Electrosurgery

 

Principle

 

Electrosurgery is the application of a high-frequency electric current to tissue as a means to remove lesions, staunch bleeding, or cut tissue. It is based on the generation of local heat dissipated by a piece of tissue when electric current is flowing through it. The tissue can be considered as an electric resistor.

To perform electrosurgery, a voltage source is applied to the fine surgical electrode or probe (electric knife) and a 2nd electrode with the tissue in between. The current can be adjusted by changing the voltage of the source. The dissipated power is P = V2/R (in Watts) which directly follows from Ohms law (V = iR) and the relation P = iV).

The electrode does not heat up since the resistance of the metal electrode and metal wire is so much smaller than that of the tissue that very little power is expended inside the metal conductors. Electrosurgery is performed using a device called an Electrosurgical Generator, sometimes referred to as an RF Knife.

 

The change in temperature of an object is inversely proportional to its (specific) heat capacity (in J∙kg−1∙K−1). For water (near 20 oC) this is 4184 J∙kg−1∙K−1. This value can also be used for watery tissues.  Since the heat needed is proportional to the mass of the object, the heated mass is limited by using small probes. By applying a high current density (current/area), which is achieved by a small electrode tip, the resistance of the small volume f tissue adjacent to the tip is subjected to a large current density. The generated heat will now easily burn the tissue at the electrode.

 

The human nervous system is very sensitive to low-frequency (0 Hz to about 1000 Hz) electricity, which stimulates the nervous system. At even low currents low-frequency electricity causes electric shock, which may involve acute pain, muscle spasms, and/or cardiac arrest. The sensitivity decreases with increasing frequency and at frequencies above 100 kHz, electricity does not stimulate the nervous system. To avoid electric shock, electrosurgical equipment operates in the frequency range of 200 kHz to 5 MHz.

 

Fig. 1    Modes of operation.

 

Fig. 1 illustrates the various operation modes for the various aims.

 

Application

 

Electrosurgery can be used to cut, coagulate, desiccate, or fulgurate tissue. Its benefits include the ability to make precise cuts with limited blood loss.

Although electrosurgical devices may be used for the cauterization of tissue in some applications (e.g. hemorrhoid surgery), electrosurgery is usually used to refer to a quite different method than that used by many dedicated electrocautery devices. The latter uses heat conduction (see Body heat conduction …) from a hot probe heated by a direct current (much in the manner of a soldering iron), whereas electrosurgery uses alternating current to directly heat the tissue itself (electric diathermy), while the probe tip remains relatively cool.

Different waveforms of the electric current can be used for different electrosurgical procedures. For cutting, a continuous single frequency sine wave is generated. This produces rapid heating. At the cellular level, rapid heating causes tissue cells to boil and burst. At a larger scale, the ruptured cells create a fine tear in the tissue, creating a clean incision.

For coagulation, the sine wave is modulated by turned on and off in rapid succession. The overall effect is a slower heating process, which causes cells to coagulate. The proportion of on time to on+off time, the duty cycle, can be varied to allow control of the heating rate.

 

Dermatological applications are removal of skin tags, removal/destruction of benign skin tumors, and the removal of warts. For several aims it is now often preferred by dermatologists over laser surgery and cryosurgery.

 

Savety

High power monopolar surgery requires a good electrical contact between a large area of the body and the return electrode to prevent severe burns (3rd degree) in unintended areas on the skin and beneath the skin of (anesthetized) patients. To prevent unintended burns, the skin should be clean and dry and a conductive jelly should be used. Proper electrical grounding practices must be followed in the electrical wiring of the building. It is also recommended to use a newer electrosurgical unit that includes alarms for ground circuit interruption.

Safety is essentially improved when the electric circuit is inductively uncoupled. This is performed by a primary coil in the generator and a secondary coil in the circuit of the probe.

 

 

Fig. 2   Bipolar setup with the electrosurgery circuit uncoupled from the mains by coils. The double headed arrow indicates the currents direction which alternates every cycle of the current (A.C. or alternating current).

 

A strongly simplified calculation yields the order of magnitude of the strength of currents and voltages applied. Suppose that a small blood vessel (say 2 mm in diameter) should be coagulated. This is performed by applying a current, which coagulates at 237 K a sphere with a diameter of 2.8 mm (volume 0.23 ml). This requires 0.23 g x 4.2 J/(g x K) x 200 K = 10 J (tissue density is 1 g/ml). This is applied in 5 s, so 2 J/s, is 2 Watt. Supposing an electrode resistance of 10 KOhm (bipolar set up), then 140 V (= (10000 x 2)0.5) is needed and the current is 140 V/10 KOhm = 14 mA. This holds for continues current (DC or direct current), whereas the currents is applied intermittent in high frequency bursts. Supposing a duty cycle of 10% and a sinusoidal current, then the amplitude of the voltage is 20.5 X 10 X 140/2 is about 1000 V. The 1000 V (generating 140 mA effectively) again underlines the absolute necessity of save grounding and shows that uncoupling is the ultimate save approach.

 

More Info

 

Electrosurgical modalities

Monopolar and Bipolar   There are two circuit topologies: monopolar and bipolar. The bipolar modality is used less often. Voltage is applied to the patient using a special forceps, with one tine connected to one pole of the A.C. (alternating current) voltage source and the other tine connected to the other pole of the voltage source. When a piece of tissue is held by the forceps, a high frequency electrical current flows from one to the other forceps tine, through the intervening tissue. In the monopolar modality the patient lies on top of the return electrode, a relatively large metal plate or a relatively large flexible metalized plastic pad (somewhere attached to the body), which is connected to the other wire of the current source. The surgeon uses a single, pointed, probe to make contact with the tissue. The electrical current flows from the probe tip, through the body and then to the return electrode. In the monopolar modality the heating is also very precisely confined to the tissue that is near the probe tip since the current rapidly spreads out laterally in the body, causing a quadratic decrease in the current density with the distance.

Spark gap or fulguration modality   It is a low-powered monopolar electrosurgery performed on conscious outpatients (dermatology). At low-power, this technique requires no return electrode or patient-contact-plate since at the very high frequencies and low currents, the parasitic capacitance between the patient’s body and the machine's ground potential is large enough to close the electric circuit. If such a spark is small, it can cause relatively minor shocks or burns, but these are not a problem in a low-powered setting with conscious patients because they are immediately noticed. For high-power or surgical anesthesia settings, however, a ground pad is always necessary to insure that all such stray ground currents enter the machine safely through a large-skin-surface contact, and dedicated wire.