Abstract:
A compressor driven by a motor sends to a nasal mask a breathable gas at a low positive relative pressure whereby the motor is controlled to maintain the pressure in the delivery pipe of the compressor substantially equal to a set point, independently of the inspiration and expiration of the patient, a computer receiving on an input a motor speed signal as a parameter representative of the respiratory activity of the patient and analyzing the motor speed variations whereby the computer will increase the pressure set point if necessary or reduces the pressure set point by a predetermined amount depending upon whether there is a hypopnoea or the absence thereof.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 08/839,459 filed Apr. 14, 1997, now U.S. Pat. No. 6,283,119; which is a continuation of U.S. application Ser. No. 08/360,720 filed Dec. 12, 1994, now abandoned, which is a §371 application claiming priority to PCT/FR93/00547 filed Jun. 9, 1993 and claims priority to foreign application FRANCE 92 07184 filed Jun. 15, 1992. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a breathing aid apparatus, in particular for treating people which are prone to the disease called “sleep apnoea”. 
     Sleep apnoea syndrome (SAS) is the accumulation of signs as well as their consequences due to the periodic interruption of respiration during sleep. The re-establishment of respiration generally only occurs when the person concerned wakes up. This phenomenon can occur several hundred times per night, with interruptions of 10 seconds or more each time. 
     Three types of apnoea syndrome exist, each corresponding to a particular pathology. 
     The first type, which is the most common, is obstructive apnoea. It results from an obstruction of the upper respiratory tracts caused by a collapse of the tongue and the palate. The respiratory movements continue, but because of this obstruction, air can neither enter nor leave the lungs. 
     The second type, which is rarer, is called “central apnoea”. It is produced when the respiratory center of the brain no longer controls respiration. In the absence of a signal originating from the brain, the respiratory muscles do not function and air can neither enter nor leave the lungs. 
     The third type is mixed apnoea which is a combination of the two previous types, the start of the apnoea being of central type. 
     In the case of obstructive apnoea and mixed apnoea, treatment by continuous positive pressure is the most commonly used. This technique consists of permanently applying, via a nasal mask connected by a pipe to a pressure generating apparatus, a low positive relative pressure in the upper respiratory tracts in order to avoid their obstruction. This pressure prevents the tongue and palate from sticking together. The result is immediate: interrupted respiration is re-established, the lungs receive the oxygen they need and the person sleeps much better. 
     The optimum value of the pressure corresponds to the minimum allowing the suppression of apnoeas and the oxygen desaturations which result in the blood. 
     Determination of this optimum pressure is carried out in the laboratory, by subjecting the patient to a polygraph recording, and by progressively raising the level of pressure applied to the patient until the disappearance of respiratory incidents. 
     The treatment described previously, which consists of applying a constant pressure level to the patient throughout the night, has certain deficiencies. 
     In fact, the frequency and extent of apnoeas vary during the night according to the stage of sleep the patient is in. Also, they vary over time as a function of the development of the condition of the patient (gain or loss of weight, absorption of alcohol before going to sleep . . . ). 
     Therefore, the treatment pressure determined by the prescription is not necessarily adequate subsequently. Now, control recordings cannot be carried out regularly, due to their cost and the significant burden on sleep laboratories, connected with the large number of patients to be treated. 
     In addition, the patient is subjected to an identical pressure all night, whereas depending on the stages of his sleep, a lower pressure may be sufficient, or a higher pressure may be necessary. Now, the lower the average pressure applied during the night is, the better the patient&#39;s comfort will be and therefore his acceptance of the treatment, and the more the deleterious effects linked with too high a pressure will be minimised. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to propose a breathing aid apparatus which allows the treatment to be optimized as a function of the effective needs of the patient at each stage of treatment. 
     According to the invention, the breathing aid apparatus, in particular for treating sleep apnoea, comprising means of producing a flow of breathable gas under a low positive relative pressure, and means for leading this flow to a respiratory mask, is characterized in that in addition the apparatus comprises means of acquiring a parameter representative of the respiratory activity of the patient, and automatic adjustment means for increasing the pressure applied at least when the representative parameter is indicative of a hypopnoea, and for reducing the applied pressure when the representative parameter is indicative of normal respiration over a predetermined time. 
     The term “hypopnoea” encompasses the phenomena of the total disappearance of respiration, and can also include certain phenomena of partial disappearance of respiration, due to a partial obstruction of the upper respiratory tracts. 
