Abstract:
A ventilator for ventilating a patient has means integrated therewith for carrying out a determination of the functional residual capacity of the patient using an inert gas wash in/wash out technique. To this end, the ventilator operates to alter the inert gas content of breathing gases provided to the patient. The amount of inert gas expired by the patient is obtained and used to determine functional residual capacity on a breath-by-breath basis. A graph of the functional residual capacities for a given number of breaths is produced. Thereafter, the inert gas levels in the breathing gases are returned to the original levels and further functional residual capacity determinations and a graph of same provided. The functional residual capacity information may also be provided in tabular form. A log of functional residual capacity determinations and ventilator settings or patient treatments affecting same may also be provided.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The present application claims the priority of U.S. Provisional Application Ser. No. 60/719,322, filed Sep. 21, 2005. 
     
    
     BACKGROUND AND SUMMARY  
       [0002]     The present invention relates to an apparatus and method for determining and displaying functional residual capacity data and other pulmonary parameters, such as positive end expiratory pressure (PEEP) data, for patients breathing with the aid of a mechanical ventilator, such as a critical care ventilator. The invention also determines and displays relationships between these and other parameters.  
         [0003]     Functional residual capacity (FRC) is the gas volume remaining in the lungs after unforced expiration or exhalation. Several methods are currently used to measure functional residual capacity. In the body plethysmography technique, the patient is placed in a gas tight body box. The patient&#39;s airway is sealingly connected to a breathing conduit connected to the exterior of the body box. By measuring lung pressures and pressures in the box, at various respiratory states and breathing gas valve flow control conditions, the functional residual capacity of the patient can be determined.  
         [0004]     Another technique for measuring functional residual capacity is the helium dilution technique. This is a closed circuit method in which the patient inhales from a source of helium of known concentration and volume. When the concentration of helium in the source and in the lungs has reached equilibrium, the resulting helium concentration can be used to determine the functional residual capacity of the patient&#39;s lungs.  
         [0005]     A further technique for determining functional residual capacity is the inert gas wash-out technique. This technique is based on a determination of the amount of gas exhaled from the patient&#39;s lungs and corresponding changes in gas concentrations in the exhaled gas. The gas used for the measurement is inert in the sense that it is not consumed by metabolic activity during respiration. While a number of gases may be used for such a measurement of functional residual capacity, it is convenient to use nitrogen for this purpose.  
         [0006]     In a straightforward example in which the patient is initially breathing air, the lung volume forming the functional residual capacity of the lung will contain nitrogen in the same percentage as air, i.e. approximately 80%, the remaining 20% of air being oxygen. In a wash-out measurement, the subject commences breathing gases in which oxygen is at a different concentration than 20%. For example, the patient commences breathing pure oxygen. With each breath, nitrogen in the lungs is replaced by oxygen, or, stated conversely, the nitrogen is “washed out” of the lungs by the oxygen. While the breathing of pure oxygen could continue until all nitrogen is washed out of the lungs, in most cases, the breathing of oxygen continues until the nitrogen concentration in the exhaled breathing gases falls below a given concentration. By determining the volume of inert gas washed out of the lungs, and knowing the initial concentration of the inert gas in the lungs, the functional residual capacity of the lungs may be determined from these quantities.  
         [0007]     Methods for determining functional residual capacity in this manner are well known and are described in such literature as The Biomedical Engineering Handbook, CRC Press, 1995, ISBN 0-8493-8346-3, pp. 1237-1238, Critical Care Medicine, Vol. 18, No. 1, 1990, pp. 8491, and the  Yearbook of Intensive Care and Emergency Medicine , Springler, 1998, ISBN 3-540-63798-2, pp. 353-360. By analogy to the above described wash out measurement technique, it is also possible to use a wash in of inert gas for measurement of functional residual capacity. Such a method and apparatus is described in European Patent Publication EP 791,327.  
         [0008]     The foregoing methods are used with spontaneously breathing patients and are typically carried out in a respiratory mechanics laboratory. But in many cases, patients that could benefit from a determination of functional residual capacity are so seriously ill as to not be breathing spontaneously but by means of a mechanical ventilator, such as a critical care ventilator. This circumstance has heretofore proven to be a significant impediment in obtaining functional residual capacity information from such patients. Additionally, the patient&#39;s illness may also make it impossible or inadvisable to move the patient to a laboratory or into and out of a body box for the determination of functional residual capacity.  
         [0009]     It would therefore be highly advantageous to have an apparatus and method by which the functional residual capacity of mechanically ventilated patients could be determined. It would be further advantageous to associate the apparatus for carrying out the determination of functional residual capacity with the ventilator to reduce the amount of equipment surrounding the patient and to facilitate set up and operation of the equipment by an attending clinician. Such apparatus would also enable the determination of functional residual capacity to be carried out at the bedside of the patient, thus avoiding the need to move the patient.  
         [0010]     A single determination of functional residual capacity provides important information regarding the pulmonary state of the patient. However, it is often highly desirable from a diagnostic or therapeutic standpoint to have available trends or changes in the functional residual capacity of a patient over time.  
         [0011]     It would also be helpful to be able to relate functional residual capacity to other pulmonary conditions existing in the lungs or established by the ventilator and to changes in these conditions. For example, it is known that the pressure established by the ventilator in the lungs at the end of expiration, the positive end expiratory pressure or PEEP, affects the functional residual capacity of the lungs.  
