Regional anesthesia is a great post-op pain reliever for pediatric patients, because they often can't communicate their needs, nor can they use a patient-controlled analgesia device. Regional also boosts blood flow to the operative site, which can translate to faster recoveries. Though children must be sedated or anesthetized for most regional blocks - a normal 4-year-old simply won't cooperate sufficiently - nerve stimulators and ultrasound guidance minimize the chances of nerve damage and inaccurate block placement. Differences in technique and pharmacology due to anatomical and physiological differences between children and adults are the real issues. Here's a guide to understanding and dealing with those differences.

Central (spinal mediated) regional techniques
Of the three methods of central regional analgesia, only caudal epidural administration (done after induction of general, with the child in the lateral or prone position) is practical for outpatients. You can buy pediatric epidural kits with 18- or 20-gauge Touhy or Weiss needles, though you can use a typical adult tray with a 17-gauge, 3.5-inch needle, even in small infants. Local anesthetics with or without epinephrine are preferred; the addition of clonidine or opioids is reserved for inpatients.

Caudal epidural anesthesia is used in combination with general anesthesia mostly to control post-op pain, but when administered before the surgical procedure, it may decrease the total amount of general anesthesia required. It's useful for procedures below the level of the umbilicus and is relatively easy to perform in the prone or lateral position once you've identified the coccyx and the sacral hiatus between the two sacral cornua.

The anesthesia provider inserts a small needle 1mm to 2mm caudal from a point midway between the cornua, at a 30-degree to 45-degree angle, then advances it through the sacrococcygeal membrane into the epidural space. The dural sac may extend as far down as the S3 or S4 level in small infants, so the needle must not be advanced too far. Some anesthesiologists prefer to make a small nick in the skin with an 18-gauge needle to serve as an entrance for the block needle and to prevent tissue coring. To rule out misplacement of the needle in the intravascular or intrathecal space, the anesthesia provider should gently aspirate to detect blood or cerebrospinal fluid. A "test dose," which consists of a small amount of local anesthetic with epinephrine, will cause heart rate and ECG changes if the solution has been injected intravascularly.

An appropriate concentration and volume based on the height and density of the blockade are key to a successful caudal epidural block. The volume of anesthetic determines the height of the block, which depends on the level of the surgical incision. Volumes of 1.2 mL/kg to 1.5 mL/kg provide analgesia and anesthesia to the T4-T6 dermatome. Doses of 1 mL/kg will relieve post-op pain for inguinal procedures; 0.5 mL/kg to 0.75 mL/kg is sufficient for lower-extremity procedures. No more than 2.5 mg/kg of bupivacaine, ropivacaine or levobupivacaine should ever be used.

Side effects from epidural analgesia include motor blockade of the lower extremities and urinary retention. Complications include unintentional intravascular injection and intraneural injection.

Peripheral nerve blocks
Peripheral nerve blocks are associated with less incidence of side effects and complications than with central blocks. A nerve stimulator should be used for the best accuracy, which means avoiding neuromuscular blockers. More recently, ultrasound guidance has been used to further enhance accuracy and reduce the amount of local anesthetic solution required. The use a nerve stimulator may increase the risk of intraneural needle-tip placement, so a small syringe should be used initially to inject local anesthetic; unexplained resistance will warrant adjustment of the needle position. Here's a look at the individual techniques.

