DETAILED DESCRIPTION

[0069] Throughout the description, like reference numerals and letters indicate corresponding structure throughout the several views, and such corresponding structure need not be separately discussed. Furthermore, any particular feature(s) of a particular exemplary embodiment may be equally applied to any other exemplary embodiment(s) of this specification as suitable. In other words, features between the various exemplary embodiments described herein are interchangeable as suitable, and not exclusive.

[0070] The invention provides devices, systems and methods that control tissue temperature at a tissue treatment site during a medical procedure. This is particularly useful during surgical procedures upon tissues of the body, where it is desirable to coagulate and shrink tissue, to occlude lumens of blood vessels (e.g., arteries, veins), airways (e.g., bronchi, bronchioles), bile ducts and lymphatic ducts.

[0071] The invention includes electrosurgical procedures, which preferably utilize RF power and electrically conductive fluid, to treat tissue. Preferably, a desired tissue temperature range is achieved by adjusting parameters, such as conductive fluid flow rate, to affect the temperature at the tissue/electrode interface.

[0072] In one embodiment, the invention provides a control device, the device comprising a flow rate controller that receives a signal indicating power applied to the system, and adjusts the flow rate of conductive fluid from a fluid source to the electrosurgical device. The invention also provides a control system comprising a flow rate controller, a measurement device that measures power applied to the system, and a pump that provides fluid at a selected flow rate.

[0073] The invention will be discussed generally with reference to FIG. 1 , which shows a block diagram of one exemplary embodiment of a system of the invention. Preferably, an electrically conductive fluid 24 is provided from a fluid source 1 through a fluid line 2 to a pump 3 , which has an outlet fluid line 4 a that is connected as an input fluid line 4 b to electrosurgical device 5 . In a preferred embodiment, outlet fluid line 4 a and input fluid line 4 b are flexible and are made from a polymeric material, such as polyvinylchloride (PVC) or polyolefin (e.g., polypropylene, polyethylene) and the conductive fluid comprises a saline solution. More preferably, the saline comprises sterile, and even more preferably, normal saline. Although the description herein will specifically describe the use of saline as the fluid 24 , other electrically conductive fluids, as well as non-conductive fluids, can be used in accordance with the invention.

[0074] For example, in addition to the conductive fluid comprising physiologic saline (also known as “normal” saline, isotonic saline or 0.9% sodium chloride (NaCl) solution), the conductive fluid may comprise hypertonic saline solution, hypotonic saline solution, Ringers solution (a physiologic solution of distilled water containing specified amounts of sodium chloride, calcium chloride, and potassium chloride), lactated Ringer's solution (a crystalloid electrolyte sterile solution of distilled water containing specified amounts of calcium chloride, potassium chloride, sodium chloride, and sodium lactate), Locke-Ringer's solution (a buffered isotonic solution of distilled water containing specified amounts of sodium chloride, potassium chloride, calcium chloride, sodium bicarbonate, magnesium chloride, and dextrose), or any other electrolyte solution.

[0075] While a conductive fluid is preferred, as will become more apparent with further reading of this specification, fluid 24 may also comprise an electrically non-conductive fluid. The use of a non-conductive fluid is less preferred than a conductive fluid, however, the use of a non-conductive fluid still provides certain advantages over the use of a dry electrode including, for example, reduced occurrence of tissue sticking to the electrode of device 5 . Therefore, it is also within the scope of the invention to include the use of a non-conducting fluid, such as, for example, deionized water.

[0076] Returning to FIG. 1 , energy to heat tissue is provided from an energy source, such as an electrical generator 6 which preferably provides RF alternating current energy via a cable 7 to an energy source output measurement device, such as a power measurement device 8 that measures the RF alternating current electrical power. In one exemplary embodiment, preferably the power measurement device 8 does not turn the power off or on, or alter the power in any way. A power switch 15 connected to generator 6 is preferably provided by the generator manufacturer and is used to turn generator 6 on and off. The power switch 15 can comprise any switch to turn the power on and off, and is commonly provided in the form of a footswitch or other easily operated switch, such as a switch 15 a mounted on electrosurgical device 5 . The power switch 15 or 15 a may also function as a manually activated device for increasing or decreasing the rate of energy provided from device 5 . Alternatively, internal circuitry and other components of generator 6 may be used for automatically increasing or decreasing the rate of energy provided from device 5 . A cable 9 preferably provides RF energy from power measurement device 8 to electrosurgical device 5 . Power, or any other energy source output, is preferably measured before it reaches electrosurgical device 5 .

[0077] When capacitation and induction effects are negligibly small, from Ohm's law, power P, or the rate of energy delivery (e.g., joules/sec), may be expressed by the product of current times voltage (i.e., I×V), the current squared times resistance (i.e., I 2 ×R), or the voltage squared divided by the resistance (i.e., V 2 /R); where the current I may be measured in amperes, the voltage V may be measured in volts, the electrical resistance R may be measured in ohms, and the power P may be measured in watts (joules/sec). Given that power P is a function of current I, voltage V, and resistance R as indicated above, it should be understood, that a change in power P is reflective of a change in at least one of the input variables. Thus, one may alternatively measure changes in such input variables themselves, rather than power P directly, with such changes in the input variables mathematically corresponding to a changes in power P as indicated above.

[0078] The RF electrical energy is preferably provided within a frequency band (i.e., a continuous range of frequencies extending between two limiting frequencies) in the range between and including about 9 kHz (kilohertz) to 300 GHz (gigahertz). More preferably, the RF energy is provided within a frequency band in the range between and including about 50 kHz (kilohertz) to 50 MHz (megahertz). Even more preferably, the RF energy is provided within a frequency band in the range between and including about 200 kHz (kilohertz) to 2 MHz (megahertz). Most preferably, RF energy is provided within a frequency band in the range between and including about 400 kHz (kilohertz) to 600 kHz (kilohertz). It should be understood that, for any frequency band identified above, the range of frequencies may be further narrowed in increments of 1 (one) hertz anywhere between the lower and upper limiting frequencies.

