Patent Publication Number: US-2007110612-A1

Title: External circulation apparatus

Description:
FIELD OF THE INVENTION  
      The present invention generally relates to an external circulation apparatus. More particularly, the invention pertains to an external circulation apparatus that includes a blood pump for transferring and circulating blood externally of a body, a defoaming device for defoaming the blood externally circulated, and control means for controlling the actions of the blood pump.  
     BACKGROUND DISCUSSION  
      In cardiosurgery operations, for example, a blood pump is activated to perform artificial lung external blood circulation in which blood is extracted from the vein (e.g., large vein) of a patient, subjected to gas exchange in an artificial lung, and then returned to the artery of the patient.  
      A circuit (an external circulation circuit) for the artificial lung external blood circulation is equipped with a defoaming device for removing (or separating) foam in the extracted blood. This defoaming device includes a housing or container body, and a filter member disposed in the housing for partitioning the housing interior into a blood inflow space for the blood to flow in and a blood outflow space for the blood to flow out. In this known defoaming device such as described in Japanese Application Publication No. 64-8562, foam is collected in the housing by applying centrifugal force to the blood and then the foam is removed.  
      Moreover, the defoaming device described above is usually equipped with a foam sensor for detecting the foam residing in the blood inflow space. One foam sensor includes an ultrasonic transmission unit and an ultrasonic reception unit disposed opposite the ultrasonic transmission unit with a gap between the ultrasonic transmission unit and an ultrasonic reception unit.  
      The ultrasonic reception unit receives the ultrasonic waves transmitted from the ultrasonic transmission unit and, making use of the fact that the liquid (blood) and the gas (foam) have different transmissivities to ultrasonic waves, the foam sensor detects whether the substance in the gap between the ultrasonic transmission unit and the ultrasonic reception unit is blood or foam. As a result, when foam is collected in the blood inflow space so that the liquid surface comes down to the position of the foam sensor, this can be detected by the foam sensor so that the gas (foam) can be prevented from being excessively accumulated in the blood inflow space.  
      If foam excessively accumulates in the blood inflow space, the foam may pass through the filter member. The foam may not be sufficiently or reliably removed, but may be released together with the blood that has passed through the filter member and may pass out of the defoaming device.  
      The foam sensor described above is a sensor which uses ultrasonic waves. This foam sensor using ultrasonic waves is liable to receive potential adverse influences of the environment, such as noises. Therefore, the foam sensor may erroneously detect that the liquid surface has dropped to the position of the foam sensor when in fact the liquid surface has not dropped to such position.  
      In the external circulation circuit, air in the circuit is replaced by physiological saline before the blood is circulated, that is before the cardiosurgery operations. As a result, the air in the external circulation circuit can be prevented from being sent to the human body.  
      The cardiosurgery operations are started after the external circulation circuit has been filled up with the physiological saline.  
      In the external circulation circuit filled up with the physiological saline, when the cardiosurgery operations are started, an interface is established between the physiological saline and the blood in the defoaming device due to the difference in the specific gravity between the physiological saline and the blood. When this interface goes up or rises to the position of the foam sensor which uses ultrasonic waves, erroneous detections frequently occur such that the foam sensor senses that the liquid surface has dropped to the position of the foam sensor, though the liquid surface has not in fact dropped to such position.  
      In addition, in this external circulation circuit, each time the erroneous operation or erroneous detection of the foam sensor occurs, the blood pump is interrupted, or the clamp for blocking the external circulation circuit midway is activated to stop the circulation of the blood so that the availability is lowered. As a result, the blood circulation in the patient may become unstable.  
     SUMMARY  
      An external circulation apparatus comprises a blood pump for circulating blood externally of a body, a defoaming device for defoaming the externally circulated blood, and control means for controlling the actions or operations of the blood pump. The defoaming device includes a device body having an internal space for the blood to flow in, a foam reserving chamber formed on the upper side of the device body for temporarily reserving the foam floated from the device body, and detecting means for detecting the liquid level of the blood in the foam reserving chamber or information on the liquid level. The detecting means includes a first electrode portion having at least its portion exposed to the inside of the foam reserving chamber, a second electrode portion having at least a portion exposed to the inside of the device body or the foam reserving chamber, and a power feed unit for feeding electricity between the first electrode portion and the second electrode portion. The control means controls the operation of the blood pump on the basis of the information obtained from the detecting means.  
      The control means maintains the operation of the blood pump when a decision unit decides a conductive state exists between the first electrode portion and the second electrode portion through a liquid, and stops the operation of the blood pump when the decision unit decides the non-conductive state does not exist between the first electrode portion and the second electrode portion. The blood pump can be a centrifugal pump.  
      According to another aspect, an external circulation apparatus comprises a line through which blood is transferred to outside a body, a clamp for shielding a portion of the line, a defoaming device for defoaming the blood, and control means for controlling the operation or action of the clamp. The defoaming device includes a body portion having an internal space for the blood to flow in, a foam reserving chamber formed on the upper side of the body portion for temporarily reserving the foam floating from the device body, and detecting means for detecting the liquid level of the blood in the foam reserving chamber or information on the liquid level. the detecting means includes a first electrode portion having at least a portion exposed to the inside of the foam reserving chamber, a second electrode portion having at least a portion exposed to the inside of the body portion or the foam reserving chamber, and a power feed unit for feeding electricity between the first electrode portion and the second electrode portion. The control means controls the action or operation of the clamp on the basis of the information obtained from the detecting means.  
      The control means preferably includes a decision unit for deciding whether or not the first electrode portion and the second electrode portion conduct electricity through a liquid. Preferably, the current applied by the power feed unit is an AC current. In addition, the control means includes a conversion unit for converting the AC current between the first electrode portion and the second electrode portion into an AC voltage, and a rectification unit for full-wave rectifying the converted AC voltage.  
      The defoaming device preferably includes a first communication portion disposed in the body portion for communicating the crest of the body portion with the foam reserving chamber thereby to pass foam floated from the apparatus body therethrough, and a second communication portion disposed in the body portion communicating the circumferential wall portion of the body portion with the foam reserving chamber. The foam floated from the body portion flows through the first communication portion into the foam reserving chamber whereas the blood in the foam reserving chamber returns to the body portion through the second communication portion.  
      The defoaming device preferably includes a negative pressure chamber disposed on the upper side of the foam reserving chamber and connectable to deaeration means so that it is held under a negative pressure, and a filter member disposed to separate the foam reserving chamber and the negative pressure chamber for passing the gas in the foam reserving chamber therethrough but not the blood. The first electrode portion and the second electrode portion are preferably positioned in the vicinity of the lower portion of the foam reserving chamber.  
      The first electrode portion and the second electrode portion are preferably individually made of stainless steel.  
      According to the invention, the current between the paired electrodes disposed in the defoaming device can be detected to relatively reliably detect the liquid level of the liquid in the foam reserving chamber of the defoaming device.  
