Caudal Anesthesia - continued

The sacral canal shares in the general engorgement of extradural veins that occurs in late pregnancy, or in any clinical condition in which the inferior vena cava (IVC) is partially obstructed. Since the effective volume of the caudal canal is markedly diminished during the latter part of pregnancy, the caudal dosage should be reduced proportionately in women at term. The segmental spread of local anesthetics may increase increase substantially in pregnant women at term, necessitating a 28-33% decrease of dose requirement in this patient population.[1] The choice of a continuous catheter or a single-shot technique during active labor is limited by the relative lack of sterility at the sacral hiatus, which may be contaminated by feces and meconium.

Rare cases of Horner syndrome have been noted when large doses of local anesthetics are injected caudally during labor.[1] This is most likely to occur if injection is made with the patient on her back (engorgement of epidural venous plexus and IVC compression are maximal). The so-called dual technique (lumbar and caudal) of epidural block for labor is no longer widely used. Since the pain of uterine contractions is mediated by sympathetic nervous system fibers originating from T10 to L2, a lumbar epidural catheter suffices for both stage I and stage II of parturition, with dosage adjustments being made depending on the exact circumstances and requirements.

Characteristics of the Blockade

Unlike in adults, the segmental spread of analgesia following caudal administration is more predictable in children up to about 12 years of age. Studies suggest that the cephalad spread of caudal solutions in children is not hampered by the same anatomic constraints that develop from puberty onward. Before puberty, anatomic impedance at the lumbosacral junction has not yet developed to a marked degree, and caudal solutions can flow freely upward into the higher recesses of the spinal canal. As a consequence, the rostral spread of caudal anesthesia is more extensive and more predictable in children than in adults.

In children, caudal block is usually combined with light general anesthesia with spontaneous ventilation. During lower abdominal and genitourinary surgery in children, caudal block with 0.25% bupivacaine (2 mg/kg) was shown to lower the metabolic and endocrine responses to stress, as measured by glucose concentrations, mean prolactin, insulin, and cortisol concentrations, as compared with general anesthesia alone.[32] Thoracic placement of catheters is possible in neonates and small children. However, one radiographic study of 115 infants found 10 caudally placed catheters to be in the high thoracic or low cervical region, when their intended site was in the lower thoracic segments.[33]

Caudal block with bupivacaine (4 mg/kg) and morphine (150 mcg/kg) was found to lower fentanyl requirements during cardiac surgery and shorten extubation times in a group of 30 pediatric patients randomized to receive general anesthesia alone or a combination of general and caudal block.[34]

Anesthetic dose requirements are about 0.1 mL/ segment/year of age for 1% lidocaine or 0.25% bupivacaine.[1] The dose may also be calculated based on body weight. The relationship between age and dose requirements is strictly linear with a high degree of correlation up to 12 years old. Plasma bupivacaine concentrations in children receiving caudal block with 0.2% of the local anesthetic (2 mg/kg) were less than equivalent doses administered via ilioinguinal-iliohypogastric block for pain control following herniotomy or orchidopexy. Additionally, the times to peak plasma concentrations were faster in the peripheral nerve block group, indicating that caudal block is a safe alternative to local infiltration techniques in inguinal surgery.[35] In a study of children age 1-6 years who underwent orchidopexy, a caudal block using larger volumes of dilute bupivacaine (0.2%) was shown to be more effective than a smaller volume of the standard (0.25%) concentration in blocking the peritoneal response to spermatic cord traction, with no change in the quality of postoperative analgesia. In that study the total bupivacaine dose was identical in both groups (20 mg).[36]

Ropivacaine 0.5% was shown to provide a significantly longer duration of analgesia following inguinal herniorrhaphy in children age 1.5-7 years compared with 0.25% ropivacaine or 0.25% bupivacaine.[37] All children received 0.75 mL/kg of the local anesthetic. Unfortunately, however, the times to first voiding and to standing were significantly delayed in the group receiving 0.5% ropivacaine, and there was one case of motor block of the lower extremities. This demonstrates the trade-off when one attempts to maximize analgesia by altering local anesthetic concentration or total dose.

Ropivacaine has also been used for caudal block for hypospadias repair in a double-blind, randomized study in 26 children.The minimal effective local anesthetic concentration of ropivacaine was found to be 0.11% under general anesthesia with a 0.5 monitored anesthesia care of enflurane.[38] Plasma concentrations of ropivacaine after caudal block in 20 children 1-8 years of age, using 2 mg/mL, 1 mL/kg, demonstrated free fractions to be 5%, clearance of 7.4 mL/min/kg, and terminal half-life of 3.2 h, well below those associated with toxic symptoms in adults.[39] Clonidine has been added to bupivacaine in 36 children undergoing elective surgery. A caudal catheter was placed using 1 mg/kg bupivacaine 0.125% with an equal volume of either clonidine (2 mcg/kg) or normal saline. No benefit of adding the clonidine was found, and, in addition, more children in the clonidine group vomited in the first 24 h postoperatively.[40]

The local anesthetics typically administered for singleshot caudal blocks in pediatric patients are listed in Table 3.

