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This study was designed to evaluate different doses of hypobaric 0.15% bupivacaine administered to achieve unilateral orthopaedic surgery under subarachnoid block. Using a randomised, double-blinded protocol, 150 ASA I to II patients scheduled for elective unilateral orthopaedic surgery were allocated to received a subarachnoid block with hypobaric bupivacaine 0.15% 4.5 mg (3 ml) (Group 1), 6.0 mg (4 ml) (Group 2) or 7.5 mg (5 ml) (Group 3). Measurements included cardiovascular and haemodynamic stability, incidence of unilateral anaesthesia, time to recover from motor block, postoperative urinary retention, transitory neurological symptoms and postdural puncture headache. There was a dose-dependent increase in the duration of the block, from 1:55 +/- 00:20 hours (4.5 mg) to 2:15 +/- 00:22 hours (6.0 mg) and 3:15 +/- 00:31 hours (7.5 mg). The sensory block in the operated limb was significantly higher than that in the non-operated limb at all times. The increasing dose resulted in an increase of the motor block score. Seventy percent of patients had unilateral block. It was concluded that the spinal anaesthesia in all groups was suitable for single limb orthopaedic surgery. The smallest dose (4.5 mg) of hypobaric 0.15% bupivacaine resulted in more unilateral blocks, with narrower distribution and shorter duration. [ABSTRACT FROM AUTHOR]