The effect of local anesthetic volume on the duration of single shot axillary brachial plexus block.

Peripheral nerve block (PNB) as an anesthetic technique plays an important role
in modern regional anesthesia. The most important prerequisites for the use of
peripheral regional anesthesia in the daily clinical practice are success rate
and safety. Both issues are closely related to the administered volumes of
local anesthetics.
Ultrasound guidance is rapidly gaining popularity among anesthesiologists for
the placement of peripheral nerve blocks because it offers several potential
advantages compared with conventional methods. Nerves and other anatomical
structures such as vessels, muscles, bones, fascias and tendons, can be
visualized directly. Also, local anesthetic (LA) spread around the nerves can
be assessed with the possibility of repositioning the needle in cases of
maldistribution. Ultrasound guidance shortens the block performance time,
reduces the number of needle insertions and shortens the block onset time. It
probably even reduces complication rates, because the needle can be visualized
at all times and intraneural and intravascular injection can be avoided. Recent
publications illustrate that the volume of local anesthetic (LA) can be
significantly reduced when particular regional anesthetic techniques are
performed with ultrasound guidance.

The axillary approach to block the brachial plexus is widely used to provide
anesthesia for surgery of the forearm, wrist and hand. To block the brachial
plexus in the axilla, deposition of local anesthetic is required adjacent to
the nerves (the median, radial, ulnar, musculocutaneous and medial antebrachial
cutaneous nerves). The procedure is relatively safe and, if dose limits are
observed, complications are uncommon.
Before the introduction of nerve stimulation and/or ultrasound guidance, blind
injection of the local anesthetic around the axillary artery (infiltration
technique) was the predominant method used to block the axillary brachial
plexus. Failures or incomplete blocks were thought to be caused by imprecise
needle placement or septation of the brachial plexus sheath, leading to
malposition of the local anesthetic. Recent research is largely concerned with
identifying volumes of local anesthetic and techniques to *fill* the axillary
brachial plexus sheath to *capacity*. Furthermore, investigators attempt to
define the relationship between dose, volume, and concentration of the local
anesthetic and reliability, quality, and duration of the blockade.

To date, there are no data considering duration of peripheral nerve blocks in
low volumes using ultrasound guidance. There may be a linear relationship, but
it is also conceivable that with higher doses, a large part of the local
anesthetic will be absorbed and redistributed without contributing to nerve
blockade.

The purpose of the present study is to compare the duration of sensory and
motor block with 15 and 40 millilitres mepivacaine 1.5% for axillary brachial
plexus block using ultrasound guidance. Our hypothesis (H0) is that there is no
difference (less than 60 min) in duration of AXB using the different amounts of
local anesthetic under ultrasound guidance.

The design of this study is parallel, prospective, randomized and single blind.
Thirty patients scheduled for single shot axillary brachial plexus block for
hand, wrist or forearm orthopaedic surgery will be studied.

Currently, many peripheral nerve blocks are perfomed under ultrasound guidance.
This allows for significant reduction of the volume of local anesthetic (LA),
wich diminishes the risk for neural toxicity. It is important however, to
maintain sufficient block duration for postoperative pain control. Block
duration will be tested every 30 minutes postoperatively in patients who
participate in this study until the block has resolved completely. We think
this minor burden does not outweigh the information we can obtain to optimally
treat future patients.