METHODS AND APPARATUS FOR CONTROLLING MASK LEAK IN CPAP TREATMENT

BACKGROUND OF THE INVENTION

[0001] This application claims the benefit of Australian Application No.

AU2006900774, filed February 16, 2006, Australian Application No. AU2005903089, filed June 14, 2005, Australian Application No. 2005906122, filed November 4, 2005, and Australian Application No. 2005906193, filed November 8, 2005, each of which is incorporated herein by reference in its entirety.

1. Field of the Invention

[0002] This invention relates to mechanical ventilation of sleep disordered breathing

(5DB), and in particular to methods and apparatus for improving patient compliance in Continuous Positive Airway Pressure (CPAP) treatment.

2. Description of Related Art

[0003] A comprehensive background discussion of mechanical ventilation can be found in "Principles and Practice of Mechanical Ventilation" (1994) edited by Martin J Tobin, published by McGraw-Hill Inc., ISBN 0-07-064943-7.

[0004] The use of nasal Continuous Positive Airway Pressure (CPAP) to treat

Obstructive Sleep Apnea (OSA) was invented by Cohn Sullivan, see U.S. Patent No. 4,944,310. Generally, the treatment involves providing a supply of air or breathable gas from a blower to a patient via an air delivery conduit and a patient interface, such as a full-face or nasal mask, or nasal prongs. While treatment is effective, some patients find it uncomfortable. Improving patient comfort and compliance is a continuing challenge.

[0005] One way to improve comfort is to provide a more comfortable patient interface. In this regard, the ResMed MIRAGE™ masks have provided significant improvement in comfort. See U.S. Patent Nos. 6,112,746; 6,357,441; 6,581,602; and

6,634,358. A more recent development is the ResMed MIRAGE™ ACTIVA™mask series. See International Patent Application WO 2001/97893.

[0006] In the early days of nasal CPAP systems for treating OSA, patients were first titrated in a clinical study to determine an optimal treatment pressure. Titration involves a patient sleeping overnight in a clinic and being tested with a mask and CPAP device. The treatment pressure provided by the CPAP device is adjusted until apneas are eliminated. The treatment pressure is usually in the range of 4-20CmH ₂ O. A device would be set to that pressure and given to the patient to take home. A subsequent development was the automatically adjusting device that a patient could take home. The automatically adjusting device will raise and/or lower the treatment pressure based on indications of obstructive sleep apnea, such as snoring. Such CPAP devices are sometime generically referred to as Automatic Positive Airway Pressure (APAP) devices. See U.S. Patent Nos. 5,245,995; 6,398,739; and 6,635,021.

[0007] Another type of nasal CPAP device provides a first pressure during inhalation

(sometimes termed an IPAP) and a second, lower pressure during exhalation (sometimes termed an EPAP). Examples of these include the ResMed VP AP™ series, and the Respironics BiPAP series. Bilevel CPAP devices may be prescribed for patients who do not comply with single pressure CPAP devices. Some patients perceive that the lower pressure during exhalation is more comfortable, at least while they are awake.

[0008] Another way of improving patient comfort and compliance is to start each therapy session at a low therapeutic pressure, e.g., 4cmH ₂ O, and ramp up to fill therapeutic pressure over the first hour, to allow the patient to adjust to the sensation while falling asleep. Alternatively, the device may set to implement a time delay before fill therapeutic pressure is applied, to allow the patient time to fall asleep before full therapeutic pressure is applied. See U.S. Patent Nos. 5,199,424 and 5,522,382.

[0009] Another form of automatically adjusting CPAP device is the ResMed

AUTOSET™ SPIRIT™ device. In this device, the CPAP pressure is automatically increased or decreased in accordance with indications of flow limitation, such as flow flattening, snore, apnea and hypopnea. See U.S. Patent Nos. 5,704,345; 6,029,665; 6,138,675; and 6,363,933.

An advantage of an automatically adjusting system is that over time the treatment pressure required may vary for a particular patient and a correctly functioning automatic system can obviate the need for the patient to return for a subsequent sleep study. These patents also describe a method and apparatus for distinguishing between so-called "central" and obstructive apneas.

[0010] The contents of all of the aforesaid patents are incorporated by cross- reference.