[0079] While RF electrical energy is preferred, it should be understood that the electrical energy (i.e., energy made available by the flow of electric charge, typically through a conductor or by self-propagating waves) may comprise any frequency of the electromagnetic spectrum (i.e., the entire range of radiation extending in frequency from 10 23 hertz to 0 hertz) and including, but not limited to, gamma rays, x-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and any combinations thereof.

[0080] Heating of the tissue is preferably performed by electrical resistance heating. That is, the temperature of the tissue increases as a result of electric current flow through the tissue, with the electrical energy being absorbed from the voltage and transformed into thermal energy (i.e., heat) via accelerated movement of ions as a function of the tissue's electrical resistance.

[0081] Heating with electrical energy may also be performed by dielectric heating (capacitation). That is, the temperature of the tissue increases through the dissipation of electrical energy as a result of internal dielectric loss when the tissue is placed in a varying electric field, such as a high-frequency (e.g., microwave), alternating electromagnetic field. Dielectric loss is the electrical energy lost as heat in the polarization process in the presence of the applied electric field. In the case of an alternating current field, the energy is absorbed from the alternating current voltage and converted to heat during the polarization of the molecules.

[0082] However, it should be understood that energy provided to heat the tissue may be from surgical devices other than electrosurgical devices, energy sources other than generators, energy forms other than electrical energy and mechanisms other than resistance heating. For example, thermal energy can be provided to the tissue from an energy source having a higher temperature. Such may be provided, for example, by a heated device which heats tissue through direct contact (conduction), through contact with a flowing fluid (convection), or from a remote heat source (radiation).

[0083] Also, for example, providing energy to the tissue may be provided via mechanical energy which is transformed into thermal energy via accelerated movement of the molecules, such as by mechanical vibration provided, for example, by an energy source such as a transducer containing a piezoelectric substance (e.g., a quartz-crystal oscillator) that converts high-frequency electric current into vibrating ultrasonic waves which may be used by, for example, an ultrasonic surgical device.

[0084] Also, for example, energy can be provided to the tissue via radiant energy (i.e., energy which is transmitted by radiation/waves) which is transformed into thermal energy via absorption of the radiant energy by the tissue. Preferably the radiation/waves comprise electromagnetic radiation/waves which include, but are not limited to, radio waves, microwaves, infrared radiation, visible light radiation, ultraviolet radiation, x-rays and gamma rays. More preferably, such radiant energy comprises energy with a frequency of 3×10 11 hertz to 3×10 16 hertz (i.e., the infrared, visible, and ultraviolet frequency bands of the electromagnetic spectrum). Also preferably the electromagnetic waves are coherent and the electromagnetic radiation is emitted from energy source such as a laser device.

[0085] Referring again to FIG. 1, a flow rate controller 11 preferably includes a selection switch 12 that can be set to achieve desired levels of percentage fluid boiling (for example, 100%, 98%, 80% boiling). Preferably, flow rate controller 11 receives an input signal 10 from power measurement device 8 and calculates an appropriate mathematically predetermined fluid flow rate based on percentage boiling indicated by the selection switch 12 . In a preferred embodiment, a fluid switch 13 is provided so that the fluid system can be primed (e.g., air eliminated) before turning on generator 6 . The output signal 16 of flow rate controller 11 is preferably sent to pump 3 motor to regulate the flow rate of conductive fluid, and thereby provide an appropriate fluid flow rate which corresponds to the amount of power being delivered.

[0086] In one embodiment, flow rate controller 11 is configured and arranged to be connected to a source of RF power (e.g., generator 6 ), and a source of fluid (e.g., fluid source 1 ), for example, a source of conductive fluid. The device of the invention receives information about the level of RF power applied to electrosurgical device 5 , and adjusts the flow rate of fluid 24 to electrosurgical device 5 , thereby controlling temperature at the tissue treatment site.

[0087] In another embodiment, elements of the system are physically included together in one electronic enclosure. One such embodiment is shown by enclosure within the outline box 14 of FIG. 1 . In the illustrated embodiment, pump 3 , flow rate controller 11 , and power measurement device 8 are enclosed within an enclosure, and these elements are connected through electrical connections to allow signal 10 to pass from power measurement device 8 to flow rate controller 11 , and signal 16 to pass from flow rate controller 11 to pump 3 . Other elements of a system can also be included within one enclosure, depending upon such factors as the desired application of the system, and the requirements of the user.

[0088] Pump 3 can be any suitable pump to provide saline or other fluid at a desired flow rate. Preferably, pump 3 is a peristaltic pump. With a rotary peristaltic pump, typically a fluid 24 is conveyed within the confines of a flexible tube (e.g., 4 a ) by waves of contraction placed externally on the tube which are produced mechanically, typically by rotating rollers which intermittently squeeze the flexible tubing against a support with a linear peristaltic pump, typically a fluid 24 is conveyed within the confines of a flexible tube by waves of contraction placed externally on the tube which are produced mechanically, typically by a series of compression fingers or pads which sequentially squeeze the flexible tubing against a support. Peristaltic pumps are generally preferred, as the electro-mechanical force mechanism (e.g., rollers driven by electric motor) does not make contact the fluid 24 , thus reducing the likelihood of inadvertent contamination.

[0089] Alternatively, pump 3 can be a “syringe pump”, with a built-in fluid supply. With such a pump, typically a filled syringe is located on an electro-mechanical force mechanism (e.g., ram driven by electric motor) which acts on the plunger of the syringe to force delivery of the fluid 24 contained therein. The syringe pump may be a double-acting syringe pump with two syringes such that they can draw saline from a reservoir (e.g., of fluid source 1 ), either simultaneously or intermittently. With a double acting syringe pump, the pumping mechanism is generally capable of both infusion and withdrawal. Typically, while fluid 24 is being expelled from one syringe, the other syringe is receiving fluid 24 therein from a separate reservoir. In this manner, the delivery of fluid 24 remains continuous and uninterrupted as the syringes function in series. Alternatively, it should be understood that a multiple syringe pump with two syringes, or any number of syringes, may be used in accordance with the invention.

