Abstract:
A continuous positive airway pressure (CPAP) apparatus for respiratory assistance of a pattern is disclosed. There is a blower having an output connectible to a ventilation mask wearable by the patient. A first pressure sensor measures blower pressure at the output of the blower, and a second pressure sensor that is connectible to the ventilation mask measures mask pressure therein. A pressure controller is connected to the first pressure sensor and the second pressure sensor, and a patient inspiratory phase and a patient expiratory phase is be detectable by the pressure controller to regulate therapeutic pressure at the patient mask, based upon pressure differentials between the mask pressure and the blower pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present disclosure relates generally to the treatment of respiratory and cardiovascular conditions, and more particularly, to methods and systems for continuous positive airway pressure (CPAP) therapy selectively providing a pressurized flow of breathable gas to a patient utilizing dual pressure sensors at a source and on a ventilation mask. 
         [0005]    2. Description of the Related Art 
         [0006]    Mechanical ventilators comprise medical devices that either perform or supplement breathing for patients. Early ventilators, such as the “iron lung,” created negative pressure around the patient&#39;s chest to cause a flow of ambient air through the patient&#39;s nose and/or mouth into the lungs. However, the vast majority of contemporary ventilators instead use positive pressure to deliver gas to the patient&#39;s lungs via a patient circuit between the ventilator and the patient. The patient circuit typically consists of one or two large bore tubes (e.g., 22 mm inner diameter for adults; 15 mm inner diameter for pediatrics) that interface to the ventilator on one end and a patient mask on the other end. 
         [0007]    Ventilators may support either a single limb or a dual limb patient circuit. Single limb patient circuits are typically utilized for less acute clinical requirements such as the treatment of obstructive sleep apnea or respiratory insufficiency. In further detail, the single limb patient circuit, as its nomenclature suggests, involves gas flow from the ventilator to the patient and patient mask over a single conduit. The patient inspires fresh gas from the patient circuit, and expires carbon dioxide-enriched gas that is purged from the system through vent holes in the mask. 
         [0008]    One particular application of ventilator devices is in the treatment of obstructive sleep apnea (OSA) syndrome, where the patient&#39;s upper airway narrows or collapses during sleep. There are repetitive pauses in breathing that may extend in duration up to half a minute. Although some degree of apnea is considered normal, in more severe cases, daytime sleepiness and fatigue may result as a consequence of reduced blood oxygen saturation, as well as constant interruptions to sleep cycles. In order to retain the patient&#39;s airway and ensure normal, uninterrupted breathing during sleep, continuous positive airway pressure (CPAP) therapy may be prescribed. 
         [0009]    Generally, CPAP involves the application of positive pressure to open the patient&#39;s airway to prevent its collapse, as would otherwise occur during apnea. In a basic implementation, CPAP therapy applies a constant pressure that is not tied to the patient&#39;s normal breathing cycle. The positive airway pressure is desired in the inspiratory phase when the pressure differences between the lungs and the nose contribute to the collapse of the intermediate airway. However, supplying positive pressure flow into the patient during the expiratory phase generates resistance to the patient&#39;s breathing efforts, causing discomfort. Furthermore, toward the end of the patient&#39;s expiratory phase, flow and pressure in the airway is naturally minimal, such that positive pressure can cause additional discomfort. Notwithstanding the clinician&#39;s best efforts to prescribe a CPAP treatment flow rate that minimizes such extraneous pressure augmentation while ensuring the proper splinting of the airway during inspiration, the patient is still subject to higher pressures than needed throughout the breathing cycle. 
         [0010]    Partially in response to this deficiency, CPAP systems that varied the pressure augmentation depending on the patient flow, i.e., inspiration or expiration, were developed. One such system is described in U.S. Pat. No. 6,932,084 to Estes, et al., which is understood involve pressure augmentation during inspiration and pressure relief during exhalation based upon a patient flow estimator. The delivered pressure, which itself is measured and utilized by a pressure controller in a feedback loop to confirm accuracy, is a calculated as a function of a constant CPAP prescription pressure and a proportional value of patient flow. A relief or augmentation constant defines the degree thereof, and the patient flow is estimated. The value of the constant is zero during inspiration, thus providing no pressure augmentation and the delivered pressure is equivalent to the base or prescription pressure. During expiration, the value of the constant is non-zero, and the pressure delivered to the patient is the prescription pressure less a proportional amount of the estimated instantaneous patient flow. A signal representative of the delivered pressure drives the blower hardware delivering therapeutic air flow to the patient. Although the method disclosed by Estes, et al. allows the delivered therapeutic pressure to the patient to be tuned to a greater degree, the patient nevertheless experiences discomfort. 
