ELECTRO SURGICAL GENERATOR TESTING

Richard Smith is a service engineer with Unitech where he encounters many diathermy units but Unitech does not market or use any tester other than that composed of the components as specified in IEC-601-2-2. With the widespread use of Electrosurgical Generators in the Operating Theatre, it is vital that the Biomedical Engineer understands the need to test these generators from both an operational and safety standpoint. Richard Smith is familiar with the intricacies of diathermy testing.

Introduction:

Electrosurgery is defined as the passage of high-frequency (or radio frequency) current through the tissue for the purpose of cutting or coagulating. It combines the principles of electricity and cautery. Cautery uses heat or caustic substances to destroy tissue (necrosis) or coagulate blood (haemostasis).

As early as 3000 B.C. ancient physicians treated battle wounds with heated stones or swords and cauterised skin ulcers and breast tumours.

Hippocrates (460 - 370 B.C.) cauterised wounds to destroy abscessed growths and stop bleeding.

Cautery first merged with electricity in the nineteenth century with the invention of the electrocautery device. Instead of using heated irons, surgeons could now cauterise with a wire loop heated electrically. However, in electrocautery, the current flows through the instrument and not through the patient. The first documented case of both coagulation and cutting by passing high frequency current through the body tissue dates from 1910. AM radio operates from 550 kHz to 1.6 MHz. Because neuromuscular stimulation ceases at approx. 100 KHz, electrosurgery can operate safely at 'radio' frequencies. At these frequencies, the body will feel only the heat generated during surgery and not the current that at lower frequencies would cause neuromuscular stimulation.

During the 1920s, electrophysicist William Bovie, and neurosurgeon Harvey Cushing collaborated in the development of modern electrosurgery. From 1926 to 1968, all electrosurgical generators, now nicknamed for their inventor, essentially followed Bovie's design, though to this day, the principle of operation remains unchanged.

Most modern electrosurgical procedures today are 'monopolar' procedures, i.e. the current flows from the generator to an active electrode, through the patient to a return plate and then back to the generator.

At present the safety of high frequency surgical equipment is specified in IEC 601-2-2, 1991. The most relevant tests in this standard include:

1 : Output power measurement

2: High frequency leakage current measurement

3: Low frequency leakage current measurement

4: Return electrode monitoring circuit verification

The test equipment used to verify compliance of the above is:

Keithley 2001 wideband True RMS. multimeter (Bandwidth - 2MHz)

Pearson 411 wideband current transformer (0. 1 volts per amp bandwidth - 5MHz)

Dale NH250 non inductive 250 watt resistors (reactive phase angle < 20 degrees @ 750 KHz)

Sifam isolated 0 - 250mA RF milliammeter 0 0 - I K Multi turn potentiometer

30pF 6KV capacitor 0 IEC 601-1 safety tester

Power output is measured by placing a relevant load resistance across the active output and measuring the current via the current transformer. This should be carried out for a range of impedance's from 50 to 2000 ohms for monopolar output and from 10 to 1000 ohms for bipolar output. The resultant power curve can be a very useful diagnostic aid if for example there was a complaint of poor cutting performance on obese patients. In this case the power curve would graphically show the high impedance roll off point possibly occurring too early.

High frequency leakage current measurement is carried out by measuring the current through a 200 ohm resistor from each individual output to ground. This is done to ensure that HF leakage does not exceed 150 mA, which could result in 'other site' burns. The Standard specifies that this test be carried out on an 'RF leakage' table with all patient leads extended. Since this is quite cumbersome, it has been found that a 'field approximation' can be achieved by introducing a series 30pF 6KV capacitor into the circuit. If a suitable capacitor is not available, the test can be carried out directly from the output terminals, with a maximum allowable leakage current of 100mA. If this method is used, it is advised to use the shortest leads possible to increase reproducibility.

Low frequency leakage current is measured using a readily available safety tester, bearing in mind that according to IEC 601-2-2, 1991 Section 3, para 19.3

"for type BF equipment the limit for the patient auxiliary current of 1OuA is replaced by 100uA, provided it has a frequency of not less than 0. 1 Hz "

Return electrode monitoring circuits vary from one manufacturer to another, but basically, they all cut out when an interruption of the return electrode connection or its cable occurs. This is easily verified by introducing test impedance's according to manufacturers specifications in series with the return electrode.

Testing electrosurgical generators is not best performed by putting a bar of soap on a return plate and activating the output to see if there are sparks. It is therefore in the biomedical engineers best interest to be able to reliably test their electrosurgical generators (or at least have them tested by the supplier) on a regular basis. There are many commercial testers available on the market. In a preliminary study, two such systems were investigated: the METRON QAES and the DYNATECH NEVADA tester. The generators used were a Valleylab Force 2 and an Eschmann TD 411 -S. In the case of the Eschmann generator, the testers were compared to the Eschmann tester, and in the case of the Valleylab generator, the system described above was used.

Since the testing of a generator can take some time, the evaluation was only carried out for monopolar output levels.

Table 1 - Note: Load impedance = 400 ohms On the Eschmann tester there is no possibility to change load impedance.

Table 2 - Note: Load impedance = 300 ohms

While there are no test results for High Frequency leakage, and Return Electrode Monitor testing, both the Metron QA ES and the Dynatech Nevada testers have these facilities. The Metron QA ES has an in-built range of load impedance's up to 5200. The Dynatech Nevada has an in-built range up to 1550, but an add-on module can be purchased to increase this.

These results are not an indication of the performance of a particular electrosurgical generator nor are they conclusive regarding the performance of one generator tester against another, but they do give an indication of the ability of each tester to measure RF output from an electrosurgical generator.

If, as a Biomedical Engineer you feel that you need to purchase an Electrosurgical Generator tester, you should consider the following points

How much time can you devote to generator testing;

Are there enough generators in the hospital to warrant the purchase of a tester;

Would a service contract with the generator supplier be more cost effective;

Would generator repair be a requirement once test equipment is purchased.

If after considering these points, it is still felt that a hospital owned tester is the best option, then 1 would strongly recommend evaluating the testers yourself, so that you can get the one that best suits your requirements.