Risk of Electrosurgical Burns at Needle Electrode Sites
Hazard [Health Devices Aug-Sep 1994;23(8-9):273-4]
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.
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.
- 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
- In any electrosurgical
procedure, use the lowest power output setting that provides the desired
- 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.
- 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.
- Becker CM, Malhotra IV, Hedley-Whyte J. The distribution of
radiofrequency current and burns. Anesthesiology 1973
- 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