Abstract:
A ventilator apparatus, method, system, and computer program for determining leakage in a flow circuit providing pressurized gas to a patient having breathing disorder. The present invention determines the leakage by calculating a ratio between a measured flow of gas and a determined flow of gas related to a standard leakage. The determined standard leak flow may be calculated from a formula derived from Bernoulli&#39;s theorem. The invention may further be arranged to use a volume difference between inspiration and expiration phases in the compensation process.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a method for determining the current leakage present in a ventilator and method for compensating for this leakage. 
       BACKGROUND OF THE INVENTION 
       [0002]    Patients suffering from different forms of breathing disorders can be subject to several types of treatments depending on the illness or disorder present. Such treatments include surgical procedures, pharmacologic therapy, and non-invasive mechanical techniques. Surgical techniques to remedy breathing disorders constitute a considerable risk for the patient and can lead to permanent injury or even mortality. Pharmacologic therapy has in general proved disappointing with respect to treating certain breathing disorders, e.g. sleep apnea. It is therefore of interest to find other treatments, preferably non-invasive techniques. 
         [0003]    A mechanical ventilator represents a non-invasive technique for treatment of certain breathing disorders such as ventilatory failure, hypoventilation, and periodic breathing during sleep and awake and in sleep apnea that occurs exclusively during sleep. Ventilatory failure includes all forms of insufficient ventilation with respect to metabolic need whether occurring during wake or periods of sleep. Hypoventilation and periodic breathing, in its most frequently occurring form referred to as Cheyne-Stokes ventilation, may occur periodically or constantly during wake or sleep. Conditions associated with hypoventilation, in particular nocturnal hypoventilation include e.g. central nervous system disorders such as stroke, muscular dystrophies, certain congenital conditions, advanced chronic obstructive pulmonary disease (COPD), etc. Cheyne-Stokes ventilation or various forms of central apnea are commonly associated with cardiac and circulatory disorders, in particular cardiac failure. 
         [0004]    Ventilatory failure is a potentially life threatening condition. The general comorbidity in patients with failing ventilation is considerable. The condition is highly disabling in terms of reduced physical capacity, cognitive dysfunction in severe cases and poor quality of life. Patients with ventilatory failure therefore experience significant daytime symptoms but in addition, the majority of these cases experience a general worsening of their condition during state changes such as sleep. The phenomenon of disordered breathing during sleep, whether occurring as a consequence of ventilatory failure or as a component of sleep apnea in accordance with the description above causes sleep fragmentation. Daytime complications include sleepiness and cognitive dysfunction. Severe sleep disordered breathing occurring in other comorbid conditions like obesity, neuromuscular disease, post polio myelitis states, scoliosis or heart failure may be associated with considerable worsening of hypoventilation and compromised blood gas balance. Sleep apnea has been associated with cardiovascular complications including coronary heart disease, myocardial infarction, stroke, arterial hypertension, thrombosis, and cardiac arrhythmia. It is therefore of both immediate and long-term interest to reduce the exposure to sleep disordered breathing. 
         [0005]    Recent advancement in mechanical non-invasive ventilator techniques includes administration of continuous positive airway pressure (CPAP) in different forms of sleep disordered breathing. During CPAP administration an elevated airway pressure is maintained throughout the breathing phase during a period coinciding with sleep. In sleep apnea this procedure may provide appropriate stabilization of the upper airway thereby preventing collapse. This, so called mono-level CPAP therapy, provides an almost identical pressure during inhalation and exhalation. Not only may CPAP be uncomfortable for the patient due to a sensed increased work of breathing during ventilation, specifically expiration. Some forms of apnea, mainly including those of central origin, and most forms of hypoventilation are only poorly controlled by CPAP. A more recently developed bi-level CPAP system administers different pressure levels during inhalation and exhalation. Bi-level CPAP provides increased comfort for most patients and not infrequently, an improved clinical response. Bi-level CPAP provides two pressure levels, Inspiratory Positive Airway Pressure (IPAP) and Expiratory Positive Airway Pressure (EPAP). IPAP is administered during the inhalation phase while EPAP is given during the exhalation phase. 
