Diathermy

Introduction

Diathermy is the use of high frequency alternate polarity radio-wave electrical current to cut or coagulate tissue during surgery. It allows for precise incisions to be made with limited blood loss and is now used in nearly all surgical disciplines.

Mechanism of Action

Diathermy uses very high frequencies (around 0.5-3 MHz) of electrical current. This allows diathermy to avoid the frequencies used by body systems generating electrical current, such as skeletal muscle and cardiac tissue, allowing body physiology to be broadly unaffected during its use.

The radio-frequencies generated by the diathermy heat the tissue to allow for cutting and coagulation, by creating intracellular oscillation of molecules within the cells. Depending on the temperature reached, different results occur: at 60^oc, cell death occurs (fulgurate), between 60-99^oc, dehydration occurs and the tissues coagulate, and at around 100 ^oc, the tissues vaporise (cutting).

Due to the small surface area at the point of the electrode, the current density is high, producing a focal effect and allowing the tissues to heat up rapidly. In monopolar diathermy, as the current passes through the body, the current density decreases rapidly as the surface area the current acts across increases. Consequently, this allows for focused heating of tissues at the point of use, without heating up the body systemically.

Monopolar vs Bipolar

The configuration of the diathermy devices can be either monopolar or bipolar; both actions require the electrical circuit to be completed, but vary in how this is achieved.

In monopolar action, the electrical current oscillates between the surgeon’s electrode, through the patient’s body, until it meets the ‘grounding plate’ (typically positioned underneath the patient’s leg) to complete the circuit.

SnowBink [CC0]

Figure 1 – A monopolar diathermy instrument

In bipolar diathermy, the two electrodes are found on the instrument itself. The bipolar arrangement negates the need for a dispersive electrode, instead a pair of similar sized electrodes are used in tandem. The current is then passed between the electrodes.

Bipolar is most commonly used in operations of the digits (to avoid monopolar current focused over a smaller region), in patients with pacemakers (to avoid electrical involvement with the pacemaker), or in microsurgery.

Cutting vs Coagulation

The main two settings of diathermy* are cutting and coagulation.

Cutting uses a continuous waveform with a low voltage. In cutting mode, the electrode reaches a high enough power to vaporise the water content. Hence it is able to perform a clean cut but is less efficient at coagulating. The cutting mode focuses heat at the surgical site, using sparks being the more focussed way to distribute heat; the cutting mode should therefore be used with the tip slightly away from the tissue.

Coagulation alternatively uses a pulsed waveform with a high voltage. In coagulation, the waveform is at a lower average power, not generating enough heat for explosive vaporisation, but enough for thermal coagulation. The tip should be held slightly away from the tissue, however the sparks are spread over a wider area causing charring rather than cutting.

*There is also a mixed (or blend) mode, acting in between as both cutting and coagulating, however this is not widely used

Laparoscopic Diathermy

In laparoscopic surgery, insulated equipment must be used and checked regularly to ensure it is intact. Not insulated metallic equipment can potentially create an alternative electrical pathway, so should be kept at a safe distance from the active electrode.

Capacitive coupling can occur when alternative current is passed from an insulated instrument to a non-insulated one through a capacitor. Metallic trocars should therefore have good contact with the abdominal wall to avoid burns, and non-conducting trocars must ensure they are in good condition to avoid burns.

Limitations

In monopolar mode, good contact between the patient and the ground plate is essential. If this surface area is decreased, for example if the ground plate slips partially off the patient, then this can result in severe burns. Fortunately, many modern machines monitor impedance and will stop working if this occurs.

The ground plate should be positioned as close to the operative site as possible, however it should not be placed over a bony prominence, metal prosthesis, distal to a tourniquet, over scar tissue, on hairy surfaces, or on pressure points, to minimise the risk of burns. The plate should also be kept dry.

There is also the theoretical risk of smoke and contaminant inhalation when using diathermy, to both surgeon and theatre staff, hence appropriate masks should be used in well ventilated areas.

Jclemens [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Figure 2 – Monopolar diathermy in use intra-operatively

Contraindications

Implantable devices, such as pacemakers and spinal cord stimulators, are at risk of damage from the electrical current in diathermy, therefore bipolar should be used where possible (if not, then specialised magnets can be placed over pacemakers to reduce this risk).

Implantable cardioverter-defibrillator (ICDs) can interpret the diathermy current as an unstable cardiac rhythm and potentially result in the patient being shocked. Consequently, such devices need to be deactivated prior to surgery and specialist input received regarding its use; if the ICD is deactivated, the patient must have defibrillator pads applied intra-operatively.