In a routine case of cardiac ablation, an ablation catheter electrode is used to ablate various target sites in the heart and a dispersive electrode is placed on the patient's body. Both are connected to an electrosurgical unit (ESU) that provides a radio-frequency (RF) current (typically 0.5 to 1 MHz) to ablate the tissue in contact with the active electrode. In the case presented here, the patient received a severe electrosurgical burn associated with the single dispersive electrode positioned on the right-upper back over the scapular bone.
The procedure was performed to treat atrial fibrillation, which usually involves ablation of numerous neurological target sites in the atrium, resulting in multiple ablation sessions and lengthy ablation times. The patient had an RF current approaching one ampere (high, in electrosurgical terms) passing from the catheter electrode in the heart to the dispersive electrode on the back for nearly an hour and a half. Many of the ablation sessions lasted five minutes, with very short rest periods in between. After the ablation was completed, a significant skin burn was discovered on the patient's back, along the medial edge of the dispersive electrode pad. ECRI Institute tested the ESU generator and the incident pad, neither of which exhibited any significant malfunctions.
In reviewing the ablation data for the procedure, ECRI Institute found that the patient had been exposed to a high RF current for a long period of time, cumulatively. Calculations demonstrated that the exposure far exceeded the current dispersing and heat-dissipation capacity of a typical, adult-size dispersive electrode. Given the RF exposure, it was not surprising that a skin burn had occurred.
To minimize the risk of electrosurgical burns at the dispersive electrode, there are two main rules to follow, says Bruce C. Hansel, PhD, CCE, executive director of ECRI Institute's accident and forensic investigation services: "First, minimize the current density and duration, and second, optimize the placement of the dispersive electrode."
The current density flowing through the skin is a function of the dispersive pad surface area and the amount of current passing through the dispersive electrode. In cases where high levels of current and extended activation times are expected, more than one dispersive electrode may be needed to effectively disperse the current and prevent skin injury. Furthermore, users should always use the lowest ESU power setting sufficient to accomplish the treatment.
Optimizing dispersive electrode placement primarily involves identifying the best location for the electrode(s) and then ensuring good adherence to the patient's skin. When possible, dispersive electrodes should be placed on non-weight-bearing skin surfaces where they can be examined during the procedure. Placing the dispersive pads on load-bearing skin surfaces reduces heat loss to the environment and also diminishes heat loss by capillary blood flow through the tissue—each contributing to heating that could be avoided. Dispersive pads should be placed over areas with underlying large muscles, such as the lower back, abdomen, flank, or thighs. Although it is not uncommon for dispersive electrodes to be placed on a patient's back during cardiac RF ablation, positioning the electrode over large bones, such as the scapula, is not optimal and is counter to generally accepted practices for dispersive electrode placement.
Furthermore, dispersive electrodes should not be placed in close proximity to the active electrode because contiguous placement can result in insufficient current dispersion. In this case, the proximity of the pad to the active electrode in the heart and short distance between the active electrode and dispersive pad likely hindered the current from being dispersed uniformly across the entire surface of the pad.
"Although it is not uncommon to use a single dispersive electrode on the patient's back during cardiac ablation, ECRI Institute recommends using two dispersive electrodes—one on each anterior thigh, where there is better underlying muscle mass, less heat entrapment, and better electrode accessibility—during the procedure," says Hansel. "This placement does not guarantee that a burn cannot occur, but it does lessen the risk and allow clinicians to monitor the area, if needed, during the procedure."
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