Abstract:
A dynamic respiratory control device includes a fast-response valve capable of dynamically imposing multiple resistive loads on the flow of respiratory gas to and from a patient. The resistive loads are applied in response to measured flow rates, patient lung volumes, and/or mouthpiece pressures. The device can precisely constrain tidal breathing, provide precise volumetric control of the airway, and impose multiple specific inspiratory and/or expiratory loading functions to evaluate respiratory function. The device is useful for pulmonary function testing, CT and MRI imaging of the chest, combined CT imaging/interventional radiology, radiotherapy delivery to the thorax/abdomen, and/or as a resistive muscle trainer for weaning patients off ventilators and for respiratory muscle training.

Description:
RELATED APPLICATION DATA 
     This application is based on U.S. Provisional Patent Application No. 60/093,214, filed Jul. 17, 1998, which is herein incorporated by reference. 
    
    
     BACKGROUND 
     This invention relates to systems and methods for respiratory function analysis and control, and in particular to systems and methods for dynamically analyzing and controlling the respiration of a patient. 
     Controlling a patient&#39;s respiration is useful for many applications, including pulmonary function testing and evaluation, CT and MRI imaging of the chest, respiratory muscle training, and weaning patients off ventilators. Most currently available systems for controlling and evaluating respiratory function are relatively inflexible, and do not have the capability to precisely and dynamically control respiratory function. Moreover, some available methods of controlling respiratory function can be uncomfortable for the patient, particularly methods requiring the patient to hold his or her breath for extended periods of time. 
     SUMMARY 
     The present invention provides systems and methods for dynamically and accurately controlling a patient&#39;s respiratory function. The methods allow substantial flexibility in the evaluation process. The methods further allow limiting patient discomfort during respiratory function control procedures. 
     A dynamic respiratory control device includes a fast-response valve capable of dynamically imposing multiple resistive loads on the flow of respiratory gas to and from a patient. The resistive loads are applied according to measured flow rates, patient lung volumes, and/or mouthpiece pressures. The device can precisely constrain tidal breathing, provide precise volumetric control of the airway, and impose multiple specific inspiratory and/or expiratory loading functions to evaluate respiratory function. The device is useful for pulmonary is function testing, CT and MRI imaging of the chest, combined CT imaging/interventional radiology, radiotherapy delivery to the thorax/abdomen, and/or as a resistive muscle trainer for weaning patients off ventilators and for respiratory muscle training. 
     The present invention provides a dynamic respiratory control apparatus comprising: a respiratory function valve for dynamically controlling a respiratory gas flow for a patient; a flow rate monitoring device positioned in a flow path of the respiratory gas, in fluidic communication with the valve, for measuring a flow rate of the respiratory gas; and a control unit electrically connected to the monitoring device and the valve, for receiving flow rate data from the monitoring device and for dynamically controlling the valve to apply an intermediate resistive load to the flow according to the flow rate data. Further provided is a dynamic respiratory control method comprising: generating flow rate data characterizing a flow of a respiratory gas between a respiratory function valve and a patient; and dynamically controlling the valve to apply an intermediate resistive load to the flow according to the flow rate data. The real-time feedback and flexibility in applying multiple inspiratory and/or expiratory resistive loads allow improved respiratory function evaluation and control, as well as improved respiratory muscle training. 
     The present invention further provides a control unit electrically connected to the monitoring device and the valve, for receiving flow rate data from the monitoring device, determining a lung volume of the patient from the flow rate data, and dynamically controlling the valve to maintain the lung volume between a first predetermined value and a second predetermined value. Further provided is a dynamic respiratory control method comprising: generating flow rate data characterizing a flow of a respiratory gas between a respiratory function valve and a patient; determining a lung volume of the patient from the flow rate data; and dynamically controlling the valve to maintain the lung volume between a first predetermined value and a second predetermined value. Actively maintaining the patient&#39;s lung volume between two predetermined values allows limiting the range of motion of the patient&#39;s organs during imaging or therapy procedures, without requiring the patient to hold his or her breath. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1-A is a schematic diagram of a preferred dynamic respiratory control system of the present invention. 
