A. Nerve Mapping in Children
For many years there has been a lack of specific material to perform regional anesthesia in infants and children. Radial artery catheterization sets, epidural kits, and peripheral and central venous catheter sets have been used for continuous peripheral nerve blocks. Nowadays for a safe performance it is mandatory to use dedicated pediatric tools. Any peripheral nerve can be blocked either by infiltrating a local anesthetic within a compartment space through which the nerve runs or by precisely locating the nerve. Compartment blocks such as intercostal block, intrapleural block, fascia iliaca compartment block, penile block, etc., depend on the localization of the fascial plane; when the relevant fascia is unique with no underlying vital structure, different needles such as an IM needle, can be safely used. When there are several fascial planes or a danger of damaging important anatomical structures, such as with an ilioinguinal block, only a short-beveled or pinpoint needle should be selected. Precise localization of a plexus or a nerve trunk must not be performed by seeking paresthesias with standard IM needles because of the danger of direct nerve damage; only short-beveled needles, insulated and connected to a nerve stimulator are suitable. For most peripheral nerve blocks in children, 21- to 23-gauge, 35- to 50-mm long needles are used, depending on the type of block and on the age of the child. Eliciting a motor response using a nerve stimulator is the most useful and safe technique for performing a pediatric nerve block. Since the plexuses in children are quite superficial, especially the brachial plexus at the axilla, before introducing the needle, try to detect the position of the plexus with the transcutaneous technique. After having introduced the tip of the needle, connect it to the neurostimulator and set it to stimulate at a frequency of 2 Hz. Starting with a current of 1 to 1.5 mA, the needle is advanced until distinct contractions of the nerves to be blocked are noticed. The optimum nerve location is achieved by adjusting the needle so that these contractions are still visible with currents of 0.4 mA. It is now possible to inject the bolus dose of local anesthetic. Remember that, as nerves are thin and very closely linked to each other without sheaths dividing them, one twitch of a single nerve is enough to administer the drug without looking for a multiple-twitch technique.
Mapping of Nerves in Pediatric Regional Anesthesia
In the new century, regional anesthesia in the pediatric field has reached worldwide acceptance. Safety and efficacy have been evidenced in a major survey showing that pediatric regional anesthesia has a low rate of complications and no major sequelae or deaths. Light sedation or anesthesia plus a block offers optimal pain control throughout surgery, allowing also good postoperative analgesia. Peripheral blocks are employed more and more, but thus far they are still less common than central blocks even though in the same survey their safety was superior—no complications in more than 9,000 blocks.
One of the problems connected with the performance of a peripheral block, even with the mandatory use of a nerve stimulator (NS), is a thorough knowledge of the anatomy of children; specifically, the closeness of different structures—nerves, veins, arteries. The small distance between the skin and the nerves can cause, in inexperienced hands, severe injuries while detecting a plexus with the needle.
Adrian Bosenberg published in 2002 a simple but very effective method to improve experience and reduce mistakes during the performance of a peripheral block. The technique, called nerve mapping, requires the use of the unblunted tip of the negative electrode of the NS. It involves increasing the mA of the NS up to 3 mA or more; the skin is touched close to the nerve plexus, causing stimulation until motor responses are elicited, and then the voltage is reduced to detect the best point to perform the block.
In the same year (2002) Urmey and Grossi described the same technique in adults using a device with a needle-through passage obtaining an even more successful performance; in this case they employed a higher voltage, 4 to 5 mA, due to the thickness of the skin in adults; in 2003 they described a modified tool for the same technique.
Figure 46-1. Pen-like nerve stimulator.
More recently new devices have been produced by industrial companies using a pen-like stimulator instead of the negative electrode allowing an easier mapping. The NS must be set at 3 mA, 2 Hz, 1 ms (while 0.1 ms is usually used for performing the block with the needle); changing slightly the position of the tip of the pen, a motor response is elicited. The best position can then be chosen (and the best motor response according to the needs of surgery) and marked with a pen (Fig. 46-1).
Nerve mapping is a painless method as the mAs used produce a motor but not a sensory response, and can also be performed in an awake patient. Moreover, children are very often under sedation or anesthesia so that any stress is avoided; in the meantime we can teach the anatomy of a plexus allowing young doctors to detect the nerves without any skin damage.
In this way we can find the different nerve responses of a plexus; radial, medial, ulnar, and musculocutaneous nerves can be elicited for the upper arm, and femoral and sciatic with its components—peroneal and tibial—for the lower limb.
The success rate of blocks in children can be increased, keeping in mind that malformations may make it difficult to place the needle (e.g., arthrogryposis).
We use this technique for the axillary and the parascalene approach of the brachial plexus, the femoral approach, and all the more distal detection of nerves (i.e., popliteal level) including the “small blocks.”
It takes only a few minutes, is not time consuming, and can be considered one of the tools for daily clinical practice—a new and extremely useful technique that can increase the efficacy and safety of peripheral blocks in children.
Bosenberg A, Raw R, Boezaart AP. Surface mapping of peripheral nerves in children with a nerve stimulator. Paed Aneasth 2002;12(5):398–403.
Giaufre E., Dalens B, Gombert A. Epidemiology and morbidity of regional anesthesia in children. A one year prospective survey of the French Language Society of Pediatric Anesthesiologists. Anesth Analg 1996;83:904–912.
Urmey WF, Grossi P. Percutaneous electrode guidance: a noninvasive technique for prelocation of peripheral nerves to facilitate peripheral plexus or nerve block. Reg Anesth Pain Med 2002; 27:261–267.
Urmey WF, Grossi P. Percutaneous electrode guidance and subcutaneous stimulating electrode guidance: modifications of the original technique. Reg Anesth Pain Med 2003;28:253–255.
