RF Leakage in Electrosurgical
RF leakage is radio frequency energy that flows from the electrosurgical generator to earth ground rather than returning to the generator as part of the therapeutic path (generator active accessory patient return electrode generator). Theoretically, all therapeutic current delivered by the generator should return to the generator via the patient return electrode. Unfortunately, due to the inherent primary-secondary capacitance of isolation transformers, as well as the construction properties of the generator, some stray leakage inevitably occurs. RF leakage from an electrosurgical generator is of concern to users because an excessive amount can place the patient, the surgeon, and other personnel coming in contact with the patient at risk for an alternate site burn.
Almost all electrosurgical generators sold today have outputs isolated from ground to prevent RF (radio frequency) leakage. By itself, an isolated output is not enough to completely eliminate RF leakage. Other methods must also be used. However, some of these methods can have a dramatic effect on the performance of the generator.
RF Leakage Current Measurement Setup
|1. Supply mains
2. Table, made of insulating material
3. HF surgical equipment
4. Active electrode
5. Neutral electrode, metallic or in contact with metal foil of the same size
6. Measuring resistance, 200
7. HF current meter
8. Earthed conductive plane
The potential hazard from RF leakage was initially recognized almost twenty years ago. To minimize potential injuries, domestic and international safety standards were developed. These standards require that the monopolar RF leakage be less than 150 mA (0.150 Amps) as measured by a well-defined method. The value 150 mA was chosen to be less than the minimum amount of RF current that could cause an alternate site burn given a patient impedance of 200 ohms. The test uses the worst case scenario for RF leakage, which is open circuit activation at the maximum power setting. There is a higher risk of RF leakage with high voltage waveforms such as the fulgurate and spray coagulation modes.
In order to pass leakage standards, manufacturers use several methods of measurement. Some electrosurgical generators are capable of detecting an open circuit condition and, after a finite period of time, lower their peak output voltage. Since it is the peak output voltage that initiates the sparking required for the fulgurate and spray coagulation effect, using this leakage control mechanism could potentially degrade generator performance. Due to the amount of time the generator needs to sense the open circuit condition, momentary spikes of voltage can occur. As the surgeon moves through gaps in the tissue or when the active electrode is withdrawn from the tissue, high voltage spikes can momentarily cause the RF leakage current to exceed the recommended specification.
To minimize RF leakage current, Valleylab generators modify the waveform duty cycle that is, the amount of time the RF energy is on versus the time it is off. This controls excessive RF leakage while retaining peak output voltage and the desired clinical effect. By continuously monitoring the electrical system for open circuit or excessive high voltage conditions using advanced electrical circuit techniques, Valleylab generators achieve low RF leakage current while maintaining the desired surgical effect.
The monopolar cut and bipolar modes of the Valleylab Force FX with Instant Response technology are handled differently from the monopolar coagulation modes. These Instant Response controlled modes, by design, control RF leakage by the precise regulation of all three output parameters: current, power, and voltage. Unlike generators from other manufacturers, a separate RF leakage control system is not required to meet all of the relevant standards requirements.
The technique used to minimize RF leakage should not compromise the ability of the generator to meet the surgeons expectations. Due to its reduction in coagulating affect, limiting the output voltage may not be the best method for RF leakage control. A careful review of the methods used to reduce RF leakage is important for understanding generator performance. The method of RF leakage control should emphasize both generator performance and personnel safety.
Charles D. Allen has been involved in engineering research and development for over 20 years and currently coordinates electrosurgical product development for Valleylab.