      Since the liquid level of the liquid in the foam reserving chamber of the defoaming device can be detected, the operation of the blood pump can be controlled according to the detection result so that the external circulation apparatus is capable of excellent operational ability.  
      Since the liquid level of the liquid in the foam reserving chamber of the defoaming device can be detected, moreover, the action or operation of the clamp can be controlled according to the detection result so that the external circulation apparatus is capable of excellent operational ability.  
      According to another aspect, a method of controlling circulation of blood comprises circulating external of a body blood which has been removed from the body, defoaming the blood to separate foam from the blood, determining whether a level of blood in a chamber containing the foam which has been separated from the blood is at or above a predetermined level by electrical conduction, and controlling circulation of the blood external of the body based on whether the level of the blood is determined to be at or above the predetermined level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       FIG. 1  is a schematic diagram illustration of one embodiment of an external circulation apparatus disclosed herein.  
       FIG. 2  is a cross-sectional side view of a defoaming device forming a part of the external circulation apparatus shown in  FIG. 1 .  
       FIG. 3  is a bottom or lower face view of the defoaming device as seen from the direction of arrow A in  FIG. 2 .  
       FIG. 4  is a cross-sectional view taken along the section line B-B in  FIG. 2 .  
       FIG. 5  is a cross-sectional view taken along the section line C-C in  FIG. 3 .  
       FIG. 6  is a cross-sectional view taken along the section line C-C in  FIG. 3 .  
       FIG. 7  is a block diagram illustrating portions of the external circulation apparatus shown in  FIG. 1 .  
       FIG. 8  is a flow chart showing a control program of a control device of the external circulation apparatus shown in  FIG. 1 .  
       FIG. 9  is a cross-sectional view of the vicinity of an electrode portion of a defoaming device according to another embodiment of the external circulation apparatus.  
    
    
     DETAILED DESCRIPTION  
      A schematic illustration of an embodiment of an external circulation apparatus disclosed herein is shown in  FIG. 1 , with additional aspects of the apparatus shown in  FIGS. 2-8 . For convenience of description, the upper sides in  FIG. 2 ,  FIG. 5  and  FIG. 6  are referred to as “upper” or “upward” while the lower sides are referred to as “lower” or “downward”.  
      Referring to  FIG. 1 , the illustrated embodiment of the external circulation apparatus  100 A disclosed herein includes a centrifugal pump (a blood pump)  101  for feeding or transferring blood, a blood extraction line  102  connecting the suction port of the centrifugal pump  101  and a patient, a blood feed line  103  connecting the discharge port of the centrifugal pump  101  and the patient, a defoaming device  1 A disposed midway of the blood extraction line  102 , an artificial lung  104  disposed along an intermediate portion of the blood feed line  103  for carrying out gas exchange with the blood (i.e., the addition of oxygen to the blood and the removal of carbon dioxide from the blood), a flow meter  105  disposed along an intermediate portion of the blood feed line  103 , a recirculation line  106  for shortening the blood extraction line  102  near the suction port of the centrifugal pump  101  and the blood feed line  103  near the exit of the artificial lung  104 , several clamps  107 ,  108 ,  109  for pinching/releasing tubes composing one or more of the lines to thereby close/open the passages, and a control device or control means  110  for controlling the operation of the clamps  107 ,  108 ,  109  and the centrifugal pump  101 . Here, the circuit from the blood extraction line  102  to the blood feed line  103  of the external circulation apparatus  100 A may be called the “external circulation circuit  117 ”.  
      The defoaming device  1 A removes foam in the blood that is externally circulated. This defoaming device  1 A can be employed for external circulation in which blood is not circulated to the heart of the patient and gas is not exchanged in the patient&#39;s body and in which blood circulation and gas exchange with the blood (i.e., oxygen addition and/or carbon dioxide removal) are carried out by the external circulation apparatus. This defoaming device  1 A can also be employed for external circulation (or the auxiliary circulation) in which blood is circulated to the heart of the patient and gas is exchanged in the patient&#39;s body and in which blood circulation and gas exchange with the blood are carried out also by the external circulation apparatus.  
      As shown in  FIG. 2 , the defoaming device  1 A includes a body or housing  40 , a foam reserving chamber  5  disposed on the upper side of the body  40  (i.e., on the upper side of a swirling flow establishing chamber  2 ), a negative pressure chamber  8  disposed on the upper side of the foam reserving chamber  5 , a liquid reserving chamber  15  communicating with the negative pressure chamber  8 , for example through a connecting pipe  18 , a first filter (a filter member or degasifying film)  9  disposed to isolate the foam reserving chamber  5  and the negative pressure chamber  8 , a second filter  16  disposed in the liquid reserving chamber  15 , and detecting means  17 A (shown in  FIG. 1 ) for detecting the liquid level Q of the blood in the foam reserving chamber  5 .  
      The material(s) forming the body  40 , the foam reserving chamber  5 , the negative pressure chamber  8 , the connecting pipe  18  and the liquid reserving chamber  15  is not particularly limited, but may, preferably, be a relatively hard resin material such as polycarbonate, acrylic resins, polyethylene terephthalate, polyethylene, polypropylene, polystyrene, polyvinyl chloride, acryl-styrene copolymer or acryl-butadiene-styrene copolymer. The material may also, preferably, be a substantially transparent material so that the state of internal blood or the like can be visibly confirmed.  
      The body  40  is equipped with the swirling flow establishing chamber  2  forming an internal space, an inlet port  3  for introducing blood into the swirling flow establishing chamber  2 , an exit port  4  for discharging the blood in the swirling flow establishing chamber  2  to the outside of the defoaming device  1 A, and a first communication portion  6  and a second communication portion  7  for affording communication between the swirling flow establishing chamber  2  and the foam reserving chamber  5 .  
      The swirling flow establishing chamber  2  is a compartment having a rotor-shaped or annular internal space, i.e., an internal space having a generally circular cross-sectional shape, for establishing a swirling flow in the incoming blood. The defoaming device  1 A is employed in a position (i.e., oriented) such that the center axis  20  of the swirling flow establishing chamber  2  is vertical (in the upward/downward direction). The plane normal to the center axis  20  of the swirling flow establishing chamber  2  is referred to as the “horizontal plane”.  
      This swirling flow establishing chamber  2  is formed to include a disc-shaped diametrically enlarged portion  21  positioned substantially at the same height as that of the inlet port  3 , a frusto-conical portion  22  disposed on the upper side of (or above) the diametrically enlarged portion  21 , and a trunk portion  23  disposed on the lower side of (or below) the diametrically enlarged portion  21 .  
      The internal space of the frusto-conical portion  22  is generally frusto-conical in shape such that its internal diameter is gradually reduced upward. In the shown constitution, the internal space of the frusto-conical portion  22  is a frustum of a substantially complete circular cone. However, the internal space of the frusto-conical portion  22  need not be completely a frustum of a circular cone, but may have a rounded circumference in side view.  
