior of each passenger.

To achieve the desired object, the invention provides a method of protecting airplane passengers against the effects of depressurization of the cabin at high altitude, in which method: 'the passengers are provided with breathing gas inhalation devices adapted so that each of the passengers can perform at least a breathing cycle comprising an inhalation and an exhalation; each inhalation device is fed with breathing gas via a flow rate limiting constriction and an economizer bag connected to the breathing gas inhalation device by means of a feed duct; during the beginning of exhalation, an initial fraction only of the breathed-out gas is collected in a reinhalation bag in communication with the breathing gas inhalation device, the volume in the inflated state of the reinhalation bag being not less than the dead volume comprising the combined volume of the inhalation device plus the airways of a passenger; and surplus exhaled gas is exhausted from each economizer bag and reinhalation bag, said surplus corresponding substantially to the alveolar volume; the method being characterized by the fact that the content of the reinhalation bag is caused to be reinhaled via the feed duct only after substantially all of the content of the economizer bag has been inhaled.

By means of the invention, it is possible to recover from the reinhalation bag, a fraction of the oxygen that is breathed out during exhalation in order to reinhale it on the following inhalation, while avoiding any excessive increase in the carbon dioxide content of the inhaled gas, i. e. while limiting hypercapnia to a level that does not lead to physiological disorders, even after a long duration. It has been found that in order to protect

passengers against a major malfunction of the system for conditioning the atmosphere in the cabin, leading to rapid depressurization, inhalation hypercapnia must not exceed 2 kilopascals (kPa) on average over the entire volume of gas that contributes to alveolar gas exchange (alveolar volume). The term"alveolar volume"is used to designate that fraction of the volume of gas breathed in which actually reaches alveolar gas exchange zones, in contrast to"dead volume"which is the volume that remains in the anatomical airways and in the gas ducts external to the subject and which, by definition, does not contribute to gas exchange.

The invention thus makes it possible to reduce the flow rate of oxygen delivered by the source compared with the usual values, e. g. by acting on the feed pressure supplied by the source and/or on the right section of the constriction which constitutes a sonic throat causing the flow that passes through it to take on a value that depends only on the flow section and/or the upstream pressure.

In addition, by delaying opening of communication between the reinhalation bag and the feed duct, reinhalation from the reinhalation bag is delayed until nearly all of the content of the economizer bag has been absorbed. This result can be obtained by placing a check valve between the reinhalation bag and the feed duct, which check valve is rated to open only when the suction created by breathing in exceeds a threshold which is reached only once the economizer bag has been emptied, but which is still not sufficient for air to be sucked in from the ambient atmosphere.

This disposition, which makes it possible to avoid taking the content of the reinhalation bag until the economizer bag is empty, is particularly well suited to low levels of ventilation, where a residue in the economizer bag after inhalation automatically prevents operation of the reinhalation bag. This disposition

makes it possible to adapt to the ventilation behavior of each passenger, and in particular to that of children, given that children breath small volumes.

The method of the invention may include one, and/or the other of the following dispositions: the inhalation devices are fed with oxygen at a rate which is an increasing function of cabin altitude; and 'the breathing gas is oxygen coming from a chemical generator designed, starting from the instant at which it is put into operation, to deliver a flow that decreases as a function of time in application of a relationship that is determined as a function of the nominal descent profile of the airplane from its nominal cruising altitude, and which is a fraction only of the flow rate that would be necessary in the absence of the reinhalation bag.

When the invention is implemented in a system that delivers air that is highly enriched in oxygen, as opposed to oxygen that is practically pure, the capacity of the economizer bag and the optimum volume for the reinhalation bag need to be reduced and the flow rate which feeds the inhalation device needs to be increased accordingly.

The term"practically pure oxygen"is used to mean gas in which the volume content of oxygen is that supplied by the source. In order to comply with FAR regulations, a flow of oxygen that is not diluted (except by water vapor) corresponding to the entire needs of the passengers must be supplied from 40,000 feet, i. e.

12, 200. meters.