         [0006]    All ventilator systems exhibit leakage during administration of pressurized breathing gas and a suitable method for measuring the current leakage and compensating for the same is of interest. Several systems exists that measures and compensates for the leakage present in the ventilator/human setup. 
         [0007]    Some methods are using some specific sample points as references and thus depend strongly on the sample interval of the detection system. With a limited sample frequency there is a risk that the exact breathing cycle point is missed and a measurement is done slightly away from the correct point and thus a measurement is made that contains an error. Other systems determine the shift of the overall breathing cycle from a baseline. These systems typically give unstable feedback giving a compensation that moves up and down slowly continuously. 
         [0008]    One such method is described in the U.S. Pat. No. 6,945,248, where a method and apparatus for determining leak and respiratory airflow are disclosed. The non-linear conductance of a leak path is estimated dividing a low pass filtered instantaneous airflow by the low pass filtered square root of the instantaneous pressure. The value of the instantaneous leak is then obtained by multiplying the non-linear conductance by the square root of the instantaneous pressure. Finally, the respiratory air flow is calculated as the difference between the instantaneous air flow and the instantaneous leak flow. However, since an instantaneous leak flow is calculated from measured instantaneous values for the air-flow and the pressure, this method will suffer from the aforementioned deficiencies connected to unstable feedback. 
         [0009]    The object of the invention is to overcome some of the deficiencies associated with known technology. 
       SUMMARY OF THE INVENTION 
       [0010]    This object is achieved by a ventilator for supplying pressurized breathing gas which comprises a flow generator for producing pressurized breathing gas to be delivered to an interface, an interface for delivering the pressurized breathing gas to a patient, a first interface connected to the flow generator and arranged to deliver the breathing gas to a patient, at least one second interface connected to a processing unit and adapted to receive at least one signal indicative of the flow of pressurized breathing gas from the patient and to deliver the signal to a processing unit and a processing unit for controlling the pressure from the ventilator based on the signal received from the second interface, where the processing unit is arranged to compensate for leakage in the ventilator by using a ratio between the measured flow of pressurized breathing gas and a flow related to a reference standard leak. 
         [0011]    In one embodiment of the invention the at least one interface connected to the flow generator and arranged to deliver the breathing gas to a patient may be located in said ventilator. 
         [0012]    In one embodiment of the invention, the first interface for delivering the pressurized breathing gas to a patient may be connected to tubing or any other type of closed gas conductor suitable for delivering the pressurized breathing gas to the patient. 
         [0013]    In another embodiment of the invention the at least one second interface connected to a processing unit and adapted to receive at least one signal indicative of the flow of pressurized breathing gas from the patient and to deliver the signal to a processing unit. . 
         [0014]    As an option, the second interface above may also be arranged to receive signals indicative of the physiological state of the patient which for example may be data obtained from EEG, EMG, EOG and ECG-measurements, data indicative of the patient&#39;s eye movements, body temperature and other data suitable for characterizing the physiological state of the patient. The first and second interfaces may for example be wired or wireless interfaces. Also, processing unit may additionally comprise a computational device for analyzing the data received from the second interface. This computational device may also calculate the standard reference leak flow mentioned above by using the Bernoulli&#39;s equation for a stream in a tube and the fact that the energy going into the tube is equal to the energy going out of the tube. The mass flow may then be calculated according to the following formula: 
         [0000]    
       
