Surgical generator for ultrasonic and electrosurgical devices

1. A method for controlling electrical power provided to tissue via a surgical device, the method comprising:

providing a drive signal to the surgical device;

receiving an indication of an impedance of the tissue;

calculating a rate of increase of the impedance of the tissue; and

modulating the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to a first predetermined constant, wherein the modulating comprises increasing a power of the drive signal when the rate of increase of the impedance of the tissue is below the first predetermined constant.

2. The method of claim 1 , further comprising modulating the drive signal to hold the rate of increase of the impedance of the tissue equal to the first predetermined constant.

3. The method of claim 1 , further comprising:

determining a first total energy provided to the tissue via the surgical device at a first time;

conditioned upon the first total energy exceeding a first energy threshold, determining if the impedance of the tissue has reached a first impedance threshold; and

conditioned upon the tissue not having reached the first impedance threshold, modulating the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to a second predetermined constant, wherein the second predetermined constant is greater than the first predetermined constant.

4. The method of claim 3 , further comprising:

determining a second total energy provided to the tissue via the surgical device at a second time;

conditioned upon the second total energy exceeding a second energy threshold, determining if the impedance of the tissue has reached a second impedance threshold; and

conditioned upon the tissue not having reached the second impedance threshold, modulating the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to a third predetermined constant, wherein the third predetermined constant is greater than the first predetermined constant.

5. The method of claim 1 , further comprising:

modulating the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to the first predetermined constant for a first predetermined time period; and

at an expiration of the first predetermined time period, modulating the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to a second predetermined constant.

6. The method of claim 5 , wherein the second predetermined constant is greater than the first predetermined constant.

7. The method of claim 5 , further comprising:

at an expiration of the first predetermined time period, modulating the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to the second predetermined constant for a second predetermined time period; and

at an expiration of the second predetermined time period, modulating the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to a third predetermined constant.

8. The method of claim 7 , wherein the third predetermined constant is greater than the second predetermined constant.

9. The method of claim 7 , wherein the third predetermined constant is less than the second predetermined constant.

10. The method of claim 1 , wherein providing the drive signal comprises providing the drive signal to first and second electrodes, wherein the tissue is in electrical communication with the first and second electrodes.

11. A surgical system, the surgical system comprising:

a generator comprising at least one processor and operatively associated memory, wherein the memory comprises instructions that, when executed by the at least one processor, cause the generator to:

provide a drive signal to a surgical device;

receive an indication of an impedance of tissue;

calculate a rate of increase of the impedance of the tissue; and

modulate the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to a first predetermined constant, wherein the modulating comprises increasing a power of the drive signal when the rate of increase of the impedance of the tissue is below the first predetermined constant.

12. The system of claim 11 , wherein the memory further comprises instructions that, when executed by the at least one processor, cause the generator to modulate the drive signal to hold the rate of increase of the impedance of the tissue equal to the first predetermined constant.

13. The system of claim 11 , wherein the memory further comprises instructions that, when executed by the at least one processor, cause the generator to:

determine a first total energy provided to the tissue via the surgical device at a first time;

conditioned upon the first total energy exceeding a first energy threshold, determine if the impedance of the tissue has reached a first impedance threshold;

conditioned upon the tissue not having reached the first impedance threshold, modulate the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to a second predetermined constant, wherein the second predetermined constant is greater than the first predetermined constant.

14. The system of claim 13 , wherein the memory further comprises instructions that, when executed by the at least one processor, cause the generator to:

determine a second total energy provided to the tissue via the surgical device at a second time;

conditioned upon the second total energy exceeding a second energy threshold, determine if the impedance of the tissue has reached a second impedance threshold; and

conditioned upon the tissue not having reached the second impedance threshold, modulate the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to a third predetermined constant, wherein the third predetermined constant is greater than the first predetermined constant.

15. The system of claim 11 , wherein the memory further comprises instructions that, when executed by the at least one processor, cause the generator to:

modulate the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to the first predetermined constant for a first predetermined time period; and

at an expiration of the first predetermined time period, modulate the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to a second predetermined constant.

16. The system of claim 15 , wherein the second predetermined constant is greater than the first predetermined constant.

17. The system of claim 15 , wherein the memory further comprises instructions that, when executed by the at least one processor, cause the generator to:

at an expiration of the first predetermined time period, modulate the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to the second predetermined constant for a second predetermined time period; and

at an expiration of the second predetermined time period, modulate the drive signal to hold the rate of increase of the impedance of the tissue greater than or equal to a third predetermined constant.

18. The system of claim 17 , wherein the third predetermined constant is greater than the second predetermined constant.

19. The system of claim 17 , wherein the third predetermined constant is less than the second predetermined constant.

20. The system of claim 11 , wherein providing the drive signal comprises providing the drive signal to first and second electrodes, wherein the tissue is in electrical communication with the first and second electrodes.