         [0012]     Typically, an increase in PEEP increases functional residual capacity. There are two components to the increased functional residual capacity as PEEP is increased. One component is due to stretching of the lung by the increased pressure. A second component, particularly in diseased lungs, occurs from the effect of PEEP during breathing by the patient. As a patient expires, the pressure in the lungs drops until it approaches airway pressure. As the pressure within the lungs drops, the alveoli or air sacs in the lungs deflate. If alveolar sacs collapse completely, more pressure is required upon inspiration to overcome the alveolar resistance and re-inflate the alveolar sacs. If this resistance cannot be overcome, the volume of such sacs are not included in the functional residual capacity of the patient&#39;s lungs.  
         [0013]     By applying PEEP in the patient&#39;s airway, the additional pressure in the patient&#39;s lungs keeps more of these alveolar sacs from completely collapsing upon expiration and, as such, allows them to participate in ventilation. This increases the functional residual capacity of the patient&#39;s lungs and the increase is often described as “recruited volume.” Volume reductions are termed “de-recruitments.” 
         [0014]     However, setting the PEEP too high can cause excessive lung distension. There may also be compression of the pulmonary bed of the lung, loading the right side of the heart and reducing the blood volume available for gas exchange. Either of these circumstances present the possibility of adverse consequences to the patient.  
         [0015]     Still further, action such as performing a suction routine, administering a nebulized medication, or changing the ventilation parameters of the ventilator can also influence functional residual capacity and it would be helpful to be able to easily determine the effect of such actions on functional residual capacity.  
         [0016]     An apparatus and method that would possess the foregoing characteristics and that would easily and cogently make such information available would be highly beneficial in conveniently obtaining a full understanding of the pulmonary condition of the patient and how the patient is reacting to the mechanical ventilation and to any associated therapeutic measures. The clinician could then carry out appropriate action beneficial to the patient in a timely and informed manner.  
       BRIEF DESCRIPTION OF THE PRESENT INVENTION  
       [0017]     An embodiment of the present invention comprises an apparatus and method that achieves the desired, highly advantageous features noted above. Thus, with the present invention the functional residual capacity of a mechanically ventilated patient may be determined at the bedside of the patient without the need to move the patient to a laboratory. By associating the apparatus with the ventilator, only a single device need be employed to both ventilate the patient and determine functional residual capacity.  
         [0018]     The determined functional residual capacity may be advantageously displayed in conjunction with earlier determinations and in conjunction with other pulmonary conditions, such as PEEP. Changes, or trends, in functional residual capacity over time may thus be discerned, along with changes in the other pulmonary conditions.  
         [0019]     Additionally, the apparatus and method provides a log of events having the potential to impact the functional residual capacity of the patient and/or its accurate determination. Such events may include suctioning the patient, administering a nebulized medication to the patient, performing a lung recruitment maneuver, and altering the PEEP or other ventilator parameters. The apparatus may also automate functional residual capacity measurement in conjunction with these types of events. For example, it is desirable to provide that functional residual capacity measurements be automatically conducted immediately before and after nebulized drug therapy in order to precisely gauge the effect of the nebulization treatment.  
         [0020]     The foregoing provides an attending clinician with significant information for assessing the state of, and trends in, the functional residual capacity of the patient, as well as the relationship between the patient&#39;s residual capacity and the other factors, so that the clinician can fully discern the functional residual capacity condition of the patient.  
         [0021]     Further features of the apparatus and method of the present invention will be apparent from the following detailed description, taken in conjunction with the associated drawing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a general diagram of a mechanical ventilator and associated apparatus for ventilating a patient.  
         [0023]      FIG. 2  shows an endotracheal tube with a tracheal pressure sensor suitable for use in the present invention.  
         [0024]      FIG. 3  shows a ventilator display unit presenting an initial display screen for use in the present invention.  
         [0025]      FIG. 4  is a chart showing the relationship among a plurality of screens employed in the present invention.  
         [0026]      FIG. 5  shows a display screen for displaying functional residual capacity data and related data.  
         [0027]      FIG. 6  shows a display for use in scaling the display shown in  FIG. 5 .  
         [0028]      FIG. 7  is a flow chart showing the steps for carrying out a measurement of functional residual capacity in accordance with a method of the present invention.  
         [0029]      FIG. 8  shows a display displaying a log of events and actions that may impact the determination of functional residual capacity.  
         [0030]      FIG. 9  shows a display showing spirometry data.  
         [0031]      FIG. 10  shows a display for making setup adjustments for the screen shown in  FIG. 9 .  
         [0032]      FIG. 11  shows a tabular display displaying fictional residual capacity values with periodically obtained ventilator operating data and/or patient condition data.  
         [0033]      FIG. 12  shows a graphic display displaying functional residual capacity values with periodically obtained ventilator operating data and/or patient condition data. 
     
    
     DETAILED DESCRIPTION  
     The Mechanical Ventilator and Airway Gas Module  
       [0034]      FIG. 1  shows mechanical ventilator  10  for providing breathing gases to patient  12 . Ventilator  10  receives air in conduit  14  from an appropriate source, not shown, such as a cylinder of pressurized air or a hospital air supply manifold. Ventilator  10  also receives pressurized oxygen in conduit  16  also from an appropriate source, not shown, such as a cylinder or manifold. The flow of air in ventilator  10  is measured by flow sensor  18  and controlled by valve  20 . The flow of oxygen is measured by flow sensor  22  and controlled by valve  24 . The operation of valves  20  and  24  is established by a control device such as central processing unit  26  in the ventilator.  