     FIG. 1-B shows a schematic diagram of control electronics and an imaging/treatment device according to an embodiment of the present invention. 
     FIGS. 2-A and  2 -B show schematic top and side views, respectively, of a respiratory control device comprising an active valve according to the preferred embodiment of the present invention. 
     FIGS. 3-A and  3 -B illustrate isometric and top views, respectively, of the active valve of FIG. 2-A. 
     FIGS. 4-A and  4 -B show front and top sectional view, respectively, of the valve of FIG. 3-A in an open position. 
     FIGS. 5-A and  5 -B show front and top sectional view, respectively, of the valve of FIG. 3-A in a closed position. 
     FIGS.  6 -A-C are flowcharts illustrating a method of controlling a patient&#39;s lung volume between two predetermined values, according to the present invention. 
     FIG. 7-A shows the variation of patient lung volume with time over multiple breaths for a method in which the patient&#39;s lung volume is constrained between predetermined values, according to the present invention. 
     FIG. 7-B shows three potential variations of resistive loads with lung volume over part of one breath, according to the present invention. 
     FIG. 7-C shows potential patterns for time variations in resistive loads over multiple breaths, suitable for respiratory muscle training according to the present invention. 
     FIGS. 8-A and  8 -B show top and side schematic views, respectively, of a respiratory control device having two active iris valves, according to an alternative embodiment of the present invention. 
     FIGS. 8-C and  8 -D illustrate an iris valve of the device of FIG. 8-A in closed and open positions, respectively. 
     FIG. 9 shows a top schematic views of part of a respiratory control device according to an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, the term “intermediate resistive load” is understood to refer to a resistive load between the maximum and minimum resistive loads applied during the operation of a system of the present invention. Intermediate resistive loads are understood to be deliberately applied for finite, controlled periods of time, and are not merely incidental to the rapid opening or closing of a valve. The term “butterfly valve” is understood to refer to a valve having an occluding structure capable of pivoting about a central axis perpendicular to the local direction of gas flow. The statement that a patient&#39;s respiration is controlled dynamically is understood to mean that a resistive load is imposed in the patient&#39;s respiratory path in real time in response to an electric signal characterizing the magnitude of the resistive load. The terms “electronics” and “control unit” are understood to encompass any combinations of hardware and software—special-purpose hardware and programmed general-purpose hardware. The statement that a valve has a certain response time is understood to mean that the valve is capable of moving between its extreme positions (fully open/fully shut) within that response time. Actions taken according to some original data are understood to encompass actions taken according to the original data in unaltered form, as well as data derived from the original data. 
     The following description illustrates embodiments of the invention by way of example and not necessarily by way of limitation. 
     FIG. 1-A is a schematic diagram illustrating a presently preferred dynamic respiratory control system  20  of the present invention. System  20  comprises a respiratory control device  22  and a control unit  24  electrically connected to device  22 . Device  22  comprises a breathing conduit  32 , monitoring components  34 , and an active respiratory function valve  36 , all in fluidic communication with the respiratory system of patient  30 . 
     Breathing conduit  32  provides a path for the flow of a respiratory gas from a gas source to the respiratory system of patient  30 , and from patient  30  to a respiratory gas sink. The respiratory gas source and sink are preferably the atmosphere, but may include a ventilator or other devices. The respiratory gas is preferably air, but may be pure oxygen and may include other gases. 
     FIGS. 2-A and  2 -B show schematic top and side views of device  22 , respectively, showing breathing conduit  32 , a monitoring device  34 , and valve  36 . A conventional mouthpiece  38  is attached to the proximal end of breathing conduit  32 . During the operation of device  22 , mouthpiece  38  is held in the patient&#39;s mouth while the patient&#39;s nose is clamped shut. Valve  36  is secured to breathing conduit  32  opposite mouthpiece  38 , such that breathing conduit  32  provides a fluidic connection between the patient and valve  36 . Valve  36  is preferably attached to breathing conduit  32  through an air-tight snap-on connection. 