[0090] Furthermore, fluid 24 , such as conductive fluid, can also be provided from an intravenous (IV) bag full of saline (e.g., of fluid source 1 ) that flows by gravity. Fluid 24 may flow directly to electrosurgical device 5 , or first to pump 3 located there between. Alternatively, fluid 24 from a fluid source 1 such as an IV bag can be provided through an IV flow controller that may provide a desired flow rate by adjusting the cross sectional area of a flow orifice (e.g., lumen of the connective tubing with the electrosurgical device 5 ) while sensing the flow rate with a sensor such as an optical drop counter. Furthermore, fluid 24 from a fluid source 1 such as an IV bag can be provided through a manually or automatically activated device such as a flow controller, such as a roller clamp, which also adjusts the cross sectional area of a flow orifice and may be adjusted manually by, for example, the user of the device in response to their visual observation (e.g., fluid boiling) at the tissue treatment site or a pump.

[0091] Similar pumps can be used in connection with the invention, and the illustrated embodiments are exemplary only. The precise configuration of pump 3 is not critical to the invention. For example, pump 3 may include other types of infusion and withdrawal pumps. Furthermore, pump 3 may comprise pumps which may be categorized as piston pumps, rotary vane pumps (e.g., axial impeller, centrifugal impeller), cartridge pumps and diaphragm pumps. In some embodiments, pump 3 can be substituted with any type of flow controller, such as a manual roller clamp used in conjunction with an IV bag, or combined with the flow controller to allow the user to control the flow rate of conductive fluid to the device. Alternatively, a valve configuration can be substituted for pump 3 .

[0092] Furthermore, similar configurations of the system can be used in connection with the invention, and the illustrated embodiments are exemplary only. For example, the fluid source 1 , pump 3 , generator 6 , power measurement device 8 or flow rate controller 11 , or any other components of the system not expressly recited above, may be present as a part of the electrosurgical device 5 . For example, fluid source 1 may be a compartment of the electrosurgical device 5 which contains fluid 24 , as indicated at reference character 1 a . In another exemplary embodiment, the compartment may be detachably connected to electrosurgical device 5 , such as a canister which may be attached via threaded engagement with device 5 . In yet another embodiment, the compartment may be configured to hold a pre-filled cartridge of fluid 24 , rather than the fluid directly.

[0093] Also for example, with regards to alternative for the generator 6 , an energy source, such as a direct current (DC) battery used in conjunction with inverter circuitry and a transformer to produce alternating current at a particular frequency, may comprise a portion of the electrosurgical device 5 , as indicated at reference character 6 a . In one embodiment the battery element of the energy source may comprise a rechargeable battery. In yet another exemplary embodiment, the battery element may be detachably connected to the electrosurgical device 5 , such as for recharging. The components of the system will now be described in further detail. From the specification, it should be clear that any use of the terms “distal” and “proximal” are made in reference from the user of the device, and not the patient.

[0094] Flow rate controller 11 controls the rate of flow from the fluid source 1 . Preferably, the rate of fluid flow from fluid source 1 is based upon the amount of RF power provided from generator 6 to electrosurgical device 5 . Referring to FIG. 2 , there is illustrated a relationship between the rate of fluid flow Q and the RF power P. More precisely, as shown in FIG. 2 , the relationship between the rate of fluid flow Q and RF power P may be expressed as a direct, linear relationship. The flow rate Q of conductive fluid 24 , such as saline, interacts with the RF power P and various modes of heat transfer away from the target tissue, as described herein.

[0095] Throughout this disclosure, when the terms “boiling point of saline”, “vaporization point of saline”, and variations thereof are used, what is actually referenced for explanation purposes, is the boiling point of the water (i.e., 100° C.) in the saline solution given that the difference between the boiling point of normal saline (about 100.16° C.) and the boiling point of water is negligible.

[0096] FIG. 2 shows the relationship between the flow rate of saline, RF power to tissue, and regimes of boiling as detailed below. Based on a simple one-dimensional lumped parameter model of the heat transfer, the peak tissue temperature can be estimated, and once tissue temperature is estimated, it follows directly whether it is hot enough to boil saline. The total RF electrical power P that is converted into heat can be defined as:

P=ΔT/R+ρc ρ Q l ΔT+ρQ b h v (1)

[0097] where P=the total RF electrical power that is converted into heat.

[0098] Conduction. The term [ΔT/R] in equation (1) is heat conducted to adjacent tissue, represented as 70 in FIG. 2 , where:

[0099] ΔT=(T−T ∞ ) the difference in temperature between the peak tissue temperature (T) and the normal temperature (T ∞ ) of the body tissue (° C.); normal temperature of the body tissue is generally 37° C.; and

[0100] R=Thermal resistance of surrounding tissue, the ratio of the temperature difference to the heat flow (° C./watt).

[0101] This thermal resistance can be estimated from published data gathered in experiments on human tissue (see for example, Phipps, J. H., “Thermometry studies with bipolar diathermy during hysterectomy,” Gynaecological Endoscopy , 3:5-7 (1994)). As described by Phipps, Kleppinger bipolar forceps were used with an RF power of 50 watts, and the peak tissue temperature reached 320° C. For example, using the energy balance of equation (1), and assuming all the RF heat put into tissue is conducted away, then R can be estimated:

R=ΔT/P =(320-37)/50=5.7≈6° C./watt

[0102] However, it is undesirable to allow the tissue temperature to reach 320° C., since tissue will become desiccated. At a temperature of 320° C., the fluid contained in the tissue is typically boiled away, resulting in the undesirable tissue effects described herein. Rather, it is preferred to keep the peak tissue temperature at no more than about 100° C. to inhibit desiccation of the tissue. Assuming that saline boils at about 100° C., the first term in equation (1) (ΔT/R) is equal to (100−37)/6=10.5 watts. Thus, based on this example, the maximum amount of heat conducted to adjacent tissue without any significant risk of tissue desiccation is 10.5 watts.

