Electrosurgical generator with adaptive power control

Abstract

An electrosurgical generator has an output power control system that causes the impedance of tissue to rise and fall in a cyclic pattern until the tissue is desiccated. The advantage of the power control system is that thermal spread and charring are reduced. In addition, the power control system offers improved performance for electrosurgical vessel sealing and tissue welding. The output power is applied cyclically by a control system with tissue impedance feedback. The impedance of the tissue follows the cyclic pattern of the output power several times, depending on the state of the tissue, until the tissue becomes fully desiccated. High power is applied to cause the tissue to reach a high impedance, and then the power is reduced to allow the impedance to fall. Thermal energy is allowed to dissipate during the low power cycle. The control system is adaptive to tissue in the sense that output power is modulated in response to the impedance of the tissue.

1. An electrosurgical generator for applying output power to tissue to desiccate tissue, the electrosurgical generator comprising:
 a controller for cycling the output power to cause a cycling of the tissue impedance; and  
 a tissue impedance measurement circuit coupled to said controller for measuring impedance of the tissue;  
 wherein the controller cycles the output power by applying a predetermined amount of output power to said tissue, lowering the output power upon an output voltage reaching a predetermined maximum, re-applying the predetermined amount of output power to said tissue of a measured tissue impedance does not indicate desiccation of the tissue, and terminating output power when the measured tissue impedance exceeds a predetermined value, the predetermined value corresponding to a desiccated condition of tissue.  
 
     
     
       2. The generator of  claim 1 , wherein the controller changes the output voltage to cycle the output power. 
     
     
       3. The generator of  claim 1 , wherein the controller changes the output current to cycle the output power. 
     
     
       4. The generator of  claim 1 , wherein the output voltage is cycled by lowering the output voltage once it reaches a predetermined maximum and raising the output voltage if the reduction in measured tissue impedance does not indicate desiccation of the tissue. 
     
     
       5. The generator of  claim 1 , wherein the output power is cycled at a frequency that is between 1 and 20 Hz. 
     
     
       6. The generator of  claim 1 , wherein the output voltage does not exceed 120 volts. 
     
     
       7. The generator of  claim 1 , further comprising a comparator wherein the measured tissue impedance value is compared to a first signal representative of a desired tissue impedance value by the comparator and a difference signal is produced. 
     
     
       8. The generator of  claim 7 , wherein the difference signal is input to the controller which generates a signal to adjust the power. 
     
     
       9. The generator of  claim 7 , wherein the first signal has a cyclic pattern. 
     
     
       10. The generator of  claim 9 , wherein the first signal is a sine wave. 
     
     
       11. An electrosurgical generator for treating tissue by applying energy comprising:
 a desiccation detector for measuring a degree of desiccation of tissue; and  
 a controller for minimizing the burning of tissue comprising power control circuitry for repeatedly increasing and decreasing output power to the tissue to be treated, the power control circuitry coupled to the desiccation detector and operating to adjust the output power in response to the degree of desiccation of the tissue by applying a predetermined amount of output power to said tissue, lowering the output power upon an output voltage reaching a predetermined maximum, and re-applying the predetermined amount of output power to said tissue if said desiccation detector does not indicate desiccation of the tissue.  
 
     
     
       12. The generator of  claim 11 , wherein the output power is terminated by said controller upon detection of desiccated tissue. 
     
     
       13. The generator of  claim 12 , wherein the desiccation detector further comprises impedance measuring circuitry, wherein the degree of desiccation of the tissue is determined by the impedance of the tissue measured by the impedance measuring circuitry. 
     
     
       14. The generator of  claim 13 , wherein the circuitry adjusts the output power by adjusting the output voltage within a predetermined voltage range. 
     
     
       15. The generator of  claim 11 , wherein the output power is repeatedly increased and decreased by the circuitry at a frequency between 1 and 20 Hz. 
     
     
       16. A method for applying energy to tissue to treat tissue, the method including supplying a generator having a power control system to produce an adaptive oscillatory power curve to minimize the heating effect on tissue, the method comprising:
 a) applying a high current into a low impedance load until a maximum power is reached;  
 b) adjusting the output voltage to maintain constant output power as impedance increases as tissue begins to desiccate;  
 c) dropping the output power in response to a rapid rise in tissue impedance indicating the boiling of tissue;  
 d) allowing the tissue impedance to fall to a predetermined minimum value and then raising the output power to cause an increase in tissue impedance;  
 e) repeating steps b and c until impedance reaches a maximum value.  
 
     
     
       17. A method for applying energy to tissue to treat tissue, the method including supplying a generator having a power control system to produce an adaptive oscillatory power curve to minimize the heating effect on tissue, the method comprising:
 a) applying a high current into a low impedance load until a maximum power is reached;  
 b) adjusting the output voltage to maintain constant output power as impedance increases as tissue begins to desiccate;  
 c) dropping the output power if the output voltage exceeds a maximum value;  
 d) raising the output power after a predetermined period of time to cause an increase in tissue impedance; and  
 e) repeating steps b and c until impedance reaches a maximum value.  
 
     
     
       18. A method for applying energy to tissue to treat the tissue, comprising steps of:
 providing an electrosurgical generator;  
 coupling an output of the generator to tissue to be treated; and  
 coupling electrosurgical energy from the generator into the tissue in an oscillatory manner until an impedance of the tissue rises to a value that indicates that the tissue is desiccated, wherein the electrosurgical energy is coupled into the tissue with an oscillatory frequency that lies within a thermal bandwidth of the tissue.  
 
     
     
       19. A method as in  claim 18 , wherein the frequency is within a range of about one Hertz to about 20 Hertz. 
     
     
       20. A method of applying energy to tissue to treat the tissue, the method including supplying a generator having a power control system to produce an adaptive oscillatory power curve to minimize the heating effect on tissue, the method comprising the steps of:
   applying a high current into a low impedance load until a maximum power is reached;        adjusting the output voltage to maintain constant output power as impedance increases as tissue begins to desiccate;        dropping the output power in response to a rapid rise in tissue impedance indicating the boiling of tissue;        allowing the tissue to cool until the tissue impedance reaches a predetermined minimum value and then raising the output power to cause an increase in tissue impedance; and        repeating the adjusting and dropping steps until the tissue impedance reaches a maximum value.     
     
     
       21. A method of applying energy to tissue to treat tissue, the method comprising the steps of:
   supplying a generator having a power control system to produce an adaptive oscillatory power curve to minimize the heating effect on tissue;        coupling electrosurgical energy from the generator into the tissue in an oscillatory manner until an impedance of the tissue rises to a value that indicates that the tissue is desiccated;        adjusting an output voltage to apply output power to the tissue in a cyclical fashion such that there are periods of increased power application and periods of reduced power application; and        allowing thermal energy to dissipate during the periods of reduced power application.     
     
     
       22. The method of  claim 21  wherein the electrosurgical energy is coupled into the tissue with an oscillatory frequency that lies within a thermal bandwidth of the tissue. 
     
     
       23. The method of  claim 22  wherein the frequency is within a range of about one Hertz to about  20  Hertz. 
     
     
       24. The method of  claim 21  wherein the output voltage of the generator is limited to  120  volts to further minimize the heating effect on the tissue.