Baricity

Baricity is a term used in medicine, specifically in anesthesiology, to describe the density of a local anesthetic solution relative to the density of cerebrospinal fluid (CSF). It is a crucial factor determining how a local anesthetic spreads within the subarachnoid space after a spinal injection, influencing the resulting block height and distribution of anesthesia.

A solution's baricity is calculated by dividing the density of the local anesthetic solution by the density of CSF at a specific temperature, usually 37°C (body temperature). CSF density is approximately 1.003 g/mL.

Based on this ratio, solutions are classified into three categories:

• Hyperbaric: A hyperbaric solution is denser than CSF (baricity > 1.003). Due to gravity, a hyperbaric solution will tend to sink to the most dependent part of the subarachnoid space. Clinically, this means that if a patient is positioned with their head down after a spinal injection of a hyperbaric solution, the anesthetic will tend to spread cephalad (towards the head), resulting in a higher block.

• Isobaric: An isobaric solution has a density approximately equal to that of CSF (baricity ≈ 1.000). Isobaric solutions tend to stay where they are injected and their spread is less predictable and more influenced by other factors such as the volume of the injected solution, patient position, and anatomical factors within the spinal canal.

• Hypobaric: A hypobaric solution is less dense than CSF (baricity < 1.003). A hypobaric solution will tend to float or rise within the subarachnoid space. If a patient is positioned with their head up after a spinal injection of a hypobaric solution, the anesthetic will tend to spread caudad (towards the feet), resulting in a lower block.

The baricity of a local anesthetic solution is often adjusted by adding substances like glucose (to make it hyperbaric) or sterile water (to make it hypobaric). Understanding and manipulating baricity allows anesthesiologists to tailor the spinal anesthetic block to the specific needs of the patient and surgical procedure.