in reference to fig1 , mask flow is measured using a flow sensor 4 f and / or pressure sensor 4 p with a pneumotachograph and differential pressure transducer or similar device . a flow signal f ( t ) is derived and mask pressure is measured at a pressure tap using a pressure transducer to derive a pressure signal p mask ( t ). the pressure sensor 4 p and flow sensor 4 f have only been shown symbolically in fig1 since those skilled in the art would understand how to measure flow and pressure . flow f ( t ) and pressure p mask ( t ) signals are sent to a controller or microprocessor 6 which then determines how to adjust the blower . alternatively , it is preferred that a flow signal f ( t ) and pressure signal p mask ( t ) be estimated or calculated in relation to the blower motor by monitoring power supplied to the motor and / or the speed of the motor as disclosed in u . s . pat . no . 5 , 740 , 795 , 6 , 332 , 463 or 6 , 237 , 593 , without the provision of flow and pressure sensors as described above . the controller 6 is configured and adapted to implement the methodology described in more detail herein and may include integrated chips , a memory and / or other instruction or data storage medium . for example , programmed instructions with the control methodology may be coded on integrated chips in the memory of the device ( e . g ., firmware ) or loaded as software . the pressure delivery device includes a blower 8 , which preferably is an impeller . the impeller 8 is controlled by a servo 10 , receives ambient air through an inlet 12 and delivers pressurized air through an outlet 14 defined by an air delivery conduit 16 and a mask 18 with an integrated exhaust vent 20 . the impeller , motor , and controller assembly define a blower assembly and are located within the blower housing 22 . various switches 24 and displays 26 are provided in the blower housing . a number of sensors are provided within the blower to monitor , among other things , snore 28 , motor speed 30 , and motor current 32 . various devices known in the art can serve as these types of sensors . a communication interface 34 allows data to be transferred between the apparatus and an external device , such as a computer or controller . preferably , the device delivers a generally constant therapeutic level of continuous positive airway pressure ( cpap ) during any given treatment session . however , consistent with the control principles of the invention as described herein , other types of pressure treatment may be implemented in the apparatus , such as bi - level cpap treatment or other variants of natural patient - synchronized pressure changes . as illustrated in fig2 , the pressure treatment apparatus implements control based on historic ahi determinations . as shown in step 20 , airway pressure treatment is provided to the patient during a first treatment session . in such a session , in the absence of historic ahi , the pressure treatment level will be set to a default low or minimum level , or a level prescribed by a physician or clinician . preferably , no adjustments to the treatment pressure are made to change the level of therapy in response to a current detection of an sdb event during the current session . during the treatment session , in step 22 , sleep disordered breathing events are detected and an index of these events is determined . preferably , apnea and hypopnea events are detected and an ahi , the ahi being initialized for the current session , is incremented by the number of such detected events . in step 24 , a new or subsequent treatment session is initiated with the apparatus . in this subsequent session , a therapeutic level of the treatment is set automatically as a function of the sdb event related index that was determined in the prior treatment session . in the preferred embodiment of the invention , each of the previously described treatment sessions is a different night &# 39 ; s treatment with the device . thus , an ahi may be recorded during use of the treatment apparatus during a single night and saved at the conclusion of the session . this saved ahi may then be utilized to set the treatment pressure in the next use of the device , such as during the next night . to distinguish such sessions , the device may be configured to store the ahi on power down . then it will utilize a previously recorded ahi , if it exists , in setting the treatment pressure after the device is powered on but before or as treatment is commenced . alternatively , other schemes for ensuring the use of an ahi from a previously recorded session may be implemented . in one alternative scheme , date and / or time of every determined ahi from all sessions are recorded and stored . during a subsequent use , checking is performed for the most recent ahi . similarly , this may be implemented by checking the date of an ahi against an internal clock to permit the use of a previous day &# 39 ; s ahi in setting treatment pressure . while additional pressure treatment adjustments may be made in a current session based on a current ahi determination or on detection of an sdb related event , it is preferred that no such adjustments be made until a subsequent session . similarly , the ramping of pressure from a low pressure up to the set therapeutic treatment pressure such that the patient can fall asleep before reaching the therapeutic level may also be implemented by the device . b . determination of an apnea hypopnea index ( ahi ) in a first session as previously noted , the