Patent Publication Number: US-2023157575-A1

Title: Systems for evaluating respiratory function using forced oscillation technique (fot) oscillometry

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. 119(e) of U.S. provisional application No. 63/282,409, filed Nov. 23, 2021, the contents of which are incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     Forced oscillation technique (FOT) oscillometry is a method of respiratory oscillometry for measuring the mechanical properties of the lungs and airways. FOT often is used to diagnose asthma, COPD, and other respiratory ailments. FOT involves superimposing a stimulus, in the form of a sinusoidally-varying pressure waveform, over the normal tidal breathing of a patient. The response of the patient&#39;s respiratory system to the FOT pressure waveform is determined by measuring the resulting changes in air flow and the pressure waveform as the patient is tidal breathing. Based on these measurements, the impedance of the respiratory system can be calculated, which in turn can assist physicians and other clinicians in evaluating lung function and diagnosing respiratory ailments. 
     The FOT pressure waveform typically is a sinusoidally-varying pressure fluctuation of low-frequency, and low amplitude. For example, typical FOT pressure waveforms can have a frequency of about 5 Hz to about 50 Hz, and a peak-to-peak amplitude of about 0.1 cm H 2 O to about 2 cm H 2 O. Pressure waveforms with these characteristics are difficult to produce using rotating machinery such as blowers, due to difficulties in controlling the rotating mass of the blower with the accuracy and stability needed to reliably produce such low-frequency, low-amplitude pressure fluctuations. Thus, systems and devices used to perform FOT diagnostics typically employ a speaker or a vibrating mesh to produce the FOT pressure waveform. 
     SUMMARY 
     In one aspect of the disclosed technology, a system for evaluating the respiratory function of an individual using forced oscillation technique oscillometry includes a blower having a casing, an impeller mounted for rotation within the casing, and a motor configured to, during operation, rotate the impeller. The system also incudes a ventilation interface, and a connecting member defining a passageway in fluid communication with the blower and the ventilation interface. 
     The system further incudes a control unit communicatively coupled to the motor and configured to control a rotational speed of the impeller to meet a set of rotational speed setpoints for the impeller so that the blower produces a time-varying pressure waveform in the passageway, the time-varying pressure waveform including a sinusoidally-varying pressure fluctuation to be superimposed on a respiratory flow of the patient by way of the ventilation interface, and an offset pressure selected to maintain a positive air pressure in the passageway during operation of the system. 
     In another aspect of the disclosed technology, the control unit is further configured to control the rotational speed of the impeller to meet the set of rotational speed setpoints by generating a control input based on a desired air pressure with the passageway, and a known relationship between the rotational speed of the impeller and an air pressure produced by the blower. The motor is configured so that the motor varies the rotational speed of the impeller in response to the control input. 
     In another aspect of the disclosed technology, the control input is a single-frequency signal. 
     In another aspect of the disclosed technology, the control input is a multi-frequency signal. 
     In another aspect of the disclosed technology, the controller is further configured to generate the control input by combining at least a first and a second signal. 
     In another aspect of the disclosed technology, an amplitude, a phase, and a waveform of the first signal are different than a respective amplitude, phase, and waveform of the second signal. 
     In another aspect of the disclosed technology, the offset pressure is about 0.5 cm H 2 O to about 40 cm H 2 O. 
     In another aspect of the disclosed technology, the offset pressure is substantially constant. In another aspect of the disclosed technology, a maximum pressure amplitude of the time varying pressure waveform is about 0.1 cm H 2 O to about 2 cm H 2 O. 
     In another aspect of the disclosed technology, the system further includes an outlet port in fluid communication with the passageway in the connecting member, and an ambient environment around the system. 
     In another aspect of the disclosed technology, the outlet port has a length of about zero to about three inches. 
     In another aspect of the disclosed technology, the passageway and the outlet port form an airflow pathway between the ventilation interface and the ambient environment around the system; and the system the further includes an obstruction located within the outlet port and configured to partially restrict a passage of air from the airflow pathway and to the ambient environment. 
     In another aspect of the disclosed technology, the obstruction is at least one of: a plate having one or more orifices formed therein; and a mesh screen. 
     In another aspect of the disclosed technology, the outlet port is configured so that a total resistance of the system to normal tidal breathing of the individual is about 1 cm H 2 O/L/s or less. 
     In another aspect of the disclosed technology, the control unit is further configured to control the rotational speed of the impeller to produce pseudorandom noise within the passage. 
