# Patent

## Title
Autotitrating method and apparatus

## Abstract

An apparatus and method of controlling the delivery of therapeutic gas delivered to a patient undergoing positive airway pressure therapy is described. The method includes providing a flow of gas to a patient's airway at a pressure, obtaining information from the range of 0 to 25 Hz of the frequency domain of the flow, and adjusting the pressure based on the information. The apparatus includes a blower for providing a flow of gas to a patient's airway at a pressure, a sensor to measure a characteristic of the flow, a controller to obtain information from the range of 0 to 25 Hz of the frequency domain of the characteristic, and a pressure regulator for adjusting the pressure based on the information.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. provisional patent application Ser. No. 60/599,356, filed on Aug. 6, 2004, which is incorporated herein by reference.


FIELD OF INVENTION
This invention is generally directed to a method and apparatus for controlling the positive air pressure applied to a patient undergoing positive airway pressure therapy.
BACKGROUND OF THE INVENTION
Obstructions in some patients' airways during sleep can cause limited airflow, leading to apnoea, hypopnoea, or snoring. The obstruction is often a collapsed pharynx. The obstruction may be a partial airway obstruction, leading to altered characteristics of the airflow. A hypopnoea is a reduction of flow that is greater than fifty percent, but not complete. An apnoea, however, is a complete cessation of airflow. Each of these conditions frequently leads to sleep deprivation.
It is well known to treat patients suffering from sleep deprivation with positive airway pressure therapy (“PAP”). This therapy can be Continuous Positive Airway Pressure (“CPAP”), Variable Positive Airway Pressure (“VPAP”), Bi-level Positive Airway Pressure (“BiPAP”), or any of numerous other forms of respiratory therapy. The application of positive pressure to the patient's pharynx helps minimize or prevent this collapse. Positive airway pressure therapy is currently applied by means of an apparatus containing a pressure source, typically a blower, through a tube to a mask, which the patient wears in bed.
It is desired to control the applied pressure. Too little pressure tends not to solve the problem. Too much pressure tends to cause discomfort to the patient, such as drying out of the mouth and pharynx, as well as difficulty in exhaling against the applied pressure. The difficulty in applying optimum pressure is that incidents of airway obstruction come and go through the course of a night's sleep. One solution is to try to find an optimum pressure for a particular patient and maintain that pressure. This method requires the patient's stay at a sleep clinic, where sleep specialists can monitor the patient's course of breathing throughout one or more night's sleep, prescribe the appropriate pressure for that patient, and then set the apparatus to deliver the appropriate pressure. This method is, of course, inconvenient as well as expensive to the patient and tends to be inaccurate, as a typical patient will not sleep the same when away from familiar bedding and surroundings.
Accordingly, it is desirable to be able to adjust the applied pressure without requiring the patient to attend at a sleep center. Various methods of in-home adjustments have been considered. One method generally thought to be effective is to monitor the patient to try to anticipate the onset of an obstructed airway, and to adjust the pressure in response. When an elevated upper airway resistance or flow obstruction is anticipated or underway, the apparatus increases the applied pressure. When the patient returns to normal sleep, the applied pressure is reduced. The problem then, is to determine when a flow obstruction is occurring or is about to occur. It is desired to anticipate correctly in order to avoid the problems set forth above for when too much or too little pressure is applied.
Various methods have been proposed to solve this problem. In U.S. Pat. No. 5,107,831 to Halpern, an apparatus monitors the airflow to the patient and posits an event of airway obstruction when the patient's breath fails to meet a predetermined threshold of flow rate or duration. In U.S. Pat. No. 5,1345,995 to Gruenke, an apparatus monitors the airflow to the patient and analyzes the shape of the flow versus time waveform. If the shape of this waveform tends to be flattened, that is, more similar to a plateau than to a sinusoid, the apparatus posits an event of airway obstruction. In U.S. Pat. No. 5,245,995 to Sullivan, an apparatus monitors the patient's sound with a microphone. If audible snores are detected, the apparatus posits an event of airway obstruction. Similarly, in U.S. Pat. No. 5,953,713 to Behbehani, an apparatus measures the total pressure within an interface placed over a patient's airway and inputs frequency data in the range 100 to 150 Hz into a neural network to determine the presence of a pharyngeal wall vibration (a snore) which, according to Behbehani, is a precursor to sleep disorder breathing.
These methods have not proven totally satisfactory in controlling the applied pressure during PAP therapy. For example, the '713 patent, by measuring in the range of 100 to 150 Hz, essentially tests for snoring and does not measure or analyze any information concerning partial airway obstruction (as described within the present application), as this information is found in the lower frequency range 0 to 25 Hz. FIGS. 1 and 2 are plots in the frequency domain of energy v. frequency of typical breathing. As can be seen, there is a marked difference between normal breathing and breathing characterized by a partial airway obstruction, all in low frequencies. The present application exploits this difference to control the delivery of therapeutic gas.
Moreover, the methods of the prior art are unsatisfactory in analyzing a signal in a high-noise environment. The inventors herein have discovered an alternate way to detect the onset of an event of airway obstruction and to control the applied pressure from a high-noise signal such as results from a person's breathing over the course of a night. Accordingly, the method and apparatus of the present invention fulfill the need for analyzing a signal from a patient in order to control the applied pressure during PAP therapy.
SUMMARY OF THE INVENTION
The present invention in one embodiment is a method of controlling positive airway pressure therapy by providing a flow of gas to a patient's airway at a pressure, obtaining information from the frequency range of zero to 25 HZ in the frequency domain of the flow, and adjusting the pressure based on the information. In another embodiment, the present invention is an apparatus for providing controlled positive airway pressure therapy, having a blower for providing a flow of gas to a patient's airway, a sensor to measure a characteristic of the flow, a controller to obtain information from the frequency range of zero to 25 HZ in the frequency domain of the characteristic, and a pressure regulator for adjusting the pressure based on the information.