In another aspect, the invention provides an installation for protecting airplane passengers against the effects of cabin depressurization at high altitude, the installation comprising: a supply unit delivering, in operation, an adjustable continuous flow to a general distribution pipe

from a source under pressure of pure oxygen or of highly enriched air; inhalation devices; economizer bags, each of the economizer bags being connected firstly to the general distribution pipe and secondly to an inhalation device via a supply duct; - reinhalation bags, each reinhalation bag communicating with an inhalation device and being of a volume such that it stores only an initial fraction of the gas breathed out on each exhalation into the inhalation device, said initial fraction corresponding substantially to the dead volume comprising the combined volume of the inhalation device plus the airways of a passenger; and check valve means enabling the initial fraction of the breathed-out gas to entire freely into the reinhalation bags; the installation being characterized by the fact that each reinhalation bag communicates with the feed duct, and that control means enable the content of each reinhalation bag to be sucked via the feed duct and the inhalation device, without allowing the content of the economizer bag to pass into the reinhalation bag.

In this installation, the supply unit optionally includes regulator means for operating as a function of the ambient pressure to which the wearers of inhalation devices are subjected so as to limit the flow rate of additional oxygen that is conveyed to the inhalation devices to a fraction only of the flow rate that would be necessary in the absence of reinhalation.

In another aspect, the invention provides a device for protecting airplane passengers against effects of cabin depressurization at high altitude, the device comprising : an inhalation device; an economizer bag connected to the inhalation device via a feed duct ;-and

'a reinhalation bag communicating with the inhalation device and having a volume such that it stores only an initial fraction of the gas breathed out on each exhalation into the inhalation device, said initial fraction corresponding substantially to the dead volume comprising the combined volume of the inhalation device plus the airways of a passenger; the device being characterized by the fact that the' reinhalation bag communicates with the feed duct, and that control means enable the content of each reinhalation bag to be sucked via the feed duct and the inhalation device, without allowing the content of the economizer bag to pas's to the reinhalation bag.

This device may include one or more of the following dispositions: the inhalation device is a breathing face mask or a mouthpiece; the economizer bag and the reinhalation bag are defined in a common inextensible outer envelope by a flexible separation membrane; the control means are constituted by a control valve adapted to enable the content of each reinhalation bag to be sucked via the feed duct and the inhalation device, without allowing the content of the economizer bag to pass to the reinhalation bag, and the reinhalation bag communicates with the inhalation device via a check valve distinct from said control valve and adapted to enable the initial fraction to flow without resistance into the reinhalation bag; and the economizer bag opens out into the inhalation device via a check valve, and it includes a breathe-out valve leading to the atmosphere and provided with resilient return means adapted to retard exhaust to the atmosphere.

The above characteristics and others will appear more clearly on reading the following description of particular embodiments of the invention given as non-

limiting examples. The description refers to the accompanying drawing, in which: Figure 1 is a diagram of an installation in accordance with the present invention; Figure 2 is a diagrammatic section view of a first embodiment of a protection device in accordance with the invention; and Figure 3 shows a second embodiment of a protection device in accordance with the invention.

In order to achieve the desired result, the invention makes use of the result of an analysis of the breathing cycle which has shown that the gas breathed out presents a partial pressure of carbon dioxide that varies. In order to show more clearly the essential elements, there follows a brief summary of breathing physiology and an analysis of the consequences thereof.

The airways of the human being comprise pulmonary alveoli, alveolar ducts, bronchi, the trachea, and the anatomical airways. Only the alveoli and the end portions of the alveolar ducts contribute to exchanging gases. The fraction of the volume that has been breathed in and that remains in the other portions of the airways at the end of an intake of breath is merely rejected to the outside without its composition being modified at the beginning of the following exhalation. This entire fraction which does not contribute to gas exchange is referred to as a dead volume VD. The term alveolar volume VA is used to designate the volume of gas that participates in such exchange. The total volume breathed in VT = VD + VA.

With an approximation that is sufficient for explaining the mechanisms put to advantage by the invention, it can be assumed that breathing out comprises in succession expelling the"dead"volume that has no CO2, a transient stage, and then a stage of breathing out the alveolar volume.

The invention makes use of the existence of the volume VD to enable a fraction of the previously breathed-out volume to be reinhaled since it has been enriched with little or no CO2. When it is desired not to exceed a partial pressure of 2 kPa in the gas admitted into the pulmonary alveoli, hypercapnia remains at an acceptably low level.' It can be shown that by means of the invention, the' amount of oxygen that needs to be delivered to a wearer of an inhalation device in accordance with the invention can be reduced to substantially three-eighths the quantity that is needed in the absence of reinhalation.

