Abstract:
System and method for prevention of surgical fires inside a patient&#39;s airway. The system includes a specially adapted endotracheal tube which carries sublines (in addition to the main line carrying anesthetic gases) for carrying an air sample back from the distal end of the tube to a remote oxygen sensor. Upon the sensor sensing an undesirably or dangerously high level of oxygen within the patient&#39;s airway, the system operates alarms to alert the surgical personnel, and also operates a controllable valve to admit an inert gas into other sublines associated with the endotracheal tube and which deliver the inert fire suppressing gas to the distal end of the tube, proximal to the cuff, inside the patient&#39;s airway.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a medical apparatus and specifically to a system for preventing surgical fires involving anesthetic gases. 
       BACKGROUND 
       [0002]    The problem of surgical or operating room fires due to the combination of anesthetic gases and an ignition source is well known, see for instance Gedebou US2006/0058784, Lampotang et al. US2006/0150970 and Foltz US7,296,571. Typically the ignition source is a heat-producing surgical instrument, such as an electrosurgery cautery device or another electrosurgery tool or a laser. Oxygen is present since it is often administered to a patient during surgery as part of the anesthesia. The combustible material is, for instance, the patient&#39;s tissue or parts of the anesthesia equipment. Such fires occur when the oxygen administered to the patient leaks into the patient&#39;s upper airway or the operating room, causing a highly oxidized environment and increased flammability of human tissue and surgical equipment. A fire can ignite when this fuel is exposed to an ignition source. The oxygen is administered in a number of ways. One is by use of a cannula which is applied to the nose. Another method is a face mask. Another method is an endotracheal tube which is inserted into the patient&#39;s mouth and down into the throat so as to administer the oxygen well down into the patient&#39;s airway. Near the distal end of such tubes there is typically a cuff which is inflated to seal the patient&#39;s airway to the outer circumference of the tube to prevent the anesthesia gases from leaking out of the patient&#39;s esophagus and into the patient&#39;s throat and/or ambient atmosphere. However, often there are gas leaks because the cuff does not properly engage with the patient&#39;s anatomy, allowing the oxygen to leak past the cuff, causing an increased risk for fire. 
       SUMMARY 
       [0003]    Therefore surgical fires are traumatic and a well known risk of surgical procedures. In addition to pure oxygen, nitrous oxide (also used in anesthesia) can act as the fire oxidizer. Much surgical equipment is made of plastic and becomes more flammable in an oxidized environment, such as when anesthesia gas leaks into the ambient atmosphere. Therefore, both the patient&#39;s tissues and much surgical equipment can serve as a fuel source. It is known that ear, nose and throat surgeries commonly lead to surgical fires due to poor ventilation of the throat and airway. In these procedures the electrosurgical device operates especially in close proximity to the plastic (e.g., PVC) endotracheal tube used to deliver the anesthetic gases, potentially resulting in damaging airway fires inside the patient&#39;s throat or mouth. This can occur when the anesthetic gases leak around the cuff provided on such tubes into the patient&#39;s upper airway. 
         [0004]    The tube cuff designs are not particularly efficient due to variability in patient anatomy. The cuff pressures are supposed to be monitored by the anesthesiologist but this is not done as frequently as desired. There have been a number of endotracheal tubes developed to reduce gas leaks, but none are particularly effectively. Further, the above referenced patent documents do not deal especially with endotracheal tubes but instead focus on oxygen administered by face mask or nose cannula. 
         [0005]    The present system detects and prevents surgical fires, with use of an endotracheal tube. The system includes an oxygen sensor to detect a flammable atmosphere by the level of oxygen. Upon detecting a predetermined dangerously high oxygen concentration proximal to the endotracheal tube cuff, the system triggers an automated response to reduce or eliminate the presence of the oxygen. The response includes auditory and visual warnings, such as an audio alarm, a gauge, or set of lights, such as LEDs to provide a real time display of the risk level to the surgical personnel. Further, the active response of the system includes delivery of an inert gas to the site of the anesthetic gas leak to suppress the possibility of a fire. A typical inert gas is nitrogen. The system is operated and controlled using a conventional micro-controller driven by the oxygen sensor. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0006]      FIG. 1  shows a system in accordance with the invention. 
           [0007]      FIG. 2  shows the endotracheal tube of  FIG. 1  in detail. 
           [0008]      FIG. 3  shows a cross section of a portion of  FIG. 2  along lines AA. 
           [0009]      FIG. 4  shows detail of the controller of the  FIG. 1  system. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  shows in a block diagram a system in accordance with the present invention.  FIG. 1  does not show the conventional electrosurgery device, or the usual anesthesia equipment except for the endotracheal tube  10 , which in several respects here is not a standard endotracheal tube. However, this endotracheal tube  10  has distal end  12  which is inserted into the patient&#39;s throat and the surrounding cuff  14 , both conventional. The proximal end  16  of tube  10  is conventionally coupled to a source of anesthetic gases, such as oxygen, nitrous oxide, etc. 
