temporarily physically intact after electrosurgical application. the depth of tissue death is typically a few millimeters near the contact of the electrode. cut if the voltage level is high enough, the heat generated can create a vapour pocket. the vapour pocket typically reaches temperatures of approximately 400 degrees celsius, which vaporizes and explodes a small section of soft tissue, resulting in an incision. coag when the system is operating in "coag mode" the voltage output is usually higher than in cut mode. tissue remains grossly intact, but cells are destroyed at the point of contact, and smaller vessels are destroyed and sealed, stopping capillary and small-arterial bleeding. electrosurgical waveforms different waveforms can be used for different electrosurgical procedures. for cutting, a continuous single frequency sine wave is often employed. rapid tissue heating leads to explosive vaporization of interstitial fluid. if the voltage is sufficiently high (> 400 v peak-to-peak) the vapor sheath is ionized, forming conductive plasma. electric current continues to flow from the metal electrode through the ionized gas into the tissue. rapid overheating of tissue results in its vaporization, fragmentation and ejection of fragments, allowing for tissue cutting. in applications of a continuous wave the heat diffusion typically leads to formation of a significant thermal damage zone at the edges of the lesion. open circuit voltage in electrosurgical waveforms is typically in the range of 300–10,000 v peak-to-peak. higher precision can be achieved with pulsed waveforms. using bursts of several tens of microseconds in duration the tissue can be cut, while the size of the heat diffusion zone does not exceed the cellular scale. heat accumulation during repetitive application of bursts can also be avoided if sufficient delay is provided between the bursts, allowing the tissue to cool down. the proportion of on time to off time can be varied to allow control of the heating rate. a related parameter, duty cycle, is defined as the ratio of the on time to the period (the time of a single on-off cycle). in the terminology of electrical engineering, the process of altering this ratio to achieve an average amplitude, instead of altering the amplitude directly is called pulse-width modulation. for coagulation, the average power is typically reduced below the threshold of cutting. typically, sine wave is turned on and off in rapid succession. the overall effect is a slower heating process, which causes tissue to coagulate. in simple coagulation/cutting mode machines, the lower duty cycle typical of coagulation mode is usually heard by the ear as a lower frequency and a rougher tone than the higher frequency tone typical of cutting mode with the same equipment. many modern electrosurgical generators provide sophisticated wave forms with power adjusted in real time, based on changes of the tissue impedance. prevention of unintended harm burns for the high power surgical uses during anesthesia the monopolar modality relies on a good electrical contact between a large area of the body (typically at least the entire back of the patient) and the return electrode or pad (also known as dispersive pad or patient plate). severe burns (3rd degree) can occur if the contact with the return electrode is insufficient, or when a patient comes into contact with metal objects serving as an unintended (capacitative) leakage path to earth/ground. to prevent unintended burns, the skin is cleaned and a conductive gel is used to enhance the contact with the return electrode. proper electrical grounding practices must be followed in the electrical wiring of the building. it is also recommended to use a modern electrosurgical unit that includes a return electrode monitoring system that continuously tests for reliable and safe patient contact. these systems interrogate the impedance of a split or dual-pad return electrode and will alarm out, disabling further generator output in case of fault. prior generators relied on single pad return electrodes and thus had no means of verifying safe patient connection. return electrodes should always have full contact with the skin and be placed on the same side of the body and close to the body part where the procedure is occurring. if there is any metal in the body of the patient, the return electrode is placed on the opposite side of the body from the metal and be placed between the metal and the operation site. this prevents current from passing selectively through metal on the way to the return electrode. for example, for a patient who has had a right sided hip replacement who is scheduled for surgery, the return electrode is placed on the left side of the body on the lateral side of the lower abdomen, which places the return electrode between the location of the metal and the surgical site and on the opposite side from the metal. if there is metal on both sides of the body, the return electrode is placed between the metal and the procedure site when possible. common return electrode locations include lateral portions of the outer thighs, abdomen, back, or shoulder blades. the use of the bipolar option does not require the placement of a return electrode because the current only passes between tines of the forceps or other bipolar output device. electrosurgery should only be performed by a physician who has received specific training in this field and who is familiar with the techniques used to prevent burns. smoke toxicity concerns have also been raised regarding the toxicity of surgical smoke produced by electrosurgery. this has been shown to contain chemicals which may cause harm by inhalation by the patients, surgeon or operating theatre staff. fire hazard electrical knives should not be used around flammable substances, like alcohol-based disinfectants. history development of the first commercial electrosurgical device is credited to william t. bovie, who developed the first electrosurgical device while employed at harvard university. the first use of an electrosurgical generator in an operating room occurred on october 1, 1926 at peter bent brigham hospital in boston, massachusetts. the operation—removal of a mass from a patient’s head—was performed by harvey cushing. the low powered hyfrecator for office use was introduced in 1940.