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Baricity refers to the density of a substance compared to the density of human cerebral spinal fluid. Baricity is used in anesthesia to determine the manner in which a particular drug will spread inthe intrathecal space.
Solutions that have a baricity approaching 1.000 are referred to as isobaric, as the density of the cerebral spinal fluid is approximately 1.0003+/- 0.0003. Solutions with abaricity less than 0.999 are termed hypobaric, and are usually created by mixing the local anesthetic with distilled water. Hyperbaric solutions are created by mixing dextrose 5-8% with the desiredlocal anesthetic.
Hyperbaric solutions will flow in the direction of gravity and settle in the most dependent areas of the intrathecal space. Conversely, hypobaric mixtures will rise in relation togravitational pull. These properties allow the anesthesia provider to preferentially control the spread of the block by choice of mixture and patient position

¨Baracity- the ratio comparing the densityof one solution to another.
¨Exerts one of the greatest effects on the spread of local anesthetic within the subarachnoid space.
¨Dependent upon the specific gravity of the solution in relation tothe CSF environment.
¨CSF at 37 degrees C has a specific gravity of 1.003-1.008.
¨Based on the normal specific gravity of CSF (1.003-1.008) a local anesthetic solution may be hyperbaric, isobaric,or hypobaric.
¨Hyperbaric means that the solution is ‘heavier’ than the CSF. This is one of the most common forms of local anesthetic used. A local anesthetic is made to be hyperbaric by addingdextrose to the solution.
¨Isobaric solutions generally have the same specific gravity as the CSF. Generally the medication will spread within the area of injection. The solution may be alreadymanufactured as an isobaric solution or you may mix your local anesthetic solution with CSF in a 1:1 ratio.
¨Hypobaric solutions are ‘lighter’ than the CSF. This means they will move in a cephalad...
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