Neuromuscular Stimulation The frequency at which most machines operate is approximately 500 kilohertz (KHz) which is part of a wide radio frequency (RF) band width which could be used for electrosurgery. This range starts at 100 KHz and goes up to approximately 5 Megahertz (MHz) with very little difference in the clinical performance of the generators. Output frequencies below 100 KHz are not used because they stimulate muscles and nerves into spasms.

There are certain conditions under which muscle stimulation can take place. This is caused by introduction of the low frequency harmonics into the output to the patient. Most machines are designed to overcome this problem by inbuilt "filters" which remove the low frequencies generated (usually by the fulguration output). The sparking to tissue is a random process and inevitably not every spark hits tissue. The missing sparks cause low DC voltages to be built up across the total RF spectrum including the low frequency. It is these voltages which are filtered out.

However, if the active cable or the return cable are broken and there is a metal to metal break, then a situation occurs in which there is no means of filtering out the low frequency. Checking for cable breaks is therefore a vital procedure to prevent neuromuscular stimulation.

Checkpoints - If Neuromuscular Stimulation is Suspected

(Normally to be carried out by Hospital Electronics Dept.)

Check :

• connections of active cable and return cable.
• continuity of cables.
• for AC mains abnormal leakage currents between :

1. Case to ground.
2. Active patient leads and ground.

• If the mains AC leakage currents are normal, the low frequency filter capacitors in the output circuit should be checked.