Abstract:
Devices and methods are disclosed which relate to the detection of cardiovascular efficiency and risk of disease. The rate of volumetric change of the heart can be determined by measuring the parallel conductance across electrodes attached to the heart. Measurements from a lumen would consider the total conductance. The rate of volumetric or lumen cross-section area change can then be compared to an average model to determine the health of a patient.

Description:
PRIORITY 
       [0001]    The present application is related to and claims the benefit of U.S. Provisional Patent Application Ser. No. 60/881,841, entitled “DEVICES, SYSTEMS, AND METHODS TO EVALUATE CARDIOVASCULAR FUNCTION,” filed Jan. 23, 2007. 
     
    
     BACKGROUND 
       [0002]    The disclosure of the present application relates generally to vessel and heart efficiency and risk of disease. More particularly, the disclosure of the present application relates to techniques for evaluating cardiovascular function. 
         [0003]    Many cardiovascular diseases, including diabetes, hypertension, and heart failure, have impaired arterial vasoactivity, namely vasoconstriction and vasodilation. Hypertension, for example, is associated with changes in vasomotor tone and typically attenuates vasodilation. The vasoactivity may also be altered under physiological conditions, such as in normal growth, exercise, etc. The regulation of the vasomotor tone in medium-sized arteries is of particular interest because of the clinical relevance to vasospasm and atherosclerosis. 
         [0004]    In addition to the active component (vasoactivity) of blood vessels, there is great interest in the elasticity of vessels. One of the reasons for the great interest stems from the observation that increased stiffness of large elastic arteries represents an early risk factor for cardiovascular diseases. Specifically, increased aortic stiffness is associated with aging, hypertension, diabetes, hyperlipidemia, atherosclerosis, heart failure, and smoking. Furthermore, arterial stiffness has also been shown to be an independent risk factor for cardiovascular events such as primary coronary events, stroke, and mortality. Therefore, the assessment of the passive and active mechanical properties of vessels is particularly important for understanding the mechanisms of cardiovascular disease. 
         [0005]    Clinically, the compliance or stiffness of blood vessels is used as an index of vascular mechanics, and hence, vessel function. These measurements can be made from imaging (e.g., ultrasound) to obtain the deformation (change of dimension) and loading (pressure). The endothelial function is typically measured by the degree of vasodilation or reactive hyperemia (namely the change of diameter from imaging) post cuff occlusion. Unfortunately, these measurements can be quite variable and the theoretical basis for the measurements is not well founded. Hence, there is a need to determine a theoretically-based parameter that quantifies the function of blood vessels. 
         [0006]    Regarding the heart, much effort has gone into quantifying myocardial function, independent of ventricular loading conditions. In the left ventricle (LV), the peak first time-derivative of LV intracavitary pressure, dP/dt max , is a sensitive cardiac index of inotropicity and the current detection ‘gold standard.’Currently, the ability to obtain an accurate determination of dP/dt max  requires measurement of intraventricular LV pressure using invasive cardiac catheterization. In general, it is very difficult to accurately assess ventricular pressure non-invasively. 
         [0007]    An additional difficulty with LV dP/dt max  is that it is not preload-independent. Conceivably, LV pressure-volume relationship and elastance reflect LV contractile function more accurately formalized as the time-varying elastance of the ventricle, by defining elastance, E. Elastance is defined as E(t)=P(t)/(V(t)−V d ), where P(t) and V(t) are ventricular pressure and volume that vary with time (t), respectively. V d  is the LV volume corresponding to zero LV pressure obtained by drawing a tangent to the pressure-volume curves at the end-ejection. 
         [0008]    It has been shown that the end-systolic pressure volume (ESPV) relationship, which is the loci of pressure and volume points at end-systole, is insensitive to variations of both the end-diastolic volume (preload) and the mean arterial pressure (afterload). The ESPV relationship is usually a straight line with a slope of E es . It is found that E es  remains essentially constant if the preload and afterload are allowed to vary within the physiologic range, but is sensitive to inotropic agents and ischemia. Hence, E es  has been proposed as a “load independent” index of contractility of the ventricle. Elastance measures also require cardiac catheterization for measurement of pressure which further reduces their clinical utility. An additional limitation of E es  is that it is not easy to change afterload and obtain multiple pressure-volume data points in a given subject while maintaining a constant contractility. As such, it is impractical to use E es  clinically for patient-specific LV catheterization-ventriculography data. Hence, there is a need for a cardiac index that is more readily accessible and practical. 
       SUMMARY 
       [0009]    The disclosure of the present application measures an index of vessel and heart function to evaluate the efficiency of the cardiovascular system and risk of disease. The measurements are taken with an impedance catheter. The catheter may be inserted into the lumen of the vessel or heart chamber. Alternatively, the catheter may be inserted into the pericardial space or directly placed on the heart as during open heart surgery. A patch containing the excitation and detection electrodes can be made to adhere to the surface through glue that is introduced through the lumen of the catheter into pores of the patch if the percutaneous approach is used. Alternatively, the patch may be glued on by hand with the open surgery approach. The electrodes are then interfaced with an impedance module to measure voltage differences. The voltage differences are then either compared to an average model, or combined with other measurements to create an average model. 
         [0010]    In at least one embodiment of a device for determining the index of a heart and/or vessel function according to the present disclosure, the device comprises an impedance catheter comprising a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode, and a conductance reader in connection with the catheter, the conductance reader operable to detect conductance from the first detection electrode and the second detection electrode, whereby an assessment of the index of a heart and/or vessel function may be determined based upon the conductance detected from the first detection electrode and the second detection electrode. In another embodiment, the conductance reader comprises a data acquisition and processing system. In yet another embodiment, the data acquisition and processing system comprises a processor, a storage medium operably connected to the processor, the storage medium capable of receiving and storing conductance data, and a program stored upon the storage medium, the program operable by the processor upon the conductance data to compare the conductance data to a rate of volumetric change of a heart and/or vessel. 
         [0011]    In at least one embodiment of a device for determining the index of a heart and/or vessel function according to the present disclosure, the processor compares the conductance data from conductance acquired from the epicardial surface of a heart. In another embodiment, the processor compares the conductance data from conductance acquired from the lumen surface of a heart. In yet another embodiment, the conductance reader comprises a parallel conductance reader, and wherein the parallel conductance reader is operable to detect parallel conductance. In an additional embodiment, the parallel conductance reader comprises a data acquisition and processing system. In another embodiment, the data acquisition and processing system comprises a processor, a storage medium operably connected to the processor, the storage medium capable of receiving and storing parallel conductance data, and a program stored upon the storage medium, the program operable by the processor upon the parallel conductance data to compare the parallel conductance data to a rate of volumetric change of a heart and/or vessel. 
         [0012]    In at least one embodiment of a device according to the present disclosure, the processor compares the parallel conductance data from parallel conductance acquired from the epicardial surface of a heart. In a further embodiment, the processor compares the parallel conductance data from parallel conductance acquired from the lumen surface of a heart. In another embodiment, the patch is positioned upon the epicardial surface of a heart, and wherein the conductance reader is operable to detect conductance from the epicardial surface of the heart. In yet another embodiment, the patch is positioned upon the lumen surface of a heart, and wherein the conductance reader is operable to detect conductance from the lumen surface of the heart. In an additional embodiment, the patch is positioned upon the epicardial surface of a heart, and wherein the parallel conductance reader is operable to detect parallel conductance from the epicardial surface of the heart. 
