Abstract:
An integrated respiratory ventilation system and method for its calibration is described. The system includes a ventilator having air conduits, transducers adapted to emit signals proportional to the level of air pressure or air flow within the air conduits, and inspiration, expiration and exhaust ports. A processor-controlled calibrator is in communication with one or more of the air conduits. A preferred calibrator includes a water-filled outer column and a vertically aligned inner column extending into the outer column, the inner column having an upper end in communication with at least one of the ports. A processor controls the opening and closing of the ports and the level of air pressure in, and air flow from, the inner column to measure the level of transducer signals at two known pressures and two known flows. The processor uses the measured transducer signals to calibrate the transducer.

Description:
[0001]    This application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/926,369, filed Apr. 26, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    (1) Field of the Invention 
         [0003]    The present invention relates to a method and apparatus for automatically calibrating the pressure and flow within a ventilator, and in particular to a method and apparatus for automatically calibrating the flow, lung pressure, mouth pressure and reservoir pressure signal transducers within a ventilator responsive to calibration measurements. 
         [0004]    (2) Description of the Prior Art 
         [0005]    Researchers often evaluate the performance of the lung under various test conditions. One common way to evaluate the lung requires the subject to be anesthetized and ventilated with a respirator. In addition, compounds may be delivered through a special aerosol apparatus which is integrated with the ventilator. During the testing, the animal&#39;s airflow in and out of his lung is measured along with one of many possible lung pressures. Which lung pressure to measure is dependent upon the experimental needs of the researcher. Typically, this apparatus is connected to a host PC to perform the data collection. 
         [0006]    A ventilator is generally comprised of a source of predetermined air pressure, an inspiration port connectable to the trachea of a test subject, a conduit from the air source to the inspiration port, and means to control the volume of air flowing through the conduit from the air source to the inspiration port. The ventilator also includes an expiration port in communication with the test subject to remove inspired air, and an exhaust port to discharge air or for collection of the expired air for analysis. 
         [0007]    The inspiration and expiration ports are normally connected via a Y-tube to the subject&#39;s trachea. The exhaust port may be vented to the atmosphere or connected to means to capture expired gases or to maintain a minimum positive pressure on the lung, which is known as positive end-expiratory pressure, or PEEP. The ventilator also optionally includes a nebulizer inline between the ventilator inspiration port and the trachea. 
         [0008]    Flow and pressure signals from the subject apparatus, e.g., an enclosure surrounding the subject, are conditioned by a preamplifier prior to reading and analysis of the results by a computer, normally the host PC. The ventilator, preamplifier and a nebulizer controller can be combined with a central processor into a control unit that is in communication with, and receives commands and parameters from the host computer. 
         [0009]    Operation of the ventilator is controlled in accordance with a preset program by a processor, which determines the pressure of the air source, the volume of air flowing out of the inspiration port, and opening and closing of inspiration and expiration ports. Control of the air pressure and flow is normally achieved by opening and closing of valves within conduits in the ventilator, with the pressure and flow within the conduits at a given time being measured by inline transducers that transmit electrical signals proportional to pressure or flow to the processor. 
         [0010]    In order for the ventilator to function properly, these pressure and flow signals must be precise. Therefore, before operation of the ventilator, it is the practice to calibrate each of the transducers to ensure that the electrical signals accurately reflect the measured pressure or flow. Historically, this calibration has been done manually by applying a predetermined pressure or flow within the relevant conduits and measuring the transducer voltages at given known pressures and flows. This procedure is time consuming and may be inaccurate due to human error. 
         [0011]    Therefore, there is a need for a method and apparatus for automatically calibrating a ventilator, and specifically for a method and apparatus for calibrating the pressure and flow transducers signals in a ventilator. 
       SUMMARY OF THE INVENTION 
       [0012]    Generally, these objectives are achieved by an apparatus comprised of a ventilator, a processor and a calibration means, or calibrator, attachable to the ventilator and operable by the processor to calibrate the ventilator transducer signals. 
         [0013]    The ventilator of the invention is comprised of inspiration and expiration ports connectable to a test subject, a pressurized air source, a conduit, e.g., tubing, for connecting the air source to the inspiration port to enable the flow of pressurized air from the air source. The ventilator also includes an expiration port connectable to the test subject for removing expired air from the test subject, and an exhaust port for exhausting expired air. Means is also provided for measuring pressure and flow of expired air and for transmitting the pressure and flow information, normally via a preamplifier, to the processor. 
