An energy-based tissue-sealing system for measuring the relative level of blood circulation in tissue with an energy based instrument for determining a tissue parameter, the system comprising a power supply; an energy output stage; the energy based instrument, having jaw members (203, 204) configured to grasp and compress tissue and a vessel, wherein the jaw members include electrodes or contacts (205, 206) that are electrically coupled to the energy output stage of the system such that the electrodes or contacts receive energy from the energy output stage for application to the tissue and the vessel to seal the tissue and the vessel; a probing signal source configured to apply a probing signal to the tissue by the electrodes or contacts (205, 206); means for monitoring a cardiac signal related to cardiac contractions; a response signal monitor configured to monitor a response signal over an interval longer than an interval between two successive cardiac contractions, wherein the probing signal interacts with tissue to create the response signal; and a processor configured to: determine the amplitude of the response signal at the frequency of the cardiac contractions; correlate the response signal and the cardiac signal; determine a level of blood circulation in the tissue based upon the result of correlating the response signal and the cardiac signal; and determine a tissue parameter based upon the level of blood circulation, wherein the system is configured so that, to evaluate blood circulation, a given volume of tissue can be grasped between the jaw members (203, 204) of the energy-based instrument, and the system is configured to apply, to evaluate blood circulation, a pressure to the tissue by the jaw members (203, 204) that is selected to provide electrical contact between the electrodes or contacts (205, 206) and the tissue. an energy output stage; the energy based instrument, having jaw members (203, 204) configured to grasp and compress tissue and a vessel, wherein the jaw members include electrodes or contacts (205, 206) that are electrically coupled to the energy output stage of the system such that the electrodes or contacts receive energy from the energy output stage for application to the tissue and the vessel to seal the tissue and the vessel; a probing signal source configured to apply a probing signal to the tissue by the electrodes or contacts (205, 206); means for monitoring a cardiac signal related to cardiac contractions; a response signal monitor configured to monitor a response signal over an interval longer than an interval between two successive cardiac contractions, wherein the probing signal interacts with tissue to create the response signal; and a processor configured to: determine the amplitude of the response signal at the frequency of the cardiac contractions; correlate the response signal and the cardiac signal; determine a level of blood circulation in the tissue based upon the result of correlating the response signal and the cardiac signal; and determine a tissue parameter based upon the level of blood circulation, wherein the system is configured so that, to evaluate blood circulation, a given volume of tissue can be grasped between the jaw members (203, 204) of the energy-based instrument, and the system is configured to apply, to evaluate blood circulation, a pressure to the tissue by the jaw members (203, 204) that is selected to provide electrical contact between the electrodes or contacts (205, 206) and the tissue. determine the amplitude of the response signal at the frequency of the cardiac contractions; correlate the response signal and the cardiac signal; determine a level of blood circulation in the tissue based upon the result of correlating the response signal and the cardiac signal; and determine a tissue parameter based upon the level of blood circulation, wherein the system is configured so that, to evaluate blood circulation, a given volume of tissue can be grasped between the jaw members (203, 204) of the energy-based instrument, and the system is configured to apply, to evaluate blood circulation, a pressure to the tissue by the jaw members (203, 204) that is selected to provide electrical contact between the electrodes or contacts (205, 206) and the tissue.