Abstract:
Methods and systems of catheterization include a flexible catheter adapted for insertion into a heart of a living subject. The catheter has a lumen for passing an electrically conductive fluid therethrough with the fluid exiting the catheter at the distal portion. The lumen is connectable to an irrigation pump to form a fluid communication therewith. A fluid reservoir connected to the lumen supplies the fluid to the catheter. Electrocardiogram circuitry is connectable to the subject for monitoring electrical activity in the heart. An electrically conductive cable diverts induced charges in the fluid from the catheter electrodes, for example by connection to an isolated ground of the electrocardiogram.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to medical catheterization. More particularly, this invention relates to electrocardiographic monitoring during medical catheterization procedures. 
         [0003]    2. Description of the Related Art 
         [0004]    The meanings of certain acronyms and abbreviations used herein are given in Table 1. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Acronyms and Abbreviations 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 ECG 
                 Electrocardiogram 
               
               
                   
                 PIU 
                 Patient Interface Unit 
               
               
                   
                 RF 
                 Radiofrequency 
               
               
                   
                   
               
             
          
         
       
     
         [0005]    Medical catheterizations are routinely carried out today. For example, in cases of cardiac arrhythmias, such as atrial fibrillation, which occur when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm. Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathway for such signals. By selectively ablating cardiac tissue by application of energy, e.g., radiofrequency energy via a catheter, it is sometimes possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions. 
         [0006]    A known difficulty in the use of radiofrequency energy for cardiac tissue ablation is controlling local heating of tissue. There are tradeoffs between the desire to create a sufficiently large lesion to effectively ablate an abnormal tissue focus, or block an aberrant conduction pattern, and the undesirable effects of excessive local heating. If the radiofrequency device creates too small a lesion, then the medical procedure could be less effective, or could require too much time. On the other hand, if tissues are heated excessively then there could be local charring effects due to overheating. Such overheated areas can develop high impedance, and may form a functional barrier to the passage of heat. The use of slower heating provides better control of the ablation, but unduly prolongs the procedure. 
         [0007]    Commonly assigned application Ser. No. 13/339,782, which is herein incorporated by reference, discloses the use of an irrigation pump to cause irrigation fluid to flow through a lumen of the catheter in order to cool the ablation site. 
       SUMMARY OF THE INVENTION 
       [0008]    There is provided according to embodiments of the invention a catheterization system, which avoids spurious electrical interference in electrical monitoring circuitry when a peristaltic pump is operating to irrigate an ablation site. The system includes a flexible catheter adapted for insertion into a heart of a living subject, the catheter having a lumen for passing an electrolyte-containing fluid therethrough to exit the catheter at its distal portion. A fluid reservoir is connected to the irrigation pump for supplying the electrolyte-containing fluid to the catheter lumen. Electrocardiogram circuitry is connectable to the subject for monitoring electrical activity in the heart. A conductive cable electrically connects the electrolyte-containing fluid with the input of the electrocardiogram circuitry. 
         [0009]    According to a further aspect of the system, the electrically conductive cable leads from the fluid reservoir to an isolated ground through a resistor, and the electrocardiogram circuitry is connected to the isolated ground. 
         [0010]    According to yet another aspect of the system, the resistor has a resistance of between 0Ω and 10 KΩ. 
         [0011]    According to still another aspect of the system, the resistor has a resistance of between 0Ω and 3 MΩ. 
         [0012]    According to another aspect of the system, a drip chamber is connected to the fluid reservoir for receiving the electrolyte-containing fluid therein, and the electrically conductive cable is connected to the electrolyte-containing fluid downstream of the drip chamber. 
         [0013]    According to yet another aspect of the system the irrigation pump has an inlet hydraulic line and an output hydraulic line, and an electrically conductive link between the electrolyte-containing fluid in the inlet hydraulic line and the electrolyte-containing fluid in the output hydraulic line. The electrically conductive link may be connected to an isolated ground of the electrocardiogram circuitry. 
