Abstract:
An air management system is described for removing and reintroducing a desired amount of air into an air passage of a pressure measurement catheter. More specifically, the system includes a cylindrical housing, check valves in communication with the housing and an moveable shaft with multiple sealing members along its length. As the shaft is moved within the housing, a piston member causes the residual air in the catheter air passage to be evacuated to a defined negative pressure. Further movement of the shaft causes a piston member to inject a predetermined volume of air into the catheter air passage. In this respect, periodic adjustments of the piston member returns the volume of air in a variable volume chamber to one that is unaffected by residual volume in the chamber and that provides an optimum volume of air to maximize the time the sensor can function accurately between recharging events.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to U.S. patent application Ser. No. 13/752,079 filed Jan. 28, 2013 entitled Catheter Air Management System, which is a continuation of U.S. patent application Ser. No. 12/606,169 filed Oct. 26, 2009 entitled Catheter Air Management System (now U.S. Pat. No. 8,360,988 issued Jan. 29, 2013), which claims priority to U.S. Provisional Application Ser. No. 61/197,042 filed Oct. 24, 2008 entitled Catheter Air Management System, and to U.S. Provisional Application Ser. No. 61/209,839 filed Mar. 12, 2009 entitled Catheter Air Management System, all of which are hereby incorporated herein by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Presently, biologically compatible air-based pressure monitoring catheters are used in a number of medical applications to monitor pressure at various locations within a mammalian body. For example, air-based pressure monitoring catheters may be inserted into the skull of a patient thereby permitting the external monitoring of intra-cranial pressure. 
         [0003]    Currently, a number of air-based pressure monitoring catheters have been developed. Generally, these air-based pressure monitoring catheters comprise a catheter having an air lumen formed therein which communicates with a bladder positioned at or near its distal end. In addition, the catheter includes a connector located at or near its proximal end which may be connected to an external pressure transducer. During use, the volume of the bladder attached to the catheter changes as pressure varies in accordance with Boyle&#39;s Law (P 1 V 1 =P 2 V 2 ). As a result, the pressure of the gas within the catheter becomes equal to that of the environment surrounding the bladder. The media surrounding the bladder must be capable of movement to accommodate the variations in bladder volume as pressure changes. 
         [0004]    The use of air-based pressure monitoring catheters in low or negatively pressurized environments has proven problematic. When the proximal connector is open to atmospheric pressure in the process of periodically replacing air lost by diffusion through the bladder, the external pressure extant in the body site monitored on a bladder will expel residual air from the bladder. If the pressure is low or negative, a significant amount of residual air may remain in the bladder. The amount of air injected is intended to be sufficient to keep the bladder in an active state for a period of 8 hours. If this volume is added to the residual air in a bladder that has not been completely collapsed by the environment around it, the sum of the residual air and injected air exceed the intrinsic volume of a fully shaped bladder. Should this happen, a positive pressure is established in the bladder. The bladder is now unable to read pressure below the internal pressure created. 
         [0005]    Air management systems such as those seen in U.S. Pub. No. 2007/0208270 and U.S. Pat. No. 6,447,462, which are both herein incorporated by reference, allow a user to adjust the amount of air in a system. For example, these systems allow a user to vent the air passage of the catheter to the open environment, then charge the passage with an amount of air. While these systems function adequately when residual air in the bladder is adequately expressed by pressure from the monitored body site, they over pressurize the bladder if the monitored site pressure has not sufficiently collapsed the bladder prior to the injection of air. An optional design that avoids over pressurizing the bladder by reducing the amount of air injected is not attractive as the lessor amount of air injected reduces the effective operating time between air recharging events in a case where the bladder has been largely collapsed by pressure in the body site measured. The intent of the invention is to preserve the desired operating time between air-charging events in a manner that precludes the possibility of pressurizing the bladder. 
