Abstract:
A respiratory assistance device is disclosed. There is a variable speed blower with an output, and a patient ventilation interface configured for fitment on a patient respiratory passageway. A gas passage conduit couples the output of the blower to the patient ventilation interface. A pilot line from the gas passage conduit is coupled to a piloted exhalation valve of the patient ventilation interface. A pressure sensor measures a mask pressure in the patient ventilation interface, and a blower speed sensor measures a speed of the blower. A pressure controller in communication with the pressure sensor and the blower speed sensor detects a patient inspiratory phase and a patient expiratory phase from at least one of the measured speed of the blower and a set speed of the blower. The pressure controller adjusts an operating speed of the blower and actuates the piloted exhalation valve based upon the measured mask pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation-in-part application of U.S. patent application Ser. No. 13/411,257 entitled “DUAL PRESURE SENSOR CONTINUOUS POSITIVE AIRWAY PRESSURE (CPAP) THERAPY,” the entirety of the disclosure of which is incorporated by reference herein. 
     
    
     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 pressure sensing based continuous positive airway pressure (CPAP) therapy selectively providing therapeutic pressure of breathable gas to a patient utilizing a pressure sensor 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 a pressure sensor on a ventilation mask to control an exhalation valve. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    In accordance with various embodiments of the present disclosure, a respiratory assistance device is contemplated. The device may include a variable speed blower with an output, as well as a patient ventilation interface configured for fitment on a patient respiratory passageway. The patient ventilation interface may include a piloted exhalation valve. Furthermore, the device may include a gas passage conduit that can couple 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. There may be a pressure sensor that measures a mask pressure in the patient ventilation interface, and a blower speed sensor that measures a speed of the blower. The device may further include a pressure controller that is in communication with the pressure sensor and the blower speed sensor. A patient inspiratory phase and a patient expiratory phase may be detectable from at least one of the measured speed of the blower and a set speed of the blower. The pressure controller can adjust an operating speed of the blower and actuate the piloted exhalation valve based upon the measured mask pressure. 
         [0014]    According to another embodiment, a continuous positive airway pressure (CPAP) apparatus for respiratory assistance of a patient is disclosed. The apparatus may include a blower with an output connectible to a ventilation mask wearable by the patient. The blower may have a variable speed. There may also be a pressure sensor that is connected to the ventilation mask for measuring mask pressure therein, and a blower speed sensor connected to the blower for measuring the variable speed of the blower. The apparatus may include a blower controller that is connected to the pressure sensor and the blower speed sensor. A patient inspiratory phase and a patient expiratory phase may be detectable from at least one of the measured speed of the blower and a set speed of the blower to set a therapeutic pressure at the patient. 
         [0015]    Yet another embodiment of the present disclosure contemplates a method for administering continuous positive airway pressure (CPAP) therapy to a patient. The method may include a step of receiving a first CPAP therapeutic pressure value. There may also be a step of measuring a blower speed from the blower that is generating therapeutic pressure to the patient, as well as a step of measuring a mask pressure value at a ventilation mask worn by the patient. The method may include evaluating a patient respiratory state from the blower speed. The patient respiratory state may be one of an inspiration state and an expiration state. Furthermore, there may be a step of selectively adjusting the blower speed to deliver the therapeutic pressure to the patient in response to the evaluated patient respiratory state. The speed of the blower may generate a quantity of the therapeutic pressure corresponding 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 graph illustrating the pressure cycles at the patient mask over a typical breathing sequence including inspiratory phases and expiratory phases; 
           [0021]      FIG. 4  is a graph plotting the operating speed of a ventilation source over the typical breathing sequence in accordance with various embodiments of the present disclosure; 
           [0022]      FIG. 5  is a graph plotting the operating speed of the ventilation source with a superimposed threshold that defines when triggering and cycling occur; 
           [0023]      FIG. 6  is a graph plotting the generated pressure and flow rate for a series of operating speeds of the ventilation source; 
           [0024]      FIG. 7  is a control loop block diagram depicting pressure sensor and speed corresponding to the blower speed and a mask pressure as inputs to control devices; 
           [0025]      FIG. 8  is another control loop block diagram showing a closed-loop controller for mask pressure; 
           [0026]      FIG. 9  is a flowchart illustrating the processing steps of the control loop shown in  FIG. 7 ; 
           [0027]      FIG. 10  is a graph showing the operating speed of the ventilation source and a corresponding pressure relief target; and 
           [0028]      FIG. 11  is a block flow diagram showing the interrelated components of the ventilation unit. 