         [0014]    There is further provided according to embodiments of the invention a catheterization system, including a flexible catheter adapted for insertion into a heart of a living subject. The catheter has a lumen for passing an electrically conductive fluid therethrough to exit the catheter at its distal portion. The lumen is connectable to an irrigation pump to form a fluid communication therewith. A fluid reservoir supplies the electrically conductive fluid to lumen the catheter with the aid of the irrigation pump. Electrocardiogram circuitry is connectable to the subject for monitoring electrical activity in the heart. An electrical shield is disposed about the fluid reservoir and connected to the input of the electrocardiogram circuitry. 
         [0015]    According to one aspect of the system, the electrocardiogram circuitry is connected to the subject via a metallically shielded electrical conductor leading through the catheter to an electrode at the distal portion thereof, and the shielded electrical conductor is incorporated in the hydraulic line. 
         [0016]    There is further provided according to embodiments of the invention a catheterization system, including a flexible catheter adapted for insertion into a heart of a living subject, the catheter having and a lumen for passing an electrically conductive fluid therethrough to exit the catheter at its distal portion. The lumen is connectable to an irrigation pump to form a fluid communication therewith. A fluid reservoir is connected to the irrigation pump for supplying the electrically conductive fluid to the catheter. Electrocardiogram circuitry is connectable to the subject for monitoring electrical activity in the heart, and an electrically conductive cable links the electrically conductive fluid of the fluid reservoir to a body surface electrode on the subject. 
         [0017]    There is further provided according to embodiments of the invention a method of catheterization, which is carried out by Inserting a flexible catheter into a heart of a living subject, pumping an electrolyte-containing fluid from a fluid reservoir through a lumen of the catheter using a peristaltic pump, connecting electrocardiogram circuitry to the subject for monitoring electrical activity in the heart, connecting an electrically conductive cable between the electrolyte-containing fluid of the fluid reservoir and the input of the electrocardiogram circuitry, and while pumping the electrolyte-containing fluid processing electrical data from the subject in the electrocardiogram circuitry. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0018]    For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
           [0019]      FIG. 1  is a pictorial illustration of a system for performing catheterization procedures on a heart of a living subject, which is constructed and operative in accordance with an embodiment of the invention; 
           [0020]      FIG. 2  is a schematic diagram of a system for reducing electrocardiogram noise, in accordance with an embodiment of the invention; 
           [0021]      FIG. 3  is a schematic diagram of a test arrangement for measuring electrocardiogram noise reduction, in accordance with an embodiment of the invention; 
           [0022]      FIG. 4  is a schematic diagram of a connector for establishing electrical continuity between fluid and an electrical cable, which is constructed in accordance with an embodiment of the invention; 
           [0023]      FIG. 5  shows two bar charts indicating performance of the test arrangement shown in  FIG. 3 ; 
           [0024]      FIG. 6  shows two tables showing the performance of versions of the test arrangement shown in  FIG. 3 ; 
           [0025]      FIG. 7  is a schematic diagram of a system for reducing electrocardiogram noise, in accordance with an alternate embodiment of the invention; and 
           [0026]      FIG. 8  is a schematic diagram of a system for reducing electrocardiogram noise, in accordance with an alternate embodiment of the invention. 
           [0027]      FIG. 9  is a schematic diagram of reducing electrocardiogram noise, in accordance with an alternate embodiment of the invention. 
           [0028]      FIG. 10  is a schematic diagram of a test arrangement of an infusion system in accordance with an alternate embodiment of the invention. 
           [0029]      FIG. 11  is a schematic diagram of a pump which has been modified for noise reduction, in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention. It will be apparent to one skilled in the art, however, that not all these details are necessarily always needed for practicing the present invention. In this instance, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the general concepts unnecessarily. 
         [0031]    Aspects of the present invention may be embodied in software programming code, which is typically maintained in permanent storage, such as a computer readable medium. In a client/server environment, such software programming code may be stored on a client or a server. The software programming code may be embodied on any of a variety of known nontransitory media for use with a data processing system, such as USB memory, hard drive, electronic media or CD-ROM. The code may be distributed on such media, or may be distributed to users from the memory or storage of one computer system over a network of some type to storage devices on other computer systems for use by users of such other systems. 
       Definitions 
       [0032]    “Noise” is a disturbance, including a random and persistent disturbance that obscures or reduces the clarity of a signal. 
