in the example shown in fig1 , the breathing aid device comprises a patient circuit 1 which itself comprises a patient connection 2 , namely a facial or nasal mask , or an intubation or tracheotomy tube , connected to an inspiratory branch 3 and to an expiratory branch 4 by the intermediary of a bidirectional branch 5 . the expiratory branch 4 comprises an expiration device 6 which , in a way which is not shown , comprises an expiration valve and means of controlling this valve . the expiration valve is closed during the inspiratory phases of the patient &# 39 ; s breathing . during the expiratory phases of the patient &# 39 ; s breathing , the expiration valve can either be open so that the patient expires at atmospheric pressure , or it can operate like a discharge valve to oblige the patient to expire at a certain predetermined excess pressure . the inspiratory branch 3 is connected , at its end furthest from the mask 2 , to a unit 8 for ventilation through inspiratory aid which comprises means , such as an adjustable speed motor - turbine set , for supplying breathable gas through the inspiratory branch 3 at an adjustable pressure , in the direction of the mask 2 , means of detecting the patient &# 39 ; s respiratory reflexes , for example from instantaneous flow rate variations , and means of controlling the expiration valve of the expiration means 6 and an inspiration valve placed in the inspiratory branch 3 in order to open the inspiration valve and to close the expiration valve during the inspiratory phase , and to close the inspiration valve and to release the expiration valve during the expiratory phases . thus , in the inspiratory phase , the patient is connected in a gas - tight manner with the inspiratory branch 3 , and the volume flowing in the inspiratory branch 3 corresponds to the volume of gas inspired . and during the expiratory phases , the patient is connected in a gas - tight manner with the expiratory branch 4 and the volume flowing in the expiratory branch 4 corresponds to the volume of gas expired . such inspiratory aid devices , or inspiratory aid devices of the same kind are described in the prior art , in particular in fr - a - 2 695 830 . the ventilation unit 8 can comprise pressure control means by means of which the pressure p detected in the inspiratory branch 3 by a detector 10 is compared with a pressure command ai in order to adjust , for example , the speed of rotation of the motor - turbine set in the direction tending to make the pressure p equal to the command ai . according to the invention , the breathing aid device comprises means 11 of regulating the patient &# 39 ; s breathed volume . the regulating means 11 comprise a control unit 9 for controlling the pressure command ai which the ventilation unit 8 must apply to the inspiratory branch 3 during the inspiratory phases . the regulating means 11 furthermore comprise a unit 12 for measuring the volume vti inspired by the patient during each breathing cycle . the unit 12 provides the control unit 9 with a signal indicative of the volume vti . the control unit 9 comprises an input 13 for receiving the signal vti , and three inputs 14 , 16 , 17 , allowing the user of the device to enter a minimum breathed volume command into the control unit , in the form of a minimum inspired volume per cycle vtimini , a minimum inspiratory pressure command aimini , and a maximum inspiratory pressure command aimaxi . in general , the control unit 9 compares the measured volume vti with the command vtimini and adjusts the pressure command ai in the direction tending to bring the measured volume vti towards the command vtimini , without however causing the command ai to move outside of the range included between the two extreme values aimini and aimaxi . within this range , the control unit 9 tends to increase the command ai when the measured volume vti is lower than the command vtimini , and to reduce the pressure command ai in the opposite case . when starting up the device , the commands vtimini and aimini are chosen such that the breathed volume vti is established at a value higher than vtimini when the pressure command ai is equal to aimini . thus , if the patient breathes as expected , the pressure command ai stabilises at aimini with a breathed volume above the minimum command vtimini . it is only in the event of a breathing anomaly or incident , for example a partial obstruction of the breathing channels , that the measured breathed volume vti is likely to become lower than vtimini , thus causing an increase in the command ai generated by the control unit 9 . when the breathing becomes normal again , the breathed volume again becomes higher than the command vtimini , such that the control unit 9 returns the pressure command ai more or less rapidly to the value aimini . the flowchart used by the control unit 9 will now be described in greater detail with reference to fig2 . at the start , ai is made to equal to aimini ( step 18 ). then , at the end of each breathing cycle , or during each expiratory phase , the measurement vti of the volume inspired during the preceding inspiratory phase is acquired ( step 19 ) and is then compared with the command vtimini by the test 21 . if the measured volume vti is greater than or equal to vtimini , in other words if the volume inspired by the patient is satisfactory , a test 22 determines if the pressure command ai is or is not greater than the minimum aimini . if the pressure command is equal to the minimum , the conditions are therefore ideal ( volume at least equal to the minimum , minimum pressure ) and the sequence therefore returns directly to step 19 for acquiring the next inspired volume measurement . in the