[0103] Referring again to FIG. 2 , RF power to tissue is represented on the X-axis as P (watts) and flow rate of saline (cc/min) is represented on the Y-axis as Q. When the flow rate of saline equals zero (Q=0), there is an “offset” RF power that shifts the origin of the sloped lines 76 , 78 , and 80 to the right. This offset is the heat conducted to adjacent tissue. For example, using the calculation above for bipolar forceps, this offset RF power is about 10.5 watts. If the power is increased above this level with no saline flow, the peak tissue temperature can rise well above 100° C., resulting in tissue desiccation from the boiling off of water in the cells of the tissue.

[0104] Convection. The second term [ρc ρ Q l ΔT] in equation (1) is heat used to warm up the saline without boiling the saline, represented as 72 in FIG. 2 , where:

[0105] ρ=Density of the saline fluid that gets hot but does not boil (approximately 1.0 gm/cm 3 );

[0106] c ρ =Specific heat of the saline (approximately 4.1 watt-sec/gm-° C.);

[0107] Q l =Flow rate of the saline that is heated (cm 3 /sec); and

[0108] ΔT=Temperature rise of the saline. Assuming that the saline is heated to body temperature before it reaches the electrode, and that the peak saline temperature is similar to the peak tissue temperature, this is the same ΔT as for the conduction calculation above.

[0109] The onset of boiling can be predicted using equation (1) with the last term on the right set to zero (no boiling) (ρQ b h v =0), and solving equation (1) for Q l leads to:

Q l =[P−ΔT/R]/ρc ρ ΔT (2)

[0110] This equation defines the line shown in FIG. 2 as the line of onset of boiling 76 .

[0111] Boiling. The third term [ρQ b h v ] in equation (1) relates to heat that goes into converting the water in liquid saline to water vapor, and is represented as 74 in FIG. 2 , where:

[0112] Q b =Flow rate of saline that boils (cm 3 /sec); and

[0113] h v =Heat of vaporization of saline (approximately 2,000 watt-sec/gm).

[0114] A flow rate of only 1 cc/min will absorb a significant amount of heat if it is completely boiled, or about ρQ b h v =(1) (1/60) (2,000)=33.3 watts. The heat needed to warm this flow rate from body temperature to 100° C. is much less, or ρc ρ Q l ΔT=(1) (4.1) (1/60) (100−37)=4.3 watts. In other words, the most significant factor contributing to heat transfer from a wet electrode device can be fractional boiling. The present invention recognizes this fact and exploits it.

[0115] Fractional boiling can be described by equation (3) below: 1 $\begin{array}{cc}{Q}_1=\frac{\left\{P-\Delta T/R\right\}}{\left\{\rho c_p\Delta T+\rho h_vQ_b/Q_1\right\}}& \left(3\right)\end{array}$

[0116] If the ratio of Q b /Q l is 0.50 this is the 50% boiling line 78 shown in FIG. 2 . If the ratio is 1.0 this is the 100% boiling line 80 shown in FIG. 2 .

[0117] As indicated previously in the specification, use of a fluid to couple energy to tissue inhibits undesirable effects such as sticking, desiccation, smoke production and char formation. Tissue desiccation, which occurs if the tissue temperature exceeds 100° C. and all the intracellular water boils away, is particularly undesirable as it leaves the tissue extremely dry and much less electrically conductive.

[0118] As shown in FIG. 2 , one control strategy or mechanism which can be employed for the electrosurgical device 5 is to adjust the power P and flow rate Q such that the power P used at a corresponding flow rate Q is equal to or less than the power P required to boil 100% of the fluid and does not exceed the power P required to boil 100% of the fluid. This control strategy targets using the electrosurgical device 5 in the regions of FIG. 2 identified as T<100° C. and T=100° C., and includes the 100% boiling line 80 . That is, this control strategy targets not using the electrosurgical device 5 only in the region of FIG. 2 identified as T>>100° C.

[0119] Another control strategy that can be used for the electrosurgical device 5 is to operate device 5 in the region T<100° C., but at high enough temperature to shrink tissue containing Type I collagen (e.g., walls of blood vessels, bronchi, bile ducts, etc.), which shrinks when exposed to about 85° C. for an exposure time of 0.01 seconds, or when exposed to about 65° C. for an exposure time of 15 minutes. An exemplary target temperature/time for tissue shrinkage is about 75° C. with an exposure time of about 1 second. A determination of the high end of the scale (i.e., when the fluid reaches 100° C.) can be made by the phase change in the fluid from liquid to vapor. However, a determination at the low end of the scale (e.g., when the fluid reaches, for example, 75° C. for 1 second) requires a different mechanism as the temperature of the fluid is below the boiling temperature and no such phase change is apparent. In order to determine when the fluid reaches a temperature that will facilitate tissue shrinkage, for example 75° C., a thermochromic material, such as a thermochromic dye (e.g., leuco dye), may be added to the fluid. The dye can be formulated to provide a first predetermined color to the fluid at temperatures below a threshold temperature, such as 75° C., then, upon heating above 75° C., the dye provides a second color, such as clear, thus turning the fluid clear (i.e., no color or reduction in color). This color change may be gradual, incremental, or instant. Thus, a change in the color of the fluid, from a first color to a second color (or lack thereof) provides a visual indication the user of the electrosurgical device 5 as to when a threshold fluid temperature below boiling has been achieved. Thermochromic dyes are available, for example, from Color Change Corporation, 1740 Cortland Court, Unit A, Addison, Ill. 60101.