      The internal space of the diametrically enlarged portion  21  is formed to possess a generally disc-shape configuration having a larger inner diameter than the internal diameter of the lower end of the frusto-conical portion  22 .  
      The internal space of the trunk portion  23  is generally cylindrical in shape (or a generally columnar shape), having a smaller internal diameter than that of the diametrically enlarged portion  21 . The lower portion of the trunk portion  23  is funnel shaped and is equipped at its lower end with the protruding exit port  4 .  
      As depicted in  FIG. 3 , the inlet port  3  is disposed to protrude generally tangentially to the inner circumference of the diametrically enlarged portion  21  of the swirling flow establishing chamber  2 .  
      With the disclosed embodiment of the body  40 , blood that has flown from the inlet port  3  into the swirling flow establishing chamber  2  can reliably be formed into a swirling flow.  
      The foam reserving chamber  5  is a compartment for temporarily reserving the foam that have floated from the swirling flow establishing chamber  2 . This foam reserving chamber  5  is filled up, when no foam is contained in the blood flowing into the swirling flow establishing chamber  2 , with the blood.  
      The foam reserving chamber  5  has the generally disc-shaped internal space. The foam reserving chamber  5  has its upper portion covered by the first filter  9 . Since the foam reserving chamber  5  is generally disc-shaped or possesses a generally circular shape, the area of the first filter  9  can be retained relatively large while reducing the charge or priming volume. In addition, foam residue at the time of charging the priming liquid can be relatively reliably prevented due to the absence of angled or sharp corners. Of course, while the described shape of the foam reserving chamber  5  provides certain functional or operational advantages, the foam reserving chamber  5  is not limited to the general disc shape, and may also be, for example, a polygonal plate shape.  
      This foam reserving chamber  5  has its center axis  50  offset (to the left side in  FIG. 2 ) with respect to the center axis  20  of the swirling flow establishing chamber  2 . As a result, the foam that has flown into the foam reserving chamber  5  is liable to gather on one side (or on the offset side, i.e., on the left side in  FIG. 2 ) of the foam reserving chamber  5  so that the foam can efficiently pass through the first filter  9 .  
      Moreover, the center axis  50  of the foam reserving chamber  5  is inclined with respect to the center axis  20  of the swirling flow establishing chamber  2 . This inclination is so directed that portions of the foam reserving chamber  5  located farther from the center axis  20  of the swirling flow establishing chamber  2  are located at a higher height. Thus, relative to the illustration in  FIG. 2 , the foam reserving chamber  5  is inclined upwardly and to the left. As a result, foam that has flown into the foam reserving chamber  5  can be collected more smoothly and quickly on one side of the foam reserving chamber  5 .  
      The angle α of inclination of the center axis  50  of the foam reserving chamber  5  with respect to the center axis  20  of the swirling flow establishing chamber  2  is not particularly limited, but is preferably about 0 to 50 degrees (preferably greater than zero degrees) and more preferably about 5 to 20 degrees.  
      The foam reserving chamber  5  has its bottom face  51  inclined so that the depth of the foam reserving chamber  5  increases towards the end closest to the swirling flow establishing chamber  2 .  
      The frusto-conical portion  22  of the swirling flow establishing chamber  2  communicates near its crest (upper portion) with the foam reserving chamber  5  through the first communication portion  6 . This first communication portion  6  is shaped as a circular opening formed in the bottom face  51  of the foam reserving chamber  5  as also shown in  FIG. 4 .  
      When the blood undergoes swirling flow in the swirling flow establishing chamber  2 , the foam in the blood is collected, by the centrifugal force action, at the central portion due to the gas-liquid density difference. By virtue of buoyancy, the foam thus collected at the central portion floats and flows through the first communication portion  6  into the foam reserving chamber  5  as generally illustrated by dotted lines in  FIG. 2 .  
      The foam that flows into the foam reserving chamber  5  is collected, by buoyancy, toward the higher portion (i.e., the left side in  FIG. 2 ) of the foam reserving chamber  5 .  
      The swirling flow establishing chamber  2  and the foam reserving chamber  5  further communicate with each other through the second communication portion  7 . This second communication portion  7  opens in the vicinity of the circumferential wall portion (inclined wall portion) at the left side of  FIG. 2  of the frusto-conical portion  22 . This second communication portion  7  provides communication between the foam reserving chamber  5  at the portion opposed to the first communication portion  6  through the central axis  50  and the circumferential wall portion of the frusto-conical portion  22 .  
      Since the capacity of the foam reserving chamber  5  is naturally constant, the blood of the same capacity as that of the foam which has floated from the swirling flow establishing chamber  2  has to return, when it flows into the foam reserving chamber  5  through the first communication portion  6 , in place of the foam from the foam reserving chamber  5  to the swirling flow establishing chamber  2 .  
      By virtue of the second communication portion  7 , the blood in the foam reserving chamber  5  can return through the second communication portion  7  into the swirling flow establishing chamber  2  (as indicated by the shorter dotted lines in  FIG. 2 ), as the foam which has floated from the swirling flow establishing chamber  2  flows through the first communication portion  6  into the foam reserving chamber  5 .  
      When the foam which has floated from the swirling flow establishing chamber  2  flows into the foam reserving chamber  5 , a generally one-way flow of blood can be established along a route from the frusto-conical portion  22 , to the first communication portion  6 , to the foam reserving chamber  5 , to the second communication portion  7  and to the frusto-conical portion  22 , in that order, so that the foam in the swirling flow establishing chamber  2  can be introduced relatively efficiently, smoothly and quickly into the foam reserving chamber  5 . Since the aforementioned one-way flow is established, it is possible to help prevent the possibility of blood residing in the foam reserving chamber  5 . This thus contributes to achieving a secondary effect of making it difficult for blood coagulation to occur.  
      The second communication portion  7  communicates with the circumferential wall portion of the frusto-conical portion  22  so that the vicinity of the exit of the second communication portion  7  is closer to the center axis  20 . Therefore, the swirling flow has a relatively slow speed near the exit of the second communication portion  7  so that the blood emanating from the second communication portion  7  can relatively smoothly enter the frusto-conical portion  22  while neither flowing backward nor disturbing the swirling flow.  
      The exit of the second communication portion  7  may be directed either normal in a top plan view to the circumferential wall of the frusto-conical portion  22 , or tangential to the circumferential wall of the frusto-conical portion  22 , i.e., in the direction of the swirling flow.  
      In the absence of the second communication portion  7 , when the foam in the swirling flow establishing chamber  2  flows through the first communication portion  6  into the foam reserving chamber  5 , blood returning from the foam reserving chamber  5  to the swirling flow establishing chamber  2  would pass through the first communication portion  6  in a direction opposite the foam. As a result, the flow in the vicinity of the first communication portion  6  may be disturbed and thus block the smooth passage of the foam.  