pressure treatment device preferably detects sleep disordered breathing events including apneas and hypopneas , by determining an apnea - hypopnea index . optionally , other sdb related indices may be used , for example , an apnea index , a hypopnea index or some other sdb related index . the preferred determination methodology is illustrated in the flow chart of fig3 . at the beginning of a treatment session with the device , a current sdb index or ahi is reset or initialized in step 30 . during the delivery of pressure treatment , flow is continuously measured or determined in step 32 . with flow information or the flow signal ( e . g ., from a differential pressure transducer or derived from blower speed or power to the blower motor ), measures of ventilation ( e . g ., an average flow determined over a period of time ) are calculated in a step 34 . preferably , these ventilation measures include a short term measure and a long term measure . in one embodiment , a suitable recent ventilation measure or a short term average may be a low pass filtered flow signal utilizing a low pass filter having a time constant which is short with respect to the duration of a breath , e . g ., about 2 to 8 seconds . a suitable longer term ventilation or longer term average measure of flow may be a low pass filtered flow signal utilizing a low pass filter having a time constant which is long with respect to the duration of a breath , e . g ., about 110 seconds . these ventilation measures , including a short term measure and a long term measure , are for purposes of comparing a more recent average measure with a longer term average . from the results of such a comparison , either apneas or hypopneas may be detected in steps 36 a and / or 36 b respectively . for example , in detecting a hypopnea , if the short term average measure falls below the longer term average such that it is less than 50 % of the longer term average , a hypopnea may be tallied or detected . similarly , if the short term average falls below the longer term average such that it is less than 20 % of the longer term average , an apnea may be tallied or detected . in one embodiment of the invention an ahi scoring scheme may be implemented as follows : i . an apnea is scored if a 2 second moving average ventilation drops below 25 % of a recent average ventilation ( time constant = 100 s ) for at least 10 consecutive seconds , ii . a hypopnea is scored if an 8 second moving average drops below 50 % but not more than 25 % of the recent average ventilation for 10 consecutive seconds . those skilled in the art will recognize other methods or modifications for detecting hypopneas or apneas and determining an ahi or an sdb index which will otherwise indicate severity in the patient &# 39 ; s sdb symptoms . after detecting either an apnea or a hypopnea event , an ahi may be incremented in step 37 . after incrementing the ahi , the system determines whether the session has ended at step 38 . at this point , the system terminates at step 39 . if the session is ongoing , the system cycles back to step 32 to continue detecting apnea and hypopnea events and incrementing the ahi . the total of these detected apneas and hypopneas for any given session would make up the ahi used in the adjustment of treatment pressure in a subsequent session . preferably , the ahi is determined by adding the total number of apneas and hypopneas the patient experienced over a treatment period covering a single night . optionally , the ahi may be a function of time such as an average hourly ahi determined over the total time for any given treatment session with the device such as a period of sleep or a single night of sleep . c . adjustment of treatment pressure in response to ahi in a subsequent session as previously noted , preferably automated adjustments to the therapeutic level of the treatment pressure are only made or only take effect for treatment in a subsequent session or subsequent night of treatment based on an ahi determined in a prior session or previous night of treatment . that is , the automated dynamic pressure changes are on a night - by - night basis rather than a breath - by - breath basis . thus , the device implements an algorithm for adjusting the treatment pressure in a subsequent session . for example , as illustrated in fig4 , after starting a new or entering a treatment session in step 40 , the device may then evaluate a previously recorded ahi in an evaluation step 42 . based on the detected ahi from a prior session a new treatment pressure will be set . optionally , for a first use or first session , the ahi may have a default of 0 such that no changes to the treatment pressure will be implemented in the first session . similarly , a default pressure setting for the first use may be a low non - therapeutic value ( e . g ., about 1 - 3 cmh 2 o ) or some other physician or clinician set value in a therapeutic range of about 4 - 20 cmh 2 o . in evaluating the historic ahi in step 42 , if a prior session results in no detected apneas or hypopneas or only a few ( e . g ., ahi = 0 or less than 8 ), no pressure changes will be implemented and the treatment pressure setting will remain from the prior session . alternatively , for purposes of determining the minimum pressure necessary to prevent sdb , for an ahi of 0 or an ahi of less than 8 from the prior session , the device may lower the pressure in the new session . in lowering the pressure , the device may decrement the pressure by a fixed amount , ( e . g ., 0 . 