Background
Early surgical intervention after brachial plexus injury is the best predictor of
a favourable functional outcome after a trial of conservative management. Electrodiagnostic
studies like sensory evoked potentials (SEP), electromyography (EMG) and nerve compound
action potentials (NCAPs) are performed intraoperatively to aid in monitoring, guiding,
identifying and localizing nerve function.[[1]] Though these diagnostic modalities have contributed immensely to the improved surgical
outcomes following brachial plexus repair, their use may prove cumbersome and prone
to errors of interpretation. Direct observation of muscle belly contraction after
nerve stimulation remains the gold standard to detect intact neuronal function.
Phrenic nerve identification is a key step during the supraclavicular approach for
brachial plexus surgery. Capnography is a technique to record end-tidal carbon dioxide
(ETCO2) and is one of the standards of monitoring in anesthetic care. The authors describe
the use of capnography as an aid in the intraoperative localization of the phrenic
nerve.
Methods
Three adult patients with diagnosed traction panbrachial plexus lesions were scheduled
for supra and infraclavicular exploration and neurotization of the suprascapular,
axillary and musculocutaneous nerves.
The general anaesthetic technique was tailored to allow intraoperative electrophysiological
techniques to guide the localization and repair of the injured nerves. Consequently,
muscle relaxants were avoided. Anaesthesia was induced with propofol and fentanyl
and the airway was secured with a laryngeal mask. Anaesthesia was maintained with
a propofol infusion and intermittent boluses of fentanyl. Routine monitoring included
heart rate, non-invasive blood pressure, pulse oximetry and time capnography.
Supraclavicular exploration was commenced in the supine position with the head extended
and turned to the opposite side and the injured arm in an adducted position. The skin
incision was extended inferiorly over the lower 1/3rd of the posterior border of the clavicular head of the sternocleidomastoid and then
curved laterally over the medial 2/3rd of the superior surface of the clavicle. The platysma was incised and the supraclavicular
pad of fat was dissected sharply under the microscope away from the carotid sheath
and the subclavian vein and retracted posterolaterally. The omohyoid bellies were
then identified and their common tendinous insertion was divided between ligatures.
The Scalenus anticus was then sought as the musculofascial structure behind the phrenic
nerve. In view of the extensive scarring, the visual identification of the phrenic
nerve was not possible at first. Hence, blind bipolar electrical stimulation using
a handheld bipolar nerve stimulator was used to localize the same by eliciting diaphragmatic
contraction. The nerve stimulator was initially used at low amplitude (1 mA) and the
capnographic wave form was observed. The changes in waveform were monitored by the
neuroanesthetist as the stimulating current was gradually increased. Simultaneously,
the presence of diaphragmatic contraction was judged by the surgical assistant with
his hand placed over the patient’s draped epigastrium. Both the neuroanesthetist and
the surgical assistant were blinded as to when the bipolar stimulating electrode was
actually in use. Once the phrenic nerve was approximately localized, sharp dissection
was commenced to identify the same.
Results
In all patients, the capnographic wave form revealed a notch at a low electrical stimulating
current of about 2–4 mA. This became progressively jagged with increasing current
strengths till diaphragmatic contraction could be palpated by the blinded surgical
assistant at about twice the amplitude (6–7 mA). ([Fig 1])
Figure 1
Fused capnograms as seen on the patient monitor. The top row is normal. After electrical stimulation, the middle row reveals progressive
notching of the wave form (subclinical diaphragmatic contraction) which degenerates
into frank spikes with increasing current corresponding to palpable diaphragmatic
contractions. The progressive drop in end tidal CO2 from a baseline of 39 mm Hg to 31 mm Hg is noteworthy.
Discussion
Brachial plexus lesions most frequently affect the supraclavicular region rather than
the retroclavicular or infraclavicular levels.[[2]] Hence, the supraclavicular approach is the most commonly performed for traumatic
brachial plexus repair. Intraplexal and extraplexal nerve-transfers are increasingly
being utilized for brachial plexus reconstruction aimed at restoring elbow flexion
and shoulder abduction.[[3]] Commonly used donor nerves are the thoracic intercostals, the medial pectoral,
the phrenic and the spinal accessory nerves.