       
    
    
       [0029]    Common reference numerals are used throughout the drawings and the detailed description to indicate the same elements. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    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. 
         [0031]    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 VENTILLATION MASK WITH INTEGRATED PILOTED EXHALATION VALVE and U.S. patent application Ser. No. 13/411,407 entitled VENTILLATION 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. 
         [0032]    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 . 
         [0033]    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 ventilation source  22 . There is an inlet port  22   a  coupled to the sound suppressor  20 , and an 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 ventilation source  22  capable of generating the gas flow and pressure suitable for CPAP treatment in accordance with the present disclosure may be utilized, including centrifugal fans and other like blowers. The ventilation source  22  is driven electrically and its actuation is governed by a programmable controller  26 , which implements the various methods of CPAP treatment contemplated by the present disclosure as will be described in further detail below. 
         [0034]    The flow of breathing gas that is output from the ventilation source  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. 
         [0035]    As will be explained in further detail below, detection of the inspiration and expiration is contemplated to be a function of the pressure at the patient ventilation interface and the speed of the ventilation source  22 . In order to read the pressure at the ventilation interface, the presently contemplated CPAP system  10  includes a mask or patient interface pressure sensor  34 . The patient interface pressure sensor  34  is 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. 
         [0036]    The block diagram of  FIG. 2  illustrates the various electrical components of one typical embodiment 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  44 . A power supply  46  is connected to the power source  44 , 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 ventilation source  22  utilizes a higher DC voltage than control logic devices, and thus the power supply  46  is connected to a power source logic  48 . A first output  50  of the power source logic  48  is connected to an integrated circuit voltage regulator  52  that steps down the DC voltage to the logic device level of 5V. A second output  54  of the power source logic  38  is the existing high DC voltage directly from the power supply  46 , and is connected to a motor control circuit  56 . 
         [0037]    The ventilation source  22  is comprised of several electrical components, including a motor  58  and the aforementioned motor control circuit  56 . In accordance with one embodiment, the motor  58  is a brushless DC or electrically commutated motor. It will be recognized that the speed of rotation of the motor  58  is based upon input logic signals provided to the motor control circuit  56 , 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 the outlet port  24 , which is coupled to the gas conduit  28 . As described above, the patient interface pressure sensor  34  is connected to the pneumatic circuit between the motor  58  and the patient  13 . 
         [0038]    The motor control circuit  56  has a motor drive output  60  that is connected to the motor  58 . The rotational position of the motor  58  is detected by a Hall-effect sensor that is incorporated into the motor  58 . An output voltage  62  from the Hall-effect sensor is fed back to the motor control circuit  56 , which ensures that the actual position corresponds to the intended or commanded position. 
         [0039]    The controller  26  and its functionality may be implemented with a programmable integrated circuit device such as a microcontroller or control processor  64 . Broadly, the control processor  64  receives certain inputs, and based upon those inputs, generates certain outputs. The specific operations that are performed on the inputs may be programmed as instructions that are executed by the control processor  64 . In this regard, the control processor  64  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)  66  may be connected to the control processor  64  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  64  is powered by a low voltage DC supply from the voltage regulator  54 . 
         [0040]    As mentioned above, in order to set the operational parameters of the ventilation unit  14 , and to initiate or terminate certain functions, a graphical user interface is provided. Such graphical user interface is generated on a display screen  68 , which may be of a liquid crystal type (LCD). Any type of graphic may be shown on the display screen  68 , though for more specific indicators, a simple light emitting diode (LED) device  70  may be utilized. It will be recognized that alarm conditions, power status, and the like may be indicated with the LED device  70 . Audible outputs may also be produced with audio transducers  72  that are likewise connected to the control processor  64 . Among the contemplated outputs that may be generated on the audio transducer  72  include simple beeps and alarms, as well as sophisticated voice prompts that provide information and instructions. 
         [0041]    An operator may interact with the graphical user interface through different input devices such as a touch screen interface  74  that is overlaid on the display screen  68 . It will be recognized that various graphic elements may be generated on the display screen  68 , with touch inputs/interactions corresponding in position to those graphic elements being evaluated as a selection or activation of the same. Various touch screen interfaces, some of which may be directly integrated with the display screen  68 , are known in the art. Besides touch screen inputs, buttons  76  may also be connected to the control processor  64  for similarly receiving user inputs. It is understood that the audio transducer  72  may also accept sound input in the form of voice commands, the processing of which is performed may be performed by the control processor  64 . 
