Abstract:
The present invention relates to a method of compensating the pressure drop in an ordinary ventilator tube. The method comprises the measurement of a first air flow at a first pressure on the apparatus&#39; side of the ventilator tube and with the respiratory mask taken off. Moreover, a parameter is calculated in a function which provides the pressure drop in the ventilator tube in dependence on the air flow. The parameter for the air flow and pressure is calculated. Later, the air flow is calculated with the respiratory mask being put on. By means of the function a correction pressure is calculated from the air flow. Finally, a target pressure is set which results as a sum of the selected mask pressure and the correction pressure. The invention moreover relates to ventilator for performing the aforementioned method and to a memory medium for storing a corresponding program.

Description:
CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS  
       [0001]     This application is a continuation of international application number PCT/DE03/003592 (publication number: WO 2004/045670 A2) filed on Oct. 29, 2003 and entitled METHOD FOR COMPENSATING A PRESSURE DROP IN A VENTILATOR TUBE, VENTILATOR AND MEMORY MEDIUM and claims the benefit of the above-mentioned international application and the corresponding German national patent application number 102 53 947.2-09 filed on Nov. 19, 2002 and entitled VERFAHREN ZUR KOMPENSATION DES DRUCKABFALLS AN EINEM BEATMUNGSSCHLAUCH, BEATMUNGSGERAT SOWIE SPEICHERMEDIUM the contents of which are expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     According to a first aspect the invention relates to a method for ventilator for the compensation of a pressure drop in a ventilator tube. The invention moreover relates to a ventilator performing such a method, and to a memory medium for storing a corresponding program.  
       BACKGROUND OF THE INVENTION  
       [0003]     Known are ventilators or respirators for the mechanical, artificial respiration for all forms of an oxygen deficiency state. They are, inter alia, applied for the long-time respiration. (Roche Medical Dictionary, 4 th  Edition, edited by Hoffmann-La Roche AG and Urban &amp; Fischer, Urban &amp; Fischer, Munich, Stuttgart, Jena, Lübeck, Ulm).  
         [0004]     A particular form of ventilator, so-called CPAP apparatus, are used to avoid obstructive respiratory disturbances during sleep.  
         [0005]     Obstructive respiratory disturbances result in apneas which make the sleeping person wake up. Frequent apneas prevent the sleeping person from falling into the relaxing deep sleep. People suffering from apneas during sleep therefore are tired during the day, which may result in social problems at work and, in the worst case, in deadly accidents, for example with professional motorcar drivers.  
         [0006]     The CPAP (continuous positive airway pressure) therapy was developed for the treatment of apneas and is described in Chest. Volume No.  110 , pages 1077-1088, October 1996 and in Sleep, Volume No.  19 , pages 184-188. A CPAP-apparatus generates a positive overpressure up to approximately 30 mbar by means of a fan and administers said positive pressure, preferably via a humidifier, via a ventilator tube and a nose or face mask to the respiratory tract of the patient.  
         [0007]     This positive pressure is to ensure that the upper respiratory tract remains fully opened during the whole night, so that no apneas will occur (DE 198 49 571 A1). The required positive pressure is also called therapeutic pressure p t  and depends, inter alia, on the sleeping stage and the position of the body of the sleeping person.  
         [0008]      FIG. 1  shows a CPAP-apparatus  1  during therapeutic use. The CPAP-apparatus  1  comprises a housing  4 , a ventilator tube  9 , and a nose or face mask  18 . The housing  4  includes a fan  8  also being called a compressor, blower, ventilator or turbine. A pressure sensor  11  for measuring the positive pressure, generated by the fan, with respect to the ambient pressure is provided in the proximity of the ventilator tube connection inside the housing. The measured positive pressure will hereinafter be called actual pressure. The air conveyed by the fan  8  is supplied via a ventilator tube  9  to the face mask  18  worn by the patient  19  himself. In the face mask  18 , or in the proximity thereof, an expiration opening  2  is provided, through which a permanent air flow occurs from the ventilator tube into the ambiance. This air flow ensures that the air expired by the patient is carried off into the ambiance and prevents the accumulation of CO 2  in the ventilator tube  9 . A microcontroller  5  controls the number of revolutions of the fan such that the actual pressure measured by the pressure sensor  11  corresponds to a target pressure.  
         [0009]     With all known CPAP-apparatus, and according to the patent literature, so far two methods are used so as to determine the pressure in the face mask of CPAP-apparatus.  
         [0010]     According to one thereof a pressure sensor detects the pressure of the air conveyed to the patient directly in the apparatus. In the factory, for a frequently used combination of ventilator tube and mask the pressure difference is measured at a medium volume flow. This differential value is deducted from the pressure measured in the apparatus, and the result is interpreted as mask pressure. Errors in the pressure adjustment are inevitable because the air flow permanently fluctuates due to the respiration so that the pressure loss in the ventilator tube varies. It is thereby particularly annoying for the patient that the pressure in the face or nose mask is lower during inspiration as compared to expiration. Particularly with this form of pressure control the patient therefore has the feeling that he has to respire against a resistance.  
         [0011]     According to other methods, one of which is described in WO 00/66207, the detection of the pressure is effected in the proximity of the end of the ventilator tube in front of the mask or in the mask. The pressure detection is thereby very exact. The pressure control also compensates leakages etc. The pressure measurement itself may be accomplished with a pressure sensor the electrical connecting leads of which have to be guided with the ventilator tube to the ventilator. In particular with apparatus from MAP is a separate thin tube guided from the measuring point to a pressure sensor disposed in a CPAP-apparatus. A drawback of these variants resides in that special tubes are necessary so as to bring the connecting leads or the tube, respectively, back into the CPAP-apparatus. These custom-made products are more expensive than ordinary ventilator tubes. Moreover, their use with other CPAP-apparatus is possible only to a limited extent. Finally, the cleaning is more complicated.  