       SUMMARY OF THE INVENTION 
       [0006]    A preferred embodiment according to the present invention includes an air management system for removing and reintroducing a desired amount of air into an air passage of a pressure measurement catheter. More specifically, the system includes three cylinders within which can move a shaft with three sealing members along its length. The system includes check valves in communication with one of the cylinders. As the shaft is moved within the set of cylinders, it causes the residual air in the catheter air passage to be evacuated to a negative pressure defined by one check valve. Further movement of the piston member injects a predetermined volume of air into the catheter air passage. This air is from excess air present in the evacuation cylinder and held by a check valve at a predetermined pressure (e.g., 0.5 PSI), not from exposure to the open atmosphere. Evacuating the catheter to a fixed negative pressure prior to injecting air eliminates the possibility that variations in the residual volume in the bladder can affect the pressure reading or the operating time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    These and other aspects, features and advantages of which embodiments of the invention are capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which 
           [0008]      FIG. 1  illustrates a perspective view of a catheter air management system according to a preferred embodiment of the present invention; 
           [0009]      FIG. 2  illustrates a perspective view of the internal components of a body housing of the system of  FIG. 1 ; 
           [0010]      FIG. 3  illustrates a perspective view of piston members and piston cylinders from  FIG. 2 ; 
           [0011]      FIG. 4  illustrates a side view of the piston members and piston cylinders from  FIG. 3 ; 
           [0012]      FIG. 5  illustrates a cross sectional view of the piston members and piston cylinders from  FIG. 4  taken along lines A-A; 
           [0013]      FIGS. 6-8  illustrate simplified cross sectional views of the piston members in various positions within the piston cylinders; and, 
           [0014]      FIG. 9  is an example pressure graph of an air management system according to a preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0015]    Specific embodiments of the invention will now be described with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The terminology used in the detailed description of the embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, like numbers refer to like elements. 
         [0016]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0017]      FIG. 1  illustrates an air catheter management system  100  according to a preferred embodiment of the present invention. The system  100  allows a user to charge an air passage in communication with a catheter having a pressure sensing lumen (e.g., such as that shown in U.S. Provisional Applications 61/197,039 and 61/197,041, the contents of which are hereby incorporated by reference). The air passage of the catheter is connected to the catheter connection port  104 . 
         [0018]    The system  100  also includes a monitor cable  108  that communicates between a pressure monitor (not shown) and a transducer  115  ( FIG. 2 ) within the housing  102 . The monitor cable preferably includes multiple wires that provide an excitation voltage for the transducer and voltage measurements to the monitor. A monitor connector  110  (e.g., a Marquette connector) connects to a desired monitor type to display a pressure reading to a user. 
         [0019]    Proper operation of an air-based pressure sensor requires that a predetermined amount of air reside in the bladder. Over time, air can diffuse out of the catheter, the pressure lumen of the catheter, or other portions of the arrangement. The system  100  allows the user to periodically replace air lost by diffusion by pulling out a handle  106 , then pushing that handle back into to the housing  102  of the system  100 . This action removes the air in the system  100  to a predetermined vacuum pressure, and then adds a predetermined amount of air back into the system. 
         [0020]    Preferably, the housing further includes a set of indicators  105 , such as lights, that indicate when the expected operating life of the sensor between recharging events has been reached and a recharging event is required. Preferably, the housing  102  (or optionally the transducer) may include a timer that counts a predetermined period of time (e.g., 8 hours) before signaling that it is time to recharge (i.e., pull out the handle  106  and push in the handle  106 ). 
         [0021]    Further, the housing  102  may include a microprocessor or other electronics that connects to the transducer  115  to obtain a pressure reading, then convert this reading to a form recognizable to the pressure monitor (e.g., mV/mmHg/V). This further allows the indicators  105  on the housing  102  to signal a need for a recharge of the system  100  at different times. For example, if a catheter is not connected to the port  104 , the pressure will be zero (i.e., 1 ATM) and therefore the indicator will turn red, signaling to the user that the catheter is currently connected. In another example, if the catheter is connected to the port  104  and inserted into the patient, a positive pressure may register and cause the indicator to signal a recharge to place the desired amount of air into the system. Alternately the form of the pressure wave can be monitored to determine if the catheter is connected to the port  104 . 