         [0033]    A “patient interface unit” (PIU) provides an interface between analog input signals and a digital data processing system. 
       System Description 
       [0034]    Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  10  for performing exemplary catheterization procedures on a heart  12  of a living subject, which is constructed and operative in accordance with a disclosed embodiment of the invention. The system comprises a catheter  14 , which is percutaneously inserted by an operator  16  through the patient&#39;s vascular system into a chamber or vascular structure of the heart  12 . The operator  16 , who is typically a physician, brings the catheter&#39;s distal tip  18  into contact with the heart wall at an ablation target site. Electrical activation maps, anatomic positional information, i.e., of the distal portion of the catheter, and other functional images may then be prepared using a processor  22  located in a console  24 , according to the methods disclosed in U.S. Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whose disclosures are herein incorporated by reference. One commercial product embodying elements of the system  10  is available as the CARTO® 3 System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765, which is capable of producing electroanatomic maps of the heart as required. This system may be modified by those skilled in the art to embody the principles of the invention described herein. 
         [0035]    Areas determined to be abnormal, for example by evaluation of the electrical activation maps, can be ablated by application of thermal energy, e.g., by passage of radiofrequency electrical current from a radiofrequency (RF) generator  40  through wires in the catheter to one or more electrodes at the distal tip  18 , which apply the radiofrequency energy to the myocardium. The energy is absorbed in the tissue, heating it to a point (typically about 50° C.) at which it permanently loses its electrical excitability. When successful, this procedure creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathway causing the arrhythmia. 
         [0036]    The catheter  14  typically comprises a handle  20 , having suitable controls on the handle to enable the operator  16  to steer, position and orient the distal end of the catheter as desired for the ablation. To aid the operator  16 , the distal portion of the catheter  14  contains position sensors (not shown) that provide signals to a positioning processor  22 , located in the console  24 . 
         [0037]    Ablation energy and electrical signals can be conveyed to and from the heart  12  through the catheter tip and an ablation electrode  32  located at or near the distal tip  18  via cable  34  to the console  24 . Pacing signals and other control signals may be also conveyed from the console  24  through the cable  34  and the ablation electrode  32  to the heart  12 . Sensing electrodes  33 , also connected to the console  24  are disposed between the ablation electrode  32  and the cable  34 . 
         [0038]    Wire connections  35  link the console  24  with body surface electrodes  30  and other components of a positioning sub-system. The electrode  32  and the body surface electrodes  30  may be used to measure tissue impedance at the ablation site as taught in U.S. Pat. No. 7,536,218, issued to Govari et al., which is herein incorporated by reference. A temperature sensor (not shown), typically a thermocouple or thermistor, may be mounted on or near each of the electrode  32 . 
         [0039]    The console  24  typically contains one or more ablation power generators  25 . The catheter  14  may be adapted to conduct ablative energy to the heart using radiofrequency energy. Such methods are disclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and 7,156,816, which are herein incorporated by reference. 
         [0040]    The positioning processor  22  is an element of a positioning subsystem in the system  10  that measures location and orientation coordinates of the catheter  14 . 
         [0041]    In one embodiment, the positioning subsystem comprises a magnetic position tracking arrangement that determines the position and orientation of the catheter  14  by generating magnetic fields in a predefined working volume and sensing these fields at the catheter, using field generating coils  28 . The positioning subsystem may employ impedance measurement, as taught, for example, in U.S. Pat. No. 7,756,576, which is hereby incorporated by reference, and in the above-noted U.S. Pat. No. 7,536,218. 
         [0042]    As noted above, the catheter  14  is coupled to the console  24 , which enables the operator  16  to observe and regulate the functions of the catheter  14 . The processor  22  is typically a computer with appropriate signal processing circuits. The processor  22  is coupled to drive a monitor  29 . The signal processing circuits typically receive, amplify, filter and digitize signals from the catheter  14 , including signals generated by the above-noted sensors and a plurality of location sensing electrodes (not shown) located distally in the catheter  14 . The digitized signals are received via cable  38  and used by the console  24  and the positioning system to compute the position and orientation of the catheter  14  and analyze the electrical signals from the electrodes, and generate desired electroanatomic maps. 