BRIEF DESCRIPTION OF THE DRAWINGS
The organization and manner of the structure and operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, wherein like reference numerals identify like elements in which:
FIG. 1 is a plot in the frequency domain of energy v. frequency for normal breathing and breathing characterized by partial airway obstruction;
FIG. 2 is also a plot in the frequency domain of energy v. frequency for normal breathing and breathing characterized by partial airway obstruction;
FIG. 3 is a diagram of an exemplary positive airway pressure apparatus of the preferred embodiment of the present invention;
FIG. 4 is a block diagram of the main algorithm of the method of the preferred embodiment of the present invention, showing the interaction of the five algorithms;
FIG. 5 is a block diagram of the Breath Detection Algorithm of the preferred embodiment of the present invention;
FIG. 6 is a block diagram of the Partial Airway Obstruction Algorithm of the preferred embodiment of the present invention;
FIG. 7 is a block diagram of the Apnoea Detection Algorithm of the preferred embodiment of the present invention;
FIG. 8 is a block diagram of the Hypopnoea Detection Algorithm of the preferred embodiment of the present invention;
FIGS. 9a, 9b, 9c, and 9d are block diagrams of the Pressure Adjusting Algorithm of the preferred embodiment of the present invention;
FIG. 10 is a diagram of airflow versus time, illustrating Tstart and Mf; and
FIG. 11 is a diagram of airflow versus time, illustrating Tend, t1, t2 and Mt1-t2.



DETAILED DESCRIPTION
While the invention may be susceptible to embodiment in different forms, there is shown in the drawings, and herein will be described in detail, a specific embodiment with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that as illustrated and described herein.
A positive airway pressure apparatus 100 of the preferred embodiment of the present invention is shown in FIG. 3 in which the patient 1 receives humidified, pressurized gas through an inspiratory conduit 3. It should be understood that the delivery systems could be CPAP (Continuous Positive Airway Pressure), VPAP (Variable Positive Airway Pressure), BiPAP (Bi-level Positive Airway Pressure), or any of numerous other forms of respiratory therapy. The apparatus 100 and method 200 of the present invention will be described as used for CPAP but an artisan of ordinary skill in the art will readily adapt both for use with VPAP, BiPAP, or another positive airway pressure therapeutic system.
Inspiratory conduit 3 is attached at one end to a mask 2, preferably one such as is described in U.S. Pat. No. 6,662,803. Inspiratory conduit 3 connects at its other end to the outlet 4 of a humidification chamber 5, which contains a volume of water 6. Inspiratory conduit 3 may contain heating heater wires (not shown) or other suitable heating elements that heat the walls of the conduit to reduce condensation of humidified gases within the conduit. Humidification chamber 6 is preferably formed from a plastic material and may have a highly heat-conductive base (for example an aluminum base) that is in direct contact with a heater plate 7 of humidifier 8.
Electronic controller 9 controls the various components of the apparatus 100. Controller 9 may be a microprocessor-based controller containing, as is well known in the art, RAM, ROM, an ALU, one or more registers, a data bus, counters (including at least a breath number counter 120 and a pressure decrease counter 130), and one or more buffers (including at least a circular buffer 110). Controller 9 executes computer software commands stored in its RAM and ROM.
Controller 9 receives input from sources such as user input dial 10 through which a user of the device may, for example, set a predetermined required value (preset value) of various characteristics of the gases supplied to the patient 1, such as initial airflow, pressure, humidity, or temperature of the gases. Controller 9 preferably receives input relating to airflow from differential pressure sensor 11, which is preferably located in blower 15. Differential pressure sensor 11 could alternatively be located elsewhere, upstream of mask 2, such as within conduit 3 or anywhere on mask 2. Alternatively, controller 9 may receive input related to airflow by direct measurement of flow at any point from blower 15 to mask 2. Controller 9 may also receive input from other sources, for example temperature sensors 12 through connector 13 and heater-plate temperature sensor 14.
In response to the user-set inputs and the other inputs, controller 9 determines when (or to what level) to energize heater plate 7 to heat the water 6 within humidification chamber 5. As the volume of water 6 within humidification chamber 5 is heated, water vapor begins to fill the volume of the chamber 5 above the water's surface and is passed out of the outlet 4 of humidification chamber 5 with the flow of gases (for example air) provided from a gas supply device such as blower 15, which gases enter the chamber 5 through inlet 16. Exhaled gases from the patient 1 are passed directly to ambient surroundings in FIG. 3.
Blower 15 is provided with a variable-pressure regulating device such as variable speed fan 21, which draws air or other gases through blower inlet 17. The speed of variable speed fan 21 is controlled by electronic controller 9 in response to inputs from the various components of apparatus 100 and by a user-set predetermined required value (preset value) of pressure or fan speed via dial 19.
Controller 9 is programmed with five algorithms:

    
    
        1. Breath Detection Algorithm;
        2. Apnoea Detection Algorithm;
        3. Hypopnoea Detection Algorithm;
        4. Partial Airway Obstruction Detection Algorithm; and
        5. Pressure Adjusting Algorithm.
    