         [0022]      FIG. 2  illustrates an internal view of the housing  102  and some of its components. The monitor cable  108  enters the housing  102  and connects to electronics, such as a microprocessor, that are in communication with the transducer  115 . Hence, the electronics can communicate with both the transducer  115  and the pressure monitor. The transducer  115  preferably includes a manifold  107  over its top which provides a connection interface for a tube (not shown) with the port  104 . This passage through the port  104  and the transducer  115  within the housing  102  is further connected to a charging assembly  112  via connecting passage  118  ( FIG. 3 ). Hence, as the handle  106  is moved out and in, the air passage of the system  100  is charged with a predetermined amount of air. 
         [0023]      FIGS. 3 and 4  illustrate various views of the charging assembly  112 . Preferably, the charging assembly  112  is composed of an outer piston housing or piston cylinder  113  through which a piston member  120  moves into and partially out of. Preferably, this piston cylinder  113  is generally cylindrical in shape. The piston member  120  is connected to the handle  106  at end  122 . 
         [0024]    As described in more detail below, the charging assembly  112  achieves a desired air pressure through passage  118  by the movement of the piston member  120  and two check valves  114  and  116  that are set to open at different pressures. Preferably, these check valves also include air filters  115  to prevent any particles, dirt or other contamination into the system ( FIG. 4 ). 
         [0025]    Turning to  FIG. 5 , a cross sectional view of the charging assembly  112  is shown. The piston member  120  preferably includes a first section  121  adjustably connected to a second section  130  (e.g., via a thread) to allow the length of the piston member  120  to be adjusted. As will become clear with the explanation below, this adjustment can modify an amount of air moved by the piston member  120 . Also, this threading allows the piston member  120  to be easily assembled within the housing  113 . 
         [0026]    The piston member  120  also includes at least three sealing members (e.g., o-rings): a first sealing member  124 , a second sealing member  126  and a third sealing member  128 . These sealing members  124 ,  126  and  128  contact an inner surface of the housing  113  so as to prevent passage of air and create two sealed air spaces between each other. Preferably, these sealing members are located in recessed areas of the piston member  120  and are composed of fluroelastomer (Viton). 
         [0027]    Preferably, the passage within the housing  113  has three different areas of different diameter: area  132 , area  134  and area  136 . Preferably area  134  is located between the other two areas  132  and  136  and has a smaller diameter than these two areas  132  and  136 . Since the piston member  120  also has areas of different diameters, the piston member  120  is restricted from moving (i.e., restricted from being pushed in or pulled out beyond a predetermined position). 
         [0028]    The section  134  also includes an opening in communication with passage  118 , through which air is pulled from or pushed into the air passage of the system  100 . Further, this area is coupled to the first check valve  114  configured to allow outside air to enter the system at a predetermined air pressure and the second check valve  116  configured to allow air in the housing  113  to be released into the outside environment at a predetermined air pressure. In other words, the first check valve  114  opens at a predetermined negative pressure and the second check valve  116  opens at a predetermined positive pressure. For example, the valve  114  may open when a pressure in the housing  113  reaches −0.5 PSI and the valve  116  may open when a pressure in the housing  113  reaches +0.5 PSI. 
         [0029]      FIGS. 6-8  illustrate a simplified drawing of the charging assembly  112  in various positions. Beginning with  FIG. 6 , the piston member  120  is illustrated in a “pushed in” position (i.e., the handle  106  is located near the housing  102 ). In this position, the passage  118  is isolated by sealing members  124  and  126 , preventing air from entering or leaving the passage of the system  100 . 