       SUMMARY OF THE INVENTION  
       [0012]     According to an embodiment of the invention a method of compensating the pressure drop in a ventilator tube is provided. A first air flow at a first pressure on the apparatus&#39; side of a ventilator tube is measured with the respiratory mask taken off. A parameter in a function providing the pressure drop in said ventilator tube in dependence on the air flow is calculated. The parameter for the first air flow and the first pressure is calculated. Then the air flow with the respiratory mask being put on is measured. A correction pressure by means of the function for the measured air flow is calculated. A target pressure is determined as sum of the selected mask pressure and the correction pressure. This target pressure is set.  
         [0013]     According to another embodiment of the invention a ventilator is provided. The ventilator comprises a housing, a fan within said housing, a mask, a ventilator tube, a flow sensor and a controller. The ventilator tube connects said fan and said mask. The flow sensor measures an air flow in said ventilator tube; said flow sensor measuring a first air flow at a first pressure while the respiratory mask is being taken off. The controller calculates a parameter in a function which provides the pressure drop in said ventilator tube in dependence on the air flow. The parameter for the first air flow and the first pressure is calculated. The flow sensor measures the air flow with the respiratory mask being put on. The controller calculates a correction pressure by means of the function for the measured air flow. The controller sets a target pressure determined as sum of the selected mask pressure and the correction pressure.  
         [0014]     According to a further embodiment of the invention a memory medium for use with a CPAP-apparatus. The memory medium stores commands. The CPAP-apparatus, comprises a central processing unit for processing the commands stored in the memory medium. During the processing of the commands stored in the memory medium the CPAP-apparatus measures a first air flow at a first pressure on the apparatus&#39; side of a ventilator tube with the respiratory mask taken off. The CPAP-apparatus calculates a parameter in a function providing the pressure drop in said ventilator tube in dependence on the air flow, wherein the parameter for the first air flow and the first pressure is calculated. The CPAP-apparatus measures the air flow while the respiratory mask is being put on and calculates a correction pressure by means of the function for the measured air flow. The CPAP-apparatus sets a target pressure determined as sum of the selected mask pressure and the correction pressure.  
         [0015]     An advantage of the invention resides in that the costs of production of the CPAP-apparatus are reduced because no special ventilator tubes have to be used.  
         [0016]     An advantage in the measurement of the air flow at two different pressures resides in that the two parameters C and a in equation 6 are determined on the basis of measured values, so that the pressure drop determined for a measured flow during the therapy corresponds to the actual pressure drop in the ventilator tube more exactly.  
         [0017]     The higher the number of pressures is, at which the flow is measured during the initializing phase when the respiratory mask is taken off, the more exact is the pressure compensation during the therapy.  
         [0018]     The fitting of the parameter C or the parameters C and a to a plurality of pressure and flow values reduces the influence of possible outliers on the measured values and avoids the searching expenditure for the correct interval for a function defined section-wise.  
         [0019]     The rejection of pressures measured during the initialization reduces the memory expenditure in an advantageous manner. By the section-wise fitting of the parameter C or the parameters C and a in equation 6 to intervals, at the lower and upper end of which both the pressure and the flow were measured, results in a very exact correspondence between the calculated and the actual pressure drop in the ventilator tube. Outliers in the pressure or flow measurement only influence the adjacent intervals.  
         [0020]     The criterion to discontinue the initializing phase given a significant difference between the target pressure and the actual pressure is advantageous, because by this the entire available flow range is covered by measuring points, and it is avoided that several measuring points are recorded with a maximum flow. The latter entails an unnecessarily long initialization.  
         [0021]     The control, whether the parameter C is in a reasonable range, already during the measurement of the pressure and flow values during the initializing phase and the discontinuance of the initializing phase, if the parameter C is not in a reasonable range, ensures, for example, that the patient has not put on the mask during the initialization already.  
         [0022]     The use of a stored parameter value is a sensible fallback position if the initialization cannot be performed. This is the case, for example, when the patient has put on the mask already while the apparatus is turned off.  
         [0023]     The use of actual pressure values instead of target pressure values according to the inventive method avoids errors with deviations between the actual pressure and the target pressure.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     Preferred embodiments of the invention will hereinafter be explained in more detail with reference to the enclosed drawings, wherein like numerals represent like parts.  
         [0025]      FIG. 1  shows a CPAP-apparatus;  
         [0026]      FIG. 2  shows a flow diagram of an initialization method according to the invention; and  
         [0027]      FIG. 3  shows a flow diagram of a method according to the invention for compensating the pressure drop in a ventilator tube. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     As was mentioned above, the pressure drop in the ventilator tube fluctuates with the air flow through the ventilator tube. The air flow, again, changes during a respiratory cycle. It is high in particular during inspiration and low during expiration, and may even change its sign during expiration. If only the pressure supplied by the ventilator is kept constant, the pressure in the respiratory mask is higher during expiration as compared to inspiration, which is unpleasant for the patient. In this respect the invention takes corrective measures in that the desired therapeutic pressure p t , which is to be equal to the mask pressure p m , if possible, is corrected by the pressure drop in the tube. The sum of the therapeutic pressure p t  and the pressure drop Δp is set as target pressure p: 
 