[0120] It is also noted that the above mechanism (i.e., a change in the color of the fluid due to a dye) may also be used to detect when the fluid reaches a temperature which will facilitate tissue necrosis; this generally varies from about 60° C. for an exposure time of 0.01 seconds and decreasing to about 45° C. for an exposure time of 15 minutes. An exemplary target temperature/time for tissue-necrosis is about 55° C. for an exposure time of about 1 second.

[0121] In order to reduce coagulation time, use of the electrosurgical device 5 in the region T=100° C. of FIG. 2 is preferable to use of the electrosurgical device 5 in the region T<100° C. Consequently, as shown in FIG. 2 , another control strategy which may be employed for the electrosurgical device 5 is to adjust the power P and flow rate Q such that the power P used at a corresponding flow rate Q is equal to or more than the power P required to initiate boiling of the fluid, but still less than the power P required to boil 100% of the fluid. This control strategy targets using the electrosurgical device 5 in the region of FIG. 2 identified as T=100° C., and includes the lines of the onset of boiling 76 and 100% boiling line 80 . That is, this control strategy targets using the electrosurgical device 5 on or between the lines of the onset of boiling 76 and 100% boiling line 80 , and not using the electrosurgical device 5 in the regions of FIG. 2 identified as T<100° C. and T>>100° C.

[0122] For consistent tissue effect, it is desirable to control the saline flow rate so that it is always on a “line of constant % boiling” as, for example, the line of the onset of boiling 76 or the 100% boiling line 80 or any line of constant % boiling located in between (e.g., 50% boiling line 78 ) as shown in FIG. 2 . Consequently, another control strategy that can be used for the electrosurgical device 5 is to adjust power P and flow rate Q such that the power P used at a corresponding flow rate Q targets a line of constant % boiling.

[0123] It should be noted, from the preceding equations, that the slope of any line of constant % boiling is known. For example, for the line of the onset of boiling 76 , the slope of the line is given by (ρc p ΔT), while the slope of the 100% boiling line 80 is given by 1/(ρc p ΔT+ρh v ). As for the 50% boiling line 78 , for example, the slope is given by 1/(ρc p ΔT+ρh v 0.5).

[0124] If, upon application of the electrosurgical device 5 to the tissue, boiling of the fluid is not detected, such indicates that the temperature is less than 100° C. as indicated in the area of FIG. 2 , and the flow rate Q must be decreased to initiate boiling. The flow rate Q may then decreased until boiling of the fluid is first detected, at which time the line of the onset of boiling 76 is transgressed and the point of transgression on the line 76 is determined. From the determination of a point on the line of the onset of boiling 76 for a particular power P and flow rate Q, and the known slope of the line 76 as outlined above (i.e., 1/ρc p ΔT), it is also possible to determine the heat conducted to adjacent tissue 70 .

[0125] Conversely, if upon application of the electrosurgical device 5 to the tissue, boiling of the fluid is detected, such indicates that the temperature is approximately equal to 100° C. as indicated in the areas of FIG. 2 , and the flow rate Q must be increased to reduce boiling until boiling stops, at which time the line of the onset of boiling 76 is transgressed and the point of transgression on the line 76 determined. As with above, from the determination of a point on the line of the onset of boiling 76 for a particular power P and flow rate Q, and the known slope of the line 76 , it is also possible to determine the heat conducted to adjacent tissue 70 .

[0126] With regards to the detection of boiling of the fluid, such may be physically detected by the user (e.g., visually by the naked eye) in the form of either bubbles or steam evolving from the fluid coupling at the electrode/tissue interface. Alternatively, such a phase change (i.e., from liquid to vapor or vice-versa) may be measured by a sensor which preferably senses either an absolute change (e.g., existence or non-existence of boiling with binary response such as yes or no) or a change in a physical quantity or intensity and converts the change into a useful input signal for an information-gathering system. For example, the phase change associated with the onset of boiling may be detected by a pressure sensor, such as a pressure transducer, located on the electrosurgical device 5 . Alternatively, the phase change associated with the onset of boiling may be detected by a temperature sensor, such as a thermistor or thermocouple, located on the electrosurgical device 5 , such as adjacent to the electrode. Also alternatively, the phase change associated with the onset of boiling may be detected by a change in the electric properties of the fluid itself. For example, a change in the electrical resistance of the fluid may be detected by an ohm meter; a change in the amperage may be measured by an amp meter; a change in the voltage may be detected by a volt meter; and a change in the power may be determined by a power meter.

[0127] Yet another control strategy which may be employed for the electrosurgical device 5 is to eliminate the heat conduction term of equation (1) (i.e., ΔT/R). Since the amount of heat conducted away to adjacent tissue can be difficult to precisely predict, as it may vary, for example, by tissue type, it may be preferable, from a control point of view, to assume the worst case situation of zero heat conduction, and provide enough saline so that if necessary, all the RF power could be used to heat up and boil the saline, thus providing that the peak tissue temperature will not go over 100° C. significantly. This is shown in the schematic graph of FIG. 3 .

[0128] Stated another way, if the heat conducted to adjacent tissue 70 is overestimated, the power P required to intersect the 100% boiling line 80 will, in turn, be overestimated and the 100% boiling line 80 will be transgressed into the T>>100° C. region of FIG. 2 , which is undesirable as established above. Thus, assuming the worse case situation of zero heat conduction provides a “safety factor” to avoid transgressing the 100% boiling line 80 . Assuming heat conduction to adjacent tissue 70 to be zero also provides the advantage of eliminating the only term from equation (1) which is tissue dependent, i.e., depends on tissue type. Thus, provided ρ, c p , ΔT, and h v are known as indicated above, the equation of the line for any line of constant % boiling is known. Thus, for example, the 98% boiling line, 80% boiling line, etc. can be determined in response to a corresponding input from selection switch 12 . In order to promote flexibility, it should be understood that the input from the selection switch preferably may comprise any percentage of boiling. Preferably the percentage of boiling can be selected in single percent increments (i.e., 100%, 99%, 98%, etc.).

