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.
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.
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.
Factors which influence the effect of diathermy
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.
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.
Type of tissue
Tissue moisture is the major factor affecting tissue resistance; the higher the moisture content, the lower the resistance and the higher the current flow. The cervix in a postmenopausal woman will have lower water content than that of a young nulliparous woman. A lower wattage will be required to perform a loop biopsy in the latter case.
Duration of application
The longer the duration of application of electrosurgical current, the greater the extent of thermal injury. Research on uterine tissue shows that the duration of exposure of the tissue to current, rather than the wattage used, was the most important factor in producing tissue damage (Duffy et al 1991).
Size and shape of the diathermy electrode
The smaller the point of contact, the greater the current density. Thus, if the points of diathermy scissors are brought close to tissue, a cutting effect will result, whilst if the convex part of the scissor blades is used, a lower current density will result in a coagulating effect. Similarly, when using a wire loop to biopsy the cervix, the thickness of the wire or the diameter of the loop will influence the current. A thicker wire will need a higher current than a finer wire to produce a cutting effect because of the lower current density.
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.
Diathermy safety
Three major safety issues with the use of monopolar diathermy have become apparent with the evolution of electrosurgery in laparoscopic surgery. Each of them involves inadvertent discharge of diathermy current. These issues are:
Insulation failure
Defects in insulation are most likely to cause a discharge of current when higher power is employed. The use of coagulation current carries a greater risk than cutting current. Insulation failure can occur at any point from the electrosurgical unit to the active electrode. The most common site of failure is in the instrument that contains the active electrode. With conventional surgical instruments, the most common site is the joint on diathermy graspers where repeated use wears away the insulation material. Any insulation breakdown, seen as sparks from the joint area, is usually clearly visible because the whole instrument is within the surgeon’s field of view. This contrasts with laparoscopic surgery where the field of view is much smaller and only a small part of the instrument containing the active electrode may be visible. Repeated discharges from a break in the insulation may occur without the surgeon being aware. Damage to insulation most commonly occurs when moving an instrument through the laparoscopic port. Although the port valve may cause damage, the most likely cause is scraping against the sharp edge of the inner end of the port, particularly one constructed of metal.
Concern about insulation failure has fuelled the debate about disposable and reusable instruments. Disposable instruments clearly have an advantage in that the insulation sheath does not have to withstand both repeated use and repeated cleaning cycles. However, disposable instruments are built to much less robust specifications and the insulating material may not withstand harsh treatment in a long case. It is important that surgeons and theatre nurses are constantly vigilant for evidence of trauma to insulation, and reusable instruments must be checked on a regular basis both during and between cases.
Whilst a metal port may cause more trauma to the instruments passing through it, it will allow discharge of diathermy current in the event of insulation failure. The large area of port contact with the skin should enable the current to be dissipated without serious thermal injury. A plastic port would not facilitate such a discharge and the current might therefore flow to adjacent bowel, causing damage that may not be recognized at the time. Sigmoid colon is the most vulnerable piece of bowel because of its proximity to a left lateral port. If a metal port cannula is used, a plastic retaining sleeve must not be used because this prevents any discharged diathermy current passing through the skin back to the ground plate.
Direct coupling
Direct coupling involves the transmission of diathermy current from one instrument to another. Many surgeons use direct coupling to coagulate small vessels which have been grasped with a small forceps, when opening a wound for example. The diathermy instrument is then placed against the small forceps and the pedal pressed. Such a practice carries a real risk of a diathermy burn to the surgeon or assistant. If the surgeon’s glove has a perforation, the diathermy current may flow through the surgeon to ground either through the feet or through contact with the operating table. The surgeon should also be aware that theatre gowns do not provide insulation against high-frequency diathermy current.
The risk of direct coupling will be reduced in laparoscopic surgery if insulated instruments alone are used when diathermy is employed. Since many instruments, such as needle holders, are not commonly produced with insulation, this may restrict instrument choice. The risk of direct coupling to a metal port is increased when the working part of the laparoscopic instrument is large. The surgeon must always try to keep the whole of the metal part visible when using diathermy so that any inadvertent discharge will be seen.
Capacitive coupling
The concept of capacitive coupling is new to most gynaecologists. A capacitor consists of two conductors separated by an insulator. A metal instrument surrounded by an insulated sheath passing through a metal port is a capacitor. When a current passes through the instrument, a capacitive current may develop in the metal port, particularly if it is insulated from the skin. The higher the current passing through the instrument, the greater the capacitive current. If the port is insulated from the skin (by a plastic sleeve), capacitive current may be discharged to adjacent bowel, causing thermal injury. In common with insulation failure, the sigmoid colon is most vulnerable to capacitive current injury due to its proximity to a left lateral port.
A capacitive current can also develop in a gloved hand holding a non-insulated instrument through which diathermy current is passed. This risk is greater if the hand is moist and there is contact between the surgeon and the operating table or the floor.
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