 p=p   s   +Δp   (1) 
 
 The pressure drop Δp is a function of the air flow: 
 
 Δp=f ( {dot over (V)} )  (2) 
 
 For measuring the air flow the CPAP-apparatus according to the invention is equipped with an air flow sensor  16 . The air flow sensor  16  can determine the air flow on the basis of the thermal loss of a heating filament  17 . As was mentioned above, the air flow fluctuates with the respiration of the patient. The pressure drop Δp therefore is to be calculated more often during a respiratory cycle. A respiratory cycle typically lasts between 3 and 5 seconds so that the pressure drop and the target drop are to be calculated at least twice per second. 
 
         [0029]     According to the invention an initialization is performed after the apparatus is switched on and before the patient  19  puts on the respiratory mask  18 . The flow diagram of the initialization is illustrated in  FIG. 2 . If the mask is taken off, the mask pressure p m  is equal to the ambient pressure. From this results that the pressure measured on the pressure sensor  11  is equal to the pressure drop Δp in the ventilator tube. According to an embodiment the target pressure is increased in several steps (step  32 ) up to a maximum value during the initialization, and the actual pressure Δp i  and the air flow {dot over (V)} i  (steps  26 ,  28 ,  28 ) are measured in each step. After the initialization the measured pressure and flow values Δp i  and {dot over (V)} i , respectively, can be stored for further use.  
         [0030]     During the therapy, i.e. when the patient has put on the respiratory mask  18 , the nearest air flow {dot over (V)} i  measured during the initialization can be searched for with respect to a measured air flow {dot over (V)} so that the pressure drop results as Δp i . It may also occur that negative flows are measured during the expiration phase. In this case {dot over (V)} i  nearest to the absolute value of {dot over (V)} is searched for and Δp i  receives the sign of the measured flow. A drawback of this method resides in that plenty of pressure and flow values have to be measured during the initialization so as to allow a sufficiently exact determination of the pressure drop.  
         [0031]     In order to be able to do with fewer pressure and flow values the pressure drop Δp between the measured flow values can be calculated, for example, by linear interpolation according to equation (3). However, before applying equation (3), the index i still has to be determined, i.e. the correct interval for the measured flow {dot over (V)} must be found.  
               Δ   ⁢           ⁢     p   (     V   •     )       =         Δ   ⁢           ⁢     p   i       +           Δ   ⁢           ⁢     p     i   +   1         -     Δ   ⁢           ⁢     p   i               V   •       i   +   1       -       V   •     i         ⁢     (       V   •     -       V   •     i       )     ⁢           ⁢   wherein   ⁢           ⁢       V   •     i         ≤     V   •     &lt;       V   •       i   +   1                 (   3   )             
 
 The factor sign ({dot over (V)}) thereby either adopts value 1 for an air flow to the patient or −1 for an air flow from the patient so as to take into account the direction of the flow. 
 