         [0013]    In at least one embodiment of a device for determining the index of a heart and/or vessel function according to the present disclosure, the patch is positioned upon the lumen surface of a heart, and wherein the parallel conductance reader is operable to detect parallel conductance from the lumen surface of the heart. In another embodiment, the processor is operable to evaluate the maximum rate of volumetric change of the heart. In yet another embodiment, the processor is operable to evaluate the maximum rate of volumetric change of the heart. In an additional embodiment, the wherein the processor compares the conductance data from conductance acquired from the outer surface of a vessel. 
         [0014]    In at least one embodiment of a device for determining the index of a heart and/or vessel function according to the present disclosure, the processor compares the conductance data from conductance acquired from the lumen surface of a vessel. In another embodiment, the processor compares the parallel conductance data from parallel conductance acquired from the outer surface of a vessel. In yet another embodiment, the processor compares the parallel conductance data from parallel conductance acquired from the lumen surface of a vessel. In an additional embodiment, the patch is positioned upon the epicardial surface of a vessel, and wherein the conductance reader is operable to detect conductance from the epicardial surface of the vessel. 
         [0015]    In at least one embodiment of a device for determining the index of a heart and/or vessel function according to the present disclosure, the patch is positioned upon the lumen surface of a vessel, and wherein the conductance reader is operable to detect conductance from the lumen surface of the vessel. In another embodiment, the patch is positioned upon the epicardial surface of a vessel, and wherein the parallel conductance reader is operable to detect parallel conductance from the epicardial surface of the vessel. In yet another embodiment, the patch is positioned upon the lumen surface of a vessel, and wherein the parallel conductance reader is operable to detect parallel conductance from the lumen surface of the vessel. In an additional embodiment, the processor is operable to evaluate the maximum rate of lumen cross-sectional area change of a vessel. In a further embodiment, the processor is operable to evaluate the maximum rate of lumen cross-sectional area change of a vessel. 
         [0016]    In at least one embodiment of a device for determining the index of a heart and/or vessel function according to the present disclosure, the device further comprises a current source, the current source operable to provide a supply of electrical current to the first excitation electrode and the second excitation electrode to facilitate the detection of conductance from the first detection electrode and the second detection electrode. In another embodiment, the device further comprises a current source, the current source operable to provide a supply of electrical current to the first excitation electrode and the second excitation electrode to facilitate the detection of parallel conductance from the first detection electrode and the second detection electrode. In yet another embodiment, the first excitation electrode, the second excitation electrode, the first detection electrode, and the second detection electrode each comprise a wire, and wherein each wire is insulated from the other wires. 
         [0017]    In at least one embodiment of a device for determining the index of a heart and/or vessel function according to the present disclosure, the device comprises an impedance catheter comprising a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode, and a conductance reader in connection with the catheter, the conductance reader operable to detect conductance from the first detection electrode and the second detection electrode, wherein the conductance reader comprises a data acquisition and processing system comprising a processor, a storage medium operably connected to the processor, the storage medium capable of receiving and storing conductance data, and a program stored upon the storage medium, the program operable by the processor upon the conductance data to compare the conductance data to a rate of volumetric change of a heart and/or vessel, whereby an assessment of the index of a heart and/or vessel function may be determined based upon the conductance detected from the first detection electrode and the second detection electrode. 
         [0018]    In at least one embodiment of a device for determining the index of a heart and/or vessel function according to the present disclosure, the device comprises an impedance catheter comprising a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode, and a parallel conductance reader in connection with the catheter, the parallel conductance reader operable to detect parallel conductance from the first detection electrode and the second detection electrode, wherein the parallel conductance reader comprises a data acquisition and processing system comprising a processor, a storage medium operably connected to the processor, the storage medium capable of receiving and storing parallel conductance data, and a program stored upon the storage medium, the program operable by the processor upon the parallel conductance data to compare the parallel conductance data to a rate of volumetric change of a heart and/or vessel, whereby an assessment of the index of a heart and/or vessel function may be determined based upon the parallel conductance detected from the first detection electrode and the second detection electrode. 
         [0019]    In at least one embodiment of a system for determining the index of a heart and/or vessel function according to the present disclosure, the system comprises an impedance catheter assembly, the impedance catheter assembly comprising a catheter, the catheter comprising a patch, and a conductance reader in connection with the catheter assembly, the conductance reader operable to detect conductance from the impedance catheter assembly, whereby an assessment of the index of a heart and/or vessel function may be determined based upon the conductance detected from the catheter assembly. In another embodiment, the patch comprises a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode. In yet another embodiment, the conductance reader is operable to detect conductance from the first detection electrode and the second detection electrode, and whereby the assessment of the index of a heart and/or vessel function may be determined based upon the conductance detected from the first detection electrode and the second detection electrode. 
         [0020]    In at least one embodiment of a system according to the present disclosure, the conductance reader comprises a data acquisition and processing system. In another embodiment, the data acquisition and processing system comprises a processor, a storage medium operably connected to the processor, the storage medium capable of receiving and storing conductance data, and a program stored upon the storage medium, the program operable by the processor upon the conductance data to compare the conductance data to a rate of volumetric change of a heart and/or vessel. In an additional embodiment, the processor compares the conductance data from conductance acquired from the epicardial surface of a heart. In a further embodiment, the processor compares the conductance data from conductance acquired from the lumen surface of a heart. 
         [0021]    In at least one embodiment of a system for determining the index of a heart and/or vessel function according to the present disclosure, the conductance reader comprises a parallel conductance reader, and wherein the parallel conductance reader is operable to detect parallel conductance. In another embodiment, the parallel conductance reader comprises a data acquisition and processing system. In yet another embodiment, the data acquisition and processing system comprises a processor, a storage medium operably connected to the processor, the storage medium capable of receiving and storing parallel conductance data, and a program stored upon the storage medium, the program operable by the processor upon the parallel conductance data to compare the parallel conductance data to a rate of volumetric change of a heart and/or vessel. In an additional embodiment, the processor compares the parallel conductance data from parallel conductance acquired from the epicardial surface of a heart. In yet an additional embodiment, the processor compares the parallel conductance data from parallel conductance acquired from the lumen surface of a heart. 
         [0022]    In at least one embodiment of a system for determining the index of a heart and/or vessel function according to the present disclosure, the patch is positioned upon the epicardial surface of a heart, and wherein the conductance reader is operable to detect conductance from the epicardial surface of the heart. In another embodiment, the patch is positioned upon the lumen surface of a heart, and wherein the conductance reader is operable to detect conductance from the lumen surface of the heart. In even another embodiment, the patch is positioned upon the epicardial surface of a heart, and wherein the parallel conductance reader is operable to detect parallel conductance from the epicardial surface of the heart. In yet another embodiment, the patch is positioned upon the lumen surface of a heart, and wherein the parallel conductance reader is operable to detect parallel conductance from the lumen surface of the heart. 