         [0043]    The system  10  may include an electrocardiogram (ECG) monitor  37 , coupled to receive signals from one or more body surface electrodes. The ECG signal is typically received through an interface with the console  24 , e.g., a patient interface unit  42  having an analog input and an isolated ground may be used to provide an ECG synchronization signal to the console  24 . The patient is normally grounded to the isolated ground. 
         [0044]    An electrically conductive fluid, e.g., saline, Ringer&#39;s lactate, is delivered through a lumen  44  in the catheter  14  from a reservoir  46  via a hydraulic line  48 . The electrically conductive fluid is generally referred to herein as “saline” for convenience, it being understood that this is by way of example and not of limitation. The lumen  44  terminates in exit pores  50  through which the liquids emerge to cool the electrode  32  and the ablation site. A pump  52  is connected to the hydraulic line  48  and causes the fluid to be delivered to the catheter  14  through an entrance port  54  at a desired rate. One difficulty with such an arrangement is that operation of equipment in the environment, e.g., the pump  52 , produces electrical emissions, which produce noise that can be picked up by the hydraulic line  48  and interfere with the analysis and display of the ECG on the monitor  37 . 
       First Embodiment 
       [0045]    The inventors have found that connecting an electrically conductive wire  56  between the electrolytic fluid, e.g., between the reservoir  46 , and the isolated ground of the analog front end of the interface unit  42  effects a significant reduction in the electrical interference. 
         [0046]    In order to minimize the number of conductors in the area of operation, the wire  56  may be incorporated in the hydraulic line  48  leading from the reservoir  46 . 
         [0047]    Reference is now made to  FIG. 2 , which is a schematic diagram of a system  78  for reducing electrocardiogram noise, in accordance with an embodiment of the invention. The system operates in an environment in which an electromagnetic field  80  exists, and which may be produced in part by a hydraulic pump  82 , which propels an electrolytic fluid, e.g., a saline solution  84  from a reservoir  86 , such as an intravenous bottle or bag. The saline solution  84  flows through line  88 , and through a catheter  90 . An electrical conductor  92  extends from the distal portion of the catheter  90  to ECG circuitry  94 . 
         [0048]    The electrical conductivity of the saline solution  84  is sufficient for it to function as an effective antenna. As the catheter  90  may be several meters in length, the saline solution  84  in the catheter  90  can pick up and radiate the electromagnetic emissions  80 , which is then perceived as noise on the cardiac electrogram measured from the catheter&#39;s tip electrode  96  and on an electrocardiogram employing standard leads  98 . 
         [0049]    Capacitive coupling may occur between the line  88  and ECG leads  98  and ECG pads  100  and between the line  88  and the conductor  92  within the catheter  90  that may connect to a programmable interface unit (PIU) input  102 . Such coupling, represented by mutual impedances (Z)  104 ,  106 , may account in part for communication of electrical noise originating in the pump  82  or RF generator  40  ( FIG. 1 ) or elsewhere in the environment to the ECG circuitry  94 . In general the impedances  104 ,  106  do not have the same magnitude or phase angle. 
         [0050]    A short circuit connects the electrolyte to the ECG circuitry  94 , e.g., via the PIU input  102 , using a low impedance wire  108 . The connection to the reservoir  86  should be made using a connector  110  disposed downstream of a drip chamber  112 . When the wire  108  is connected as shown in  FIG. 2 , substantial reduction in noise is experienced when the pump  82  or other equipment relating to the catheterization procedure, e.g., the RF generator  40  ( FIG. 1 ) is in operation. 
       EXAMPLE 
       [0051]    Reference is now made to  FIG. 3 , which is a schematic diagram of a test arrangement  114  using an RF signal generator, in accordance with an embodiment of the invention. 
         [0052]    An intravenous infusion pack  116 , containing saline, constitutes an electrolyte fluid reservoir, as described above. A cable  118 , leading from the intravenous infusion pack  116 , is connected to an isolated ground by a conductive cable  118  that extends from a metal connector  120  through an adjustable resistor  122 . The saline flows from the drip chamber  112  through the connector  120  to reach a line  124 , and is in electrical contact with the cable  118 . The line  124  extends from the connector  120  to a pump  126 . 
