Abstract:
Methods and systems facilitate catheterization of a living subject by passing a fluid through an irrigation conduit. Heat energy is delivered to the conduit to create a heated pod of irrigation fluid that propagates downstream from the heat source. A departure time of the pod from a first location in the conduit is recorded, and an arrival time of the pod is detected at a second location that is downstream from the first location. A transit time of the pod from the first location to the second location is determined, and the flow of the fluid is adjusted responsively to the transit time.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to tissue ablation systems. More particularly, this invention relates to monitoring of flow rates of irrigation fluid within an invasive probe within the body. 
         [0003]    2. Description of the Related Art 
         [0004]    Cardiac arrhythmias, such as atrial fibrillation, occur when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm. 
         [0005]    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 radio-frequency 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]    Previous approaches to controlling local heating include the inclusion of thermocouples within the electrode and feedback control, signal modulation, local cooling of the catheter tip, and fluid-assisted techniques, for example irrigation of the target tissue during the energy application, using chilled fluids. Typical of the last approach is Mulier, et al. U.S. Pat. No. 5,807,395. 
         [0008]    Commonly assigned U.S. Pat. No. 6,997,924, which is herein incorporated by reference, describes a technique of limiting heat generated during ablation by determining a measured temperature of the tissue and a measured power level of the transmitted energy, and controlling the power output level responsively to a function of the measured temperature and the measured power level. 
         [0009]    More recently, commonly assigned U.S. Patent Application Publication No. 2010/0030209 by Govari et al., which is herein incorporated by reference, describes an insertion tube, having an outer surface with a plurality of perforations through the outer surface, which are typically about 100 μm in diameter, and are distributed circumferentially and longitudinally over the distal tip. A lumen passes through the insertion tube and is coupled to deliver an irrigation and cooling fluid to the tissue via the perforations. 
       SUMMARY OF THE INVENTION 
       [0010]    There is provided according to embodiments of the invention a method of catheterization, which is carried out by inserting a catheter into a body of a living subject, causing a flow of a fluid through an irrigation conduit. The method is further carried out by delivering heat energy to the conduit from a heat source that is external to the conduit to create a heated pod of irrigation fluid that propagates downstream from the heat source, recording a departure time of the pod from a first location in the conduit, detecting externally to the conduit an arrival time of the pod at a second location that is downstream from the first location, determining a transit time of the pod from the first location to the second location measured by the departure time and the arrival time, and adjusting the flow of the fluid responsively to the transit time. 
         [0011]    In a further aspect of the method, the catheter has an ablation electrode. The method is further carried out by actuating the ablation electrode to deliver energy to an ablation site. 
         [0012]    According to yet another aspect of the method, detecting the arrival time is performed by recording signals from the first and second thermocouples to determine passage of the pod by their respective first and second locations. 
         [0013]    According to still another aspect of the method, the first location corresponds to a location of the heat source and the second location corresponds to a location of a thermocouple, and detecting is performed by recording temperature signals from the thermocouple to determine passage of the pod by the second location responsively to the temperature signals. 
         [0014]    In another aspect of the method adjusting the flow is performed by establishing an objective that is representative of a desired flow through the conduit, calculating a deviation from the objective based on the transit time, and adjusting the flow to reduce the deviation. 
         [0015]    According to an additional aspect of the method, the objective is a desired transit time between the first location and the second location. 
         [0016]    In one aspect of the method, adjusting the flow is carried out by establishing a desired flow through the conduit, calculating an actual flow based on the transit time, determining a deviation between the desired flow and the actual flow, and adjusting the flow to reduce the deviation. 
         [0017]    There is further provided according to embodiments of the invention an ablation apparatus, including a flexible catheter adapted for insertion into a heart of a living subject and having an ablation electrode disposed at the distal segment of the catheter, which can be brought into contact with a target tissue in the heart. The catheter has a lumen for passage of irrigation fluid therethrough, the fluid exiting from the catheter at the distal segment for cooling of the target tissue. The apparatus includes an ablator, operative to apply a dosage of energy to the target tissue to ablate the target tissue, a flow processor for adjusting a flow of the irrigation fluid through the lumen, and a sensor unit, disposed in a non-contacting relationship with the irrigation fluid, including a heat source that delivers heat energy to the irrigation fluid flowing in the lumen, and a thermocouple disposed downstream from the heat source in a direction of flow of the irrigation fluid for measuring a temperature of the irrigation fluid. 