[0011] Some OSA patients find treatment with the above devices uncomfortable and they become non-compliant with the therapy. Other patients such as cardiovascular patients with Congestive Heart Failure, patients with REM Hypoventilation, and patients with Respiratory Insufficiency could also benefit from a more comfortable and/or effective form of therapy.

[0012] One hurdle to patient acceptance of the therapy is in the initial fitting of the patient interface (e.g., mask), where the mask fit is tested under relatively low pressure and high flow. This may result in noisy operation of the device and high flow air leaks as the mask is adjusted to the patient, which can be a noisy and unsettling initial experience for the patient.

[0013] Another impediment to patient comfort and compliance may be the patient or bed partner being disturbed by air leaks from the mask.

SUMMARY _. OF THE INVENTION

[0014] One aspect of the invention relates to a method and apparatus to overcome or ameliorate one or more of these disadvantages.

Mask leak control

[0015] A first aspect of the invention relates to a method of operating a device for treating sleep disordered breathing (SDB), wherein said device provides continuous positive airway pressure during sleep, the method comprising :

determining whether treatment pressure at the patient interface is below a desired value;

in response to said determining, increasing pressure generated by the flow generator;

in response to said increasing, making a determinatiori that the increase in flow generator pressure has resulted in less than a desired increase in treatment pressure at the patient interface; and

in response to said determination, reducing or inhibiting further increase of the flow generator pressure.

[0016] A further aspect of the invention relates to a method of operating a device for treating sleep disordered breathing (SDB), wherein said device provides continuous positive airway pressure during sleep, the method comprising :

determining whether treatment pressure at the patient interface is below a desired value;

in response to said determining, increasing pressure generated by the flow generator;

in response to said increasing, making a determination that the increase in flow generator pressure has resulted in a greater than desired increase in air leak from the patient interface, or has increased the air leak from the patient interface; and

in response to said determination, reducing or inhibiting further increase of the flow generator pressure.

[0017] In one form, making a determination is made by comparison of a mask leak parameter to a mask leak threshold. The mask leak parameter may be a mask leak impedance parameter.

[0018] In an embodiment, the mask leak threshold is determined with reference to a patient arousal index, such as an Apnea Hypopnea Index (AHI), and/or a sleep phase index of the patient.

[0010] A further aspect of the invention relates to a method of operating a device for treating sleep disordered breathing (SDB), wherein said device provides continuous positive airway pressure during sleep, the method comprising:

comparing a mask leak parameter to a mask leak threshold to determine excessive leak at the patient interface;

in response to said determination of excessive leak, reducing the flow generator pressure; and

continuing to compare said mask leak parameter to said mask leak threshold at said reduced flow generator pressure.

[0020] In an embodiment, the mask leak parameter is a mask leak impedance parameter.

[0021] In one embodiment, reducing the flow generator pressure includes reducing the pressure to a ramp start pressure, and the method further includes ramping up the pressure to a treatment pressure over a predetermined ramp time while continuing to compare the mask leak parameter to the mask leak threshold.

[0022] The method may further include halting the pressure ramping upon further determination of excessive leak at the patient interface.

[0023] In another embodiment, reducing the pressure includes reducing the pressure by a pressure increment to a first reduced pressure, and optionally incrementally further reducing the pressure if excessive leak is detected at the first reduced pressure.

[0024] A further aspect of the invention relates to a method of assessing the fit of a patient interface (e.g., a mask) to a patient for a device for treating sleep disordered breathing (SDB), wherein said device provides continuous positive airway pressure during sleep, the method comprising : generating a treatment pressure at a flow generator of the device;

limiting gas flow applied to the patient to a predetermined flow rate;

determining that treatment pressure at the patient interface is below a desired value; and

making a determination that the seal of the patient interface to the patient is inadequate.

[0025] In an embodiment, the flow rate is determined with reference to a vent flow rate of the patient interface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings facilitate an understanding of the various embodiments of this invention. In such drawings:

[0027] Fig. 1 illustrates a ventilator apparatus according to an embodiment of the present invention for implementing methods according to embodiments of the invention;

[0028] Fig. 2 is a graph of a mask pressure against blower set pressure, showing mask leak; and

[0029] Fig. 3 is a flowchart illustrating mask leak control according to an embodiment of the invention.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Hardware

[0030] A positive airway pressure (PAP) device in accordance with an embodiment of the invention includes a blower and blower-controller. The blower can deliver a supply of air at positive pressure 2-40 CmH ₂ O, but generally in the range of 4-20 CmH ₂ O to a patient interface via an air delivery conduit.