     A monitoring device  34  is mounted within conduit  32 , between mouthpiece  38  and valve  36 . Positioning device  34  between mouthpiece  38  and valve  36  increases the accuracy of device  34 . Monitoring device  34  is electrically connected to control unit  24  through a conventional electrical connection (not shown). Monitoring device  34  preferably comprises a conventional mass flow sensor such as a hot-wire anemometer, for dynamically measuring flow rates of respiratory gas through conduit  32 . 
     Device  22  further comprises a pressure sensing device such as a pneumotachograph (pneumotach) or differential pressure transducer device, for measuring pressures within breathing conduit  32 . A conventional mesh screen  42  and a pressure measurement tube  40  are connected to conduit  32  adjacent to mouthpiece  38 . A mouthpiece pressure sensor (not shown) is connected to conduit  32  through tube  40 . The pressure sensor measures the pressure within conduit  32  near mouthpiece  38 . The mouthpiece pressure is indicative of the direction of flow within conduit  32 . Outside of valve  36 , device  22  is conventional. Suitable breathing conduits, mass flow sensors, pressure sensors, and associated components are available for example from SensorMedics, Yorba Linda, Calif. 
     Valve  36  is capable of dynamically controlling the flow of respiratory gas through conduit  32 , to and from patient  30 . In response to received control signals, valve  36  is capable of introducing dynamically variable resistive loads into conduit  32 , thus modulating the flow rate within conduit  32  in real time. Valve  36  is capable of completely opening/closing within 100 ms (milliseconds), preferably in less than 50 ms, ideally in 10-30 ms. 
     Preferably, valve  36  has a relatively good resolution, i.e. ability to finely modulate the flow rates to and from patient  30 . Finely modulating the flow rates allows tightly controlling the volume within the patient&#39;s lungs and the motion of the patient&#39;s lungs and/or other internal organs. Control over organ motion is particularly desirable in treatment applications such as highly localized radiation and laser therapy. Valve  36  is preferably capable of constraining the patient volume within 100 ml, ideally 10-50 ml or less. A fast and sensitive valve allows dynamically controlling the respiration and lung volume of patient  30  in response to variable patient breathing efforts. Valve  36  is also relatively robust, such that its time and volume sensitivities do not substantially degrade over a large number of operation (breathing) cycles. Valve  36  is preferably on the order of 2-3 cm in diameter for adult patients, and about 1-2 cm in diameter for infants or small children. 
     Valve  36  is preferably a butterfly valve. Other fast valves such as iris (with or without seals), solenoid, and scissors valves are also suitable for use with the present invention. Iris valves allow reducing the transfer of gas associated with valve closures and openings. Potentially relevant valve parameters include minimum resistive load imposed on flows, variability of resistance, response time, maximum shutoff pressure, valve size, torque required to move, full range motion distance, difficulty in designing a suitable seal, torque required to hold the seal, and complexity of development and/or manufacturing. Butterfly valves offer a small size, simple design, low minimum resistance to flow, and good flow variability and speed. 
     FIGS. 3-A and  3 -B show isometric and top plan views, respectively, of a preferred butterfly valve  36 . Valve  36  comprises a main valve housing  44  defining a flow channel  46 . A shaft (pivot)  48  is mounted within valve housing  44 . Shaft  48  is perpendicular to the local, direction of respiratory gas flow within flow channel  46 . An occluding structure  50  is rotatably mounted on shaft  48 , for controllably occluding channel  46 . Structure  50  pivots around shaft  48  between fully open and fully closed positions. The position of structure  50  determines the resistive load imposed by valve  36  on the flow of respiratory gas through channel  46 . The fully open position corresponds to a minimal resistive load, while the fully closed position corresponds to a maximal resistive load. 
     As shown in FIG. 3-B, a digital encoder  52  is mechanically coupled to shaft  48  and housing  44 . The body of encoder  52  is secured to housing  44 , while its code wheel is coupled to shaft  48 . An adapter  54  couples valve shaft  48  to the shaft of a motor  58 . Motor  58  is a conventional DC motor with pulse-width-modulated (PWM) control. The housing of motor  58  is secured to a mounting plate  60 . Mounting plate  60  is in turn secured to valve housing  44  by screws (not shown). The screws extend through holes  62  and  64  in mounting plate  60  and valve housing  44 , respectively, as shown in FIG. 3-A. Encoder  52  and motor  58  are electrically connected to control unit  24  (not shown). Control unit  24  controls DC motor  58 , and receives from encoder  52  digital data indicative of the position of shaft  48 . The position of shaft  48  is in turn indicative of the position of valve  36 . 