     Thanks to the invention, the treatment apparatus is no longer a simple constant pressure generator, but becomes an apparatus capable of detecting hypopnoeas and of adjusting the pressure level in order to suppress the hypopnoeas. 
     In this way, thanks to the apparatus, each time a hypopnoea is detected, the pressure is increased, preferably by increments, until the hypopnoea ceases. When no hypopnoea has occurred for a defined period of time, the pressure is reduced by a predetermined value. 
     This process allows hypopnoeas to be put to an end while permanently minimizing the applied pressure. 
     Preferably, the pressure cannot go below a lower threshold defined by the consultant and set on the apparatus, and of course it cannot exceed the maximum value that the apparatus is capable of delivering, or a maximum value defined by the doctor. 
     Other characteristics and advantages of the invention will become apparent from the description below, with reference to the non-limitative examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings: 
     FIG. 1 is a diagram of an apparatus according to the invention; 
     FIG. 2 is a flow chart for the operation of the computer of FIG. 1; 
     FIGS. 3 and 4 are diagrams similar to FIG. 1 but relating to two other embodiments; and 
     FIG. 5 is a flow chart of the operation of the computer of the embodiment of FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The apparatus represented in FIG. 1 comprises a compressor  1  capable of producing through its delivery pipe  2  a breathable gas at a positive relative pressure, i.e. measured relative to atmospheric pressure, which depends on the rotational speed of the drive motor  3 . In a non-represented manner, the compressor  1  is of a type which produces the positive relative pressure by a turbine for propelling breathable gas. The delivery pipe  2  is connected to a nasal mask  4  by a flexible tube  6 . The nasal mask  4  is intended to be applied to the patient&#39;s face, for example by means of a strap. The mask  4  includes an opening  7  allowing the patient to expire despite the flow in the opposite direction coming from the compressor  1 . 
     A comparator  8  permanently compares the pressure P m  detected in the delivery pipe  2  of the compressor  1  by a pressure detector  9  with a pressure set point P c  applied to the other input  11  of the comparator  8 . As a function of the result of the comparison, the comparator  8  supplies at its output  12  a signal applied to a motor control device  13  to reduce the rotational speed of the motor  3  when the pressure measured by the detector  9  is greater than the pressure set point, and to increase the rotational speed of the motor  3  and therefore the pressure at the delivery pipe  2  when the pressure measured by the detector  9  is lower than the pressure set point. 
     In this way, the pressure at the delivery pipe  2  and therefore in the nasal mask  4 , is approximately the same during the inspiration phases and during the expiration phases of the patient. 
     During the inspiration phases, a relative low pressure tends to be created at the delivery pipe  2  of the compressor  1 , and maintaining the pressure at the set point value requires an increase in the rotational speed of the motor  3 . 
     On the other hand, during the expiration phases of the patient, an excess pressure tends to be created at the delivery pipe  2 , and maintaining the pressure at the set point value requires a decrease in the rotational speed of the motor  3 . 
     Consequently, when the respiration of the patient is normal, the rotational speed of the motor  3  follows a periodical curve. 
     According to the embodiment in FIG. 1, a signal representative of the rotational speed of the motor  3  is applied by the control device  13  to the input  14  of a computer  16  whose function is to analyze the curve of the speed of the motor  3  as a parameter representative of the respiratory activity of the patient, and to modify the pressure set point P c  applied to the input  11  of the comparator  8  as a function of the result of this analysis. 
     In a general fashion, when the analysis of the curve of the rotational speed of the motor reveals a hypopnoea situation, the computer  16  increases the pressure set point. 
     On the other hand, if the analysis of the curve of the speed of the motor reveals an absence of hypopnoea for a certain predetermined period of time, the computer reduces by a predetermined amount the pressure set point. 
     The computer  16  is connected to a manual control  17  allowing the minimum pressure set point P min  authorized by the doctor for each patient to be adjusted. 
     There will now be described with reference to FIG. 2, the flow chart according to which, essentially, the computer  16  is programmed. 
     In what follows, by “hypopnoea” is meant the symptom consisting either of an abnormal lowering (for example by 50%) of the respiratory activity, or the symptom of total apnoea consisting of the complete disappearance of respiratory activity. 
     At the start, the pressure set point P c  is chosen to be equal to P min , i.e. the minimum pressure set point chosen using the manual control  17  (stage  18 ). 