         [0011]    Also included in the system is a controller housed in a controller housing  20  and including a driver for an audio alarm located inside the housing such as a loud speaker, not shown, and a visual indicator of the oxygen level, such as a gauge  26  or set of light-emitting diodes  22 , as explained further below. 
         [0012]    The controller housing is connected via a port (not shown) to a conventional external source of suction  32 , such as a suction pump or the suction mains typically provided in an operating room. Also provided is a connection  36  to a conventional source of a fire suppressant (inert) gas  40 . Source  40  may be a conventional inert gas source provided in the operating room. The internal arrangements of the controller are explained in detail below. A conventional oxygen sensor, for instance, a partial pressure oxygen sensor, of the type commercially available, is housed inside the controller housing  20  and monitors fluctuations in the concentration of oxygen on the outside circumference of the proximal end of the endotracheal tube cuff, to sample the oxygen concentration proximal to the cuff, via suction. Similar tubes deliver the inert gas to the same location. Hence although the oxygen sensor is located inside the controller housing  20 , it continuously receives samples of the atmosphere inside the patient&#39;s throat at the proximal end of cuff  14  via line  42  at port  64 , as explained further below. The oxygen sensor conventionally generates a voltage signal directly proportionally to ambient sensed oxygen concentration. The source of suction  32  connected to the controller housing  20  constantly pulls air through the flow-through head of the oxygen sensor from line  42 . The suction source  32  is connected to the housing via a port in the back of the housing  20 . A tube on the inside of the housing connects to this source  32  and couples suction to one side of the oxygen sensor flow-through head. On the other side of the flow-through head a different tube leads to the front port  64  from the inside of the housing. On the outside of the housing at the front port  64  a tube  70  is connected that leads to the endotracheal tube  10 . Tube  70  branches to smaller diameter tubes that line the outside circumference of the endotracheal tube or are manufactured inside the walls of the endotracheal tube. Therefore the suction pump draws a sample of air through the oxygen sensor. 
         [0013]      FIG. 2  shows detail of endotracheal tube  10  of  FIG. 1  with similar elements carrying the same reference numbers. Tube  10  carries the anesthetic gases from their source  89  via a coupling  16 , all of which are conventional. Also conventional is adapter fitting  84  to couple to an air syringe to inflate the endotracheal tube cuff  14  via line (tube)  110  which here extends along or in the wall of tube  10  and extends to the inside of the balloon cuff  14 . This structure is conventional also. From the outside of port  64 , tube  42  is divided at coupling  106  into smaller tubes  100  and  102 . The end of these tubes  100 ,  102  is immediately adjacent the proximal end of the balloon cuff  14 . Inert gas source  40  feeds to a port in the back of the housing. A tube inside the housing connects the gas source to the solenoid and is coupled to the front port  64 . Front port  64  is connected to line  70  which branches at coupling  94  to smaller tubes  88  and  90  that run along the outside or inside the wall of the endotracheal tube. The end of these smaller tubes is immediately adjacent the proximal end of the balloon cuff. Hence port  64  couples to two separate lines, one to deliver the inert gas and the second to couple the suction. 
         [0014]      FIG. 3  shows a cross section of the tube  10  along line A-A of  FIG. 2 . Central channel  80  carries the anesthetic gases. Tube  10  conventionally has a wall  116  in which are defined sampling lines (channels)  88  and  90 , and fire suppression gas supply lines (channels)  100  and  102 . Also defined in the tube wall  116  is cuff inflation line  110 . Of course, this provision of lines or channels in the wall  116  of tube  10  is not limiting. The various lines can be provided by other means such as being independent tubes attached to the inside or outside of the wall of tube  10 . Typically tube  10  is molded of plastic such as polyvinyl chloride (PVC) and is a disposable item. The actual dimensions of the various structures shown in  FIG. 2  are largely conventional. The diameters of the various lines  70 ,  88 ,  90 ,  42 ,  100 ,  102 ,  110  is a matter of engineering choice, so long as sufficient airflow is provided for oxygen sampling purpose and sufficient inert gas is provided. Exemplary diameters of tubes  88 ,  90 ,  100 ,  102  are outside diameter 3/32″ (2.5 mm), inside diameter 1/32″ (0.8 mm). Tubes  42  and  70  have an exemplary inside diameter of ⅛″ to ¼″ (3 to 6 mm) and corresponding outer diameter. The diameters of the tubes are not critical. The number of lines (tubes) associated with tube  10  for inert gas delivery and air sampling is also a matter of engineering choice. 
         [0015]    The structure of  FIGS. 2 and 3  is a subsystem of the  FIG. 1  system and may be sold separately since it is usually disposable, and typically used for only one surgical procedure, while the remainder of the  FIG. 1  system is typically reused many times, for instance installed in an operating room or surgical suite. 