         [0023]    In at least one embodiment of a system for determining the index of a heart and/or vessel function according to the present disclosure, the processor is operable to evaluate the maximum rate of volumetric change of the heart. In another embodiment, the processor is operable to evaluate the maximum rate of volumetric change of the heart. In yet another embodiment, the processor compares the conductance data from conductance acquired from the outer surface of a vessel. 
         [0024]    In at least one embodiment of a system according to the present disclosure, the processor compares the conductance data from conductance acquired from the lumen surface of a vessel. In another embodiment, the processor compares the parallel conductance data from parallel conductance acquired from the outer surface of a vessel. In yet another embodiment, the processor compares the parallel conductance data from parallel conductance acquired from the lumen surface of a vessel. 
         [0025]    In at least one embodiment of a system for determining the index of a heart and/or vessel function according to the present disclosure, the patch is positioned upon the epicardial surface of a vessel, and wherein the conductance reader is operable to detect conductance from the epicardial surface of the vessel. In another embodiment, the patch is positioned upon the lumen surface of a vessel, and wherein the conductance reader is operable to detect conductance from the lumen surface of the vessel. In yet another embodiment, the patch is positioned upon the epicardial surface of a vessel, and wherein the parallel conductance reader is operable to detect parallel conductance from the epicardial surface of the vessel. 
         [0026]    In at least one embodiment of a system according to the present disclosure, the patch is positioned upon the lumen surface of a vessel, and wherein the parallel conductance reader is operable to detect parallel conductance from the lumen surface of the vessel. In an additional embodiment, the processor is operable to evaluate the maximum rate of lumen cross-sectional area change of a vessel. In yet an additional embodiment, the processor is operable to evaluate the maximum rate of lumen cross-sectional area change of a vessel. 
         [0027]    In at least one embodiment of a system according to the present disclosure, the system further comprises a current source, the current source operable to provide a supply of electrical current to the first excitation electrode and the second excitation electrode to facilitate the detection of conductance from the first detection electrode and the second detection electrode. In another embodiment, the system further comprises a current source, the current source operable to provide a supply of electrical current to the first excitation electrode and the second excitation electrode to facilitate the detection of parallel conductance from the first detection electrode and the second detection electrode. In yet another embodiment, the first excitation electrode, the second excitation electrode, the first detection electrode, and the second detection electrode each comprise a wire, and wherein each wire is insulated from the other wires. 
         [0028]    In at least one embodiment of a system for determining the index of a heart and/or vessel function according to the present disclosure, the system comprises an impedance catheter assembly, the impedance catheter assembly comprising a catheter, the catheter comprising a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode, and a conductance reader in connection with the catheter assembly, the conductance reader operable to operable to detect conductance from the first detection electrode and the second detection electrode, wherein the conductance reader comprises a data acquisition and processing system comprising a processor, a storage medium operably connected to the processor, the storage medium capable of receiving and storing conductance data, and a program stored upon the storage medium, the program operable by the processor upon the conductance data to compare the conductance data to a rate of volumetric change of a heart and/or vessel, whereby an assessment of the index of a heart and/or vessel function may be determined based upon the conductance detected from the first detection electrode and the second detection electrode. 
         [0029]    In at least one embodiment of a system for determining the index of a heart and/or vessel function according to the present disclosure, the system comprises an impedance catheter assembly, the impedance catheter assembly comprising a catheter, the catheter comprising a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode, and a parallel conductance reader in connection with the catheter assembly, the parallel conductance reader operable to operable to detect parallel conductance from the first detection electrode and the second detection electrode, wherein the parallel conductance reader comprises a data acquisition and processing system comprising a processor, a storage medium operably connected to the processor, the storage medium capable of receiving and storing parallel conductance data, and a program stored upon the storage medium, the program operable by the processor upon the parallel conductance data to compare the parallel conductance data to a rate of volumetric change of a heart and/or vessel, whereby an assessment of the index of a heart and/or vessel function may be determined based upon the parallel conductance detected from the first detection electrode and the second detection electrode. 
         [0030]    In at least one embodiment of a program having a plurality of program steps to be executed on a computer having a processor and a storage medium to analyze conductance data according to the present disclosure, the program is operable to receive conductance data from a conductance reader, and analyze the conductance data to determine the index of heart and/or vessel function. In another embodiment, the program is further operable to evaluate the maximum rate of volumetric change of the heart and/or vessel. In yet another embodiment, the program is further operable to evaluate the maximum rate of lumen cross-sectional area change of a vessel. 
         [0031]    In at least one embodiment of a program having a plurality of program steps to be executed on a computer having a processor and a storage medium to analyze parallel conductance data according to the present disclosure, the program is operable to receive parallel conductance data from a parallel conductance reader, and analyze the parallel conductance data to determine the index of heart and/or vessel function. In an additional embodiment, the program is further operable to evaluate the maximum rate of volumetric change of the heart and/or vessel. In yet an additional embodiment, the program is further operable to evaluate the maximum rate of lumen cross-sectional area change of a vessel. 
         [0032]    In at least one embodiment of a method of determining an index of heart function according to the present disclosure, the method comprises the steps of introducing an impedance catheter into a pericardial space on the surface of a heart, measuring a parallel conductance during a cardiac cycle, and generating an efficiency model of the heart from the parallel conductance. In another embodiment, the impedance catheter comprises a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode. In yet another embodiment, the step of measuring a parallel conductance is performed by obtaining parallel conductance from the first detection electrode and the second detection electrode. 
         [0033]    In at least one embodiment of a method of determining an index of heart function according to the present disclosure, the step of generating an efficiency model further comprises the step of comparing the parallel conductance to a rate of volumetric change of the heart. In another embodiment, the step of measuring a parallel conductance comprises multiple parallel conductance measurements to determine the volume of the heart. In yet another embodiment, the step of measuring a parallel conductance comprises the use of a parallel conductance reader operably coupled to the impedance catheter. In even another embodiment, the step of generating an efficiency model further comprises the step of evaluating the maximum rate of volumetric change of the heart. 
         [0034]    In at least one embodiment of a method of determining an index of heart function according to the present disclosure, the method comprises the steps of introducing an impedance catheter into a pericardial space on the surface of a heart, measuring a general conductance during a cardiac cycle, and generating an efficiency model of the heart from the general conductance. In another embodiment, the impedance catheter comprises a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode. In yet another embodiment, the step of measuring a general conductance is performed by obtaining general conductance from the first detection electrode and the second detection electrode. In a further embodiment, the step of generating an efficiency model further comprises the step of comparing the general conductance to a rate of volumetric change of the heart. 
