Abstract:
Alternate embodiments of an impedance spectroscopy method are disclosed for monitoring ischemic mucosal damage in hollow viscous organs. In each embodiment, a sensor catheter is placed inside a patient&#39;s hollow viscous organ. Afterwards, the catheter is electrically driven to obtain a complex impedance spectrum by causing two electrodes in the tip of the catheter to inject a current into the mucosal tissue at different frequencies, while two other electrodes measure the resulting voltages. A pattern recognition system is then used to analyze the complex impedance spectrum and to quantify the severity of the mucosal injury. Alternatively, the complex impedance spectrum can be appropriately plotted against the spectrum of normal tissue, allowing for a visual comparison by trained personnel.

Description:
This application is a divisional application of U.S. application Ser. No. 10/990,253, filed Nov. 16, 2004, now U.S. Pat. No. 6,965,795, which is a divisional of U.S. application Ser. No. 09/907,781, now U.S. Pat. No. 6,882,879, filed Jul. 18, 2001, which claims priority from a Provisional Patent Application Ser. No. 60/219,281 filed Jul. 19, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to systems and internal sensors for monitoring and quantifying ischemic damage in tissues. 
     BACKGROUND OF THE INVENTION 
     The gastrointestinal mucosa is at great risk of ischemia in the critically ill, and its disruption has been shown to be the motor of multiple organ failure, a leading cause of death. Knowledge of the level of damage can help guide therapy, reversing moderate damage and/or preventing further complications. For example, as indicated by path A in  FIG. 6 , the status of a healthy person&#39;s mucosa changes little, if at all, over time. Path C shows how the damage level of an ill person&#39;s ischemic mucosa greatly increases over the course of several hours if unchecked. However, as shown by path B, further damage can be arrested if the ischemic damage is detected and an appropriate course of treatment is undertaken. Unfortunately, there exists no clinically suitable method to directly monitor ischemic mucosal damage in the gastrointestinal tract of the critically ill patient. 
     Impedance spectroscopy has been used to detect ischemia (a condition of inadequate blood flow and oxygen delivery to a given tissue) in biological tissues using different instrumental methods. Impedance spectroscopy differs from other impedance measurements (which have long been used for a variety of biomedical applications such as cardiac output estimation, body fat measurement, and plethismography) in that it involves multiple measurements over a range of frequencies that as a whole contain significantly more information of the structural and electrical properties of the sample. For example, U.S. Pat. No. 5,454,377 to Dzwoczyk et al. teaches the assessment of ischemia in the myocardium, U.S. Pat. No. 5,807,272 to Kun et al. teaches the assessment of ischemia in directly accessible tissues (surface or subjacent tissue), and U.S. Pat. No. 6,055,452 to Pearlman shows the general characterization of the status and properties of tissues. However, none of these references show or describe a clinically acceptable method for impedance spectroscopy measurements of the inner wall of hollow viscous organs such as the gastrointestinal mucosa, in vivo or in situ. 
     On the other hand, several other methods have been devised to detect and/or monitor gastrointestinal ischemia using different measurement technologies. These include tonometry (as shown in U.S. Pat. Nos. 5,788,631 and 6,010,453 to Fiddian-Green), direct in situ measurement using an electrochemical sensor (as shown in U.S. Pat. No. 5,158,083 to Sacristán), and direct in situ measurement using an optochemical sensor (as shown in U.S. Pat. No. 5,423,320 to Salzman et al.) Additionally, U.S. Pat. No. 5,771,894 to Richards et al. shows external, non-invasive measurement using a magnetometer. 
     Numerous gastrointestinal catheter combinations, using electrodes or other sensors, have been used over the years for various measurements and medical applications. For example, U.S. Pat. No. 5,657,759 to Essen-Moller discloses a gastrointestinal output catheter, U.S. Pat. Nos. 5,848,965 and 5,438,985, both to Essen-Moller, show a gastric pH sensor/catheter combination, and U.S. Pat. No. 5,477,854 to Essen-Moller discloses a helicobater pylori gastric infection sensor. Furthermore, U.S. Pat. No. 5,833,625 to Essen-Moller shows a gastric reflux monitor, U.S. Pat. No. 6,010,453 to Fiddian-Green shows a pressure nasogastric sump and tonometer combination, U.S. Pat. No. 5,158,083 to Sacristán et al. discloses a miniature pCO 2  probe and catheter, and U.S. Pat. No. 5,423,320 to Salzman et al. shows an air tonometry sensor and catheter. 