     In another aspect of the disclosed technology, the control unit is further configured to calculate an impedance of a respiratory system of the individual based on a measured pressure and a measured volumetric flowrate of the air within the passageway. 
     In another aspect of the disclosed technology, the ventilation interface includes at least one of a mouthpiece, a facemask, an endotracheal tube, a tracheal tube, a tracheostomy adapter, a tubing adapter, and a connection to a standard ventilatory interface. 
     In another aspect of the disclosed technology, the control unit includes a microcontroller. In another aspect of the disclosed technology, the microcontroller comprises a motor controller; and the control unit further comprises a gate driver communicatively coupled to the motor controller; and one or more field effect transistors communicatively coupled to the gate driver and configured to provide electrical current to the motor of the blower. 
     In another aspect of the disclosed technology, the control unit is further configured to implement a first feedback loop to control the rotational speed of the impeller to meet the set of rotational speed setpoints for the impeller. 
     In another aspect of the disclosed technology, the control unit is further configured to implement a second feedback loop to update the one or more rotational speed setpoints to achieve a target pressure for air within the passageway. 
     In another aspect of the disclosed technology, the control unit is further configured to implement the second feedback loop to at least one of: update the one or more rotational speed setpoints to a next value in the sequence of rotational speed setpoints; and compensate for changes in an actual pressure of the air within the passageway due to respiration of the individual. 
     In another aspect of the disclosed technology, the control unit is further configured to update the second feedback loop based a difference between the target pressure for the air within the passageway and a measurement of an actual pressure of the air within the passageway. 
     In another aspect of the disclosed technology, an update frequency of the first feedback loop is greater than an update frequency of the second feedback loop. 
     In another aspect of the disclosed technology, the update frequency of the second feedback loop is sufficient to permit the impeller to stabilize at each of the setpoints. 
     In another aspect of the disclosed technology, a method for evaluating the respiratory function of an individual using forced oscillation technique oscillometry includes providing a ventilation interface configured to direct breathing air to and from the individual, and providing a connecting member defining a passageway in fluid communication with the ventilation interface. The method further includes producing a substantially constant pressure offset in the passageway, and on a simultaneous basis with the production of the substantially constant pressure offset in the passage, further producing a time-varying pressure waveform in the passageway. 
     In another aspect of the disclosed technology, the time-varying pressure waveform is a forced oscillation technique waveform. 
     In another aspect of the disclosed technology, the method further includes providing a blower in fluid communication with the passageway of the connecting member, and controlling a speed of an impeller of the blower to produce the pressure offset and the time-varying pressure waveform in the passageway. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings are illustrative of particular embodiments of the present disclosure and therefore do not limit the scope of the present disclosure. The drawings are not to scale and are intended for use in conjunction with the explanations in the following detailed description. 
         FIG.  1    is a side view of a portable handheld system for evaluating the respiratory function of patients using forced oscillation technique oscillometry. 
         FIG.  2    is a diagrammatic illustration of the system shown in  FIG.  1     
         FIG.  3    is a block diagram of various electrical and electronic components of the system shown in  FIGS.  1  and  2   . 
         FIG.  4    is a perspective view of a blower of the system shown in  FIGS.  1 - 4   . 
         FIG.  5    is an exploded view of the blower shown in  FIG.  5 A , 
         FIG.  6 A  depicts a single-frequency pressure waveform that can be produced by the system shown in  FIGS.  1 - 5   . 
         FIG.  6 B  depicts a multiple-frequency pressure waveform that can be produced by the system shown in  FIGS.  1 - 5   . 
         FIG.  6 C  depicts a pseudorandom noise waveform that can be produced by the system shown in  FIGS.  1 - 5   . 
         FIG.  7    is a flow diagram depicting a process for generating a waveform of blower speed setpoints to produce the pseudorandom noise waveform shown in  FIG.  6 C . 
         FIG.  8    is a block diagram of various electrical and electronic components of the system shown in  FIGS.  1 - 5   . 
         FIG.  9    is a diagrammatic illustration of the system shown in  FIGS.  1 - 5  and  8   , depicting two feedback loops implemented by the system. 
         FIG.  10    is a diagrammatic illustration of the system shown in  FIGS.  1 - 5 ,  8 , and  9    implementing a first and second feedback loop to generate an FOT waveform. 