    


These algorithms interact as diagramed in FIG. 4. When the patient 1 turns on the apparatus 100, the controller 9 receives data from pressure sensor 11 and starts the main algorithm (step 200). Pressure sensor 1 is preferably a differential pressure sensor, and controller 9 converts differential pressure data to airflow data. Controller 9 samples the raw analogue flow signal at 50 Hz (step 202) and calculates bias flow (step 204). Bias flow, such as occurs from leaks in the mask 2 or elsewhere in the apparatus 100, is obtained preferably via a Butterworth low-pass filter (with a rise time of approximately thirty seconds). The information stored in circular buffer 110 in controller 9 is therefore net airflow data, as the controller 9 removes the bias flow (step 206). Circular buffer 110 in controller 9 is continuously updating data and storing that data for 15 seconds (step 208). Accordingly, throughout these algorithms, the flow being analyzed does not contain the bias flow. That is, the flow oscillates about zero flow.
The incoming flow data is continuously checked for the presence of a leak (step 210). If a significant leak is detected the algorithm is paused until the leak is resolved.
If there are no leaks and fewer than ten breaths have passed, the current data is analyzed by the Breath Detection Algorithm (step 300), as will be described in connection with FIG. 5. The Breath Detection Algorithm determines where the oldest breath begins and ends.
If no breath is detected (step 212), the main algorithm starts over with sampling the raw analogue flow signal (step 202). If a breath is detected (step 212), a breath number counter 120 is incremented (step 214), and the main algorithm starts over with sampling the raw signal (step 202).
Since it is assumed that the patient 1 will breathe a minimum of ten breaths before any apnoeas or hypopnoeas occur, the main algorithm of the preferred embodiment counts to determine if at least ten breaths have occurred (step 216). If more than ten breaths have occurred, the apparatus proceeds to the Apnoea Detection Algorithm (step 500), as will hereinafter be described in connection with FIG. 7. If fewer than ten breaths have occurred, circular buffer 110 in controller 9 continues to sample raw analogue data (step 202).
Once ten breaths have occurred, the main algorithm proceeds as diagramed in FIG. 4. The Apnoea Detection Algorithm (step 500) constantly checks the real-time incoming flow to see if an apnoea is occurring (step 218), as will be described in greater detail in connection with FIG. 7. If an apnoea is occurring, the Pressure Adjusting Algorithm (step 700) is called, as will be described in connection with FIG. 9. Once the apnoea has finished (step 220), the main algorithm starts over with sampling raw data (step 202). If no apnoea is occurring, the Breath Detection Algorithm (step 300) is called.
If no new breath has been detected (step 222), the algorithm checks to see if 2.5 minutes have passed since the last partial obstruction or apnoea (step 224). If not, the main algorithm starts over with sampling raw data (step 202). If so, the Pressure Adjusting Algorithm (step 700) is called. If a breath is detected, the Hypopnoea Detection Algorithm is called (step 600), as will be described in connection with FIG. 8, followed by the Partial Airway Obstruction Algorithm (step 400) as will be explained in connection with FIG. 6.
The Hypopnoea Detection Algorithm (step 600) checks to see if a breath is possibly part of a hypopnoea. The Partial Airway Obstruction Algorithm (step 400) is called to check for partial airway obstruction (step 232). If the Hypopnoea Detection Algorithm finds that a hypopnoea has occurred (step 226), the main algorithm checks to see if any breaths in the hypopnoea showed partial airway obstruction (step 228). If so, the Pressure Adjusting Algorithm is called (step 700). If not, the main algorithm checks to see if 2.5 minutes have passed since the last partial obstruction or apnoea event (step 230). If so, the Pressure Adjusting Algorithm is called (step 700). If not, the main algorithm starts over with sampling raw data (step 202).
The Partial Airway Obstruction Algorithm checks for partial airway obstruction (step 232) in the event a hypopnoea has not occurred. If the current breath shows a partial airway obstruction, the main algorithm checks to see if the previous two breaths have shown a partial airway obstruction (step 234). If so, the Pressure Adjusting Algorithm (step 700) is called. If the current breath does not show a partial airway obstruction (step 232) or if the previous two breaths do not show a partial airway obstruction (step 234), the main algorithm checks to see if 2.5 minutes have passed since the last partial obstruction or apnoea event (step 224). If so, the Pressure Adjusting Algorithm (step 700) is called; if not, the main algorithm starts over with sampling raw data.
Using the above algorithms, the applied positive airway pressure is at the lowest pressure required by the patient 1 to achieve therapeutic treatment. The details of the algorithms will now be explained.
Breath Detection Algorithm
Two routines are used, as diagramed in FIG. 5, depending on how many breaths have been detected since the program was initiated. The two routines differ, in that one incorporates the breathing period of the patient 1.
The Breath Detection Algorithm (step 300) initially determines if a previous breath's end is still contained within the flow buffer (step 302). If a previous breath's end point is still in the flow buffer, the start of the next breath (beginning of inspiration, or Tstart) will be the data point following the end point of the previous breath (step 304). If the previous breath's end point is not in the buffer (such as if an apnoea occurred), the new end point is determined, once a piece of flow data greater than five liters per minute is immediately followed by a piece of flow data less than 5 liters per minute has occurred (step 306), by searching the flow buffer to find Ef where Ef is 0.15 times the maximum flow in the buffer (step 308), and where flow is increasing, that is, flow is less than Ef, followed by flow greater than Ef (step 310). The new end point Tend is then set as the start of the next breath (step 304).
At this point, the algorithm determines whether more than twenty breaths have occurred (step 312). If so, the algorithm searches to find Mf, the maximum flow over the last one-quarter of the average breathing period after Tstart (step 314). If twenty or fewer breaths have occurred, Mf is defined as the maximum flow in the next second after Tstart (step 316).
The end point of expiration, Tend, is determined by searching between two reference points (step 318) (reference points t1, t2 are shown in FIG. 10, a plot of airflow to the patient, as determined from differential pressure sensor 11, versus time). Each reference point t1, t2 is identified by determining the occurrence of a flow data value greater than the reference value followed by a flow data value less than the reference value (steps 320, 322), where t2 is after t1, which is after Tstart.
The reference value is given by:

reference value=0.2×Mf 

where Mf is the maximum flow in 0.25× average breathing period since the beginning of inspiration (as found in step 314).

Mf is illustrated in FIG. 10, also a plot of airflow to the patient 1, as determined from differential pressure sensor 11, versus time.
The period between t1 and t2 should be greater than 0.5 sec (step 326). If not, t2 is found again (step 322). The maximum flow, greater than zero, between the two reference points t1, t2 is calculated and used to determine the end of expiration Ef (step 328). The end of expiration is:

Ef=0.15×Mt1-t2 

where

    
    
        Mt1-t2=maximum flow between t1 and t2 

Mt1-t2, t1, and t2 are illustrated in FIG. 11.

    
    


A flow data value less than Ef immediately followed by a flow data value greater than Ef indicates the end of the breath Tend (step 330). The breath is therefore from Tstart to Tend (step 332). The apparatus then stores the maximum flow Mt1-t2, provided the breath is not part of a hypopnoea, as determined by the Hypopnoea Detection Algorithm (step 600), as will be hereinafter described, and stores the period of the breath (step 334).
The period of the breath and the maximum inspiratory flow are used by the Apnoea Detection Algorithm (step 500) and the Hypopnoea Detection Algorithm (step 600), as will be described.
Partial Airway Obstruction Algorithm
The Partial Airway Obstruction Detection Algorithm (step 400) is diagramed in FIG. 6. It analyzes a breath, previously detected by the Breath Detection Algorithm (step 300), for the presence of a partial airway obstruction (partial obstruction of the upper airway).
When the patient 1 breathes, pressure gradients are generated between the lungs and atmosphere. The physiology of the upper airway combined with these pressure gradients and Bernoulli's Effect can result in partial collapse of the upper airway during inspiration. This partial collapse is prevalent in people with obstructive sleep apnoea.
In order to determine if a breath contains a partial airway obstruction, Fourier analysis is used to analyze the inspiratory flow for features specific to partial airway obstruction. Once a signal has been mapped to the frequency domain via a Fourier transform, there are many ways to represent and analyze the frequency domain information. One could analyze the direct result of the Fourier transform, which would give the amplitude of the Fourier transform's sine component (information representative of the odd component of the original signal) and the amplitude of the Fourier transform's cosine component (information representing the even component of the original signal). Alternatively, from the Fourier transform, one could construct a phase v. frequency plot and an energy v. frequency plot (energy spectrum). The phase and energy information could be used to analyze the original waveform. An alternative to the energy v. frequency plot is to construct a magnitude v. frequency plot. In the preferred embodiment an energy spectrum is used to determine the presence of partial airway obstruction. Partial airway obstructions can be detected from analysis of the energy spectrum at low frequencies, as illustrated in FIGS. 1 and 2.
In particular, energy statements involving groupings of the frequency harmonics of the Fourier transform of the flow of therapeutic gas to the patient are generated from frequency-domain considerations. This technique allows analysis of signals that might have a considerable amount of background noise. All processing and analysis is done in the frequency domain based upon observed relationships between the patient's responses and the character of the energy spectrum in the frequency domain.
Additionally, severe airway obstruction often results in a reduced peak flow-rate during inspiration, which results in a prolongation of time spent inspiring relative to expiring. This increase in inspiratory time is incorporated in the Partial Airway Obstruction Detection Algorithm.
To obtain information solely from the inspiratory phase of the respiratory cycle, Fourier analysis is performed on a waveform consisting of two inspiratory phases oppositely combined. The result is an odd function defined as

f(−x)=−f(x)  (1)

The standard Fourier series definition is




  
    
      
        
          f
          ⁡
          
            (
            x
            )
          
        
        =
        
          
            
