Antenna coupling from monopolar instruments explains unintended thermal injury caused by common O.R. monitoring devices

Nicole T Townsend, MD, Edward Jones, MD, Alessandro Paniccia, MD, Greg V Stiegmann, MD, Thomas N Robinson, MD. University of Colorado.

INTRODUCTION: Unintended thermal injury from patient monitoring devices connected to wires extending off the surgical field (e.g., neuromonitoring leads, EKG pads) is reported in the literature without a clear understanding of this injury’s mechanism. The monopolar “bovie” instrument emits radiofrequency energy that can couple to nearby non-electrically active cables without direct contact by capacitive and antenna coupling. The PURPOSE of this study was to determine if, and to what extent, radiofrequency energy couples to common patient monitoring devices as well define practical steps to minimize the risk of this unintended thermal injury.

METHODS AND PROCEDURES: In a porcine model, monopolar radiofrequency energy was delivered to a handheld ‘bovie’ pencil. Non-electrically active neuro- and cardiac-monitoring leads were placed in proximity to the bovie and its cord. Temperature changes of tissue were measured using a thermal camera at site of patient contact of the monitoring device. The experimental aims altered the relationship of the active electrode’s cord and the patient monitoring device’s cord by changing angulation between the cords and changing the distance of separation of parallel cords. Additional aims included comparing different monopolar generator power settings and comparing alternate energy sources.


CONCLUSION: Stray current couples between the monopolar active electrode and its cord to commonly used patient monitoring devices. The energy transferred can increase tissue temperature by almost 40°C and causes visible thermal injury using standard settings (30 Watts coag mode). The most effective ways to decrease this stray current are decreasing power settings, increasing angulation between wires, increasing distance between wires, and choice of instrument. Antenna coupling may explain the rare but devastating thermal injuries at patient monitoring device sites reported in the literature. Awareness of the interactions between active electrode cord and patient monitoring lead can reduce the risk of unintended thermal injury.

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