     In stage  19 , the values An−8, An−7, . . . , An−1 of the amplitude of the motor speed variation during the eight respiratory cycles before the one which is currently being analyzed, are arbitrarily set equal to a value A 0  which is relatively low. 
     Then, in stage  21 , the average of the amplitudes of the eight previous cycles (average M) is calculated and two thresholds S 1  and S 2  are calculated with for example: 
     
       
         S 1 =0.8 M 
       
     
     
       
         S 2 =0.7 M 
       
     
     In stage  22 , the extreme values of the rotational speed of the motor are sought. 
     In order to do this, the rotational speed of the motor at each execution cycle of the program is stored in memory. A maximum or minimum is only validated if the speed has then varied sufficiently so as to be back from this maximum or minimum by a value at least equal to threshold S 2 . 
     In other words, as the threshold S 2  is greater than half of the average of the previous amplitudes, a given extreme value will only be processed if the speed again then reaches a value beyond that of the average of the speeds. In particular, if respiration stops (total apnoea), the speed of the motor assumes its average value and the previous extreme value is not validated. More generally, if an amplitude lower than threshold S 2  tends to become established, it will no longer be possible to validate the extreme values. 
     After a period of time T 1  equal for example to 10 seconds, this is detected in the following test  23 . In the absence of an extreme value for 10 seconds, one follows the path “detection of strong hypopnoea”  24  of the flow chart, in which the four amplitudes An−8 . . . An−5 which are the oldest values still in memory are reduced to the relatively low value of A 0 . The aim of this is to reduce the thresholds S 1  and S 2  for the next calculation cycle so as to make the resumption of respiratory activity easier to detect. 
     Returning to test  23 , if an extreme value was found within the 10 previous seconds and if this extreme value is the same as that already processed during the previous calculation cycle, one returns to stage  23  in order to search for extreme values. 
     If, on the other hand, the extreme value is new, one passes via stage  26  for calculating the new amplitude An, then, stage  27 , storing in memory the amplitude An while simultaneously deleting the oldest amplitude in memory An−8. 
     In stage  28 , the newly-calculated amplitude An is compared with the largest S 1  of the two thresholds. 
     If the newly-calculated amplitude An is greater than threshold S 1 , one follows normal respiration path  29  which will be described further on. 
     In the opposite case, i.e. if the amplitude is between thresholds S 1  and S 2 , it is considered that a weak hypopnoea  31  exists. 
     Whether strong hypopnoea  24  or weak hypopnoea  31  has been recorded, a test  32  is carried out in order to determine whether there was already a hypopnoea during the previous 30 seconds. If the result is negative a number MAP is reset to zero. MAP corresponds to the total increase in pressure in the previous 30 seconds. 
     If, on the other hand, there was hypopnoea during the previous 30 seconds, the MAP number is not reset to zero. 
     The following stage  33  consists of adding a relatively high increment to the MAP number if strong hypopnoea was detected, and a relatively low increment if weak hypopnoea was detected. Then, in stage  34 , a test is carried out to establish whether the MAP number is greater than 6 cm of water (6 hP a ). If the result is negative, stage  36 , an increment X, being high or low depending on the strength of the hypopnoea, is added to the pressure set point P c . If, on the other hand, MAP exceeds 6, the pressure set point P c  is only increased to the extent that the total increase in the previous 30 seconds is equal to 6 (stage  37 ). 
     The aim of this is to avoid increasing the pressure excessively to treat a single hypopnoea: if an increase of more than 6 cm of water is necessary to treat a hypopnoea, it is because there is some anomaly and it would be better to wake the patient up. 
     Then, the new pressure set point is applied to the comparator  8  in FIG. 1 on the condition that it does not exceed the maximum pressure set point P max . If the pressure P c  exceeds P max , the set point applied to the comparator  8  is equal to P max  (stage  38 ). One is then returned to stage  21  in which the thresholds are calculated. If the strong or weak hypopnoea which was detected during the previous cycle is still not alleviated, the pressure set point will be increased by a new increment and so on until the total pressure increase MAP within 30 seconds reaches 6 cm of water or until the hypopnoea is alleviated. 
     In this way, the amplitude is compared to two different thresholds, one to detect strong hypopnoeas, including the total hypopnoeas, and to apply a relatively swift increase in the pressure set point, the other to detect weak hypopnoeas, resulting from a partial obstruction of the upper respiratory tract, and to apply a clearly milder increase in pressure. 