         [0035]     The air and oxygen are mixed in conduit  28  of ventilator  10  and provided to inspiratory limb  30  of breathing circuit  32 . Inspiratory limb  30  is connected to one arm of Y-connector  34 . Another arm of Y-connector  34  is connected to patient limb  36 . During inspiration, patient limb  36  provides breathing gases to lungs  38  of patient  12 . Patient limb  36  receives breathing gases from the lungs of the patient during expiration. Patient limb  36  may include components such as a humidifier for the breathing gases, a heater for the breathing gases, a nebulizer, or a water trap (not shown). The breathing gases expired by patient  12  are provided through patient limb  36  and Y-connector  34  to expiratory limb  46  of breathing circuit  32 . The expired breathing gases in expiratory limb  46  are provided through valve  54  and flow sensor  56  for discharge from ventilator  10 . Valve  54  may be used to establish the PEEP for patient  12 .  
         [0036]     Patient limb  36  includes gas flow and pressure sensor  57  which may be of the type shown in U.S. Pat. No. 5,088,332. A pair of pressure ports and lines  58 ,  60  are placed on either side of a flow restriction in the sensor and the pressure difference developed across the flow restriction is used by flow measurement unit  62  in gas module  64  to measure gas flow in patient limb  36 . One of the pressure lines is connected to pressure measurement unit  66  to measure the pressure in patient limb  36 . Sensor  57  also provides for a gas sampling line  68  which is connected to gas analyzer  70 . Gas analyzer  70  may measure the amount of oxygen and carbon dioxide in the breathing gases. Knowing the amounts of oxygen and carbon dioxide in the breathing gases enables the amount of nitrogen to be determined as the total amount less the amounts of carbon dioxide and oxygen. Respiratory and metabolic gas module  64  may comprise that made and sold by GE Healthcare as a Datex-Ohmeda MCOVX gas module. The output of gas module  64  is provided in data bus  72  to central processing unit  74  in ventilator display unit  76 . Central processing unit  26  in ventilator  10  is also connected to central processing unit  74  via data bus  78 .  
       The Endotracheal Tube  
       [0037]     To obtain an accurate indication of the pressure in lungs  38  of the patient  12 , endotracheal tube  90  shown in  FIG. 2  may be used. Endotracheal tube  90  has end  92  for connection to patient limb  36 . In use, endotracheal tube  90  extends through the mouth and into the trachea of patient  12  to provide an airway passage to lungs  38 .  
         [0038]     Endotracheal tube  90  includes pressure sensor catheter  94  that extends from end  96  to provide a pressure sampling point that is close to lungs  38  of patient  12  when the endotracheal tube is inserted in the patient and can thus obtain a highly accurate indication of the pressure in the lungs. An intermediate portion of catheter  94  may lie within endotracheal tube  90 . The proximal portion exits the endotracheal tube and is connected via A-A to a pressure transducer and to an auxiliary input to ventilator display unit  76 . The pressure obtained from catheter  94  is termed Paux. While  FIGS. 1 and 2  show a connection to ventilator display unit  76  for this purpose, the connection may, alternatively, be to gas module  64 .  
         [0039]     An endotracheal tube of the type shown in  FIG. 2  is described in U.S. Pat. No. 6,315,739.  
       Ventilator Display Unit  
       [0040]     Display unit  76  of ventilator  10  receives information from the ventilator and gas module  64  and is used by the clinician to control the pneumatic control components of ventilator  10  that deliver breathing gases to patient  12  via data bus  78 . Additionally, central processing unit  74  in display unit  76  carries out the determination of functional residual capacity, recruited/de-recruited volumes, and other quantities employed in the present invention. It will be appreciated that other CPU configurations, such as a single CPU for the ventilator and its display unit may be used, if desired.  
         [0041]     Ventilator display unit  76  includes user interface  100  and display  102 . Display  102  is shown in greater detail in  FIG. 3 . Display  102  is divided into a number of display portions  102   a - g  for displaying inputted, sensed, and computed information. Display portions  62   a  though  102   f  relate primarily to the operation of ventilator  10  and the ventilation of patient  12  and are discussed briefly below. Display screen portion  102   g  displays information and relationships in accordance with the present invention, as described in detail below.  
         [0042]     Display portion  102   a  provides for the display of operating information of ventilator  10 . The portion shows the type of ventilation being performed by ventilator  10 , in the exemplary case of  FIG. 3 , synchronized, intermittent, mandatory ventilation, or SIMV-volume controlled ventilation. Portion  102   a  also provides a display of operating information inputted into ventilator  10  including the percentage of oxygen for the breathing gases, tidal volume (TV), breathing rate, inspiration time (T insp ), amount of positive end expiratory pressure (PEEP) and the pressure limit (P limit ) set for the volume controlled ventilation. To input these operating parameters into ventilator  10 , an appropriate one of buttons  104   a  through  104   f  is actuated. Control knob  106  is rotated to enter a desired value for the selected option and pressed to confirm the new parameter value. Further ventilator functions may be controlled by pressing a button that controls a specialized function such as ventilator setup button  72  that establishes other ventilation modes for patient  12 , spirometry button  74  for showing and controlling the display of spirometry information, 100% 0 2  button  76 , nebulizer button  78 , and procedures button  80  that controls specialized procedures for ventilator  10 .  