         [0035]    In at least one embodiment of a method of determining an index of heart function according to the present disclosure, the step of measuring a general conductance comprises multiple general conductance measurements to determine the volume of the heart. In another embodiment, the step of measuring a general conductance comprises the use of a general conductance reader operably coupled to the impedance catheter. In yet another embodiment, the step of generating an efficiency model further comprises the step of evaluating the maximum rate of volumetric change of the heart. 
         [0036]    In at least one embodiment of a method of determining an index of heart function according to the present disclosure, the method comprises the steps of introducing an impedance catheter into a lumen of a heart measuring a parallel conductance during a cardiac cycle, and generating an efficiency model of the heart from the parallel conductance. In another embodiment, the impedance catheter comprises a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode. In yet another embodiment, the step of measuring a parallel conductance is performed by obtaining parallel conductance from the first detection electrode and the second detection electrode. 
         [0037]    In at least one embodiment of a method of determining an index of heart function according to the present disclosure, the step of generating an efficiency model further comprises the step of comparing the parallel conductance to a rate of volumetric change of the heart. In another embodiment, the step of measuring a parallel conductance comprises multiple parallel conductance measurements to determine the volume of the heart. In even another embodiment, the step of measuring a parallel conductance comprises the use of a parallel conductance reader operably coupled to the impedance catheter. In yet another embodiment, the step of generating an efficiency model further comprises the step of evaluating the maximum rate of volumetric change of the heart. 
         [0038]    In at least one embodiment of a method of determining an index of heart function according to the present disclosure, the method comprises the steps of introducing an impedance catheter into a lumen of a heart, measuring a general conductance during a cardiac cycle, and generating an efficiency model of the heart from the general conductance. In an additional embodiment, the impedance catheter comprises a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode. In yet an additional embodiment, the step of measuring a general conductance is performed by obtaining general conductance from the first detection electrode and the second detection electrode. 
         [0039]    In at least one embodiment of a method of determining an index of heart function according to the present disclosure, the step of generating an efficiency model further comprises the step of comparing the general conductance to a rate of volumetric change of the heart. In another embodiment, the step of measuring a general conductance comprises multiple general conductance measurements to determine the volume of the heart. In yet another embodiment, the step of measuring a general conductance comprises the use of a general conductance reader operably coupled to the impedance catheter. In even another embodiment, the step of generating an efficiency model further comprises the step of evaluating the maximum rate of volumetric change of the heart. 
         [0040]    In at least one embodiment of a method of determining an index of vessel function according to the present disclosure, the method comprises the steps of introducing an impedance catheter into a lumen of a vessel, measuring a parallel conductance during a cardiac cycle, and generating an efficiency model of the vessel from the parallel conductance. In another embodiment, the impedance catheter comprises a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode. In yet another embodiment, the step of measuring a parallel conductance is performed by obtaining parallel conductance from the first detection electrode and the second detection electrode. 
         [0041]    In at least one embodiment of a method of determining an index of vessel function according to the present disclosure, the step of generating an efficiency model further comprises the step of comparing the parallel conductance to a rate of volumetric change of the vessel. In an additional embodiment, the step of measuring a parallel conductance comprises a single parallel conductance measurement. In another embodiment, the step of measuring a parallel conductance comprises the use of a parallel conductance reader operably coupled to the impedance catheter. In yet another embodiment, the step of generating an efficiency model further comprises the step of evaluating the maximum rate of volumetric change of the vessel. 
         [0042]    In at least one embodiment of a method of determining an index of vessel function according to the present disclosure, the method comprises the steps of introducing an impedance catheter into a lumen of a vessel; measuring a general conductance during a cardiac cycle, and generating an efficiency model of the vessel from the general conductance. 
         [0043]    In another embodiment, the impedance catheter comprises a patch, the patch comprising a first excitation electrode, a second excitation electrode, a first detection electrode, and a second detection electrode. In yet another embodiment, the step of measuring a general conductance is performed by obtaining parallel conductance from the first detection electrode and the second detection electrode. In an additional embodiment, the step of generating an efficiency model further comprises the step of comparing the general conductance to a rate of volumetric change of the vessel. 
         [0044]    In at least one embodiment of a method of determining an index of vessel function according to the present disclosure, the step of measuring a general conductance comprises a single general conductance measurement. In another embodiment, the step of measuring a general conductance comprises the use of a general conductance reader operably coupled to the impedance catheter. In yet another embodiment, the step of generating an efficiency model further comprises the step of evaluating the maximum rate of volumetric change of the vessel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]      FIG. 1A  shows an embodiment of an engagement catheter and an embodiment of a delivery catheter as disclosed herein; 
           [0046]      FIG. 1B  shows a percutaneous intravascular pericardial delivery using another embodiment of an engagement catheter and another embodiment of a delivery catheter as disclosed herein; 
           [0047]      FIG. 2A  shows a percutaneous intravascular technique for accessing the pericardial space through a right atrial wall or atrial appendage using the engagement and delivery catheters shown in  FIG. 1A ; 
           [0048]      FIG. 2B  shows the embodiment of an engagement catheter shown in  FIG. 2A ; 
           [0049]      FIG. 2C  shows another view of the distal end of the engagement catheter embodiment shown in  FIGS. 2A and 2B ; 
           [0050]      FIG. 3A  shows removal of an embodiment of a catheter as disclosed herein; 
           [0051]      FIG. 3B  shows the resealing of a puncture according to an embodiment as disclosed herein; 
           [0052]      FIGS. 4A ,  4 B, and  4 C show a closure of a hole in the atrial wall using an embodiment as disclosed herein; 
           [0053]      FIG. 5A  shows an embodiment of an engagement catheter as disclosed herein; 
           [0054]      FIG. 5B  shows a cross-sectional view of the proximal end of the engagement catheter shown in  FIG. 5A ; 
           [0055]      FIG. 5C  shows a cross-sectional view of the distal end of the engagement catheter shown in  FIG. 5A ; 
           [0056]      FIG. 5D  shows the engagement catheter shown in  FIG. 5A  approaching a heart wall from inside of the heart; 
           [0057]      FIG. 6A  shows an embodiment of a delivery catheter as disclosed herein; 
           [0058]      FIG. 6B  shows a close-up view of the needle shown in  FIG. 6A ; 
           [0059]      FIG. 6C  shows a cross-sectional view of the needle shown in  FIGS. 6A and 6B ; 
           [0060]      FIG. 7  shows an impedance catheter according to at least one embodiment of the present disclosure placed on the surface of the heart; and 
           [0061]      FIG. 8  shows an impedance catheter with multiple sets of detection leads according to at least one embodiment of the present disclosure placed on the surface of the heart; and 
           [0062]      FIG. 9  shows a data acquisition and processing system according to at least one embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0063]    The disclosure of the present application measures an index of vessel and heart function to evaluate the efficiency of the cardiovascular system and risk of disease. For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the present disclosure is thereby intended. 
       Vessel Contractility 
       [0064]    Regarding vessel contractility, an assumption is made that an artery as a thick-walled cylindrical shell consisting of incompressible, homogeneous, isotropic, elastic material. The inner and outer radii of the shell are denoted by r i  and r e , respectively. The outer surface is considered load-free white the inner surface is subjected to blood pressure P(t), where t is time. The circumferential wall stress (σ θ ) can be expressed at any transmural radial position in the wall, r, as Lame&#39;s formula: 
         [0000]    
       