         [0018]    According to one aspect of the apparatus, the sensor unit is external to the lumen. 
         [0019]    According to yet another aspect of the apparatus, the sensor unit is disposed in the handle of the catheter. 
         [0020]    A further aspect of the apparatus includes a controller operative for energizing the heat source of the sensor unit for a predetermined time interval to create a heated pod of irrigation fluid that propagates downstream from the heat source, recording a departure time of the pod from a first location, detecting a temperature signal from the thermocouple indicative of a change in the temperature of the irrigation fluid, determining an arrival time of the pod at a second location responsively to the temperature signal, determining a transit time of the pod from the first location to the second location measured by the departure time and the arrival time, and commanding the flow processor to adjust the flow of the irrigation fluid responsively to the transit time. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0021]    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: 
           [0022]      FIG. 1  is a pictorial illustration of a system for performing diagnostic and therapeutic procedures on a heart of a living subject, which is constructed and operative in accordance with an embodiment of the invention; 
           [0023]      FIG. 2  is a detailed schematic view of the flow sensor module shown in  FIG. 1  in accordance with an embodiment of the invention; 
           [0024]      FIG. 3 , which is a detailed schematic view of the flow sensor module shown in  FIG. 1  in accordance with an alternate embodiment of the invention; and 
           [0025]      FIG. 4  is a flow chart of a method for monitoring flow rates of irrigation fluid in a cardiac catheter in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    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. 
         [0027]    Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  10  for performing diagnostic and therapeutic procedures on a heart  12  of a living subject, and 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. Optionally, electrical activation maps may then be prepared, 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. This system may be modified by those skilled in the art to embody the principles of the invention described herein. 
         [0028]    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 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. The principles of the invention can be applied to different heart chambers to treat many different cardiac arrhythmias. 
         [0029]    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 a console  24 . 
         [0030]    Ablation energy and electrical signals can be conveyed to and from the heart  12  through one or more ablation electrodes  32  located at or near the distal tip  18  via cable  34  to the console  24 . Pacing signals and other control signals may be conveyed from the console  24  through the cable  34  and the electrodes  32  to the heart  12 . Sensing electrodes  33 , also connected to the console  24 , are disposed between the ablation electrodes  32  and have connections to the cable  34 . 
         [0031]    Wire connections  35  link the console  24  with body surface electrodes  30  and other components of a positioning sub-system. The electrodes  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 electrodes  32 . 
         [0032]    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 any known ablation technique, e.g., radiofrequency energy, ultrasound energy, and laser-produced light 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. 
         [0033]    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 . 
         [0034]    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. 
         [0035]    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 . Console  24  includes a processor, preferably a computer with appropriate signal processing circuits. The processor 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 and used by the console  24  and the positioning system to compute the position and orientation of the catheter  14  and to analyze the electrical signals from the electrodes. 
         [0036]    A lumen or conduit  41  within the catheter  14  conducts irrigation and cooling fluid to the tip of the catheter, where it exits via apertures  43 , which are located near the ablation electrode  32 . The fluid serves to cool the tissues during ablation in order to prevent the undesirable effects mentioned above. Temperature control of the ablation site is facilitated by regulating the rate and hence volume of flow through the conduit  41  and the apertures  43 . A higher flow rate cools the area more than a lower flow rate. A flow sensor module  45  measures the flow rate and transmits signals indicating the measurement to the console  24 , for example via the cable  34 . A controller in the console  24  responds to the signals and varies the volume of fluid flowing through the conduit  41 . 
