Investigating the role of capacitive coupling between the operating table and the return electrode of an electrosurgery unit in the modification of the current density distribution within the patients' body

Abstract

Background: Electrosurgery units are widely employed in modern surgery. Advances in technology have enhanced the safety of these devices, nevertheless, accidental burns are still regularly reported. This study focuses on possible causes of sacral burns as complication of the use of electrosurgery. Burns are caused by local densifications of the current, but the actual pathway of current within patient's body is unknown. Numerical electromagnetic analysis can help in understanding the issue.

Methods: To this aim, an accurate heterogeneous model of human body (including seventy-seven different tissues), electrosurgery electrodes, operating table and mattress was build to resemble a typical surgery condition. The patient lays supine on the mattress with the active electrode placed onto the thorax and the return electrode on his back. Common operating frequencies of electrosurgery units were considered. Finite Difference Time Domain electromagnetic analysis was carried out to compute the spatial distribution of current density within the patient's body. A differential analysis by changing the electrical properties of the operating table from a conductor to an insulator was also performed.

Results: Results revealed that distributed capacitive coupling between patient body and the conductive operating table offers an alternative path to the electrosurgery current. The patient's anatomy, the positioning and the different electromagnetic properties of tissues promote a densification of the current at the head and sacral region. In particular, high values of current density were located behind the sacral bone and beneath the skin. This did not occur in the case of non-conductive operating table.

Conclusion: Results of the simulation highlight the role played from capacitive couplings between the return electrode and the conductive operating table. The concentration of current density may result in an undesired rise in temperature, originating burns in body region far from the electrodes. This outcome is concordant with the type of surgery-related sacral burns reported in literature. Such burns cannot be immediately detected after surgery, but appear later and can be confused with bedsores. In addition, the dosimetric analysis suggests that reducing the capacity coupling between the return electrode and the operating table can decrease or avoid this problem.

Figure 1

Current density distribution (A/m²) within patient’s body (mid-sagittal section) resulted by the FDTD analysis: (a) with a non-conductive operating table; (b) with a conductive operating table considered at ground potential. The active electrode is located about at sternum (note the high current-density spots in both figures) while the return electrode is at the patient’s back. For the reported example a 400 kHz frequency is assumed.

Figure 2

3D views of isosurfaces obtained at different current density thresholds in the case of conductive table: (a) 1500 A/m²; (b) 800 A/m²; (c) 600 A/m². The isosurfaces are colored in gray and represented as opaque, while the different tissues of the patient assume different colors and are represented as semi-transparent. The right lateral edges of the mattress and the operating table appear as green and blue horizontal lines. In (a) a yellow dashed circle surrounds the small isosurface located just behind the sacrum.

Figure 3

Sagittal (a) and coronal (b) sections of the patient representing the difference between the two current densities (A/m²) (with and without conductive table). A high current density spot is evident behind the sacrum. High values of current densities results also in the patient head and in correspondence of the thoracic intervertebral discs (much more conductive than vertebrae).

Figure 4

Qualitative illustration of current distribution inside patient’s body and capacitive coupling with a conductive operating table. Continuous lines draw the primary current path connecting the two electrodes. The capacity between patient’s body and the table (even if it is distributed) is depicted as concentrated capacitors. Capacitive effects are more relevant where the body closer face the table surface (i.e. head, shoulder and pelvis). The dashed lines represent alternative paths for the current generated by the electro surgical device. Note that all capacitive currents must close through the capacitance coupling of the neutral electrode (which can be opportunely reduced). Local values of the current density inside the patient’s body depends on the specific conductivity of the tissues (e.g. sacrum bone is less conductive than surrounding tissues).