[0129] Upon determination of the line of the onset of boiling 76 , the 100% boiling line 80 or any line of constant % boiling there between, it is generally desirable to control the flow rate Q so that it is always on a particular line of constant % boiling for consistent tissue effect. In such a situation, flow rate controller 11 will adjust the flow rate Q of the fluid 24 to reflect changes in power P provided by the generator 6 , as discussed in greater detail below. For such a use flow rate controller 11 may be set in a line of constant boiling mode, upon which the % boiling is then correspondingly selected.

[0130] As indicated above, it is desirable to control the saline flow rate Q so that it is always on a line of constant % boiling for consistent tissue effect. However, the preferred line of constant % boiling may vary based on the type of electrosurgical device 5 . For example, if with use of the device 5 , shunting through saline is not an issue, then it can be preferable to operate close to or directly on, but not over the line of the onset of boiling, such as 76 a in FIG. 3 . This preferably keeps tissue as hot as possible without causing desiccation. Alternatively, if with use of the device 5 shunting of electrical energy (e.g., from one jaw to an opposing jaw of certain copative bipolar devices) through excess saline is an issue, then it can be preferable to operate along a line of constant boiling, such as line 78 a in FIG. 3 , the 50% line. This simple proportional control will have the flow rate determined by equation (4), where K is the proportionality constant:

Q l =K×P (4)

[0131] In essence, when power P goes up, the flow rate Q will be proportionately increased. Conversely, when power P goes down, the flow rate Q will be proportionately decreased.

[0132] The proportionality constant K is primarily dependent on the fraction of saline that boils, as shown in equation (5), which is equation (3) solved for K after eliminating P using equation (4), and neglecting the conduction term (ΔT/R): 2 $\begin{array}{cc}K=\frac{1}{\left\{\rho c_p\Delta T+\rho h_vQ_b/Q_1\right\}}& \left(5\right)\end{array}$

[0133] Thus, the present invention provides a method of controlling boiling of fluid, such as a conductive fluid, at the tissue/electrode interface. In a preferred embodiment, this provides a method of treating tissue without use of tissue sensors, such as temperature or impedance sensors. Preferably, the invention can control boiling of conductive fluid at the tissue/electrode interface and thereby control tissue temperature without the use of feedback loops.

[0134] In describing the control strategy of the present invention described thus far, focus has been drawn to a steady state condition. However, the heat required to warm the tissue to the peak temperature (T) may be incorporated into equation (1) as follows:

P=ΔT/R+ρc ρ Q l ΔT+ρQ b h v +ρc ρ VΔT/Δt (6)

[0135] where ρc ρ VΔT/Δt represents the heat required to warm the tissue to the peak temperature (T) 68 and where:

[0136] ρ=Density of the saline fluid that gets hot but does not boil (approximately 1.0 gm/cm 3 );

[0137] c ρ =Specific heat of the saline (approximately 4.1 watt-sec/gm-° C.);

[0138] V=Volume of treated tissue

[0139] ΔT=(T−T ∞ ) the difference in temperature between the peak tissue temperature (T) and the normal temperature (T ∞ ) of the body tissue (° C.). Normal temperature of the body tissue is generally 37° C.; and

[0140] Δt=(t−t ∞ ) the difference in time to achieve peak tissue temperature (T) and the normal temperature (T ∞ ) of the body tissue (° C.).

[0141] The inclusion of the heat required to warm the tissue to the peak temperature (T) in the control strategy is graphically represented at 68 in FIG. 4 . With respect to the control strategy, the effects of the heat required to warm the tissue to the peak temperature (T) 68 should be taken into account before flow rate Q adjustment being undertaken to detect the location of the line of onset of boiling 76 . In other words, the flow rate Q should not be decreased in response to a lack of boiling before at least a quasi-steady state has been achieved as the location of the line of onset of boiling 76 will continue to move during the transitory period. Otherwise, if the flow rate Q is decreased during the transitory period, it may be possible to decrease the flow Q to a point past the line of onset of boiling 76 and continue past the 100% boiling line 80 which is undesirable. In other words, as temperature (T) is approached the heat 68 diminishes towards zero such that the lines of constant boiling shift to the left towards the Y-axis.

[0142] FIG. 5 is an exemplary graph of flow rate Q versus % boiling for a situation where the RF power P is 75 watts. The percent boiling % is represented on the X-axis, and the saline flow rate Q (cc/min) is represented on the Y axis. According to this example, at 100% boiling the most desirable predetermined saline flow rate Q is 2 cc/min. Also according to this example, flow rate Q versus % boiling at the remaining points of the graft illustrates a non-linear relationship as follows: 1

[0143] Typical RF generators used in the field have a power selector switch to 300 watts of power, and on occasion some have been found to be selectable up to 400 watts of power. In conformance with the above methodology, at 0% boiling with a corresponding power of 300 watts, the calculated flow rate Q is 69.7 cc/min and with a corresponding power of 400 watts the calculated flow rate Q is 92.9 cc/min. Thus, when used with typical RF generators in the field, a fluid flow rate Q of about 100 cc/min or less with the present invention is expected to suffice for the vast majority of applications.

[0144] As discussed herein, RF energy delivery to tissue can be unpredictable and vary with time, even though the generator has been “set” to a fixed wattage. The schematic graph of FIG. 6 shows the general trends of the output curve of a typical general-purpose generator, with the output power changing as load (tissue plus cables) impedance Z changes. Load impedance Z (in ohms) is represented on the X-axis, and generator output power P (in watts) is represented on the Y-axis. In the illustrated embodiment, the electrosurgical power (RF) is set to 75 watts in a bipolar mode. As shown in the figure, the power will remain constant as it was set as long as the impedance Z stays between two cut-offs, low and high, of impedance, that is, for example, between 50 ohms and 300 ohms in the illustrated embodiment. Below load impedance Z of 50 ohms, the power P will decrease, as shown by the low impedance ramp 28 a . Above load impedance Z of 300 ohms, the power P will decrease, as shown by the high impedance ramp 28 b . Of particular interest to saline-enhanced electrosurgery is the low impedance cut-off (low impedance ramp 28 a ), where power starts to ramp down as impedance Z drops further. This change in output is invisible to the user of the generator and not evident when the generator is in use, such as in an operating room.