The invention also takes account of the fact that the wearer of the inhalation device might be a child or an anxious person. In addition, other work has made it possible to determine the conditions that need to be satisfied in the event of a subject who is very anxious or is a child; with a child, the value VT for the breathing cycle is much smaller than that for a normal adult subject (400 cubic centimeters (cm3)). This constraint is nevertheless attenuated if means are provided for delaying reinhalation until the content of the economizer bag has been used up.

By way of non-limiting example, an embodiment of a protection installation in accordance with the invention is described below.

The embodiment of an installation 1 in accordance with the invention is shown in Figure 1. In this embodiment, the installation 1 comprises a supply system 2 that delivers, in operation, a continuous adjustable flow to a general distribution pipe 3 taken from a source 4 under pressure of pure oxygen or of air that is highly enriched.

The installation 1 also includes inhalation devices, in this case breathing masks 5. These breathing masks are connected firstly to the general distribution pipe 3

via economizer bags 6, and secondly to reinhalation bags 7.

The gas exchange scheme in each breathing mask 5 is illustrated in Figure 2.

In the embodiment shown in Figure 2, the two bags, i. e. the economizer bag 6 and the reinhalation bag 7, are interconnected, thereby providing the advantage, amongst others, of making them easier to store.

For example, the two bags 6 and 7 are defined in a common inextensible outer envelope 8 and by a flexible separation diaphragm 9. The envelope 8 may be rigid, but for storage purposes, it is generally flexible. Thus, the two bags 6 and 7 are connected together both functionally and structurally.

The economizer bag 6 opens out into the breathing mask 5 via a feed duct 10 provided with a check valve 11 that provides practically no resistance to breathing in.

The breathing mask 5 has a breathe-in valve 12a which is adapted, if the volume of the economizer bag 6 and of the reinhalation bag 7 is less than the instantaneous breathe-in demand of the subject, to inhale an additional quantity of outside air, thereby avoiding suffocation.

The breathe-in mask 5 further includes a breathe-out valve 12 leading to the atmosphere and provided with resilient return means for retarding exhaust to the atmosphere, so as to allow the reinhalation bag 7 to be filled before excess breathed-out air is exhausted to the atmosphere, which excess corresponds essentially to the alveolar volume VA.

The reinhalation bag 7 communicates with the breathing mask 5, firstly via a valve 13 enabling the initial fraction of the breathed-out gas to enter freely into the reinhalation bag 7, which fraction corresponds essentially to the dead volume VD, and secondly via the feed duct 10.

Between the reinhalation bag 7 and the feed duct 10, control means 14 enable the content of the reinhalation bag 7 to be sucked in via the feed duct 10, without allowing the content of the economizer bag 6 to pass into the reinhalation bag 7.

These dispositions make the following possible: the initial fraction of the breathed-out gas is stored, which means that no resistance should be opposed' to filling; and the transfer of the gas stored in the reinhalation bag 7 to the mask 5 is delayed until all of the content of the economizer bag 6 has been breathed in.

Thus, the reinhalation bag 7 can be filled only if the economizer bag 6 was emptied during the preceding stage of breathing in. This disposition makes it possible to adapt operation automatically to small volumes being ventilated, and specifically to protecting children. Because of their small total volume VT, they will breathe essentially oxygen coming via the economizer bag 6.

Additional studies have made it possible to determine values that are close to optimum from the point of oxygen consumption, while taking account of the need not to exceed a partial pressure of CO2 of about 2 kPa.

In practice, the volume of the reinhalation bag 7 when full should lie in the range 400 cm3 to 600 cm3.

Correspondingly, the volume of the economizer bag 6 can be small. In general, an economizer bag 6 and a reinhalation bag 7 should be used such that the sum of their volumes in the inflated state is of the same order of magnitude as twice the volume of a present day economizer bag, i. e. about 1000 cm3 to 1600 cm3.

In the embodiment shown in Figure 3, the inhalation devices are mouthpieces 15. Each mouthpiece 15 replaces the mask 5 in the embodiment of the protection device described with reference to Figure 2. Each mouthpiece 15 is connected to the feed duct 10. Breathe-out valves 12

and breathe-in valves 12b can be mounted in the feed duct 10 and/or the mouthpiece 15. Each mouthpiece 15 is optionally provided with a nose clip 16 and/or a protective head cover 17 of transparent plastics material.