Chapter 4 Diathermy and lasers

Introduction

For many years, diathermy was the technique of choice for tissue resection and destruction in both gynaecology and general surgery. The advent of the laser in the 1960s threatened this supremacy, but, during the last decade, advances in diathermy have led to a more balanced position between the two modalities. The aim of this chapter is to briefly explain the operation of these instruments, tissue effects, common system types, some of the safety aspects involved and practical clinical use of both techniques in gynaecology.

Diathermy

Diathermy has been used in surgical procedures for over 100 years (d’Arsonval 1893) for cutting and coagulation of tissues. Harvey Cushing pioneered the use of electrosurgery in neurosurgery, using a generator designed by Bovie in the 1920s, and this name is still synonymous with diathermy to some surgeons.

Although diathermy is still a much-used tool in surgical practice, few surgeons have received formal training in its use. Operating theatres will often have a variety of diathermy units for which ‘standard’ settings are used without reference to the operating conditions. In conventional open surgery, this has only presented a few hazards. The development of laparoscopic and hysteroscopic surgery has greatly increased the number of applications for electrosurgery and has presented new dangers. Only by having an understanding of the principles of this energy source will surgeons avoid the risk of injury to the patient, theatre staff and themselves.

Electrosurgery refers to both types of diathermy, monopolar or bipolar, in which current passes through the patient’s tissue. In monopolar diathermy, the active electrode and return electrode are some distance apart. In bipolar diathermy, the two electrodes are only millimetres apart. Electrocautery refers to the use of a heating element in which no current passes through the patient.

Monopolar diathermy

In monopolar diathermy, one electrode is applied to the patient who becomes part of the circuit. The surface area of the electrode plate is much greater than the contact area of the diathermy instrument to ensure that heating effects are confined to the end of the active electrode (Figure 4.1). The advantage of monopolar diathermy is that it can be used to cut as well as to coagulate tissues.

Figure 4.1 Monopolar and bipolar diathermy systems.

Tissue effects of diathermy

When household electrical current of 50 Hz frequency passes through the body, it causes an irreversible depolarization of cell membranes. If the current is sufficiently large, depolarization of cardiac muscles will occur and death may result. If household current is modified to a higher frequency, above 200 Hz, depolarization does not occur; instead, ions are excited to produce a thermal effect. This is the basis of diathermy. Figure 4.2 illustrates how the frequency of a current influences the effects on the body.

Figure 4.2 Current frequency spectrum.

Factors which influence the effect of diathermy

Cutting and coagulation current

• Type of tissue

Tissue moisture

• Duration of application

• Size and shape of diathermy electrode

Diathermy current

The amount of damage or thermal injury that diathermy produces is determined by the current density and the size of the current.

The current density at the tip of a needle electrode will be very high because the current is concentrated into a small point. The plate used for the return electrode in monopolar diathermy has a large surface area of contact, resulting in a much lower current density. Any thermal effect will therefore be widely dissipated. This highlights the need for the whole surface of the plate to be attached securely to the body. If the plate becomes partially detached, the current density will be greater in the remaining attached part and a burn may result.

The size of the current is influenced by the voltage potential and the resistance to current flow according to the following equation:

Turning up the power output of an electrosurgical unit will increase the size of the diathermy current if the resistance remains constant. Some electrosurgical units can automatically alter the voltage potential to keep the current constant if the resistance changes; however, an alternative design approach allows for control of this voltage (voltage regulation).

The resistance of tissue varies particularly with its water content. Dry tissue has high resistance and moist tissue has lower resistance. Thus, during diathermy of an area of tissue, it is desiccated by the thermal effect and its resistance will increase. To prevent the current flow from falling, some modern electrosurgical units will increase the voltage output. The surgeon should be aware of this because there are additional hazards to working with higher voltages. Insulation failure, capacitance coupling and direct coupling are all more likely with a higher voltage.