         [0043]     Display portion  102   b  of display  102  shows airway pressure data as measured from sensor  57 . Portion  102   c  shows textual information relating to the flow of breathing gases to the patient obtained from sensor  57 , and portion  102   d  shows pressure data from catheter  94  in the endotracheal tube  90  during ventilation of patient  12 .  
         [0044]     Portion  102   e  of display  102  shows the information in regions  102   b ,  102   c , and  102   d  in graphic form and includes an indication of certain other operating information, such as the mode of ventilation SIMV-VC, and whether certain features of the present invention are operational or not.  
         [0045]     Display portion  102   f  of display  62  shows additional data as selected by the clinician. In the example of  FIG. 3  end tidal CO 2 (E t CO 2 ), lung compliance, expiratory alveolar minute volume (MVe (alv)), respiratory rate, total positive end expiratory pressure, and inspiratory alveolar minute volume (MVi (alv)) are being shown.  
         [0046]     Display portion  102   a - f  remain generally unchanged as the present invention is practiced although, as noted above, the clinician may select the information to be shown in certain portions, such as portion  102   f.    
       Display Screen of Present Invention  
       [0047]     Display screen  102   g  is the part of display  102  employed in the present invention. As shown in  FIG. 4  and in  FIGS. 5, 6 ,  8 ,  9 , and  10 , the content of this screen will change, depending on the inventive feature being utilized, the different content in screen  102   g  being identified as  102   g   1 ,  102   g   2 ,  102   g   3 , etc. in the appropriate figures of the drawing.  
         [0048]     In general, each screen  102   g  will include a menu or control portion  108 , a graphic portion  110  and tabular portion  112 . For this purpose, graphic portion  110  contains a pair of orthogonal axes by which data can be graphically presented. The clinician may navigate and control the screen using control knob  106 . Control knob  106  is rotated to scroll through the menu options displayed in menu portion  108 , depressed to select a menu option, rotated again to establish a numerical value for the selected option when appropriate, and depressed again to enter the value into ventilator display unit  76  or to confirm selection of the menu option.  
         [0049]      FIG. 3  shows an initial content for screen  102   g  relating to spirometry. As hereinafter noted, spirometry illustrates the relationship between inspired gas volumes and the pressure in the lungs as the patient breathes. The graphic form of the data is normally in a loop, a portion of which is formed during inspiration and the other portion of which is formed during expiration in the manner shown in  FIG. 10 . The tabular portion  112  provides fields in which various obtained and computed ventilation and lung properties may be displayed.  
         [0050]     Menu portion  108  allows the clinician to select a number of options with respect to the display and use of the information shown in graphic and tabular portions  110  and  112 . Menu portion  108  also allows the clinician to select a further screen at  116  for adjusting the scaling for the abscissa and ordinate of graph  110  and the setup for spirometry measurements at  118 .  
         [0051]     From menu portion  108 , the clinician may also select screens that allow the functional residual capacity (FRC) features of the present invention and the spirometry features of the present invention to be carried out by selecting items  120  and  122 , respectively. The spirometry features of the present invention are identified by applicant as SpiroDynamics or the abbreviation SpiroD.  
         [0052]      FIG. 4  shows the architecture of the screens  102   g  used in the present invention. As noted above, the spirometry screen shown in  FIG. 3  as screen  102   g   1  is the initial screen appearing as screen  102   g . As noted above, associated with this screen are screens for spirometry scaling and spirometry setup.  
         [0053]     By means of menu items  120  and  122 , the clinician can select either a screen relating to functional residual capacity, namely screen  102   g   2  shown  FIG. 5  or a screen relating to SpiroDynamics comprising screen  102   g   3  of  FIG. 9 . The screen format of  FIG. 5  is termed “FRC INview.” The view of  FIG. 9  is termed “spiroD”.  
         [0054]     The FRC INview showing of  102   g   2  includes screen shown in  FIG. 6  that allows for scaling of the quantities shown graphically in  FIG. 5 .  
         [0055]     A further selection on the FRC INview screen allows the clinician to select the FRC log screen shown in  FIG. 8  as screen  102   g   4 .  
       FRC Determination and Display  
       [0056]     The flow chart of  FIG. 7  shows a method of the present invention for determining and displaying functional residual capacity information for patient  12 . The clinician uses a screen in the format of  102   g   2  of  FIG. 5 . It is assumed that the clinician has previously established an oxygen percentage for the breathing gases to be provided by ventilator  10  using button  104   a , control knob  106  and screen region  102   a , at step  200 . In the example shown in  FIG. 3 , the oxygen percentage is 50%. Ventilator  10  can be operated with the set percentage of oxygen to provide breathing gases to patient  12  at step  202 .  