         [0042]    Several modalities for connecting to and communicating with other data processing devices such as general-purpose computers are also contemplated. Accordingly, the control processor  64  may be connected to a universal serial bus (USB) controller  78 . For more basic communications, there may be a serial RS-232 transceiver  80 . Through these data communications modalities, the configuration options of the ventilation unit  14  may be set, operating profiles may be downloaded, and so forth. Notwithstanding the specific reference to USB and RS-232 communications modalities, any other communications modality including wireless systems may be substituted without departing from the present disclosure. 
         [0043]    The functions of the ventilation unit  14  depend on proper synchronization, and so the control processor  70  is connected to a real time clock  82  that maintains a common clock cycle. Although a primary feature of the real time clock  82  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  82  may be powered independently of the primary power source  44 , and there is accordingly a battery backup  84 . Under heavy processing loads or unexpected program conditions, the control processor  64  may become unable to execute critical programmed steps in real-time. Thus, the control processor  64  may include a processor supervisor  86  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  86  may cause a predetermined routine to be executed. 
         [0044]    As mentioned above, the motor  58  is driven by the motor control circuit  56 , which generates different outputs depending on signals received from the control processor  64 . The signal to drive the motor  58  is generated on a current command line  88 . For control processing on a broader level, feedback from the ventilation source  22  is utilized, and in the specific form of a speed or current measurement input  90  from the motor control circuit  56 . Furthermore, as detailed below, pressure readings at the patient  13  is utilized to reach control decisions. Accordingly, the patient interface pressure sensor  34  is connected to the control processor  70 . The ventilation source  22  is activated and deactivated via a motor enable line  92 . To ensure that the temperature of the motor  58  remains within operational parameters, a motor cooling fan  94  may be driven directly by the control processor  64 . In some embodiments, there may be additional control circuitry that isolates the power source of the motor cooling fan  94  from the control processor  64 . The decision to activate and deactivate the motor cooling fan  94  may be made in response to temperature readings from the motor  58 , and so there is a motor temperature reading  96  passed to the control processor  64 . 
         [0045]    Referring now to the pressure diagram of  FIG. 3 , a first plot  98  illustrates the pressure cycle at the patient ventilation interface  12 , and is characterized by an inspiration region  100  and an expiration region  102 . As will be appreciated, pressure at the patient ventilation interface  12  decreases during inspiration, and increases during expiration. Henceforth, the first plot  98  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). This pressure is given in terms of cmH 2 O, as are the other pressure measurements discussed herein. 
         [0046]    As shown in the diagram of  FIG. 4 , the operating speed of the ventilation source  22 , on the other hand, is understood to exhibit an opposite response as shown by a second plot  104 . The blower operating speed increases during the inspiration region  100  and decreases in the expiration region  102 . The second plot  104  and the measurement represented thereby will be referred to as V Blower , with the pressure value at any particular time t being referred to as P Blower (t). As can be seen, the operating speed of the ventilation source  22  exhibits a generally a reciprocal relationship with respect to the pressure at the patient ventilation interface  12 . That is, when P Mask  peaks, V Blower  is at its lowest, and vice versa. 
         [0047]    In accordance with various embodiments of the present disclosure, the indication of leakage at the patient ventilation interface  12  is contemplated. As further shown in the graph of  FIG. 4 , the second plot  104  has an average value  106 , which corresponds to a leak constant  107 . Generally, the greater the average operating speed of the ventilation source  22 , the greater the leakage, and this relationship serves as a basis for the quantified comparisons that define the degree of leakage. 
         [0048]    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 speed, 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 . 
         [0049]    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  108  represents the blower speed V Blower  over a time period t. 
         [0050]    It is contemplated that the ventilation source  22  is operated in the linear region, that is, the operating speed of the ventilation source  22  pressure directly corresponds to the pressure generated thereby. Certain embodiments envision pressure differentials to be induced between the ventilation source  22  and the patient ventilation interface  12 . However, because of the direction relationship between the pressure the speed, any evaluations for determining inhalation and exhalation state can be substituted with the speed of the ventilation source  22 . The particular configuration of the CPAP system  10  in which minimal air flow rates are required is understood to make possible operating the ventilation source  22  in its linear region. 