     FIGS. 4-A and  4 -B show front and side sectional views, respectively, of valve  36  in its fully open position. As illustrated in FIG. 4-B, structure  50  comprises two flaps  50   a-b  symmetrically mounted on opposite sides of shaft  48 . Flaps  50   a-b  have corresponding major surfaces  66   a-b  for occluding the flow of respiratory gas through corresponding apertures  68   a-b . The effective sizes of apertures  68   a-b  can be varied by rotating shaft  48 . 
     FIGS. 5-A and  5 -B show front and side sectional views, respectively, of valve  36  in its fully closed position. When valve  36  is closed, flaps  50   a-b  establish two separate seals along corresponding closed sealing perimeters  72   a-b . Sealing perimeters  72   a-b  are situated at the interface between major surfaces  66   a-b  and protruding edges  74   a-b . Edges  74   a-b protrude from housing  44  into channel  46 , to allow the establishment of seals along major surfaces  66   a-b . Protrusions  74   a-b  are lined with an elastomeric or foam material along sealing perimeters  72   a-b , for facilitating the establishment of a seal along perimeters  72   a-b . Establishing seals along major surfaces  66   a-b  removes the need for a soft sealing material along the edges of occluding structure  50 . The major-surface seals allow reducing the minimal resistive load imposed by valve  36 . 
     Referring back to FIG. 1, control unit  24  comprises dynamic respiratory control electronics for receiving data from device  22  and for dynamically controlling the operation device  22 . Control unit  24  comprises measurement electronics  38  electrically connected to flow monitoring components  34 , valve control electronics  40  electrically connected to measurement electronics  38  and valve  36 , and a personal computer  44  electrically connected to control electronics  40 . Personal computer  44  serves as a processing/control device, for determining resistive loads to be imposed by valve  36  according to monitoring data received from measurement electronics  38 . Personal computer  44  also serves as an input and output device, for displaying and transmitting monitoring and control data, and for receiving processing instructions. Generally, the different components of control unit  24  may be spatially separated or integrated in a single housing. Generally, control unit  24  may be implemented using dedicated special-purpose hardware or may be integrated in a general-purpose computer, as will be apparent to the skilled artisan. 
     Control unit  24  receives from device  22  monitoring data including motor shaft position, flow rate, and/or pressure information. Control unit  24  then sends motor drive signals to motor  58  for controlling valve  36  to apply desired inspiratory and/or expiratory resistive loads according to the received monitoring data and stored information and instructions. 
     In the preferred embodiment, control unit  24  periodically determines the patient&#39;s current lung volume. Preferably, measurement electronics  38  integrate flow rate data over time to generate the patient&#39;s current volume. The integration step employs pressure data indicative of flow directions. Control unit  24  is then capable of applying a predetermined resistive load for each patient lung volume, according to a stored table of inspiratory and/or expiratory resistive load values to be imposed at specific lung volumes during inspiration and/or expiration. Control unit  24  can then also dynamically adjust the imposed resistive load so as to establish a desired time-dependence for the patient lung volume. Control unit  24  may determine the applied resistive load according to other parameters such as time, flow rates, or mouthpiece pressures. 
     FIG. 1-B shows a control unit  24 ′ according to an alternative embodiment of the present invention. Control unit  24 ′ comprises triggering electronics  39  electrically connected to measurement electronics  38  and to an external imaging and/or treatment device  41 . Imaging/treatment device  41  can, be a computer tomography (CT), magnetic resonance imaging (MRI), laser therapy, or radiotherapy device. Output signals produced by triggering electronics  39  are used to trigger the imaging/therapeutic functions of the external device  41  at predetermined patient lung volumes. Measurement electronics  38 , control electronics  40 , and PC  44  may also transmit measurement or valve control data to an external imaging or therapy device. 
