Spinal Anesthesia: Level of block

The level of block achieved by a spinal anesthetic is based on factors in three major categories: 1) the properties of the local anesthetic used in the injection, 2) patient characteristics, and 3) technique.

Of the important factors, the most important factors for local anesthetic spread are the baricity of the solution, the positioning of the patient during and immediately after the injection, and the dose of the injection injected.

The baricity of the injected fluid is determined by the density of the fluid divided by the density of CSF at 37 degrees Celsius. Injected fluids can be hypobaric, isobaric, or hyperbaric. Hypobaric solution will rise against gravity, isobaric solutions will tend to remain at the level injected, and hyperbaric solutions will follow gravity after the injection.

Patient positioning can also greatly affect spread of local anesthetic. For hypobaric solutions, the fluid will rise against gravity; that is, will move in the caudal direction for patient in Trendelenburg and will move cephalad in patients in reverse Trendelenburg. The reverse is true for hyperbaric solutions.

Both dose of local anesthetic and volume of injected fluid affect spread in a proportional manner. Studies have shown that the dose of the anesthetic is more important than the volume given.