Abstract:
The invention concerns a method for open-loop regulation of a breathing aid apparatus which consists in constituting a compression chamber by dividing it into two compartments each connected to an intake of air or breathing mixture and to an outlet of compressed air, using an clastic floating diaphragm guiding said floating diaphragm by fixing its periphery to the wall of said chamber fixing a field coil at the center of said floating diaphragm, placing said field coil in an air gap which is oriented in the direction deforming said floating diaphragm, measuring the instantaneous flow rate of air leaving through said outlet and powering said field coil continuously calculating the instantaneous intensity and the direction of the supply current on the basis of the set pressure of compressed air, of said instantaneous flow rate and of the constants of said apparatus.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation application of PCT/IB01/02627 filed Dec. 20, 2001, claiming priority of European Application No. 00811237.7 filed Dec. 22, 2000, which are included in their entirety by reference made hereto. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an open loop pressure regulation method for a respiratory assistance apparatus, and to a compression device of a compression apparatus for implementing this method. 
     2. Description of the Related Art 
     The problem encountered with respiratory assistance apparatuses which are required to supply a variable air flow rate at constant pressure is that of the response time. It is in fact necessary to succeed in producing an endotracheal reference pressure which can be adjusted by the practitioner, which is independent of the instantaneous inhalation flow rate demanded by the patient, the exhalation passing through an exhalation valve, the inhalation valve then being closed. 
     There are two types of respiratory assistance apparatuses. The apparatuses of the first type comprise a pressurized respiratory gas supply, the flow rate and the pressure of which are regulated by a regulating valve with a variable constriction. The apparatuses of the second type have no pressurized gas supply, but a compressor with variable pressure and flow rates. 
     Existing apparatuses operate with pressure feedback, which requires a compromise between stability of the closed-loop system and its response time. The response time of such systems is about 50 to 150 ms, while the response time of the valve is about 4 to 10 ms. 
     It would not be possible to operate in open-loop mode with such a system, given the friction which is not a fixed parameter and the measurement of which would be too complex. To operate in open-loop mode, it is therefore first of all essential to find a compression device operating virtually without mechanical friction. FR-2733688 has already proposed a respiratory assistance apparatus whose pressurized gas source is a compressor with an electromagnetically actuated membrane. This compressor comprises a casing which contains two chambers of variable volume having a guide shaft which passes through a soft iron core placed coaxially at the center of an annular magnet and which bears at each of its ends a rigid circular plate fitted with an annular membrane, the periphery of which is fastened to the inner wall of the casing, thus defining, inside the casing, two chambers, the respective volumes of which vary according to the displacement of the plates and membranes. Each chamber comprises at least one inlet valve and at least outlet valve in order to control the inlet and outlet of the air. 
     Such a compression device does not meet the requirements of a system operating in open-loop mode, given that the guide shaft is a significant source of friction. 
     BRIEF SUMMARY OF THE INVENTION 
     The aim of the present invention is to make it possible to provide a solution to regulating a respirator comprising a membrane compressor, so that its response time is reduced virtually to that of the membrane of the compression device. 
     To this end, the subject of the present invention is first of all a method of regulating, in open-loop mode, a respiratory assistance apparatus, as claimed claim  1 . The subject of the invention is also a respiratory assistance apparatus as claimed in claim  3 . 
     The advantage of this method and of the apparatus for its implementation arises from the fact that in the absence of mechanical friction, it is enough to measure the flow rate and to know the reference pressure in order to calculate the instantaneous supply current of the driving coil, the other parameters consisting of the constants of the respiratory assistance apparatus. 
     The appended drawing illustrates, schematically and by way of example, one embodiment of the respiratory assistance apparatus for implementing the method which is the subject of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view in axial section of the compression device which is the subject of the present invention; 
     FIG. 2 is a view along II—II of FIG. 1; 
     FIG. 3 is a block diagram of the system for regulating the respirator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To make it possible to implement the regulation method according to the invention, a compression device as illustrated in FIG. 1 is produced which comprises a casing C, inside which is made a compression chamber  1  divided into two compartments  3  and  4  by a floating elastic membrane  2  made of silicone, the periphery of which is fastened to the wall of this chamber  1 . This floating membrane  2  provides the seal between the two compartments. This membrane  2  is called floating since it is entirely guided by the fastening of its periphery to the wall of the chamber  1 , to the exclusion of any other guiding generating mechanical friction constituting a significant and essentially variable parameter which does not allow regulation in open-loop mode. 
