Like the slightly less complex CPAP-apparatus, bi-level apparatus serve the pneumatic splinting of the respiratory tract to avoid obstructive respiratory troubles during sleep.
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 compressor or turbine and administers the same, preferably via a humidifier, via a hose and a nose mask, to the respiratory tract of the patient. This overpressure is to make sure that the upper respiratory tract remains fully opened during the whole night, so that no apneas disturbing the patient's sleep will occur (DE 198 49 571 A1). The required overpressure depends, inter alia, on the sleeping phase and the body position of the sleeping person.
Frequently, the overpressure is a nuisance to the patient. To adjust the overpressure as low as possible but as high as necessary, so-called auto-CPAP-apparatus (compare FIG. 1) were developed. Algorithms for the adjustment of an optimum overpressure are known, for example, from WO 00/24446 A1, WO 02/00283 A1 and WO 02/083221 A1. As compared to the less complex CPAP-apparatus, which measure the air pressure, auto-CPAP-apparatus additionally measure the airflow to the patient. During the processing of the airflow, maximums and minimums are searched for in the time derivative of the airflow, and the inspiration and expiration phases are determined on the basis of these extremes.
Another approach to make the pneumatic splinting for the patient as comfortable as possible relates to so-called bi-level apparatus. Bi-level apparatus support the respiration of the patient by administering a slightly higher pressure to the patient during the inspiration as compared to the expiration.
Due to the different pressures during the inspiration and the expiration bi-level apparatus require the determination of the inspiration and expiration phases. In the bi-level apparatus known from WO 98/35715 A1 and EP 0 656 216 A2 the time derivative of the airflow is compared with threshold values so as to make a distinction between inspiration and expiration. According to WO 98/35715 A1 this method used to be the common one in the prior art (bridging paragraph from page 1 to page 2).
In connection with bi-level apparatus it is technically difficult to exactly detect the transitions between inspiration and expiration by means of the airflow, because the pressure is to be changed exactly during these transitions. By changing the pressure, the air in the respiratory hose and the patient's lungs is slightly compressed or expanded, so that the airflow generated by the respiration is superimposed by an airflow generated by the pressure change. The airflow generated by the pressure change is now particularly high exactly at those times that are to be detected with exactness.
It is also problematical that a derivative acts like a high-pass filtering, which results in a roughening of the signal so that noise occurs more strongly. Due to the roughness of the signal the simple comparison with threshold values may provide incorrect results. In WO 02/083221 A2 the derivative is therefore combined with a low-pass filtering, which is called “estimation of the derivative”. On the other hand, a low-pass filtering has the disadvantage that it delays the rise or fall of a signal.
It is desirable to detect the transition moments between inspiration and expiration more exactly and thus determine the inspiration and expiration phases more exactly.