Abstract:
An expiratory flow sensor system is disclosed herein. The expiratory flow sensor includes an expiratory channel adapted to transfer an expiratory gas, a fresh gas channel adapted to transfer a fresh gas, and a sensing element disposed within the fresh gas channel so that the sensing element is never directly exposed to the expiratory gas. The sensing element is configured to measure a fresh gas flow rate. The fresh gas flow rate may be implemented to estimate an expiratory flow rate in a manner that minimizes imprecision attributable to sensing element contamination.

Description:
FIELD OF THE INVENTION 
       [0001]    This disclosure relates generally to an expiratory flow sensor system and method for a medical ventilator. 
       BACKGROUND OF THE INVENTION 
       [0002]    Medical ventilators are used to provide respiratory support to patients undergoing anesthesia and respiratory treatment whenever the patient&#39;s ability to breath is compromised. The primary function of the medical ventilator is to maintain suitable pressure and flow of gases inspired and expired by the patient. Gas flow may, for example, be maintained based on feedback from an expiratory flow sensor. The gases measured by the expiratory flow sensor can contain contaminants such as water vapor, mucus, arasolized drugs, etc. One problem is that the contaminants can interfere with the sensing element thereby reducing the precision with which expiratory flow is estimated and/or the reliability of the sensing element. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification. 
         [0004]    In an embodiment, an expiratory flow sensor system includes an expiratory channel adapted to transfer an expiratory gas, a fresh gas channel adapted to transfer a fresh gas, and a sensing element disposed within the fresh gas channel so that the sensing element is never directly exposed to the expiratory gas. The sensing element is configured to measure a fresh gas flow rate. The fresh gas flow rate may be implemented to estimate an expiratory flow rate in a manner that minimizes imprecision attributable to sensing element contamination. 
         [0005]    In another embodiment, an expiratory flow sensor system includes an expiratory channel adapted to transfer an expiratory gas, and a flow restrictor disposed within the expiratory channel. The expiratory flow sensor system also includes a fresh gas channel adapted to transfer a fresh gas, and a sensing element disposed within the fresh gas channel such that the sensing element is never directly exposed to the expiratory gas. The sensing element is configured to measure a fresh gas flow rate. The expiratory flow sensor system also includes a controller operatively connected to the sensor. The controller is configured to estimate an expiatory flow rate based on the fresh gas flow rate. 
         [0006]    In another embodiment, a method includes providing an expiratory channel, providing a fresh gas channel pneumatically coupled with the expiratory channel, and providing a sensing element disposed within the fresh gas channel. The method also includes transferring a fresh gas through the fresh gas channel, implementing the sensing element to measure a fresh gas flow rate, and estimating an expiratory gas flow rate based on the fresh gas flow rate. 
         [0007]    Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram illustrating a ventilator system connected to a patient in accordance with an embodiment; 
           [0009]      FIG. 2  is a detailed schematic illustration of an expiratory flow sensor in accordance with an embodiment; and 
           [0010]      FIG. 3  is a detailed schematic illustration of an expiratory flow sensor in accordance with another embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken as limiting the scope of the invention. 
         [0012]    Referring to  FIG. 1 , a schematically illustrated ventilator system  10  is shown connected to a patient  12  in accordance with an exemplary embodiment. The ventilator system  10  includes a ventilator  14 , a breathing circuit  16 , and an expiratory flow sensor  18 . 
         [0013]    The ventilator  14  provides breathing gasses to the patient  12  via the breathing circuit  16 . The ventilator  14  regulates the volume of gasses transferred into the breathing circuit  16  and/or the pressure level of the gasses within the breathing circuit  16  based in part on feedback from the expiratory flow sensor  18 . 
         [0014]    The breathing circuit  16  includes an inspiratory branch  20 , an expiratory branch  22 , a Y-connector  24 , a patient branch  26 , and an interface  28 . The interface  28  is the portion of the breathing circuit  16  that is directly coupled with the patient  12 . According to the embodiment depicted, the interface  28  comprises an endotracheal tube, however it should be appreciated that other known devices may also be implemented for the interface  28 . 
