Spinal Anesthesia

Anatomic Considerations

The subarachnoid space is incompletely divided by the denticulate ligament laterally and the subarachnoid septum medially. The volume of CSF is doubled in infants (4 mL/kg) compared with adults (2 mL/kg), and this difference is further enhanced by the fact that 50% of the total volume of CSF is located within the spinal subarachnoid space in infants, compared with 25% in adults. These differences dramatically influence the pharmacokinetics of intrathecally administered local anesthetics. The CSF hydrostatic pressure ranges from 30 to 40 cm H₂ O in the dorsal recumbent position, which is significantly less than in adults.^[186] This pressure is further decreased in patients under general anesthesia and when the patient is placed in the Trendelenburg position. As in adults, vascularization of the lumbar part of the spinal cord often depends on one small artery (i.e., Adamkiewicz artery) entering the spinal canal between T8 and L3. Any lesions of this artery can result in spinal ischemia and subsequently in permanent paraplegia.

Indications and Contraindications

The main indications for spinal anesthesia include surgery on the lower part of the body, especially irreducible inguinal hernia repair and operations on the lower limbs and the spine.^{[187] [188] [189]} Spinal anesthesia is of major interest in premature babies whose conceptual age is less than 60 weeks, especially those who experienced neonatal respiratory distress syndromes or presenting with anemia (i.e., hematocrit below 30%). These patients are more prone to develop postoperative apnea after general anesthesia, including sevoflurane anesthesia,^[190] than after pure spinal anesthesia. However, only 15% were operated with spinal anesthesia according to the Hernia Survey of the Section on Surgery of the American Academy of Pediatrics.^[191] In older patients, there are few indications for spinal anesthesia. Some anesthesiologists recommend using the technique for the management of minor surgery (e.g., day-surgery cases) because discharge criteria are met within 3 to 4 hours after surgery.^[192] However, these patients can be managed as efficiently with ilioinguinal and iliohypogastric nerve blocks, without the potential hazards of a neuraxial block. In some developing countries, the technique is commonly used because it is the only available and safe technique of anesthesia. Children undergoing major operations (e.g., open heart surgery, spinal fusions) may benefit from the administration of intrathecal narcotics ^{[193] [194]} for postoperative pain relief. This is a serious decision that requires careful postoperative monitoring because of the risks of delayed respiratory depression (see "Opioids"). Spinal blocks have the same contraindications and complications as epidural blocks. The limitations of the technique in regard to the extension and duration of the block can be regarded as contraindications. The younger the patient, the shorter is the duration of the blocks. In premature infants, spinal anesthesia does not last longer than 45 minutes with lidocaine and 60 to 75 minutes with bupivacaine (and probably the same duration with levobupivacaine).

Block Procedure

The spinal anesthesia technique is a lumbar puncture that can be performed in the same positions as for lumbar epidural approaches. It is easier when the baby is in the sitting position, but it is safer, especially in high-risk infants, when performed in the lateral decubitus position. In either case, considerable attention must be paid to the position of the head, which must be maintained extended throughout the duration of the procedure. Intrathecal catheters have very limited indications in children.^[195] The number of local anesthetics recommended for spinal anesthesia has dramatically decreased, and only tetracaine and bupivacaine in hyperbaric solutions or in standard solutions are currently recommended. Levobupivacaine and possibly ropivacaine may become candidates for spinal anesthesia in the future. The recommended doses of local anesthetics are displayed in Table 45-10 . Classically, hyperbaric solutions are used for spinal anesthesia, but isobaric solutions are as effective^[196] and limit the danger of displacement of the upper limit of blockade in case of inadvertent moving of the lower limbs in infants. Epinephrine increases the duration of

TABLE 45-10 -- Usual doses of local anesthetics for spinal anesthesia in children *

Local Anesthetic	0–5 kg	5–15 kg	>15 kg
Plain tetracaine (1%)
Dose (mg/kg)	0.5	0.4	0.3
Volume (mL/kg)	0.05	0.04	0.03
Duration (min)	75	80	85
Tetracaine (1%) with epinephrine
Dose (mg/kg)	0.5	0.4	0.3
Volume (mL/kg)	0.05	0.04	0.03
Duration (min)	120	120	125
Bupivacaine (0.5%)
Dose (mg/kg)	0.5	0.4	0.3
Volume (mL/kg)	0.1	0.08	0.06
Duration (min)	65–75	70–80	75–85

*In the very young, many anesthesiologists give twice the doses mentioned (i.e., up to 1 mg/kg of bupivacaine or tetracaine) without apparent problems.

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Adverse Effects and Complications

The main limitation of spinal anesthesia is its short duration. Nociceptive stimulation returns soon after the procedure, and injection of parenteral analgesics should better be avoided. For this reason, some physicians recommend using caudal anesthesia instead of spinal anesthesia in awake patients. Another effective option is to perform an ilioinguinal/iliohypogastric nerve block immediately after the spinal block. This precludes any pain problem in case of surgical difficulties, provides adequate postoperative analgesia, and does not significantly increase the danger of systemic toxicity. The most common adverse effect of spinal anesthesia in infants is failure due to an inability to enter the subarachnoid space, which occurs in more than 25% of cases.^[199] A specific danger of the technique results from imprudent movement of the infant's legs during surgical draping, which can cause upward spread of the hyperbaric local anesthetic with subsequent respiratory arrest, requiring tracheal intubation and assisted ventilation until recovery. Other complications are the same as those described with epidural anesthesia. PDPH occurs infrequently and requires a blood patch. Backache, meningitis, shivering, and neurologic sequelae are extremely unusual.