[0031] The device also includes a flow sensor to measure the flow of air along the conduit, and pressure sensors to measure the pressure of air at the blower outlet.

[0032] In one form, the device alternatively includes an additional pressure sensor to detect the pressure in the patient interface.

[0033] For example, Fig. 1 illustrates a ventilator device according to an embodiment of the invention. As illustrated, the ventilator device may include a servo-controlled blower 2, a flow sensor 4f, pressure sensor 4p, a mask 6, and an air delivery conduit 8 for connection between the blower 2 and the mask 6. Exhaust gas is vented via exhaust 13.

[0034] Mask flow may be measured by a flow sensor, such as a pneumotachograph and differential pressure transducer to derive a flow signal F(t). Alternatively, the pneumotachograph may be replaced by a bundle of small tubes aligned in parallel with the flow from the blower with the pressure difference measured by the differential pressure transducer across the bundle.

[0035] Mask pressure is preferably measured at a pressure tap using a pressure transducer to derive a pressure signal P _ma sk(t). The pressure sensor 4p and flow sensor 4f have been shown only symbolically in Fig. 1 since it is understood that those skilled in the. art would understand how to measure flow and pressure.

[0036] Flow F(t) and pressure P _ma sk(t) signals are sent to a controller or microprocessor - referred to herein as processor 15 - to derive a pressure request signal

PRequ _es t(t)- The controller or processor is configured and adapted to perform the methodology described in more detail herein. The controller or processor may include integrated chips, a memory and/or other instruction or data storage medium to implement the control methodology. For example, programmed instructions with the control methodology are either coded on integrated chips in the memory of the device or loaded as software. As those skilled in the art will recognize, analogue devices may also be implemented in the control apparatus.

[0037] The controller or processor 15 is further adapted to derive parameters indicative of the patient's breathing and sleep pattern, such as for deriving indications of flow limitation, such as flow flattening, snore, apnea and hypopnea and the Apnea Hypopnea Index (AHI), and for distinguishing between REM and non-REM sleep. See U.S. Patents 5,704,345 and 6,029,665.

[0038] The apparatus of Fig. 1 includes other sensors, communication interfaces and displays, a servo, etc., and functional blocks the details of which are not necessary for an understanding of the present invention.

[0039] In an embodiment, the controller may be programmable by the clinician using the menu system to alter the parameters of the 'first timer' mode, for example, to set the initial therapy session pressure and/or the daily pressure increment according to the severity of the patient's sleep disordered breathing and the clinician's opinion of how long the patient may take to acclimatize to the sensations of the CPAP therapy.

[0040] At the commencement of the therapy session, the session timer of the controller is started.

[0041] Where the flow generator is of the type which provides for this, and where this feature is selected, the treatment pressure is ramped up over a 5 to 20 minute period at the beginning of each treatment session, starting at a low pressure, for example 4cmH ₂ O, and ramps up to the desired full therapeutic pressure for the therapy session.

Mask leak control

[0042] In an embodiment of the invention, described with reference to Figs.

2 and 3, the response of the blower to changes indicative of excessive air leakage at the patient interface is controlled for improved patient comfort and compliance and reduced possibility of disturbance.

[0043] Fig. 2 is a schematic graph of mask pressure against flow generator pressure illustrating mask leak where the mask is not sitting properly and sealing completely on the patient's face, for example where the mask and headgear has not been optimally adjusted or where the patient may have partially dislodged the mask during sleep.

[0044] With reference to Fig. 2, the blower set pressure is on the x axis and the mask pressure on they axis.

[0045] It can be seen that the pressure lost due to the mask leak is the vertical distance between the mask pressure curve and the 45° line. In a typical situation, the mask leak at lower blower pressures will be approximately linear with blower pressure, but above a certain blower pressure will begin to depart more significantly from the blower pressure as the increased pressure and flow causes a significant increase in mask leakage. The point at which this departure commences, and the gradient of this departure, will depend on the type and fit of the mask to the patient, and may also vary from one session to the next, or within a therapy session, depending on the patient's sleeping position.