     
    
    
     DETAILED DESCRIPTION 
     The following drawings are illustrative of particular embodiments of the present disclosure and do not limit the scope of the present disclosure. The drawings are not to scale and are intended for use in conjunction with the explanations provided herein. Embodiments of the present disclosure will hereinafter be described in conjunction with the appended drawings. 
     As used in this document, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. As used in this document, the term “comprising” (or “comprises”) means “including (or includes), but not limited to.” When used in this document, the terms “exemplary” and “for example” are intended to mean “by way of example” and is not intended to indicate that a particular exemplary item is preferred or required. 
       FIGS.  1 - 5    depict a portable handheld system  10  for evaluating respiratory function using forced oscillation technique (FOT) oscillometry. The system  10  comprises a blower  11 ; and a control unit  12 . The blower  11  is depicted in  FIGS.  1 ,  2 ,  4 , and  5   ; the control unit  12  is depicted in  FIGS.  3  and  8 - 10   . The blower  11  acts as an air oscillation source that superimposes a sinusoidally varying pressure fluctuation on the normal tidal breathing of a patient or other individual through the large and small airways of the lungs. The control unit  12  is configured to control the speed of the blower  11  so that the blower  11  produces the desired time-varying pressure profile, or FOT pressure waveform. 
     In general, an FOT pressure waveform needs to have at least one frequency, and needs to be generated from at least from one sinusoidal signal. More advanced FOT pressure waveforms, containing more than one frequency and based on multiple sinusoidal signals, may be used to evaluate respiratory system behavior within the same breath. As used herein, the term “pressure waveform” is intended to mean the pressure variation, over time, generated by the blower  11  or other apparatus capable of generating a sinusoidally-varying pressure oscillation. The waveform can be generated by the blower  11  or other apparatus based on one signal provided to the blower  11  as a single control input; or a combination or summation of two or more signals provided to the blower  11  as single control input, where each signal may be periodic or aperiodic and may provide frequency content at one or more frequencies. 
     The control unit  12  is further configured to process the response of the patient&#39;s lungs to the FOT pressure waveform, as indicated by measurements of the pressure and volumetric flowrate of the air being inhaled and exhaled by the patient as the FOT pressure waveform is superimposed on the patient&#39;s normal tidal breathing. Specifically, the control unit  12  determines the mechanical impedance of the patient&#39;s respiratory system based on these measurements. Knowledge of the mechanical impedance of the patient&#39;s respiratory system can assist a clinician in diagnosing and treating conditions such as asthma and COPD. As discussed below, in alternative embodiments of the system  10 , the calculation of the mechanical impedance of the patient&#39;s respiratory system can be made by a control unit other than the control unit  12 , located remotely from the system  10 . 
     Referring to  FIGS.  1  and  2   , the system  10  further comprises a ventilation interface in the form of a mouthpiece  14 ; and a connecting member  16 . The connecting member  16  is coupled physically to the blower  11  and the mouthpiece  14 , and defines an internal passageway  17  that places the blower  11  in fluid communication with the mouthpiece  14 . The connecting member  16  directs the FOT pressure waveform generated by the blower  11  toward the mouthpiece  104 , so that the FOT pressure waveform can be superimposed on the patient&#39;s normal respiratory flow during inhalation and exhalation. 
     The mouthpiece  14  is placed in the patient&#39;s mouth during the diagnostic process, so that the patient inhales air transmitted to the mouthpiece  14  via the connecting member  16 , and exhales into the connecting member  16  via the mouthpiece  14 . The mouthpiece  14  is configured to form an airtight seal between the mouthpiece  14  and the patient&#39;s lips, so that substantially all of the air being inhaled and exhaled through the patient&#39;s mouth passes through the mouthpiece  14  and into the connecting member  16 . (During the diagnostic process, the patient&#39;s nasal breathing typically will be blocked so that substantially all of the air being inhaled and exhaled by the patient passes through the mouthpiece  14  and the connecting member  16 .) The mouthpiece  14  can be equipped with a viral/bacterial filter (not shown). 
