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.
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.
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.
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).
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.
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.
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.
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.
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).
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
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
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).
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.
Tissue change can be determined by measurement of the electrical resistance of the tissue (impedance). Protein denaturing during the coagulation process results in destruction of the cell membranes, which in turn induces an improvement of the electrical conductivity. The tissue drying out process on reaching the boiling point, on the other hand, reduces the electrical conductivity of the tissue. These processes make it possible to monitor the coagulation process from start to finish. Tissue resistance provides direct information on the tissue change itself, in contrast to the temperature or energy input, physical parameters which induce the said tissue change.
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