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Nerve Monitors and
Peripheral Blockade:
Assuring Optimal Needle Placement
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MALIKAH LATMORE, MD
D. MATT LEVINE, MBCHB
Regional Anesthesia Fellow
St. Luke’s-Roosevelt Hospital Center
New York, New York
Regional Anesthesia Fellow
St. Luke’s-Roosevelt Hospital Center
New York, New York
T
JEFF GADSDEN, MD,
FRCPC, FANZCA
Associate Professor of
Anesthesiology
Duke University Medical Center
Durham, North Carolina
raditionally, the goal of peripheral
the needle are potentially hazardous
nerve blockade (PNB) has been
and can lead to histologic and clinical
to deposit local anesthetic as
nerve injury.1-4 This recognition has
resulted in a paradigm shift during
a successful block. It was once widely
needle placement from “as close as
taught that needle–nerve contact was
possible” to “close enough, but not too
necessary for a reliable block (eg, “no
close.”5 This article reviews the monitors
paresthesia, no anesthesia”). However,
that are currently available to provide
it has become apparent that needle
information about the relationship
puncture, intraneural injection, and
between the needle and the nerve
even simple contact of the nerve with
during nerve block procedures.
d.
close as possible to the nerve to ensure
Dr. Gadsden has consulted for B.Braun Medical Inc. and received honoraria from Cadence Pharmaceuticals.
Drs. Latmore and Levine report no relevant financial conflicts of interest.
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Injection Pressure
Electrical stimulation has been used for decades
as a method for localizing nerves during PNB. Several clinical and animal studies have demonstrated
that the distance from the nerve and the intensity of
motor response are poorly correlated. That is, a needle
tip can be placed intraneurally yet provoke no motor
response.6-8 However, data consistently show that the
presence of a motor response at a very low current
(such as <0.2 mA) during PNB is highly specific for
needle-nerve contact or intraneural needle placement
and is associated with histologic injury.9-12 In other
words, the absence of a motor response is not necessarily reassuring, but if a response is obtained at less
than 0.2 mA, the needle likely is either intraneural or
engaged in the epineurium and should be withdrawn
(Figure 1). As such, we believe the value of nerve stimulation lies less in localization and more in ensuring the
absence of a motor response during PNB.
Opening pressure (OP) is the pressure in the needle–tubing–syringe system immediately before the initiation of flow. It is independent of the dimensions of the
syringe or needle, and is equal from the needle tip to
the plunger. Once flow begins, however, pressure at the
needle tip will vary based on factors including syringe/
needle length and diameter, and compliance of the tubing. An OP greater than 20 psi during the injection of
local anesthetic has been associated with intrafascicular injection and neurologic injury.2 Moreover, an inability to generate flow with a pressure of less than 15 psi
was 97% sensitive for needle–nerve contact in a study
of PNBs.4 As such, it is recommended that high-pressure injection be avoided.
Assessing OP by “hand feel” is both subjective and
unreliable.15 Two objective methods for measuring injection pressure are the “compressed air injection technique” and commercially available in-line pressure
manometers (BSmart, B.Braun Medical; Figures 5-7).16
Injection pressure monitoring is sensitive for detecting
contact between the needle and nerve, and for intrafascicular placement of the needle tip. Yet it lacks specificity, as there are many other causes of high injection
pressure, such as a blocked needle.
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Nerve Stimulation
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Ultrasound
The introduction of ultrasound revolutionized the
practice of regional anesthesia, allowing practitioners to visualize in real time the nerve and the needle as it approaches the target, as well as the spread
of local anesthetic in a space or plane adjacent to the
target. Despite its potential as a visual aid, successful
use of ultrasound depends largely on the skill of the
practitioner. Proficiency requires training, as well as
good hand–eye coordination and 3-dimensional spatial reasoning.
Needle visualization is critical for safety; an in-plane
approach to the femoral nerve has been shown to
result in less needle–nerve contact than an out-of-plane
approach (Figures 2 and 3).13 Observing the spread of
local anesthetic during injection may help clinicians
determine the proximity of the needle to the nerve. If
the needle is close to the nerve, but not inside it, local
anesthetic will be seen spreading around the nerve or
within a sheath (Figure 4). However, intraneural injection may be visualized as swelling of the nerve itself.14
Conclusion
Three objective monitors can help regional anesthesiologists avoid intraneural injection. However, none is
perfect: Ultrasound depends on the ability of the user;
nerve stimulation is specific but not particularly sensitive; and injection pressure monitoring is sensitive but
not specific. Because of the limitations of each of these
monitors, we recommend that they be used simultaneously during PNBs to provide the most comprehensive protection possible against neurologic injury
(Figure 8). We feel that approach is analogous to the
simultaneous use of pulse oximetry, electrocardiography, and blood pressure and other routine monitoring
during the administration of general anesthesia to avoid
complications.
d.
Figure 1. The use of electrical
nerve stimulation to prevent
needle–nerve contact and/or
intraneural injection.
