Abstract:
A portable critical care medical ventilator with autonomous control of oxygenation is provided. The system and method provide for significantly reducing the O 2  required to maintain an adequate O 2  concentration and as a result delivery of O 2  to the body&#39;s tissues and does not require the clinician to enter initial values.

Description:
This application claims priority on U.S. Provisional Application No. 61/148,069 filed on Jan. 29, 2009. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present disclosure relates generally to medical ventilator systems, and more particularly, to a medical ventilator with autonomous control of oxygenation. 
     2. Description of the Related Art 
     Achieving adequate oxygenation is one of the primary goals of mechanical ventilation. This goal is accomplished through the adjustment of the fraction of inspired oxygen concentration (FlO 2 ), positive end-expiratory pressure (PEEP), and mean airway pressure. Titration of these variables is guided by continuous noninvasive monitoring of oxygen saturation by pulse oximetry (SpO 2 ) and intermittent arterial blood sampling for arterial oxygen tension (PaO 2 ) and measured oxygen saturation (SaO 2 ). Traditional application of this technology has sought to maintain the SpO 2  between 98 and 100% to assure adequate oxygenation and prevent desaturation events (SpO 2 &lt;88%) which has been shown to have a deleterious effect on patient outcome. Previous work has focused on preventing hypoxemia and hyperoxia which has tended to maintain the SpO 2  on the flat portion of the oxyhemoglobin dissociation curve. Maintaining the SpO 2  in this area allows for transient changes in PaO 2  caused by shunting while maintaining SpO 2  and therefore the O 2  concentration in the blood. 
     Goals for oxygenation vary with specific disease states and philosophy of the attending heath care personnel. In adults, adequate oxygenation is typically considered an SaO 2 &gt;90% and PaO 2 &gt;60 mm Hg. In neonates, where both hypoxia and hyperoxia are associated with adverse outcomes, arterial oxygen is more tightly controlled. Similarly, in patients with head injuries, even a single hypoxemic event is associated with poor outcome. In these patients, hyperoxia may be indicated both to prevent hypoxemia and to assure adequate brain tissue oxygenation. Previous automated control methods have required the clinician to enter an SpO 2  target and range of acceptable values. These methods presuppose the attending clinician is capable of determining what is best for the patient and adjusting the device accordingly. 
     Oxygen toxicity is a common concern in intensive care, but the effects of elevated FlO 2  in critically ill patients is controversial. A common goal of PEEP titration is reduction of FlO 2  to nontoxic levels (e.g., &lt;0.60); however, the exact level of O 2  that may be construed as safe is unknown. In adults, the potential effects of hyperoxia are always preferable to the immediate effects of hypoxemia. 
     Although maintenance of adequate O 2  delivery and prevention of hypoxemia are the primary goals for the battlefield casualty, military medical operations have unique concerns. In civilian US hospitals, under normal conditions, O 2  reserves are plentiful. In military medical and flight operations, O 2  is a limited resource that needs to be conserved. During deployed military medical operations, the logistical considerations required to provide and sustain O 2  resources in forward areas are considerable. It has been estimated that O 2  containers and O 2  generation equipment comprise approximately fifteen to thirty percent of the entire logistical footprint of weight and space (cube) necessary to provide medical care in the field during combat operations. These considerations become even more acutely focused during transport when both the patient, medical equipment and O 2  need to be moved in air and ground vehicles that are called into service to move the casualty. Mission length can vary depending upon the tactical and strategic consideration(s) of the theater and the criticality of the patient. The necessity of accounting for potential required O 2  resources as well as the physical transport of O 2  tanks and liquid O 2  is an important component of any mission. Mission planning commonly involves a calculation of required O 2  needs and then doubling that value as a margin of safety. Similar O 2  supply problems exist with mass casualty incidents and disaster relief. These applications have needs that are very similar to the military&#39;s. Immediately following hurricane Katrina for example, affected area hospitals ran out of O 2  as did the few ambulances that supported the evacuations from the area. Florida had similar situations following its hurricanes: (1) the loss of electrical power prevented hospitals from distributing gaseous O 2  from their liquid O 2  tanks, (2) downed electrical lines prevented restoration of power which prevented the use of O 2  concentrators and, (3) power lines and trees blocked roads which prevented deliveries of bottled gaseous O 2 . Also, rural areas that routinely transport patients over great distances to hospitals sometimes take in excess of 4 hours which could marginalize supplies of on-board O 2 . 
