Tissue Heating By Electric Current
When voltage is applied across the material it produces electric field which exerts force on charged particles. A flow of free charge carriers – electrons and ions - is called electric current. In metals and semiconductors the charge carriers are primarily electrons, whereas in liquids the charge is carried predominantly by ions. Electrical conduction in biological tissues is primarily due to the conductivity of the interstitial fluids, and thus is predominantly ionic. Transition between the electronic and ionic conduction is governed by electrochemical processes at the electrode–electrolyte interface. Value of electric current, I, is determined by the applied voltage, V, and material’s resistance, R, according to Ohm's law:
Electric current of a constant polarity is referred to as direct current (DC). A current of alternating polarity is referred to as alternating current (AC). Its frequency is measured in cycles per second or hertz (Hz).
Current flowing through a resistor causes the generation of Joule heating. In other words, the resistance of the tissue converts the electric energy of the voltage source into heat (thermal energy) which causes the tissue temperature to rise. The deposited electric power (energy per time) can be calculated using:
where P represents the electric power, typically measured in watts.
In absence of heat conduction, the rate of temperature rise, dT/dt, in a heated object is proportional to the deposited power P, and inversely proportional to which is in turn proportional to the mass m of the object and its specific heat capacity c:
Larger amount of heat is required to increase the temperature of a heavier object. Thus when heat is generated in a small region of an object, the temperature of that localized region will rise much faster than if the same amount of heat is evenly dispersed over the entire object.
Current density, j is a measure of the concentration of electric current. A higher current density results in a higher concentration of Joule heating. Power density generated by electric current in the material, p is proportional to the square of the current density, and to the material's resistivity, g:
In absence of heat conduction, the rate of local temperature rise is proportional to the power density, p, produced in that region of tissue, and inversely proportional to its specific heat capacity and density .
Read more about this topic: Electrosurgery
Famous quotes containing the words tissue, heating, electric and/or current:
“Whether or not his newspaper and a set of senses reduced to five are the main sources of the so-called “real life” of the so- called average man, one thing is fortunately certain: namely, that the average man himself is but a piece of fiction, a tissue of statistics.”
—Vladimir Nabokov (1899–1977)
“If the factory people outside the colleges live under the discipline of narrow means, the people inside live under almost every other kind of discipline except that of narrow means—from the fruity austerities of learning, through the iron rations of English gentlemanhood, down to the modest disadvantages of occupying cold stone buildings without central heating and having to cross two or three quadrangles to take a bath.”
—Margaret Halsey (b. 1910)
“It requires a surgical operation to get a joke well into a Scotch understanding. The only idea of wit, or rather that inferior variety of the electric talent which prevails occasionally in the North, and which, under the name of “Wut,” is so infinitely distressing to people of good taste, is laughing immoderately at stated intervals.”
—Sydney Smith (1771–1845)
“It is not however, adulthood itself, but parenthood that forms the glass shroud of memory. For there is an interesting quirk in the memory of women. At 30, women see their adolescence quite clearly. At 30 a woman’s adolescence remains a facet fitting into her current self.... At 40, however, memories of adolescence are blurred. Women of this age look much more to their earlier childhood for memories of themselves and of their mothers. This links up to her typical parenting phase.”
—Terri Apter (20th century)