      In this embodiment, a groove  53  is formed in the bottom face  51  of the foam reserving chamber  5 . This groove 53  is on the side of the center axis  50  opposite the first communication portion  6 . The bottom surface of the groove  53  forms an inclined face  52  of the groove  53  that continues to the second communication portion  7  such that it is inclined downward to the second communication portion  7  with respect to a horizontal plane. The inclined face  52  allows the blood in the foam reserving chamber  5  to flow down more smoothly and quickly into the second communication portion  7 .  
      The angle β of inclination of the inclined surface  52  is not particularly restricted, but may preferably be 0 to 90 degrees (i.e., greater than zero degrees and less than or equal to 90 degrees), more preferably 5 to 40 degrees.  
      The first filter  9  is a film member, which permits the passage of air (or gas), but prevents the passage of blood. This first filter  9  (or the second filter  16 ) is preferably treated to have a hydrophobic surface or is a hydrophobic film.  
      Examples of materials for the hydrophobic film include polytetrafluoroethylene (PTFE), copolymer (FEP) of tetrafluoroethylene and hexafluoropropylene, copolymer (PFA) of tetrafluoroethylene and perfluoroalkylvinylether, polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), copolymer of (ETFE) ethylene and tetrafluoroethylene, copolymer (ECTFE) of ethylene and chlorotrifluoroethylene, or polypropylene (PP). The first filter  9  is preferably prepared by making those materials porous by the extension method, the micro-phase separation method, the electron beam etching method, the sintering method or the argon plasma particle method.  
      The hydrophobic treating method is not particularly limited. An example includes a method in which the surface of the first filter  9  is coated with a hydrophobic component material.  
      The first filter  9  is disposed vertically above the foam reserving chamber  5  with reference to the center axis  50  of the foam reserving chamber  5 . The first filter  9  is inclined with respect to the plane (horizontal plane) that is normal to the center axis  20  of the swirling flow establishing chamber  2 . The foam that has flown into the foam reserving chamber  5  is thus able to move along the inclined first filter  9  to one side (i.e., the left hand side in  FIG. 2 ) of the foam reserving chamber  5  so that the foam can be collected more smoothly and quickly.  
      Moreover, the first filter  9  permits the passage of the gas in the foam reserving chamber  5 , as described hereinbefore, so that any evaporation or water vapor from the foam reserving chamber  5  can pass through the first filter  9 . Water vapor having passed through the first filter  9  condenses into a liquid L which can move along the inclined first filter  9  to the side opposed to the foams (i.e., to the right side in  FIG. 2 ), that is to the side of the liquid reserving chamber  15 . As a result, the liquid L can easily flow into the liquid reserving chamber  15 .  
      The negative pressure chamber  8  is a compartment having an internal space which is separated from the foam reserving chamber  5  by the first filter  9 . The internal space in the negative pressure chamber  8  possesses a planar or flat three-dimensional configuration. In the illustrated embodiment, this negative pressure chamber  8  is disposed concentrically with the foam reserving chamber  5 . Thus, the center axis of the negative pressure chamber  8  is also inclined with respect to the center axis  20  of the swirling flow establishing chamber  2 . As a result, the liquid L in the internal space of the negative pressure chamber  8  can move toward the liquid reserving chamber  15  so that it can relatively easily flow into the liquid reserving chamber  15 .  
      The negative pressure chamber  8  does not admit the blood. In other words, the lower surface  92  of the first filter  9  contacts blood, but the upper surface  91  of the first filter  9  contacts blood.  
      The foam (or air) that is located in the foam reserving chamber  5  is sucked through the first filter  9  into the negative pressure chamber  8 , by virtue of the negative pressure in the negative pressure chamber  8 , and is discharged to the outside of the defoaming device  1 A through a deaeration port  153  of the liquid reserving chamber  15 .  
      As illustrated in  FIG. 2 , one end of the inclined negative pressure chamber  8  (i.e., the lower end at the right side of the negative pressure chamber  8 ) is connected to a connecting pipe  18  which protrudes from the negative pressure chamber  8 .  
      In the illustrated embodiment, no step is established between the bottom face  181  of the connecting pipe  18  and the upper surface  91  of the first filter  9 . In other words, it is preferable that the bottom surface  181  of the connecting pipe  18  forms a smooth continuation of the upper surface  91  of the first filter  9  so that the two are flush with one another, with the connecting pipe  18  and the liquid reserving chamber  15  being inclined at the same angle. The liquid L can thus be prevented from residing in the negative pressure chamber  8 . That is, the liquid L can smoothly flow from the upper surface  91  of the first filter  9  to the bottom surface  181  of the connecting pipe  18  so that the liquid L can be reliably discharged to the liquid reserving chamber  15 .  
      Moreover, the liquid reserving chamber  15  is connected or attached to the negative pressure chamber  8  through the connecting pipe  18 .  
      The liquid reserving chamber  15  is equipped with a reservoir chamber body portion  151 , a check valve mounting portion  152  for mounting a check valve  30 , and the deaeration port  153  connected with a deaeration means. According to one example, the deaeration means can be the wall suction of an operation room. The wall suction is one of the medical piping facilities for gases such as oxygen, medical air or nitrogen or for suction, that is the pipes arranged in the wall of the operation room for suctioning (or discharging). The deaeration means may also be constituted by a vacuum pump(s).  
      In the illustrated embodiment, the reserve chamber body portion  151  is box-shaped. This reserving chamber body  151  is adapted to reserve or hold the liquid L which flows out of the negative pressure chamber  8  thereinto through the connecting pipe  18 . As a result, the liquid L is reliably trapped or held in the reserving chamber body  151  so that the liquid L can be prevented from flowing out of the defoaming device  1 A.  
      The check valve mounting portion  152  is a cylindrical portion disposed in the upper portion  155  of the reserving chamber body  151 . Moreover, the check valve mounting portion  152  is inclined in the same direction as the protruding direction of the connecting pipe  18 .  
      The deaeration port  153 , which possesses a cylindrical shape, extends or protrudes from the end portion  154  of the check valve mounting portion  152 . This arrangement of the deaeration port  153  helps facilitate the connection of the tube of the deaeration means to the deaeration port  153 . The inside of the negative pressure chamber  8  is kept under a negative pressure so that gas (or air) in the negative pressure chamber  8  is discharged from the deaeration port  153 .  
      The protruding direction (angle of inclination) of the deaeration port  153  is substantially identical to that of the connecting pipe  18  (or the check valve mounting portion  152 ). Moreover, the inner and outer diameters of the deaeration port  153  that are smaller than the inner and outer diameters respectively of the check valve mounting portion  152 .  
      The second filter  16  and the check valve  30  are mounted in the liquid reserving chamber  15  thus constituted, there are mounted the second filter  16  and the check valve  30 , the former of which. The second filter  16  is a film member made similar to that of the first filter  9  to permit the passage of air (or gas), but not the liquid L. The check valve  30  is a valve member which permits only the flow of gas to the deaeration means.  
