The relation between electrical current and efficacy of stimulating catheters for brachial plexus block

Peripheral nerve block (PNB) as an anesthetic technique is rapidly gaining
popularity among anesthesiologists and patients. Compared to general anesthesia
or central neuraxis blockade, interference of PNB with vital functions is
minimal and postoperative analgesia is excellent.
PNB can be administered as a single shot or continuously. When postoperative
pain is mild to absent or expected to be short-lived, the single shot approach
is sufficient. However, when pain is expected to be moderate to severe for
several days, a continuous technique using a PNB catheter is more appropriate.

Nerve stimulation (NS), alone or in combination with Ultrasound, is at present
the most widely used method in PNB to ensure close proximity of the needle tip
to the nerve before injecting the local anesthetic. Using a current of 100-150
nC at the tip of the advancing needle, an appropriate motor response is sought
to find the approximate location of the target nerve. By gradually reducing the
current while advancing the needle and maintaining the appropriate motor
response, the needle tip is positioned close enough to the nerve to ensure
adequate blockade when injecting the local anesthetic dose through the needle.
There is general consensus that the minimal current eliciting the appropriate
motor response for an adequate nerve block has to be between 20 - 50 nC.

There are stimulating and non-stimulating PNB catheters. Non-stimulating
catheters are inserted a couple of centimeters blindly through the needle,
usually after the loading dose has been administered. Whether the catheter tip
is correctly placed, i.e. in close proximity to the nerve, does not become
apparent until after the effect of the loading dose has worn off, usually late
at night. If the catheter is not correctly placed, patients will have no, or
little benefit of the local anesthetics they receive through the catheter.
The advantage of a stimulating catheter is that it can be inserted while
stimulating at the tip of the catheter; this allows monitoring of the path of
the advancing catheter, i.e. staying close to the nerve or moving away from it.
From a theoretical perspective, a stimulating catheter offers several
advantages: if during advancement an appropriate motor response can be
maintained, the success rate of the catheter will be very high, i.e.
postoperative analgesia is expected to be adequate. When the motor response is
lost, the needle tip can be manipulated in order to change the path of the
advancing catheter to one where motor response is maintained. However,
sometimes it is not possible to maintain a motor response and in that case
there is little difference between a stimulating and a non-stimulating
catheter. Another advantage of an appropriately inserted stimulating catheter
is that the loading dose can be fractionated, reducing the incidence of
systemic toxicity.
The literature is mixed about the advantages of stimulating catheters. Some
studies found an advantage compared to non-stimulating catheters (1-8), whereas
others did not (9-12).
Obviously the tip of the catheter needs to be in the vicinity of the nerve to
be effective. When an appropriate motor response can be elicited with a low
current at the tip of the catheter, close proximity is evident. However, when
the necessary current is relatively high or an appropriate motor response is
absent, the tip of catheter may still be close enough to the nerve to be
effective, it may be at an intermediate distance with a partial effect, or it
may be too far off and inadequate for postoperative analgesia.

Whether there is a relation between the minimal current necessary to elicit an
appropriate motor response and the efficacy of the catheter has not yet been
investigated.
Since stimulating catheters are more expensive than non-stimulating catheters,
resolving this issue is not only important in terms of patient care, but also
from an economical perspective.

The purpose of the present study is to investigate whether there is a relation
between the minimal current at the tip of the stimulating catheter necessary to
elicit an appropriate motor response, and the efficacy of the PNB catheter.

The design of the study is prospective and single blind. Forty patients
scheduled for Cuff- or stability repair of the shoulder will be studied. The
study will be conducted at the Sint Maartenskliniek Nijmegen according to the
Declaration of Helsinki and later revisions thereof and in accordance with the
ICH guidelines for Good Clinical Practice. No patients will be recruited before
written approval has been obtained from the local Medical Ethics Review
Committee as well as from the board of Directors of the Sint Maartenskliniek
Nijmegen.
Patients will be assessed for eligibility during the preoperative screening
visit, at least 48 h before the planned date of surgery. Patients will be
informed about the study verbally and in writing. At a later date, at least 24
h before the planned date of surgery, patients will be asked to participate in
the study by the investigators and when affirmative will be asked to sign the
written informed consent form.

Procedures before and during surgery

Intravenous access and routine monitoring will be established in all patients.
Using Ultrasound guidance and NS, a brachial plexus catheter will be inserted
five cm past the needle tip using an in-plane technique in the interscalene
area by an experienced anesthesiologist (RS, NJ).
Brachial plexus block will be established by injecting a total volume of 20 mL
ropivacaine 0.75 % in fractionated doses. Time is designated T = 0 upon
conclusion of the loading dose.
Surgery will be performed under general anesthesia with propofol, remifentanil
and a laryngeal mask airway.

Treatment schedule

After the brachial plexus catheter has been inserted five cm past the needle
tip, the minimal current necessary to evoke an appropriate motor response
(deltoid, biceps or triceps muscle) will be determined and registered. The
observer of motor response (KS) is blinded for the current.
One hour after administration of the brachial plexus loading dose, a continuous
infusion of ropivacaine 0.2 % 8 mL/h will be connected to the brachial plexus
catheter and maintained until T = 24 h. Upon arrival in the recovery, a PCA
morphine device will be connected to the intravenous cannula. Patients will be
instructed in the use of the PCA device preoperatively and to maintain
postoperative painscores (NRS 0-10) at or below 3.
At T = 24h, the catheter will be removed and the PCA device will be
disconnected by the investigator (KS). The total amount of asked and received
boluses of morphine will be registered.

Not applicable.