     Preferably, this floating membrane  2  is subject to some pre-tension when it is fastened to the wall of the chamber  1 . To this end, the membrane is stretched diametrally by 3 to 8%, preferably 4 to 5%. The aim of this pre-tension is to limit the dead flutter of the membrane when the pressure exerted thereon is unused. 
     Each compartment  3 ,  4  communicates with the outside by two openings  5 ,  6  or  7 ,  8 , respectively. Preferably, each compartment comprises a plurality of inlet and outlet valves. Each of these openings  5 - 8  is controlled by a nonreturn valve  9  to  12 , respectively. The two valves  9 ,  10  or  11 ,  12  of each compartment  3 ,  4  respectively operate inversely to each other, such that the valves  9  and  11  allow air to enter the respective compartments  3 ,  4  but prevents it from leaving, while the valves  10  and  12  allow the air to leave these same compartments  3 ,  4  but not to enter them. 
     The intake openings  5 ,  7  communicate with the atmosphere, while the openings  6  and  8  open out into an outlet duct  13  intended to take the pressurized air into the patient&#39;s trachea. 
     The floating membrane  2 , preferably of circular shape, bears a cylindrical driving coil  14 . This driving coil  14  is wound on a plastic hollow cylinder  15 , (FIG.  1 ), the bottom  15   a  of which is secured to the floating membrane  2 . Two arcuate springy arms  17 ,  18 , made of a Cu—Be alloy, respectively, connect two copper half-disks  16   a ,  16   b , placed between the bottom  15   a  of the hollow cylinder  15  and the floating membrane  2 , to two half-rings  19 ,  20 . As a variant, each of these arcuate arms  17 ,  18  could also be divided into several parallel arms. These two springy arms  17 ,  18  are symmetrical with respect to the center of the floating membrane  2 . The two half-rings  19 ,  20  are fastened between two parts of the casing C and are connected to the two respective poles of a current source I for supplying the driving coil  14 . 
     These two arms  17 ,  18  also serve to center the moving element formed by the floating membrane  2  and the driving coil  14  and to guide this element during its displacement. 
     This driving coil  14  is placed in a gap E made between a soft iron core  21  and a soft iron yoke  22  which are connected respectively to the two poles of a permanent magnet  23  forming an electrodynamic motor where the magnetic force is essentially independent of the coil position. 
     The compression device also comprises a light-emitting diode  24  placed opposite a reflecting surface borne by the floating membrane  2  and a photodiode  25  intended to pick up the light reflected by the reflecting surface, depending on the position of the floating membrane  2 , whose displacement is induced by the driving coil  14  as a function of the current intensity I which it receives. 
     As may be noted from the description of the compression device described above, this device, the moving element of which consists of the floating membrane  2 , has no other moving parts generating mechanical friction, the only friction being that which occurs in the material of the floating membrane  2  and the springy arms  17 ,  18 , but this friction is included in the constants of the device and has a very low value. 
     Furthermore, since the compression device comprises only a single floating membrane, the moving mass is reduced to a minimum, as is the noise emission. The floating membrane  2  requires a driving coil  14  of small height in the gap E, so that the distribution of the force exerted on the floating membrane  2  is uniform whatever the position of this floating membrane  2 . It is necessary that h gap ≧travel coil =h coil . 
     A coil  14  of low height also has the advantage of decreasing its resistance, and therefore the Joule heating losses (I 2 R), thus improving the overall efficiency of the compression device. 
     By virtue of the fact that, with such a compression device, the only variable, for a given reference pressure of the pressurized air supplied to the outlet duct  13 , is the demanded flow rate which depends on the instantaneous suction, therefore on the low pressure generated by the patient, it becomes quite possible to control the respiratory assistance apparatus by an open-loop system, which makes it possible to reduce the response time compared to a regulating system with feedback, by between 5 and 30 times. 
     The flow rate can advantageously be measured by detecting the position of the floating membrane  2 , the surface of which may be reflecting or may be combined with a reflecting element, with the help of the light-emitting diode  24  which sends a light spot having a particular angle of incidence, for example 60°, onto the reflecting surface, the photodiode  25  receiving the light reflected by the floating membrane  2  as a function of the position of this floating membrane  2 . Preferably, the amplitude of the floating membrane  2  is fixed. 