     Several therapies have been proposed to limit or reverse the gastrointestinal mucosal damage and/or the associated complications in critical patients, including, for example, aggressive hemodynamic resuscitation (as shown in Gutierrez et al.), NO synthase modulators (as shown in U.S. Pat. No. 5,585,402 to Moncada et al.), rBPI protein (as shown in U.S. Pat. No. 6,017,881 to Ammons et al.), oral glutamine (as shown in U.S. Pat. No. 5,981,590 to Panigrahi et al.), and DHEA (as shown in U.S. Pat. No. 5,922,701 to Araneo). All of these can be optimally effective if they are administered within ideal treatment time windows depending on the status of the mucosa. 
     Accordingly, it is a primary object of the present invention to provide a unique method not only for detecting ischemia, but also for monitoring and quantifying ischemic mucosal damage, that is of great clinical value as a therapeutic guide for patients with intestinal ischemia and/or shock. 
     Another primary object of the present invention is to provide such a method that uses impedance spectroscopy with a sensor catheter inside hollow viscous organs. 
     Yet another primary object of the present invention is to provide such a method that uses an impedance spectroscopy system, and a sensor catheter, to monitor the level of damage of the gastric mucosa in critically ill patients. 
     SUMMARY OF THE INVENTION 
     A method is disclosed that uses impedance spectroscopy to monitor ischemic mucosal damage in a hollow viscous organ. In the preferred embodiment, the preferred method comprises: electrically driving a sensor catheter, inside a hollow viscous organ, to obtain a complex impedance spectrum of tissue proximate the catheter; and using the complex impedance spectrum to determine the extent to which the tissue is damaged, as opposed to just determining if the tissue is ischemic. More specifically, as mentioned above, ischemia is a condition of inadequate blood flow and oxygen delivery to a given tissue, which may or may not result in tissue damage (i.e., ischemic tissue can be undamaged, and vice versa). Thus, detecting tissue ischemia does not result in a measurement of tissue damage, and a different process, as implemented in the present invention, must be utilized to do so. 
     The preferred catheter, which is configured to be inserted into any hollow viscous organ, comprises four Ag/AgCl electrodes positioned on an end tip of the catheter. The electrodes are functionally ring-shaped, and are coaxially spaced apart a short distance from one another. The outer two ring electrodes inject current into the tissue, and the inner two electrodes measure the resulting voltage. Leads, electrically connected to the electrodes, extend along the wall of the catheter tubing or in a lumen portion of the tubing, and terminate at an interface plug suitable for connection to the impedance spectrometer. Once the catheter is in place in one of a patient&#39;s hollow viscous organs, the impedance spectrometer causes the electrodes in the tip of the catheter to inject a current into the mucosal tissue at different frequencies, allowing for the measurement of the tissue&#39;s complex impedance spectrum. The spectrum contains information of the structural and metabolic status of the tissue, and can be used to quantify the level of damage. More specifically, the spectrum can be appropriately graphically plotted against the spectrum of normal tissue, allowing for a direct visual comparison by trained personnel, and, therefore, an indication or measurement of damage. Alternatively, a standard pattern recognition system or the like may be used to automatically analyze the complex impedance spectrum and quantify the severity of the mucosal injury. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with respect to the following description, appended claims, and accompanying drawings, in which: 
         FIG. 1  is a schematic view of an impedance spectroscopy system for monitoring ischemic mucosal damage in hollow viscous organs; 
         FIG. 2A  is a cross-sectional elevation view of a catheter for use with the impedance spectroscopy system; 
         FIG. 2B  is a perspective view of an electrode portion of the catheter; 
         FIG. 2C  is a cross-sectional plan view of an alternative upper portion of the catheter; 
         FIG. 3  is a cross-sectional elevation view of a second embodiment of the catheter; 
         FIG. 4A  is an exploded view of a third embodiment of the catheter; 
         FIG. 4B  is an elevation view, partly in cross-section, of a portion of the catheter shown in  FIG. 4A , once assembled; 
         FIG. 4C  is a detail view of a portion of  FIG. 4A ; 
         FIGS. 5A-5C  are perspective views of a fourth embodiment of the catheter; 
         FIG. 6  is a diagrammatic graph showing mucosal structure (e.g., as lining an intestinal wall) and different courses of mucosal ischemic pathogenesis; 
         FIG. 7  is a schematic illustration of the operation of the catheter; and 
         FIGS. 8A-11C  are various graphs or plots illustrating how ischemic mucosal damage in hollow viscous organs is detected and/or quantified according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This application concerns a method referred to in non-elected claims of the “parent” Utility patent application Ser. No. 09/907,781, filed Jul. 18, 2001. 