     The ventilation interface can be any type of device suitable for allowing the patient to exhale and inhale to and from the connecting member  16 , and alternative embodiments of the system  10  can include a ventilation interface other than the mouthpiece  14 . For example, the ventilation interface can be a facemask, an endotracheal tube, a tracheal tube, a tracheostomy adapter, a tubing adapter, etc. As another example, the ventilation interface can be a connection to a standard ventilatory interface based on the ISO 5637 standard or other standards. The connecting member  16  can be formed from a rigid polymeric material. The internal passageway  17  within the connecting member  16  can have, for example, a circular, oval, or rectangular cross section. The passageway  17  can be configured so that the airflow within the passageway  17  remains laminar during normal operation of the system  10 , and dead-space and flow obstructions within the passageway  17  are minimal or non-existent. Alternative embodiments of the connecting member  16  can have shapes, sizes, relative proportions, etc. other than those of the connecting member  16 , and can be formed from semi-rigid and non-rigid materials. Also, the connecting member  16  can be unitarily formed with, or otherwise made a part of the ventilation interface or the blower  11 , in other alternative embodiments. 
     The connecting member  16  includes an impedance port or outlet port  18 , as shown in  FIGS.  1  and  2   . The outlet port  18  provides a pathway between the internal passageway  17  within the connecting member  16 , and the ambient environment around the system  10 . The pathway provided by the outlet port  18  permits the patient to breathe through the connecting member  16  and the mouthpiece  14 . The outlet port  18  is sized, located, and otherwise configured so that the patient can breathe normally, while minimizing dissipation of the FOT pressure waveform generated by the blower  11 . 
     The outlet port  18  is designed and configured as an impedance port. The term “impedance port,” as used herein, is distinct from an “inertance port.” Inertance ports are commonly used in loudspeaker-based FOT systems as low-pass filters to allow the passage of respiratory flow, at breathing frequencies, to the ambient environment, while allowing higher-frequency oscillatory flow from an FOT source to pass to the user. Inertance ports typically are configured as long tubes, to provide a volume of air sufficient to produce a desired inertive load. 
     The outlet port  18  has an impedance that is primarily resistive, i.e., more resistive than reactive. Due to its resistance, the outlet port  18  functions as an attenuating element for the oscillatory and respiratory flows within the system  10 , without acting as a low-pass filter. The use of a blower, such as the blower  11 , as a source of oscillatory flow helps to ensure that a sufficient level of oscillatory flow can delivered to the user without the use of a low-pass filter. In setting a desired maximum air pressure delivered to the user, as measured by a pressure sensor  20  of the system  10 , the operational settings of the blower  11  can be adjusted to compensate for oscillatory flow that may be lost through the outlet port  18 . 
     High inertance is not a required feature for the impedance port, i.e., the outlet port  18 . Thus, the outlet port  18  can be substantially shorter than the tube used in a typical inertance port. For example, the length of the outlet port  18  can be as low as 0 to 3 inches, which can help to promote handheld form factors for the system  10 . An optimal or otherwise desired impedance for a particular application can be achieved by tailoring the diameter of the outlet port  18 , and/or placing an obstruction or obstructive element  19  in the outlet port  18 , to vary the impedance of the system  10 . The obstructive element  19  is visible in  FIG.  1     
     It is desirable to reduce the work required by a user to breathe through the system  10 , by providing a low resistance in the breathing path between the user to the ambient environment. This resistance includes a series resistance due to an airflow sensor  22  of the system  10 , and the viral/bacterial filter (if so equipped); and the parallel resistance of the blower  11  and the outlet port  18 . The resistance of the blower  11  may be difficult to adjust once the blower  11  is fabricated; the resistance of the outlet port  18 , however, can be adjusted more easily. Thus, the impedance of the outlet port  18  can be set so that the total resistance of the system  10  to the patient&#39;s normal tidal breathing is low, e.g., less than about 1 cm H 2 O/L/s. 
     The impedance of the impedance port, i.e., the outlet port  18 , can be set by positioning the obstructive element  19  within the outlet port  18 , to restrict the passage of air to the ambient environment by way of the airflow pathway within the system  10 . For example, the obstructive element  19  can be a mesh in the form of woven nylon or stainless steel screens available, for example, from Component Supply Company of Sparta, Tennessee; Tex Tech Industries, Inc., of Kernersville, N.C.; and Saati S.p.A., of Appiano Gentile, Italy. Alternatively, the obstructive element  19  can be an orifice plate with one or more small, fixed-diameter openings, positioned in the airflow pathway and available, for example, from O&#39;Keefe Controls Co., of Monroe, Connecticut; Pfeiffer Vacuum Inc., of Nashua, N.H.; Werner Solken. 