         
           
             m 
             = 
             
               
                 ∫ 
                 
                   A 
                   c 
                 
               
                
               
                 ρ 
                  
                 
                     
                 
                  
                 
                   u 
                    
                   
                     ( 
                     
                       r 
                       , 
                       x 
                     
                     ) 
                   
                 
                  
                 
                     
                 
                  
                 
                    
                   Ac 
                 
               
             
           
         
       
     
         [0015]    where m is the mass flow through a pipe, p the volume density of the fluid in the pipe, u(r,x) the velocity profile for the fluid in the pipe and Ac the cross sectional area for the flow) and where said calculated mass flow is divided by pressure for said pressurized breathing gas to obtain a normalized mass flow. 
         [0016]    Of course the computational device may also be adapted to retrieve values for the standard reference leak flow from a table of values representing the standard reference leak flow at a certain pressure for the pressurized breathing gas. 
         [0017]    This approach would have the advantage of accelerating the calculation of the ratio between the measured instantaneous mass flow and the standard reference leak flow. 
         [0018]    In a further embodiment of the ventilator according to the present invention, the processing unit may additionally comprise a data storage unit for later analysis and inspection of the measured signals delivered by the second interface indicative of the instantaneous mass flow for the pressurized breathing gas, the physiological state of the patient and the aforementioned ratio between the measured signal indicative of the instantaneous mass flow for the pressurized breathing gas and a standard reference leak flow. This data storage unit may be a non-volatile memory device, such as for example a hard-disk or some other type of suitable memory device. 
         [0019]    In yet another embodiment of the invention the processing unit above may include a first communication device for communication with an external sensing device, such as a flow sensor. Also, the processing unit above may additionally include a second communication device for communication with the ventilator from en external computational device for retrieving data and results for analysis and/or inspection. 
         [0020]    These communication devices may be wired or wireless communication devices and may work according to different connection standards for wired or wireless communication. 
         [0021]    In another aspect of the present invention a ventilation system is provided, which comprises a mechanical ventilator for supplying pressurized breathing gas, a tubing for guiding the pressurized breathing gas connected to the mechanical ventilator, a device connected to the tubing for administrating the pressurized breathing gas to a patient, at least one sensing device arranged to measure at least a signal indicative of the instantaneous flow for the pressurized breathing gas and further arranged to send the signal to the mechanical ventilator and a processing unit arranged to receive the signal indicative of flow for controlling the pressure or flow from the mechanical ventilator, where the processing unit is arranged to compensate for leakage in the ventilator system using a ratio between said measured flow of pressurized breathing gas and a flow related to a reference standard leak. 
         [0022]    In one embodiment of the invention, the sensing device above may comprise a flow sensor. This flow sensor may be located either in or nearby the mechanical ventilator mentioned above or nearby the device connected to the tubing for administrating the pressurized breathing gas to a patient mentioned above. 
         [0023]    One may also arrange two such flow sensors, one nearby the interface for receiving at least one signal indicative of the flow of pressurized breathing gas. In this fashion one could measure the flow of the breathing gas by calculating the difference between the flow measured by the flow sensor near the mechanical ventilator and the flow measured by the sensor near the patient interface, which for example may be a face mask or the like. 
         [0024]    In another embodiment of the invention the device connected to the tubing for administrating the pressurized breathing gas to a patient may be a breathing mask, where such a breathing mask may cover the face or the nose of the patient. Also, the mask may only cover the nose or the nostrils of the patient. However, instead of using such a mask, it is also possible to use a hood covering a part or the whole of the patient&#39;s body. 
         [0025]    The advantage of a mask would be the relatively easy positioning of the mask on the patients face and the small cost involved in using face masks. 
         [0026]    One advantage of using the hood would be an even better control of the leakage occurring due to the imperfect fit of the mask or hood administering pressurized breathing gas to the patient. 
         [0027]    In yet another aspect of the present invention a method for determining current leakage in a ventilator is provided, where the method comprises the steps of 
         [0028]    measuring the mass flow through the ventilator 
         [0029]    comparing values from a standard leak calculation for a standard leak in the ventilator, 
