Another case of electromagnetic interference in the operating theatre

Abstract

Electromagnetic interference in the operating theatre can be a hazard for patients. Understanding of the phenomena that can lead to it is of paramount importance. I highlight a form of interference that to my knowledge has not been reported before and which posed a risk of injury to the patient. A 61-year-old male patient with non-insulin-dependent diabetes mellitus was anaesthetised for evacuation of a right chronic subdural haematoma. There was no focal deterioration and the Glasgow Coma Scale (GCS) was 14–15. The anaesthetic technique consisted of induction with propofol, fentanyl and atracurium and maintenance of anaesthesia with a mixture of nitrous oxide and isoflurane in oxygen. Neuromuscular blockade was maintained with incremental doses of atracurium, as indicated by monitoring of the neuromuscular junction. The peripheral nerve stimulator used was a Dakmed 750 Digital, attached just below the lateral aspect of the right knee, over the common peroneal nerve. Train-of-four (TOF) stimulation was applied on demand for the assessment of neuromuscular blockade. During the intervals between testing, the stimulator was switched on but idle. The diathermy device used was a Valleylab Force 2 CEM Electrosurgical Generator. The indifferent electrode paddle was sited at the left thigh. During the course of the anaesthetic, it was noticed that the right foot was jerking vigorously and the peripheral nerve stimulator was discharging in an erratic way. Just prior to that, TOF stimulation had shown clinical recovery of only the first twitch and no additional muscle relaxant was used, because the operation was nearing an end. Triggering of chaotic peripheral nerve stimulator activation coincided with activation of the monopolar diathermy. On closer observation, it was noticed that the cable of the monopolar probe of the diathermy was lying behind the stimulator, almost in contact with it. Activation of the diathermy after the diathermy cable was moved away from the stimulator did not affect the stimulator or cause jerking of the foot; the malfunction of the stimulator and movements of the foot recurred if the cable was returned to its original position. Interference between diathermy equipment and almost any electronic device in use in the theatre environment has been described before. There is extensive literature on the effects and safety of pacemakers and implantable cardioverter-defibrillators when diathermy is used [1]. Interference between a peripheral nerve stimulator and pacemaker that led to cardiac arrest has been described [2]. In this case, the unintentional proximity of the cable of the monopolar diathermy with the peripheral nerve stimulator caused uncontrolled discharge from the stimulator. The vigorous jerking of the foot may have several explanations, one of which is post-tetanic potentiation. I am not able to say if the voltage output of the stimulator was also increased during the incident (transformer-like inductive coupling effect could explain such a phenomenon). The fact that neuromuscular function was recovering facilitated the diagnosis; however, we do not know if harmful current was applied over the peroneal nerve during earlier phases of the operation. Prolonged application of an excessive voltage over a nerve could result in transient or permanent nerve damage. The diathermy device uses frequencies of 0.5–10 MHz, well above the 100 Hz tetanic stimulus used by a peripheral nerve stimulator. This frequency could be applied indirectly by inductive coupling to the nerve stimulator and then directly to the nerve through the stimulator leads. We do not know what are the effects of prolonged application of such frequencies to human nerves. Clinical examination revealed no damage to the patient's peroneal nerve. Nerve conduction studies were not performed. This incident is an example of a potentially harmful event in the operating theatre environment. Electromagnetic interference is ‘invisible’ even to an otherwise experienced eye and can have catastrophic results [3]. Diathermy cables should always be kept well separated from any other electronic equipment. Separation criteria should be followed for all electronic devices [4]. Special care should be taken in the presence of implantable electronic devices. At present, health workers are not trained to be aware of problems arising due to electromagnetic interference. Standards on electromagnetic compatibility and interference prevention of electronic devices exist, i.e. the European standard EN55011 [5], but more attention needs to be given in healthcare practice and planning to minimise risks.