It is rather difficult to predict precisely the effects of electrical energy on tissue and that too in a clinical settting because of the variables (both known and unknown) involved. The effects caused by electrosurgery is due to the heat generated within the tissue by an external source of energy. Since high frequency alternating current is used there is no net transfer of electrons and also there is no movement of ions across cell membranes (depolarization). In cauterization the heat is derived from an external source and transmitted to the tissue by conductance. In the case of laser it could be produced within the tissue by an external source of energy.
Part of the heat generated by the tissue is from its impedance (resistance to current flow), while majority of heat is caused by rapid vibration of molecules within the tissue due to the changing effects of electromagnetic field. There is virtually no difference in the effect of heat produced any method. Experiments proved that lesions produced by induction heating, radio frequency electrocoagulation and direct application of heat were rather similar for similar degrees of temperature elevation.
Many electrosurgical tissue effects have been described out of which only two effects i.e fulguration and vaporization (commonly known as cutting) have managed to show histological effects of desiccation and coagulation.
Description of desciccation and coagulation was first provided by Clark in 1924. This description is so complete it is accepted today as it is without any changes. Desiccation is produced by low current and a relatively high voltage applied over a broad area. The current density produced is rather low. In desiccation the cells are shrunk and shriveled with elongated nuclei. The cellular details are usually well preserved. This effect is usually caused by loss of water from the cells without any associated protein coagulation.
This occurs at higher current densities than that are used in desiccation. This results in higher tissue temperatures. The tissue fluids boil away and the proteins become denatured. A white coagulum is formed similar to that of egg white when boiled. There is loss of cellular definition as all structures fuse into formless homogenous mass with a hyalinized appearance. This is what is known classically as coagulation necrosis.
This effect is caused by the action of electrical arcs striking the tissue at widely divergent locations causing highly localized instantaneous current density. But the average current density is rather low. The characteristics of fulguration are superficial in nature with the presence of large amounts of carbonization. Carbonization is caused due to the high temperature of the tissue at the point of the strike. Superficial tissue destruction occurs because after the arc strikes, the current is dispersed widely causing rapid diminution of current density and little generation of heat in the deeper tissues. Also the thin layer of carbon and the desiccated tissue beneath it form an isulating barrier decreasing the probability of subsequent arc strikes in the same location. Fulguration requires low amperage and high voltages to overcome the resistance of teh large distances between electrode and tissue.
The cutting of tissue by electrical current is due to vaporization of cells. This is really interesting because the actual cutting mechanism is controversial. Similar to other tissue effects due to heat, the cutting action of electric current is a product of current density. A dampened (coagulating) current can be made to cut tissue by increasing its power or decreasing the size of the electrode. This comes at an expense of great lateral thermal damage to tissues. Similarly undamped sinusoidal current (cutting) will produce coagulation if the current density is low and the electrode contacts the tissue.
Tissue cutting requires a spark to be present between the electrode and the tissue. An arc may be present in coagulating currents, and is necessary in fulguration. In the formation of an arc little happens until a sufficient voltage is reached to allow the electrons to traverse the air gap between the electrode and the tissue. When the critical voltage is reached the electrons jump across the gap, causing ionization of the air molecules along its path causing the spark to occur. This ion path presents a low resistance pathway to the tissue as long as the plasma (gas composed of ionized molecules) cloud is maintained. In a cutting current, the rapidly repetetive peak voltages occcur before the plasma cloud could dissipate, so each spark tends to follow the same pathway striking at the same spot, generating a locally high current density. A dampened current reaches higher peak voltages than does an undamped one. But here the peaks are separated by a longer period of time so the plasma cloud dissipates between each peak. The low resistance path to tissue is lost and the charring and dehydration of the tissue caused by the previous peak could lead to higher resistance locally decreasing the likelihood of subsequent spark striking the same spot. The net result in this scenario is lower current density and coagulation.
Pierce theory of mechanism of cellular disruption caused by electrocautery:
He postulated that when an intense electromagnetic field impinges on an absorbing tissue, the rate of vapor formation cannot keep pace with the rate of energy input. In order to maintain thermodynamic equilibrium, an acoustic wave is generated. These waves disrupt cells.
The degree of tissue damage is also affected by the duration of the energy application, increasing levels of damage is usually evident with longer applications. The rate of tissue destruction however decreases with increasing duration of application because the resistance caused by destroyed tissue overcomes the ability of the current to penetrate it and hence no further damage occurs. Increasing levels of power also increases the tissue damage, with increasing amperage causing more damage than increased voltage. Since the current density increases inversely as the square of the radius of the electrode, tissue damage is likely to increase with increasing electrode size.
Physics involved in bipolar cautery:
This is really unique. Bipolar coagulation required less power than unipolar and would operate regardless of the medium in which it is used. It permits coagulation even in a fluid environment. Hence it can be used for wet field cautery. Major advantage of bipolar cautery is that its electric effect has limited spread. Since the tissue to be coagulated is nearly isolated from the rest of the body as it is held between the blades of the forceps the current flow is essentially limited to this area. It should also be borne in mind that there could be some amount of current leakage causing unintended tissue damage. Bipolar coagulators require less voltage and hence the likelihood of current flowing through unexpected pathways are rather rare.