     Turning now to  FIGS. 1-11C , preferred embodiments of a unique method that use an impedance spectroscopy system  10 , with catheters  12   a - 12   d , for monitoring ischemic mucosal damage monitoring in hollow viscous organs, will now be given. The catheters  12   a - 12   d  are generally similar, in that each one has two or four electrodes with annular side surfaces positioned at a distal end of the catheter. For example, the catheter  12   a  comprises a flexible tube  14  and two or four generally cylindrical electrodes  16   a - 16   d  disposed at one end thereof. The electrodes  16   a - 16   d  are electrically connected, via leads  18   a - 18   d  extending up through the tube  14 , to an impedance spectrometer  20  portion of the system  10 . The spectrometer  20  is used in conjunction with a signal processing device  22 , such as an appropriately-programmed general purpose computer, for processing the complex impedance spectrum to detect tissue damage. To monitor mucosal damage, the catheter is placed in one of a patient&#39;s hollow viscous organs  24 , and current is injected by two of the electrodes  16   a ,  16   d  at a range of frequencies. The other two electrodes  16   b ,  16   c  measure the resulting voltage spectrum, which is subsequently processed and analyzed by the spectrometer  20  and signal processing device  22 . 
       FIGS. 2A-2C  show a first embodiment  12   a  of the preferred catheter, used in the preferred method. The catheter  12   a  comprises the flexible plastic tube  14  that can be inserted in any hollow viscous organ (e.g., 14-16 french). At the distal end or tip of the tube  14  are located the two or four electrodes  16   a - 16   d  (i.e., the catheter can be provided with either two electrodes or four electrodes) that function as ionic-current-to-electronic-current transducers, such as Ag/AgCl electrodes. The electrodes are substantially identical. As best shown in  FIG. 2B , each has a cylindrical central portion (e.g.,  24   a ) having a first diameter and two annular side surfaces (e.g.,  25   a ,  25   b ), and two cylindrical extensions (e.g.,  26   a ,  26   b ) attached to the ends of the central portion and coaxial therewith. Each extension (e.g.,  26   a ,  26   b ) has a second, reduced diameter, and each electrode  16   a - 16   d  has an axial through-bore. 
     The electrodes  16   a - 16   d  are spaced equally apart from one another along the distal tip of the catheter  12   a , and are separated by spacers (short lengths of tubing)  27   a - 27   d . As best seen in  FIG. 2A , the annular side walls of the central portions  24  of the electrodes  16   a - 16   d  are the only portions thereof that are exposed to the outside of the catheter  12   a . Thus, each electrode  16   a - 16   d  is ring-like in functionality, and the distal end of the catheter (with the electrodes) is generally radially symmetric. The catheter, therefore, will provide the same measurements regardless of its radial orientation in an organ. 