         [0030]    where the a ratio between the measured mass flow through the ventilator and the values from a standard leak calculation for a standard leak flow in the ventilator is calculated and where the difference between the measured mass flow and the calculated standard leak flow is compensated for and the current leakage from said comparison is determined. 
         [0031]    It is also contemplated to use the calculated ratio between the measured mass flow through the ventilator and the values from a standard leak calculation for a standard leak in the ventilator as basis for compensation for the difference between the measured mass flow and the calculated standard leak flow. 
         [0032]    In one embodiment of the method according to the present invention some further substeps may be included, such as the sampling instantaneous values for the mass flow through the ventilator and calculation of a ratio between each sampled value for the instantaneous mass flow and a corresponding value for the standard leak flow. 
         [0033]    Further step may also provide for sampling values for the mass flow through the ventilator during one predetermined time period, calculating a ratio between sampled mass flow values above and corresponding standard leak flow values during said predetermined time period, calculating a mean value for the ratio by integrating the ratio over the predetermined time period measured and dividing it by the number of flow values sampled and calculating mass flow through the ventilator using a known relation between the mean value for the flow ratio and a standard leak flow. 
         [0034]    The standard leak flow may thereby be calculated from Bernoulli&#39;s equation along a stream in a tube and the use of the energy conservation principle as already explained previously. 
         [0035]    The efficiency of the method described above may be further enhanced by calculating the mean value for the aforementioned ratio according to the steps of: 
         [0036]    calculating a volume for the inspiration—and the expiration phases of a patient 
         [0037]    determining the volume difference between the inspiration—and expiration phases of the patient 
         [0038]    calculating the actual flow rate based on the volume difference 
         [0039]    calculating a ratio between the actual flow rate based on the volume difference and a standard leak flow and adding the ratio between the actual flow rate based on said volume difference and a standard leak flow and the mean value for said ratio obtained through integration over a pre-determined time period as described previously. Thus, the value for the volume difference between the inspiration—and expiration phases of the patient can be used to further enhance the stability of the feedback to compensate for leakage and to hold the compensation stable if the leakage is changed during operation. 
         [0040]    The method according to the present invention is especially suited to be implemented by the ventilator and the ventilation system described above. 
         [0041]    In yet another aspect of the present invention a computer program for determining a leakage in a ventilator system is provided, where the computer program comprises instruction sets for obtaining data indicative of a first mass flow of breathing gas through the ventilator system, obtaining the first mass flow through the ventilator and the second standard leak flow in the ventilator system and an instruction set for determining a leakage in the ventilator system. 
         [0042]    The computer program is specially suited to implement the method steps indicated above and to receive signals from and to control building parts included in the ventilator and the ventilation system according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0043]    In the following the invention will be described in a non-limiting way and in more detail with reference to exemplary embodiments illustrated in the enclosed drawings, in which: 
           [0044]      FIG. 1  illustrates schematic of a breathing circuit system according to the present invention; 
           [0045]      FIG. 2  is a schematic block diagram of a ventilator apparatus according to the present invention; 
           [0046]      FIG. 3  illustrates a measured and standard flow curve versus pressure; 
           [0047]      FIG. 4  illustrates a flow schematic according to the present invention; 
           [0048]      FIG. 5  illustrates a schematic breathing cycle; 
           [0049]      FIG. 6  illustrates a schematic block diagram of a method according to the present invention; and 
           [0050]      FIG. 7  illustrates in a schematic block diagram another embodiment of the method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0051]    In  FIG. 1  a schematic mechanical ventilation system used for the treatment of hypoventilation disorders is depicted. A ventilation system comprise a mechanical ventilator  4  supplying pressurized breathing gas, tubing  3  for guiding breathing gas to the patient  1 , a breathing mask  2  or similar for administrating the breathing gas to the patient  1 , sensing means  5 ,  6 ,  7 ,  8 ,  9  and  10  for determining the physiological status of the patient  1 . The number of sensors connected to the mechanical ventilator may be one or more; however, in a preferred embodiment of the present invention at least one sensor is necessary: a breathing gas flow measurement which may be located essentially anywhere along the breathing gas tubing or in the mask. A mechanical ventilator  4  is supplying breathing gas for instance as a positive airway pressure via a tubing  3  and through a mask  2  to a patient  1 . The mask  2  can be a face mask  2  covering both the mouth and nose or a nasal mask covering only the nose or nostrils depending on the patients needs. It can also be a hood covering the complete head or body of the patient. 