         
           
             
               
                 
                   
                     σ 
                     θ 
                   
                   = 
                   
                     P 
                     [ 
                     
                       
                         
                           ( 
                           
                             
                               r 
                               e 
                               2 
                             
                             / 
                             
                               r 
                               i 
                               2 
                             
                           
                           ) 
                         
                         + 
                         1 
                       
                       
                         
                           ( 
                           
                             
                               r 
                               e 
                               2 
                             
                             / 
                             
                               r 
                               i 
                               2 
                             
                           
                           ) 
                         
                         - 
                         1 
                       
                     
                     ] 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     #1 
                   
                   ] 
                 
               
             
           
         
       
     
         [0065]    The maximum wall stress occurs at the intima, and is given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       σ 
                       θ 
                     
                      
                     
                       ( 
                       ri 
                       ) 
                     
                   
                   = 
                   
                     P 
                     [ 
                     
                       
                         
                           ( 
                           
                             
                               r 
                               e 
                               2 
                             
                             / 
                             
                               r 
                               i 
                               2 
                             
                           
                           ) 
                         
                         + 
                         1 
                       
                       
                         
                           ( 
                           
                             
                               r 
                               e 
                               2 
                             
                             / 
                             
                               r 
                               i 
                               2 
                             
                           
                           ) 
                         
                         - 
                         1 
                       
                     
                     ] 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     #2 
                   
                   ] 
                 
               
             
           
         
       
     
         [0066]    The geometric relation between vessel wall volume (V w ), vessel cavity volume (V), r i  and r e  can be expressed as: 
         [0000]        V   w =π( r   e   2   −r   i   2 ) L  and V=πr i   2 L  [Equation #3] 
         [0000]    where L is the length of the vessel. If we combine Equation #2 and Equation #3, the following desired result is obtained: 
         [0000]    
       
         
           
             
               
                 
                   
                     σ 
                     θ 
                   
                   = 
                   
                     P 
                      
                     
                       [ 
                       
                         
                           
                             2 
                              
                             V 
                           
                           
                             V 
                             w 
                           
                         
                         + 
                         1 
                       
                       ] 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     #4 
                   
                   ] 
                 
               
             
           
         
       
     
         [0067]    By normalizing wall stress to blood pressure (P), an index of LV contractile function may result as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       σ 
                       θ 
                     
                     / 
                     P 
                   
                   = 
                   
                     
                       
                         2 
                          
                         V 
                       
                       
                         V 
                         w 
                       
                     
                     + 
                     1 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     #5 
                   
                   ] 
                 
               
             
           
         
       
     
         [0068]    Analogous to dP/dt max , we propose a vessel contractility index as the maximal rate of change of pressure-normalized wall stress; i.e., namely: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             
                               
                                  
                                 
                                   σ 
                                   * 
                                 
                               
                               / 
                               
                                  
                                 
                                   t 
                                   max 
                                 
                               
                             
                             = 
                             
                               
                                  
                                 
                                   ( 
                                   
                                     
                                       σ 
                                       θ 
                                     
                                     / 
                                     P 
                                   
                                   ) 
                                 
                               
                               
                                  
                                 t 
                               
                             
                           
                            
                         
                         max 
                       
                       = 
                       
                         
                           2 
                           
                             V 
                             w 
                           
                         
                          
                         
                           
                              
                             V 
                           
                           
                              
                             t 
                           
                         
                       
                     
                      
                   
                   max 
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     #6 
                   
                   ] 
                 
               
             
           
         
       
     
         [0069]    Since the length of the vessel remains constant, Equation #6 can be written in terms of lumen area, CSA, as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                            
                           
                             σ 
                             * 
                           
                         
                         / 
                         
                            
                           
                             t 
                             max 
                           
                         
                       
                       = 
                       
                         
                           2 
                           
                             
                               V 
                               w 
                             
                              
                             L 
                           
                         
                          
                         
                           
                              
                             CSA 
                           
                           
                              
                             t 
                           
                         
                       
                     
                      
                   
                   max 
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     #7 
                   
                   ] 
                 
               
             
           
         
       
     
         [0070]    As such, the maximum rate of change of the vessel lumen cross-sectional area is an important index of contractility, and hence, vascular function. 
         [0071]    Conventional clinical imaging (magnetic resonance imaging (MRI), computed tomography (CT), ultrasound (US), etc.) can be used in conjunction with Equation #7 to yield an index of vessel function of a patient. This index can be determined under resting conditions during the cardiac cycle, after a cuff occlusion to specifically examine endothelial function, or after a pharmacological challenge to evaluate the vasoactive tone of vessel. 
       Cardiac Contractility 
       [0072]    The formulation as described above may also be used to evaluate heart function. The disclosure of the present application reveals that a similar equation (Equation #6) results if a cylinder or a spherical geometry is assumed but with a different proportionality constant. Hence, a similar strategy of combining current non-invasive imaging (CT, MRI, US, etc.) with Equation #6 to yield a patient specific contractility index. 
       Contractility Index Based on Electrical Impedance 
       [0073]    Vessel 
         [0074]    As referenced by prior studies, the conductance of current flow through the organ lumen and organ wall and surrounding tissue is parallel. For example, 
         [0000]    
       
         
           
             
               
                 
                   
                     G 
                      
                     
                       ( 
                       
                         z 
                         , 
                         t 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           CSA 
                            
                           
                             ( 
                             
                               z 
                               , 
                               t 
                             
                             ) 
                           
                         
                         · 
                         
                           C 
                           b 
                         
                       
                       L 
                     
                     + 
                     
                       
                         G 
                         p 
                       
                        
                       
                         ( 
                         
                           z 
                           , 
                           t 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     #8 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    where G p (z,t) is the effective conductance of the structure outside the bodily fluid (organ wall and surrounding tissue), C b  is the specific electrical conductivity of the bodily fluid, CSA is the lumen cross-sectional area of the organ and L is the distance between the detection electrodes. This concept was previously used to determine luminal area. However, the disclosure of the present application identifies that the same concept can be applied here for blood vessels with the use of Equation #7 to determine the function of blood vessels during percutaneous catheterization. Since only the change of CSA is required, Equation #8 can be reduced to: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             
                                
                               CSA 
                             
                             
                                
                               t 
                             
                           
                            
                         
                         max 
                       
                       = 
                       
                         
                            
                           G 
                         
                         
                            
                           t 
                         
                       
                     
                      
                   
                   max 
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     #9 
                   
                   ] 
                 
               
             
           
         
       
     
