Cardiac Monitoring

Equipment is prone to interference from all electrosurgery units due to the principle on which each type of equipment is based. The cardioscope unit detects low frequency voltages millivolts, whereas the electrosurgery generator is using very high voltages to give the cutting/fulgurating effect kilovolts. Whilst the generator works at a much higher frequency than the cardioscope detects, the generator wave-form does include lower frequencies due to the sparking to the tissue (and hence some rectification by the body to produce the low volts). If there is no sparking to the tissue, as in the case of desiccation, then interference to the cardioscope will in most cases be removed.

Many new cardiac monitors "filter out" most of the effects of the generator but there may still be output traces which are blamed on the electrosurgery unit. Normally, the trace will only be affected by a sparking output where the generator is activated.

Any interference should be checked as to its source -

1. Is it continuous or is it only when the unit is sparking to tissue?
2. If continuous, does it take the form of a blurred trace or thickened trace? If the trace is continuous, the interference is electrical and the Hospital Engineering Department should be asked to check all electrical equipment, starting with the suspect equipment first, to see if the grounding is defective. If the interference is only when the unit is activated, the following points should be checked -

• a. Check active and return cables and connections to ensure that there is no metal to metal break (and hence sparking).
• b. Have the Engineer ground the patient plate with a rest cable. If this reduces interference, it indicates a problem with the ECG unit which may need re-balancing.
• c. If the generator is activated but not touching the patient, there will be no sparks and hence no low frequencies will be produced. The implication is therefore that the ECG is responding to radio-frequencies (this will be especially true with less efficient and older ECG units).

Various manufacturers produce RF choke filters which are connected into the ECG leads. In many cases, they remove most of the interference caused by the generators, as in point (c).