Cause of Device-Related Incident
Device factors; Support system failures; User errors

Clinical Specialty or Hospital Department
Anesthesia; Clinical/Biomedical Engineering; OR / Surgery

Device Factors
Device interaction

Document Type
Hazard Reports

External Factors
*Not stated

Mechanism of Injury or Death
Burn (electrical, thermal, chemical); Monitoring failure

Support System Failures
Use of inappropriate devices

Tampering and/or Sabotage
*Not stated

User Errors
Failure to read label; Inappropriate reliance on an automated feature; Incorrect clinical use

UMDNS
Electrodes, Electroencephalographic [11-440]; Electrodes, Electromyographic [11-441]; Electrodes, Neurological [11-453]; Electrosurgical Units [11-490]; Electrosurgical Units, Bipolar [18-230]; Electrosurgical Units, Monopolar [18-229]; Electrosurgical Units, Monopolar/Bipolar [18-231]; Evoked-Potential Units [11-614]

Risk of Electrosurgical Burns at Needle Electrode Sites



Hazard [Health Devices Aug-Sep 1994;23(8-9):273-4]

Problem

We investigated an incident in which a patient received electrosurgical burns at the site of subcutaneous needle electrodes used for monitoring evoked potentials (evoked responses) during the procedure. The burns were caused by radio-frequency (RF) leakage current, which is normally present when an electrosurgical unit (ESU) is operated in the monopolar configuration (i.e., with an active and a dispersive electrode). The leakage current in this case was sufficient to cause a high current density through the needles, which had a small area of tissue contact.

Because of their small contact area, needle electrodes present a serious risk of burns when used in the presence of electrosurgery. Conventional adhesive electrodes, such as those used in ECG monitoring, have sufficient contact area to ensure that the current density at the electrode resulting from RF leakage current does not reach a value that places the patient at risk of a skin burn.

Technical Background

Although needle electrodes are no longer used during intraoperative electrocardiograph monitoring, they are being used during neurosurgical and orthopedic procedures to monitor evoked potentials and the integrity of neural transmission pathways during the course of the surgery. RF leakage current may pass from the patient through the needle electrodes to ground for two reasons: 1) The monitoring circuitry to which the needles are connected may be inadequately isolated at the higher frequencies typical for electrosurgery (300 kHz and higher), despite good isolation at power line frequency. 2) The high-frequency currents used during electrosurgery can flow from the electrode leads to ground through capacitive coupling (e.g., to a grounded cable shield). Even if the cable is disconnected from the monitor, the capacitive coupling to the surroundings may be great enough that leakage to ground may result.

The Association for the Advacement of Medical Instrumentation (AAMI)/American National Standards Insitute (ANSI) standard for electrosurgical devices, the International Electrotechnical Commission (IEC) standard for medical electrical equipment, and ESU manufacturers' product literature typically warn against the use of needle electrodes for monitoring, particularly when monopolar electrosurgery is being performed.(1) Although the current ESU standards restrict the RF leakage that would be accessible to ancillary electrodes on the patient's body surface, their permissible limit is 150 mA. This limit will protect against injury during use of conventional ECG electrodes, but not from needle electrodes. In addition, devices manufactured before the current standards were implemented may present leakage currents that exceed the 150 mA limit. A current density of 100 mA/cm2 for 10 sec is cited as capable of causing a lesion.(2)  Leakage current at 150 mA through a 22 G needle electrode inserted to a depth of ¾ inch results in a current density of 350 mA/cm2 (diameter = 0.71 mm, surface area = 0.43 cm2); leakage current through a 25 G needle inserted to the same depth results in a current density of about 490 mA/cm2. Thus, the risk of a lesion is unacceptably high using needle electrodes.

The conditions that lead to excess ground-seeking RF leakage current differ for the two types of ESUs (i.e., ground referenced and isolated). Generally, with ground-referenced ESUs, excess ground-seeking RF leakage current occurs when the active electrode is in contact with tissue, and it is exacerbated when an excessive length of the dispersive cable is coiled (for reasons of neatness or convenience), thus introducing inductance into the return circuit. On the other hand, with isolated-output ESUs, excess RF leakage current occurs during open-circuit activation (i.e., when the active electrode is not in contact with tissue).

These risks are associated only with the use of monopolar electrosurgery. With bipolar electrosurgical current delivery, lower power outputs are typically used, and both electrodes are positioned at the treated site. Some patient monitoring cables may incorporate RF chokes that mitigate the risk during monopolar surgery. Although unlikely to be a problem, users should verify that chokes (if used) do not reduce the bandwidth necessary to preserve the integrity of the monitored variables.

Recommendations

  1. In any electrosurgical procedure, particularly when monopolar delivery is used, avoid the use of needle electrodes for monitoring or other nonelectrosurgical purposes. Be aware that the risk cannot always be avoided by disconnecting the monitoring cable at the monitor end when electrosurgery is being used. If the use of needle electrodes is deemed necessary, avoid using monopolar electrosurgery following needle placement. Instead, use a scalpel, laser, or bipolar electrosurgery.
  2. In any electrosurgical procedure, use the lowest power output setting that provides the desired surgical effect.
  3. In any electrosurgical procedure, minimize open-circuited activation of isolated-output ESUs; ground-seeking RF leakage currents, which are greatest under this condition, can thereby be easily avoided. Reduce the risk of unavoidable ground-seeking leakage currents from ground-referenced ESUs by refraining from coiling any excess length of dispersive cable.

Notes

  1. Association for the Advancement of Medical Instrumentation (AAMI). American national standard for electrosurgical devices. Arlington (VA): AAMI, 1993; ANSI/AAMI HF18--1993. International Electrotechnical Commission (IEC). Medical electrical equipment, Part 2: Particular requirements for the safety of high frequency surgical equipment [IEC standard]. 2nd ed. Geneva, Switzerland: IEC, 1991;IEC 601-2-2.
  2. Becker CM, Malhotra IV, Hedley-Whyte J. The distribution of radiofrequency current and burns. Anesthesiology 1973 Feb;38(2):106-22.

UMDNS Terms

  • Electrodes, Electroencephalographic [11-440]
  • Electrodes, Electromyographic [11-441]
  • Electrodes, Neurological [11-453]
  • Electrosurgical Units [11-490]
  • Electrosurgical Units, Bipolar [18-230]
  • Electrosurgical Units, Monopolar [18-229]
  • Electrosurgical Units, Monopolar/Bipolar [18-231]
  • Evoked-Potential Units [11-614]

Cause of Device-Related Incident

Device factor: Device interaction

User errors: Failure to read label; Inappropriate reliance on an automated feature; Incorrect clinical use

Support system failure: Use of inappropriate devices

Mechanism of Injury or Death

Burn; Monitoring failure


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