     The diameters of the central electrode portions (e.g.,  24   a ) are about the same as the outer diameter of the tube  14 . This ensures that the outer surface of the catheter  12   a  is relatively smooth, e.g., that it has no more than minor surface roughness or undulations. The electrodes  16   a - 16   d  are respectively electrically connected to the leads  18   a - 18   d  (via soldering, welding, or the like) in the electrodes&#39; axial through-bores. The leads from the distal three electrodes  16   b - 16   d  extend through the axial through-bores of the other electrodes, as applicable. The electrodes  16   a - 16   d , leads  18   a - 18   d , and short portions of tubing are kept in place and stabilized via an epoxy or plastic fill  28 . 
     The catheter  12   a  may be a stand alone sensor catheter, or it may be provided as part of a feeding/sump tube or some other type of life support tube or catheter. For example, as shown in  FIG. 2A , the catheter  12   a  doubles as a feeding tube. More specifically, the end of the catheter  12   a  is provided with the electrodes  16   a - 16   d , while the remainder of the tube  14  is left hollow to act as a feeding line  29 . Additionally, the catheter tube  14  may include a second lumen for sampling and feeding, like a Levin type gastric catheter, and/or a third lumen for a vented feeding/sump tube, as in a Salem type gastric catheter. For example, as shown in  FIG. 2C , the electrical leads  18   a - 18   d  extend down through a side wall portion of the tube  14  having a vent lumen  30  and a feeding/sump lumen  32 . 
     To manufacture the catheter  12   a , the leads  18   a - 18   d  are fed through the tube  14 , if needed (since the leads may be provided as part of the tube  14  during the tube&#39;s manufacturing process), and through the electrode through-bores, as applicable. The leads are subsequently electrically connected to the respective electrodes. Then, the proximate electrode  16   a  is inserted in the end of the tube  14 , one of the short lengths of tubing is affixed to the proximate electrode  16   a , and so on. Adhesive may be used to hold the components together in this manner. Finally, the plastic or epoxy fill  28  is injected into the space between the tubing portions, electrodes, and partially into the tube  14 , and is allowed to set. The end of the fill  28  is rounded, as shown in  FIG. 2A , to ease insertion of the catheter into a patient. As will be appreciated by those of skill in the manufacturing arts, the catheter  12   a  can be manufactured according to a number of different methods and/or steps. For example, the catheter could be extruded from a machine. 
     If the catheter  12   a  is provided with four electrodes, the two outer ring electrodes  16   a ,  16   d  inject a current into the tissue, and the two inner electrodes  16   b ,  16   c  measure the resulting voltage, as shown schematically in  FIG. 7 . In the two electrode configuration (not shown), the electrodes are used for both current source and voltage measurement. As mentioned above, the electrodes  16   a - 16   d  are respectively connected to the leads  18   a - 18   d  that provide an electrical connection to the other end of the catheter along the wall of the tubing or in the lumen. At the other, proximal end of the catheter, the leads  18   a - 18   d  end in an electrical multi-channel connector  34  that can be plugged into the impedance spectrometer  20 . 
       FIG. 3  shows a second, alternative embodiment of the catheter  12   b . Here, the catheter  12   b  has four tubular or ring-like electrodes  36   a - 36   d  simply placed over (and adhered to) the exterior surface of the tubing  14 , with the leads extending from the electrodes down through the tube wall. In this case, the electrodes would have to be as thin as possible to minimize surface roughness. 
