DENSITY OF LUMBAR CEREBROSPINAL-FLUID IN PREGNANT AND NONPREGNANT HUMANS

Background: Dextrose-free local anesthetics and opioids, alone and in combinations, are being used increasingly to provide subarachnoid anes thesia and analgesia. These dextrose-free drugs have been described as hypobaric by some and isobaric by others. To accurately classify anes thetics with regard to baricity, the density of cerebrospinal fluid (C SF) must be known. The authors sought to determine the exact density o f human CSF, and determine whether CSF density is altered by pregnancy . Methods: Density measurements accurate to 0.00001 g/ml were made at 37.00 degrees C, using a mechanical oscillation resonance frequency de nsity meter. Cerebrospinal fluid samples were obtained from 44 patient s during spinal anesthesia. Five groups were studied: men, and premeno pausal, postmenopausal, term pregnant, and postpartum women. Results: Mean CSF densities in men (1.00064 +/- 0.00012 g/ml), postmenopausal w omen (1.00070 +/- 0.00018 g/ml), and nonpregnant premenopausal women ( 1.00049 +/- 0.00004 g/ml) were significantly greater than in term preg nant (1.00030 +/- 0.00004 g/ml) and postpartum (1.00034 +/- 0.00005 g/ ml) women.Cerebrospinal fluid density did not correlate with age. Conc lusions: Mean CSF density varies in different patient subpopulations. Pregnancy and the immediate postpartum period are associated with the lowest CSF densities. In addition, the cutoff values defining hypobari city (mean CSF density minus three standard deviations) are greater th an previously reported. Accurate CSF density values should be used whe n considering baricity as a mechanism for clinical observations of dex trose-free intrathecal local anesthetics and opioids. Gestational stat us also should be considered.