         [0014]    The ventilator also includes various valves to control the flow of air through different parts of the ventilator and measurement means, normally transducers, for measuring pressure or flow within the different ventilator conduits. Means is also included for communication between the processor and the various ventilator valves and measurement means to enable the processor to control the flow of air through the ventilator. 
         [0015]    The processor acts in accordance with a software program and user inserted parameters to control the flow of air through the ventilator conduits, including the length of time that the air is permitted to flow. During operation, the processor also receives signals from the various measurement means in the ventilator conduits proportional to the pressure or flow within the conduits. This information is used by the processor to quantitatively determine the pressure or flow based on the level of the signals received. 
         [0016]    For example, if the measurement means are transducers, each transducer will transmit an electrical signal, e.g., a voltage signal, to the processor that is proportional to the pressure or flow in the conduit. The processor is then able to calculate the pressure or flow from this voltage signal from the relationship between the voltage signal and the pressure or flow. 
         [0017]    The measurement means signal, e.g., the voltage signal from transducers, must accurately reflect the level of pressure or flow. That is, each measurement means, before use on the system for ventilation of a test subject, must be calibrated so that the signal level in fact accurately corresponds to the pressure or flow level. 
         [0018]    Instead of calibrating the transducers manually, the present system also incorporates a means for automatically calibrating the transducers, thereby saving time and increasing accuracy. Basically, the present system includes a calibrator attachable to the inspiration and expiration ports of the ventilator to enable the processor to apply known pressures and flows to the system, and determine the signal level of each transducer upon application of the known pressure or flow. 
         [0019]    From this signal and the signal level when there is no applied pressure or flow, a linear relationship between the signal level and pressure or flow can be calculated. Then, during use of the ventilator for ventilating a test subject, the processor can determine the precise level of pressure or flow using this relationship and compare the level to the desired level. 
         [0020]    Generally, the calibrator is comprised of a water-filled outer column with a vertically aligned inner column, or rigid tube, extending into the outer column, a piece of tubing connecting the main test chamber to the exhaust port on the ventilator, and a plug which blocks the tracheal port of the Y connector. The inner column includes an upper end above the level of the water in the outer column, and a lower end in the water. The upper end is in communication with the inspiration port of the ventilator. 
         [0021]    Using this calibration apparatus, two known pressure levels and two known flows can be measured, along with the signal levels of the transducers at these levels. Assuming a straight line relationship between the pressure levels and the signal levels, the pressure at any signal level can then be calculated. 
         [0022]    A first pressure level is measured when nothing is flowing into the inner column and the water level in the inner column is the same as the water level in outer column, the apparatus is applying 0 cm H 2 O and zero flow. The signal levels of the transducers to be calibrated are read at this first pressure level. 
         [0023]    To measure a second pressure level, the processor opens the inspiration port valve fully, while simultaneously closing the reservoir bleed port fully. The processor applies a known pressure level as provided by calibration apparatus by flowing air through the inspiration port and closing expiration port, and waiting until the air bubbles out the bottom of the inner column. At this time, the inspiration port is closed, trapping the air in the inner column, with no bubbles flowing, and the second pressure reading is taken. The signal levels of the transducers to be calibrated are also read at this second pressure level. 
         [0024]    When the air flowing into the inner column is stopped and the air is allowed to flow out of the inner column, then a known amount of air will be forced by the water pressure out the top of the inner column. This known volume of air will be forced out within a limited and known amount of time. Since the volume of the inner column and the maximum time it takes to force the volume out are known, the average flow rate that the air moved during that time can be calculated. That average flow rate is the second known flow for calibration, the first flow being zero flow. 
         [0025]    Lung pressure, mouth pressure, and reservoir pressure transducers are calibrated using the above procedure with the signals measured at the first and second pressure levels. Subject air flow is calibrated from the linear relationship between the above two known calibration flows and the signals taken at the subject air flow transducer at each flow. In calibrating subject air flow, the processor closes the expiration port and applies a known flow with the inflation port open until air bubbles exit the bottom of the inner column. The inspiration port is then closed and the expiration port is opened, which releases the air in the inner column and allows the air to be forced into the expiration port out of the exhaust port. Since the exhaust port is connected to the test chamber or the subject flow measurement apparatus, the flow from the inner column is in turn forced through the flow measurement apparatus. The average flow over a region of time can then be calculated based on the known volume of the inner column and the time that it takes to exhaust the air. 
