Baricity
Encyclopedia
Baricity refers to the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 of a substance compared to the density of human cerebral spinal fluid. Baricity is used in anesthesia
Anesthesia
Anesthesia, or anaesthesia , traditionally meant the condition of having sensation blocked or temporarily taken away...

 to determine the manner in which a particular drug will spread in the intrathecal
Intrathecal
Intrathecal is an adjective that refers to something introduced into or occurring in the space under the arachnoid membrane of the brain or spinal cord...

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 a baricity 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 desired local 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 to gravitational pull. These properties allow the anesthesia provider to preferentially control the spread of the block by choice of mixture and patient positioning.
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