     The pressure sensor  20  and the airflow sensor  22  are communicatively coupled to the control unit  12 . The pressure sensor  20  and the airflow sensor  22  are depicted in  FIGS.  1 - 3   . The pressure sensor  20  and the airflow sensor  22  are in fluid communication with the internal passageway  17  within the connecting member  16 , so that the pressure sensor  20  and the airflow sensor  22  can sense the respective pressure and volumetric flowrate of the air being inhaled and exhaled by the patient. More specifically, the pressure sensor  20  and the airflow sensor  22  can measure changes in the pressure and volumetric flowrate of the airflow in response to the FOT pressure waveform introduced into the airflow by the blower  11 . 
     The pressure sensor  20  is a differential pressure sensor. The pressure sensor  20  can be mounted on the connecting member  16 . Alternatively, the pressure sensor  20  can be connected to the connecting member  16  by tubing or piping as shown in  FIG.  1   . The opening in the connecting member  16  that facilitates fluid communication between the pressure sensor  20  and the internal passageway  17  of the connecting member  16  is located proximate the end of the connecting member  16  that adjoins the mouthpiece  14 , as can be seen in  FIGS.  1  and  2   . The pressure sensor  20  can be any type of differential pressure sensor suitable for use within the range of pressures present within the connecting member  16  during normal operation of the system  10 . 
     The airflow sensor  22  can be a dynamic-impedance pneumotachometer, with a heated wire to prevent condensation from forming on the pneumotach screen. Other suitable types of airflow sensors can be used in alternative embodiments. 
     Referring to  FIGS.  3 - 5  and  10   , the blower  11  comprises a casing  24 , an impeller  26  mounted for rotation within the casing  24 , and a three-phase electric motor  28  configured to rotate the impeller  26  in relation to the casing  24 . The blades of the impeller  26  can have a “squirrel cage” or “centrifugal” configuration. The blower  11  can be equipped with a brushless AC (BLAC) three-phase, asynchronous electric motor, such as in the AIRMAX™ P28-AC-ID blower available from Moog Inc., or the 5 kPa CPAP blower available from ASPINA. 
     Alternatively, the blower  11  can be equipped with a brushless DC (BLDC) electric motor, such as in the AIRMAX™ P45 series of fans and blowers, available from Moog Inc.; or the Model U71MX-024KX-4 miniature radial blower, available from Micronel AG. Other types of blowers from these and other manufacturers can be used in the alternative. 
     Referring to  FIGS.  4  and  5   , the “centrifugal” blower  11  is configured to draw ambient air from outside of the casing  24  of the blower  11 , through an inlet port  32  oriented perpendicular to the casing  24 . An outlet port  33  of the blower  11  is positioned within the center of the rotating impeller  26  of the blower  11 . The centrifugal impeller  26  has a drum shape, and comprises a plurality of blades positioned around the outer circumference of the impeller  26 . The air from the inlet port  32  is transported through the impeller  26 , to the outlet port  33 , and into the passageway  17  of the connecting member  16 . The centrifugal blower  11  uses kinetic energy to increase or decrease the velocity and pressure of the air passing through the blower  11 , thus differentiating it from a positive displacement fan or blower in an axial configuration, which uses mechanical energy to physically move air from the inlet to the outlet. The dimensions of the impeller  26  are selected to tightly match the dimensions of the adjacent internal surfaces the casing  24 , to facilitate the efficient movement of air through the blower  11  and the transfer of air velocity and pressure into the passageway  17 . 
     Details of the bower  11  are provided for illustrative purposes only. Alternative embodiments of the system  10  can be equipped with blowers having configurations other than that of the blower  11 . 
     Referring to  FIGS.  3  and  8 - 10   , the control unit  12  comprises a microcontroller in the form of a motor controller IC, or motor controller  29  communicatively coupled to the pressure transducer  20 . The control unit  12  also comprises a gate driver  30  communicatively coupled to the motor controller  29 ; and high and low-side field effect transistors (FETs)  34 , such as but not limited to MOSFETs, communicatively coupled to the gate driver  30 . The FETs  34  can provide current to the three-phase motor  28 . 
     The gate driver  30  can be, for example, a model DRV8301 or DRV8302 gate driver available from Texas Instruments Incorporated. The gate driver  30  is configured to receive digital inputs from the motor controller  29 , and to provide outputs to the FETs  34 . In response, the FETs  34  control each phase of the 3-phase motor  28  by varying the current to each phase, so as to cause the impeller  26  to rotate at a rotational-speed setpoint determined by the motor controller  29 . 
