[1] Solutions that have a density very close to that of cerebrospinal fluid have a baricity approaching 1.0 and are referred to as isobaric.
[2] Hyperbaric solutions are created by mixing dextrose 5-8% with the desired local anesthetic; such mixtures tend to have densities far above the theoretical lower boundary.
Plain (no distilled water or dextrose added) solutions of bupivacaine (a commonly used spinal anesthetic) with a density of 0.9993 g litre−1 at 37°C are hypobaric in all patients.
[2] Hyperbaric solutions will flow in the direction of gravity and settle in the most dependent areas of the intrathecal space.
These properties allow the anesthesia provider to preferentially control the spread of the block by choice of mixture and patient positioning.