         [0057]     As noted above, in order to determine the functional residual capacity of patient  12  by a gas wash-out/wash-in technique, it is necessary to alter the composition of the breathing gases supplied to patient  12 . To this end, the clinician sets a different level for the oxygen content of the breathing gases. This is performed by selecting the FRC 0 2  field  206  in menu portion  68  of screen  102   g   1  and appropriately establishing the FRC 0 2  value. The amount of change may be an increase or decrease from the previously set level established at step  200 ; however it must be an amount sufficient to perform the functional residual capacity analysis. A change of at least 10% is preferable in order to obtain an accurate indication of the functional residual capacity. To ensure that appropriate oxygen concentrations are supplied to patient  12  it is usually desired to increase the oxygen level and, unless the current oxygen level is very high (greater than 90%), a default setting of a 10% increase over the current setting may be provided. The level of oxygen set by the clinician “tracks” changes made in the oxygen content of the breathing gases at the ventilator, as for example by actuating button  104   a . Thus, for example, if the ventilator oxygen is originally 50% as shown in  FIG. 3 , and the FRC 02 shown in  FIG. 5  is 60%, if the ventilator oxygen setting is later changed to 70%, the FRC 02 amount will automatically move to 80%.  
         [0058]     Lowering the ventilator oxygen setting, however, will not result in lowering the FRC 02 amount, thereby avoiding the possibility of low oxygen breathing gases for the patient. The alteration of the oxygen content of the breathing gases is carried out in step  208  of  FIG. 7 .  
         [0059]     For exemplary purposes, below, an alteration in the form of an increase to 75% O 2  is shown in  FIG. 5 .  
         [0060]     Next, the clinician must select the frequency, or interval, at which the functional residual capacity measurements will be carried out. This is performed at step  210 . A single functional residual capacity determination by the present method may be selected by the appropriate field  212  in menu  68 . Alternatively, a series of FRC determinations or cycles may be selected, with a series interval, set in field  214 , between each determination. The interval is typically between one and twelve hours in increments of one hour but may be more frequent. The time when the next functional residual capacity determination begins is shown in field  115 .  
         [0061]     Alternatively, functional residual capacity measurements can be set to occur automatically in conjunction with certain procedures controlled by ventilator  10 , such as immediately prior and/or after a period of nebulized drug therapy, recruitment maneuvers, a suction procedure, or a change in ventilator setting. Functional residual capacity measurement may be initiated, terminated, delayed, interrupted, or prevented in accordance with the occurrence of events, such as those noted above, that may affect the accuracy of the functional residual capacity measurement. For example, a functional residual capacity measurement may be terminated for a high oxygen procedure for patient  12  and then resumed or started after a “lock out” period.  
         [0062]     The initial or base line amount of nitrogen in the expired breathing gases is determined at step  216 . As noted above this may be determined by subtracting the amounts of oxygen and carbon dioxide, as determined by gas analyzer  70 , from the total amount of the breathing gases, as determined using flow sensor  62 .  
         [0063]     While the present invention is described using nitrogen as the inert gas, it will be appreciated that other inert gas may also be used. For example, the breathing gases for patient  12  may include the inert gas helium and amounts of helium expired by the patient could be used in a functional residual capacity measure in the manner described herein.  
         [0064]     To commence the determination of functional residual capacity, breathing gases having the increased amount of oxygen shown in data field  105  are provided to patient  12  in step  218 . The increased percentage of oxygen in the breathing gases will wash a portion of the nitrogen or other inert gas out of lungs  38  of patient  12  with each breath of the patient. The amount of breathing gases inspired and expired by patient  12  with each breath, i.e. the tidal volume, is a lung volume that is in addition to the residual volume of the lungs found after expiration. The tidal volume is also smaller than the residual volume. For a healthy adult a typical tidal volume is 400-700 ml whereas the residual volume or functional residual capacity is about 2000 ml. Therefore, only a portion of the nitrogen in the lungs  38  of patient  12  is replaced by the increased amount of oxygen with each breath.  
         [0065]     The amount of nitrogen washed out of the lungs in each breath is determined by subtracting the amount of oxygen and carbon dioxide from the amount of breathing gases expired by patient  12  during each breath obtained using flow sensor  68 . See step  220 . Knowing the amounts of expired breathing gases, the initial amount of expired nitrogen and the amount expired in each expiration by patient  12 , a functional residual capacity quantity can be determined for each successive breath in steps  222   a ,  222   b  . . .  222   n . Any inert gas wash out/wash in functional residual capacity measurement technique may be used, a suitable technique for determining functional residual capacity for use in the present invention being described in U.S. Pat. No. 6,139,506.  
         [0066]     The functional residual capacity quantity as determined after each successive breath, will tend to increase as nitrogen continues to be washed out of the lungs of the patient by the increased oxygen in the breathing gases. This results from the fact that the breathing gases that are inspired by patient  12 , i.e., the tidal volume, are not fully equilibrated inside the entire functional residual capacity volume before being exhaled by the patient. In particular, functional residual capacity volume that lies behind intrinsic lung resistance does not mix as quickly with inspired gases compared to functional residual capacity volume that is pneumatically connected to the trachea through a lower resistance path. As such, the magnitude of breath-to-breath increases in functional residual capacity that are noted are an indication of the amount of intrinsic resistance within the lung gas transfer pathways. Thought of another way, additional functional residual capacity volume that is registered many breaths into the functional residual capacity measurement procedure is lung volume that is not participating well in the gas transfer process.  