         [0053]    Measurements of electrical noise were conducted using values of 0Ω and 10 KΩ for the resistor  122 . In practice values of up to 5 MΩ are usable to avoid signal distortion that would result from a direct liquid connection to ground. For testing purposes values of 0-10 KΩ were chosen. Hydraulic lines  124 ,  128  interconnect the intravenous infusion pack  116 , and the pump  126 . An electrical Line  130  connects a handle  132  and a catheter  134  with an RF generator  136 . The line  128  extends from the pump  126  to the handle  132  of catheter  134 . The distal end of the catheter  134  is inserted into an aquarium  150  containing saline  138 , which emulates a human subject. The cable  118  and resistor  122  may be embedded into the wall of hydraulic lines or may be external to the hydraulic lines. 
         [0054]    A test subsystem  140  includes ECG circuitry  142 , which is connected to a display  144 . Four ECG leads  146  are connected to the ECG circuitry  142  and to metal patches  148  that are mounted on the internal surfaces of an aquarium  150  in contact with the saline  138 . The cable  118  connects the intravenous infusion pack  116  to an isolated ground  152  in the ECG circuitry  142  via resistor  122  that can have a value of 0-10 KΩ. 
         [0055]    Reference is now made to  FIG. 4 , which is a schematic diagram of the connector  120  ( FIG. 3 ), which is constructed in accordance with an embodiment of the invention. The connector  120  is tubular, having an outer metal shell  154 , and a lumen filled with an electrically conductive net or sponge  156 . The sponge  156  assures extensive physical contact with saline flowing in the lumen of the connector  120 , and increases its conductance. An electrical connector  158  is provided on the metal shell  154  so that electrical continuity exists between the saline in the lumen, the sponge  156  and the cable  118 . 
         [0056]    A suitable test protocol for evaluating the embodiments described herein follows: 
       Test Setup. 
       [0057]    Connect the Catheter to the PIU magnetic navigation catheter (MAP) input. Fill the aquarium with saline. Connect all four ECG limb channels (right leg, right arm, left arm and left leg) to different sides of the interior wall of aquarium using previously installed metal patches, which are in contact with the saline  138 . Connect the RF generator to the PIU. Connect the irrigation pump to the catheter, and set the flow rate to 30 ml/min. 
       Test Procedure. 
       [0058]    Set up a MATLAB® Application adapter DLL (Mex-DLL) to acquire ECG data from electrodes M1-M6. 
         [0059]    Open relevant ECG channels. Set sampling rate to 1 KHz. Acquire 1200 packets (20 packets per second, 60 seconds test) of ECG data. 
         [0060]    Calculate bipolar noise between the next couples: M2−M1. 
       Data Analysis 
       [0061]    Every 400 ms, calculate the following cumulative distribution function (CDF95): 
         [0000]      MAX(BiPolarNoise[uV])=Max( M 2 −M 1)−MIN( M 2 −M 1); and
 
         [0000]      CDF95(BiPolarNoise[uV])=CDF95 i ( M 2 −M 1). 
         [0062]    Reference is now made to  FIG. 5 , which is a collection of two bar charts indicating performance of the test arrangement  114  ( FIG. 3 ), in accordance with an embodiment of the invention. Charts  160 ,  162  indicate noise levels when the leads  146  are set up in unipolar and bipolar configurations, respectively. All configurations show improvement, when compared to the nominal values at the left of the charts. Nominal values were obtained using a conventional arrangement in which the cable  118  was not connected. The CDF95 values shown on the vertical axis of chart  162  reflect the noise level that includes 95% of the observed noise. 
         [0063]    Reference is now made to  FIG. 6 , which shows two tables  164 ,  166 , showing the performance of versions of the test arrangement  114  ( FIG. 3 ) with respect to noise reduction in bipolar and unipolar configurations, respectively. 
       Second Embodiment 
       [0064]    Reference is now made to  FIG. 7 , which is a schematic of a system  168  for reducing electrocardiogram noise, in accordance with an alternate embodiment of the invention. A metal shield  170  surrounds the reservoir  86  and is connected to an isolated ground  172  in the PIU input  102  by a shielded cable  174 . With the shield  170  in place during operation, noise picked up by the ECG circuitry  94  is effectively reduced. 