     The gate driver  30  implements a first feedback loop, discussed below, that facilitates adjustment of the rotational speed of the impeller  26  to the setpoints determined by the motor controller  29 , using the current amplifiers of the gate driver  30 , and the FETs  34 . The rotational speed of the impeller  26  can be monitored using the back-electromotive force, or back-EMF, of the motor  28  as monitored on any or all of the 3 phases of the motor by the gate driver  30 , or in alternative embodiments, by a separate IC using a differential current amplifier. The gate driver  30  can have thermal and current sensing capabilities, to help prevent damage to the motor  28  and other components of the system  10 . 
     The motor controller  29  continuously receives pressure data from the pressure sensor  20  and airflow data from the airflow sensor  22 , and implements a second feedback loop, discussed below, to update the rotational speed setpoints for the impeller  26  to obtain a target pressure for the airflow within the connecting member  16 . 
     The control unit  12  also can include provisions to protect the electronic components of the system  10  from back-EMF, using generally known techniques. 
     The control unit  12  further includes a battery  36  to power the motor  28 , the controller  12 , and the other electronic components of the system  10 . The battery  36  is depicted in  FIG.  3   . The battery  36  can be, for example, a lithium polymer battery. Alternative embodiments can be configured to be powered by standard 120-volt, 60 Hz household current in lieu of, or in addition to the battery  36 . 
     The blower  11  is controlled so that it superimposes a sinusoidally-varying pressure waveform on the normal tidal breathing of the patient. The blower  11  is controlled and stabilized using a first feedback loop implemented by the system  10  and depicted diagrammatically in  FIG.  9   . The first feedback loop controls the blower  11  to produce FOT pressure waveforms with maximum pressure amplitudes typically ranging, for example, from about 0.1 cm H 2 O to about 2 cm H 2 O; and offset pressures typically ranging, for example, from about 0.5 cm H 2 O to about 40 cm H 2 O. 
     The first feedback loop controls the rotational speed of the impeller  26  to meet a set of rotational speed setpoints using, for example, a proportional-integral-derivative (PID) control algorithm. The update frequency of the first feedback loop is relatively fast, e.g., less than about 1 millisecond (ms). 
     The control unit  12  can be further configured to implement a second feedback loop, also depicted in  FIG.  9   . The second feedback loop updates the rotational speed setpoints to obtain a targeted pressure for the airflow being inhaled and exhaled by the patient, as measured by the pressure sensor  20 . The second feedback loop is optional, i.e., second feedback loop can be implemented on a continuous basis, on a periodic or intermittent basis, or not all. When the second feedback loop is not being implemented, the rotational speed setpoints can be set to either pre-calibrated or uncalibrated values to allow a user to decrease the pressure waveform during FOT testing. 
     The update frequency of the second feedback loop is slower than the update frequency of the first feedback loop. For example, the updated frequency of the second control loop can be about every 10 ms to 20 ms. 
     The targeted air pressure is continuously updated by the motor controller  29 , based on the desired sinusoidal pressure fluctuation that is to be superimposed on the normal tidal breathing of the patient. The air pressure developed by the blower  11  approximately follows the square of the speed of the blower  11  (as reflected by the speed of the impeller  26 ). Thus, a particular pressure waveform can be stored in the memory of the motor controller  29  as a set of motor speed setpoints for the blower  11 . Alternatively, the motor speed setpoints can be calculated on demand by one or more internal signal generators  31  within the motor controller  29 , as depicted in  FIG.  10   . 
     The internal signal generator(s)  31  can generate a variety of periodic signals corresponding to the set of motor speed setpoints needed to produce a particular pressure waveform. These periodic signals can have, for example, square waveforms, sine waveforms, triangle waveforms, sawtooth waveforms, etc. Parameters of the signals, such as frequency and amplitude, can be adjusted to compensate for frequency-variant behavior of the blower  11  and its drive electronics, i.e., the gate driver  30 , to generate frequency content of consistent amplitudes across all frequencies of interest. As also can be seen in  FIG.  10   , the internal signal generator  31  can impose a substantially constant, i.e., static or non-time-variant, offset on the pressure waveform produced by the blower  11 , which can help ensure stability of the blower  11 . 
     The internal signal generator  31  can be used for tuning coefficients or parameters of the first or second feedback loop as implemented by the gate driver  30 , FETs  34 , or the motor controller  29 . To perform this tuning, the internal signal generator  31  can be set to generate a sequential set of signals with varying frequency components, amplitudes, offsets, or signal shapes, and then the measured pressure and flow data can be used to evaluate the overall response of the system  10 . Tuning the feedback loop or loops in this way can enable more rapid transitions between specific pressure levels, improve performance across frequencies, and decrease the energy lost during electronic braking of the blower motor. 