         [0030]    As seen in  FIG. 7 , as the user begins to pull out the piston member  120 , air within the larger-volume area  136  is forced into the smaller volume area of area  134 , thereby increasing the air pressure. Once the air pressure between the sealing members  126  and  128  reaches a predetermined positive pressure, the check valve  116  releases any excess air to maintain a pressure at the predetermined positive pressure. As the piston member  120  continues to be pulled out, the sealing member  126  passes the passage  118  and therefore opens the area between sealing members  126  and  128  to the air passage of the system  100  (i.e., the passage  118 , the air passage of the catheter and the transducer  115 ). Since the air pressure has been limited by the check valve  116 , the pressure lumen or bladder on the pressure measuring catheter is not damaged. Hence, the predetermined opening pressure of this check valve  116  (and as discussed below valve  114 ) should be selected, in part, based on the pressure rating of the pressure catheter. 
         [0031]    In  FIG. 8 , the piston member  120  is pulled out from the charging assembly  112  as far as possible. In this position, the area between the sealing member  124  and sealing member  126  becomes open to the air passage of the system  100  (i.e., the passage  118 , the air passage of the catheter and the tranducer  115 ). In this respect, the area between sealing member  124  and  128  is at about the positive predetermined positive pressure that check valve  116  opens (e.g., +0.5 PSI). 
         [0032]    Once the piston member  120  has been pulled out as far as it will go, it can be pushed back in. Returning once again to  FIG. 7 , as the piston member  120  is pushed in, the sealing member  126  once again engages area  134 . The air between sealing member  126  and  128  moves from smaller diameter region  134  to larger diameter region  136  and therefore reduces the pressure in the system  100 . As the pressure reaches a negative predetermined amount (e.g., −0.5 PSI), the check valve  116  opens to let in additional air to maintain that predetermined pressure. Hence, the passages of the system  100  remain at the predetermined air pressure. 
         [0033]    As the piston member  120  continues to be pushed inwards, pressure builds between sealing member  124  and  126  as the air moves from the larger diameter region  132  to the smaller diameter region  134 . As seen in  FIG. 6 , the sealing member  126  passes passage  118 , allowing the high pressure air between sealing members  124  and  126  to pass into the system  100 . In this respect, the system  100  progresses from a predetermined negative pressure followed by the injection of a known volume of air, since the volume of air between the sealing members  124  and  126  is added to the system  100 . In other words, system  100  has been evacuated to a defined negative pressure, then injected with a predetermined quantity of air. 
         [0034]      FIG. 9  illustrates an example pressure wave measurement of an air management system during movement of the piston member  120 . For example, area  1  illustrates the pressure after full injection of the known air volume of the piston member  120 . Area  2  illustrates the initial retraction of the piston member where pressure decreases due to volume increases in the housing  113  (the system remains isolated from the check valve). Area  3  illustrates a continued retraction as the system pressure increases due to volume decreases in the housing  113  (the check valve  116  maintains a position predetermined pressure). Area  4  illustrates the initial actuation inwards of the piston member  120  where the pressure is maintained. Area  5  illustrates the continued inward movement of the piston member  120  where the check valve  114  limits the negative pressure. Finally, area  6  illustrates the final movements of the piston member  120  as the known volume of air is injected back into the system (i.e., the catheter). 
         [0035]    Depending on the predetermined opening pressure of the check valves  114  and  116 , the diameter of the areas  132 ,  134  and  136 , and the spacing of the sealing members  124 ,  126  and  128 , the final pressure in the system can be determined. The injection volume is equal to the area between sealing member  124  and  126  times the distance from transition of area  132  to area  134  and the final location of  126  at the end of the stroke. Preferably, this final pressure is a value that allows the transducer and the bladder/lumen on the distal end of the measurement catheter to achieve accurate results. In this respect, a second shut off valve, which is typically required in use with prior art air management systems, is unnecessary. 
         [0036]    In one example, the sealing members  124  and  126  are spaced at about 0.160 inches from each other and sealing members  126  and  128  are spaced at about 0.800 inches from each other. The diameter of the piston member  120  between the sealing members  124  and  126  is about 0.204 inches and the diameter of the piston member  120  between the sealing members  126  and  128  is about 0.173 inches. The diameter of the first area  132  is about 0.205 inches, the diameter of the second area  134  is about 0.173 inches and the diameter of the third area  136  is about 0.238 inches. 
         [0037]    Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.