     Given the preceding considerations, it would seem apparent that O 2  conservation takes on greater importance in prehospital and transport environments. In these environments the goals of O 2  therapy are to prevent hypoxemia, to assure adequate arterial oxygenation and to minimize O 2  usage. 
     SUMMARY 
     A portable critical care medical ventilator with autonomous control of oxygenation is provided. The system and method provides for significantly reducing the O 2  required to maintain an adequate O 2  concentration and as a result delivery of O 2  to the body&#39;s tissues and does not require the clinician to enter initial values. 
     According to one aspect of the present disclosure, a method for delivering O 2  to a patient with a mechanical ventilator includes (a) supplying fractionally inspired oxygen (FlO 2 ) to a patient based on a target O 2  saturation (SpO 2 ), wherein the target SpO 2  is set a lower limit of clinical acceptability of SpO 2  for the patient to conserve O 2 ; (b) monitoring SpO 2  returned from the patient; (c) determining an error between the monitored SpO 2  and the target SpO 2  and while the error is greater than a predetermined error iteratively performing, (d) determining a gain factor such that increases to FlO 2  are greater than decreases for a magnitude of the determined error; (e) determining a correction factor by multiplying the determined gain factor by the magnitude of the error; and (f) applying the corrector factor to the supplied FlO 2 . In one aspect, the method waits a first predetermined time period before iteratively performing steps (c)-(f). 
     In another aspect, the gain factor is determined such that increases to FlO 2  are three times as great as decreases for the magnitude of the determined error. 
     According to a further aspect of the present disclosure, a system for delivering O 2  to a patient is provided. The system includes a ventilator for supplying fractionally inspired oxygen (FlO 2 ) to a patient based on a target O 2  saturation (SpO 2 ), wherein the target SpO 2  is set to the lower limit of clinical acceptability of SpO 2  for the patient to conserve O 2 ; and an O 2  concentrator coupled to the ventilator for supplying O 2  to the ventilator, wherein the ventilator monitors SpO 2  returned from the patient, determines an error between the monitored SpO 2  and the target SpO 2  and while the error is greater than a predetermined error, the ventilator iteratively determines a gain factor such that increases to FlO 2  are greater than decreases for a magnitude of the determined error, determines a correction factor by multiplying the determined gain factor by the magnitude of the error and applies the corrector factor to the supplied FlO 2 . The system further includes a patient ventilation circuit including a supply tube for supplying FlO 2  to the patient from the ventilator and a bidirectional connection for sending sensed or measured parameters of the patient to the ventilator. 
     In one aspect, the ventilator controls an output of the O 2  concentrator to maintain the corrected supplied FlO 2  In another aspect, the ventilator mixes the output of the O 2  concentrator with ambient air to maintain the corrected supplied FlO 2 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings. 
         FIG. 1  is a diagram of a system for O 2  generation having closed loop control of FlO 2  in accordance with an embodiment of the present disclosure; 
         FIG. 2  is a block diagram of the ventilator in accordance with an embodiment of the present disclosure; 
         FIG. 3  is a flow diagram of an exemplary method for controlling FlO 2  supplied to a patient in accordance with an embodiment of the present disclosure; and 
         FIG. 4  is a flow diagram of an exemplary method of a safety loop employed in the ventilator in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     To facilitate understanding, identical reference numerals have been used, where possible to designate identical elements that are common to the figures, except that alphanumerical suffixes may be added, when appropriate, to differentiate such elements. The images in the drawings are simplified for illustrative purposes and are not depicted to scale. 
     The appended drawings illustrate exemplary embodiments of the present disclosure and, as such, should not be considered as limiting the scope of the disclosure that may admit to other equally effective embodiments. Correspondingly, it has been contemplated that features or steps of one embodiment may beneficially be incorporated in other embodiments without further recitation. 
     In some embodiments, particular method steps of the discussed methods are performed in the depicted order. In alternate embodiments, in the respective methods, at least two method steps or portions thereof may be performed contemporaneously, in parallel, or in a different order. 
     This description is presented as an example of the principles of the invention. It will be appreciated that those skilled in the art will be able to devise various other arrangements that embody the principles of the disclosure and that are included within the spirit and scope of this invention. 