      The second filter  16  is disposed between the negative pressure chamber  8  and the deaeration means. That is, the second filter  16  is disposed on the upper portion  155  side of the opening  182  in which the connecting pipe  18  of the reserving chamber body  151  opens to the reserving chamber body portion  151 . As a result, the liquid L from the connecting pipe  18  can flow into the reserving chamber body portion  151  without any contact with the second filter  16 . Therefore, the liquid L can be reliably held in the reserving chamber body  151  while being prevented from flowing to the outside of the defoaming device  1 A.  
      In the illustrated embodiment, the second filter  16  is arranged generally in parallel with the first filter  9 . That is, the second filter  16  is inclined at the same angle with respect to the horizontal direction as the first filter  9 . Since the second filter  16  is mounted in such a position, any liquid L which touches the second filter  16  can relatively quickly leave the inclined second filter  16 , which is disposed at an angle α of inclination). Thus, the second filter  16  can be prevented from being damaged in its air permeability (or its defoaming ability).  
      The second filter  16  is positioned on the upper side of the first filter  9  relative to its thickness direction, i.e., in the direction of the center axis  50 . The second filter  16  has its uppermost end portion  161  positioned lower than the uppermost end portion  93  of the first filter  9 , relative to a horizontal axis passing through the uppermost end portion  161 . The lowermost end portion  162  of the second filter  16  is positioned substantially at the same height as the lowermost end portion  94  of the first filter  9  so that a horizontal axis passing through the lowermost end portion  162  of the second filter  16  also passes through the lowermost end portion  94  of the first filter  9 .  
      Moreover, the first filter  9  and the second filter  16  are disposed at positions in which they are spaced apart from one another in the direction parallel to the center axis  50 . That is, the first filter  9  and the second filter  16  lie in respective planes that are spaced apart from one another (i.e., the planes are not coplanar). As a result, the liquid L on the first filter  9  can be prevented from coming into contact with the second filter  16 .  
      The check valve  30  is disposed between the deaeration port  153  and the second filter  16 , i.e., in the check valve mounting portion  152 . As a result, gas discharged or removed by the deaeration means can be reliably prevented from flowing backward into the negative pressure chamber  8  so that the gas can be removed from the defoaming device  1 A. Moreover, the negative pressure state in the liquid reserving chamber  15  can be held at a relatively stable level.  
      In the illustrated embodiment, the check valve  30  is a duck bill valve as shown in  FIG. 2 . However, the check valve  30  is not limited in this regard as it may be formed as a different valve member allowing the flow of gas only to the side of the deaeration means.  
      In the illustrated defoaming device  1 A, the frusto-conical portion  22  is disposed in the upper portion of the swirling flow establishing chamber  2 , and foam can be collected through centrifugal force and the buoyancy so that the collected foam can be efficiently fed through the first communication portion  6  to the foam reserving chamber  5 .  
      It has been found that foam, as collected at the center portion by the action of the swirling flow in the swirling flow establishing chamber  2 , becomes a generally column-shaped lump, which is formed to have a diameter generally equal to the internal diameter d 2  of the first communication portion  6 . If, therefore, the internal diameter d 2  of the first communication portion  6  is approximately equal to or larger than the maximum diameter of the swirling flow establishing chamber  2 , the foam lump expands entirely into the swirling flow establishing chamber  2  thereby lowering the gas-liquid separating efficiency.  
      From this view point, it is preferable that the ratio of the internal diameter (or the maximum internal diameter) d 1  of the trunk portion  23  of the swirling flow establishing chamber  2  to the internal diameter d 2  of the first communication portion  6  is d 1 :d 2 =about 1:1 to 10:1, and is more preferably about 2:1 to 4:1.  
      The apex angle θ of the frusto-conical portion  22  is preferably 10 to 170 degrees, more preferably 30 to 150 degrees, and even more preferably 40 to 120 degrees.  
      If the apex angle θ of the frusto-conical portion  22  is excessively large, the frusto-conical portion  22  approaches a flattened shape having a small height and so it may be difficult to introduce the foam into the foam reserving chamber  5  by making effective use of the buoyancy. If the frusto-conical portion  22  has an excessively small apex angle θ, its height is increased to increase the charge.  
      A disc  11  is disposed in the trunk portion  23  of the swirling flow establishing chamber  2  and a connecting member  12  connects the disc  11  to the bottom portion of the swirling flow establishing chamber  2 . The disc  11  acts to define the lower end of the foam lumps collected at the center portion. The disc  11  is disposed at a position normal to the center axis  20  of the swirling flow establishing chamber  2 . The disc  11  is preferably disposed concentrically relative to the swirling flow establishing chamber  2 , but may also be eccentrically disposed.  
      The disc  11  helps prevent the foam lumps from being formed below the disc  11  so that the collected foams can be more reliably prevented from flowing out of the exit port  4 .  
      The upper face of the disc  11  is preferably positioned at the same height as or lower than the lower surface (end)  31  of the inlet port  3 . As a result, the disc  11  does not block the formation of the swirling flow. The diameter of the disc  11  is preferably the same as or larger than the internal diameter of the first communication portion  6 . As described above, the diameter of the foam lump is about as large as the internal diameter of the first communication portion  6 . Therefore, the diameter of the disc  11  is made equal to or greater than the internal diameter of the first communication portion  6  so that the diameter of the disc  11  is made equal to or greater than the foam lump. Therefore, the foam lump can be more reliably prevented from being formed below the disc  11 .  
      The disc  11  is fixed at the upper end portion of the connecting member  12 . This connecting member  12  is a cylindrical member having an outer diameter substantially equal to that of the outer diameter of the disc  11 , and its lower end is fixed on the bottom surface of the swirling flow establishing chamber  2 . The circumferential wall of the connecting member  12  is provided with a plurality of slits or openings through which the blood flows from the outer circumferential side to the inner circumferential side of the connecting member  12  and further to the exit port  4 .  
      Filters impermeable to the foam may be disposed in the slits or the openings of the connecting member  12 . This connecting member  12  may also be formed as a plurality of spaced apart members or legs for supporting the disc  11 .  
      The annular-shaped (or cylindrical) passage formed between the inner circumferential surface of the trunk portion  23  and the outer circumferential surfaces of the disc  11  and the connecting member  12  has a cross-sectional area larger than that of the passage of the inlet port  3 . This arrangement can help reduce the flow resistance in that annular-shaped passage.  
      The defoaming device  1 A is equipped with the detecting means  17 A for detecting the liquid level Q of the blood in the foam reserving chamber  5 . This detecting means  17 A is equipped with a first electrode portion  19   a , a second electrode portion  19   b , and a power supply unit  171  for supplying electricity between the first electrode portion  19   a  and the second electrode portion  19   b.    