         [0015]    According to one embodiment, breathing gasses are transferred from the ventilator  14 , through the inspiratory branch  20 , the Y-connector  24 , the patient branch  26 , the interface  28 , and are then delivered into the patient&#39;s lungs (not shown). After the breathing gasses are delivered into the patient&#39;s lungs, the patient  12  passively exhales due to the elasticity of his or her lungs. The exhaled gas from the patient&#39;s lungs is transferred through the interface  28 , the patient branch  26 , the Y-connector  24 , the expiratory branch  22 , the expiratory flow sensor  18 , and is then vented to atmosphere  30  or a collection system (not shown). Before the exhaled gas is vented to atmosphere  30 , the expiratory flow sensor  18  estimates one or more characteristics of the expiratory gas flow. Data corresponding to the estimated expiratory flow characteristics is transmitted from the expiratory flow sensor  18  to the ventilator  14 . The ventilator  14  may, for example, implement this data to regulate the volume of breathing gasses delivered during a subsequent breathing cycle or report an independent exhaled volume measurement. 
         [0016]    Referring to  FIG. 2 , the expiratory flow sensor  18  is shown in accordance with an embodiment. The expiratory flow sensor  18  includes an expiratory channel  40 , a fresh gas channel  42 , a sensing element  44 , a source of fresh gas  46 , and a flow restrictor  48 . According to one embodiment, the expiratory flow sensor  18  also includes a controller  50  operatively connected to the sensing element  44 . It should, however, be appreciated that the controller  50  may alternatively be remotely located and incorporated, for example, as a component of the ventilator  14  (shown in  FIG. 1 ). 
         [0017]    The source of fresh gas  46  may, for example, comprise a pressurized storage tank or pump adapted to deliver a generally contaminant free gas. For purposes of this disclosure, a contaminant free gas is one that does not include any of the contaminants commonly associated with ventilation expiration such as mucus, arasolized drugs, etc. 
         [0018]    The expiratory channel  40  is pneumatically coupled in parallel with the fresh gas channel  42 . The diameter of the expiratory channel  40  is generally larger than that of the fresh gas channel  42  in order to allow for a higher flow rate, however the respective diameters may be configurable to meet the needs of a particular application. The fresh gas channel  42  includes a fresh gas inlet  52  coupled with the source of fresh gas  46 . The flow restrictor  48  is disposed within the expiratory channel  40 , and the sensing element  44  is disposed within the fresh gas channel  42 . The expiratory channel  40  is adapted to accommodate an expiratory flow represented by the arrow  54  and/or a fresh gas flow represented by the arrow  55 . The fresh gas channel  42  is exclusively adapted to accommodate a fresh gas flow represented by the arrow  56 . 
         [0019]    The flow restrictor  48  forms a constriction reducing the effective inner diameter of the expiratory channel  40 . Fluid flow through the constriction generates a pressure differential across the flow restrictor  48 . The pressure differential would tend to divert at least a portion of any expiratory gasses from the patient  12  (shown in  FIG. 1 ) into the fresh gas channel  42  in the absence of the fresh gas flow described in detail hereinafter. 
         [0020]    The source of fresh gas  46  is configured to introduce fresh gas into the fresh gas inlet  52  at a rate sufficient to prevent expiratory gasses from entering the fresh gas channel  42 . The fresh gas introduced into the fresh gas inlet  52  takes the path of least resistance through one or both of the fresh gas channel  42  and the expiratory channel  40 . As an example, if no expiratory gas is passing through the expiratory channel  40 , half of the fresh gas may pass through the fresh gas channel  42  and the other half of the fresh gas may pass through the expiratory channel  40 . Similarly, if a high volume of expiratory gas is passing through the expiratory channel  40 , all the fresh gas introduced into the fresh gas inlet may be passed through the fresh gas channel  42 . 
