An ESU, or electrosurgical unit, is the device surgeons use to cut tissue and stop bleeding with high-frequency electrical current instead of a traditional scalpel. It’s one of the most commonly used instruments in modern operating rooms, appearing in nearly every surgical specialty from general surgery to ophthalmology. The device works by converting standard electrical power into a radiofrequency current (typically between 100 kHz and 3 MHz) that generates intense, focused heat at the point of contact with tissue.
How an ESU Works
At its core, an ESU is a specialized electrical generator connected to a handheld instrument with a small electrode tip. When the surgeon activates the device, current flows from the electrode into the tissue and generates heat. The key to electrosurgery is the frequency of the current. Standard household electricity at 50 or 60 Hz would cause muscles to contract and nerves to fire, which would be dangerous and painful. By operating above 100 kHz, the ESU avoids stimulating nerves and muscles entirely, producing only a heating effect in the tissue it contacts.
What happens to tissue depends on how hot it gets. In the 60 to 95°C range, proteins lose their structure and coagulate, which is how the device seals blood vessels and stops bleeding. At higher temperatures, cells vaporize, allowing the surgeon to cut through tissue cleanly. At around 200°C, tissue carbonizes, forming a dark crust called eschar on the surface.
Cutting vs. Coagulation Modes
Most ESUs offer at least two primary modes that produce very different tissue effects, controlled by how the electrical output is delivered.
In cutting mode, the device sends a continuous stream of current at a consistent voltage. This produces rapid, intense heating at a focused point, vaporizing cells and creating a clean incision through tissue, similar to what a scalpel would achieve but with the added benefit of sealing small blood vessels as it goes.
Coagulation mode works differently. Instead of continuous output, the device pulses the current on and off. During the brief “on” periods, the voltage is high enough to create tiny electrical arcs that jump from the electrode tip across a small air gap to the tissue surface. These arcs spread heat over a wider area at lower power density, raising tissue temperature enough to seal vessels and stop bleeding without cutting through. A “spray coagulation” setting pushes this further, creating an arc that deposits a thin layer of eschar over a bleeding surface. Many units also offer a “blend” mode that mixes characteristics of both, giving surgeons a middle ground between pure cutting and pure coagulation.
Monopolar and Bipolar Configurations
ESUs operate in one of two circuit configurations, and the choice matters for both surgical technique and patient safety.
In monopolar electrosurgery, the active electrode is the small tip at the surgical site. Current flows from that tip, through the patient’s body, and exits through a large adhesive pad (called a return electrode or dispersive pad) placed on the patient’s skin, usually on the thigh or back. Because the return pad has a large surface area, the current density there is very low, so no heating occurs at that site. All the thermal effect concentrates at the tiny active electrode where the surgeon is working. Monopolar mode is the most versatile and widely used configuration.
In bipolar electrosurgery, both the active and return electrodes are built into the same instrument, typically a pair of forceps. Current passes only between the two tips, through the small piece of tissue grasped between them. This means no current travels through the rest of the patient’s body, and no return pad is needed. Bipolar instruments are especially useful for delicate procedures near sensitive structures, such as operations on the brain, eyes, or around nerves, where stray current could cause serious damage.
Risks of Electrosurgery
While ESUs are reliable and well-established, they carry specific risks that surgical teams actively manage. Three complications deserve attention, particularly during laparoscopic (keyhole) surgery where the surgeon’s view of surrounding tissue is limited.
Insulation failure occurs when the protective coating on a monopolar instrument develops a crack or defect. Current can leak through the damaged spot and burn tissue the surgeon can’t see. During abdominal laparoscopy, this is a major cause of unintended bowel injury, which can lead to delayed perforation and peritonitis, a potentially life-threatening infection.
Direct coupling happens when the active electrode accidentally touches another metal instrument during surgery, transferring current to an unintended location.
Capacitive coupling is more subtle. Even with intact insulation, electrical energy can transfer from the active electrode through the insulation to nearby conductive material without any direct contact. During laparoscopic surgery, this can cause the current to accumulate at the skin around the trocar (the small tube inserted through the abdominal wall), resulting in burns at the insertion site.
Surgical Smoke: A Hidden Hazard
Every time an ESU heats tissue, it generates a visible plume of smoke. This isn’t just an annoyance. Surgical smoke contains a cocktail of hydrocarbons, fatty acids, and chemicals including formaldehyde, benzene, toluene, and polyaromatic hydrocarbons, several of which are known carcinogens. The toxicity of this plume is comparable to cigarette smoke. Destroying just one gram of tissue with electrosurgery produces toxic exposure equivalent to three to six cigarettes, depending on the technique used.
Beyond chemical toxins, surgical smoke can contain viable biological material. Blood fragments, bacteria (including Staphylococcus), and viruses have been recovered from the plume. Multiple cases of human papillomavirus (HPV) transmission from patient to surgeon have been documented. Standard surgical masks filter out more than 90% of smoke particles, but they don’t seal tightly against the face. N95 respirators offer better protection. Smoke evacuation systems, which suction the plume away from the surgical field, are the most effective safeguard, though many operating rooms still don’t use them consistently. The Association of periOperative Registered Nurses (AORN) issued updated guidelines in 2025 specifically addressing surgical smoke evacuation and filtration.
Special Precautions for Patients With Implanted Devices
Patients with pacemakers, implantable defibrillators, or cardiac resynchronization devices require additional planning before any procedure involving an ESU. The radiofrequency energy from electrosurgery can cause electromagnetic interference with these devices, potentially disrupting heart rhythm management. Surgical teams follow specific protocols that include preoperative assessment of the device, adjustments to the device settings before surgery, and strategic choices about which electrosurgical mode to use. Bipolar instruments are generally preferred in these patients because the current path is confined to a small area and doesn’t travel through the chest.
Why ESUs Replaced Traditional Cutting in Many Procedures
The ability to cut and control bleeding simultaneously gives ESUs a significant advantage over cold steel instruments. Before electrosurgery became widespread, surgeons relied on ligatures (tied-off sutures) and manual pressure to control bleeding, which added time and complexity to every procedure. An ESU lets a surgeon move through vascular tissue quickly while maintaining a clear operative field. In laparoscopic and minimally invasive surgery, where access is limited and manual pressure isn’t practical, electrosurgery is often the only feasible way to manage bleeding in real time. The trade-off is the set of electrical risks described above, which is why proper equipment inspection, correct pad placement, and smoke management are standard parts of every surgical team’s routine.