      As shown in  FIGS. 5 and 6 , the first electrode portion  19   a  and the second electrode portion  19   b  are arranged in opposing or confronting relation to each other in the groove  53  of the foam reserving chamber  5 . As shown in  FIG. 2 , moreover, the first electrode portion  19   a  and the second electrode portion  19   b  are positioned near the lower portion  523  of the inclined face  52  (or the lower portion of the foam reserving chamber  5 ). As described in more detail below and as schematically shown in  FIG. 7 , a processing unit  114  is also provided.  
      The first electrode portion  19   a  and the second electrode portion  19   b  are rod-shaped or plate-shaped and are made of a conductive material such as a metal material or a carbon material. The first electrode portion  19   a  and the second electrode portion  19   b  are also equipped with an insulating layer on their outer circumferences. The first electrode portion  19   a  and the second electrode portion  19   b  extend through the wall portion  54  of the groove  53  so that their end faces  191  are exposed to the wall face  541  (or into the groove  53 ).  
      When the liquid surface of the blood or liquid in the foam reserving chamber  5  is higher than the first electrode portion  19   a  and the second electrode portion  19   b  (or the liquid level Q), as shown in  FIG. 5 , the end faces  191  of the first electrode portion  19   a  and the second electrode portion  19   b  contact the blood. Since the blood generally has a conductivity, although low, the first electrode portion  19   a  and the second electrode portion  19   b  conduct electricity (referred to hereinafter as the “conductive state”) through the blood while a voltage is applied between the two electrodes.  
      When the liquid surface of the blood (or liquid) in the foam reserving chamber  5  is lower than the first electrode portion  19   a  and the second electrode portion  19   b , or when the liquid surface is lower than the first electrode portion  19   a  or the second electrode portion  19   b , as shown in  FIG. 6 , the end faces  191  of the first electrode portion  19   a  and the second electrode portion  19   b  do not contact the blood. At this time, the first electrode portion  19   a  and the second electrode portion  19   b  do not conduct electricity (referred to hereinafter as the “non-conductive state”).  
      In the non-conductive state, more specifically, the resistance between the first electrode portion  19   a  and the second electrode portion  19   b  becomes the maximum. When the electrodes come to the conductive state, on the other hand, the resistance between the first electrode portion  19   a  and the second electrode portion  19   b  becomes lower.  
      Thus, the first electrode portion  19   a  and the second electrode portion  19   b  can take the conductive and non-conductive states in accordance with the height of the liquid (blood) level. As a result, the external circulation apparatus  100 A (or the detecting means  17 A) can detect whether or not the liquid level is at or above the liquid level Q.  
      Examples of the materials for making the first electrode portion  19   a  and the second electrode portion  19   b  include stainless steel, titanium, a titanium alloy (e.g., a nickel-titanium alloy) or platinum, of which the stainless steel is preferred.  
      Because of excellent biological adaptability, stainless steel can be properly used for the first electrode portion  19   a  and the second electrode portion  19   b  to contact the blood.  
      Also, in case the first electrode portion  19   a  and the second electrode portion  19   b  are made of the stainless steel, their production cost can be reduced.  
      As shown in  FIG. 5  and  FIG. 6 , the current applied between the first electrode portion  19   a  and the second electrode portion  19   b  is AC current. The AC current is less likely to hurt or cause damage to the cells in the blood than DC current.  
      In the embodiment described above, the current to be applied between the first electrode portion  19   a  and the second electrode portion  19   b  is desirably AC current. However, the applied current is not limited to AC current, as DC current may be applied.  
      In case the current applied between the first electrode portion  19   a  and the second electrode portion  19   b  is DC current, the resistance between the first electrode portion  19   a  and the second electrode portion  19   b  is measured. In the conductive state, the resistance is lower than that in the non-conductive state.  
      Therefore, the control device  110  is able to detect the liquid level Q by setting the threshold value at a predetermined resistance and deciding relative to the threshold value whether or not the measured resistance is large.  
      The clamp  107  is disposed in the blood extraction line  102  near the exit port  4  of the defoaming device  1 A. The clamp  108  is disposed in the blood feed line  103  near the exit of the artificial lung  104 . The clamp  109  is disposed in the recirculation line  106 .  
      The clamps  107 ,  108  and  109  are individually controlled between their opened/closed states by the control device  110 .  
      The clamps  107 ,  108  are normally controlled individually to be in the opened state. On the other hand, the clamp  109  is normally controlled to be in the closed state.  
      The deaeration port  153  of the defoaming device  1 A is connected through a deaeration line  111  to the wall suction (or the deaeration means). A negative pressure regulator  112  is disposed at an intermediate point along the deaeration line  111 . The negative pressure regulator  112  regulates the pressure in the negative pressure chamber  8 .  
      As shown in  FIG. 7 , the control device  110  includes a decision unit  113  comprised of a CPU (Central Processing Unit), and the processing unit  114  for processing the AC current generated between the first electrode portion  19   a  and the second electrode portion  19   b  in the conductive state.  
      The processing unit  114  includes a current-voltage converter (or conversion unit)  115  and a full-wave rectifier (or rectification unit)  116 .  
      The current-voltage converter  115  converts the AC current between the first electrode portion  19   a  and the second electrode portion  19   b  into an AC voltage. This current-voltage converter  115  is composed of, for example, two operation amplifiers, with one operation amplifier converting the AC current inputted from the first electrode portion  19   a  and the second electrode portion  19   b  into an AC voltage, and the other operation amplifier amplifying the converted AC voltage and outputting the amplified voltage to the full-wave rectifier  116 .  
      The full-wave rectifier  116  rectifies, in the full-wave manner, the AC voltage converted by the current-voltage converter  115 . This full-wave rectifier  116  includes a transformer having its input side connected with the current-voltage converter  115 , and a diode connected with the output side of the transformer. When an AC voltage is applied to the input side (or the primary side) of the transformer, an AC voltage according to the winding ratio of that transformer is generated and is rectified by the diode so that it is outputted.  
      The full-wave rectifier  116  should not be limited to the aforementioned one using the transformer, but may be of a type which performs the full-wave rectification with a rectifying diode bridge and a capacitor or may be a full-wave rectifier utilizing an operation amplifier to correct the forward voltage drop of a diode. Alternatively, the analog signal of the current-voltage converter  115  may be subjected to an A/D conversion, and to a full-wave rectification by a digital signal processing.  
      Thus, in the conductive state, the external circulation apparatus  100 A can establish the AC current and accordingly the AC voltage. In the non-conductive state, on the other hand, the AC current value is substantially zero, so that the according AC voltage is hard to generate.  
      Referring to  FIG. 8 , the decision unit  113  decides on the basis of the output signal from the full-wave rectifier  116  whether or not the conductive state exists.  
      For example, the decision unit  113  compares the output signal from the full-wave rectifier  116  and the threshold value of the voltage (referred to as the “voltage threshold value”) stored in advance in the control device  110 , and decides the conductive state exists if the output signal is at or above the voltage threshold value, and determines that the non-conductive state exists if the output signal is less than the voltage threshold value (or zero).  