         [0011]    Another approach to the issue of excess pressure at the expiration stage is disclosed in U.S. Pat. No. 7,128,069 to Farrugia et al. When a transition from the inspiration phase to the expiration phase is detected, i.e., when the pressure at the patient&#39;s mouth begins to drop, the motor that controls the blower is understood to be de-energized and allowed to free-wheel. After the pressure increases back to a predetermined level, which in the Farrugia et al. disclosure is 3 cm H 2 O, the blower motor is restarted to bring the pressure at the patient&#39;s mouth to prescription levels (10 cm H 2 O). Thus, the temporary stopping of the blower is understood to function as a pressure relief during expiration. However, due to the wind-up and wind-down times associated with starting and stopping an electrical motor, patient comfort is not optimized due to the existence of residual pressure. 
         [0012]    Accordingly, there is a need in the art for an improved methods and systems for continuous positive airway pressure (CPAP) therapy including the use dual pressure sensors at a source and on a ventilation mask to control an exhalation valve. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    In accordance with one embodiment of the present disclosure, there is contemplated a respiratory assistance device. There may be a blower with an output, in addition to a patient ventilation interface that can be configured for fitment on a patient respiratory passageway. The patient ventilation interface may also include a piloted exhalation valve. Furthermore, there may be a gas passage conduit that couples the output of the blower to the patient ventilation interface. A pilot line from the gas passage conduit may be coupled to the pilot of the exhalation valve. The respiratory assistance device may also include a first pressure sensor that can measure a mask pressure in the patient ventilation interface, as well as a second pressure sensor that can measure a blower pressure at the output of the blower. There may be a pressure controller that is in communication with the first pressure sensor and the second pressure sensor that can detect a patient inspiratory phase and a patient expiratory phase based upon pressure differentials between the mask pressure and the blower pressure. The pressure controller can regulate therapeutic airflow delivered to the patient and exhausted through the piloted exhalation also based upon such pressure differentials. Furthermore, the pressure controller may relieve mask pressure by reducing therapeutic airflow delivered to the patient according to a function of the blower pressure. 
         [0014]    In accordance with another embodiment of the present disclosure, there is a continuous positive airway pressure (CPAP) apparatus for respiratory assistance of a patient. The apparatus may include a blower having an output connectible to a ventilation mask that is wearable by the patient. There may also be a first pressure sensor that can measure blower pressure at the output of the blower, in addition to a second pressure sensor that is connectible to the ventilation mask for measuring mask pressure therein. The apparatus may further include a pressure controller that can be connected to the first pressure sensor and the second pressure sensor. A patient inspiratory phase and a patient expiratory phase may be detectable by the pressure controller to regulate therapeutic airflow delivered to the patient based upon pressure differentials between the mask pressure and the blower pressure. 
         [0015]    According to yet another embodiment of the present disclosure, there is contemplated a method for administering continuous positive airway pressure (CPAP) therapy to a patient. The method may begin with receiving a first CPAP therapeutic pressure value. There may also be a step of measuring a blower pressure value at an output of a blower that generates therapeutic gas flow for the patient. Additionally, there may be a step of measuring a mask pressure value at a ventilation mask that is worn by the patient. The method may further include generating a pressure differential value from the blower pressure value and the mask pressure value, and then evaluating a patient respiratory state from the pressure differential value. The patient respiratory state may be one of an inspiration state and an expiration state. The method may further include selectively applying a quantity of the therapeutic gas flow to the patient in response to the evaluated patient respiratory state. The quantity of the therapeutic gas flow generated by the blower may correspond to the first CPAP therapeutic pressure value. 