         [0032]     The linear interpolation merely is a mathematical method which can be applied in general, however, which ignores the physical backgrounds of the air flow through a tube. For further reducing the number of required measured values Δp i  and {dot over (V)} i  the pressure drop in a tube such as the ventilator tube  9  can be calculated from the following formula (Technische Strömungsmechanik 1, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig):  
               Δ   ⁢           ⁢   p     =       ξ   ⁢           ⁢     ρ   2     ⁢     v   a       =           λ   ⁢           ⁢   l     d     ·     ρ   2       ⁢       v   a     ·     sign   ⁡     (   v   )                     (   4   )             
 
 Δp thereby is the pressure dropping in the tube, ξ is a pressure loss correction value of the tube, λ is a pipe friction value of the tube, l is the length of the tube, d is the diameter of the tube, ρ is the density of the flowing medium, i.e. in CPAP-apparatus approximately 1.2 kg/m 3  for air, and v is the flow velocity from the CPAP-apparatus in the direction of the mask averaged over the cross-section. a has the value 2 for turbulent flows and 1 for laminar flows. In practice a may also adopt intermediate values, as an ideal-typical form of flow is rare. Under the typical conditions of CPAP-apparatus turbulent flows are prevailing, so that a≈ 2 . Equation (4) is also known from Strömungslehre, J. H. Spurk, 4 th  Edition, Springerverlag, Berlin 1996, wherein λ here is called the flow resistance coefficient. The averaged flow velocity is connected with the air flow {dot over (V)} as follows: 
 
 {dot over (V)}=v ·π·( d/ 2) 2   (5) 
 
 V itself stands for an air volume. The point designates the derivative with respect to time d/dt. {dot over (V)} is detected by the flow sensor  16 . 
 
         [0033]     If one inserts (2) in (1), one obtains the following quadratic dependence of the pressure drop Δp on {dot over (V)}. The dependencies of λ, l, d and ρ were combined to the constant C with C being a parameter for the used tube: 
 
 Δp=C·|{dot over (V)}|   a ·sign( {dot over (V)} )  (6) 
 
 Equation (6) contains the two parameters C and a. Therefore, at least two measuring points at different pressures Δp i  and Δp i+1  as well as the corresponding flow values {dot over (V)} i  and {dot over (V)} i+1  are required to determine both parameters. According to an embodiment the parameters a i  and C i  for the interval i can thus be determined for a flow {dot over (V)} between {dot over (V)} i  and {dot over (V)} i+1  according to equations (7) and (8). The pressure drop Δp then results from equation (9).  
               a   i     =       ln   ⁡     (       Δ   ⁢           ⁢     p   i         Δ   ⁢           ⁢     p     i   +   1           )         ln   ⁡     (         V   •     i         V   •       i   +   1         )                 (   7   )                 C   i     =       Δ   ⁢           ⁢     p   i           (       V   •     i     )       a   i                 (   8   )                 Δ   ⁢           ⁢   p     =           C   i     ·            V   •            a   i       ·     sign   (     V   •     )       ⁢           ⁢   wherein   ⁢           ⁢       V   •     i       ≤     V   •     &lt;       V   •       i   +   1                 (   9   )             
 
 As can be seen from equation (9) the measured pressure values of the Δp i  here no longer occur. After the initialization, therefore, merely the flow values {dot over (V)} i  and the parameter values C i  and a i  have to stored for further use (step  31 ). 
 