     In a particular application, system  20  constrains the patient&#39;s lung volume between two predetermined values. Desired resistive loads may be applied at the same time. FIGS. 6-A through  6 -C are flowcharts illustrating a preferred method of dynamically controlling valve  36  for such an application. 
     FIG. 6-A shows a subroutine  100  for determining whether the patient is inhaling or exhaling. Subroutine  100  preferably runs in the background of the main program controlling the operation of valve  36 , and executes with a frequency of at least 20 Hz (every 50 ms). If the flow rate measured by monitoring device  34  (shown in FIG. 2-A) is higher than a predetermined positive threshold, the patient is inhaling. If the flow rate measured by monitoring device  34  is lower than a predetermined negative threshold, the patient is exhaling. If the flow rate is not measurable, the mouth pressure measured through tube  40  (see FIG. 2-A) is used to determine whether the patient is trying to inhale or exhale. If the mouth pressure is higher than a predetermined positive threshold, the patient is trying to exhale. If the mouth pressure is lower than predetermined negative threshold, the patient is trying to inhale. If the mouth pressure measurement is inconclusive, subroutine  100  uses the last known inhalation state. 
     The results of subroutine  100  are used in a subroutine  102  illustrated in FIG. 6-B. Subroutine  102  ensures that the patient&#39;s lung volume is maintained between two predetermined values. Subroutine  102  runs periodically in the background. If the patient is inhaling or trying to inhale and the lung volume has exceeded the maximum allowable limit, control unit  24  fully closes valve  36 . Similarly, control unit  24  fully closes valve  36  if the patient&#39;s lung volume is below the minimum allowable limit and the patient is exhaling or trying to exhale. Otherwise, subroutine  102  allows the main program of control unit  24  to maintain control of the position of valve  36 . 
     FIG. 6-C shows a main program  104  for controlling the position of valve  36  in a time-dependent fashion. The index n refers to the position of valve  36 , and thus the resistive load imposed by valve  36 . At each valve position (n), program  104  checks whether a timer has expired and whether the patient&#39;s breathing direction has changed. If the patient&#39;s breathing direction changes, program  104  checks whether the patient is inhaling or exhaling, and then enters the appropriate inhalation or exhalation loop. The valve position (n) is incremented at predetermined timer intervals. The dependence of (n) with time determines the resistive load pattern imposed by valve  36 . 
     Generally, the step of incrementing n can be dependent on any measured or derived parameters characterizing time, flow rates, patient lung volumes, or pressures. If it is desired to control valve  36  in a volume-dependent fashion, the step of incrementing n is made dependent on the current patient lung volume. The timer conditions of program  104  can be removed. The discomfort felt by the patient during sudden openings and closures of valve  36  can be reduced by gradually increasing the applied resistive load before a maximal (valve-closure) resistive load is applied to close valve  36 . The index n is incremented to apply a plurality of increasing resistive loads approaching the maximal resistive load. 
     FIG. 7-A schematically illustrates the time variation of the volume in the patient&#39;s lungs for a method in which a patient&#39;s respiration is constrained around discrete volume levels V 1-3  for predetermined time periods. For the method shown in FIG. 7-A, the volume in the patient&#39;s lungs is used to control the timing of the closures and openings of valve  36 . For example, valve  36  is closed whenever the volume in the patient&#39;s lungs approaches/reaches a volume level V 1 +ΔV 1 . Similarly, valve  36  is closed whenever the volume in the patient&#39;s lungs approaches/reaches volume V 1 . Valve  36  is otherwise at least partially open. The patient&#39;s respiratory function can be evaluated for each breathing regime. The evaluation can include measurements of flow rates, mouth pressures, and CT or MRI imaging. The evaluation data is then recorded and analyzed. The method illustrated in FIG. 7-A limits the motion of the patient&#39;s organs during imaging or therapy, without requiring the patient to hold his or her breath for extended periods of time. 
     FIG. 7-B illustrates schematically three potential dependencies of resistive load on lung volume. Each of the resistive load patterns may be applied during either or both inspiration and expiration. The first is a stairstep function, with higher resistive loads introduced at higher lung volumes. The second is a quasi-continuous linear function, with higher loads introduced at higher lung volumes. The third is a quasi-continuous curved (e.g. sinusoidal) function, with a maximal load introduced at an intermediate lung volume. For each of the illustrated resistive load patterns, the patient&#39;s respiratory function can be evaluated and measurement data can be sent to an external imaging or therapy device. 