         [0021]    Implementing fresh gas to prevent expiratory gasses from entering the fresh gas channel  42  in the manner described hereinabove has the effect of protecting the sensing element from expiratory contaminants. More precisely, by preventing the expiratory gasses from entering the fresh gas channel  42  in which the sensing element  44  is disposed, the sensing element  44  is never directly exposed to the expiratory gasses or any constituent contaminants. As the sensing element  44  only comes into contact with contaminant free fresh gas and is never exposed to expiratory gasses, the precision and reliability of the sensing element  44  cannot become diminished as a result of expiratory contaminant exposure. 
         [0022]    During operation, the sensing element  44  can be configured to measure the flow rate of the fresh gas passing through the fresh gas channel  42 . Thereafter, the controller  50  can estimate the flow rate of the expiratory gas from the patient  12  (shown in  FIG. 1 ) based on the fresh gas flow rate measurements. According to one embodiment, this conversion is accomplished by calibrating expiratory flow sensor  18  relative to a conventional flow sensor. According to another embodiment, the difference between the volume of fresh gas introduced into the fresh gas inlet  52  and the volume of fresh gas measured at the sensing element  44  may be implemented to estimate expiratory gas flow rate. 
         [0023]    Referring to  FIG. 3 , an expiratory flow sensor  58  representing an alternate embodiment of the expiratory flow sensor  18  (shown in  FIG. 2 ) is shown in detail. The expiratory flow sensor  58  includes an expiratory channel  60 , a fresh gas channel  62 , a sensing element  64 , a source of fresh gas  66 , and a flow restrictor  68 . According to one embodiment, the expiratory flow sensor  58  also includes a controller  70  operatively connected to the sensing element  64 . It should, however, be appreciated that the controller  70  may alternatively be remotely located and incorporated, for example, as a component of the ventilator  14  (shown in  FIG. 1 ). 
         [0024]    The fresh gas channel  62  is pneumatically coupled with the expiratory channel  60  at a downstream position as measured relative to the flow restrictor  68 . The diameter of the expiratory channel  60  is generally larger than that of the fresh gas channel  62  in order to allow for a higher flow rate, however the respective diameters may be configurable to meet the needs of a particular application. The fresh gas channel  62  is pneumatically coupled with the source of fresh gas  66  that may, according to one embodiment, comprise atmospheric air. The flow restrictor  68  is disposed within the expiratory channel  60 , and the sensing element  64  is disposed within the fresh gas channel  62 . The expiratory channel  60  is adapted to accommodate an expiratory flow represented by the arrow  72  and/or a fresh gas flow represented by the arrow  74 . The fresh gas channel  62  is exclusively adapted to accommodate the fresh gas flow represented by the arrow  74 . 
         [0025]    The flow restrictor  68  forms a constriction reducing the effective inner diameter of the expiratory channel  60 . Fluid flow through the constriction generates a low-pressure region downstream from the flow restrictor  68  in accordance with the Venturi effect, which is well known to those skilled in the art. This low-pressure region pulls fresh gas from the source of fresh gas  66 . It should be appreciated that the sensing element  64  disposed within the fresh gas channel only comes into contact with fresh gas drawn from the source of fresh gas  66 . As the sensing element  64  is never exposed to expiratory gasses, the precision and reliability of the sensing element  64  cannot become diminished as a result of expiratory contaminant exposure. 
         [0026]    During operation, the sensing element  64  can be configured to measure the flow rate of the fresh gas passing through the fresh gas channel  62 . Thereafter, the controller  70  can estimate the flow rate of the expiratory gas from the patient  12  (shown in  FIG. 1 ) based on the fresh gas flow rate measurements. According to one embodiment, this conversion is accomplished by calibrating expiratory flow sensor  58  relative to a conventional flow sensor. 
         [0027]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.