     All examples and conditional language recited herein are intended for teaching purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
     Moreover, all statements herein reciting principles, aspects and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
     Thus, for example, those skilled in the art who have read this application will appreciate that the block diagrams presented herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo-code, and the like represent various processes which may be substantially represented in computer readable media and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
     The functions of the various elements shown in the figures may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (“DSP”) hardware, read only memory (“ROM”) for storing software, random access memory (“RAM”), and nonvolatile storage, programmable logic or other device or devices. 
     Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any configuration or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other configurations or designs. Herein, the phrase “coupled with” is defined to mean directly connected to or indirectly connected with through one or more intermediate components. Such intermediate components may include both hardware and software based components. 
     Referring to  FIG. 1 , a system  100  for O 2  generation having closed-loop control of FlO 2  is provided. The system  100  includes a ventilator  102  for providing ventilation and supplemental O 2  to a patient and an O 2  concentrator (typically providing 90%-96% O 2 )  104  to the ventilator  102  for supplying to the ventilator  102  O 2 . The system  100  further includes a patient ventilation circuit  106  which includes a supply tube  108  for supplying FlO 2  to the patient  112  and a bidirectional connection  110  for sending sensed or measured parameters of the patient to the ventilator  102 . Although an O 2  concentrator  104  is shown in  FIG. 1 , the system  100  can use any source of medical oxygen, including liquid, chemical and gaseous sources. 
       FIG. 2  illustrates a block diagram of the ventilator  102 . Generally, the ventilator  102  includes input connections for a plurality of sensors  204 , a plurality of analog-to-digital (ND) converters  206  for converting the inputted sensors into digital form and a controller  208  controlling an output of the ventilator  102  based on the sensed parameters of the inputs. In one embodiment, the controller  208  will include a central processing unit (CPU) and/or a digital signal processor (DSP). The controller  208  is configured for receiving the digital signals from the ND converters  206  to perform the necessary calculations for controlling the overall operations of the ventilator  102 , including control of the FlO 2 . 
     A power supply  210  is also provided for providing power to each component of the ventilator  102 . In one embodiment, the power supply  210  is coupled to an external power source, e.g., 120V power source. In another embodiment, the power supply  210  is coupled to an internal rechargeable battery disposed in the housing  212  of the ventilator  102 . 
     The ventilator  102  of the present disclosure will have user interface  216  for interacting with a user and for communicating events, alarms and instructions to the user. The user interface will include a display for providing visual indications to the user. The display may include a touch screen, a liquid crystal display (LCD), a plurality of LED number segments, individual light bulbs, a plasma screen display, a cathode ray tube display, an electroluminescent display an organic LED display or any combination of these. The display may provide the information to the user in the form of alpha-numeric lines, computer-generated graphics, videos, etc. Visual information provided on the display may include but is not limited to instructional videos, operating manuals associated with an Intelligent Electronic Device (IED), a flowchart for troubleshooting, a checklist for troubleshooting, etc. Digital files including the various visual instructions are stored in either memory  216  or retrieved from a remote event server. 
     The user interface  214  will also include an audible output device, e.g., a speaker. The speaker will be coupled to the controller  208  via a digital-to-analog converter (D/A) for converting digital audio files stored in memory  216  to analog signals playable by the speaker. The audible output device  24  may simply provide audible instructions to a user when an event is detected or may provide audio with a corresponding video being displayed on the display. 
     The ventilator  102  of the present disclosure will support various file types including but not limited to Microsoft Windows Media Video files (.wmv), Microsoft Photo Story files (.asf), Microsoft Windows Media Audio files (.wma), MP3 audio files (.mp3), JPEG image files (.jpg, .jpeg, .jpe, .jfif), MPEG movie files (.mpeg, .mpg, .mpe, .mp1v, .mp2v .mpeg2), Microsoft Recorded TV Show files (.dvr-ms), Microsoft Windows Video files (.avi) and Microsoft Windows Audio files (.wav). 
     The memory  216  is configured for storing files executable by the controller and for storing files for the visual and/or audible instructions. The memory  28  includes internal storage memory, e.g., random access memory (RAM), or removable memory such as magnetic storage memory; optical storage memory, e.g., the various known types of CD and DVD media; solid-state storage memory, e.g., a CompactFlash card, a Memory Stick, SmartMedia card, MultiMediaCard (MMC), SD (Secure Digital) memory; or any other memory storage that exists currently or will exist in the future. By utilizing removable memory, the ventilator  102  can be easily upgraded with new instruction files as needed. 