         [0075]    As such, the change of conductance is desired which does not require injections as referenced by earlier studies, and can be directly determined from the change of conductance. 
         [0076]    Heart 
         [0077]    Intra-Ventricle Approach 
         [0078]    In previous studies, the catheter was placed inside of the lumen to determine the dimensional changes. This procedure can still be done for the heart with multiple leads (two outer excitation electrodes (E) but multiple sets of inner detection electrodes (D)) to add up the cross-sectional areas to provide the volume, and hence, Equation #6. Again, only the change in conductance is required which does not necessitate any saline injections. 
         [0079]    Epicardial Approach 
         [0080]    Previous studies introduced the ability to introduce a catheter in the pericardial space on the surface of the heart. Such techniques include devices, systems, and methods useful for accessing various tissues of the heart from inside the heart. For example, various embodiments provide for percutaneous, intravascular access into the pericardial space through an atrial wall or the wall of an atrial appendage. In at least some embodiments, the heart wall is aspirated and retracted from the pericardial sac to increase the pericardial space between the heart and the sac and thereby facilitate access into the space. 
         [0081]    Unlike the relatively stiff pericardial sac, the atrial wall and atrial appendage are rather soft and deformable. Hence, suction of the atrial wall or atrial appendage can provide significantly more clearance of the cardiac structure from the pericardium as compared to suction of the pericardium. Furthermore, navigation from the intravascular region (inside of the heart) provides more certainty of position of vital cardiac structures than does intrathoracic access (outside of the heart). 
         [0082]    Access to the pericardial space may be used for identification of diagnostic markers in the pericardial fluid; for pericardiocentesis; and for administration of therapeutic factors with angiogenic, myogenic, and antiarrhythmic potential. In addition, epicardial pacing leads may be delivered via the pericardial space, and an ablation catheter may be used on the epicardial tissue from the pericardial space. 
         [0083]    In the embodiment of the catheter system shown in  FIG. 1A , catheter system  10  includes an engagement catheter  20 , a delivery catheter  30 , and a needle  40 . Although each of engagement catheter  20 , delivery catheter  30 , and needle  40  has a proximal end and a distal end,  FIG. 1A  shows only the distal end. Engagement catheter  20  has a lumen through which delivery catheter  30  has been inserted, and delivery catheter  30  has a lumen through which needle  40  has been inserted. Delivery catheter  30  also has a number of openings  50  that can be used to transmit fluid from the lumen of the catheter to the heart tissue in close proximity to the distal end of the catheter. It can be appreciated that catheter system  10 , engagement catheter  20 , and delivery catheter  30  may be generally referred to as a “catheter.” 
         [0084]    As shown in more detail in  FIGS. 2A ,  2 B, and  2 C, engagement catheter  20  includes a vacuum channel  60  used for suction of a targeted tissue  65  in the heart and an injection channel  70  used for infusion of substances to targeted tissue  65 , including, for example, a biological or non-biological degradable adhesive. As is shown in  FIGS. 2B and 2C , injection channel  70  is ring-shaped, which tends to provide relatively even dispersal of the infused substance over the targeted tissue, but other shapes of injection channels may be suitable. A syringe  80  is attached to injection channel  70  for delivery of the appropriate substances to injection channel  70 , and a syringe  90  is attached to vacuum channel  60  through a vacuum port (not shown) at the proximal end of engagement catheter  20  to provide appropriate suction through vacuum channel  60 . At the distal end of engagement catheter  20 , a suction port  95  is attached to vacuum channel  60  for contacting targeted tissue  65 , such that suction port  95  surrounds targeted tissue  65 , which is thereby encompassed within the circumference of suction port  95 . Although syringe  90  is shown in  FIG. 2B  as the vacuum source providing suction for engagement catheter  20 , other types of vacuum sources may be used, such as a controlled vacuum system providing specific suction pressures. Similarly, syringe  80  serves as the external fluid source in the embodiment shown in  FIG. 2B , but other external fluid sources may be used. 
         [0085]    A route of entry for use of various embodiments disclosed herein is through the jugular or femoral vein to the superior or inferior vena cavae, respectively, to the right atrial wall or atrial appendage (percutaneously) to the pericardial sac (through puncture). 
         [0086]    Referring now to  FIG. 1B , an engagement catheter  100  is placed via standard approach into the jugular or femoral vein. The catheter, which may be 4 or 5 Fr., is positioned under fluoroscopic or echocardiographic guidance into the right atrial appendage  110 . Suction is initiated to aspirate a portion of atrial appendage  110  away from the pericardial sac  120  that surrounds the heart. As explained herein, aspiration of the heart tissue is evidenced when no blood can be pulled back through engagement catheter  100  and, if suction pressure is being measured, when the suction pressure gradually increases. A delivery catheter  130  is then inserted through a lumen of engagement catheter  100 . A small perforation can be made in the aspirated atrial appendage  110  with a needle such as needle  40 , as shown in  FIGS. 1A and 2A . A guide wire (not shown) can then be advanced through delivery catheter  130  into the pericardial space to secure the point of entry  125  through the atrial appendage and guide further insertion of delivery catheter  130  or another catheter. Flouroscopy or echocardiogram can be used to confirm the position of the catheter in the pericardial space. Alternatively, a pressure tip needle can sense the pressure and measure the pressure change from the atrium (about 10 mmHg) to the pericardial space (about 2 mmHg). This is particularly helpful for transeptal access where puncture of arterial structures (e.g., the aorta) can be diagnosed and sealed with an adhesive, as described in more detail below. 
         [0087]    Although aspiration of the atrial wall or the atrial appendage retracts the wall or appendage from the pericardial sac to create additional pericardial space, CO2 gas can be delivered through a catheter, such as delivery catheter  130 , into the pericardial space to create additional space between the pericardial sac and the heart surface. 
         [0088]    Referring now to  FIG. 3A , the catheter system shown in  FIG. 1B  is retrieved by pull back through the route of entry. However, the puncture of the targeted tissue in the heart (e.g., the right atrial appendage as shown in  FIG. 3A ) may be sealed upon withdrawal of the catheter, which prevents bleeding into the pericardial space. The retrieval of the catheter may be combined with a sealing of the tissue in one of several ways: (1) release of a tissue adhesive or polymer  75  via injection channel  70  to seal off the puncture hole, as shown in  FIG. 3B ; (2) release of an inner clip or mechanical stitch to close off the hole from the inside of the cavity; or (3) mechanical closure of the heart with a sandwich type mechanical device that approaches the hole from both sides of the wall (see  FIGS. 4A ,  4 B, and  4 C). In other words, closure may be accomplished by using, for example, a biodegradable adhesive material (e.g., fibrin glue or cyanomethacrylate), a magnetic system, or an umbrella-shaped nitinol stent. An example of the closure of a hole in the atrium is shown in  FIG. 3B . Engagement catheter  20  is attached to targeted tissue  95  using suction through suction port  60 . Tissue adhesive  75  is injected through injection channel  70  to coat and seal the puncture wound in targeted tissue  95 . Engagement catheter  20  is then withdrawn, leaving a plug of tissue adhesive  75  attached to the atrial wall or atrial appendage. 
         [0089]    Another example for sealing the puncture wound in the atrial wall or appendage is shown in  FIGS. 4A ,  4 B, and  4 C. A sandwich-type closure, having an external cover  610  and an internal cover  620 , is inserted through the lumen of engagement catheter  600 , which is attached to the targeted tissue of an atrial wall  630 . Each of external and internal covers  610  and  620  is similar to an umbrella in that it can be inserted through a catheter in its folded configuration and expanded once it is outside of the catheter. As shown in  FIG. 4A , external cover  610  is deployed (in its expanded configuration) on the outside of the atrial wall to seal a puncture wound in the targeted tissue. Internal cover  620  is delivered through engagement catheter  600  (in its folded configuration), as shown in  FIGS. 4A and 4B . Once internal cover  620  is in position on the inside of atrial wall  630  at the targeted tissue, internal cover  620  is deployed to help seal the puncture wound in the targeted tissue (see  FIG. 4C ). Engagement catheter  600  then releases its grip on the targeted tissue and is withdrawn, leaving the sandwich-type closure to seal the puncture wound, as shown in  FIG. 4C . External cover  610  and internal cover  620  may be held in place using adhesion or magnetic forces. 
         [0090]      FIGS. 5A ,  5 B,  5 C, and  5 D show another embodiment of an engagement catheter as disclosed herein. Engagement catheter  700  is an elongated tube having a proximal end  710  and a distal end  720 , as well as two lumens  730 ,  740  extending between proximal end  710  and distal end  720 . Lumens  730 ,  740  are formed by concentric inner wall  750  and outer wall  760 , as particularly shown in  FIGS. 5B and 5C . At proximal end  710 , engagement catheter  700  includes a vacuum port  770 , which is attached to lumen  730  so that a vacuum source can be attached to vacuum port  770  to create suction in lumen  730 , thereby forming a suction channel. At distal end  720  of catheter  700 , a suction port  780  is attached to lumen  730  so that suction port  780  can be placed in contact with heart tissue  775  (see  FIG. 5D ) for aspirating the tissue, thereby forming a vacuum seal between suction port  780  and tissue  775  when the vacuum source is attached and engaged. The vacuum seal enables suction port  780  to grip, stabilize, and retract tissue  775 . For example, attaching a suction port to an interior atrial wall using a vacuum source enables the suction port to retract the atrial wall from the pericardial sac surrounding the heart, which enlarges the pericardial space between the atrial wall and the pericardial sac. 