         [0016]    The present disclosure will be best understood by reference to the following detailed description when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0017]    These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which: 
           [0018]      FIG. 1  is a block diagram showing the various components of a CPAP apparatus in accordance with various embodiments of the present disclosure including a ventilation unit, a patient ventilation mask, and gas passage conduits; 
           [0019]      FIG. 2  is a block diagram illustrating the electrical components of the ventilation unit; 
           [0020]      FIG. 3  is a pressure diagram graphically illustrating the pressure cycles at the blower and the patient mask over a typical breathing sequence including inspiratory phases and expiratory phases; 
           [0021]      FIG. 4  is a pressure diagram graphically illustrating the pressure cycle differences (AP) at the blower and the patient mask over the breathing sequence; 
           [0022]      FIG. 5  is a pressure diagram graphically illustrating the pressure difference (AP) at the blower device and the patient mask with a superimposed threshold that defines when an exhalation valve is opened or closed; 
           [0023]      FIG. 6  is a control loop block diagram depicting pressure sensor variables corresponding to the blower and a mask as inputs to control devices; 
           [0024]      FIG. 7  is a flowchart illustrating the processing steps of the control loop shown in  FIG. 6 ; 
           [0025]      FIG. 8  is a pressure diagram illustrating the pressure at the blower device and a pressure relief target; and 
           [0026]      FIG. 9  is a block flow diagram showing the interrelated components of the ventilation unit. 
       
    
    
       [0027]    Common reference numerals are used throughout the drawings and the detailed description to indicate the same elements. 
       DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    The detailed description set forth below in connection with the appended drawings is intended as a description of the several presently contemplated embodiments of a system for continuous positive airway pressure (CPAP) therapy. The system delivers breathing gas to a patient for the treatment of obstructive sleep apnea (OSA) and other cardio-pulmonary conditions, and implements various methods for the selective pressure augmentation and relief throughout the breathing cycle. This description is not intended to represent the only form in which the disclosed invention may be developed or utilized. The description sets forth the functions and features in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions may be accomplished by different embodiments that are also intended to be encompassed within the scope of the present disclosure. It is further understood that the use of relational terms such as first and second and the like are used solely to distinguish one from another entity without necessarily requiring or implying any actual such relationship or order between such entities. 
         [0029]    With reference to the block diagram of  FIG. 1 , one embodiment of the present disclosure contemplates a CPAP system  10  generally comprised of a patient ventilation interface  12  and a ventilation unit  14 . The patient ventilation interface  12  may include such devices as a full-face mask or a nasal mask that can be placed in direct gas flow communication with the upper respiratory tract of the patient, i.e., the nasal cavity and the oral cavity. One embodiment of the CPAP system  10  may utilize a nasal mask such as that described in co-pending U.S. patent application Ser. No. 13/411,348 entitled VENTILATION MASK WITH INTEGRATED PILOTED EXHALATION VALVE and U.S. patent application Ser. No. 13/411,407 entitled VENTILATION MASK WITH INTEGRATED PILOTED EXHALATION VALVE, both of which were filed on Mar. 2, 2012 and the disclosures of which are hereby incorporated by reference in their entirety herein. It will be appreciated that other apparatuses that so interface the respiratory system of the patient to the ventilation unit  14  may be substituted without departing from the scope of the present disclosure, so long as certain features noted below are incorporated. 
         [0030]    Generally, the ventilation unit  14  generates a flow of breathing gas that is delivered to the patient via the patient ventilation interface  12 . The breathing gas may be ambient air a combination of ambient air enriched with oxygen, or any other suitable mixture of gas appropriate for treating the patient. Those having ordinary skill in the art will recognize the variety of options for mixing breathing gasses before delivery to the patient. In further detail, the ventilation unit  14  includes a first inlet port  16 , through which ambient air is drawn. The first inlet port  16  is in communication with an inlet filter  18  that removes particulates and other contaminants from the breathing gas that is ultimately delivered to the patient. Optionally, in line with the inlet filter  18  is a sound suppressor  20  that reduces the sound of gas flow through the ventilation unit  14 . 