     One of the important features of the invention consists of analyzing the parameter representative of respiratory activity (the speed of the motor  3 ) not by comparison with absolute thresholds, but by comparison with the respiratory activity which has just preceded the respiratory anomaly. In fact, it has been noted that respiratory activity varies greatly during sleep, to the extent that an activity which would be considered normal during a certain phase of sleep can correspond to a hypopnoea in another phase of sleep. 
     Returning to path  29  of the flow chart, this leads to a test  39  for determining whether a time T has passed without detecting a hypopnoea. If the result is negative, one returns to stage  21  in which the thresholds are calculated. 
     If, on the other hand, a time T 2 , for example equal to 30 minutes, has passed without a hypopnoea, the pressure set point is reduced by, for example, 2 cm of water. In this way one provides an opportunity to bring the pressure applied to the patient to a lower value if this is possible. 
     However, if the new pressure set point thus became lower than the minimum pressure as set with the manual control  17  of FIG. 1, the pressure set point is simply reset equal to the minimum pressure set. Then, once again, one is returned to stage  21  in which the thresholds are calculated. 
     In the example represented in FIG. 3, which will only be described with regard to its differences relative to that of FIG. 1, a flow rate detector  41  is placed on the delivery pipe  2  of the compressor  1  whose signal is sent to an input  42  of the computer. On the other hand the computer no longer receives a signal corresponding to the rotational speed of the motor. It is now the flow rate signal provided by the detector  41  which provides the computer with the parameter representative of the respiratory activity. When the patient inspires, the flow rate detector  41  reveals a higher flow rate than when the patient expires. In other words, the variations in flow rate work in the opposite sense to those of the speed of the motor  3 . Apart from that, nothing is changed, and the flow chart of FIG. 2 is valid for the embodiment of FIG. 3, with the exception that in stage  22  in which the extreme values are sought, the word “speed” must be replaced by the words “flow rate”. 
     The example of FIG. 4 corresponds to a simplified version. 
     In this example, which will only be described with regard to its differences relative to that of FIG. 1, there is no pressure regulation at the delivery pipe  2 , i.e., apart from situations of apnoea or hypopnoea, the motor  3  rotates at the same speed whether the patient inspires or expires. The pressure at the delivery pipe  2  is therefore relatively low when the patient inspires and relatively high when he expires. Therefore, the pressure at the delivery pipe  2  constitutes a parameter representative of the respiratory activity and it is, as such, detected by the pressure sensor  9 . The computer  16 , which receives the pressure signal  9  on an input  43 , analyzes the pressure curve and provides the control device  13  of the motor  3  with a signal for increasing the speed of the motor  3  when the variations in pressure indicate a situation of hypopnoea, and for decreasing the speed of the motor  3  when any situation of hypopnoea has not been alleviated within a predetermined period of time, for example 30 minutes. 
     FIG. 5 represents a schematic flow chart according to which the computer  17  of FIG. 4 can be programmed. 
     At the start, the speed V of the motor is adjusted to a value V min  (stage  44 ) set with a manual control  46  (FIG.  4 ). 
     Then one passes to stage  47  in which hypopnoeas are detected according to the amplitude of the variations in pressure. This stage can correspond to stages  21  and  22  of FIG. 2, except that it is then applied to the pressure instead of being applied to the speed of the motor. In the absence of hypopnoea, one passes via path  48  in which the speed of the motor is reduced by a predetermined value n′ if a time T 2 , for example 30 minutes, has passed without hypopnoea, without however lowering the speed to a value which is less than the set speed V min . 
     In the case of a hypopnoea being detected during a period of time greater than or equal to a value T 1  of for example 10 seconds, the speed V is incremented by a predetermined value n, without however allowing the speed to exceed a value V max . 
     Consequently, in this simplified example, only a single degree of intensity of hypopnoea is distinguished and when the hypopnoea is detected, one and the same mode of action is envisaged in every case, i.e. an incrementation of the speed of the motor according to one predetermined step and one only. 
     Of course, the invention is not limited to the examples as described and represented. 
     In the computers of the embodiments according to FIGS. 1 and 3 a program could be envisaged which distinguishes only one type of hypopnoea, or on the other hand, the embodiment according to FIG. 4 could be equipped with a program which processes in a different way the weak hypopnoeas and the strong hypopnoeas as was described with reference to FIG.  2 . 
     While particular forms of the invention have been illustrated and described, it will also be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.