         [0052]    The breathing gas may be of any suitable gas composition for breathing purposes as understood by the person skilled in the art, the composition may depend on the physiological status of the patient and the treatment of interest. 
         [0053]    The pressure or flow from the ventilator  4  is controlled by a processing unit  11  as shown in  FIG. 1 . The processing unit  11  may involve a computer program that receives one or several input parameters  5 ,  6 ,  7 ,  8 ,  9 , and  10  obtained from the patient  1  describing the physiological status of the patient and pressure/flow data indicative of breathing gas system configuration and status. Data indicative of patient status is obtained using sensors  5 ,  6 ,  7 ,  8 ,  9 , and  10  connected to the patient and transferred to the processing unit  11  via connection means  5   a,    6   a,    7   a,    8   a,  and  9   a  (connection means for sensor  10  is not depicted in  FIG. 1  since the sensor may be placed at several different locations, such as inside the ventilator apparatus) and an interface ( 15 ) in the ventilator ( 4 ). These input parameters may be for instance flow or pressure signals, data obtained from EEG, EMG, EOG, and ECG measurements, O 2  and/or CO 2  measurements in relation to the patient, body temperature, blood pressure, SpO 2  (oxygen saturation), eye movements, and sound measurements. It should be understood that the invention is not limited to the above mentioned input parameters but other input parameters may be used. In  FIG. 1  not all sensors  5 ,  6 ,  7 ,  8 ,  9 , and  10  and sensor connection means  5   a,    6   a,    7   a,    8   a,  and  9   a  are depicted, only a subset is shown in order to illustrate a schematically view of the system and the depicted locations are only given as examples and are in no way limiting to the invention, e.g. the flow signal may be measured at either the mask location or close to the mechanical ventilator or at both locations in order to deduce a differential signal if this is required. 
         [0054]    The flow sensor  10  may be located at several different positions, e.g. in the breathing air tubing  3  at any suitable position, such as close to the mechanical ventilator apparatus (or even within the ventilator housing) or in the vicinity of the mask. 
         [0055]    The input data is then supplied to a processing unit  11  via the interface ( 15 ). 
         [0056]    In  FIG. 2 , the processing unit  200  comprises at least computational means  201 , where the computational or processing means  201  analyses the measured data, preferably data from the flow measurement, according to an appropriate method, algorithm or algorithms (to be discussed in detail below) in order to determine an appropriate response and send control signal or signals to a mechanical ventilator unit  12 . This mechanical ventilator unit  12  may be a fan  12  arranged to deliver appropriate amounts of breathing gas at specified and controlled pressure levels. The processing means may for instance be a microprocessor, computer, workstation, FPGA (Field programmable array), or ASIC (Application Specific Integrated Circuit). The processing unit may be built into the ventilator or be located external of the ventilator in a stand alone unit. 
         [0057]    The processing unit  200  may also comprise a data storage unit  202  for post analysis and inspection and also a connection for an internal or external non-volatile memory device, like for instance a memory device using a USB connection, an external hard drive, a floppy disk, a CD-ROM writer, a DVD writer, a Memory stick, a Compact Flash memory, a Secure Digital memory, an xD-Picture memory card, or a Smart Media memory card. These are only given as examples, and are not limiting for the invention, many more non-volatile memory devices may be used in the invention as appreciated by the person skilled in the art. 
         [0058]    The mechanical ventilator  12  may also have input means (not shown) for manually setting control parameters and other parameters necessary for the operation of the device. 
         [0059]    Through a first and a second communication means  206  and  207  illustrated in  FIG. 2  it is possible to communicate with the device  4  to and from an external computational device or one of the flow sensors ( 5 ,  6 ,  7 ,  8 ,  9 ,  10 ) for retrieving data and results for immediate and/or later analysis and/or inspection. The communication means can be of a serial type like for instance according to the standards RS232, RS485, USB, Ethernet, or Fire wire, or of a parallel type like for instance according to the standards Centronics, ISA, PCI, or GPIB/HPIB (General purpose interface bus). It may also be any wireless system of the standards in the IEEE 802.11, 802.15, and 802.16 series, HiperLAN, Bluetooth, IR, GSM, GPRS, or UMTS, or any other appropriate fixed or wireless communication system capable of transmitting measurement data. It can also be of any proprietary non-standardized communication formats, whether it is wireless or wired. 
         [0060]    The ventilator device  4  may also have display means (not shown) for displaying measured data and obtained response parameters for use by a physician, other medical personnel, or the patient. The display means may be of any normal type as appreciated by a person skilled in the art. The data is displayed with such a high rate that a real time feedback is provided to a person monitoring the ventilator characteristics and function for immediate feedback and control. 
