A 6-kg, 3-month-old infant undergoing inguinal herniorrhaphy with a spinal anesthetic receives 0.5 mL of 0.5% bupivacaine. What is the expected duration of the block?

A 6-kg, 3-month-old infant undergoing inguinal herniorrhaphy with a spinal anesthetic receives 0.5 mL of 0.5% bupivacaine. What is the expected duration of the block?

• A. Less than 30 minutes
• B. 30 to 60 minutes
• C. 60 to 90 minutes
• D. 90 minutes to 2 hours

Answer: (C)

In infants, how long a spinal bupivacaine block lasts depends largely on the dose per kilogram and the pharmacokinetics of the immature CSF system. A smaller child has a relatively larger CSF volume and faster clearance, so for a given intrathecal dose the block tends to last shorter than in adults. Here, 0.5 mL of 0.5% bupivacaine equals 2.5 mg total. For a 6-kg infant, that’s about 0.42 mg/kg—within the typical infant spinal dose range. That dose commonly provides sensory anesthesia for roughly 1 to 1.5 hours, with motor block persisting a bit longer, so a duration around 60 to 90 minutes is expected.

In infants, how long a spinal bupivacaine block lasts depends largely on the dose per kilogram and the pharmacokinetics of the immature CSF system. A smaller child has a relatively larger CSF volume and faster clearance, so for a given intrathecal dose the block tends to last shorter than in adults. Here, 0.5 mL of 0.5% bupivacaine equals 2.5 mg total. For a 6-kg infant, that’s about 0.42 mg/kg—within the typical infant spinal dose range. That dose commonly provides sensory anesthesia for roughly 1 to 1.5 hours, with motor block persisting a bit longer, so a duration around 60 to 90 minutes is expected.