       FIGS. 4A-4C  show a third embodiment of the catheter  12   c . The catheter  12   c  is generally similar to the catheter  12   a , but has spacers (e.g.,  42   a ,  42   b ,  42   c ), provided with annular internal sockets or spacer lips (e.g.,  52   a ,  52   b ), into which flanged electrodes (e.g.,  44   a ,  44   b ) lock into place. In this sample interlocking means, each flanged electrode (e.g.,  44   a ,  44   b ) has a cylindrical central portion (e.g.,  46   a ,  46   b ) having a first diameter and two annular side surfaces, two extensions (e.g.,  48   a ,  48   b ) attached to the ends of the central portion and coaxial therewith, and an axial through-bore. The extensions (e.g.,  48   a ,  48   b ) each have a second, reduced diameter, but instead of being purely cylindrical, the extensions (e.g.,  48   a ,  48   b ) have annular electrode lips (e.g.,  50   a ,  51   a ) that face towards the central portion  46 . Additionally, the spacers (e.g.,  42   a - 42   c ) are made of flexible plastic tubing (i.e., a relative non-conductor of electricity) or the like. Each spacer has two annular, inwardly-facing shoulders, in this embodiment, which form the sockets (e.g.,  52   a ,  52   b ). The shoulders are spaced back a bit from the open ends of the spacers. As shown in  FIG. 4B , the electrode extensions (e.g.,  48   a ,  48   b ) are dimensioned to fit within the spacers (e.g.,  42   a ,  42   b ), such that the electrode lips (e.g.,  50   a ,  50   b ) abut the shoulders, locking the flanged electrodes (e.g.,  44   a ,  44   b ) to the spacers (e.g.,  42   a - 42   c ). 
     The catheter  12   c  is assembled similarly to the catheter  12   a , as described above. More specifically, the leads are electrically connected to the electrodes (e.g.,  44   a ,  44   b ) and are threaded through the spacers and electrodes, and the electrodes (e.g.,  44   a ,  44   b ) are locked into successive spacers (e.g.,  42   a ,  42   b ) to form an assembly of two or four electrodes. As should be appreciated, since the electrodes (e.g.,  44   a ,  44   b ) simply snap into the spacers (e.g.,  42   a ,  42   b ), assembly is much quicker. Finally, the assembly is filled with the epoxy or plastic fill  28  to further hold the assembly together and to provide a rounded tip, e.g., as shown in  FIG. 2A . Also, the ends of the leads are connected to the multi-channel connector  34 . 
     To give the catheter  12   c  a smooth, low-friction outer surface, the diameter of the central portions (e.g.,  46   c ) of the electrodes (e.g.,  44   a ) may be initially slightly greater than the outer diameter of the spacers (e.g.,  42   a ,  42   b ), as shown in  FIG. 4C . Then, once the catheter  12   c  is assembled, the outer surface of the catheter is sanded, remove the portions (e.g.,  54 ) of the electrodes that extend past the spacers. 
       FIGS. 5A-5C  show a fourth embodiment of the catheter  12   d . The catheter  12   d  comprises: an injection-molded, plastic tip  60 ; two or four electrodes  62   a - 62   d ; one or three spacers  64   a - 64   c  (i.e., in the case of two electrodes, one spacer is needed, while three spacers are needed for a four electrode catheter); dual-lumen tubing  66  or the like; and the cables or leads  18   a - 18   d . As best shown in  FIG. 5B , the plastic tip  60  comprises a rounded fore portion  68  and a rounded, trough-like projection  70  that extends back from the fore portion  68 . As indicated, the tip  60  can be injection molded, or it can be made via another suitable manufacturing process. The electrodes  62   a - 62   d  and spacers  64   a - 64   c  are generally similar in shape. Each has a small, cylindrical passageway (e.g.,  72   a ,  72   b ,  72   c ,  72   d ) for the cables  18   a - 18   d , as well as a rounded through-bore (e.g.,  74   a ,  74   b ,  74   c ,  74   d ) through which the trough-like projection  70  of the tip  60  is dimensioned to fit (i.e., the rounded through-bores, e.g.,  74   a ,  74   b ,  74   c ,  74   d , and projection  70  are complementary in shape). Additionally, the outer diameters of the electrodes and spacers are the same as the outer diameter of the tip  60 , which has the same outer diameter as the tubing  66 . To assemble the catheter  12   d , the cables  18   a - 18   d  are respectively electrically connected to the electrodes  62   a - 62   d , and alternating electrodes  62   a - 62   d  and spacers  64   a - 64   c  are slid over the projection  70 . Simultaneously, the cables  18   a - 18   d  are inserted through the passageways (e.g.,  72   a ,  72   b ,  72   c ,  72   d ), as applicable. Then, the portion of the projection  70  not covered by electrodes and spacers is slid into the tubing  66 , as shown in  FIG. 5A . Appropriate fastening means, such as a solvent or an adhesive, are used to hold the components of the catheter  12   d  together. 