• Axillary approach to the brachial plexus. Brachial plexus blocks are useful for pain related to procedures at or below the elbow. Using the axillary approach, the patient's arm is abducted 90 degrees from the body with the elbow flexed. The block is performed using the transarterial, one-injection, or two-injection technique. This successfully blocks the posterior cord of the brachial plexus, anesthetizing the ulnar nerve. The axillary approach usually misses the musculocutaneous nerve; this is remedied by injecting part of the local anesthetic into the coracobrachialis muscle through the same needle. The total anesthetic dose should equal 1 mL/kg of 0.25% bupivacaine, 0.25% levobupivacaine, or 0.2% ropivacaine up to a maximum of 30mL, or 0.5 mL/kg of 0.5% bupivacaine, 0.5% levobupivacaine, or 0.3% ropivacaine up to a maximum of 15mL.
• Parascalene approach to the brachial plexus. This approach anesthetizes the entire upper extremity without risk of damaging delicate neck structures. Placing the child supine with a towel roll under the shoulders and the head turned away from the side of the block, the anesthesia provider should look for the midpoint of the upper border of the clavicle and Chassaignac's tubercle (the transverse process of C6). The insertion point is the junction of the upper two-thirds and lower one-third of the imaginary line between the clavicle and Chassaignac's tubercle. An insulated needle is advanced at a 90-degree angle to the skin until there's a motor response in the distal upper extremity - between 7mm and 30mm, depending on the child's size. The needle should be withdrawn and directed more laterally if there's no response. Anesthetic choices include 0.25% or 0.5% bupivacaine up to 2 mg/kg; 0.2% or 0.3% ropivacaine up to 3 mg/kg; and 1% or 2% lidocaine up to 5 mg/kg. Risks include recurrent laryngeal nerve block, accidental vein puncture and unilateral Horner's syndrome.
• Femoral nerve, lateral femoral cutaneous nerve, 3-in-1 block. The 3-in-1 block anesthetizes the femoral, lateral femoral cutaneous and lateral femoral obturator nerves that innervate the lower extremity. The provider uses a larger volume of anesthetic than in an isolated femoral nerve block and holds pressure distal to the injection site during administration. Dose regimens include 1 mL/kg of 0.25% bupivacaine; 0.5 mL/kg of 0.5% bupivacaine up to 40mL; or 0.2% ropivacaine up to 40mL.
• Fascia iliaca block. In this femoral nerve block modification, the anesthesia provider injects the local at 1cm to 2cm below the inguinal ligament, and at least 2cm to 3cm lateral to the femoral artery. The solution spreads medially to the femoral nerve and superiorly to anesthetize the obturator and lateral femoral cutaneous nerves. Since the technique is performed several centimeters from the femoral nerve, a nerve stimulator is not used.
• Penile block. Used for circumcision or hypospadias repair, the provider anesthetizes the dorsal penile nerves by injecting 1mL to 2mL of local anesthetic below the deep fascia at the base of the penis at the 2- and 10-o'clock positions. The most common complication of this block is subcutaneous hematoma at the injection site.
  
  Another method is to inject anesthetic into the subpubic space, which contains the pudendal nerve as it exits from beneath the pubic bone. The base of the penis is gently stretched downward with one hand while the block needle is advanced along the caudal edge of the pubic bone in the midline. A "give" is felt as the needle pierces the superficial fascial layer. Another, less marked give will then be felt as the needle pierces Scarpa's fascia and enters the subpubic space. After gentle aspiration to rule out an intravascular injection, 3mL of 0.5% plain bupivacaine is injected. Epinephrine shouldn't be used because of the risk of end-arterial vasoconstriction.
• Ilioinguinal/iliohypogastric (hernia) block. Using a single injection of local anesthetic 1cm medial to the anterior superior iliac spine, the anesthesia provider can block the ilioinguinal and iliohypogastric nerves that innervate the superficial tissues overlying the inguinal ligament and proximal scrotum. By injecting half the amount of solution, the nerves of the muscle layers will be saturated. Then, without removing the needle entirely, the remaining half of the anesthetic solution is injected in a fan-like manner in a medial and cephalad direction. For this block, I typically use 1 mL/kg of 0.25% bupivacaine or 0.2% ropivacaine with or without epinephrine.

Pharmacology of local anesthetics
Local anesthetic metabolism is affected by age; low quantities of albumin and alpha-1 acid glycoprotein made in the liver inhibit protein binding of local anesthetics. This increases the plasma-free fraction of the drug and the risk of local anesthetic toxicity. Drug absorption time is also a consideration, because a rapid rise of the serum concentration is more likely to result in toxicity. Cardiac output and local blood flow in children are relatively greater than in adults, so systemic absorption of local anesthetics is relatively faster in children, as are peak plasma concentrations. When a vasoconstrictor such as epinephrine is added, the absorption rate is decreased, and the peak serum concentration is decreased by 10 to 20 percent.

Anatomical differences also affect local anesthetic pharmacology. The fat within the epidural space is sparse and loose in pediatric patients. The perineurovascular sheaths located around nerve roots and bundles are more loosely attached to underlying structures in children than in adults. Therefore, injected local anesthetics appear to spread more and innervate a greater area in children. The endoneurium is relatively loose in young children, allowing rapid exposure of local anesthetic to the nerve and more rapid onset of anesthesia than in adults.

Even with extreme caution and careful calculation, local anesthetic toxicity can still occur, so it's important to calculate the total milligram dose on a per-kilogram basis and to administer less than the recommended maximum doses.

Safety points
While the use of regional techniques is limited in children as compared to adults, it's effective as long as you take care in calculating doses. Ongoing research continues to refine techniques and give us information on optimal dosing and the most effective drug combinations.