     FIG. 7-C shows a potential variation of an inspiratory or expiratory resistive load with time over multiple breaths. The resistive load pattern includes stairsteps, step functions, and continuous functions. Additional forcing functions affecting breathing volumes or rates are also possible. Such varying resistive loads are useful for ventilator management or respiratory muscle training, for example for weaning a patient off a ventilator. Such varying loads may also be used for respiratory function evaluation or for triggering external devices. 
     FIGS. 8-A and  8 -B show top and side views of a respiratory control device  222  according to an alternative embodiment of the present invention. Device  220  comprises a T-shaped conduit  224  defining the side walls of an inspiratory limb  226 , an expiratory limb  228 , and a mouthpiece limb  230 . Arrows  244   a-b  illustrate the directions of air flow through device  220 . Mouthpiece limb  230  includes a mouthpiece  240  defining a mouthpiece aperture  242 . A cylindrical central supporting piston or hub  246  runs longitudinally through the center of limbs  226 ,  228 . Hub  246  is connected to the walls of limbs  226 ,  228  through radial spokes  248 . Hub  246  and spokes  248  serve to provide mechanical stability to conduit  224  within limbs  226 ,  228 . 
     An active inlet (inspiratory control) valve  250  is mounted within inspiratory limb  226 , for controllably occluding the passage of air through limb  226 . An active outlet (expiratory control) valve  252  is mounted within expiratory limb  228 , for controllably occluding the passage of air through limb  228 . Valves  250 ,  252  are capable of independently introducing dynamically variable resistive loads into limbs  226 ,  228 , respectively. Mouthpiece limb  230  includes a mass flow sensor  254 , a mouthpiece pressure connection tube  256 , and a mesh screen  257 . Valves  250 ,  252  and sensor  254  are electrically connected to control unit  24 . 
     Valves  250 ,  252  are iris valves. FIG. 8-C shows a front view of valve  250  in its fully closed position, while FIG. 8-D, shows valve  250  in its fully open position. Valve  252  is similar to valve  250 , but can be operated independently of valve  250 . Valve  250  comprises a plurality of overlapping blades (leaflets)  251 . As shown in FIG. 8-C, blades  251  are capable of extending into the opening of limb  226 , occluding the passage of air therethrough. When valve  250  is fully closed, blades  251  abut hub  246 . Blades  251  are also capable of retracting from the opening of limb  226 , allowing relatively unrestricted air flow through limb  226 . Extending blades  251  partially into the opening of limb  226  allows introducing desired forcing functions (resistive loads) into limb  226 . The forcing functions can be accurately modulated to control the inlet/outlet flow rates at specific lung volumes, as explained above. 
     FIG. 9 illustrates another alternative embodiment of the present invention. An inspiratory limb  326  and an expiratory limb  328  form part of a device  320 . Inspiratory limb  326  comprises plural parallel air/gas channels  327   a-b . An active inlet valve  350  is positioned within channel  327   a  but not within channel  327   b . Channel  327   b  may be connected to a different gas source than channel  327   a , such as an air, oxygen, or other gas source, or to a gas source at higher-than-atmospheric pressure. An active outlet valve  352  is positioned within limb  328 . 
     Device  320  comprises plural pressure and/or flow sensors  354   a-b  all connected to the control unit of the device. Sensors  354   a-b  can be situated at various locations within limbs  326 ,  328 . Sensor  354   a  is situated at the interface between limbs  326  and  128 . Sensor  354   b  is positioned within expiratory limb  328 , externally relative to valve  352 . Data from all sensors  354   a-b  may be used to control the resistive loads imposed by valves  350 ,  352 . 
     It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. Measuring the patient&#39;s lung volume need not require integrating a flow rate over time. Single or multiple active valves of various types can be used. Multiple valves may be independently controlled. Passive one-way valves may be used in conjunction with one or more active valves. A Pito tube, Flesch differential pressure or other known devices may be used for pressure measurements. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.