         [0031]    The force needed for drawing the ambient air through the first inlet port  16 , the inlet filter  18 , and the sound suppressor  20  is provided by a centrifugal fan or blower  22 . In this regard, the blower has a blower inlet port  22   a  coupled to the sound suppressor  20 , and a blower outlet port  22   b  that is in gas flow communication with an outlet port  24  of the ventilation unit  14 . It will be recognized that any suitable blower  22  capable of generating the gas flow and pressure suitable for CPAP treatment in accordance with the present disclosure may be utilized. The blower  22  is driven electrically and its actuation is governed by a programmable pressure controller  26 , which implements the various methods of CPAP treatment contemplated by the present disclosure as will be described in further detail below. 
         [0032]    The flow of breathing gas that is output from the blower  22  is passed through the outlet port  24  to a gas conduit  28  that is in coupled to the aforementioned mask or patient ventilation interface  12 . The gas conduit  28  is understood to be a plastic tube having a predetermined inner diameter such as 22 mm or smaller, though any other conduit of suitable material and construction may be utilized. The patient ventilation interface  12  in accordance with various embodiments of the present disclosure also includes a piloted exhalation valve  30  that is selectively actuated depending on the pressure differential between the patient ventilation interface  12  and the ventilation unit  14 . The exhalation valve  30  is connected to a pilot line  32  that branches from the gas conduit  28 . A pressure difference is generated between the patient ventilation interface and the exhalation valve, such that it is closed during inspiration and opened during expiration. 
         [0033]    In order to ascertain such pressure differentials, the presently contemplated CPAP system  10  includes dual pressure sensors, including a device pressure sensor  34  and a mask pressure sensor  36 . The device pressure sensor  34  is disposed within the ventilation unit  14 , and monitors the pressure at the blower outlet port  22   b.  The mask pressure sensor  36  is also physically disposed within the ventilation unit  14 , but is in direct gas flow communication with the mask or patient ventilation interface  12  over a pressure sensor line  38  that is connected to a second inlet port  40 . When the ventilation unit  14  is operating, gas pressure within the pressure sensor line  38  as well as the gas conduit  28  may be connected to deliver a purge flow to clear line  38 . This can be done through a purge solenoid  42  connected to both. The purge can be continuous or intermittent according to the patient&#39;s breathing phase or pressure difference between the blower pressure and the mask pressure. 
         [0034]    The block diagram of  FIG. 2  illustrates the various electrical components of the ventilation unit  14 . Power for the ventilation unit  14  may be provided from a conventional household electricity supply of either 120V or 220V alternating current (AC), at 50 Hz or 60 Hz. The block diagram denotes this supply as a power source  80 . A power supply  82  is connected to the power source  80 , and as will be recognized by those having ordinary skill in the art, the power signal is variously rectified, filtered, and stepped down to a direct current (DC) voltage. In accordance with one embodiment of the present disclosure, the DC voltage source is 24 V. It is understood that the blower  22  utilizes a higher DC voltage than control logic devices, and thus the power supply  82  is connected to a power source logic  84 . A first output  85  of the power source logic  84  is connected to an integrated circuit voltage regulator  86  that steps down the DC voltage to the logic device level of 5V. A second output  87  of the power source logic  84  is the existing high DC voltage directly from the power supply  82 , and is connected to the motor control circuit  90 . 
         [0035]    The blower  22  is comprised of several electrical components, including a motor  88  and a motor control circuit  90 . In accordance with one embodiment, the motor  88  is a brushless DC or electrically commutated motor. It will be recognized that the speed of rotation of the motor  88  is based upon input logic signals provided to the motor control circuit  90 , which drives electrical current through its windings that induce magnetic fields that translate to rotational motion of the attached rotor. A fan coupled to the rotor thus rotates and generates a flow of air through an internal conduit  27 . The internal conduit  27  is coupled to an outlet port  27 , which is coupled to the gas conduit  28 . As described above, the device pressure sensor  34  and the mask pressure sensor  36  are connected to the pneumatic circuit between the motor  88  and a patient  13 . 
         [0036]    The motor control circuit  90  has a motor drive output  91  that is connected to the motor  88 . The position of the motor  88  is detected by a Hall-effect sensor that is incorporated into the motor  88 . An output voltage  92  from the Hall-effect sensor is fed back to the motor control circuit  90 , which ensures that the actual position corresponds to the intended or commanded position. 