      The control device  110  can thus reliably determine the conductive state and the non-conductive state between the first electrode portion  19   a  and the second electrode portion  19   b . In accordance with this decision result, moreover, the control device  110  can relatively easily control the actions of the centrifugal pump  101 .  
      The following is a description of the actions of the external circulation apparatus  100 A.  
      Before the external circulation apparatus  100 A is employed, the external circulation circuit  117  usually contains or is filled up with air. In the external circulation apparatus  100 A, the air in the external circulation circuit  117  is replaced with physiological saline. This replacing method can be performed, for example, by activating the centrifugal pump  101 . At this time, the clamps  107 ,  108 ,  109  are opened.  
      With the external circulation circuit  117  having its inside filled with physiological saline, the external circulation apparatus  100 A is employed, for example, in cardiosurgery operations.  
      The control device  110  controls the clamps  107 ,  108  to normally be in the opened state and the clamp  109  to normally be in the closed state.  
      When the centrifugal pump  101  is activated to start the operations, the blood is extracted from the patient through the blood extracting catheter and flows through the blood extraction line  102  into the inlet portion  3  of the defoaming device  1 A. In this defoaming device  1 A, the foam in the blood is removed, as described hereinbefore. The blood from which the foam is removed is sent out from the exit port  4  of the defoaming device  1 A through the centrifugal pump  101  into the artificial lung  104 . In this artificial lung  104 , the blood is subjected to a gas exchange operation in which oxygen is added and carbon dioxide is removed. The gas-exchanged blood is returned to the patient through the blood feed line  103  and the blood feed catheter.  
      In the defoaming device  1 A having its inside filled up with the physiological saline, the centrifugal pump  101  is activated to extract the blood from the patient and to feed the blood back to the patient. As a result, an interface is established between the physiological saline and the blood in the foam reserving chamber  5 . This interface rises as the physiological saline in the foam reserving chamber  5  is replaced by the blood. In the case of the detecting device mounted in the conventional defoaming device utilizing the transmissivity of ultrasonic waves, the detecting device erroneously detects the interface as the liquid level when the interface rises to reach the liquid level.  
      However, with the apparatus disclosed herein, by detecting the conductive state and the non-conductive state, the external circulation apparatus  100 A is able to relatively reliably prevent the aforementioned erroneous detection from occurring.  
      In this external circulation apparatus  100 A, when the amount of foam flowing together with the extracted blood into the defoaming device  1 A is equal to the foam removing ability of the defoaming device  1 A (or the defoaming means), the liquid level is stabilized (or balanced) at a position in the foam reserving chamber  5 .  
      In this external circulation apparatus  100 A, it is preferable that the liquid level of the blood in the foam reserving chamber  5  is positioned (or kept) in the state shown in  FIG. 5 , that is at or above the liquid level Q.  
      Therefore, the control device  110  stops the action of the centrifugal pump  101  when the liquid level falls from a position at or above the liquid level Q to a position below the liquid level Q, because the foam reserving chamber  5  is so filled up with foam as to make it difficult to remove the foam quickly and sufficiently from the defoaming device  1 A. After this action of the centrifugal pump  101  is stopped, the defoaming device  1 A is quickly cleared of the foam, and the centrifugal pump  101  is quickly activated again to restore the external circulation of the blood quickly.  
      While the centrifugal pump  101  is stopped, no new foam flows into the defoaming device  1 A so that the foam in the defoaming device  1 A is removed through the first filter  9  and the second filter  16  by the foam removing means (or deaeration means). As a result, the liquid level rises to a position higher than the liquid level Q.  
      The control flows (or programs) of the control device  110  of the external circulation apparatus  100 A are described below primarily with reference to the flow chart of  FIG. 8 . When the external circulation is started, as described hereinbefore, the power supply unit  171  is activated (at Step S 500 ).  
      Next, the AC current between the first electrode portion  19   a  and the second electrode portion  19   b  is converted into an AC voltage (at Step S 501 ). Following this, the AC voltage converted at Step S 501  is subjected to a full-wave rectification (at Step S 502 ).  
      Next, the AC voltage full-wave rectified at Step S 502  and the voltage threshold value stored in advance in the control device  110  are compared, as described hereinbefore, to decide (at Step S 503 ) whether or not the conductive state is established between the first electrode portion  19   a  and the second electrode portion  19   b.    
      If it is determined at Step S 503  that the conductive state exists, the operation or active state (current speed) of the centrifugal pump  101  is maintained (at Step S 504 ).  
      After the execution of Step S 504 , the flow chart returns to Step S 501  and executes the subsequent steps sequentially.  
      If it is determined at Step S 503  that the non-conductive state exists (or the conductive state does not exist), the operation of the centrifugal pump  101  is stopped (at Step S 505 ).  
      By the control described above, the external circulation apparatus  100 A can control the actions of the centrifugal pump  101  so that the foam may be reliably prevented from excessively residing in the foam reserving chamber  5  to thereby make the operational ability of the apparatus quite excellent.  
      As described above, if the non-conductive state is determined at Step S 503 , the action of the centrifugal pump  101  is stopped. However, the invention is not limited in this regard. For example, instead of stopping the operation of the centrifugal pump  101 , the control device  110  may control the clamps  107 ,  108 ,  109  to close the clamps  107 ,  108  and open the clamp  109 . As a result, the blood having left the artificial lung  104  returns again to the suction port of the centrifugal pump  101  through the recirculation line  106 . As a result, the blood repeatedly circulates (or recirculates) through the annular passage including the centrifugal pump  101  and the artificial lung  104 .  
      By these recirculations, it is possible to inhibit or prevent the foam in the defoaming device  1 A from being sent to the patient and to suppress the damage of the blood in the centrifugal pump  101  even if the centrifugal pump  101  is continuously driven.  
      During this recirculation, the foam in the defoaming device  1 A is relatively quickly removed, and then the ordinary external circulation state is restored by returning the clamps  107 ,  108  to the opened state and the clamp  109  to the closed state.  
       FIG. 9  is a cross-sectional diagram showing the vicinity of the electrode portion of the defoaming device of an external circulation apparatus according to a second embodiment. In  FIG. 9 , the illustrated circuit is equipped with the processing unit  114  described above.  
      With reference to this drawing, the second embodiment of the external circulation apparatus is described primarily with respect to the differences between this embodiment and the embodiment described above. A detailed description of features of the second embodiment that are the same as those associated with the first embodiment is not repeated.  
      This second embodiment is similar to the foregoing first embodiment, except that the place at which is mounted the second electrode portion is different.  
      As shown in  FIG. 9 , the second electrode portion  19   b  of the detecting means  17 B of a defoaming device  1 B extends so far through the bottom portion  55  of the groove  53  of the foam reserving chamber  5  that the end face  191  is exposed to the inclined surface  52 .  
      Like the detecting means  17 A of the first embodiment, the first electrode portion  19   a  extends through the wall portion  54  of the groove  53  so that the end face  191  is exposed at the wall surface  541 .  