Although caudal block is a mainstay of perioperative pain management in pediatric surgery and represents probably 60% of all regional anesthetic techniques in this patient population, not all studies demonstrated a marked benefit of caudal block for postoperative analgesia compared with other modalities. Following unilateral inguinal herniorrhaphy, caudal block was shown to provide effective, but not superior, pain management compared with local wound infiltration in 54 children. The side effects and rescue analgesia requirements did not differ between the two groups.[42]

Caudal epidural block in children may induce significant changes in descending aortic blood flow while maintaining heart rate and mean arterial blood pressure. In a study of 10 children age 2 months to 5 years, a transesophageal Doppler probe was used to calculate hemodynamic variables after the injection of 1 mL/kg of 0.25% bupivacaine with epinephrine 5 mcg/mL. The aortic ejection volume increased, and aortic vascular resistance decreased by about 40%.[43] These data suggest that caudal block results in vasodilatation secondary to sympathetic nervous system blockade.

Radiculopathy Refractory to Conventional Therapy

When 5-10 mL of contrast medium is injected into the caudal canal through an epidural catheter, a "Christmas-tree" appearance develops as dye spreads into the perineural structures inside the bony canal and along the nerves as they exit the vertebral column.[14] Epidural adhesions prevent the spread of the dye so there is no outline of the involved nerve roots.

Once correct catheter placement in the epidural space is ensured, 1500 units of hyaluronidase in 10mL of preservative-free saline is injected rapidly. This is followed by an injection of 10 mL of 0.2% ropivacaine and 40 mg of triamcinolone. Following these two injections, an additional injection of 9 mL of 10% hypertonic saline is infused over 20 to 30 min. On the second and third days, the local anesthetic (ropivacaine) injection is followed up by the hypertonic saline solution. Antibiotic coverage is provided to reduce the possibility of epidural abscess formation.

Another unique application of caudal block is to provide postoperative analgesia in patients undergoing lumbar spine surgeries. In one series, patients received 20 mL of 0.25% bupivacaine with 0.1 mg buprenorphine via the caudal epidural approach, performed prior to surgical incision. The patients underwent posterior interbody fusion and laminotomy for spinal stenosis, and postoperative pain control was compared in the caudal group with a group treated with conventional parenteral opioids. The caudal group required less rescue analgesic medication doses over the first 12 h following surgery.[16] A reduction in blood pressure in the caudal group patients undergoing laminotomy, but not fusion, was noted in the patients with a prolonged duration (24 h) of postoperative analgesia.

Caudal epidural block has also been compared with intramuscular opioids in the treatment of pain after emergency lower extremity orthopedic surgery. The caudal group received 20 mL of 0.5% bupivacaine and had 8 h of superior analgesia with a concomitant significant reduction in the need for rescue opioid medications.[17]

Caudal injection of clonidine, 75 mcg with 7 mL bupivacaine 0.5% and 7 mL lidocaine 2% with epinephrine 5 mcg/mL has been used for postoperative analgesia after elective hemorrhoidectomy. Thirty-two adults received the clonidine-local combination while a control group received local anesthetic alone. Analgesia averaged 12 hours in the clonidine group, compared to

Caudal injections of alcohol or phenol have been used to treat intractable pain due to cancer. In a study of 67 blocks, it was found that the lower sacral roots were easily reached with the caudal injection, and that the S1 and S2 roots (contribution from the lumbosacral plexus) were spared.[19]

The complications of caudal block are similar to those occurring following lumbar epidural block and include complications related to the technique itself and complications related to related to the injectate (local anesthetic or other injected substance). Fortunately, serious complications occur infrequently. The list of possibilities includes epidural abscess, meningitis, epidural hematoma, dural puncture and postdural puncture headache, subdural injection, pneumocephalus and air embolism, back pain, and broken or knotted epidural catheters.

The incidence of local anesthetic-induced seizures occurs more frequently following caudal epidural block than it does following lumbar or thoracic approaches. In a retrospective study of 25,697 patients who received brachial plexus blocks, caudal or lumbar epidural blocks from 1985 to 1992, Brown noted 26 seizures.[44] The frequency of seizures in adults was caudal > brachial plexus block > lumbar or thoracic epidural block. Nine overall seizures were attributed to local anesthetic injection in the caudal space, eight occurring with chloroprocaine and one occurring with lidocaine. There was a 70-fold increased incidence (0.69%) of local anesthetic toxic reactions with caudal epidural anesthesia than with lumbar or thoracic epidural anesthesia in adults.

In children, however, one retrospective review identified only two toxic reactions (i.e., local anesthetic-induced seizures) in 15,000 caudal blocks.[45] Dalens' group found that inadvertent intravascular injection occurs in up to 0.4% of pediatric caudal blocks,[46] demonstrating the importance of performing epinephrine-containing test dosing in this age group. It has been suggested that an elevation of heart rate by > 10bpm or an increase in systolic blood pressure of > 15 mm Hg should be taken as indicative of systemic injection. T wave changes on the ECG occur earliest following intravascular injection, followed by heart rate changes, and lastly, by blood pressure changes.

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