1.1.1 SUBMUCOUS COAGULATION

In the process of submucous coagulation, needle-shaped electrodes puncture the surface of the organ and are subsequently positioned inside the organ (Fig. 39.1). When energy is applied, thermally induced coagulation builds up around the electrode, usually of ellipsoid shape, depending on the construction of the electrode. If positioning and energy dose are correct, the organ’s surface is conserved and the application is almost entirely free of pain. The body’s own decomposition and discharge of the necrotic tissue leads to a reduction in volume in the region treated.

Fig. 39.1 Submucous coagulation, soft palate.

1.1.2 SUPERFICIAL VAPORIZATION

The reduction in size of an organ, e.g. a tonsil, may also be accomplished by means of direct tissue ablation via vaporization (tissue vaporization) on the surface (Fig. 39.2).

Fig. 39.2 Superficial vaporization.

1.1.3 SUBMUCOUS VAPORIZATION

In the process of submucous vaporization, so-called ‘channeling’, channels are bored in the affected organ. For this purpose, a special bipolar electrode is necessary, at the tip of which a plasma ignition process takes place. When the tip comes into contact with tissue, the latter vaporizes immediately. A channel is generated by pushing the electrode forward in the tissue. More details on so-called plasma applications are to be found in Section 2.4.3.

1.1.4 ELECTROTOMY

Electrotomy is also related to superficial vaporization methods. Contrary to plane vaporization at the surface, however, cuts are generated using fine needle-shaped electrodes with the objective of separating tissue (Fig. 39.3).

Fig. 39.3 Electrotomy: tissue separation with high-frequency current.

1.2.1 VOLUME SHRINKAGE BY THE BODY’S OWN DISCHARGE OF COAGULATION NECROSIS/TISSUE STABILIZATION BY SCAR FORMATION AFTER COAGULATION NECROSIS

Submucous coagulation shows two time-delayed effects: volume shrinkage caused by the body’s own discharge of the coagulated tissue as well as stabilization of a region of tissue by the scarring process.

Following thermo-coagulation several phases take effect in the volume of tissue involved.

1.2.2 TISSUE ABLATION BY MEANS OF VAPORIZATION

Superficial ablation of tissue at the surface is used, for example, in tonsillotomy – the partial resection of hyperplastic tonsils. In this case, the tissue is removed by vaporization in layers using a special arrangement of electrodes.

1.2.3 TISSUE RESECTION BY MEANS OF ELECTROTOMY

In the process of electrotomy, a high current density is generated at the point where the tissue is touched by means of electrodes of small surface area in the form of needles or slings. The tissue is heated to well above 100°C in a very short period of time, leading to vaporization of the cell fluid and bursting of the cells concerned. If the electrode is moved through the tissue a cut is the result. Depending on the cutting power and speed of the cut, a more or less deep coagulation seam is produced at the edges of the cut, with the result that bleeding is reduced or can be avoided.

1.3.1 MONOPOLAR AND BIPOLAR APPLICATION TECHNOLOGY

Monopolar technology

Monopolar technology, where one of the two electrodes required to close the circuit is connected to the patient as a large-surface return path, is most commonly used in radiofrequency surgical applications to date. The actual working or active electrode is in the shape of a small-surface surgical instrument, e.g. in the form of a needle, a lancet or a ball. It is from the latter electrode that the radiofrequency alternating current from the generator is conducted into the patient, producing the desired surgical effect as a result of high current density at the point where the tissue is touched. The radiofrequency current disperses rapidly in the tissue and flows with lower current density through the body of the patient to the neutral electrode, whence it flows back to the generator (Fig. 39.4).

Fig. 39.4 Monopolar application technology.

A considerable number of complications may arise from the use of radiofrequency surgery, which are excluded when bipolar technology is applied.¹

Problem: neutral electrode

Attachment of a neutral electrode to the patient is necessary when monopolar technology is used. In this context, attention must be paid to the certainty of low contact resistance. The current density has to be evenly distributed over the entire surface of the neutral electrode. Perspiration, hairs or fatty substances between skin and the electrode surface may mean the current is unable to utilize the entire transfer surface, leading to current densities which are too high in certain places and which may in turn lead to burns.²

Problem: current flow through the body between active and neutral electrode

Thermal tissue damage may occur in parts of the body between the active and neutral electrodes where the cross- section available for current flow is small and electrical resistance high.² The current flow is particularly problematic in the region of head and neck: the greater part of the volume concerned here consists of bone as well as air-filled nasal and oral cavities or paranasal sinuses. In certain places only the thin mucous membrane layer remains as conducting residual cross-section.