              A
              0
            
            2
          
          +
          
            
              ∑
              
                n
                =
                1
              
              ∞
            
            ⁢
            
              (
              
                
                  
                    A
                    n
                  
                  ⁢
                  cos
                  ⁢
                  
                    
                      n
                      ⁢
                      
                        
                      
                      ⁢
                      π
                      ⁢
                      
                        
                      
                      ⁢
                      x
                    
                    T
                  
                
                +
                
                  
                    B
                    n
                  
                  ⁢
                  sin
                  ⁢
                  
                    
                      n
                      ⁢
                      
                        
                      
                      ⁢
                      π
                      ⁢
                      
                        
                      
                      ⁢
                      x
                    
                    T
                  
                
              
              )
            
          
        
      
    
    
      
        (
        2
        )
      
    
  




where n is the number of harmonics, An are the harmonic cosine coefficients, Bn are the harmonic sine coefficients, and T is the period of cycle. Modifying Equation (2) according to Equation (1) gives





  
    
      
        
          f
          ⁡
          
            (
            x
            )
          
        
        =
        
          
            ∑
            
              n
              =
              1
            
            ∞
          
          ⁢
          
            
              B
              n
            
            ⁢
            sin
            ⁢
            
              
                n
                ⁢
                
                  
                
                ⁢
                π
                ⁢
                
                  
                
                ⁢
                x
              
              T
            
          
        
      
    
    
      
        (
        3
        )
      
    
  




as all An, which represent the even part of the function, are zero.

To apply Fourier analysis to the inspiratory waveform, the algorithm of the preferred embodiment of the present invention first samples the incoming flow signal. Inspiration is then separated from expiration and manipulated as in Equation (1) to give a vector of N data points, y=[y1 y2 . . . yN], that represent a single period of a cyclic function. The data is sampled evenly in time, hence tj+1=τj where τ is the sampling interval between data points j=0, . . . −1. The discrete Fourier transform of y is defined as




  
    
      
        
          Y
          
            k
            +
            1
          
        
        =
        
          
            ∑
            
              j
              =
              0
            
            
              N
              -
              1
            
          
          ⁢
          
            
              y
              
                j
                +
                1
              
            
            ⁢
            
              e
              
                
                  -
                  2
                
                ⁢
                π
                ⁢
                
                  
                
                ⁢
                ij
                ⁢
                
                  
                
                ⁢
                
                  k
                  /
                  N
                
              
            
          
        
      
    
    
      
        (
        4
        )
      
    
  




where i is the square root of negative one and k=0, . . . , N−1. Each point Yk+1 of the transform has an associated frequency,

fk+1=k/τN  (5)

In the preferred embodiment, the fundamental frequency, k=1, is defined as f2=1/τN and the first harmonic frequency, k=2, is defined as f3=2/τN.
In order to determine whether a breath is a partial airway obstruction, the relative energy of specific frequencies and groups of frequencies is analyzed. To do this the energy spectrum is calculated,

Wk+1=|Yk+1|2  (6)

and normalized such that the total energy equals one.

In the preferred embodiment, the first 13 harmonics are considered for analysis, as the relative power in the higher harmonics is minuscule. The analyzed harmonics are in the frequency range of zero to 25 Hz. The energy distribution of an inspiratory contour of a normal breath generally will have a majority of energy situated at W2, which is associated with the fundamental frequency, and a small amount of energy is distributed among the harmonics. The present invention uses this characteristic of the energy spectrum as developed through Fourier analysis to posit that if the relative energy situated at a particular frequency or group of frequencies is above an empirically-observed threshold, the breath is deemed to be a partial airway obstruction.
Generally, for a normal breath the percentage of time spent inspiring is 40 percent and expiring is 60 percent. The patient 1 with a partially collapsed airway cannot achieve maximum inspiratory flow. Accordingly, the patient 1 extends the time spent inspiring relative to expiring. The time spent inspiring increases to 50 percent or more of the total breath during a partial airway obstruction.
Accordingly, the Partial Airway Obstruction Detection Algorithm first calculates an initial ratio, Iinsp, which is the portion of the entire breath spent on inspiration greater than the mean (step 402). Note that bias flow has been previously removed (steps 204, 206), so the mean of the breath should be zero or very close to zero. Next, the algorithm determines the inspiratory part of the breath and constructs a waveform consisting of two inspiratory phases oppositely combined (step 404). Then, the algorithm calculates the discrete energy spectrum of the oppositely combined waveform as a function of frequency f (step 406):

W(f)=|FFT(waveform)|2 

It is assumed that no significant energy is contained in the frequencies (or harmonics) above a predetermined level, preferably 13 times the fundamental frequency. Therefore, the energy spectrum is only retained, in the preferred embodiment, up to 13 times the fundamental frequency (step 408). Next, the algorithm normalizes the energy spectrum such that the total energy equals one (step 410):

normalized energy spectrum=W(f)/ΣW(f)

This calculation is done so that all breaths will be analyzed the same, even though each breath may differ from another breath in duration, tidal volume, and maximum flow.
Next, the algorithm groups energies corresponding to different harmonic frequencies into information-bearing values (step 412). These information-bearing values are compared to threshold values that are calculated in accordance with the percentage of the breath that is spend on inspiration (step 414). The information-bearing values and the threshold values are determined empirically.
In the preferred embodiment, four information-bearing values are used: Wfirst, Wsecond, Wfreq, and Whigh—freq, as follows:



  
    
      W
      first
    
    =
    
      W
      3
    
  




  
    
      W
      second
    
    =
    
      W
      4
    
  




  
    
      W
      freq
    
    =
    
      
        ∑
        
          k
          =
          2
        
        14
      
      ⁢
      
        W
        
          k
          +
          1
        
      
    
  




  
    
      W
      high_freq
    
    =
    
      
        ∑
        
          k
          =
          6
        
        14
      
      ⁢
      
        W
        
          k
          +
          1
        
      
    
  



According, Wfirst corresponds to the energy in the first harmonic, Wsecond corresponds to the energy in the second harmonic, Wfreq corresponds to the energy in the first 13 harmonics, and Whigh—freq corresponds to the energy in the harmonics five through 13. Other information-bearing values can be obtained from the energies corresponding to different harmonic frequencies using other mathematical operations.

In the preferred embodiment, two thresholds are used, Tfreq and Thigh—freq. These values vary depending on the value of Iinsp the percentage of the breath spend inspiring (calculated at step 402) and have been determined empirically to be:












Threshold
Iinsp
Value







Tfreq
Iinsp ≦ 40
0.15


Tfreq
40 < Iinsp 50
−0.005 × Iinsp + 0.35


Tfreq
Iinsp ≧ 50
0.1 


Thigh_freq
Iinsp ≦ 40
0.03


Thigh_ freq
40 < Iinsp 60
−0.001 × Iinsp + 0.07


Thigh_freq
Iinsp ≧ 60
0.01









Using these empirically-determined values, the algorithm computes the information-bearing summations to the thresholds. If Wsecond is greater than or equal to 0.1 (step 416), the breath is a partial airway obstruction (step 418). If Wfirst is greater than or equal to 0.02, Wsecond is greater than or equal to 0.02, and Wfreq is greater than or equal to 0.12 (step 420), the breath is a partial airway obstruction (step 422). If the sum of Wfirst and Wsecond is greater than or equal to 0.06 and Wfreq is greater than or equal to 0.12 (step 424), the breath is a partial airway obstruction (step 426). If the sum of Wfirst and Wsecond is greater than or equal to 0.07 and Wfreq is greater than or equal to 0.11 (step 428), the breath is a partial airway obstruction (step 430). If Wfreq is greater than or equal to Tfreq (step 432), the breath is a partial airway obstruction (step 434). If Whigh—freq is greater than or equal to Thigh—freq (step 436), the breath is a partial airway obstruction (step 438). If none of these comparisons is true, the breath is normal (step 440).
Apnoea Detection Algorithm
The Apnoea Detection Algorithm (step 500) is diagramed in FIG. 7. In order to detect an apnoea (cessation of flow), the controller 9 compares the incoming flow data (minus bias flow) with a threshold, μ1, determined by the previous peak inspiratory flow. The Breath Detection Algorithm (step 300) had previously stored the maximum or peak inspiratory flow, not part of a hypopnoea. The Apnoea Detection Algorithm calculates the threshold, μ1, as 20 percent of the average peak inspiratory flow of the oldest five breaths of the last ten breaths (step 502). The algorithm then calculates Tapnoea (step 504):

Tapnoea=1.7×(breathing period averaged over last 50 breaths)

Tapnoea, however, must be between ten and fifteen seconds.