B. Local Anesthetics
Local anesthetics are tertiary amines and are divided into esters, metabolized by plasma cholinesterases (neonates and infants up to 6 months have half of the adult levels of this enzyme), and amides, metabolized by the liver and bound by plasma proteins (neonates and infants up to 3 months have a reduced hepatic blood flow and immature degradation pathways). In children, a large amount of anesthetic remains unmetabolized and active in comparison with adults; moreover, neonates and infants are at greater risk of toxic effects due to lower levels of albumin and α-1 acid glycoprotein. As the nerve fibers in children are small and myelination is not complete, the minimum concentration necessary to obtain nerve block may be reduced and we can expect to use lower concentrations of local anesthetic. The toxic effects of local anesthetics are dependent on the total dose of drug administered and on the rapidity of absorption into the bloodstream. Few local anesthetics have been studied systematically or approved for use in pediatric age groups. Local anesthetics like mepivacaine, lidocaine, and bupivacaine are still largely used. Although adequate dose guidelines are available, case reports on toxic plasma concentrations (mainly concerning bupivacaine) have been described. Recently two new aminoamide local anesthetics, ropivacaine and levobupivacaine, have been introduced and are showing interesting characteristics in the pediatric area, too (Table 46-1). Ropivacaine and levobupivacaine have similar characteristics: both of them are isomers, S-(-) enantiomers whose main pharmacological aspects, in comparison with the racemic mixture, are the minor cardio and nervous affinity and toxicity, and a differential neural blockade with less motor than sensation block. Currently, for these two new local anesthetics specific dosage limits have been reported in the pediatric population. Although ropivacaine and levobupivacaine have similar characteristics, there are differences between them as evidenced in several investigations in adults. Levobupivacaine, as S-enantiomer of bupivacaine, maintains similar properties of the racemic formula in terms of liposolubility, protein binding, and MLAC (levobupivacaine/bupivacaine potency ratio of 0.98) but with less motor block. Ropivacaine, on the contrary, showed in adults 40% less potency in comparison with levobupivacaine and bupivacaine, thus partially reducing the advantage of a lower toxicity. On the other hand, looking at the studies in children, some differences appear between ropivacaine, levobupivacaine, and bupivacaine in comparison with adults' results.
Studies confirm an equianalgesic effect of 0.2% solution vs. 0.25% bupivacaine; this effect is probably linked to the biphasic vascular action of ropivacaine—vasoconstriction at lower concentrations is no more detectable at higher concentrations. Moreover this action adds safety delaying the uptake from the action sites. There is only one case report of inadvertent I.V. injection of ropivacaine—a continuous infusion through the I.V. line instead of through the epidural line. Authors claim no clinical signs of toxicity because of the very low dose but probably also thanks to the increased safety of this isomer.
Table 46-1. Local Anesthetics Commonly Used and Usual Doses for the Peripheral Nerve Block
So far there are very few studies in children except some with data concerning both single-shot and continuous infusion, pharmacokinetics, and dose response. One of the main characteristics of these isomers that is significant in children is the reduced motor block. At the end of surgery the motor impairment, even for a short time, is stressful both for children and parents; the use of levoenantiomers reduces this motor block, concentration dependent: it is not evident at 0.2%–0.25% while it increases with higher concentrations. Levoisomers have a vasoconstrictive activity at low concentration while they give vasodilation at higher concentration; in children normally a low concentration is used (0.1–0.2%) so that probably we have a longer duration of analgesia. There are, so far, very few studies comparing ropivacaine, levobupivacaine, and bupivacaine; some results show that onset time and analgesic duration were similar while the motor block impairment is statistically longer with bupivacaine in comparison with the two isomers.
In conclusion, levobupivacaine and ropivacaine appear to have similar characteristics in children: same analgesic duration, same reduced motor blockade, same dose required. Probably many more studies are needed to verify differences, if any, but even if there is a minimal difference in MLAC, in daily clinical practice adequate doses are used to obtain 100% success (that is, a higher concentration and dose), thus minimizing the hypothetic difference between the drugs.
1. It is mandatory to have a good theoretical knowledge and a knowledge of various techniques before approaching a peripheral block in children.
2. All peripheral blocks should be performed in a sedated child, therefore in a room with resuscitation equipment and after the placement of an intravenous line and standard monitoring (electrocardiogram, saturated oxygen, heart and respiratory rates, blood pressure).
3. Use only pediatric equipment that should be stocked in a cart specifically designed for peripheral nerve blocks.
4. All peripheral nerve blocks should be performed under aseptic conditions. After defining the anatomic landmarks and, when possible, detecting the plexus position with the transcutaneous stimulation, the gloved anesthesiologist prepares the material in an aseptic manner. The site of puncture is cleaned with an antiseptic solution.
5. Using the nerve stimulator to elicit the motor response and setting it to stimulate at a frequency of 2 Hz with a current of 1.5 mA, the needle is advanced until distinct contractions of the region to be blocked are noticed. The optimum nerve location is achieved by adjusting the needle so that these contractions are still visible with currents of 0.4 mA.
6. Make the aspiration test before any injection and after the injection of the test dose (0.5 to 1 mL). Check that the motor response disappears and that no anomalies appear on the ECG monitor for 30 to 40 seconds after this injection.
7. Inject slowly (no more than 10 mL/min) since toxicity depends mainly on the plasma peak concentration rather than the total amount of local anesthetic injected.
8. Inject with low pressure to avoid, in case of intraneural injection, irreversible damage to the nerve.
9. Follow accurate drug dose guidelines.
10. If doubt arises about any part of the procedure (abnormal resistance, pain, ECG, neuro-anomalies) the injection should be immediately stopped.
11. Be aware of any possible complications and know how to treat them.
12. Do not attempt to perform the same procedure more than three times in the same patient.
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