         [0051]    Referring briefly to the graph of  FIG. 6 , there are a series of plots  110  that represents the pressure generated (cm H 2 O) at differing operating speeds. More particularly, a first plot  110   a  represents the generated pressure for an operating speed of 25,000 rpm. As shown, the pressure remains constant for low flow rates up to around 100 lpm, after which drastic reductions in pressure occur. The same is true for an operating speed of 30,000 rpm shown in a second plot  110   b , which maintains a generated pressure of around 50 cm H 2 O in its linear region, as well as an operating speed of 35,000 rpm shown in a third plot  110   c.  Here, a pressure of around 68 cm H 2 O is exhibited in the linear region. For an operating speed of 40,000 rpm as shown in a fourth plot  110   d,  the pressure is around 88 cm H 2 O. 
         [0052]    The illustrated blower operating speeds shown in the plots  110  are presented by way of example only and not of limitation, and the ventilation source  22  can be operated in any intermediate speed. Along these lines the foregoing operating characteristics of the ventilation source  22  are also exemplary only. It is to be understood that different variants of the ventilation source  22  may have different operating characteristics, and those having ordinary skill in the art will recognize the needed accommodating modifications. 
         [0053]    With reference back to the graph of  FIG. 5 , in accordance with one embodiment, a trigger limit  112  is set or otherwise computed as an average of the blower operating speed. More particularly, the trigger limit  112  at time (t) may be the average blower operating speed also at time (t) plus a predetermined trigger constant, which may be set by the clinician or the patient. If the blower operating speed at (t) is greater than the trigger limit at the same time (t), the patient is considered to be in the inspiration phase. 
         [0054]    A cycle limit  114  is also set, and is understood to be a function of the peak value of the operating speed. In further detail, the cycle limit at time (t) is the maximum operating speed 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 the operating speed. If the operating speed is less than the set cycle limit, then the patient is determined to be in the expiration phase. 
         [0055]    The specific pressure that is to be delivered to the patient by way of the ventilation source  22  is set by the programmable controller  26 , and  FIG. 7  is a control loop block diagram thereof. The piloted exhalation valve  30  further provides pressure relief depending on the current pressure difference between the ventilation source  22  and the patient ventilation interface  12 , 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. 
         [0056]    Still referring to the control loop block diagram of  FIG. 7 , additional details pertaining to the motor control functions of the ventilation source  22 , and specifically the closed loop control circuit  116 , will now be considered. Generally, the closed loop control circuit  116  includes a first PID controller  118 , and a second PID controller  120 , both of which act upon the motor  58  to effectuate pressure changes within the patient circuit. There is a first or inner control loop  122  that is driven by the first PID controller  118  to modulate the speed of the motor  58  and thus a blower speed/operating speed  124 , as well as a second or outer control loop  126 . Together with the first PID controller  118  and the second PID controller  120 , mask pressure  128  is modulated. The inner control loop  122  and the outer control loop  126  are inter-related and together define the closed loop control circuit  116 . 
         [0057]    One objective of the closed loop control circuit  116  is to operate the ventilation source  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  130  that is provided to a first summing point  132 . The pressure at the patient ventilation interface  12 , i.e., mask pressure  128 , is measured by pressure sensor as discussed above, and also input to the first summing point  132 . An output signal  134  corresponding to the summed pressures of the input value  130  and the mask pressure  128  is passed to the second PID controller  70 . The output signal  134  is processed by the second PID controller  120 , and this processed signal is output to the motor control circuit  56  to partially regulate the ventilation source  22  in response. 
         [0058]    The output from the second PID controller  120  is input to a second summing point  136 , which also adds the operating speed of the blower, i.e., the ventilation source operating speed  124 . Another output signal  138  corresponding to these summed values is passed to the first PID controller  118 , which again is processed and output to the motor control circuit  56  to regulate the ventilation source  22 . The subsequent blower operating speed  124  measurement is again fed back to the second summing point  136 , at which point the inner loop continues. 
         [0059]    The first PID controller  118  is thus part of a closed loop control over a ventilation source speed sensor  124 , with the output thereof being the current set point for the ventilation source  22 . Furthermore, the second PID controller  120  minimizes the error between the mask pressure and the CPAP set level  130 . The output of the second PID controller  120  is the set pressure for the first PID controller  118 . The respective gains of the first PID controller  118  and the second PID controller  120  may be scheduled according to the patient breathing phase and/or the CPAP set level  130 . During expiration, the programmable controller  26  can be reconfigured to control the speed of the ventilation source  22 , blower pressure, blower flow, either alone or to different set targets. Upon detecting an inspiration phase, the CPAP set level  130  can be set to a different value than during expiration, a technique known as Bi-level CPAP. 
