Baricity and the distribution of lidocaine in a spinal canal model.

The role of the baricity of local anaesthetic solutions in determining the distribution of local anaesthetics injected into the subarachnoid space (and hence the level of anaesthesia) has been challenged. A recent study found no difference in the extent of cephalad spread of hyperbaric and isobaric solutions and concluded that density had no effect on the spread of local anesthetics. The present study, to determine the validity of this conclusion, utilized a spinal model filled with a "cerebrospinal fluid equivalent." Following the injection of hyperbaric lidocaine, the local anaesthetic was most concentrated at the lower end of the column, whereas following the injection of isobaric solution the local anaesthetic was most concentrated around the site of injection. Therefore, baricity is an important determinant of local anaesthetic distribution in the subarachnoid space.