         [0037]    The pressure controller  26  and its functionality may be implemented with a programmable integrated circuit device such as a microcontroller or control processor  94 . Broadly, the control processor  94  receives certain inputs, and based upon those inputs, generates certain outputs. The specific methods that are performed on the inputs may be programmed as instructions that are executed by the control processor  94 . In this regard, the control processor  94  may include an arithmetic/logic unit (ALU), various registers, and input/output ports. Although external memory such as EEPROM (electrically erasable/programmable read only memory)  96  may be connected to the control processor  94  for permanent storage and retrieval of program instructions, there may also be an internal random access memory (RAM). One embodiment contemplates the use of an Intel 8081 instruction set/architecture, though any other suitable instruction set or processor architecture may be substituted. As indicated above, the control processor  94  is powered by a low voltage DC supply from the voltage regulator  86 . 
         [0038]    Several output devices are envisioned for the ventilation unit  14 . In order to set the operational parameters of the ventilation unit, and to initiate or terminate certain functions, a graphical user interface is provided. Such graphical user interface is generated on a display screen  98 , which may be of a liquid crystal type (LCD). Any type of graphic may be shown on the display screen  98 , though for more specific indicators, a simple light emitting diode (LED) device  100  may be utilized. It will be recognized that alarm conditions, power status, and the like may be indicated with the LED device  100 . Along these lines, audible outputs may be produced with audio transducers  102  that are likewise connected to the control processor  94 . Among the contemplated outputs that may be generated on the audio transducer  102  include simple beeps and alarms, as well as sophisticated voice prompts that provide information and instructions. 
         [0039]    An operator may interact with the graphical user interface through different input devices such as buttons  104  that are connected to the input ports of the control processor  94 . It is understood that the audio transducer  102  may also accept sound input in the form of voice commands, the processing of which is performed may be performed by the control processor  94 . Similarly, the display screen  98  may be incorporated with touch sensors, and may also function as an input device that allows an operator to interact with the graphical user interface displayed thereon. 
         [0040]    Several modalities for connecting to and communicating with other data processing devices such as general-purpose computers are also contemplated. Accordingly, the control processor  94  may be connected to a universal serial bus (USB) controller  106 . For more basic communications, there may be a serial RS-232 transceiver  108 . Through these data communications modalities, the configuration options of the ventilation unit  14  may be set, operating profiles may be downloaded, and so forth. 
         [0041]    The functions of the ventilation unit  14  depend on proper synchronization, and so the control processor  94  is connected to a real time clock  110  that maintains a common clock cycle. Although a primary feature of the real time clock  110  is to maintain synchrony at a processor cycle level, longer term time data is also maintained. In order to retain such time data, the real time clock  110  may be powered independently of the primary power source  80 , and there is accordingly a battery backup  112 . Under heavy processing loads or unexpected program conditions, the control processor  94  may become unable to execute critical programmed steps in real-time. Thus, the control processor  94  may include a processor supervisor  114  that invokes a program execution break upon detecting such conditions. Typically, this is implemented as a step of clearing a memory variable periodically, and when that step is unable to take place because instruction execution is frozen or otherwise delayed, the processor supervisor  114  may cause a predetermined routine to be executed. 
         [0042]    As indicated above, the motor  88  is driven by the motor control circuit  90 , which generates different outputs depending on signals received from the control processor  94 . The signal to drive the motor  88  is generated on a current command line  116 . For control processing on a broader level, feedback from the blower  22  is utilized, and in the specific form of a speed measurement input  118  from the motor control circuit  90 . Furthermore, as detailed below, pressure readings at the blower  22  and the patient  13  are utilized to reach control decisions. Accordingly, the device pressure sensor  34  and the mask pressure sensor  36  are both connected to the control processor  94 . The blower  22  is activated and deactivated via a motor enable line  120 . To ensure that the temperature of the motor  88  remains within operational parameters, a motor cooling fan  122  is driven directly by the control processor  94 . In some embodiments, there may be additional control circuitry that isolates the power source of the motor cooling fan  122  from the control processor  94 . The decision to activate and deactivate the motor cooling fan  122  is made in response to temperature readings from the motor  88 , and there is a motor temperature reading  122  passed to the control processor  94 . 