         [0060]    With reference to  FIG. 8 , the present disclosure contemplates another closed loop control circuit  140  to confirm the delivery of the specified CPAP pressure  130  to the patient ventilation interface  12 . In further detail, the closed loop control circuit  140  includes a pressure controller  142  that generally corresponds to the programmable controller  26 . The pressure controller  126  acts upon the motor  58  to effectuate pressure changes within the patient circuit, and a mask pressure  28  is applied. The pressure at the patient ventilation interface  12 , i.e., mask pressure  128 , which is measured by the pressure sensor  34 , is input to a summing point  146  and compared against the specified CPAP pressure  130 . An output  148  of the summing point  146  in turn serves as the control input for the pressure controller  142 . 
         [0061]    With reference to the flowchart of  FIG. 9 , the steps involved in the control loop circuit  116  will be described. In a step  200 , the CPAP level  73  is set. The mask pressure  128  is then read in a step  202 , and the error between the set CPAP level  130  and the mask pressure  128  is computed in a step  204 , as described above in relation to the second summing point  136 . In a step  206 , the computed error is fed to the second PID controller  120 . The ventilation source  22  is modulated by the second PID controller  120  in a step  208  to minimize the error between the set CPAP level  130  and the mask pressure  128 . At this point, the blower operating speed set point is generated. Per the inner control loop  122 , the blower operating speed  124  is received in a step  210 , and an error between such value and the blower operating speed set point from the second PID controller  120  is computed in a step  212 . The amount of error is then fed to the first PID controller  118  in accordance with a step  214 . The error is minimized by further modulating the ventilation source  22  in a step  216 , thereby generating the current set point. 
         [0062]    The pressure relief target is understood to be a function of the blower operating speed. The pressure diagram of  FIG. 10  includes a plot  150  of the speed V Blower (t) over a breathing cycle. During the time the instantaneous blower operating speed is lower than average, the mask pressure target is also reduced to create a pressure relief  152 . For improved patient comfort, pressure at the patient could be further reduced according to a function of the blower operating speed. The target mask pressure at time (t) is contemplated to be the set CPAP level  130  during the inspiration phase. During the expiration phase, the target pressure at time (t) is contemplated to be the set CPAP level  130  reduced by a function of the blower operating speed V Blower (t) multiplied by a relief constant. The relief constant and a minimum mask pressure level may also be set. 
         [0063]    With reference to the flow diagram of  FIG. 11 , the operational sequence of the CPAP system  10  will be considered. As indicated above, the motor  58  is driven by a motor control circuit  56 , that is, electrical current is selectively applied by the motor control circuit  56  to the conductive elements of the motor  58  to induce a magnetic field that produces rotation. The specific sequence and manner in which the ventilation source  22  (i.e., the motor  58  and the motor control circuit  56 ) is actuated is governed by a closed loop control circuit  154  that is implemented by the programmable controller  26 . One of the inputs to the closed loop control circuit  154  is a pressure command  156 , or the therapeutic pressure that is set by a clinician. 
         [0064]    An actuation of the motor  58  changes its speed. This is sensed in a speed sensor block  157 . Furthermore, pressure readings are also made at the patient ventilation interface  12 , or a mask pressure sensor block  158 . These readings are inputs to the closed loop control circuit  154 . Additionally, the readings from the speed sensor block  140  is utilized in a breathing cycle state detector block  132 . As mentioned above, the blower operating speed 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  160  so utilizes the speed measurements and generates a breathing cycle state output  162 . 
         [0065]    Different alarm conditions may be evaluated based in part on the pressure measurements from the mask pressure sensor block  158 . These values are passed to an alarm detection logic block  164  that can trigger an alarm  166 . Besides the pressure measurements, the speed of the motor  88  calculated in the speed sensor block  157 , and the temperature of the same as calculated in a motor temperature sensor block  168 . These calculated values are also passed to the alarm detection logic block  164 . Referring back to the block diagram of  FIG. 2 , the performance of the motor  58  can be adjusted according to its temperature. The command being passed from the control processor  64  to the motor control circuit  56  may be a function of a temperature reading from the motor temperature sensor block  168 . More specifically, the maximum current applied to the motor  58  can be a function of the motor temperature reading. 
         [0066]    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.