[0046] In prior art APAP machines, air leakage at the mask is detected by a drop in the pressure at the nasal interface, and the response of the blower as governed by the controller is to increase the blower pressure to compensate for the leak and thus maintain adequate treatment pressures to normalize the patient's AHI. However, in some instances this increased pressure merely serves to further reduce the sealing of the mask onto the patient's face, and the increased pressure is wholly or largely lost by increased leakage. In such instances, the volume of air being leaked is increased with little or no net benefit to efficacy of the therapy, and with increased chance of arousal of or disturbance to the patient and/or the bed partner.

[0047] In the present embodiment of the invention, the response of the blower to detection of mask leak is modified.

[0048] Fig. 3 is a flowchart illustrating the steps of the mask leak control method according to an embodiment of the invention.

[0049] At first detection of excessive mask leak (step 301), for example as determined by low mask pressure, by an excessive differential between the blower pressure and the pressure at the mask or by a computed mask impedance parameter below a certain threshold, the blower controller causes an increase in the blower pressure (step 302) to compensate for the leak and maintain the therapeutic pressure to desired levels, thus moving further to the righf along the mask pressure curve of Fig. 2.

[0050] Another determination of mask pressure is made (step 303) and, if the increased blower pressure has corrected the mask pressure to compensate for the leakage then the higher blower pressure is continued (step 304). The patient continues to be monitored for arousal (step 305).

[0051] If however the increased blower pressure has resulted in less than a predetermined pressure increase in the mask, or a greater than predetermined flow volume increase, this means that the blower is operating at a low-gradient part, of the mask pressure curve of Fig. 2. In this case, the controller is programmed to then recognize that the pressure increase has wholly, or mostly, been ineffectual due to increased leakage at the mask.

[0052] When detecting this condition, the controller is programmed to then reduce the blower pressure (step 306) (for example, by a set percentage of the treatment pressure or to the ramp start pressure) or to inhibit further increases in the flow generator output pressure.

[0053] The mask leak threshold against which the increased mask leak is compared

(step 308) may be a fixed quantity or proportion, for example where a 2 CmH ₂ O increase in blower results in less than a 1 cmH ₂ 0 pressure increase at the mask.

[0054] Alternatively, the mask leak threshold may vary as a function of a patient sleep phase indicator so that a greater amount of mask leakage is tolerated if the patient is in a heavier phase of sleep and thus less likely to be disturbed, or if other indicators of disturbance of the patient — such as respiratory rate — remain low. Indicators of sleep phase are known per se in sleep medicine and include monitoring of brain waves and/or respiration.

[0055] In a further alternative form of this embodiment, the mask leak threshold may vary as a function of indications of flow limitation, such as flow flattening, snore, apnea and hypopnea, or the Apnea Hypopnea Index (AHI), so that a greater amount of mask leakage is tolerated if the patient is more in need of higher treatment pressure. The blower controller is preferably programmable by the clinician to adjust the mask leak threshold, or turn off the mask leak control function, for patients with historically high flow limitation measurements.

[0056] By controlling the response of the flow generator to mask leak to operate the apparatus at a lower mask leak portion of the mask pressure/flow generator pressure curve of Fig 2, the amount of flow generator energy wasted in producing undesirable outcomes of unwanted mask leak and noise is minimized.

[0057] By reducing or capping the blower set pressure in circumstances of high mask leakage, rather than increasing the pressure further as was done in the prior art, disturbance of the patient and/or bed partner by the noise or air flow caused by excessive air leakage is minimized. While the therapy may continue at sub-optimal pressure, it may continue to provide some airway support and is considered that this may in many instances be preferable to waking the patient or having the patient remove the mask during sleep. The former is a substantial patient comfort and compliance issue and the latter may deny the patient the entire benefit of the therapy. It is expected that, by reducing patient arousals, total sleep time and adherence to the treatment can be improved.

[0058] Furthermore, by reducing the blower set pressure in response to excessive mask leak, the mask may settle back into improved conformity with the patient's face and the mask leakage be reduced, thus altering the mask pressure curve shown in Fig. 2 to one closer to the 45° line.

