Plain anaesthetic agent solutions in the prone position

Abstract

We read with interest Cuvas et al.’s [1] work comparing the use of low dose plain solutions for spinal anaesthesia in the prone position. Our own work [2, 3] supports his suggestion that the lower incidence of complete motor blockade may be related to the lower dose of local anaesthetic used. Magnetic resonance imaging reveals interesting information about the anatomy of the cauda equina. The cauda equina lies symmetrically on the dorsal side of the subarachnoid space when the patient is in the supine position [3]. However, it moves to the left side of the subarachnoid space when the patient assumes the left lateral decubitus position [4]. These results suggest that the cauda equina has considerable mobility in the cerebrospinal fluid. Although this has not been studied so far, we believe that the same happens when the patient is placed prone with a pillow under the abdomen: the cauda equina assumes an anterior position, making it easier to block sensory roots. It is necessary to consider the specific gravity of the local anaesthetic agent for spinal anaesthesia. To perform dural puncture with patients in the prone position a pillow must be placed under the abdomen to correct the lordosis and increase the spinal interspace. Low dose and hypobaric anaesthetics injected in this position induce anaesthesia solely in posterior roots, avoiding anterior blockade [5]. Placing the patient in the lateral decubitus with the legs flexed 30° keeps the anaesthetic solution confined to the lumbar roots, so that the incidence of hypotension is close to zero. Consequently, the use of an 8 ml.kg−1 saline preload is unnecessary.