         [0067]     As the determination of functional residual capacity proceeds, the determined values for functional residual capacity for the breaths are displayed in graphic portion  110  of screen  102   g   2  as a capacity or volume curve  224  in steps  226   a ,  226   b  . . .  226   c  at the end of the determination for each breath. This confirms to the clinician that the determination of functional residual capacity is working properly. Also, as curve  224  forms from left to right, the shape of the curve is an indication to the clinician of the intrinsic resistance and quality of ventilation of lung functional residual capacity, as discussed above. In the example shown, the clinician can appreciate that patient  12  has a homogeneously ventilated lung volume, as indicated by the qualitative flatness of the functional residual capacity curve, with a lung capacity of about 2500 ml.  
         [0068]     The scaling of graph  110  of  FIG. 5  may be automatically altered to provide a scale appropriate to the fictional residual capacity data being shown.  
         [0069]     It will be appreciated that, if desired, the data relating breath number to the corresponding functional residual capacity value can also be displayed in tabular form in the display of ventilator display unit  76 . This could comprise a column containing the breath numbers and a column containing the corresponding functional residual capacity values.  
         [0070]     Mechanical ventilator  10  continues to supply breathing gases having increased oxygen concentration for x number of breaths, for example, 20 breaths. A final value for functional residual capacity is determined at the end of the x breaths at step  228  and volume or capacity curve  224  extends to this breath to show the final determination of functional residual capacity at the end of 20 breaths.  
         [0071]     Thereafter, at step  230  the concentration of oxygen in the breathing gases is altered to the original level of, for example 50%, set at step  208  and ventilator  10  is operated at step  232  to repeat steps  216 - 228  to make a second determination of functional residual capacity with this alteration of the oxygen concentration in the breathing gases. It will be appreciated that this determination uses a wash-in of nitrogen, rather than a wash-out. This second determination is graphed and displayed in graphic portion  110  as graph  234 , in the same manner as graph  224 , described above. The values for the two final functional residual capacity determinations are shown in data field  236  of tabular portion  112  of screen  102   g   2  in step  236 . In the example shown, these values are 2500 and 2550 ml.  
         [0072]     For future use, the final determination of functional residual capacity made in step  232  is compared to that determined in step  228 . This is carried out at step  238 . It is then determined, in step  240 , whether the difference between the two determinations of functional residual capacity is less or greater than some amount, such as 25%. If the difference is less than 25%, the two values are averaged and will be subsequently displayed in text form in data field  245  in step  244  when the determination becomes part of the chronological record following a later functional residual capacity determination.  
         [0073]     If the difference between the two values for the functional residual capacity is greater than some amount, such as than 25%, both the final value determined at step  228  and the final value determined in step  232  will be displayed by step  246  in data field  245  of  FIG. 5  and in the graph  110 . This display of the functional residual capacity determination informs the clinician that the accuracy of the functional residual capacity determination is questionable.  
         [0074]     The final value(s) for the functional residual capacity are preferably displayed in tabular portion  112  of screen  102   g   2  along with additional associated data such as the time and date at which functional residual capacity was determined, or the values of PEEPe and PEEPi existing when the functional residual capacity determination was made. PEEPe is the end expiratory pressure established by ventilator  10 . PEEPi, also known as auto PEEP, is the intrinsic end expiratory pressure and is a measurement in pressure of the volume of gas trapped in the lungs at the end of expiration to the PEEPe level.  
         [0075]     While the determination of functional residual capacity has been described as being carried out for a given number of breaths, such as 20, it can be terminated sooner if it is apparent that the functional residual capacity measurement has become stable on a breath-to-breath basis. This can be conveniently determined by measuring the O 2 content of the expired breathing gases at the end of the patient&#39;s expirations, that is, the end tidal oxygen level. When the amount of oxygen in the expired breathing gases remains unchanged for a predetermined number of breaths, it is an indication that the wash out/wash in the inert gas is complete and that the functional residual capacity determination can be terminated.  
         [0076]     Thereafter, if a series of functional residual capacity determinations has been selected at step  210 , steps  218  through  246  are repeated after the time interval indicated in data field  214  with the start of the functional residual capacity determination occurring at the time displayed in data field  248 . The predetermined time interval may be overridden or the functional residual capacity determination terminated by appropriate commands from the clinician entered into menu  68 .  
         [0077]     The volume curves, such as  224 ,  234 , and functional residual capacity data, such as that in field  236 , generated in the course of successive functional residual capacity determinations are saved by ventilator display unit  76  and, as such, can be compared to data from previous or subsequent functional residual capacity determinations. This comparison requires that a previous determination of functional residual capacity be selected as a reference curve using the time at which it was obtained as identified in data field  250 . When a reference curve is selected, an indication is made in data field  250  and that functional residual capacity curve is displayed as the reference curve  252 . Curve  252  shows a lung that is not well ventilated. Further indication of the reference curve and reference curve values may be made by a color indication for this data, different from that of the other functional residual capacity data in graph  110  and table  112 . The result is a visual indicator that can easily be referred to by the clinician to quickly assess improvement or deterioration in the functional residual capacity condition of patient  12  over time. In the example shown in  FIG. 5 , there has been an increase in the functional residual capacity of patient  12  for each eight hour interval.  