         [0026]    Calibration of inspiration flow is similar to calibration of subject air flow. This must be calibrated after the mouth pressure has been calibrated. The processor fills the inner column to some level. The processor can measure the level by measuring the pressure on the mouth pressure transducer. Knowing the pressure on the mouth pressure, we know the level inside the inner column. Since the inner column is uniformly bored, and the level inside the inner column is proportional to the volume of air in the inner column, the volume filled can be calculated directly by measuring the mouth pressure. The average flow is then known by dividing that known volume by the time it takes to fill that volume. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is an illustration of the typical system components of a prior art ventilator. 
           [0028]      FIG. 2  is an illustration of the ventilator ports. 
           [0029]      FIG. 3  is a diagram of the integrated ventilator. 
           [0030]      FIG. 4  is a detailed diagram of the components that the processor uses to ventilate the subject. 
           [0031]      FIG. 5  is a configuration of the reservoir when the subject is ventilated with normal air. 
           [0032]      FIG. 6  is a configuration of the reservoir when the subject is ventilated with a gas source. 
           [0033]      FIG. 7  illustrates the calibration apparatus used by the system to provide known pressure and flow levels. 
           [0034]      FIG. 8  illustrates the calibration apparatus when no air is flowing through it. 
           [0035]      FIG. 9  illustrates the calibration apparatus with air flowing through it. 
           [0036]      FIG. 10  shows the calibration apparatus when the pressure which was created when air was forced into tube  62  is about to be released. 
           [0037]      FIG. 11  illustrates the connection of the calibration apparatus to combined unit. 
           [0038]      FIGS. 12-16  illustrate the routines and subroutines undertaken by the processor in calibrating the ventilator with the calibrator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    In the following description, terms such as horizontal, upright, vertical, above, below, beneath, and the like, are used solely for the purpose of clarity in illustrating the invention, and should not be taken as words of limitation. The drawings are for the purpose of illustrating the invention and are not intended to be to scale. 
         [0040]      FIG. 1  shows the basic components of a system, generally  10 , commonly used for respiratory studies. The system includes a ventilator  12 , nebulizer  15  placed inline between the inspiration port  14  and subject apparatus  16  enclosing the test subject, and preamplifier  18  to condition the flow and pressure signals from enclosure  16  so that host computer  20 , typically a PC, can read the signals and analyze the results. 
         [0041]    Ventilator  10  generally has 3 ports: inspiration port  22 , expiration port  24 , and exhaust port  26 . Inspiration port  22  fills the lung. Expiration port  24  vents the lung. Exhaust port  26  may often be left open to atmosphere, but permits the researcher to either capture the expired gases, or maintain a minimum positive pressure on the lung (called positive end-expiratory pressure, or PEEP). 
         [0042]      FIG. 2  shows how ports  22 ,  24  and  26  are connected to subject apparatus  16 . Inspiration port  22  and expiration port  24  are connected with tubing  28  Y-junction which joins the two ports very near the trachea of the subject. The trachea is connected to the third port on the Y connector, with as little tubing as possible. This configuration minimizes the volume of air that the subject must take into its lungs before it gets fresh air from the ventilator, and therefore enables the animal to get maximum fresh air with each breath. Exhaust port  26  remains open to atmosphere. Another common configuration would call for tubing to be connected to exhaust port  26 , with the free end of the tubing submerged a couple centimeters below the surface of a water column. 
         [0043]    The components of ventilator  12  are preferably combined into a single unit as shown in  FIG. 3 . With the aid of dedicated central processor unit  30 , a new level of system intelligence is achieved because these previously separate tools can be coordinated to operate as one in order to achieve specific goals. Within this unit, processor  30  coordinates a set of specific actions as directed by host computer  32 . Processor  30  controls the breathing of the subject using information relating to 1) target breathing rate, 2) maximum inspiration volume, and 3) maximum mouth pressure. In addition, it is the goal of ventilator  12  to shape the inspiratory flow pattern in order to deliver the air in a way which is more comfortable to the subject. The expiratory flow pattern is passive. That is, ventilator  12  does not attempt to actively control the flow of air out of the lungs beyond simply permitting the air to flow. 