     The air pressure at the outlet of the compression device must be:                P   aw     =              Δ                   P   ETT       +     P   e                   =              R   ·       V   .          (   t   )         +       R   2     ·         V   .     2          (   t   )         +     P   e                                    
     where: 
     P aw =outlet pressure of air from the compression device 
     P e =endotracheal reference pressure 
     P ETT =pressure drop in the nozzle of the respiratory assistance apparatus, mainly the intubation cannula V(t)=y(t)·S S=effective surface area of membrane hence: {dot over (V)}(t)={dot over (y)}(t)·S 
     Newton&#39;s law; applied to the moving element, that is the driving coil  14 , the floating membrane  2  and the springy arms  17 ,  18 , gives: 
     
       
         Σ F=m·a=m·ÿ ( t ) 
       
     
     
       
           P   awS ( t )· S+BII ( t )+ k·y ( t )+η· {dot over (y)} ( t )= mÿ ( t ) 
       
     
     by inserting P aw =R{dot over (V)}(t)+R 2 {dot over (V)} 2 +P e    
     where: 
     η=internal friction of floating membrane 
     k=spring constant of the system 
     we obtain:          I        (   t   )       =     -       1     B                 l            [       S                 R                     V   .          (   t   )         +     S                   R   2              V   .     2          (   t   )         +     S                   P   e       +     k   ·     y        (   t   )         +     η   ·       y   .          (   t   )         -     m   ·       y   ¨          (   t   )           ]                                
     in which equation:                y        (   t   )       =       V        (   t   )       S                   y   .     =       V   .     S                   y   ¨     =       V   ¨     S                 that                 is        :                   I        (   t   )       =     -       1     B                 l            [         (       S                 R     +     η   S       )                       V   .          (   t   )         +     S                   R   2              V   .     2          (   t   )         +       k   s          V        (   t   )         -     m            V   ¨          (   t   )       S       +     S                   P   e         ]                                      
     Apart from:            I        (     P   e     )       =       S   ·     P   e       Bl       ,                          
     the other terms of the equation are corrections depending on the flow rate, making it possible to keep P e  constant whatever the value of t and {dot over (V)}(t). 
     The current in the driving coil  14  can be continuously calculated either by an analog computer or, advantageously, using a digital signal processing unit (DSP) or a microcontroller in which: 
     B,l,S,η,k,m are known construction constants of the respirator. 
     R,R 2  are constants dependent on the cannula introduced into the patient&#39;s trachea. These constants are entered into the regulating system by means of a keyboard and are determined by preliminary calibration before ventilating the patient. 
     V(t),{dot over (V)}(t),{umlaut over (V)}(t) are calculated on the basis of V(t)=y(t)·S where y(t) is measured by the photodiode  25 . 
     P e  is the desired endotracheal reference pressure which is input into the regulating system by the practitioner using a keyboard. 
     The instantaneous flow rate and the instantaneous frequency of the compression device described vary depending on demand from the patient. 
     As illustrated in FIG. 3, the regulating system for implementing the method which is the subject of the invention preferably comprises a digital signal processing unit DSP, the inputs of which are connected so as to receive the various values involved in calculating the intensity of the current supplying the driving coil  14 . The values introduced into the DSP comprise, on the one hand, digital values, that is the constants k,η,m,S,B,l, the values R, R 2 , P e , and on the other hand, analog values, those supplied by the photodiode  25 , that from measuring the supply current as a function of time I mes (t) and that from a safety pressure sensor P s , the role of which is to trigger an alarm if the measured pressure moved away from the reference value. 
     These analog values are input into an analog-digital converter A/D of the DSP unit and the current supplying the driving coil  14  is generated by a bidirectional power amplifier Amp supplied with voltage of 12 or 24 V, the inputs of which are connected to the output of a digital-analog converter D/A of the DSP unit, in order to modulate the intensity of the supply current as a function of the value calculated by the DSP unit. The latter has another two outputs S m , S d  which determine the direction in which the current passes through the driving coil  14  and therefore the direction of displacement of this coil  14  in the gap between the soft iron core  21  and the soft iron yoke  22  as a function of its position, thus determining the amplitude of its displacement in this gap and in some way playing the role of “electronic stops” between which the floating membrane  2  is displaced.