Spinal anaesthesia with hypobaric 0.19% or plain 0.5% bupivacaine.

Hypobaric 0.19% bupivacaine (plain 0.5% bupivacaine 3 ml + distilled water 5 ml) was compared with 0.5% plain bupivacaine 3 ml for spinal anaesthesia in 29 healthy patients undergoing orthopaedic surgery of the lower extremities. The solutions were injected at the L3-4 interspace in 40 s, and patients were kept sitting for 2 min after injection. The mean maximal cephalad spread of sensory block was to the T1 segment (SD 3.6) and to T8 (4.1) in the hypobaric and plain bupivacaine groups, respectively (P less than 0.0001). The study was interrupted after observing the sensory block of the 29th patient (hypobaric bupivacaine) spread to C2 within 5 min of injection. In most patients, the hypobaric bupivacaine block affected the upper thoracic nerves, and in three patients the cervical nerves also. The high levels of block were accompanied by marked hypotension. The extensive spread of the blocks makes this hypobaric spinal anaesthesia technique unsuitable for routine use.