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.

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

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.

Current flow to adjacent organs