         [0065]    Additionally or alternatively the line  88  may be electrically shielded, for example by including the line  88  in a metallically shielded cable  176 , which extends from the pump  82  and the reservoir  86  to the catheter  90 . The shielded cable can be, for example coaxial cable. The leads  98  may also be shielded. 
         [0066]    Further additionally or alternatively, the reservoir  86  may be connected to an isolated ground as described above. 
       Third Embodiment 
       [0067]    Reference is now made to  FIG. 8 , which is a schematic of a system  178  for reducing electrocardiogram noise, in accordance with an alternate embodiment of the invention. In this embodiment the reservoir  86  is connected to a subject  180  by an electrically conductive cable  182 , for example using a body surface electrode pad  184  attached to a limb or other portion of the body. The cable  182  may be shielded as described above. The connector  158  is placed in hydraulic line  88  downstream from the drip chamber  112 , as described above in the discussion of  FIG. 3 . The electrode  96  is connected by conductor  92  to a console  186  containing ECG circuitry and an RF ablation generator. The features of this embodiment may be combined with any of the other embodiments described above. 
       Fourth Embodiment 
       [0068]    Reference is now made to  FIG. 9 , which is a schematic of a system  188  for reducing electrocardiogram noise, in accordance with an alternate embodiment of the invention. The arrangement in  FIG. 9  is similar that of  FIG. 8 . However, the reservoir  86 , the pump  82  and a catheter handle  190  are now connected in series. The reservoir  86  and the inflow of the pump  82  are directly connected via a hydraulic line  192 . The outflow of the pump  82  is conducted to the connector  158  by a hydraulic line  194 . The connector  158  is located downstream of the pump  82  near the catheter handle  190 . The connector  158  is shorted to the electrode pad  184  on the subject  180  via the cable  182 . 
         [0069]    It is desirable to locate the electrode pad  184  as far as possible from the electrode  96 . Thus the electrode pad  184  could be placed on the distal portion of a lower extremity. 
         [0070]    In some embodiments a plurality of connectors may be placed along the line  194  between the pump  82  and the handle  190 . Electrical interference that is believed to be produced by an interaction between pump  82  and the saline in the hydraulic lines is reduced as long as the electrical resistance of the path through connectors  158 , the electrode pad  184  and the subject  180  is less than the resistance of the path through the subject  180  via the saline in the hydraulic lines and the saline-irrigated electrode  96 . 
       Fifth Embodiment 
       [0071]    Reference is now made to  FIG. 10 , which is a schematic diagram of a test arrangement  196  of an infusion system using an RF signal generator, in accordance with an alternate embodiment of the invention. This embodiment is particularly effective in eliminating pump-induced noise that does not primarily result from an antenna effect, but is believed to have other causes, possibly a piezoelectric effect caused by moving parts of the pump and the tubing. 
         [0072]    Two conductive connectors  198 ,  200  are inserted in the fluid stream upstream and downstream of the pump  126  in the lines  124 ,  128 , respectively. The connectors  198 ,  200 , which may have the same construction as the connector  120  ( FIG. 5 ), are shorted together by an electrically conductive wire  202 . The connectors  198 ,  200  may be positioned conveniently as shown in  FIG. 11 . Alternatively, the connectors  198 ,  200  may be placed immediately before and after the interface to the pump  126 . In any case the wire  202  may also connect to the isolated ground of the ECG circuitry  142 . 
         [0073]    In some embodiments, the electrical connections of the test arrangement  196  and test arrangement  114  ( FIG. 4 ) may be combined. In operation the metal patches  148  and the aquarium  150  are replaced by ECG leads applied to a subject. 
       Sixth Embodiment 
       [0074]    Reference is now made to  FIG. 11 , which is a schematic diagram of a peristaltic pump  205 , which has been modified for noise reduction, in accordance with an embodiment of the invention. The pump  205  has a metallic roller  217  an input line  207  and an output line  209  to which are attached connectors  211 ,  213 , respectively, which may have the same construction as the connector  120  ( FIG. 4 ). A link  215  shorts the connectors  211 ,  213  and may connect to the isolated ground of the EKG circuitry as described above. 
         [0075]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.