[0059] The controller may be programmed to make one or more further attempts to ,

ramp up blower pressure after a predetermined time periods, for example 15 minutes, have elapsed to reassess the quality of the mask-to-patient seal.

[0060] The controller may also be programmed to monitor in real time the patient's

AHI and other patient arousal and flow limitation indicators, to detect any reduction in effectiveness of therapy at the lower pressure. If the indicators show increased flow limitation and/or patient arousal at the lower treatment pressure, indicative of insufficient treatment pressure for airway stability, the controller will re-increase the blower pressure (step 307).

[0061] Patient arousal may also be sensed by monitoring body position. Continual movement, if detected in conjunction with high leak, may indicate patient arousal due to mask leak.

[0062] . Snoring is indicative of airway narrowing, and may be used as an indicator that the treatment pressure is too low and must be increased regardless of leak.

[0063] In essence, this embodiment seeks to apply the lowest pressure required to treat the patient effectively and limits maximum pressure in circumstances of mask leak, whereas previous treatments did only the former. In this way, a balance is achieved between patient arousal due to mask leak and effectiveness of the treatment, to help maintain patient compliance with the therapy.

[0064] The controller keeps a record of incidents where the blower pressure is reduced in response to excessive mask leak, for subsequent review by the clinician to help with mask selection and adjustment for the patient.

[0065] Also, at the end of a session where excessive mask leak has been detected, the controller may cause to be displayed on the machine a message alerting the patient to the need to adjust the fit of the mask, and/or to contact the clinician.

[0066] The CPAP machine may have one or more leak control modes. If the CPAP machine comprises a plurality of modes a control system may control selecting between these modes depending for example on patient preference and compliance levels, hi a first of these modes, which for example may be selected for less compliant patients, the machine on

detection of a leak reduces the treatment flow/pressure and maintains the pressure at a level below full-therapeutic to allow the mask to seal against the patient's face.

[0067] In the second mode of the leak control method, the flow/pressure is reduced to a sub-therapeutic pressure for a brief predetermined period, for example 15-60 seconds, to allow the mask to reseal against the patient's face, and then is ramped back up to therapeutic pressure. For example, upon detection of mask leak in excess of the mask leak threshold, the pressure may be reduced to the ramp start pressure used for the commencement of the therapy session, and then ramped back up to full therapeutic pressure over a shortened ramp time, such as 50% of the ramp time used at the commencement of therapy.

[0068] In a further mode, upon detection of mask leak in excess of the mask leak threshold, the pressure may be reduced by a set increment, e.g., by lcmHaO, and the mask leak parameter is measured again for a set period of time. If the leak remains above the threshold, the pressure is further incrementally reduced, the mask leak measured again, and so on until the leak has been corrected or fallen below the mask leak threshold.

[0069] After a predetermined period of operation below the mask leak threshold, the controller re-increases the flow generator pressure to re-assess the mask-to-patient seal, as previously discussed.

[0070] In a further embodiment, a mask seal testing and mask fitting regimen is provided wherein the mask is fitted to the patient in a clinical setting and the device switched to a "mask fitting" operational mode. In this mode, the flow generator is controlled to apply a normal range treatment pressure, with the flow rate limited to a flow approximately equal to or slightly above the vent flow rate for the mask.

[0071] If the mask seals well, the pressure at the mask will be at or near full therapeutic pressure. If there is a substantial mask leak, this may be detected by insufficient pressure in the mask, without the noisy operation and large volume air leaks which the prior art low pressure-high volume mask fitting modes regimens. In this way, the mask fit may be adjusted without undue noise and a large volume of air flow rushing past the patient's face, and a further hurdle to patient adoption of and compliance with the therapy is reduced.

[0072] In this specification, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of. A corresponding meaning is to be attributed to the corresponding words "comprise, comprised and comprises where they appear.

[0073] While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention. Also, the various embodiments described above may be implemented in conjunction with other embodiments, e.g., aspects of one embodiment may be combined with aspects of another embodiment to realize yet other embodiments. In addition, while the invention has particular application to patients who suffer from OSA, it is to be appreciated that patients who suffer from other illnesses (e.g., congestive heart failure, diabetes, morbid obesity, stroke, barriatric surgery, etc.) can derive benefit from the above teachings. Moreover, the above teachings have applicability with patients and non-patients alike in nonmedical applications.