     The motor controller  29  is configured to update the rotational speed setpoint based on the difference between the target pressure, and the actual pressure of the air within the connecting member  16  as measured by the pressure sensor  20 , as depicted in  FIG.  10   . Initial motor speed estimates are supplied from the motor controller  29  using a pre-characterized pressure vs. motor speed table based on an assumption of no-load or known-load conditions. After the first feedback loop quickly drives the motor  28  to the desired setpoint, the second feedback loop can either update the motor speed setpoint to the next value in the sinusoidal waveform, or adjust the motor speed setpoints in the present and/or future waveforms to compensate for loading due to the patient&#39;s respiratory system. The adjustment of the motor speed setpoints is highly desirable to FOT, where sinusoidal amplitude pressure maxima in the range of 1 cm H 2 O to 4 cm H 2 O typically are required to help ensure signal integrity of flow, pressure, and ultimately transpulmonary impedance data. 
     The update or adjustment of the setpoints is performed at a relatively slow rate, to allow the impeller  26  to stabilize at each setpoint. Once the setpoint is updated or adjusted, the motor controller  29 , implementing the first feedback loop, adjusts the rotational speed of the impeller  26  via the gate driver  30  and the FETs  34 , to maintain the rotational speed of the impeller  26  at the updated setpoint. 
     The slower update frequency for the second feedback loop is necessary to allow the impeller  26  to stabilize at each targeted rotational speed, before the targeted speed is again adjusted to produce the desired sinusoidal pressure fluctuation on the patient&#39;s tidal breathing. 
     As such, the rotational speed setpoints represent a variety of thresholds to be maintained individually, using the first feedback loop, so that the blower  11  produces a pressure waveform having the desired characteristics. This approach is different from that used in conventional FOT systems comprising a speaker or a vibrating mesh, where a static pressure cannot be applied, so a constantly moving surface produces the pressure waveform; and in which there is no rotating mass, and no corresponding need to account for the moment of inertia of a rotating mass as it accelerates and decelerates, as in the system  10 . 
     While conventional loudspeaker-based and piston-based FOT systems provide positive and negative pressure variations relative to the ambient pressure, the blower-based system  10  can be configured to provide a substantially constant offset, in relation to the ambient pressure, in the air being supplied by the blower  11 . The offset is added as a substantially constant value to each point in the sinusoidal FOT pressure waveform that is to be superimposed on the normal tidal breathing of the patient. The addition of a substantially constant offset to the FOT pressure waveform can provide a small bias flow to prevent patient rebreathing of stagnant air, and/or to compensate for local ambient pressure which may see significant site-to-site variation depending on altitude, weather, temperature, etc. Also, the pressure offset helps to ensure that a positive air pressure, i.e., an air pressure above ambient, is maintained in the passage  17  of the connecting member  16 , which in turn can help to minimize or substantially eliminate the potential for insufflation of the patient caused by low, or negative air pressure and airflow in the passageway  17 . 
     Also, the pressure offset imposed on the air being supplied by the blower  11  permits the blower  11  to be controlled within the tolerances required to produce the relatively small sinusoidal variations air pressure needed for FOT oscillometry. In particular, the Applicants have found that superimposing the FOT pressure oscillations on top of a low-amplitude, substantially constant pressure offset can prevent the speed of the impeller  26  from decaying to a zero or near-zero level from which it would be difficult or impossible to accelerate sufficiently to produce the required FOT pressure oscillations. Also, the pressure offset helps to improve the signal to noise ratios in the air pressure and airflow measurements ultimately used in calculating the impedance of the patient&#39;s respiratory system. 
     The system  10  can be configured to produce the conventional FOT pressure waveforms discussed in the following paragraphs. These particular waveforms are disclosed for illustrative purposes only. The system  10  can be configured to produce other types of waveforms, including waveforms simultaneously comprising more than one signal. For example, the internal signal generator  31  can support the generation of two or more simultaneous signals, such as sine signals, square signals, sawtooth signals, triangle signals, etc., with the parameters of each signal capable of being set independently of the other signal. The individual signals are summed together into one combined signal, or control input, of time-varying blower speed setpoints. Optionally, the waveform may be an arbitrary or baseline waveform downloaded to the system  10  as a time-series of blower speed setpoint values, and the waveform may or may not repeat over time. A software-settable playback rate allows the effective frequency of any of the signals to be adjusted. Each signal contained in the control input optionally may compensate for frequency-variant behavior of the blower  11  and its drive electronics, i.e., the gate driver  30 , to generate frequency content of consistent amplitudes across all frequencies of interest. 
