Transversus Abdominis Plane Block in Children Undergoing Open Pyeloplasty Surgery

Inclusion Criteria: * American Society of Anesthesiologists (ASA) classification score 1-3 * age 1 month to 6 years inclusive Exclusion Criteria: * children undergoing an additional surgical procedure at an anatomical location not covered by a unilateral TAP block during the same anesthetic * children in whom a TAP block is contraindicated, i.e. surgical scar or distorted anatomy at the site of injection * postoperative admission to the intensive care unit * children with a known allergy to bupivacaine * children with a history of chronic abdominal pain requiring opioid analgesics * children with known renal insufficiency * children with known impaired hepatic function * children with known impaired cardiac function * children known hypersensitivity to sodium metabisulfite