[0145] FIG. 7 shows the general trend of how tissue impedance generally changes with time for saline-enhanced electrosurgery. As tissue heats up, the temperature coefficient of the tissue and saline in the cells is such that the tissue impedance decreases until a steady-state temperature is reached upon which time the impedance remains constant. Thus, as tissue heats up, the load impedance Z decreases, potentially approaching the impedance Z cut-off of 50 ohms. If tissue is sufficiently heated, such that the low impedance cut-off is passed, the power P decreases along the lines of the low impedance ramp 28 a of FIG. 6 .

[0146] Combining the effects shown in FIG. 6 and FIG. 7 , it becomes clear that when using a general-purpose generator set to a “fixed” power, the actual power delivered can change dramatically over time as tissue heats up and impedance drops. Looking at FIG. 6 , if the impedance Z drops from 100 to 75 ohms over time, the power output would not change because the curve is “flat” in that region of impedances. If, however, the impedance Z drops from 75 to 30 ohms one would transgress the low impedance cut-off and “turn the corner” onto the low impedance ramp 28 a portion of the curve and the power output would decrease dramatically.

[0147] According to one exemplary embodiment of the invention, the control device, such as flow rate controller 11 , receives a signal indicating the drop in actual power delivered to the tissue and adjusts the flow rate Q of saline to maintain the tissue/electrode interface at a desired temperature. In a preferred embodiment, the drop in actual power P delivered is sensed by the power measurement device 8 (shown in FIG. 1 ), and the flow rate Q of saline is decreased by flow rate controller 11 (also shown in FIG. 1 ). Preferably, this reduction in saline flow rate Q allows the tissue temperature to stay as hot as possible without desiccation. If the control device was not in operation and the flow rate Q allowed to remain higher, the tissue would be over-cooled at the lower power input. This would result in decreasing the temperature of the tissue at the treatment site.

[0148] Flow rate controller 11 of FIG. 1 can be a simple “hard-wired” analog or digital device that requires no programming by the user or the manufacturer. Flow rate controller 11 can alternatively include a processor, with or without a storage medium, in which the determination procedure is performed by software, hardware, or a combination thereof. In another embodiment, flow rate controller 11 can include semi-programmable hardware configured, for example, using a hardware descriptive language, such as Verilog. In another embodiment, flow rate controller 11 of FIG. 1 is a computer, microprocessor-driven controller with software embedded. In yet another embodiment, flow rate controller 11 can include additional features, such as a delay mechanism, such as a timer, to automatically keep the saline flow on for several seconds after the RF is turned off to provide a post-coagulation cooling of the tissue or “quench,” which can increase the strength of the tissue seal. Flow rate controller 11 can include a delay mechanism, such as a timer, to automatically turn on the saline flow several seconds before the RF is turned on to inhibit the possibility of undesirable effects as sticking, desiccation, smoke production and char formation. Optionally, flow rate controller 11 can include a low level flow standby mechanism, such as a valve, which continues the saline flow at a standby flow level (which prevents the flow rate from going to zero when the RF power is turned off) below the surgical flow level ordinarily encountered during use of the electrosurgical device 5 .

[0149] An exemplary electrosurgical device of the present invention which may be used in conjunction with the system of the present invention is shown at reference character 5 a in FIG. 9 , and more particularly in FIGS. 9 - 13 . While various electrosurgical devices of the present invention are described with reference to use with the remainder of the system of the invention, it should be understood that the description of the combination is for purposes of illustrating the remainder of the system of the invention only. Consequently, it should be understood that the electrosurgical devices of the present invention can be used alone, or in conjuction with the remainder of the system of the invention, or that a wide variety of electrosurgical devices can be used in connection with the remainder of the system of the invention. The electrosurgical devices disclosed herein are preferably further configured for both open and laparoscopic surgery. For laparoscopic surgery, the devices are preferably configured to fit through either a 5 mm or 12 mm trocar cannula.

[0150] As shown in FIG. 8 , electrosurgical device 5 a may be used in conjunction with a cannula as illustrated at reference character 19 , during laparoscopic surgery such as, for example, a laparoscopic cholecystectomy. Cannula 19 comprises a proximal portion 19 a separated from a distal portion 19 b by an elongated rigid shaft-portion 19 c . Proximal portion 19 a of cannula 19 preferably comprises a head portion 19 d connected to rigid shaft portion 19 c , preferably by threaded engagement. Most importantly, cannula 19 has a working channel 19 e which extends through head portion 19 d and shaft portion 19 c from proximal portion 19 a to distal portion 19 b of cannula 19 . In one particular embodiment, during insertion into cannula 19 , electrosurgical device 5 a is configured to enter the proximal end of working channel 19 e , move along the channel 19 e distally, and then be extended from the distal end of the working channel 19 e . In the same embodiment, during retraction from cannula 19 , electrosurgical device 5 a is configured to enter the distal end of working channel 19 e , move along the channel 19 e proximally, and then be removed from the proximal end of working channel 19 e.

[0151] Referring back to FIG. 9 , as shown electrosurgical device 5 a is a monopolar electrosurgical device. Electrosurgical device 5 a preferably includes a rigid, self-supporting, hollow shaft 17 , a proximal handle comprising mating handle portions 20 a , 20 b and a tip portion as shown by circle 45 . Handle 20 a , 20 b is preferably made of a sterilizable, rigid, non-conductive material, such as a polymer (e.g., polycarbonate). As shown in FIGS. 10 and 11 , tip portion 45 includes a contact element preferably comprising an electrode 25 . Tip portion 45 also comprises a sleeve 82 having a uniform diameter along its longitudinal length, a spring 88 and a distal portion of shaft 17 . As shown in FIG. 10 , the longitudinal axis 31 of the tip portion 45 may be configured at an angle A relative to the longitudinal axis 29 of the proximal remainder of shaft 17 . Preferably, angle A is about 5 degrees to 90 degrees, and more preferably, angle A is about 8 degrees to 45 degrees.