         [0061]      FIG. 4  is a schematic of flow related issues in a ventilator/human setup, i.e. a ventilator connected to a patient. A ventilator is connected to a hose or tubing  402  delivering a pressurized breathing gas; this hose  402  is in turn connected to a patient ( 430 ) using a suitable mask or similar device. However, a leak  420  may be present, for instance due to that the mask does not fit exactly to the patient ( 43 ) or the patient ( 430 )has the mouth opened slightly. 
         [0062]    The current flow is sampled at the ventilator side of the hose or within the ventilator with a certain frequency and in each sample point a ratio between the measured flow and a reference standard leak flow is determined (however, the flow may also be optionally measured at the mask side of the ventilator system). This difference between the measured flow and the standard leak flow is shown in  FIG. 3 , where the upper curve shows the measured flow  310  and the lower curve the calculated flow for a standard leak  320  at a certain pressure. The area bordered by the curved and the two straight arrows depicts the measured flow  310  for one breathing cycle  330 . 
         [0063]    This series of ratio measurements is shown in  FIG. 5  for two breathing cycles.  510  depict the start of the inspection of the ratio measurements and  510  the average calculation period, which in this case is the length of breathing cycle of the patient. An average of a breathing cycle can than be determined by integrating over a cycle and dividing with the integration number (i.e. number of samples). By adding or subtracting the mean value from the flow control parameter it is possible to compensate for this average error determined from the ratio calculation. This can be done by adding the necessary flow to the entire breathing cycle. 
         [0064]    In an embodiment of the present invention, a method is provided for determining the flow leak and compensating for the same as shown in  FIG. 6 , this method can be implemented both in hardware and in software as understood by the person skilled in the art. 
         [0065]    At step  600  the sampling of data is started and sample points from the breathing cycle of the patient are gathered are gathered. 
         [0066]    At the next step  610  a ratio between the measured instantaneous mass flow for the pressurized air delivered to the patient and the calculated reference leak flow at a certain pressure is built. The values for the reference leak flow at a certain pressure may be stored in a table and simply accessed when calculating the ratio above. 
         [0067]    In case one is interested in measuring the ratio over a full breathing cycle of the patient, a mean ratio is calculated at step  620 , where the ratio is integrated over a full breathing cycle of the patient and divided by the number of samples taken during the breathing cycle. 
         [0068]    The mass flow for the pressurized air is then calculated at step  630 , where a known relation for the ratio between the measured instantaneous mass flow for the pressurized air and the reference leak flow and the reference leak flow is used. 
         [0069]    If the mass flow for the pressurized breathing gas has changed since the last measurement, the trigger baseline for the breathing cycle of the patient is adjusted at step  640 , either upwards or downwards depending on whether the mass flow has decreased or increased. 
         [0070]    In another embodiment of the method according to the present invention shown in  FIG. 7 , the above mentioned method is combined with a volume measurement method. It should be mentioned that steps  700  to  720  are identical with the steps  600  to  620  from  FIG. 6 . 
         [0071]    At step  722 , the total volume of the pressurized gas administered to the patient is calculated. 
         [0072]    Then, at step  724  the difference between the volume of the pressurized breathing gas during the inspiration and the expiration phases of the patient is calculated, which is used at step  726  to calculate the flow rate of the pressurized breathing gas. 
         [0073]    At step  728 , a ratio delta is calculated between the flow rate during the inspiration and the expiration phases of the patient. 
         [0074]    Finally, at step  730 , the ratio delta above is added to the mean ratio between the measured instantaneous mass flow and the standard reference leak flow for the pressurized breathing gas. 
         [0075]    The use of the extra delta parameter servers to further enhance the stability of the feedback to compensate for leakage and hold the compensation stable if the leakage is changed during operation. The system will determine the leakage and adjust the control parameters in such a way that it will be compensated for in a few breathing cycles. 
         [0076]    It should be noted that the word “comprising” does not exclude the presence of other elements or steps than those listed and the words “a” or “an” preceding an element do not exclude the presence of a plurality of such elements. It should further be noted that any reference signs do not limit the scope of the claims, that the invention may at least in part be implemented by means of both hardware and software, and that several “means” may be represented by the same item of hardware. 
         [0077]    The above mentioned and described embodiments are only given as examples and should not be limiting to the present invention. Other solutions, uses, objectives, and functions within the scope of the invention as claimed in the below described patent claims should be apparent for the person skilled in the art.