     As should be appreciated, the rounded through-bores  74  and projection  70  can be provided in any of a number of complementary shapes. For example, the projection and through-bores can be V-shaped, square, or circular. However, having a V- or trough-shaped projection, or a projection with another shape where the electrodes and spacers have to be oriented in a particular manner to be positioned over the projection, facilitates assembly and enhances structural stability. 
     Referring back to  FIG. 1 , the system  10  generally consists of three elements: any of the catheters  12   a - 12   d ; the impedance spectrometer  20 ; and the signal processing device  22 . The impedance spectrometer  20  is an electronic instrument that includes electrical patient isolation and can measure the impedance spectrum of the mucosa in the range of 10 Hz (or thereabouts) to 10 MHz (or thereabouts). The spectrum may be obtained by a frequency sweep from a synthesizer or by a pulse, and processed by such methods as synchronous demodulation or Fast Fourier Transform, or any other similar method. The output of the spectrometer  20  is the complex impedance spectrum measured in digital form. Spectrometers and processing methods suitable for adaptation for use in the present invention are well known to those of skill in the art, for example, as shown in U.S. Pat. No. 5,807,272 to Kun et al., U.S. Pat. No. 5,633,801 to Bottman, and U.S. Pat. No. 5,454,377 to Dzwonczyk et al. The entireties of these patents are hereby incorporated by reference. 
     Once the complex impedance spectrum is obtained, the results are processed by the signal processing device  22 . The signal processing device  22  may be an appropriately-programmed general purpose computer, or a dedicated analog and/or digital device, both of which are well known to those of ordinary skill in the art. For processing the complex impedance spectrum obtained by the spectrometer  20 , the signal processing device  22  may graph or plot the spectrum for visual analysis, as discussed in further detail below. Alternatively, the signal processing device  22  may utilize a pattern recognition algorithm or system (e.g., running as software) or the like for analyzing the complex impedance spectrum itself. The pattern recognition system uses a previously trained or calibrated algorithm to classify the impedance spectrum measured and provided by the spectrometer  20 . The output of this system is a numerical score (or other reference point) in an ischemic damage index scale  80  validated experimentally via MRI&#39;s, chemical analysis, biopsy samples, or the like. In other words, the signal processing device  22 , implementing the pattern recognition system, analyzes the impedance spectrum to determine to what extent the impedance spectrum of the analyzed tissue deviates from that of normal tissue. The degree and character of deviation provides an actual measure of tissue damage, which translates into the ischemic damage index scale  80 , as validated experimentally (e.g., heavily damaged tissue, as determined experimentally, will have a certain pattern, and slightly damaged tissue, as also determined experimentally, will have a different pattern). 
     More specifically, as discussed above, the impedance spectrum of the analyzed tissue is obtained by making multiple complex impedance measurements at different frequencies over the range of interest. At each frequency, an amplitude, Z, and a phase, φ, of the tissue response are obtained. These values are then plotted as a function of frequency, or combined and plotted in the complex plane (resistance vs. reactance) in a Nyquist or a Cole-Cole plot (this latter term applies specifically to tissue impedance spectra plots), where resistance (R) and reactance (X) are defined as:
 
 R=Z  cos φ  Eq. 1
 
 X=Z  sin φ  Eq. 2
 
Analysis of these plots shows that normal tissue spectra have a characteristic shape or pattern. According to the Cole-Cole electric model of biological tissues, this shape is the arc of a circle when plotted in the complex plane. However, if tissue is damaged after an extended period of ischemia, the spectra of the damaged tissue loses this characteristic shape. In fact, when plotted in the complex plane, the spectra of the damaged tissue become sigmoid- or S-shaped, deviating significantly from the normal tissue spectra.
 
       FIGS. 8A-11C  show averaged experimental data obtained in the small intestine of a group of test subjects subjected to a period of intestinal ischemia followed by a period of reperfusion (restored blood flow), in comparison to a group of test subjects in which normal perfusion and oxygenation was maintained. The data is presented in both the frequency plots and in the complex plane. For the Nyquist plots (complex plane), the data has been normalized so that the shapes of the curves can be more easily compared, e.g., the point at the highest measurement frequency (300 KHz) has a dimensional impedance of 1 and a phase angle of 0. 
