How an electrosurgical generator works and what it is made of

Explain how a generator cuts and coagulates with radio-frequency current, tell monopolar, bipolar, vessel sealing and argon modes apart, and name the main assemblies and how each typically fails.

Heat made inside the tissue

An electrosurgical unit (ESU, in Russian practice often called a high-frequency surgical unit) does not heat a wire and touch the patient with it. That is electrocautery, an older and different technique. An ESU passes current through the tissue itself, and the tissue heats because it has resistance. The electrode stays relatively cool; the heat appears where the current density is highest, right under the small tip of the active electrode.

Everything in this course follows from one idea: the heating effect depends on current density, that is, current per unit of contact area. A small tip concentrates the current and heats tissue fast enough to cut or coagulate. A large return pad spreads the same current over a big area, so the skin under it warms only slightly. If the pad partly lifts and the contact area shrinks, the density rises and the pad site can burn. Keep this picture in your head; lesson 2 builds the safety story on it.

The current alternates at radio frequency, typically several hundred kilohertz. At mains frequency the same current would stimulate nerves and muscles and could cause fibrillation. Nerve and muscle stimulation falls away as the frequency rises; this is general physics found in textbooks, not a figure from the standard, and it is the reason generators work in the hundreds of kilohertz. As an example of one model only, the Valleylab Force FX-C user's guide lists output waveforms of roughly 240 to 470 kHz depending on mode. Your generator's frequency is in its specification; you do not set it.

Cut, coag and blend: it is the waveform, not just the power

The surgeon chooses between cutting and coagulating, and the generator delivers this by changing the shape of the waveform, not only the power. A useful way to think about it is how fast the heat is delivered compared with how long it has to spread.

Cut uses a continuous or almost continuous sine wave at relatively low voltage. Energy arrives so quickly that cell water boils and the cells burst along a thin line, with little heating of the tissue on either side. The result is a clean incision with little haemostasis.

Coag uses short bursts of high voltage separated by long pauses. During the pauses the tissue cools a little, so instead of vaporising, the cells dry out and proteins denature. That seals small vessels. Higher voltage also lets the current jump through air as sparks, which is how fulguration (spray coagulation) works on a bleeding surface without touching it.

Blend modes sit between the two: bursts with a shorter pause, giving a cut with some haemostasis. Different manufacturers give their modes different names, but the physics is the same.

• Duty cycle: the fraction of time the generator is actually delivering RF. Pure cut is near 100%; coag modes are low.
• Crest factor: peak voltage divided by RMS voltage. A pure sine wave has a crest factor of about 1.4 (the square root of 2). Coag waveforms have much higher crest factors because the voltage comes in tall, short bursts.
• For orientation only: the Force FX-C user's guide lists a crest factor of 1.5 for its cut modes and 5 to 8 for its coag modes. Other generators publish their own values.
• Higher crest factor means higher peak voltage for the same power. That matters for insulation of cables and instruments (lesson 2 and lesson 4).

Monopolar and bipolar

In monopolar mode the current leaves the generator through the active electrode (a pencil, a loop, a laparoscopic hook), passes through the patient's body and returns through a large patient return electrode, usually a self-adhesive pad on the thigh or the upper arm. The return electrode is also called the neutral electrode, dispersive electrode or plate; Russian manuals usually say neutral or passive electrode. The whole patient is part of the circuit, which is why monopolar mode has the most safety issues: pad burns, alternate-site burns, capacitive coupling, interference with monitors and implants.

In bipolar mode both electrodes are on the instrument, typically the two tips of forceps. Current flows only through the tissue held between them. No return pad is needed for bipolar, the voltages are lower and the current path is short. Bipolar is preferred near nerves, on small structures and, in many cases, in patients with implanted devices. Many generators automatically switch off the return electrode circuit while bipolar is active, so no current can stray through the pad.