     In one embodiment, the digital audio files may be programmed directly through the ventilator  102 . In this embodiment, the ventilator  102  will include an audio input device, e.g., a microphone, for receiving spoken words in the form of analog signals. The analog signals will then be sent to an analog-to-digital converter (A/D) to convert the analog signals into digital signals understandable by the controller  208 . The controller  208  will then store the recorded digital audio file in the memory  216 . The user may associate the recorded digital file with a particular alarm through the touch screen display. Alternatively, the user may associate the recorded digital file with an event code. 
     In a further embodiment, the ventilator  102  will include an alarm  218  separate from the user interface  214  for remotely indicating alarms to the user. 
     The ventilator  102  includes an inlet  220  for receiving O 2  from the O 2  concentrator  104  and a fresh gas/emergency air intake  222  which are both piped to an air/oxygen manifold  224 . In one embodiment, the manifold combines the outputs of an O 2  valve and an air compressor. In another embodiment, the manifold may contain an O 2  and air valve or all three: air valve, O 2  valve and compressor. The controller  208  will modulate the output of the O 2  and air valves or an O 2  valve and compressor  224  to achieve a determined level of FlO 2  which will be supplied to a patient via the gas output  226 . The controller  208  determines the required change in FlO 2  based on the input sensors  204 , e.g., transducer input  228 , exhalation valve input  230  and pulse oximeter input  232 . An algorithm calculates the ratio of flow rates required from the O 2  valve and compressor to attain the required change in FlO 2  from the current FlO 2 . An output port  234 , e.g., a USB port, is provided to communicate to and control the O 2  concentrator  104  or other O 2  source. 
     The system  100  is designed to maintain oxygenation while at the same time minimizing O 2  use. This is accomplished by a predetermined control target that is at the lower limit of clinical acceptability that has been validated in clinical trials. Doing this assures that the patient is maintained at a safe limit using the least amount of O 2  which is of most importance in military and pre-hospital environments as described above. When used with an O 2  concentrator the approach also reduces the power required to maintain the patient by regulating the output of the concentrator. To safely manage patients at the lower limit of acceptable oxygenation, an asymmetric gain approach is employed. In this approach, the rate at which the delivered O 2  is decreased for a patient who has a saturation value greater than the target is less than the rate at which O 2  is increased when the saturation is less than the target value. Use of asymmetric gain is a practical compromise between maximal O 2  conservation and patient safety based on the differences in the time required to reach equilibrium for upward changes vs. downward changes that were observed in the preclinical and clinical trials. Additional time spent slightly above the SpO 2  target allows time for the lung and blood to reach equilibrium while only slightly increasing the amount of O 2  used to maintain adequate oxygenation. Decreasing the response time to SpO 2  values below the target minimizes the time spent outside of the therapeutic range. The approach is based on the slope of the oxyhemoglobin dissociation curve for SpO 2  of 94%. Small changes in arterial O 2  tension can have a dramatic affect on SpO 2 . In addition, the algorithm has a wait state associated following each FlO 2  change where it monitors the convergence toward the target value. Time between changes is required to account for latencies for diffusion of O 2  from the lung to the blood and the transit time for blood from the lung to the peripheral capillary beds where the SpO 2  signal is measured. An additional control loop automatically increases the oxygenation target when the system determines that the patient&#39;s oxygenation is too unstable to be maintained at the current target value. Whenever there are two or more desaturation events (SpO 2 &lt;88%) in a 10 minute period, the system increases the SpO 2  target by 1%. In essence, this moves the patient to a flatter portion of the oxyhemoglobin dissociation curve where transient changes in arterial O 2  tension have a less dramatic affect on the blood oxygen concentration. Lastly, a safety control loop continuously monitors for desaturation events and automatically increase the FlO 2  to 100%. Working in parallel these approaches reduce the potential for oscillation which could result in transient hypoxemic and hyperoxemic events. 
     Referring to  FIG. 3 , a flow diagram of an exemplary method for controlling FlO 2  supplied to a patient in accordance with an embodiment of the present disclosure is provided. The method includes three states: a monitoring state, a correcting state and a converging state. Initially, in step  302 , the patient&#39;s target SpO 2  is monitored and a loop timer is started which determines a waiting period for when the patient&#39;s SpO 2  is within tolerance of the target. Next, in step  304 , the error magnitude and sign of the monitored patient&#39;s SpO 2  is determined. In step  306 , if the error is greater than tolerance, e.g., 0.02, corrective action is to be taken and the method moves to the correcting state. Otherwise, the method reverts back to step  302  and continues to monitor the patient. 