(A) Median nerve motor response with a “high”
(0.48 mA) current intensity. (B) With the same
needle position, the motor response is absent
with a current intensity less than 0.2 mA, suggesting—but not proving—the needle tip is
extraneural.
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Figure 2. In-plane needle insertion.
The needle is directed from the lateral aspect of
the transducer, in the same plane as the ultrasound
beam. In the corresponding sonographic image in a
phantom, the needle is seen approaching the
target (*).
Note that the structure of each component nerve is
intact and the outer epineurium (dotted lines) has
not been violated.
The needle approaches the beam at right angles.
Therefore, only the small slice that crosses the beam is
visible as a bright dot. (The asterisk marks the target.)
Figure 5. Compressed air injection
technique.
d.
Figure 4. The spread of local
anesthetic between the tibial
(TN) and common peroneal (CPN)
branches of the sciatic nerve.
Figure 3. Out-of-plane needle
insertion.
A 20-mL syringe is filled with 10 mL of injectate and
10 mL of air. According to Boyle’s law, a compression
of the gas by half of its original volume will double
the pressure in the system (ie, increase the pressure
at the needle tip by 1 atmosphere, or roughly 15 psi).
Maintaining the gas bubble at greater than half of its
original volume should therefore prevent injection
pressures of greater than 15 psi.
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References
1.
Steinfeldt T, Graf J, Schneider J, et al. Histological consequences of
needle-nerve contact following nerve stimulation in a pig model.
Anesthesiol Res Pract. 2011:591851.
2. Hadzic A, Dilberovic F, Shah S, et al. Combination of intraneural injection and high injection pressure leads to fascicular
injury and neurologic deficits in dogs. Reg Anesth Pain Med.
2004;29(5):417-23.
3. Farber SJ, Saheb-Al-Zamani M, Zieske L, et al. Peripheral
nerve injury after local anesthetic injection. Anesth Analg.
2013;117(3):731-739.
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5. Albrecht E, Kirkham KR, Taffé P, et al. The maximum effective
needle-to-nerve distance for ultrasound-guided interscalene block:
an exploratory study. Reg Anesth Pain Med. 2014;39(1):56-60.
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Figure 6. Pressure manometer
showing opening pressure less than
15 psi.
4. Gadsden JC, Choi JJ, Lin E, et al. Opening injection pressure consistently detects needle-nerve contact during ultrasound-guided
interscalene brachial plexus block. Anesthesiology. 2014. [Epub
ahead of print]
6. Perlas A, Niazi A, McCartney C, et al. The sensitivity of motor
response to nerve stimulation and paresthesia for nerve localization as evaluated by ultrasound. Reg Anesth Pain Med.
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7. Robards C, Hadzic A, Somasundaram L, et al. Intraneural injection
with low-current stimulation during popliteal sciatic nerve block.
Anesth Analg. 2009;109(2):673-677.
8. Chan VWS, Brull R, McCartney CJL, et al. An ultrasonographic and
histological study of intraneural injection and electrical stimulation
in pigs. Anesth Analg. 2007;104(5):1281-1284
9. Tsai TP, Vuckovic I, Dilberovic F, et al. Intensity of the stimulating
current may not be a reliable indicator of intraneural needle placement. Reg Anesth Pain Med. 2008;33(3):207-210.
10. Voelckel WG, Klima G, Krismer AC, et al. Signs of inflammation after sciatic nerve block in pigs. Anesth Analg.
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11. Bigeleisen PE, Moayeri N, Groen GJ. Extraneural versus intraneural stimulation thresholds during ultrasound-guided supraclavicular
block. Anesthesiology. 2009;110(6):1235-1243.
Figure 7. Pressure manometer
showing graduated pressure
thresholds of 15 to 20 psi and
greater than 20 psi.
12. Wiesmann T, Borntrager A, Vassiliou T, et al. Minimal current intensity to elicit an evoked motor response cannot discern between
needle-nerve contact and intraneural injection. Anesth Analg
2014;118:681-686.
13. Ruiz A, Sala-Blanch X, Martinez-Ocón J, et al. Incidence of intraneural needle insertion in ultrasound-guided femoral nerve block:
a comparison between the out-of-plane versus the in-plane
approaches. Rev Esp Anestesiol Reanim. 2014;61(2):73-77.
14. Macfarlane AJR, Bhatia A, Brull R. Needle to nerve proximity: what do the animal studies tell us? Reg Anesth Pain Med.
2011;36(3):290-302.
15. Theron PS, Mackay Z, Gonzalez JG, et al. An animal model of
“syringe feel” during peripheral nerve block. Reg Anesth Pain Med.
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16. Tsui BCH, Knezevich MP, Pillay JJ. Reduced injection pressures
using a compressed air injection technique (CAIT): an in vitro
study. Reg Anesth Pain Med. 2008;33(2):168-173.
d.
Figure 8. Complementary monitoring
strategy to reduce the risk for
needle-induced nerve injury during
peripheral nerve blocks.
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