         [0016]      FIG. 4  shows in a block diagram the controller components housed within controller housing  20 . These include the oxygen sensor  50 , a micro-controller  52  typically mounted on an associated printed circuit board with the associated conventional interface components, and an alarm driver circuit  54  also mounted on the printed circuit board for driving the audio alarm and the visual alarm  22 ,  26 , both of which are conventional. Suction from source  32  is applied to pull the sampled air through the oxygen sensor  50 . This air after being sampled by oxygen sensor  50  is ventilated. Also provided, and driven by the micro-controller  52  and its interface circuitry, is a conventional solenoid valve  68  which is operated in accordance with signals sent by the micro-controller  52  to turn on gas flow from the nitrogen source  40 , which is connected at the back of the housing and thereby at port  64  to line  70  of the endotracheal tube. 
         [0017]    The micro-controller  52  (or other suitable controller) interprets the signals from oxygen sensor  50 . First, the voltage signal, for instance, from 0 to 60 millivolts amplitude supplied by oxygen sensor  50 , is conventionally amplified by an instrument operational amplifier to be a direct current voltage signal, for instance 0 to 5 volts amplitude. This range is specific to the oxygen sensor. This amplified voltage is interpreted by the micro-controller  52  firmware and digitally mapped to a corresponding bit value between 0 and 1,023. For instance, 0 volts equates to a 0 bit value and 5 volts equates to a 1,023 bit value. The bit values are mapped to a set of three designated Cases 0, 1 or 2 in the firmware associated with the micro-controller  52 , corresponding to the atmospheric oxygen concentration, and elicit different responses. For instance, Case 0 corresponds to oxygen value 0 to 341, which is 0% to 30% oxygen. Case 1 corresponds to oxygen values 342 to 682 which is 31% to 60% oxygen. Case 2 corresponds to oxygen values 683 to 1,023, which is 61% to 100% oxygen. The corresponding oxygen concentrations to the Case numbers can be varied depending on engineering choice. 
         [0018]    Formulas are applied by the micro-controller firmware to calculate these values as follows: 
         [0000]        V =(4.88×10 −3 )×Byte #
 
         [0000]      O 2 =0.05×V,where V is the voltage and O 2  is the concentration of oxygen.
 
         [0019]    Hence the three Case numbers are assigned respectively to three Cases in the associated firmware which elicit appropriate responses in the alarm driver  54  and the solenoid valve  68 . Writing suitable firmware would be routine in light of this disclosure. 
         [0020]    Solenoid valve  68  thereby controls delivery of the flame retardant gas, for instance, nitrogen from source  40 . The controller in one version uses a 12 volt direct current solenoid valve  60  controlled by the micro-controller  52  and powered by the same power supply (not shown) as conventionally connected to the other components of the controller. Typically solenoid value 68 is closed and then operated to be (open) only for Case 2 when the oxygen concentration exceeds 60%. Hence the controller  52  activates the solenoid  68  to release the nitrogen gas through the endotracheal tube gas delivery line  70  shown in  FIG. 1 . Also provided is a conventional power supply for the controller, not shown. 
         [0021]    When subsequently the oxygen sensor  50  indicates that the ambient oxygen concentration has dropped below 60%, the solenoid value 68 is deactivated (closed) by the micro-controller  52 , shutting off the supply of nitrogen gas. Thus an active feedback loop is established, effectively maintaining a safe surgical environment in terms of oxygen concentration inside the patient&#39;s airway. 
         [0022]    In one embodiment nitrogen is used as the flame retardant gas because it is a natural component of atmospheric air, readily available in most operating rooms, and cost effective. Also of course, it is compatible with patient health, unlike, for instance, high concentrations of carbon dioxide. But other inert gases may be used as a substitute for nitrogen. 
         [0023]    The same three cases which control the solenoid valve also control the alarm driver  54 . For Case 1 which is the sensed oxygen concentration below 30%, the low risk response Case 0 is activated, thus illuminating, for instance, a green LED  22  in the visual display of  FIG. 1 . Typically no audio alarm is provided at this point. When the sensed oxygen concentration is at 31%-60%, the moderate risk response which is Case 1 is activated so that, for instance, a yellow LED in the visual display  22  is activated. When the sensed oxygen concentration is detected at the danger level of above 60%, the high risk response Case 2 is activated at which point the loud speaker is activated to sound a buzzer or other type suitable audio alarm and the red LED in the visual display  22  is activated. Of course, any other type of alarms can also be provided. The LEDS are in addition to the oxygen gauge indicator  26  which provides a numeric read out. 
         [0024]    Various types of oxygen sensors may be used, for instance, a partial pressure oxygen sensor supplied by Apogee has been found suitable. Also suitable is a zirconium dioxide oxygen sensor or galvanic oxygen sensor. 
         [0025]    It has been found that using such a system, when the oxygen is sensed to be at the danger level, its concentration inside the patient&#39;s airway can be reduced to a normal or fire safe level in as little as 20 or 30 seconds. Moreover, the determination of the 60% oxygen level as the danger level, while not limiting, has been found by experiment to be a typical level above which tissue ignition will take place and below which tissue ignition is not likely to take place. PVC ignition will take place at lower oxygen concentration, e.g. 21% and this may be used as a critical level in addition or in the alternative. 
         [0026]    This disclosure is illustrative and not limiting. Further modifications will be apparent to those skilled in the art in light of its disclosure and are intended to fall within the scope of the appended claims.