         [0037]    In  FIG. 1  the flow sensor module  45  is disposed distally in the catheter  14 . This is not essential, and the flow sensor module  45  can be located anywhere along the course of the conduit  41 , for example within the handle  20  of the catheter  14  ( FIG. 1 ). A controller  47  monitors the flow sensor module  45  and sends control signals that cause adjustment of irrigation fluid, for example, by commanding a pump, pump controller or other flow rate device known in the art (not shown). 
         [0038]    Typically, the system  10  includes other elements, which are not shown in the figures for the sake of simplicity. For example, the system  10  may include an electrocardiogram (ECG) monitor, coupled to receive signals from one or more body surface electrodes, to provide an ECG synchronization signal to the console  24 . As mentioned above, the system  10  typically also includes a reference position sensor, either on an externally-applied reference patch attached to the exterior of the subject&#39;s body, or on an internally-placed catheter, which is inserted into the heart  12  maintained in a fixed position relative to the heart  12 . As noted above, conventional pumps and lines for circulating liquids through the catheter  14  for cooling the ablation site are provided. 
         [0039]    Reference is now made to  FIG. 2 , which is a detailed schematic view of the flow sensor module  45  ( FIG. 1 ), which is constructed and operative in accordance with an embodiment of the invention. It will be noted that the components of the flow sensor module  45  described below are all in a non-contacting relationship with irrigation fluid  49  in the conduit. In  FIG. 2  they are all external to the conduit  41 , and do not contact irrigation fluid  49  within the conduit  41 . In some embodiments in which the flow sensor module  45  is disposed in the interior of the catheter  14  ( FIG. 1 ) or a lumen within the catheter  14 , a suitable seal may be provided to prevent contact with irrigation fluid. A heating element  51  can be realized as a resistor in contact with the external wall of the conduit  41 . The resistor may be a simple coil wound around the conduit  41 . 
         [0040]    Two thermocouples  53 ,  55 , spaced at different distances from the heating element  51 , are attached to the wall of the conduit  41  on downstream from the heating element  51 . Typically, the thermocouples  53 ,  55  are spaced apart by 0.5-2 mm. 
         [0041]    The controller  47  ( FIG. 1 ) energizes the heating element  51  for a short time (typically 0.5-2 sec), thereby providing a “heat pulse” to the irrigation fluid  49  flowing in the conduit  41 . The heat pulse raises the temperature of the fluid in proximity to the heating element  51 , forming a localized fluid pod or cell  57 , which has an increased temperature relative to fluid outside the pod, and which flows downstream within the conduit  41 , tending to maintain its identity, at least in the interval between the thermocouples  53 ,  55 . 
         [0042]    In the example of  FIG. 2 , plots of the signals developed by the thermocouple  53 ,  55  over time are shown as curves  59 ,  61 , respectively. Transient rises in signal strength from the thermocouples  53 ,  55  can be detected using conventional pulse detection circuitry (not shown) or by software in the controller  47  ( FIG. 1 ). 
         [0043]    The cell  57  arrives at the thermocouple  53  at time t 1 , and increased fluid temperature is detected by the thermocouple  53 , as shown by the curve  59 . Then, at time t 2  the cell  57  arrives at the thermocouple  55 , which develops a transient signal as shown by the curve  61 . Rises in signal strength are determined by the controller  47 , which then registers the times at which the cell  57  passes each of the thermocouples  53 ,  55 . The time difference (t 2 −t 1 ) between corresponding points, e.g., half maxima of the curves  59 ,  61 , correlates inversely with rate of flow of the cell  57 . Moreover, the propagation of the cell  57  through the conduit  41  is representative of the rate of flow of the irrigation fluid  49  generally in the conduit  41 , so that the time difference (t 2 −t 1 ) also correlates inversely with the fluid volume passing through the conduit  41 . Flow rates of about 2 m/sec are typical. 
         [0044]    The components of the flow sensor module  45  are very small, and, as noted above, can be fitted into catheter itself, e.g., the handle  20  ( FIG. 1 ). It will be appreciated by those skilled in the art that conventional cardiac catheters and their irrigation systems may be readily retrofitted by addition of the flow sensor module  45 . Versions of the flow sensor module  45  that employ wireless transmission are particularly suitable to upgrades of such catheters, as no additional wires need be installed. Placing the flow sensor module  45  in the catheter  14  is advantageous, as it facilitates detection of tube kink, a condition which commonly occurs during the procedure. 