If the incoming flow is less than the threshold, μ1, an apnoea may be occurring. If this condition is met for time greater than Tapnoea (step 506), then an apnoea is occurring (step 508), otherwise, no apnoea occurred (step 510). If an apnoea is occurring, the algorithm checks to see when the flow has increased to more than the threshold, μ1 (step 512), indicating that the apnoea has finished.
Hypopnoea Detection Algorithm
In order to detect a hypopnoea (reduction of flow), the Hypopnoea Detection Algorithm (step 600), as diagramed in FIG. 8, compares the stored breath with a threshold, μ2, determined by the previous peak inspiratory flow (step 602). Similar to the Apnoea Detection Algorithm (step 500), the threshold, μ2, is calculated from the peak inspiratory flow for the oldest five breaths of the last ten that did not constitute part of a hypopnoea. The threshold (μ2) is then taken as 60 percent of the average peak inspiratory flow of the oldest five breaths (step 602).
If incoming flow is less than the threshold (μ2), for a period of time greater than 12 seconds (step 604), then a possible hypopnoea has occurred; otherwise, no hypopnoea is occurring (step 606). For the event to be classified as a hypopnoea, there must be an increase in flow such that flow is greater than μ2 within 30 seconds since the flow was less than μ2 (step 608). If this increase in flow is detected, a hypopnoea occurred (step 610); otherwise, the event was not a hypopnoea (step 612).
Pressure Adjusting Algorithm
If an apnoea was detected during the Apnoea Detection Algorithm (step 500), the Pressure Adjusting Algorithm (step 700) is called. Also, if a hypopnoea was detected during the Hypopnoea Detection Algorithm (step 600), and there were partial airway obstruction breaths in the hypopnoea (step 228), or if there was no hypopnoea but the current breath and two previous breaths were partial airway obstructions (steps 226, 232, 234), the Pressure Adjusting (step 700) algorithm is called. If there was no hypopnoea, and either the current breath does not show a partial airway obstruction or the previous two breaths did not show a partial airway obstruction, but is has been 2.5 minutes since the last partial airway obstruction (steps 226, 232, 234, 224), the Pressure Adjusting Algorithm is called. Also, if there was a hypopnoea, but without any partial airway obstruction breaths, and it has been longer than a predetermined period since the last partial airway obstruction event or apnoea, preferably 2.5 minutes (steps 226, 228, and 230), the Pressure Adjusting Algorithm (step 700) is called. The Pressure Adjusting Algorithm is diagramed in FIGS. 9a through 9d. 
The Pressure Adjusting Algorithm (step 700) determines whether to adjust the pressure and by how much, in order to control the therapeutic pressure delivered to the patient. As an initial rule of the preferred embodiment, this algorithm will only increase pressure to a maximum of 10 cm H2O on an event classified as an apnoea (step 702).
The algorithm first checks to determine if there have been any pressure decreases since the beginning of the period of sleep (step 704). If there have not been any such decreases, the algorithm determines if an obstructive event of any sort has been detected and whether the pressure is under a predetermined maximum, preferably ten cm H2O (step 706). If these conditions are met, the algorithm determines whether the obstructive event was a partial airway obstruction, an apnoea, or a hypopnoea with a partial airway obstruction (step 708). In the event of a hypopnoea with a partial airway obstruction, the controller 9 increases pressure by one cm H2O (step 710) and waits ten seconds before allowing another pressure change (step 712). If the event was an apnoea, the controller 9 increases pressure by two cm H2O (step 714) and waits 60 seconds before allowing another pressure change (step 716). If the event was a partial airway obstruction, the controller 9 increases pressure by one cm H2O (step 718) and waits ten seconds before allowing another pressure change (step 720).
If there have been previous pressure decreases since the beginning of the period of sleep (step 704), or if the conditions of a detected obstructive event and the pressure being less than ten cm H2O have not been met (step 706), the algorithm determines if there have been six consecutive pressure decreases. If so, total consecutive pressure-decrease counter 130 is reset to zero (step 722).
The algorithm next determines if there has been normal breathing for a predetermined period of time, preferably 2.5 minutes (step 724). If so, the controller 9 decreases the pressure by 0.5 cm H2O (step 726) (and increments pressure-decrease counter 130 by one).
If there has not been normal breathing for the predetermined period of time (step 724), then either a partial airway obstruction, an apnoea, or a hypopnoea with partial airway obstruction has occurred (step 728). The next step depends on the previous pressure changes. If the previous consecutive pressure changes have been increases totaling greater than or equal to a total of one cm H2O, and the current pressure is less than ten cm H2O (step 730), the algorithm proceeds to step 708 as described above. If not, the controller 9 proceeds to increase the pressure by an amount depending on the nature of the obstructive event and the amount of previous pressure decreases, as diagramed in FIGS. 9b, 9c, and 9d. 
If the total previous pressure decreases were more than one cm H2O (step 732), the algorithm determines if the obstructive event was a partial airway obstruction, an apnoea, or a hypopnoea with partial airway obstruction (step 734). In the event of a hypopnoea with a partial airway obstruction, the controller 9 increases pressure by one cm H2O (step 736) and waits ten seconds before allowing another pressure change (step 738). If the event was an apnoea, the controller 9 increases pressure by two cm H2O (step 740) and waits 60 seconds before allowing another pressure change (step 742). If the event was a partial airway obstruction, the controller 9 increases pressure by 0.5 cm H2O (step 744) and waits ten seconds before allowing another pressure change (step 746).
If the previous pressure decreases were more than one cm H2O but not more than 1.5 cm H2O (step 748), the algorithm determines if the obstructive event was a partial airway obstruction, an apnoea, or a hypopnoea with partial airway obstruction (step 750). In the event of a hypopnoea with partial airway obstruction, the controller 9 increases pressure by one cm H2O (step 752) and waits ten seconds before allowing another pressure change (step 754). If the event was an apnoea, the controller 9 increases pressure by two cm H2O (step 756) and waits 60 seconds before allowing another pressure change (step 758). If the event was a partial airway obstruction, the controller 9 increases pressure by 0.5 cm H2O (step 760) and waits ten seconds before allowing another pressure change (step 762).
If the previous pressure decreases were more than 1.5 cm H2O but not more than two cm H2O (step 764) (FIG. 9c), the algorithm determines if the obstructive event was a partial airway obstruction, an apnoea, or a hypopnoea with partial airway obstruction (step 766). In the event of a hypopnoea with partial airway obstruction, the controller 9 increases pressure by 1.5 cm H2O (step 768) and waits ten seconds before allowing another pressure change (step 770). If the event was an apnoea, the controller 9 increases pressure by two cm H2O (step 772) and waits 60 seconds before allowing another pressure change (step 774). If the event was a partial airway obstruction, the controller 9 increases pressure by one cm H2O (step 776) and waits ten seconds before allowing another pressure change (step 778).
If the previous pressure decreases were more than two cm H2O but less than or equal to 3.5 cm H2O (step 780), the algorithm determines if the obstructive event was a partial airway obstruction, an apnoea, or a hypopnoea with partial airway obstruction (step 782). In the event of a hypopnoea with partial airway obstruction, the controller 9 increases pressure by 1.5 cm H2O (step 784) and waits ten seconds before allowing another pressure change (step 754). If the event was an apnoea, the controller 9 increases pressure by two cm H2O (step 788) and waits 60 seconds before allowing another pressure change (step 790). If the event was a partial airway obstruction, the controller 9 increases pressure by 1.5 cm H2O (step 792) and waits ten seconds before allowing another pressure change (step 794).
If the previous pressure decreases were more than 3.5 cm H2O (step 796), the algorithm determines if the obstructive event was a partial airway obstruction, an apnoea, or a hypopnoea with partial airway obstruction (step 798). In the event of a hypopnoea with partial airway obstruction, the controller 9 increases pressure by one-half the total pressure decrease (step 800) and waits ten seconds before allowing another pressure change (step 802). If the event was an apnoea, the controller 9 increases pressure by one-half the total pressure decrease (step 804) and waits 60 seconds before allowing another pressure change (step 806). If the event was a partial airway obstruction, the controller 9 increases pressure by one-half the total pressure decrease (step 808) and waits ten seconds before allowing another pressure change (step 810).
While preferred embodiments of the present invention are shown and described, it is envisioned that those skilled in the art may devise various modifications of the present invention without departing from the spirit and scope of the appended claims.