On the generator front panel you will usually see separate sockets for monopolar active (often two, with handswitch contacts), for the return electrode and for bipolar. They are keyed differently on purpose. A socket that accepts the wrong plug, or an adapter that makes a plug fit where it should not, is a safety finding.

Vessel sealing and argon plasma

Vessel sealing is a bipolar technique for closing arteries and veins of a size that ordinary bipolar coagulation cannot reliably seal. The instrument clamps the vessel under defined pressure, and the generator delivers high current at low voltage while measuring tissue impedance many times a second. As the tissue dries, impedance rises; the generator adjusts its output and stops when the algorithm decides the seal is complete, then gives a tone. Examples are the LigaSure family used with Valleylab ForceTriad and FT10 generators, and Erbe's BiClamp with VIO generators. The instruments are often single-use and recognised by the generator through a code or chip in the connector. A sealing generator that refuses an instrument or ends a seal with an alarm is often reporting a tissue, instrument or connection problem, not a generator fault.

Argon plasma coagulation (APC) is a monopolar, non-contact technique. A stream of argon gas flows out of the probe; the high voltage ionises it, and the current travels through the ionised gas to the nearest tissue. It gives a shallow, even coagulation and is widely used in endoscopy. The extra parts are a gas supply (cylinder and pressure regulator), a flow controller and valves in the argon module, filters and the probe. Gas brings its own risks, such as over-distension of a hollow organ or gas embolism, which are managed by clinical technique and by the flow settings in the manufacturer's documentation. The engineer's part is the gas path: correct gas, leak-free connections, regulator and flow within the manufacturer's specification.

What is inside the box

Open any modern generator and you find the same blocks, whatever the brand. You do not repair the RF stage at Level 1, but you must know what each block does so that you can locate a symptom correctly and describe it in your report.

The mains input and power supply turn mains into internal DC rails: a high-voltage rail for the RF stage and low-voltage rails for the logic. The RF output stage, built around power transistors and an output transformer, makes the RF waveform; the output transformer also provides the isolation between the patient circuit and earth on HF-isolated (floating) designs. Relays or solid-state switches route the output to the selected socket. Voltage and current sensors measure what is actually delivered, so that the controller can regulate power and limit voltage. The control board runs the modes, the self-test, the alarms and the event log; on many units a small backup battery keeps its memory and clock. The return electrode monitoring circuit injects a small measuring signal into split pads and checks contact quality. Activation inputs read the footswitch and the handswitch buttons on the pencil. A speaker produces activation and alarm tones. Optional modules add argon control, vessel sealing, smoke evacuator interfaces or communication ports.

The weak points are predictable. Footswitches and handpiece cables are mechanical and get trodden on, bent and sterilised; they cause a large share of the call list. Connectors and sockets wear and corrode. Output relays and their connectors can fail open. Backup batteries age. The power supply and the RF transistors take thermal and electrical stress. The parts table below summarises this.

Your scope at Level 1

At Level 1 you inspect, test, clean, replace user-replaceable items, run the generator's self-test, measure outputs and leakage with an ESU analyser, read error messages and logs where the user interface allows, and keep records. You diagnose to the assembly and escalate.

Repair of the RF stage, output calibration and any adjustment of power or monitoring circuits are done only according to the manufacturer's procedure, by personnel authorised and trained for that model, with the specified test equipment. Entering protected service menus, activating software options or licences, and changing internal parameters are outside this course. Basic electrical safety and the general fault-finding method are covered in the esafety and fault-finding-method courses; this course assumes them.

On-site checklist

• Identify the generator's monopolar, bipolar, return electrode and accessory sockets and their keying.
• Find the output frequency, rated loads and crest factors in the manufacturer's specification.
• Note which modes the unit offers: cut, blend, coag variants, bipolar, sealing, argon.
• Check whether the output is HF-isolated (floating) or ground-referenced.
• Locate the event log or error history available at user level.
• Note whether the unit has a backup battery and its replacement interval per the manufacturer.

Check yourself