         [0045]    Although the curves  59 ,  61  are separated by an interval in  FIG. 2 , this is not necessarily the case. Overlap can be tolerated if the signals from the thermocouples  53 ,  55  can be differentiated from one another. For example, the thermocouples  53 ,  55  may report to the controller  47  on separate lines, or even by known methods of wireless communication as described, e.g., in commonly assigned U.S. Pat. No. 7,397,364 to Govari, entitled “Digital Wireless Position Sensor”, which is herein incorporated by reference. 
       Alternate Embodiment 
       [0046]    Since energizing the heating element is implemented by the controller, the controller is aware of the time of energizing. The controller may use this time determination to improve the estimate of the speed and flow rate of the fluid. Alternatively, rather than using two thermocouples, only one may be used. 
         [0047]    Reference is now made to  FIG. 3 , which is a detailed schematic view of a flow sensor module  63 , in accordance with an alternate embodiment of the invention. The construction of the flow sensor module  63  is generally similar to the flow sensor module  45  ( FIG. 2 ), except now the thermocouple  55  is omitted, and reliance is placed on the thermocouple  53  to indicate a change in temperature as the cell  57  flows downstream from the heating element  51 . 
         [0048]    In this version the heating element  51  is energized for a time interval beginning at time t 0 , which, as noted above, is known to the controller  47  ( FIG. 1 ). The cell  57  forms during the interval and its leading edge arrives at the thermocouple  53  at time t 1 . Increased fluid temperature is detected using the signal from the thermocouple  53 , as in the previous embodiment, indicated by the curve  59 . Flow rate is measured as in the previous embodiment, but using the time difference (t 1 −t 0 ). Calibration data is prepared as described above. 
       Operation 
       [0049]    Reference is now made to  FIG. 4 , which is a flow chart of a method for monitoring flow rates of irrigation fluid in a cardiac catheter in accordance with an embodiment of the invention. The method is suitable for any of the embodiments discussed above. 
         [0050]    The process begins at initial step  65 . It is assumed that a subject has been catheterized, and the ablation electrode of the catheter positioned at a target site. Then, at step  67 , an objective representing an optimum flow rate is determined. The objective may be some parameter representative of flow rate, for example a transit time of the fluid between two points or an actual flow rate. The parameter may be derived, for example, from an optimum temperature of the ablation site. The optimum flow rate usually varies dynamically during the procedure, and may be responsive, for example, to the tissue temperature of the ablation site, and to other factors, such as limitations on fluid loading of the subject. The details of this step are beyond the scope of this disclosure and are not further discussed. Additionally or alternatively an optimum transit time interval for propagation of the pod described above may be established. This may be achieved by reference to a previously constructed a transit time table or function, which correlates with flow rates through the irrigation conduit. 
         [0051]    Next, at step  69  the flow rate is measured. Step  69  comprises step  71 , step  73  and step  75 . In step  71  an energy pulse is delivered to a heating element to create a pod having elevated temperature relative to the general flow of irrigation fluid through the catheter. Step  73 , recording a thermocouple signal is performed throughout the measurement operation, at least until a rise in temperature is detected, thereby indicating arrival of the pod the thermocouple. At step  75 , the transit time between the heating element and the thermocouple is determined and may be correlated with previously determined flow rate table, or function. 
         [0052]    Next, at decision step  77 , it is determined if the flow rate or transit time measured in step  69  is within tolerance limits with respect to the desired flow rate or transit time that was set in the last performance of step  67 . If the determination is affirmative, then control returns to step  67 , where the currently required flow rate or transit time is redetermined as described above. 
         [0053]    If the determination at decision step  77  is negative then control proceeds to step  79 . A pump or other adjusting mechanism is corrected to null out the difference between the measurement obtained in step  69  and the objective that was established in step  67 . Control then returns to step  67 . 
         [0054]    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.