         [0043]    Referring now to the pressure diagram of  FIG. 3 , the typical operating pressures at the blower  22  and at the patient mask at different points in the breathing cycle is illustrated. More particularly, a first plot  44  illustrates the pressure cycle at the blower  22 , and is characterized by an inspiration region  46  and an expiration region  48 . As will be appreciated, the pressure at the blower  22  increases during inspiration, and decreases during expiration. Henceforth, the first plot  44  and the measurement represented thereby will be referred to as P Blower , with the pressure value at any particular time t being referred to as P Blower (t). This pressure is given in terms of cmH 2 O, as are the other pressure measurements discussed herein. 
         [0044]    The mask pressure at the patient will exhibit an opposite response, where it decreases in the inspiration region  46  and increases in the expiration region  48 . A second plot  50  illustrates the pressure cycle at the patient ventilation interface  12 , and is similarly characterized by the inspiration region  46  and the expiration region  48 . The second plot  50  and the measurement represented thereby will be referred to as P Mask , with the pressure value at any particular time t being referred to as P Mask (t). As can be seen, the pressure at the blower  22  P Blower  is significantly higher than the pressure at the patient ventilation interface  12  P Mask , and the two values are generally reciprocal. That is, when P Mask  peaks, P Blower  is at its lowest, and vice versa. As referenced herein, the terms patient ventilation interface  12  and patient mask are utilized interchangeably. It will be recognized that the patient mask is a specific kind of patient ventilation interface, and as explained briefly above, other types of ventilation interfaces may be utilized. Along these lines, reference to such terms as mask pressure, blower pressure, or the use of the term mask or blower to modify any other term is for purposes of convenience only and not of limitation. For instance, mask pressure is understood to refer to the pressure in the patient ventilation interface  12 , while blower pressure refers to the pressure at the output of the blower  22 . 
         [0045]    In accordance with various embodiments of the present disclosure, patient leak indication is contemplated. As shown in the graph of  FIG. 4 , a first plot  52  is representative of the difference between P Blower  and P Mask  over a time period at a given instant, and is also referred to as ΔP. The first plot  52  can also be characterized by the inspiration region  46  and an expiration region  48 . The average of the pressure difference AP is represented as a leak constant  54 . It will be recognized that the larger the value of the leak constant  54 , the greater the leakage. 
         [0046]    Referring now the graph of  FIG. 5 , the determination of the patient trigger and cycle states will now be considered. It is understood that cycling and trigger states are based on the patient&#39;s breathing cycle, and so the present disclosure contemplates a modality by which the inspiration phase and the expiration phase can be ascertained. A plot  58  represents the pressure difference ΔP between the mask pressure P Mask  and the blower pressure P Blower  over a time period. 
         [0047]    In accordance with one embodiment, a trigger limit  60  is set or otherwise computed as an average  59  of ΔP. More particularly, the trigger limit  60  at time (t) may be the average AP also at time (t) plus a predetermined trigger constant, which may be set by the clinician or the patient. If the pressure difference ΔP at (t) is greater than the trigger limit at the same time (t), the patient is considered to be in the inspiration phase. 
         [0048]    A cycle limit  61  is also set, and is understood to be a function of the peak value of ΔP. In further detail, the cycle limit at time (t) is the maximum ΔP at time (t) multiplied by a cycle constant. The cycle constant can also be set by the clinician or the patient, or otherwise computed as a function of ΔP(t). If the measured ΔP is less than the set cycle limit, then the patient is determined to be in the expiration phase. 
         [0049]    The specific pressure that is to be delivered to the patient by way of the blower  22  is set by the programmable pressure controller  26 , and  FIG. 6  is a control loop block diagram thereof. The piloted exhalation valve  30  further provides pressure relief depending on the current pressure difference between the blower pressure and the mask pressure (ΔP(t)), the details pertaining to the operation of which will be discussed more fully below. The exhalation valve  30  is configured to open to ambient pressure when the pressure difference (ΔP) is small or negative, and closes when pressure difference (ΔP) is sufficiently high. 