         [0034]     According to another embodiment the parameter a may be set at a value between 1 and 2. Only parameter C i  is separately calculated from equation (8) for each interval. In this embodiment only the one value a for all a i  and the values C i  and {dot over (V)} i  are stored for the subsequent therapeutic phase.  
         [0035]     The pressure and flow values Δp i  and {dot over (V)} i , respectively, measured during the initialization are to cover preferably the entire flow range without, however, measuring the same actual pressure and flow several times at the upper end because the ventilator cannot supply enough air so that the actual pressure cannot continue to follow the target pressure. For this reason it is checked in step  27  whether a target pressure set by the microcontroller  5  and an actual pressure measured by the pressure sensor  11  differ from one another by less than the tolerance tol. If this is the case, additional pressure and flow pairs may be measured in steps  28  and  26 . If this is not the case, the initialization is stopped in step  33 .  
         [0036]     Preferably the parameters a i  and C i , or only C i , are calculated for each measurement interval in step  29  according to equations (7) and (8) already during the initialization. Thus, it can be checked in step  30 , whether the parameter C i  for an interval i+1 is within an allowed range between a lower value C u  and an upper value C o . As was mentioned above, the parameter C combines the flow resistance coefficient η, the length of the tube l as well as the diameter of the tube d. C thus constitutes sort of a resistance of the tube. If, for example, the patient puts on the mask during the initialization, the calculated resistance value increases because it now forms a series connection of the tube  9  and the expiration opening  2 . Thus, step  30  constitutes a plausibility control as to whether correct values are still being measured. If the initialization is stopped in step  30 , the values {dot over (V)} i , C 1-1  and possibly a i−1  measured until then are stored, and the values of a preceding initialization or preprogrammed standard values are used for higher flows.  
         [0037]     As equation (6) describes a pressure drop in a tube in a broad flow range in good approximation it may sufficient that a measurement of the flow {dot over (V)} i  be effected at merely two different pressure values Δp i  so as to determine the parameters a and C for the complete flow range. None of the pressures or flows must be zero since it must not be divided by zero in equation (7) and the logarithm of zero is not defined. Also, the flows must be different. Otherwise, their proportion is 1, the logarithm of 1 is zero and a division by zero must not be made. This is the reason for steps  22  to  24  preceding the loop, in which a pair of pressure/flow variates is measured before the calculation of C 1 .  
         [0038]     According to another embodiment the parameter a may be predefined by the factory, and merely the flow with respect to a single actual pressure Δp may be determined. From this, merely a parameter C is calculated according to equation (8).  
         [0039]     According to another embodiment n pairs of pressure/flow variates Δp i  and {dot over (V)} i  can be measured, wherein n≧3. To these pairs of variates the parameters C and a, or only C, can be fitted in equation (6). In this embodiment fluctuations in the measured values, which may be due to noise, are compensated. Also, the measuring points may be divided into several intervals, with each interval containing at least three measuring points. For each interval i either one parameter C i  only, or a parameter C i  and a parameter a i  may be fitted.  
         [0040]      FIG. 3  shows the behavior of a ventilator, in particular of a CPAP-apparatus, during therapy. After the start in step  41  a therapeutic pressure p t  is set in step  42  which may, as was mentioned above, be predefined by a medical practitioner, for example. At this therapeutic pressure the air flow is measured by a flow sensor  16  in step  43 . If the function for the calculation of the pressure drop Δp from the flow {dot over (V)} is defined section-wise as in equation (9), the correct interval i is determined for the measured air flow {dot over (V)} in step  44 . If, for example during inspiration, the flow {dot over (V)} exceeds the highest flow value V n  measured during the initialization the parameters for the highest interval n−1 are used. Subsequently, the pressure drop is determined in step  45  by means of equation (9), whereupon, in step  46 , a new target pressure is calculated from equation (1).  
         [0041]     Subsequently, the flow measurement is repeated in step  43 . The endless loop formed of steps  43 ,  44 ,  45  and  46  is, as was mentioned above, looped at least twice per second so that the target pressure fluctuates in correspondence with the respiratory cycle of the patient allowing to compensate the pressure drop in the ventilator tube in real-time. Apart from the loop illustrated in  FIG. 3  another pressure control loop is provided, as was mentioned above, which controls the speed of the turbine such that the pressure measured by the pressure sensor  11  corresponds to the set target pressure p as exactly as possible.  
         [0042]     In the CPAP-apparatus according to the invention the steps shown in  FIGS. 2 and 3  are executed by the microcontroller  5 . For changing the programming thereof, the microcontroller may be connected to a plug-in slot  7  via a data line  10  into which a memory medium  6  is plugged. This may be a memory stick or a PCMCIA card. The plug-in slot  7  may, however, be an IC socket for a PROM component (PROM: programmable read only memory). According to another embodiment the microcontroller  5  may include an EPROM (electrical PROM). In this case, the microcontroller itself is the memory medium which may be exchanged.  
         [0043]     In the foregoing, the invention was explained in more detail by means of preferred embodiments. It is, however, obvious for the person skilled in the art that various alterations and modifications may be made without departing from the spirit of the invention. Therefore, the scope of protection is defined by the following claims and their equivalents.  
       LIST OF REFERENCE NUMERALS  
       [0000]    
       
           1  cpap-apparatus  
           2  expiration opening  
           4  housing  
           5  microcontroller  
           6  memory medium  
           7  plug-in slot  
           8  fan  
           9  ventilator tube  
           10  data line  
           11  pressure sensor  
           16  flow sensor  
           17  heating filament  
           18  face mask  
           19  patient  
           20  initialization  
           21 - 33  steps  
           40  therapy  
           41 - 46  steps