LUMBOSACRAL CEREBROSPINAL-FLUID VOLUME IS THE PRIMARY DETERMINANT OF SENSORY BLOCK EXTENT AND DURATION DURING SPINAL-ANESTHESIA

Background Injection of local anesthetic into cerebrospinal fluid (CSF ) produces anesthesia of unpredictable extent and duration. Although m any factors have been identified that affect the extent of spinal anes thesia, correlations are relatively poor and the extent of spread rema ins unpredictable. This study was designed to determine whether variab ility in the volume of lumbosacral CSF among individuals is a contribu ting factor in the variability of spinal anesthesia. Methods: Spinal a nesthesia was administered to 10 healthy volunteers with 50 mg lidocai ne in 7.5% dextrose. The technique was standardized to minimize variab ility in factors known to affect the distribution of spinal anesthesia . The extent of sensory anesthesia was assessed by pin-prick and by tr anscutaneous electrical stimulation. Motor blockade was assessed in th e quadriceps and gastrocnemius muscles by force dynamometry, Duration of anesthesia was assessed by pinprick, transcutaneous electrical stim ulation, and duration of motor blockade. Lumbosacral CSF volumes were calculated from low thoracic, lumbar, and sacral axial magnetic resona nce images obtained at 8-mm increments. Volumes of CSF were correlated with measures of extent and duration of spinal anesthesia using the K endall rank correlation test. Results Lumbosacral. CSF volumes ranged from 42.7 to 81.1 mi Volumes of CSF correlated with pin-prick assessme nts of peak sensory block height (P = 0.02) and duration of surgical a nesthesia las assessed by the duration of tolerance to transcutaneous electrical stimulation at the ankle (P < 0.05). Conclusions: Variabili ty in lumbosacral CSF volume is the most important factor identified t o date that contributes to the variability in the spread of spinal sen sory anesthesia.