         [0050]    Still referring to the control loop block diagram of  FIG. 6 , additional details pertaining to the motor control functions of the blower  22 , and specifically the closed loop control circuit  126 , will now be considered. Generally, the closed loop control circuit  126  includes the first PID controller  64 , and a second PID controller  70 , both of which act upon the motor  88  to effectuate pressure changes within the patient circuit. There is a first or inner control loop  62  that is driven by the first PID controller  64  to modulate blower pressure  66 , as well as a second or outer control loop  68 . Together with the first PID controller  64  and the second PID controller  70 , mask pressure  72  is modulated. The inner control loop  62  and the outer control loop  68  are inter-related and together define the closed loop control circuit  126 . 
         [0051]    One objective of the closed loop control circuit  126  is to operate the blower  22  to the extent necessary to achieve a predetermined pressure sufficient to meet the inspiratory pressure demands of the patient and the pressure losses in the ventilation system  14 . A desired pressure, which is the preset CPAP pressure as input by the clinician, is represented by an input value  73  that is provided to a first summing point  74 . The pressure at the patient mask, i.e., mask pressure  72 , is measured by pressure sensor as discussed above, and also input to the first summing point  74 . An output signal  75  corresponding to the summed pressures of the input value  73  and the mask pressure  72  is passed to the second PID controller  70 . The output signal  75  is processed by the second PID controller  70 , and this processed signal is output to the motor control circuit  90  to partially regulate the blower  22  in response. 
         [0052]    The output from the second PID controller  70  is input to a second summing point  63 , which also adds the pressure value at the blower, i.e., the blower pressure  66 . Another output signal  65  corresponding to these summed pressures is passed to the first PID controller  64 , which again is processed and output to the motor control circuit  90  to regulate the blower  22 . The subsequent blower pressure  66  measurement is again fed back to the second summing point  63 , at which point the inner loop continues. 
         [0053]    The first PID controller  64  is thus part of a closed loop control over a blower pressure sensor  66 , with the output thereof being the current set point for the blower  22 . Furthermore, the second PID controller  70  minimizes the error between the mask pressure and the CPAP set level  73 . The output of the second PID controller  70  is the set pressure for the first PID controller  64 . The respective gains of the first PID controller  64  and the second PID controller  70  may be scheduled according to the patient breathing phase and/or the CPAP set level  73 . During expiration, the pressure controller  26  can be reconfigured to control the speed of the blower  22 , blower pressure, blower flow, either alone or to different set targets. Upon detecting an inspiration phase, the CPAP set level  73  can be set to a different value than during expiration, a technique known as Bi-level CPAP. 
         [0054]    With reference to the flowchart of  FIG. 7 , the steps involved in the control loop circuit  126  will be described. In a step  200 , the CPAP level  73  is set. The mask pressure  72  is then read in a step  202 , and the error between the set CPAP level  73  and the mask pressure  72  is computed in a step  204 , as described above in relation to the second summing point  74 . In a step  206 , the computed error is fed to the second PID controller  70 . The blower  22  is modulated by the second PID controller  70  in a step  208  to minimize the error between the set CPAP level  73  and the mask pressure  72 . At this point, the blower pressure set point is generated. Per the inner control loop  62 , the blower pressure  66  is received in a step  210 , and an error between such value and the set blower pressure set point from the second PID controller  70  is computed in a step  212 . The amount of error is then fed to the first PID controller  64  in accordance with a step  214 . The error is minimized by further modulating the blower  22  in a step  216 , thereby generating the current set point. 
         [0055]    The pressure relief target is understood to be a function of the blower pressure or AP. The pressure diagram of  FIG. 8  includes a plot  78  of the pressure P Blower (t) over a breathing cycle. During the time the blower pressure is lower than average ΔP (t), the mask pressure target is also reduced to create a pressure relief  80 . For improved patient comfort, pressure at the patient could be further reduced according to a function of the blower pressure. The target mask pressure at time (t) is contemplated to be the set CPAP level  73  during the inspiration phase. During the expiration phase, the target pressure at time (t) is contemplated to be the set CPAP level  73  reduced by the blower pressure P Blower (t) multiplied by a relief constant. The relief constant and a minimum mask pressure level may also be set. 