25 cmh 2 o ) which is preferably lower than the pressure increase although they may be the same . thus , in setting the treatment pressure in step 44 , the new pressure for the current setting will be the pressure from the prior session less the fixed decrement amount . however , if the ahi from the prior session is greater than 0 or greater than some low non - adjustment range ( e . g ., 1 - 8 events ), the treatment pressure will automatically increment upwards since such an ahi score is an indication of the need for an increase in treatment . thus , based on the high ahi from the previous night , the treatment pressure can be increased by some increment . the treatment pressure increase is performed with the intent to decrease the ahi towards a clinically desirable level in the subsequent night . the ideal treatment pressure will decrease the ahi to a clinically desirable level but not be so far in excess of the minimum required pressure that it induces unnecessary discomfort . therefore , the quantum of the incremental pressure increase may vary depending upon the perceived clinical severity of the ahi . for example , the choice of incremental pressure increase may reflect the clinical observation that a relatively small increment will induce a clinically significant change where the patient has a relatively low ahi ( e . g ., a pressure increment of 0 . 5 cmh 2 o for an ahi in the range of 5 - 19 ) while a relatively larger increment will be appropriate to induce a clinically significant change where the patient has a relatively high ahi ( e . g ., a pressure increment of 2 cmh 2 o for an ahi of 40 or higher ). referring to fig4 , in the treatment pressure setting step 44 , the treatment pressure is automatically set for the new session to be the previous session &# 39 ; s pressure setting plus the increment . the treatment pressure will then be delivered during the current session in step 46 and then , at step 48 , the system cycles back to step 30 ( fig3 ) and again begins the process of determining a new ahi . the new ahi will affect the treatment pressure for the next session or treatment in the next night or future session . such a scheme allows the device to adjust itself over an unlimited number of nights , while evolving with the needs of the patient . in one embodiment , other schemes of adjustment of the pressure may be based on patterns of the ahi over more than a single night , such as two or more nights . for example , the pressure may be lowered if the ahi has been 0 for two or more consecutive sessions . similarly , pressure may be increased only if the ahi in more than one consecutive night , for example , 2 or 3 nights , suggests a need for an increase . preferably , any decay in pressure over time is slower than the increase in pressure over time . optionally , the device may be configured with an adaptation range that limits the changes to the treatment pressure that the device may automatically implement based on a previous night &# 39 ; s ahi determination . the adaptation range may be a preset variable that is determined by a physician and preferably is not changed during the many treatment sessions with the user or patient . a default range may be set into the device in the absence of such a setting by a physician . for example , the physician may preset the adaptation range to a value of 10 cmh 2 o . when a pressure change is implemented by the device , the range is checked to make certain that any increments attempted by the device never exceed the original pressure setting of the physician by the amount of the range . thus , if the pressure is set to 5 cmh 2 o for the first session with a patient , and the adaptation range is set to 10 cmh 2 o , any automated treatment pressure change that attempts to increment the treatment pressure beyond 15 cmh 2 o would be prevented . in addition , the device may be configured with an optional warning indicator to advise the patient or the physician of the attempted increase beyond the adaptation range . a device that implements the above - described treatment scheme would have many benefits for sdb patients . for example , utilizing the described algorithm would be more cost effective when compared to more complex detection and adjustment schemes . also , since the device can adjust on a night - by - night basis , it can provide adaptation for seasonal changes that may affect the patient &# 39 ; s condition . it can also adapt with the patient &# 39 ; s disease progression . the present invention may be embodied in other specific forms without departing from its spirit or essential characteristics . the described embodiments are to be considered in all respects only as illustrative and not as restrictive . the scope of the invention is , therefore , indicated by the appended claims and their combination in whole or in part rather than by the foregoing description . all changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope .
a method is disclosed for operating a device that treats sleep disordered breathing during successive treatment sessions , where the device provides continuous positive airway pressure during sleep . the method comprises the steps of applying a constant treatment pressure during a first session and deriving a sleep disorder index representative of the number of sdb episodes that occurred during the first session . if the treatment pressure should be increased based upon the derived sdi , it is increased during a second , subsequent session .