## Claims

1. A method, of controlling positive airway pressure therapy, comprising the steps of:
providing a flow of gas to a patient's airway at a supply pressure that does not include forced oscillation at a frequency greater than a breathing rate of a patient;
obtaining information comprising a discrete energy spectrum from a frequency range of 0 to 25 Hz in a frequency domain of said flow;
generating at least one information-bearing value from said frequency domain information, said information-bearing value comprising at least an energy in a first harmonic of said discrete energy spectrum and an energy in a second harmonic of said discrete energy spectrum; and
adjusting said pressure based on said information.

2. The method of claim 1, further comprising the steps of calculating a period of a breath of a patient and calculating an inspiratory portion of said period, and wherein said adjusting step comprises comparing said at least one information-bearing value to one of a plurality of thresholds, said one of a plurality of thresholds being selected according to a ratio of said inspiratory portion to said period.

3. A method of controlling positive airway pressure therapy, comprising the steps of:
providing a flow of gas to a patient's airway at a supply pressure that does not include forced oscillation at a frequency greater than a breathing rate of a patient;
obtaining information comprising a discrete energy spectrum from a frequency range of 0 to 25 Hz in a frequency domain of said flow:
generating at least one information-bearing value from said frequency domain information, wherein said at least one information-bearing value comprises an energy in a first harmonic of said discrete energy spectrum, an energy in a second harmonic of said discrete energy spectrum, a sum of energies of said first harmonic through a thirteenth harmonic of said discrete energy spectrum, and a sum of energies of a fifth harmonic through said thirteenth harmonic of said discrete energy spectrum; and
adjusting said pressure based on said information.

4. The method of claim 3, further comprising the steps of calculating a period of a breath of a patient and calculating an inspiratory portion of said period, and wherein said adjusting step comprises comparing said at least one information-bearing value to one of a plurality of thresholds, said one of a plurality of thresholds being selected according to a ratio of said inspiratory portion to said period.

5. An apparatus for controlling positive airway pressure therapy, comprising:
a blower for providing a flow of gas to a patient at a pressure that does not include forced oscillation at a frequency greater than a breathing rate of a patient;
a sensor to measure a characteristic of said flow;
a controller to obtain information comprising a discrete energy spectrum from a frequency range of 0 to 25 Hz in a frequency domain of said characteristic and to generate at least one information-bearing value from said frequency domain information, wherein said at least one information-bearing value comprises at least an energy in a first harmonic of said discrete energy spectrum and an energy in a second harmonic of said discrete energy spectrum; and
a pressure regulator controlled by said controller for adjusting said pressure based on said information.

6. The apparatus of claim 5, wherein said controller further calculates a period of a breath of a patient and an inspiratory portion of said period, and further compares said at least one information-bearing value to one of a plurality of thresholds, said one of a plurality of thresholds being selected according to a ratio of said inspiratory portion to said period.

7. An apparatus for controlling positive airway pressure therapy, comprising:
a blower for providing a flow of gas to a patient at a pressure that does not include forced oscillation at a frequency greater than a breathing rate of a patient;
a sensor to measure a characteristic of said flow;
a controller to obtain information comprising a discrete energy spectrum from a frequency range of 0 to 25 Hz in a frequency domain of said characteristic and to generate at least one information-bearing value, wherein said at least one information-bearing value comprises an energy in a first harmonic of said discrete energy spectrum, an energy in a second harmonic of said discrete energy spectrum, a sum of energies of said first harmonic through a thirteenth harmonic of said discrete energy spectrum, and a sum of energies of a fifth harmonic through said thirteenth harmonic of said discrete energy spectrum.

8. The apparatus of claim 7, wherein said controller further calculates a period of a breath of a patient and an inspiratory portion of said period, and further compares said at least one information-bearing value to one of a plurality of thresholds, said one of a plurality of thresholds being selected according to a ratio of said inspiratory portion to said period.