[0152] As shown in FIGS. 10 and 11 , for electrosurgical device 5 a , electrode 25 generally has a spherical shape with a corresponding spherical surface, a portion 42 of which is exposed to tissue 32 at the distal end of device 5 a . When electrode 25 is in the form of a sphere, the sphere may have any suitable diameter. Typically, the sphere has a diameter in the range between and including about 1 mm to about 7 mm, although it has been found that when a sphere is larger than about 4 mm or less than about 2 mm tissue treatment can be adversely effected (particularly tissue treatment time) due to an electrode surface that is respectively either to large or to small. Thus, preferably the sphere has a diameter in the range between and including about 2.5 mm to about 3.5 mm, more preferably, about 3 mm.

[0153] It is understood that shapes other than a sphere can be used for the contact element. Examples of such shapes include oblong or elongated shapes. However, as shown in FIGS. 10 and 11 , preferably a distal end surface of electrosurgical device 5 a provides a blunt, rounded surface which is non-pointed and non-sharp as shown by electrode 25 .

[0154] As shown in FIGS. 10 and 11 , electrode 25 , is preferably located in a cavity 81 of a cylindrical sleeve 82 providing a receptacle for electrode 25 . Among other things, sleeve 82 guides movement of electrode 25 . Among other things, sleeve 82 also functions as a housing for retaining electrode 25 .

[0155] Also as shown in FIG. 11, a portion 44 of electrode 25 , is retained within cavity 81 while another portion 43 extends distally through the fluid outlet opening provided by circular fluid exit hole 26 . Also as shown, sleeve 82 is connected, preferably via welding with silver solder, to the distal end 53 of shaft 17 . For device 5 a , electrode 25 , sleeve 82 and shaft 17 preferably include, and more preferably are made at least almost essentially of, an electrically conductive metal, which is also preferably non-corrosive. A preferred material is stainless steel. Other suitable metals include titanium, gold, silver and platinum. Shaft 17 preferably is stainless steel hypo-tubing.

[0156] As for cavity 81 , the internal diameter of cavity 81 surrounding electrode 25 is preferably slightly larger than the diameter of the sphere, typically by about 0.25 mm. This permits the sphere to freely rotate within cavity 81 . Consequently, cavity 81 of sleeve 82 also preferably has a diameter in the range of about 1 mm to about 7 mm.

[0157] As best shown in FIGS. 11 and 12 , in order to retain electrode 25 , within the cavity 81 of sleeve 82 , preferably the fluid exit hole 26 , which ultimately provides a fluid outlet opening, of cavity 81 at its distal end 83 comprises a distal pinched region 86 which is reduced to a size smaller than the diameter of electrode 25 , to inhibit escape of electrode 25 from sleeve 82 . More preferably, the fluid exit hole 26 has a diameter smaller than the diameter of electrode 25 .

[0158] As best shown in FIG. 12 , fluid exit hole 26 preferably has a diameter smaller than the diameter of electrode 25 , which can be accomplished by at least one crimp 84 located at the-distal end 83 of sleeve 82 which is directed towards the interior of sleeve 82 and distal to the portion 44 of electrode 25 confined in cavity 81 . Where one crimp 84 is employed, crimp 84 may comprise a single continuous circular rim pattern. In this manner, the contact element portion extending distally through the fluid outlet opening (i.e., electrode portion 43 ) provided by fluid exit hole 26 has a complementary shape to the fluid outlet opening provided by fluid exit hole 26 , here both circular.

[0159] As shown in FIG. 12 , crimp 84 may have a discontinuous circular rim pattern where crimp 84 is interrupted by at least one rectangular hole slot 85 formed at the distal end 83 of sleeve 82 . Thus, the fluid outlet opening located at the distal end of the device 5 a may comprise a first portion (e.g., the circular fluid exit hole portion 26 ) and a second portion (e.g., the slot fluid exit hole portion 85 ). As shown in FIG. 12 , preferably, crimp 84 comprises at least four crimp sections forming a circular rim pattern separated by four discrete slots 85 radially located there between at 90 degrees relative to one another and equally positioned around the fluid outlet opening first portion. Slots 85 are preferably used to provide a fluid outlet opening or exit adjacent electrode 25 , when electrode 25 is fully seated (as discussed below) and/or when electrode 25 is not in use (i.e., not electrically charged) to keep surface portion 42 of the electrode surface of electrode 25 wet. Preferably, slots 85 have a width in the range between and including about 0.1 mm to 1 mm, and more preferably about 0.2 mm to 0.3 mm. As for length, slots 85 preferably have a length in the range between and including about 0.1 mm to 1 mm, and more preferably bout 0.4 mm to 0.6 mm.

[0160] As shown in FIG. 12 , the contact element portion extending distally through the fluid outlet opening (i.e., electrode portion 43 ) extends distally through the fluid outlet opening first portion (e.g., the circular fluid exit hole portion 26 ) and does not extend distally through the fluid outlet opening second portion (e.g., the slot fluid exit hole portion 85 ). In this manner an edge 91 of slot 85 remains exposed to tissue 32 to provide a tissue separating edge as discussed below.

[0161] It should be understood that the particular geometry of fluid outlet opening provided by the fluid exit hole located at the distal end of device 5 a to the electrode is not critical to the invention, and all that is required is the presence of a fluid exit hole which provides fluid 24 as required. For example, fluid exit hole 26 may have an oval shape while electrode 25 has a different shape, such as a round shape.

[0162] As shown in FIG. 12 , in addition to slot 85 providing a fluid exit, at least one edge 91 of slot 85 may provide a tissue separating edge adjacent a blunt surface (e.g., surface portion 42 of electrode 25 ) which may