The application of monopolar high-frequency technology for patients with cardiac pacemakers or catheters may only be carried out in exceptional cases. The specialist literature contains different documented opinions and research results on the interaction between radiofrequency current and pacemakers.³

Bipolar technology

Although bipolar technology has been known for some considerable time,⁴ it was only in the mid-1980s that renewed efforts were made to press ahead and further develop bipolar radiofrequency technology for reasons of safety, both for coagulation and cutting purposes.

Bipolar application technology (Fig 39.5) is characterized in that both electrodes, integrated in an application handset, are brought as close together as possible. The current only passes immediately between the two electrodes, meaning that secondary thermal damage to the patient, both internal and external, caused by leakage currents or marked changes in impedance (cross-sections with a high percentage of bone or fat and poorly conducting residual cross-sections) can be avoided. Since the attachment of a neutral electrode is not required in bipolar radiofrequency surgery, and the current flow is restricted to the point of surgical intervention, the risks involved in monopolar technology as described above can principally be avoided (Fig. 39.6).

Fig. 39.5 Bipolar application technology.

Fig. 39.6 Bipolar electrodes.

1.3.2 PROCESS MONITORING AND POWER REGULATION

Superficial coagulation, e.g. using bipolar forceps to stop bleeding, can be directly monitored by the eye of the attending physician. Where submucous coagulation is concerned, however, the physician no longer has this possibility since no visual contact exists with the coagulation taking place underneath the tissue surface. In such a case, technical monitoring of therapy relevant parameters via the power unit is all the more important.

The size of a submucous coagulation (Fig. 39.7) depends on numerous parameters. Worth mentioning in this case are the electrode geometry, the power and the application time. Tissue resistance, tissue temperature or the energy input provide information on the tissue changes achieved.

Fig. 39.7 Ellipsoid submucous coagulation of a bipolar electrode (Celon AG).

1.4.1 CELON AG MEDICAL INSTRUMENTS

1.4.2 GYRUS ENT (SOMNUS)

1.4.3 COBLATOR (ARTHROCARE)

1.4.4 DIVERSE RADIOFREQUENCY SURGICAL EQUIPMENT

2.2.1 FARADIC EFFECT

The Faradic effect is apparent using alternating currents of lower and middle frequency between 10 Hz and 10 kHz, which induce motor stimulation of muscle and nerve cells. Figure 39.8 shows the stimulation threshold curve where current is plotted against frequency. It can be seen that the threshold current possesses a minimum between 50 and 100 Hz, meaning that very low currents are enough to cause serious damage to humans within the range of the usual power frequencies. At 50 Hz even small currents may cause ventricular fibrillation.

Fig. 39.8 Stimulating current curve.

Nernst¹³ has defined the ‘physiological stimulation’ of alternating currents as a function of the electric current I and the frequency f with the equation:

The Faradic stimulation effect increases with increasing current at constant frequency. If the current is constant and the frequency increases (from approximately 100 Hz onwards), the stimulation effect diminishes. The cellular ionic concentration gradient is reduced since ever smaller amounts of electricity can be transported within the individual half-waves of the alternating current. For frequencies in the range f>200…300 kHz, freedom from stimulation is assumed according to studies carried out by Gildemeister.¹⁴ If high-frequency alternating currents in the range between 300 kHz<f<2 MHz are used, the Faradic stimulation effect can be eliminated in the widest sense, even at higher currents. At frequencies of >2 MHz, uncontrolled ‘vagabond activity’ has to be reckoned with as a result of capacitive leakage currents, even over widely insulated stretches and beyond. The arising cable and radiation problems come into the foreground and should be avoided for safety reasons.

2.2.2 ELECTROLYTIC EFFECT

The electrolytic effect occurs mainly when direct current and low-frequency alternating currents are used. In such cases, intracellular as well as transcellular ionic migration occurs, with movement towards both electrodes, depending on the ionic charge. Cell membranes possess different permeability towards different electrolytes, with the effect that concentration tailbacks accumulate in these zones. Congestion of this nature destroys isotonia among the cells on the one hand and may lead to serious cell and tissue damage, such as tissue burns, on the other (Fig. 39.9).

Fig. 39.9 Cellular ionic migration using direct current (left) and high-frequency alternating current (right).

Ionic migration of this kind diminishes with increasing frequency on account of the ever-diminishing time between two successive current alternations until ultimately no reactive concentration changes take place which could cause a motor stimulation effect. For this reason polarization no longer occurs and stimulation effects disappear.

