Netherlands Journal of Critical Care
Submitted March 2015, accepted July 2015
CLINICAL IMAGE
Electrical burns caused by a train-of-four monitor
W.A.C. Koekkoek, D.H.T. Tjan
Intensive Care Gelderse Vallei Hospital, Ede, the Netherlands
Correspondence
WAC Koekkoek – [email protected]
Keywords - burn, train-of-four, TOF
Abstract
A 56-year-old Caucasian female was admitted to our ICU
because of acute respiratory failure due to severe pneumococcal
pneumonia. She was intubated and mechanically ventilated.
Due to progressive respiratory deterioration culminating in
arterial hypoxaemia and hypercapnia, the patient was turned
into the prone position to improve oxygenation. The patient
was sedated to a Richmond agitation and sedation score of
-4. Neuromuscular blocking agents (NMBAs) were started
within two hours of the diagnosis of acute respiratory distress
syndrome (ARDS), as this treatment may improve mortality
and decrease the duration of mechanical ventilation and
complications related to barotraumas in ARDS patients when
started early and continued for 48 hours.1
To assess the depth of neuromuscular blockade and ensure
proper medication dosing, a train-of-four (TOF) monitor was
used. The model is TOF-Watch®. The electrodes are attached
to the skin on the internal surface of the wrist, along the
course of the ulnar nerve. The contact area of the stimulating
electrodes is 7 mm (figure 1). The nerve is stimulated by four
successive stimuli delivered at 2 Hz with a current of 50 mA.
With increasing degrees of block, the twitches in the TOF
progressively fade starting with the fourth and eventually
disappear one by one. The ratio of the height of the fourth
response to the first has been defined as the TOF ratio. The
optimum level of neuromuscular blockade is reached when
80-90% of receptors are blocked; this corresponds with 1 or 2
twitches on TOF testing. When using continuous infusion of
NMBAs, TOF monitoring is recommended for neuromuscular
monitoring. A baseline measurement is started and thereafter
performed every 15 minutes until adequate depth of muscle
relaxation has been achieved, after which the measurement is
performed every four hours.
Figure 2. Second-degree burns on the wrist caused by peripheral
Figure 1. A TOF-watch
stimulation of the ulnar nerve
NETH J CRIT CARE - VOLUME 22 - NO 4 - SEPTEMBER 2015
21
Netherlands Journal of Critical Care
Electrical burns caused by a train-of-four monitor
In our patient TOF monitoring was used. However, 40 hours
after the start of NMBAs, second-degree burns were observed
on the wrist with superficial skin necrosis (figure 2). The
TOF monitor was immediately removed and neuromuscular
blockade was discontinued. The patient had been ventilated
in the prone position for 38 hours and had been turned into a
supine position two hours before discontinuation of NMBAs.
Electrical burns of the skin caused by a peripheral nerve
stimulator has been reported previously in one publication.2
However, in the past 40 years no additional cases have been
reported even though various operating manuals of TOF
monitors warn for electrical burns in case of high-current
intensity or tetanic stimulation. The burns on the wrist of our
patient seemed to be caused by the repetitive electrical pulses
sent by the TOF monitor. Pressure sores seemed unlikely as the
patient had been treated in prone position in an air fluidised
bed with arms alongside the chest and abdomen with palms
facing upwards. An allergic reaction was also unlikely as the
same electrodes were used for heart rhythm monitoring.
The reported current level was 50 mA and measurement was
performed every four hours at a frequency of 2 Hz according to
the protocol. The causes of the electrical burns in our patient
may be monitor malfunctioning with uncontrolled current
intensity or a too rapid firing frequency. Another possible
cause is high skin resistance due to dry skin or the presence of
preliminary scar tissue at the contact area of the electrodes. As
defined by Joule’s law, the higher the skin resistance the more
heat is released in the skin which causes skin injury. However,
the TOF monitor’s alarm for high skin resistance did not go
off and no malfunctioning of the monitor was found on reexamination of the device. The wound was cleaned and the skin
was viable, so no surgical exploration was needed. The burns
were treated with sulfadiazine cream. The patient’s skin healed
completely within 12 weeks.
TOF monitoring is frequently used in the ICU to assess the
depth of neuromuscular blockade. Electrical burns are a rare,
but serious complication. We therefore advise to regularly check
the site of the TOF monitor for burns and the TOF monitor
itself for malfunctioning by assessing the electrical pulse
interval and the current intensity. Immediate discontinuation of
TOF monitoring is advised.
Disclosure
All authors declare no conflict of interest. No funding or
financial support was received.
References
1.Papazian L, Forel JM, Gacouin A, et al. Neuromuscular blockers in early acute
respiratory distress syndrome. N Engl J Med. 2010;363:1107-16
2.Lippmann M, Fields WA. Burns of the skin caused by a peripheral-nerve stimulator.
Anesthesiology. 1974;40:82-4.
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