Cutting and coagulation

Coagulation and cutting can be achieved by changing the area of contact or the waveform of the current. The cutting waveform is a low-voltage, higher frequency current but the area of contact is the main factor and a cutting effect is achieved when the cutting electrode is not quite in contact with the tissue so that an electrical arc is formed. This causes the water in the cells to vaporize and the cells to explode as they come into contact with the arc. The power and current levels will rise when cutting takes place inside a liquid-filled, relatively non-conductive cavity such as the bladder or uterus. The surgeon must be aware that if the resistance increases when using cutting current (e.g. when cutting through the cervix with a wire loop), some generators will produce a higher voltage to maintain current flow against the increased resistance.

When the electrode is brought into direct contact with tissue and the waveform is modulated, coagulation occurs rather than vaporization. An intermittent waveform is used and thus bursts of thermal energy are interspersed with periods of no energy (Figure 4.3). For the power delivered to the tissue to remain constant, the electrosurgical unit must deliver a higher voltage to compensate for the episodes when no energy is delivered (up to 90% of the time with pure coagulation current). Thus, whilst cutting current at 50 W power will produce a high-frequency current of approximately 200–1000 V, a coagulation current may produce over 3000 V to deliver the same wattage to the tissue. The coagulating effect is produced by slower desiccation and shrinkage of adjacent tissue, producing haemostasis. The higher voltage produced by coagulation current carries a higher risk of inadvertent discharge of energy.

Figure 4.3 Cutting and coagulation waveforms.

If a mixture of the two types of waveform is employed, the advantages of both techniques are exploited. This is normally termed ‘blended’ output and many combinations are possible.

Heat and tissue injury

Diathermy current produces thermal injury to tissues. The temperature generated will dictate the degree of injury (Table 4.1). If carbon is seen on the tip of the diathermy electrode, the surgeon can assume that, at some stage, a temperature of 200°C has been reached.

Table 4.1 The degree of injury caused at different temperatures

Temperature (°C)	Tissue effect
44	Necrosis
70	Coagulation
90	Desiccation
200	Carbonization

Bipolar diathermy

In bipolar diathermy, the current flows between two electrodes positioned a short distance apart because both contacts are on the surgical instrument. Lower power is employed since high power would damage the tips of the instrument. This is safer because the current flow is limited to a small area and lower power is used, but a cutting effect cannot be achieved. These features encourage some surgeons to employ bipolar diathermy exclusively in the laparoscopic environment. However, there is still a small risk of aberrant current flow because the patient, table and diathermy machine are all earthed. In addition, the tissue temperatures are much higher (340°C) and this factor alone can cause unexpected effects.

Bipolar current produces tissue desiccation and has been used commonly in tubal sterilization. More recently, in laparoscopic surgery, its value in coagulating major vascular pedicles has led to its use in laparoscopic hysterectomy and laparoscopic salpingectomy for ectopic pregnancy. The lower power employed also leads to less heat spread to adjacent tissues, which reduces the risk of injury to nearby delicate structures. Engineers are endeavouring to produce reliable bipolar dissectors and scissors to compete with the range of monopolar diathermy instruments available.

Instruments are now available which coagulate a pedicle with bipolar current and utilize a non-electrosurgical blade to cut the pedicle. These instruments are often referred to as tripolar instruments, although this is a misnomer.

Bipolar instrumentation has also been introduced into hysteroscopic surgery. A bipolar electrode may be employed in the outpatient setting for the removal of endometrial polyps. The distension medium used with these devices is saline rather than glycine. Saline is isotonic and therefore reduces the risk of fluid overload associated with a hypotonic solution such as glycine.

Short-wave diathermy

Electrode redesign has led to an interest in the use of short-wave diathermy for its tissue destructive effect (Phipps et al 1990). It has been used in endometrial ablation. In this, the two electrodes form a capacitor in the output circuit with the patient providing the dielectric medium between the two plates. Frequencies of approximately 27 MHz are employed with power levels of approximately 500 W. By altering the shape and size of the electrodes, heating effects may be localized or diffused as required. However, care must be exercised as the effects are not always predictable.