         [0091]    As shown in  FIG. 5C , two internal lumen supports  810 ,  820  are located within lumen  730  and are attached to inner wall  750  and outer wall  760  to provide support to the walls. These lumen supports divide lumen  730  into two suction channels. Although internal lumen supports  810 ,  820  extend from distal end  720  of catheter  700  along a substantial portion of the length of catheter  700 , internal lumen supports  810 ,  820  may or may not span the entire length of catheter  700 . Indeed, as shown in  FIGS. 5A ,  5 B, and  5 C, internal lumen supports  810 ,  820  do not extend to proximal end  710  to ensure that the suction from the external vacuum source is distributed relatively evenly around the circumference of catheter  700 . Although the embodiment shown in  FIG. 5C  includes two internal lumen supports, other embodiments may have just one internal support or even three or more such supports. 
         [0092]      FIG. 5D  shows engagement catheter  700  approaching heart tissue  775  for attachment thereto. It is important for the clinician performing the procedure to know when the suction port has engaged the tissue of the atrial wall or the atrial appendage. For example, in reference to  FIG. 5D , it is clear that suction port  780  has not fully engaged tissue  775  such that a seal is formed. However, because suction port  780  is not usually seen during the procedure, the clinician may determine when the proper vacuum seal between the atrial tissue and the suction port has been made by monitoring the amount of blood that is aspirated, by monitoring the suction pressure with a pressure sensor/regulator, or both. For example, as engagement catheter  700  approaches the atrial wall tissue (such as tissue  775 ) and is approximately in position, the suction can be activated through lumen  730 . A certain level of suction (e.g., 10 mmHg) can be imposed and measured with a pressure sensor/regulator. As long as catheter  700  does not engage the wall, some blood will be aspirated into the catheter and the suction pressure will remain the same. However, when catheter  700  engages or attaches to the wall of the heart (depicted as tissue  775  in  FIG. 5D ), minimal blood is aspirated and the suction pressure will start to gradually increase. Each of these signs can alert the clinician (through alarm or other means) as an indication of engagement. The pressure regulator is then able to maintain the suction pressure at a preset value to prevent over-suction of the tissue. 
         [0093]    An engagement catheter, such as engagement catheter  700 , may be configured to deliver a fluid or other substance to tissue on the inside of a wall of the heart, including an atrial wall or a ventricle wall. For example, lumen  740  shown in  FIGS. 5A and 5C  includes an injection channel  790  at distal end  720 . Injection channel  790  dispenses to the targeted tissue a substance flowing through lumen  740 . As shown in  FIG. 5D , injection channel  790  is the distal end of lumen  740 . However, in other embodiments, the injection channel may be ring-shaped (see  FIG. 2C ) or have some other suitable configuration. 
         [0094]    Substances that can be locally administered with an engagement catheter include preparations for gene or cell therapy, drugs, and adhesives that are safe for use in the heart. The proximal end of lumen  740  has a fluid port  800 , which is capable of attachment to an external fluid source for supply of the fluid to be delivered to the targeted tissue. Indeed, after withdrawal of a needle from the targeted tissue, as discussed herein, an adhesive may be administered to the targeted tissue by the engagement catheter for sealing the puncture wound left by the needle withdrawn from the targeted tissue. 
         [0095]    Referring now to  FIGS. 6A ,  6 B, and  6 C, there is shown a delivery catheter  850  comprising an elongated hollow tube  880  having a proximal end  860 , a distal end  870 , and a lumen  885  along the length of the catheter. Extending from distal end  870  is a hollow needle  890  in communication with lumen  885 . Needle  890  is attached to distal end  870  in the embodiment of  FIGS. 6A ,  6 B, and  6 C, but, in other embodiments, the needle may be removably attached to, or otherwise located at, the distal end of the catheter (see  FIG. 1A ). In the embodiment shown in  FIGS. 6A ,  6 B, and  6 C, as in certain other embodiments having an attached needle, the junction (i.e., site of attachment) between hollow tube  880  and needle  890  forms a security notch  910  circumferentially around needle  890  to prevent needle  890  from over-perforation. Thus, when a clinician inserts needle  890  through an atrial wall to gain access to the pericardial space, the clinician will not, under normal conditions, unintentionally perforate the pericardial sac with needle  890  because the larger diameter of hollow tube  880  (as compared to that of needle  890 ) at security notch  910  hinders further needle insertion. Although security notch  910  is formed by the junction of hollow tube  880  and needle  890  in the embodiment shown in  FIGS. 6A ,  6 B, and  6 C, other embodiments may have a security notch that is configured differently. For example, a security notch may include a band, ring, or similar device that is attached to the needle a suitable distance from the tip of the needle. Like security notch  910 , other security notch embodiments hinder insertion of the needle past the notch itself by presenting a larger profile than the profile of the needle such that the notch does not easily enter the hole in the tissue caused by entry of the needle. 
         [0096]    It is useful for the clinician performing the procedure to know when the needle has punctured the atrial tissue. This can be done in several ways. For example, the delivery catheter can be connected to a pressure transducer to measure pressure at the tip of the needle. Because the pressure is lower and much less pulsatile in the pericardial space than in the atrium, the clinician can recognize immediately when the needle passes through the atrial tissue into the pericardial space. 
         [0097]    Alternatively, as shown in  FIG. 6B , needle  890  may be connected to a strain gauge  915  as part of the catheter assembly. When needle  890  contacts tissue (not shown), needle  890  will be deformed. The deformation will be transmitted to strain gauge  915  and an electrical signal will reflect the deformation (through a classical wheatstone bridge), thereby alerting the clinician. Such confirmation of the puncture of the wall can prevent over-puncture and can provide additional control of the procedure. 
         [0098]    In some embodiments, a delivery catheter, such as catheter  850  shown in  FIGS. 6A ,  6 B, and  6 C, is used with an engagement catheter, such as catheter  700  shown in  FIGS. 5A ,  5 B,  5 C, and  5 D, to gain access to the pericardial space between the heart wall and the pericardial sac. For example, engagement catheter  700  may be inserted into the vascular system and advanced such that the distal end of the engagement catheter is within the atrium. The engagement catheter may be attached to the targeted tissue on the interior of a wall of the atrium using a suction port as disclosed herein. A standard guide wire may be inserted through the lumen of the delivery catheter as the delivery catheter is inserted through the inner lumen of the engagement catheter, such as lumen  740  shown in  FIGS. 5B and 5C . Use of the guide wire enables more effective navigation of the delivery catheter  850  and prevents the needle  890  from damaging the inner wall  750  of the engagement catheter  700 . When the tip of the delivery catheter with the protruding guide wire reaches the atrium, the wire is pulled back, and the needle is pushed forward to perforate the targeted tissue. The guide wire is then advanced through the perforation into the pericardial space, providing access to the pericardial space through the atrial wall. 
         [0099]    Referring again to  FIGS. 6A ,  6 B, and  6 C, lumen  885  of delivery catheter  850  may be used for delivering fluid into the pericardial space after needle  890  is inserted through the atrial wall or the atrial appendage. After puncture of the wall or appendage, a guide wire (not shown) may be inserted through needle lumen  900  into the pericardial space to maintain access through the atrial wall or appendage. Fluid may then be introduced to the pericardial space in a number of ways. For example, after the needle punctures the atrial wall or appendage, the needle is generally withdrawn. If the needle is permanently attached to the delivery catheter, as in the embodiment shown in  FIGS. 6A and 6B , then delivery catheter  850  would be withdrawn and another delivery catheter (without an attached needle) would be introduced over the guide wire into the pericardial space. Fluid may then be introduced into the pericardial space through the lumen of the second delivery catheter. 
         [0100]    In some embodiments, however, only a single delivery catheter is used. In such embodiments, the needle is not attached to the delivery catheter, but instead may be a needle wire (see  FIG. 1A ). In such embodiments, the needle is withdrawn through the lumen of the delivery catheter, and the delivery catheter may be inserted over the guide wire into the pericardial space. Fluid is then introduced into the pericardial space through the lumen of the delivery catheter. 
         [0101]    The various embodiments disclosed herein may be used by clinicians, for example: (1) to deliver genes, cells, drugs, etc.; (2) to provide catheter access for epicardial stimulation; (3) to evacuate fluids acutely (e.g., in cases of pericardial tampondae) or chronically (e.g., to alleviate effusion caused by chronic renal disease, cancer, etc.); (4) to perform transeptal puncture and delivery of a catheter through the left atrial appendage for electrophysiological therapy, biopsy, etc.; (5) to deliver a magnetic glue or ring through the right atrial appendage to the aortic root to hold a percutaneous aortic valve in place; (6) to deliver a catheter for tissue ablation, e.g., to the pulmonary veins, or right atrial and epicardial surface of the heart for atrial and ventricular arrythmias; (7) to deliver and place epicardial, right atrial, and right and left ventricle pacing leads; (8) to occlude the left atrial appendage through percutaneous approach; and (9) to visualize the pericardial space with endo-camera or scope to navigate the epicardial surface of the heart for therapeutic delivery, diagnosis, lead placement, mapping, etc. Many other applications, not explicitly listed here, are also possible and within the scope of the present disclosure. 
         [0102]    If an impedance catheter is placed on the surface of the heart as shown in  FIG. 7 , the parallel conductance (Gp) will change during the cardiac cycle. Since the first term in Equation #8 will not change significantly, then: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             
                                
