What agents are commonly used for subarachnoid block?

What agents are commonly used for subarachnoid block?

The most frequently used agent for spinal anesthesia is hyperbaric tetracaine. Tetracaine is usually supplied as a 1% solution, which is made hyperbaric by addition of an equal volume of 10% dextrose in water. Anticipated duration of anesthesia is usually 1.5–2.5 hours. The addition of epinephrine (0.2–0.4 mg) will generally provide a duration of action between 2.5 and 4 hours.

Hyperbaric lidocaine is usually supplied as a 5% solu-tion and does not require mixing. Its duration of action is 0.5–1.5 hours. Adding 0.2 mg of epinephrine extends the duration of action to 1–2 hours.

Spinal bupivacaine has gained popularity and is commercially available as a 0.75% solution with dextrose. Its duration of action is similar to that of tetracaine. The addition of epinephrine only minimally prolongs bupiva-caine’s duration of action. Spinal bupivacaine provides a dense sensory block, but its motor block is less profound than that of tetracaine.

Lidocaine 2.0%, and bupivacaine 0.5%, are commonly employed isobaric preparations. Their use requires a solu-tion free of preservatives and antioxidants. Methylparaben, sodium bisulfite, or sodium metabisulfite may be neuro-toxic and are contraindicated for injection into the subarachnoid space. A large experience with isobaric preservative-free solutions of both agents has found them to be safe and effective subarachnoid medications.

Hypobaric tetracaine is prepared from the standard 1% solution, which is mixed with a large volume of distilled water. Tetracaine, 5–10 mg, is usually diluted with 5–10 mL of distilled water to form a very hypobaric solution. Dibucaine, 0.066%, an alternative to hypobaric tetracaine, is no longer available in the United States. Hypobaric and isobaric solutions are not commonly employed in the United States. However, they do provide excellent anesthesia when used properly.