       FIG.  6 A  depicts a conventional single-frequency FOT pressure waveform that can be produced by the system  10 . In this particular example, the frequency of the waveform is 5 Hz.  FIG.  6 B  depicts a conventional multiple-frequency FOT pressure waveform that can be produced by the system  10 . In this particular example, the frequencies of the waveform are 5 Hz, 11 Hz, and 19 Hz. 
       FIG.  6 C  depicts a pseudorandom noise that can be produced by the system  10 . Pseudorandom noise enables the removal of the large pressure spike that otherwise could occur as the blower  11  is started at the beginning of diagnostic FOT process. The pressure spike can make it difficult for the blower  11  to start, and can be uncomfortable to the user. The process for generating a waveform of blower setpoints to produce a pseudorandom noise waveform is shown in the flowchart depicted in  FIG.  7   . 
     Signal to noise ratio is an important consideration in FOT oscillometry. The blower stabilization time implemented by the system  10  represents a sinc filter that is applied across all frequencies generated by the blower  11 . For the single or multiple frequency waveforms, each frequency of interest optionally may be scaled to increase the signal to noise ratio; for the pseudorandom noise waveform, however, a more exact approach may be advantageous. 
     To compensate for the impact of stabilization of the blower  11  on the pseudorandom noise waveform, the magnitude of the input X may be scaled by 1/(the sinc function of the motor) in order to generate a flat band in the frequency domain. This can help ensure a consistent signal-to-noise ratio across the frequency range of interest, assuming that the noise is additive white gaussian noise. For example, the frequency variation in the pseudorandom waveform depicted in  FIG.  6 C  is substantially flat in the range of about 2 Hz to about 25 Hz. 
     The pressure and airflow values captured by the respective pressure sensor  20  and airflow sensors  22  are communicated to the control unit  12  as the FOT pressure waveform is superimposed on the tidal breathing of the patient. The control unit  12  is configured to filter the pressure and airflow measurements to identify the pressure and flow fluctuations responsive to the FOT pressure waveform, and to extract and distinguish those fluctuations from the pressure and airflow associated with the patient&#39;s tidal breathing, using conventional techniques known generally among those skilled in the art of FOT oscillometry. The control unit  12  then calculates the impedance of the patient&#39;s respiratory system based on the pressure and flow fluctuations responsive to the FOT pressure waveform, using conventional techniques generally known in the art as algorithms for measurement of transpulmonary impedance. Such techniques may include the RLS algorithm (Recursive Least Squares to minimize the squared Equation Error, sometimes called the EE algorithm), ACOE algorithm (adjustable compensator for output error of least-squares), 2SLS algorithm (two-stage least-squares), or MLR algorithm (multiple linear regression). The time domain approaches are generally based on the Equation of Motion for a transpulmonary system that relates pressure, flow, and impedance. The frequency domain approaches involve the DFT (discrete Fourier transform), the FFT (fast Fourier transform), and IQ/PQ (in-phase or quadrature decompositions) for measuring the pressure-to-flow relationship within a narrow frequency window. 
     The calculated impedance of the patient&#39;s respiratory system, and the underlying pressure and airflow data, can be stored in a memory of the control unit  12 , and/or can be downloaded or transmitted immediately, or at a later time. 
     In alternative embodiments, the pressure and airflow measurements can be transmitted a control unit remote from the system  10 , such as a smartphone, a desktop or notebook computer, a server, a mainframe, etc.; and the above-noted processing of the data to yield the impedance of the patient&#39;s respiratory system can be performed by the remote control unit, 
     The system  10  applies pressure oscillations on top of a very low pressure offset generated by the blower  11 ; and the blower  11 , the outlet port  18 , and the patient fluidly communicate by way the connecting member  16  and the mouthpiece  14 . The constant flow rate can facilitate a reduction in the size of the blower  11  and a reduction in the dead space within the airflow pathway by continuously refreshing the airflow through the parallel path of the impedance port  18  and the blower  11 . 
     The features and functions described above, as well as alternatives, may be combined into many other different systems or applications. Various alternatives, modifications, variations or improvements may be made by those skilled in the art, each of which is also intended to be encompassed by the disclosed embodiments.