         [0056]    The blower  22  can be triggered upon the pressure difference ΔP(t) reaching the triggering threshold pressure level  60 , that is, pressure augmentation during the inspiratory phase may be initiated by the patient&#39;s spontaneous breathing until the pressure difference reaches such threshold. In further detail, the blower  22  is triggered upon the pressure difference ΔP(t) increases from a level lower than the triggering threshold pressure level  60  to a level higher than the same (a positive d(t)) The triggering threshold pressure level  60  can be adjusted manually. Alternatively, an average threshold value may be calculated based upon previous breathing cycle patterns and set, and such breathing cycle patterns may be leak constant. In this regard, the triggering threshold pressure level  60  may be continuously adjusted to adapt to the patient&#39;s respiratory functions. Different embodiments further contemplate wave shape triggering, gas volume triggering, and a flow triggering, with the integral of flow being understood to be volume. 
         [0057]    With regard to flow triggering, it will be appreciated by those having ordinary skill in the art that flow sensors are typically cost-prohibitive. In combination with the aforementioned dual pressure sensor ventilation method, the blower  22  can be triggered from estimated flow. It is understood that breathable gas flow to the patient may be estimated from the speed of the blower  22 , and the detected pressure levels. The corresponding flow values may be stored in a lookup table or other like data structure. How much flow a particular blower  22  can generate depends on the configuration thereof, so multiple data sets for different blowers  22  may be developed and stored. 
         [0058]    It is also contemplated that a cycling threshold pressure level  61  may be set. When the patient&#39;s breathing cycles between the inspiratory phase and the expiratory phase, delivery of the therapeutic gas flow is stopped. The point at which this occurs is the cycling threshold pressure level  61 , and so when the pressure difference ΔP(t) transitions from a level higher than the cycling threshold pressure level  61  to a level lower than the same (a negative d(t)), the blower  22  is cycled. The cycling threshold pressure level  61  is understood to be set higher than the triggering threshold pressure level  60 . 
         [0059]    With reference to the flow diagram of  FIG. 9 , the operational sequence of the CPAP system  10  will be considered. As indicated above, the motor  88  is driven by a motor control circuit  90 , that is, electrical current is selectively applied by the motor control circuit  90  to the conductive elements of the motor  88  to induce a magnetic field that produces rotation. The specific sequence and manner in which the blower  22  (i.e., the motor  88  and the motor control circuit  90 ) is actuated is governed by a closed loop control circuit  126  that is implemented by the pressure controller  26 . One of the inputs to the closed loop control circuit  126  is a pressure command  128 , or the therapeutic pressure that is set by a clinician. 
         [0060]    Actuating the motor  88  results in a change in pressure at the blower  22 , as sensed in a device pressure sensor block  128 . Furthermore, pressure readings are also made at the patient ventilation interface  12 , or a mask pressure sensor block  130 . These readings are inputs to the closed loop control circuit  126 . Additionally, the readings from the device pressure sensor block  128  and the mask pressure sensor block  130  are utilized in a breathing cycle state detector block  132 . As mentioned above, the pressure difference (ΔP) can be utilized to determine whether the patient  13  is in an expiration (exhalation) state or an inspiration (inhalation state). The breathing cycle state detector  132  so utilizes the pressure measurements and generates a breathing cycle state output  134 . 
         [0061]    Different alarm conditions may be evaluated based in part on the pressure difference between the measurements from the device pressure sensor block  128  and the mask pressure sensor block  130 . These values are passed to an alarm detection logic block  136  that can trigger an alarm  138 . Besides the pressure differences, the speed of the motor  88  is calculated in a motor speed evaluation block  140 , and the temperature of the same is calculated in a motor temperature evaluation block  142 . These calculated values are also passed to the alarm detection logic block  136 . Referring back to the block diagram of  FIG. 2 , the performance of the motor  88  can be adjusted according to its temperature. The command being passed from the control processor  94  to the motor control circuit  90  may be a function of the motor temperature reading  124 . More specifically, the maximum current applied to the motor  88  can be a function of the motor temperature reading  124 . 
         [0062]    The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects. In this regard, no attempt is made to show details of the present invention with more particularity than is necessary, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.