       FIGS. 8A-8C  show the impedance spectra of intestine with less than ten minutes of reduced blood flow, wherein the intestine is already ischemic, with associated rising acidity. In particular,  FIGS. 8A and 8B  show the average amplitude and phase impedance spectra, respectively, of both normal intestine and the intestine subjected to reduced blood flow, while  FIG. 8C  shows the normalized Nyquist plot of the normal and ischemic intestinal tissue. As can be seen, although the intestine with reduced blood flow is ischemic, the tissue is not yet damaged, and the spectra are not easily distinguishable from the spectra of the normally perfused intestines. Note that the spectra contain some noise, but resemble the circular arc predicted by the Cole-Cole model. 
       FIGS. 9A and 9B  show the average amplitude and phase impedance spectra, respectively, of both normal intestine and intestine after 1.5 hours of severe ischemia, while  FIG. 9C  shows the normalized Nyquist plot of the normal and ischemic intestinal tissue. Here, the ischemic tissue has suffered moderate damage, and the spectra have become clearly distinguishable B the ischemic tissue spectra have lost their circular shape and have taken on a sigmoidal shape with several inflection points. 
       FIGS. 10A-10C  show similar plots for normal intestines and intestines after two hours of severe ischemia. By now, the damage is even more severe, and the spectra have become even more distorted. 
       FIGS. 11A-11C  show the spectra of normal intestines and intestines after an hour of ischemia followed by 1.5 hours of reperfusion. After an hour of ischemia, the tissue has suffered some damage. However, after being reperfused, most of this damage has been reversed, and the spectra of the damaged tissue have largely regained their characteristic shape, although they are still somewhat abnormal and are still moderately distinguishable from the spectra of the normal tissue. 
     As should be appreciated, a plot or graph of the complex impedance spectrum of potentially damaged tissue versus the spectrum of normal tissue, e.g., as shown in  FIGS. 8A-11C , can be used by appropriately-trained personnel to determine the level of damage due to ischemia, by way of a visual comparison. Accordingly, the signal processing device  22  may be configured to graph or plot the spectrum for visual analysis, accordingly the general guidelines given above, on a screen or monitor, or by way of a print-out. 
     Alternatively, the signal processing device  22  can be configured to automatically determine tissue damage, by way of the pattern recognition system or other standard signal processing techniques, such as filtering, or smoothing and extracting inflection points by analysis of derivatives. Another alternative is the use of principal component decomposition or any other method of extracting a characteristic vector describing the shape of the spectrum. Such a characteristic vector can then be analyzed by a classifying or pattern recognition algorithm to provide a score in a predetermined tissue damage scale. Such an algorithm can use one of many standard techniques for classification and/or pattern recognition, such as Bayesian statistics, neural networks, fuzzy logic, statistical classifiers, expert systems, or any combination of these. Further detail regarding a pattern recognition system suitable for use or adaptation for use in the present invention can be found in U.S. Pat. No. 5,807,272 to Kun et al., previously incorporated by reference. 
     Although the preferred catheters have been illustrated as having Ag/AgCl electrodes, one of ordinary skill in the art will appreciate that other types of electrodes could be used instead without departing from the spirit and scope of the invention. 
     Although the electrodes and spacers of the fourth embodiment of the catheter have been illustrated as having separate passageways and through-bores, one of ordinary skill in the art will appreciate that the passageways and through-bores could be connected, i.e., they do not have to be separate openings, as long as there is a space for the leads. 
     Since certain changes may be made in the above described impedance spectroscopy system and catheter for ischemic mucosal damage monitoring in hollow viscous organs, without departing from the spirit and scope of the invention herein involved, it is intended that all of the subject matter of the above description or shown in the accompanying drawings shall be interpreted merely as examples illustrating the inventive method herein and shall not be construed as limiting the invention.