         [0044]      FIG. 4  details the components that processor  30  uses to ventilate the subject. As shown in the diagram, respirator ports  22 ,  24  and  26  are shown on the right. The air which is delivered into the subject&#39;s lungs is regulated to a safe pressure in reservoir  40 . The air flows from reservoir  40  through flow transducer  42  and flow adjust  44  before it is made available to the subject. Flow transducer  42  provides the processor with the air flow into the lung at any time. Flow adjust  44  permits the processor to control the air flow by adjusting the air resistance between reservoir  40  and the inspiration port  22 . 
         [0045]    Exhaust valve  46  opens during expiration to permit the air to flow out from the lung to the exhaust port. During this time, flow adjust valve  44  is closed (infinite resistance). During inspiration, exhaust valve  46  is closed. 
         [0046]    Mouth pressure is measured at either the inspiration port  22  or the expiration port  24 , depending upon where ventilator  10  is in the breath cycle. During inspiration, when expiratory valve  46  is closed, mouth pressure transducer  48  measures the pressure at expiration port  24 . During expiration, the mouth pressure is measured at inspiration port  22 . In other words, mouth pressure transducer  48  measures pressure in static non-moving air in order to get accurate pressure measurements. 
         [0047]    Pressure reservoir  40  is designed to either operate with normal air, or with an optional external tank in the event the user wants to ventilate the subject with some special gases. One example of the former case would be if the user wants to incorporate inhaled anesthesia into the ventilation mixture. 
         [0048]      FIG. 5  shows the configuration that is used when the subject is ventilated with normal air. This diagram shows the components used to regulate the pressure in the reservoir, and the flow of air into and out of the reservoir, when ventilating the subject with normal air. In this configuration, the system senses that the pressure is too low, so it turns on pump  50 , and controls the variable bleed adjust  51  for precise pressure control. Pump  50  cannot respond too quickly so variable bleed adjust  51  is necessary to control the bleed off of excess pressure in the reservoir. 
         [0049]      FIG. 6  illustrates the configuration when the subject is ventilated with a gas source. This diagram shows the pressure regulator and the flow of air into and out of reservoir  40  when a pressurized gas tank feeds reservoir  40 . In this configuration, pump  50  is turned off. A pressurized tank of the ventilation mixture is connected to the bleed port. Processor  30  controls the variable bleed adjust  51  to permit more air through the bleed port. 
       Processor Controlled Calibration 
       [0050]    In the present invention, calibration apparatus, generally  60 , is connected to ventilator  10 , with processor  30  undertaking automated steps to precisely calibrate the following five signals: 1) flow, 2) lung pressure, 3) inspiration flow, 4) mouth pressure, and 5) reservoir pressure. 
         [0051]      FIG. 7  illustrates the calibration apparatus  60  used by this system to provide known pressure and flow levels. Calibration apparatus  60  allows the system to apply two known pressure levels, and to measure two known flows. 
         [0052]      FIG. 8  shows calibration apparatus  60  when no air is flowing through it. When no air flows through it, the water level in rigid tube  62  is the same as the water level in the water column  64 . Also, no air is bubbling up from the bottom of rigid tube  62 . In this configuration, the pressure inside the tube is 0 cm H 2 O. 
         [0053]    When air is allowed to flow into tube  62 , and bubbling up from the bottom as shown in  FIG. 9 , then stopped, the pressure in tube  62  is the distance from the water level in column  64  to the bottom of rigid tube  62 .  FIG. 9  shows calibration apparatus  60  with air flowing through it. The air flowing into tube  62  is stopped, and then the air is allowed to flow out of tube  62 , then a known amount of air will be forced by water column  64  out the top of tube  62 . This known volume of air will be forced out within a limited and known amount of time. And since we know the volume in tube  62 , and we know the time that it takes, then we know the average flow rate that the air moved in that time. That average flow rate is the second known flow for calibration. 
         [0054]      FIG. 10  shows the calibration apparatus  60  when the pressure which was created when air was forced into tube  62  is about to be released. To prepare for calibration, this apparatus should be connected to combined unit  70  and the flow transducer  72  and lung pressure transducer  74  in the following configuration.  FIG. 11  illustrates how the user should connect calibration apparatus  60  to combined unit  70 . As shown, nebulizer  14  does not need to be connected to nebulizer controller  33 . Inspiration port  22 , expiration port  24 , calibration apparatus  60  and the lung pressure transducer  74  should all be connected together using tubing. The exhaust port  26  should be connected to external airflow transducer  72 . The flow through the exhaust will provide a calibration flow which the system uses as a known flow. 