      As illustrated in  FIG. 9 , when the liquid surface of the blood (or the liquid) in the foam reserving chamber  5  is higher than the liquid level Q, the end faces  191  of the first electrode portion  19   a  and the second electrode portion  19   b  contact the blood. As a voltage is applied to the first electrode portion  19   a  and the second electrode portion  19   b , electricity is conducted between the electrode portions by way of the blood.  
      When the liquid surface of the blood (or the liquid) in the foam reserving chamber  5  is lower than the liquid level Q, at least the end face  191  of the first electrode portion  19   a  does not contact the blood. At this time, the non-conductive state prevails between the first electrode portion  19   a  and the second electrode portion  19   b.    
      Thus, the state between the first electrode portion  19   a  and the second electrode portion  19   b  can be conductive or non-conductive depending upon the height of the liquid level. In the external circulation apparatus  100 A (or the detecting means  17 B), therefore, it is possible to detect whether or not the liquid surface is at or above the liquid level Q.  
      Here, the second electrode portion  19   b  is mounted on the bottom portion  55  of the groove  53  of the foam reserving chamber  5  so that the end face  191  is exposed to the inside of the groove  53 . However, other arrangements are also possible. For example, the electrode portion  19   b  may be mounted in the wall portion of the diametrically enlarged portion  21 , in the wall portion of the frusto-conical portion  22  or in the wall portion of the trunk portion  23  so that the end face  191  is exposed to the internal space of the apparatus body  40 .  
      In the known defoaming device described in the background portion, the sensor operates according to the principle that the blood and the gas have different transmissivities to ultrasonic waves in order to detect the liquid level in the foam reserving chamber. This sensor is equipped with an ultrasonic transmission unit for transmitting ultrasonic waves and an ultrasonic reception unit for receiving the ultrasonic waves sent from the ultrasonic transmission unit, with these units being arranged to confront each other. In this sensor, the ultrasonic transmission unit and the ultrasonic reception unit have to be rather specifically mounted at a portion of the foam reserving chamber so that the place for mounting the sensor is limited.  
      In the defoaming device  1 B, however, one electrode portion (i.e., the second electrode portion  19   b ) need not always be mounted locally in the foam reserving chamber  5  with the other electrode portion (i.e., the first electrode portion  19   a ).  
      In the case of the liquid level sensor utilizing the ultrasonic waves, the transmission unit and the reception unit have to be arranged in line with one another. This thus requires an accurate positioning of the transmission unit and the reception unit. Also, in the case of this liquid level sensor, the faces of the transmission unit and the reception unit must be arranged parallel to one another and so the facing portions of defoaming device in which the transmission unit and the reception unit are arranged should be parallel. Thus, those portions of the defoaming device must be specifically and quite accurately constructed. It is also necessary for the individual confronting faces to be finished in a quite highly precise manner so that they are sufficiently smooth. Moreover, the ability to reduce the size of the liquid level sensor to a significant extent is somewhat limited.  
      This raises difficulties in the design (or manufacture) of the defoaming device so that the cost for manufacturing the die to produce the defoaming device is increased by designing it highly precisely, or the assembly cost (or manufacturing cost) of the defoaming device is increased.  
      On the other hand, in the defoaming device  1 B disclosed here, the place for mounting the second electrode portion  19   b  is not so limited in that the second electrode portion  19   b  may be mounted anywhere in the defoaming device  1 B so long as it contacts the liquid. This improves the degree of freedom for designing the defoaming device. In other words, it is not necessary to highly precisely produce parallelism between the two electrodes in the design of the defoaming device. Also, it is not necessary to precisely position the two electrodes, nor is it necessary to precisely set the roughness of the faces for mounting the two electrodes. It is also possible to utilize first and second electrode portions  19   a ,  19   b  that are relatively small in size.  
      As a result, it is possible to reduce the costs for manufacturing the mold of the defoaming device and for assembling (or manufacturing) the defoaming device.  
      In case a plate-shaped first electrode portion  19   a  is used and disposed vertically with respect to the liquid surface, the contact area of the first electrode portion  19   a  with the blood decreases as the liquid level of the blood is lowered. As a result, the current between the first electrode portion  19   a  and the second electrode portion  19   b  decreases. By detecting the current at this time in an analog manner, the position (or level) of the liquid surface at an arbitrary point of time can be detected.  
      Although the external circulation apparatus disclosed herein has been described by way of the illustrated embodiments, the invention is not limited in that regard. The individual portions constituting the external circulation apparatus can be replaced by other features capable of exhibiting the same or similar functions. Moreover, additional features or components may also be added.  
      The disclosed defoaming device is equipped with one detecting means. However, the invention is not limited in this regards as the defoaming device may be equipped with a plurality of detecting means.  
      In the case two detecting means are provided, for example, they are preferably arranged such that one detecting means detects a first liquid level whereas the other detecting means detects a second liquid level below the first one. In this case, the control device can make the following controls.  
      In case one detecting means detects that the liquid level of the blood has reached the first liquid level, the control device may control the action of the centrifugal pump so that the blood flowing into the defoaming device decreases. In case one detecting means detects that the liquid level of the blood reaches the first liquid level from the position between the first liquid level and the second liquid level, the control device may carry out a control to keep the operational state of the centrifugal pump at that time. On the other hand, in case the other detecting means detects that the liquid level of the blood reaches the second liquid level, the control device may also carry out a control to stop the operation of the centrifugal pump.  
      In case a plurality of detecting means are provided, the electricity may be selectively fed between pairs of electrode portions.  
      In the case of providing plural detecting means, moreover, each detecting means may be equipped with the power feed unit, or the plural detecting means may share one power feed unit.  
      Moreover, the external circulation apparatus (or the control device) may function to prevent an overcurrent between the electrodes. The method for preventing this overcurrent is not particularly limited, but the method may involve comparing the output signal from the full-wave rectifier and the threshold value of the voltage stored in advance in the control device to decide whether or not the output signal is higher than the threshold value of the voltage.  
      Moreover, the external circulation apparatus may have a self-diagnosing function (or the diagnostic function) to detect, before it is employed, whether or not the detecting means (or the power feed unit) is normally operating.  
      The liquid reserving chamber may be equipped with a discharge port for discharging the reserved liquid. As a result, the reserved liquid can be discharged from the liquid reserving chamber before it contacts with (or arrives at) the second filter.  
      This discharge portion may be ordinarily closed, but may be opened after, or example, an operation to eliminate the reserved liquid.  
      The liquid reserving chamber may be equipped with cooling means for cooling the inside of the liquid reserving chamber. As a result, steam can be reliably condensed in the liquid reserving chamber so that the steam can be reliably prevented from passing through the second filter. Here, the cooling means can be exemplified by disposing a heat sink around the body of the liquid reserving chamber or by mounting a Peltier element.  
      The principles, preferred embodiments and modes of operation have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.