2.2.3 THERMAL EFFECT

Radiofrequency surgery uses the thermal effect based on Joule’s law. Joule’s law states that the overall electrical output power P is released in the form of heat when an electric current I flows through an electrical resistance R. The relationship between power, resistance and current is as follows:

(1)

If a current I flows through a resistance R for a time interval Δt, the amount of heat ΔQ arises, where:

(2)

The quotient of current I and surface A is designated as electric current density J, where:

(3)

The smaller the cross-sectional area A, the larger is the current density. This means that for the same current I a higher current density J is generated by a small electrode than by an electrode with a large surface area.

The electrical resistance R of a cylinder-shaped tissue element as shown in Figure 39.10 can be calculated from the specific resistance ρ, the surface area A through which the current passes and the cylinder length L, where:

Fig. 39.10 Cylinder-shaped tissue model with resistance R.

(4)

By setting equations (4) and (3) into equation (2) and taking into account that the cylindrical volume is given by V=A 3 L, the heat generated per volume and time unit can be calculated from ΔQ/(V 3 Δt). This tissue heating is directly proportional to the product of the specific resistanceρ and the square of the electric current density J.

(5)

(heating ∼ρ 3 J²)

In other words, if current flows into the biological tissue from an active electrode with a small cross-section the current density immediately in front of the electrode is large. In accordance with Joule’s law a large heat quantity is created here.

Equation (5) also shows that the heating is larger, the larger the specific resistance ρ of the conductor through which the current passes. In other words, if an electric current flows from a metal electrode into biological tissue, the heating takes place almost exclusively in the tissue itself since the specific resistance of the metal is many times smaller than that of the tissue.

2.4.1 ELECTROTOMY

In electrotomy (Fig. 39.14), the cutting of tissue, a high current density is generated by means of small-surface electrodes at the point of transfer to the tissue. The electric power required for this purpose must be sufficient to heat the tissue above 100°C in the shortest time. As a result of this rapid heating or vaporization of the cell fluid, cell rupture, the actual cutting effect, occurs. In the above process, the electric power introduced generates a coagulation seam of greater or lesser depth at the cutting edges, which in turn seals smaller vessels and leads to a reduction of bleeding as a result.

Fig. 39.14 Principle of electrotomy.

As a result of the almost explosive nature of cell fluid vaporization immediately after starting the flow of current, an electrically insulating steam cushion is created which effectively lifts the cutting electrode away from the tissue. At sufficiently high generator voltage, this steam layer is penetrated by an electric arc. The arc concentrates the total electrical energy on a point in such a way that the tissue in the vicinity of the arc’s impact point either vaporizes or burns. In this process, more steam is created at the said location with the result that the insulating layer becomes much thicker here and the distance between electrode and tissue correspondingly larger. The result of this is that another point on the electrode reaches higher field strength and the spark flashover now takes place at another location. Over the entire length of the contact zone, tiny arcs are set off wherever the distance between electrode and tissue is momentarily least. In this way, the overall effective tissue contact surface is scanned by flashovers during the cutting process. Such flashovers occur predominantly in the cutting direction, making tissue excision possible as a result of the circumstances described above.

2.4.2 COAGULATION

In the process of electrocoagulation large-surface (several mm²) electrodes of cylindrical or conical shape are used as a rule (Fig. 39.15). The contact surfaces between electrode and tissue are larger and the high-frequency voltage smaller than those used in electrotomy, with the result that lower current densities are involved. It is a matter of good sense that the current densities worked with are such that the tissue is gradually heated above the coagulation temperature (approximately 60°C) and do not lead to vaporization (>100°C). The objective of coagulation is hemostasis by the closure of vessels, superficial coagulation or in-depth coagulation, by means of which the large-volume destruction of benign or malignant tumor tissue is usually carried out.

Fig. 39.15 Coagulation using spherical electrode.

2.4.3 ‘PLASMA’ SURGERY

Plasma is understood as a partially or fully ionized gas wherein the electrons and ions present are separated from each other. This means that plasmas are electrically conductive.

If the expression ‘plasma’ is referred to in radiofrequency surgery, this concerns a gas discharge in the form of a spark discharge. Electrons are torn from their atomic union and accelerated by the high electric field strength prevailing at the electrode. Collision with further atoms generates further electrons. This process, similar to that of an avalanche, creates a spatially restricted discharge channel, generally referred to as a spark or lightning.

Superficial plasma vaporization as well as so-called ‘plasma channeling’ is comparable to the spark erosion used in metal processing. Ignition to form a visible spark can occur in two ways.