         [0078]     Also, it is common practice to alter, usually increase, the PEEP to improve ventilation of lungs  38  of patient  12  by opening areas of the lung that are not being properly ventilated. Tabulating the actual measured values for PEEPe and PEEPi, along with the corresponding functional residual capacity determination, as shown in  FIG. 5 , allows the clinician to see the effect, if any of applied PEEPe therapy on the volume of the functional residual capacity of the patient&#39;s lungs, as well as on the intrinsic PEEP. As also shown in  FIG. 5 , a history of a certain number of functional residual capacity determinations and PEEP pressures are shown in display region  70  to present trends and the history of these quantities. In the example shown there, an increase in PEEPe has resulted in an increase in functional residual capacity of patient  12 .  
       FRC Events Log  
       [0079]     Certain clinical or other events can affect the value for functional residual capacity determined from the method steps shown in  FIG. 7 . Such events may include performing a suction routine on patient  12  to remove accumulated secretions, administering a nebulized medication, changing the ventilation mode, or changing one or more ventilation parameters, such as tidal volume (TV), breath rate, PEEP, or other parameter.  
         [0080]     By selecting the FRC Log field  252  in menu  68  of screen  102   g   2  shown in  FIG. 5 , screen  102   g   4  of  FIG. 8  will be shown to provide a log of the events that may effect functional residual capacity in data field  254  along with the time(s). and date(s) the event took place. The log also includes the time, date and value of any periodic functional residual capacity determinations made in the manner described above. The clinician may scroll through the events of the log using control knob  106  to review the functional residual capacity event history in relation to the measured values of functional residual capacity to determine if specific actions had a positive or negative effect on the determined functional residual capacity for the patient.  
       Trends Log Display  
       [0081]     The functional residual capacity value(s) determined in the above manner can also be provided in conjunction with a tabular and/or graphic display of periodic ventilator operating data and/or patient condition data, as shown in  FIGS. 11 and 12 . For example, the display may show ventilator and/or patient data existing at points of time spaced at five minute intervals. Such a display is helpful in documenting and identifing trends in the treatment and condition of the patient over time and hence is termed a “Trends” log.  
         [0082]     A tabular trends log  300  is shown in  FIG. 11  showing numerical data values obtained at five minute intervals for a period of an hour in tabulation field  302 . The data column exemplarily show a plurality of airway pressure conditions in columns  304 ,  306  and  308 , including PEEP in column  308 . As functional residual capacity determinations become available, they can be entered in the trends tabulation in column  310 , as at  312 .  
         [0083]     While tabulation field shows data for one hour, data for a much longer period, such as  14  days, may be stored in a memory in ventilator  10  or display unit  76 . Cursor  314  allows the clinician to scroll through the stored data to display data from a desired time period. Tabulation field  302  is accompanied by an appropriate menu  316  operable by control knob  106  for selecting desired data to be shown and other properties of the trends log display.  
         [0084]      FIG. 12  shows a graphical trends log display  320  in which functional residual capacity data  322  is graphically shown for a period of time, such as three hours, along with other data from ventilator  10  or patient  12 , such as respiratory rate (RR)  324 . The same menu  316  may be used with this display.  
         [0085]     The trends log displays may be placed in screen portion  102   g  by actuating an appropriate button in display unit  76  such as vent setup button  72  or spirometry button  74 .  
       Spirometry Display  
       [0086]     It may also be helpful for the clinician to have a better idea of how much of an increase in functional residual capacity is due to distension of the lung by increased PEEP and how much is due to making previously closed alveolar sacs available, i.e., opening of the lung by “recruitment” of lung volume. Such information can be obtained using the spirometry aspects of the present invention, as shown in the SpiroD screen  102   g   3  of  FIG. 9 .  
         [0087]     In general, spirometry is used to determine the mechanics of a patient&#39;s lungs by examining relationships between breathing gas flows, volumes, and pressures during a breath of a patient. A commonly used relationship is that between inspired/expired breathing gas flows and volumes that, when graphed, produces a loop spirogram. The size and shape of the loop is used to diagnose the condition of the lung.  
         [0088]     A relationship also exists between inspired/expired gas volumes and pressure in the lungs. In the past, a problem with the use of this relationship has been that pressure has been measured at a point removed from the lungs so that the measured pressure may not be an accurate reflection of actual pressure in the lungs thus lessening the diagnostic value of the pressure-volume loop. Through the use of catheter  94  extending from endotracheal tube  90  shown in  FIG. 2 , a far more accurate indication of lung pressure is obtained. For a healthy lung, a graph of the relationship between volume and pressure is roughly an elongated, narrow loop of positive uniform slope. That is, constant increments of inspired volume increase lung pressure by constant increments. The loop is formed because there remains some amount of lung resistance below the pressure sensing point at the end of catheter  94 . In a diseased lung, the loop may be wider and may also reflect a non-linear lung volume pressure relationship. For such a lung, the volume-pressure relationship over the course of an inspiration/expiration may be in a form such as that shown in  FIG. 9  by 420, and a curve illustrating the volume-pressure relationship resulting from a mathematical computation using loop data is plotted, as shown in  FIG. 9  by reference numeral  422 . The curve  422  shown in  FIG. 9  in often termed a “dynostatic curve” and is used for diagnostic purposes. A typical dynostatic curve is shown in  FIG. 9  to contain a middle portion of somewhat linear positive slope and a pair of inflection points separating end portions of differing slopes. The dynostatic curve and its generation is described in Practical Assessment of Respiratory Mechanics by Ola Stenqvist, British Journal of Anesthesia 91(1), pp. 92-105 (2003) and “The Dynostatic Algorithm in Adult and Paediatric Respiratory Monitoring” by Soren Sondergaard, Thesis, University Hospital, Gothenburg University, Sweden (2002).  
