Chapter 39 Fundamentals of minimally invasive radiofrequency applications in ear, nose and throat medicine

Ear, nose and throat (ENT) medicine currently has several different systems at its disposal for minimally invasive treatments which apply high frequency electric current to achieve a therapeutic effect. This chapter is intended to provide a survey of the technical fundamentals and tissue effects of such systems.

With the purpose of deepening the understanding of the use of high-frequency current for medical applications, an overview of the basics of high-frequency surgery is included in the second section.

1 SYSTEM OVERVIEW

The therapy systems currently available on the market are distinguished in terms of their types of application, their therapy effects and the technology of the equipment used.

1.1 TYPE OF APPLICATION

Four types of application of high-frequency energy can be distinguished:

• Submucous coagulation.

• Superficial vaporization.

• Submucous vaporization.

• Electrotomy (electroexcision)

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.

1.2 THERAPY EFFECT

Various different therapy effects are derived from the four types of application mentioned above: the delayed reduction in volume caused by the body’s own discharge of thermo-necrotic tissue, the stiffening and tightening of a region of tissue by scar formation as well as the removal of layers of tissue by superficial vaporization and the resection of parts of organs (e.g. uvula or tonsils) using electrotomy.

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.

• Tissue coagulation becomes apparent immediately after its production by a lighter color resulting from the induced protein denaturation.

• After one day, a clearly defined lesion is evident, bordered by a hyperemic edge surrounding the destroyed tissue.

• About 10 days after treatment, the lesion is surrounded with connective tissue.

• After approximately 3 weeks, the dead muscle tissue has been replaced completely by connective tissue (collagen deposits). The scarring process is complete; the region of tissue involved has decreased in size and hardened.

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 EQUIPMENT TECHNOLOGY

Radiofrequency systems can be fundamentally divided into monopolar and bipolar systems with regard to the equipment or applicator technology used. Further subdivision relates to the possibilities of therapy monitoring and power regulation.

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).