       Calibration of Lung Pressure, Mouth Pressure, and Reservoir Pressure 
       [0055]    It is assumed that relationship between the signals from the pressure transducers and pressure is a straight line relationship. Therefore, the pressure transducers require only two known pressure calibration levels to establish the calibration. Processor  30  applies 0 cm H 2 O pressure to all the transducers. This is done by fully opening the inspiration valve, and simultaneously fully opening the reservoir bleed port. 
         [0056]    Processor  30  applies a known pressure level as provided by calibration apparatus  60  by flowing air through inspiration port  22  and closing expiration port  24 , and waiting until the air bubbles out the bottom of the rigid tube in calibration apparatus  60 . At this time, the reservoir bleed port is adjusted until no bubbles flow out of the bottom of rigid tube  62 . The pressure reading should be measured when the flow through rigid tube  62  is stopped, but the air still fills rigid tube  62 . 
         [0057]    The lung pressure, mouth pressure and reservoir pressure transducers are then calibrated by reading the transducer signals at each of these known levels and assuming a straight line relationship between the signal level and pressure. 
       Calibration of Subject Air Flow 
       [0058]    Like the pressure transducers, air flow transducer  72  requires two known calibration flows to establish the calibration. Processor  30  applies 0 ml/sec flow through air flow transducer  72  by closing expiration port  24 . This causes any air flowing through the exhaust port  26  to cease. 
         [0059]    Processor  30  applies a second known flow through the air flow transducer  72  by performing the following: 
         [0000]    1) Close the expiration port  24 ,
 
2) Turn on Inspiration flow  22  until air bubbles out the bottom of rigid tube  62  in calibration apparatus  60 ,
 
3) Shut off the inspiration flow,
 
4) Open expiration port.
 
         [0060]    Step 4 releases the air in rigid tube  62  and allows it to be forced out expiration port  24  and in turn out the exhaust port  26  and through air flow transducer  72 . While that flow is not constant, we do know the volume, and we know the maximum time it takes to force that volume through. So by calculating the average flow over a region of time, we have a known flow we can use to calibrate. 
         [0061]    The subject air transducer is then calibrated by reading the transducer signals at each of these known levels and assuming a straight line relationship between the signal level and subject air flow. 
       Calibration of Inspiration Flow 
       [0062]    Calibration of inspiration flow, i.e., the inspiration flow transducer, is similar to calibration of subject air flow. However, this must be calibrated after the mouth pressure has been calibrated. Processor  30  applies 0 ml/sec through inspiration flow transducer  72  by closing inspiration port  22 . The Processor  30  fills the inner column to some level. The processor can measure the level by measuring the pressure on the Mouth Pressure  48 . Knowing the pressure, we know the level inside the inner column. Since the inner column is uniformly bored, the level inside the inner column is proportional to the volume of air in the inner column. So the volume filled can be calculated directly by measuring the mouth pressure. The known volume has been filled, so the average flow is then known by dividing that known volume by the time it takes to fill that volume. 
       Deep Breath Cycles 
       [0063]    When assessing lung function, researchers commonly challenge the subject with a drug which causes the lungs to contract. They may challenge the subject several times at increasing doses of the drug in order to see how the reaction changes. Between each challenge, it is often necessary to open the lungs by forcing a few deep breaths. With current systems, researchers perform these deep breaths by covering the exhaust port  26  for about 3 breaths. 
         [0064]    By covering exhaust port  26 , the researcher prevents the subject from exhaling, and so, the subject takes a breath with 3 times as much volume as one breath. Processor  30  controls the deep breath in the same way that it controls a regular breath, except that the maximum volume and maximum pressure conditions are increased, and the desired breathing rate is disregarded. 
       Nebulizer Breath Cycles 
       [0065]    While nebulizer  15  is producing aerosol, the ventilator can force the subject to breathe more deeply in order to deliver the aerosol more deeply into the lung. In addition, nebulizer  15  can be triggered to produce aerosol synchronous with the ventilator to most efficiently deliver the material to the lungs. 
       Electronically Controlled PEEP 
       [0066]    Because the mouth pressure measurement switches to measure pressure at inspiration port  22  during expiration, this respirator can maintain PEEP without the use of a water column on the exhaust port  26  (which is typically done). 
       Integrated Blood Pressure or ECG Preamplifier 
       [0067]    This system incorporates an integrated Blood Pressure or ECG preamplifier to monitor the life status of the subject. 
         [0068]    Certain modifications and improvements will occur to those skilled in the art upon a reading of the foregoing description. It should be understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope of the following claims.