         [0089]     In graph  110  of  FIG. 9 , the abscissa of the graph is lung pressure measured at the end of catheter  94  connected to the auxiliary input A of ventilator display unit  76  and is termed “Paux”. The ordinate is scaled in volume of breathing gases inspired/expired by patient  12 . It will be appreciated that this volume comprises the tidal volume for the patient. The tidal volume moves into and out of the lungs in a manner that can be described as being “above” the functional residual capacity. That is, for normal breathing, a patient starts a breath with the volume of the lungs at the functional residual capacity which may, for example be 2000 ml. During inhalation, the volume of the lungs increases by the tidal volume of, for example 500-700 ml, and during exhalation, the volume of the lungs decreases by approximately that amount. The same situation occurs when a patient is being provided with breathing gases from a mechanical ventilator, such as ventilator  10 . It must thus be appreciated that the ordinate of the graph  110  in  FIG. 9  is scaled in the relative volume of inspiration/expiration for which the origin of the graph is zero, not in absolute volume that would also take into consideration functional residual capacity and for which the origin of a graph would be the amount of the functional residual capacity. The scaling of graph  110  of  FIG. 9  may be automatically altered to provide a scale appropriate to the spiromety data being shown.  
         [0090]     With PEEP applied to patient  12  by ventilator  10 , there will be a movement of the graph away from the origin of the axes along the abscissa. The graph will move right by the amount of the PEEP, i.e. the lung pressure at the end of expiration by patient  12 .  
         [0091]     The menu portion  108  of SpiroD screen  102   g   3  shown in  FIG. 9  allows the user to open up a set up menu, shown in  FIG. 10  that allows the clinician to turn a purge flow through catheter  94  on or off or to zero the Paux sensor connected to catheter  94  when the purge flow is on and endotracheal tube  90  has been inserted in patient  12 . The SpiroD set-up menu also allows the clinician to set the scaling for the graphical portions of the display. A “Paux Alarm” screen, reached from the SpiroD setup screen of  FIG. 10 , allows the clinician to set appropriate alarms for patient lung pressure, as sensed by catheter  94 .  
         [0092]     Various other selections on menu  108  of screen  102   g   3  of  FIG. 9  allow the clinician to save the current data and to view this information as a first or second reference for use and display with subsequently obtained data. Up to a given number of loops, for example, six loops and curves, may be saved for analytical purposes. The “erase reference” option allows the user to determine which information to save and which to delete.  
         [0093]     The “SpiroD loops” and “SpiroD curves” menu items may be turned on or off. Selecting “on” for both the curve and loop will display both the loop and the curve at once in the manner shown in  FIG. 9 . For easier comparison among loops and curves obtained at various times, either the loop or curve showing may be turned “off.” The “cursor” option allows the clinician to scroll along the horizontal axis and display the actual pressure and volume measurements associated with the loops or curves that are displayed.  
         [0094]     For the graphical showing of graph  110  of the screen  102   g   3  in  FIG. 9 , volumes and pressures are obtained from sensor  57  and catheter  94  and the spirometry data, computed and displayed for every third breath if the respiratory rate is less than some desired number, for example, 15 breaths per minute. If the respiratory rate is greater than that number, every fifth breath used. The loop  420  for a complete inspiratory/expiratory breathing cycle is displayed in the graph of screen  102   g   3  of  FIG. 9 . The dynostatic curve  422  is then calculated for display in graph  110 .  
         [0095]     Various compliance values for the patient&#39;s lungs are shown in the table  112  of screen  102   g   3  of  FIG. 9 . Compliance can be seen as the amount by which the volume of the lung increases for an incremental increase in lung pressure. The data necessary to determine compliance can be obtained from sensor  57  and gas module  64 . Compliance is represented by the slope of dynostatic curve  422 . It is an indication of the stiffness or elasticity of the lung. In a stiff lung, an incremental increase in pressure results in a smaller increase in volume over a lung that is more elastic and the slope of curve  422  is more horizontal. In an elastic lung, the reverse is true. To aid the clinician in analyzing the lungs of patient  10 , the compliance is computed at the beginning, middle, and end of the respiratory cycle of the patient. As shown in the example in  FIG. 10 , the middle portion of dynostatic curve  422  indicates a portion of greater compliance than the end portions. This is reflected in the greater slope of the middle portion over those of the end portions. The table of the screen sets out numerical values. Ordinarily, the highly compliant, middle portion of curve  422  shown in  FIG. 10  is that in which the lung is most effectively ventilated.  
         [0096]     The table  112  of display  102   g   3  of  FIG. 9  also shows the peak pressure achieved in the lungs during the breath, the PEEP pressure, and the airway resistance, Raw. The airway resistance is the pressure drop experienced by breathing gas flow of the lungs and is expressed in units of pressure per unit of flow. Airway resistance can also be determined with data from sensor  57  and gas module  64  in a manner described in the Stenqvist reference noted above.  
         [0097]     Various alternatives and embodiments are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.