Experimental studyThe local thermal effect of using monopolar electrosurgery in the presence of a deep brain stimulator: Cadaveric studies on a lamb brain
Introduction
Since 1985, the use of deep brain stimulators (DBS) has been gaining ground for the treatment of movement disorders refractory to medical management. In DBS, an electrical current is delivered to target brain tissue via an electrode. The electrical field generated is believed to modify neuronal activity via depolarisation, synaptic inhibition or a combination of both. The current intensity used is in the order of milliamperes. Any external source of energy which increases the delivered current can result in thermal injuries to the target tissue. Documented harmful interactions include medical diathermy and magnetic resonance imaging [1], [2], [3], [4], [5].
Treating patients who have DBS therefore has potential risks, and this dilemma is set to become increasingly common. Over the last three decades an estimated 150,000 patients worldwide have already been treated with (DBS) [6]. Furthermore, the population is ageing. By 2040 an estimated 25% of the population will be over 65, with a corresponding rise in the prevalence of conditions such as Parkinson’s Disease and tremor. In addition, the indications for DBS are increasing, with new targets being identified for treatment of other movement disorders, psychiatric conditions, and medical diagnoses [7], [8], [9].
Medtronic (Medtronic Ltd, Minneapolis, USA) is an industry lead for DBS which has produced safety recommendations. However, many of these are post hoc and a small but growing body of literature documents the various adverse events to date [1], [4], [10]. Incidences of reported damage at the implant site suggest that it is the conversion of energy to heat that causes harm [2], [4], [5]. This has specific implications for the use of diathermy.
There are two distinct types of diathermy: medical diathermy and electrosurgery. Despite the clear differences between the two, both “diathermies” are grouped in the probable harm category of the Medtronic guidelines [11]. The original treatment via heat, or diathermy, was developed by Nagelschmidt in 1907. It employs the diffuse application of heat to an entire tissue bed to promote healing or to aid therapies. Numerous device variations exist, but the essence is transdermally applied, electrically generated shortwave diathermy with power settings of up to 1000 W. The aim is a prolonged increase in temperature of >3 °C, though some authors advocate achieving temperatures of up to 45 °C [12].
Twenty years after Nagelschmidt, William T Bovie and Harvey Cushing developed electrosurgery. Here electric energy flows from an active electrode to a dispersive electrode to cut and to coagulate. In bipolar use, the dispersive electrode is one arm of the forceps. In monopolar use the dispersive electrode is the “diathermy pad” applied at a distal point on the patient. Cutting monopolar employs a constant electrical current in a sine wave configuration. Coagulation monopolar uses a rapid on off sequence to achieve its effects. The energy involved is significantly more for the cutting configuration than for coagulation.
In principle, electrosurgery affects tissue immediately adjacent the active electrode. In bipolar electrosurgery, the current passes through the tissue between the tips of the coagulating forceps. The degree of current spread is small. In monopolar electrosurgery, the excess energy passes from the monopolar device to the dispersive pad. As the energy passes through tissue, a portion of the current is converted to heat. The presence of any conducting material between the monopolar device and the dispersive pad can divert the pathway and act as a current sink. This may result in unintended heating of tissue outwith the surgical target. The use of monopolar electrosurgery in patients with deep brain stimulators could conceivably result in thermal brain injuries. This theoretical risk has resulted in all DBS manufacturers advising against its use.
A review of the safety guidelines, published literature and further personal consultations with representatives of the major DBS manufacturers has failed to provide strong clinical or laboratory evidence supporting this ban. This led us to design a cadaveric model solely to explore the thermal effects of monopolar electrosurgery on the brain in the presence of a DBS system. Our model is not directly applicable to other implanted stimulators such as spinal cord stimulators or cardiac pacemakers.
Section snippets
Methods
The Eschmann E30 electrosurgery unit (Eschmann Holdings Ltd, Lancing UK) is used in our hospital. Technical aspects of use are described in the product manual. Communication with the Eschmann technical team clarified the energy delivery of the settings for monopolar electrosurgery. The information was analysed for its implications for tissue handling and potential heat transmission.
The current safety guidelines for clinicians were obtained from Medtronic, an industry lead for DBS systems.
Results
Multiple exchanges occurred between the authors and the technical teams at Eschmann and Medtronic. The technical aspects of energy delivery for the electrosurgery unit are defined and precisely controlled, but there is significant variability in tissue response to the applied energy. The team was not aware of any experimental evidence quantifying this response. The general guidelines from Medtronic were informed by a combination of experimental data collection and documented adverse events. The
Discussion
The concern for the management of patients with DBS systems is the inadvertent transmission of energy to the hardware with damaging consequences. Energy is measured in joules, where one joule is the energy required to heat 1 g of water by 1° C. On the Eschmann E30 a setting of “10” on monopolar cutting mode equates to 10 J/second, or watts. Delivered energy is therefore defined. However, the effect on tissue is affected by its resistance, or “rated load”. Although this is estimated at 300 Ohms
Conclusion
The risk of thermal injury in the brain when monopolar electrosurgery is used in the presence of deep brain stimulators is less than previously assumed. Further in vivo studies could define safe limits for settings, position, and duration of electrosurgery use in the presence of DBS.
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