opposite case , advantage will be taken of the fact that the inspired volume is satisfactory in order to attempt to reduce the pressure command by a step 23 in which there is applied to the pressure command ai , expressed in relative value , a variation equal in percentage and opposite in sign to the difference between the measured inspired volume vti and the command vtimini . the formula is such that , in the particular case in which the measured volume vti is equal to vtimini , no modification is applied to the pressure command ai ( 0 % variation ). returning now to the test 21 on the measured volume vti , if the latter is lower than the command vtimini , an attempt will be made to increase the pressure command ai in order to assist the patient more . but prior to this , by a test 24 , it will be checked that the pressure command ai has not already reached the maximum aimaxi . if the answer is yes , an alarm is triggered ( step 26 ) to indicate the necessity of an urgent intervention . on the other hand , if the pressure command ai is not yet equal to aimaxi , the sequence returns as before to step 23 in which there will be applied to the command ai a variation equal in percentage and opposite in sign to the difference between the measured volume vti and the command vtimini . before actually applying the command ai , reduced or increased such as it has been computed in step 23 , to the input of the ventilation unit 8 , it will firstly be checked , by a test 27 , that the new computed ai value does not exceed the maximum aimaxi and , by a test 28 , that it is not less than the minimum aimini . if the new ai value has gone beyond one or other of these extreme values , the command ai which will be applied to the ventilation unit 8 will be equal to the extreme value in question ( steps 29 and 31 ). the example shown in fig3 will only be described where it differs with respect to the example shown in fig1 . in the example of fig3 , the breathed volume is no longer measured by means of the volume inspired in each cycle but by means of the volume vte expired in each cycle . for this purpose , the vti measuring unit 12 in the inspiratory branch 3 has been eliminated and it has been replaced by a vte measuring unit 32 in the expiratory branch 4 , which sends the measured vte , indicated at 33 , to the control unit 9 . the minimum breathed volume command applied to the control unit 9 is therefore the command vtemini for the volume expired per cycle , in order to be able to be compared directly with the measured vte 33 provided by the unit 32 . it can be advantageous to select , case by case , measurement of the inspired volume or measurement of the expired volume . this is the solution proposed by the embodiment shown in fig4 , which will be described only where it differs with respect to the example shown in fig1 . the measuring unit 42 is this time installed in the bidirectional branch 5 of the patient circuit 1 and it comprises means 43 of selecting the direction of flow in which the volume is to be measured . in accordance with this selection , the unit 42 provides , by choice , a measurement of vti or of vte , indicated at 44 . in accordance with the operating mode of the measuring unit 42 , the control unit 9 interprets the input applied at 14 as an inspired volume command or as an expired volume command . there is no longer any measuring unit in the inspiratory branch 3 nor in the expiratory branch 4 . in all of the described embodiments , the speed of execution of the flowchart in fig2 is sufficient for the measurement carried out in each breathing cycle to make it possible to correct the pressure applied during the following inspiratory phase . when the measurement is based on the expired volume , it is however possible that the pressure correction will occur only during , and not from the start , of the following inspiratory phase . the invention is applicable to all ventilators capable of measuring the volumes delivered and of automatically controlling the value of the insufflation pressure . the invention is applicable to all methods of ventilation using pressure control , and in particular to “ inspiratory aid ” and “ controlled pressure ” methods . inspiratory aid is a method consisting in maintaining a substantially constant pressure in the patient circuit during the insufflation , the patient initiating the start and end of the insufflation by his respiratory reflexes . the controlled pressure method is identical to the inspiratory aid method except that the patient does not initiate the end of the insufflation , the latter being determined by a fixed time . it would also be conceivable for the control unit , instead of adjusting the pressure command ai applied to the ventilation unit , to adjust , for example , the speed of rotation of the motor turbine set , or the electrical power supplied to it . it would then be possible to avoid abnormal pressures in the inspiratory branch 3 by comparing the pressure in the inspiratory branch 3 with limits such as aimini and aimaxi , and by initiating a corrective modification of the speed or of the power of the motor turbine set in the case of exceeding , or of risk of exceeding such limits .
the ventilation module regulates the inspiratory pressure according to an inspiratory pressure order . a control module compares the breathed volume at each cycle with a minimum volume order and varies the pressure order in the direction tending to maintain the breathed volume just over the minimum , but keeping the pressure order within the interval comprised between the two extreme values . utilization to combine the advantages of the pressure mode ventilation with those of the volumetric ventilation .