                               G 
                             
                             
                                
                               t 
                             
                           
                            
                         
                         max 
                       
                       = 
                       
                         
                            
                           
                             G 
                             p 
                           
                         
                         
                            
                           t 
                         
                       
                     
                      
                   
                   max 
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     #10 
                   
                   ] 
                 
               
             
           
         
       
     
         [0103]    Since Gp is proportional to the cross-sectional area, Equation #10 will yield the change of cross-sectional area. If an impedance catheter with multiple sets of detection leads is used as shown in  FIG. 8 , the desired rate of change of volume evaluate at the maximum point will be determined as an index of heart function. 
         [0104]    Referring now to  FIG. 9 , there is shown a diagrammatic view of an embodiment of data acquisition and processing system  900  of the present disclosure. In the embodiment shown in  FIG. 9 , data acquisition and processing system  900  comprises user system  902 . In this exemplary embodiment, user system  902  comprises processor  904  and one or more storage media  906 . Processor  904  operates upon data obtained by or contained within user system  902 . Storage medium  906  may contain database  908 , whereby database  908  is capable of storing and retrieving data. Storage media  906  may contain a program (including, but not limited to, database  908 ), the program operable by processor  904  to perform a series of steps regarding conductance data as described in further detail herein. By way of example, the program may be operable by processor  904  to analyze conductance data, including analysis of such data in accordance with Equations #1-10 as described herein. 
         [0105]    Any number of storage media  906  may be used with data acquisition and processing system  900  of the present disclosure, including, but not limited to, one or more of random access memory, read only memory, EPROMs, hard disk drives, floppy disk drives, optical disk drives, cartridge media, and smart cards, for example. As related to user system  902 , storage media  906  may operate by storing conductance data for access by a user and/or for storing computer instructions. Processor  904  may also operate upon data stored within database  908 . 
         [0106]    Regardless of the embodiment of data acquisition and processing system  900  referenced herein and/or contemplated to be within the scope of the present disclosure, each user system  902  may be of various configurations well known in the art. By way of example, user system  902 , as shown in  FIG. 9 , comprises keyboard  910 , monitor  912 , and printer  914 . Processor  904  may further operate to manage input and output from keyboard  910 , monitor  912 , and printer  914 . Keyboard  910  is an exemplary input device, operating as a means for a user to input information to user system  902 . Monitor  912  operates as a visual display means to display the conductance data and related information to a user using a user system  902 . Printer  914  operates as a means to display conductance data and related information. Other input and output devices, such as a keypad, a computer mouse, a fingerprint reader, a pointing device, a microphone, and one or more loudspeakers are contemplated to be within the scope of the present disclosure. It can be appreciated that processor  904 , keyboard  910 , monitor  912 , printer  914  and other input and output devices referenced herein may be components of one or more user systems  902  of the present disclosure. 
         [0107]    It can be appreciated that data acquisition and processing system  900  may further comprise one or more server systems  916  in bidirectional communication with user system  902 , either by direct communication (shown by the single line connection on  FIG. 9 ), or through a network  918  (shown by the double line connections on  FIG. 9 ) by one of several configurations known in the art. Such server systems  916  may comprise one or more of the features of a user system  902  as described herein, including, but not limited to, processor  904 , storage media  906 , database  908 , keyboard  910 , monitor  912 , and printer  914 , as shown in the embodiment of data acquisition and processing system  900  shown in  FIG. 9 . Such server systems  916  may allow bidirectional communication with one or more user systems  902  to allow user system  902  to access conductance data and related information from the server systems  916 . It can be appreciated that a user system  902  and/or a server system  916  referenced herein may be generally referred to as a “computer.” 
         [0108]    The catheter can be inserted into the pericardial space, as outlined in previous studies, or directly placed on as during open heart surgery. The patch containing the excitation electrodes (E) and detection electrodes (D) can be made to adhere to the surface through glue that is introduced through the lumen of the catheter into pores of the patch if the percutaneous approach is used. Alternatively, the patch may be glued on by hand with the open surgery approach. The electrodes are then interfaced with an impedance module to measure voltage differences as noted in prior studies. 
         [0109]    The foregoing disclosure of the exemplary embodiments of the present application has been presented for purposes of illustration and description and can be further modified within the scope and spirit of this disclosure. It is not intended to be exhaustive or to limit the present disclosure to the precise forms disclosed. This application is therefore intended to cover any variations, uses, or adaptations of a device, system and method of the present application using its general principles. Further, this application is intended to cover such departures from the present disclosure as may come within known or customary practice in the art to which this system of the present application pertains. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the present disclosure is to be defined only by the claims appended hereto, and by their equivalents. 
         [0110]    Further, in describing representative embodiments of the present disclosure, the specification may have presented the method and/or process of the present disclosure as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be ‘limited’ to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present disclosure should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present disclosure.