Intraoperative monitoring of nerve repair using electrodiagnostic techniques aids
the surgeon in the dissection, identification and localization of nerves and also
helps in assessing nerve function. Electrodiagnosis proves valuable, more so, in a
setting of extensive fibrosis in the supraclavicular compartment frequently encountered
after traction brachial plexus injuries. This makes identification of the Scalenus
anticus, behind which the C5 and C6 nerve roots lie, very difficult especially when
this key muscle is fibrosed and merges with the surrounding neuroma. The muscle is
then indirectly identified as the tissue lying behind the phrenic nerve. The phrenic
nerve is the only structure in the medial supraclavicular area which passes from lateral
to medial. Thus, phrenic nerve identification is the crucial initial step in the supraclavicular
approach for brachial plexus repair. Direct visualization may not be possible even
under high magnification as the phrenic nerve too is often encased by scar tissue.
Hence, blind stimulation using a hand held bipolar electrical stimulator and judging
the contractile response of the diaphragm manually is a useful aid in initial localization
before attempting scar tissue release with sharp dissection.
Other surgical techniques to identify the phrenic nerve include following the supraclavicular
nerve proximally till the C4 root in order to identify the Phrenic nerve.[[4]] However, most brachial plexus surgeons prefer to use intraoperative electrical
stimulation.
Monitoring phrenic nerve stimulation using lower chest wall electrodes may produce
false-positive results due to co-activation of the brachial plexus.[[5],[6]] Other possible technical problems include overstimulation, stimulus artifacts,
electrical noise, and high recording electrode impedance which may diminish reliability
and increase the duration of the procedure.[[7]]
On the other hand, capnography is a routine and mandatory anaesthetic monitoring device.
In this study, a notch in the capnograph could be obtained at a lower stimulus intensity
than palpable diaphragmatic contraction. Phrenic nerve stimulation in an anesthetized
non-paralyzed patient produces sub-clinical diaphragmatic contraction which mimics
inspiration. This produces a drop in ETC02 which is reflected as a notch on the time capnograph. These observations were similar
and reproducible in all the three patients. The authors could not come across any
report in medical literature utilizing this attribute of capnography as an indicator
of phrenic nerve stimulation in brachial plexus surgery in a non-paralysed patient.
Electromyographic electrode placement to detect phrenic nerve activity may also be
affected by concurrent stimulation of the other intraplexal nerves such as the thoracodorsal.[[8]] A notch in the capnogram, however, cannot be produced upon stimulation of the brachial
plexus, thereby rendering this technique not only highly sensitive but also highly
specific. The phrenic nerve also has a large number of motor axons and thus serves
as an excellent donor nerve.[[9]] Capnography thus may help prevent inadvertent damage to the same by alerting the
surgeon to its presence in difficult cases with extensive scarring.
Conclusion
Capnography is a sensitive intraoperative test for localizing the phrenic nerve during
the supraclavicular approach to the brachial plexus.
Competing interests
Financial competing interests
In the past five years have you received reimbursements, fees, funding, or salary
from an organization that may in any way gain or lose financially from the publication
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No.
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financially from the publication of this manuscript, either now or in the future?
If so, please specify.
No.
Do you hold or are you currently applying for any patents relating to the content
of the manuscript? Have you received reimbursements, fees, funding, or salary from
an organization that holds or has applied for patents relating to the content of the
manuscript? If so, please specify.
No.
Do you have any other financial competing interests? If so, please specify.
No.
Non-financial competing interests
Are there any non-financial competing interests (political, personal, religious, ideological,
academic, intellectual, commercial or any other) to declare in relation to this manuscript?
If so, please specify.
No.
Authors’ contributions
HB made the original observation on the capnograph, was the blinded anesthetist and
helped edit the manuscript. AA wrote the manuscript’s first draft, carried out the
literature search and was a blinded anesthetist. MSS conceived the concept, elucidated
the methodology and edited the manuscript.