     In step  308 , the correction factor is scaled such that increases to FlO 2  are 3 times as great as decreases for the same distance from the target SpO 2 . Once the loop gain is determined in step  308 , the correction factor is determined in step  310 . It is to be noted that the adjustment is never to be more than 20%. In step  312 , it is determined how much of an adjustment was made and, if the FlO 2  is adjusted by 10% or more in the last 10 minutes, an alert is issued to the operator. If the correction factor is within an acceptable limit, the correction factor is applied and the method moves to the converging state, in step  314 . 
     In step  316 , the loop timer is initiated again and the system will wait for the patient to respond. After the loop timer expires, the current error will be calculated, in step  318 . 
     After the error is determined, in step  320 , it is determined if the current error is within tolerance of the target. If the error is within the tolerance of the target, the method returns to the monitoring state and step  302 . Otherwise, if the error is greater than the tolerance of the target, the method moves to step  322 . In step  322 , it is determined if the change was in the right direction by comparing the current error to the previous error. If the change was in the right direction, the method will revert to step  316  and will wait to allow the patient to respond to the current settings. However, if the change was in the wrong direction, the method will return to the correcting state at step  308  and the system will attempt to make further corrections. 
     Referring to  FIG. 4 , a flow diagram of an exemplary method of a safety loop employed in the ventilator in accordance with an embodiment of the present disclosure is illustrated. Initially, in step  402 , it is determined if the closed loop of  FIG. 3  is disabled or not. If enabled, a counter is initially reset, in step  404 , and the system will wait a predetermined time period, e.g., 1 second, for proper readings of the inputs, in step  406 . 
     In step  408 , the system will check the SpO 2  signal status using cyclic redundancy checking and additional error detection algorithms inherent in the SpO 2  sensor. If the SpO 2  signal is not valid, the closed loop control of the ventilator  102  will be disabled and an alert is issued. Otherwise, if the SpO 2  signal is valid, the closed loop control of the ventilator  102  is enabled. Once the closed loop is enabled, the controller  208  will check the SpO 2  signal against a safety threshold, e.g., is SpO 2  less than 88%. If the SpO 2  level is acceptable, the method returns to step  404  and continues to check for a valid signal. If the SpO 2  level is below 88%, a counter is started and incremented at step  414  and the SpO 2  level is checked again when the counter reached 20 seconds, at step  416 . If the SpO 2  level falls below 0.88 for 20 seconds, the FlO 2  is immediately adjusted to 1.0 while an alert is issued, in step  420 , to minimize the time and magnitude of the desaturation event. In step  422 , an alarm is presented to the operator that the desaturation threshold has been exceeded and corrective action must be taken. 
     Although the disclosure herein has been described with reference to particular illustrative embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present disclosure. Therefore numerous modifications may be made to the illustrative embodiments and other arrangements may be devised without departing from the spirit and scope of the present disclosure, which is defined by the appended claims. 
     Furthermore, although the foregoing text sets forth a detailed description of numerous embodiments, it should be understood that the legal scope of the invention is defined by the words of the claims set forth at the end of this patent. The detailed description is to be construed as exemplary only and does not describe every possible embodiment, as describing every possible embodiment would be impractical, if not impossible. One could implement numerous alternate embodiments, using either current technology or technology developed after the filing date of this patent, which would still fall within the scope of the claims. 
     It should also be understood that, unless a term is expressly defined in this patent using the sentence “As used herein, the term ‘ —————— ’ is hereby defined to mean . . . ” or a similar sentence, there is no intent to limit the meaning of that term, either expressly or by implication, beyond its plain or ordinary meaning, and such term should not be interpreted to be limited in scope based on any statement made in any section of this patent (other than the language of the claims). To the extent that any term recited in the claims at the end of this patent is referred to in this patent in a manner consistent with a single meaning, that is done for sake of clarity only so as to not confuse the reader, and it is not intended that such claim term be limited, by implication or otherwise, to that single meaning. Finally, unless a claim element is defined by reciting the word “means” and a function without the recital of any structure, it is not intended that the scope of any claim element be interpreted based on the application of 35 U.S.C. § 112, sixth paragraph.