Electrocautery devices: The way they work

Key Points

A better grasp of the basic design theory of cautery-the use of a hot instrument to destroy tissue-will help you perform electrosurgery more safely. The scientist William Bovie didn't invent cautery, which dates back to the Egyptians in 3,000 BC. But he and neurosurgeon Harvey Cushing did develop an electrocautery unit that could be used easily in the operating room and that could reliably cut through and coagulate tissue.

Cautery has come a long way in performance and reliability since Bovie's device was introduced in 1920 and used to remove a brain tumor in 1926, but the basic design theory is unchanged. How do these units, which we take for granted, really work? What differentiates a unipolar or bipolar "Bovie," a laser, a Harmonic Scalpel (Ethicon Endo-Surgery, Inc., Cincinnati, Ohio), and LigaSure (ValleyLab, Boulder, Colo.)? To answer these questions, we'll start with a little information about human cell physiology and about electricity.

Heating things up

When you heat a cell above its normal physiologic temperature range, changes begin to occur. How you heat a cell determines which thermal effect you'll produce. Usually, heating a cell up to 45°C won't cause permanent damage or prevent cellular function. At that point, cells can recover. Above that temperature, permanent damage does occur. Between 45°C and 60°C, cellular proteins denature and cells die.

If you slowly continue heating a cell to 90°C, the intracellular water will slowly vaporize, desiccating the cell. When a cell is heated to 100°C, the intracellular water turns into water vapor, expanding the cell and putting excessive pressure on the cell membrane. When expansion is rapid, the cell can't dissipate the increased forces, and it ruptures. That's the basic principle behind all cautery devices

Transfer of energy from electricity

Newer cautery devices produce heat by transferring energy from electricity, following Joule's law:

Energy = current density* (squared) X resistance X time

(*Current density is defined as the current divided by the cross-sectional area.)

Applying Joule's law to our cautery units results in four main variables related to performance:1. The amount of energy delivered to the tissue,
2. The amount of time the energy is being delivered,
3. The area over which the energy is being delivered, and
4. The composition of the tissue being heated/thermal spread.

Three criteria for judging a device

It's intuitive that you can get a greater tissue effect using the most energy delivered to as small an area as possible. The amount of time over which the energy is delivered is also important. A defibrillator can deliver 400 joules of energy to a patient in a fraction of a second. A 500-W laser switched on for a nanosecond can burn a hole through a wall. A 500-W lightbulb switched on for 8 seconds will deliver the same amount of energy, but less dramatically. How we impart the energy is the determining factor in what we want to achieve surgically. But how we impart it can also create negative side effects that we want to avoid.