Densities of cerebrospinal fluid and spinal anaesthetic solutions in surgical patients at body temperature

Purpose: To determine the densities of cerebrospinal fluid (CSF) in patients for surgery under spinal anaesthesia. The densities of the CSF were compared with the densities of local anaesthestic solutions and their mixtures with commonly used spinal opioids. Method: One ml of CSF was collected from 131 consecutive patients that consented to the study at the time of spinal anaesthesia, Densities were; measured at 37 degrees C in a Density Meter that displayed density to the fourth decimal point and was accurate to 0.00003 g . ml(-1). The densities of a selection of spinal anaesthetic drugs were also measured. Results: The mean CSF density in the study population was 1.00059 +/- SD 0.00020. In men of all ages, the mean CSF density was 1.00067 +/- 0.00018 g . ml(-1); in postmenopausal women 1.00060 +/- 0.00015 g . ml(-1); in premenopausal non-pregnant women 1.00047 +/- 0.00076 g . ml(-1); and in pregnant women 1.00033 +/- 0.00010 g. ml(-1). There were differences between the CSF densities in pregnant-women compared with men (P = 0.0001), postmenopausal women (P = 0.0001) and non-pregnant premenopausal women (P = 0.03). Local anaesthetic solutions that contain sugar (glucose or dextrose) were all hyperbaric. In the absence of sugar, all local anaesthetic solutions were hypobaric except for lidocaine CO2 which was slightly hyperbaric; Opioids were all hypobaric except meperidine which was hyperbaric. Conclusion: Pregnant women have slightly lower CSF densities than do men and postmenopausal women, and non-pregnant premenopausal women. In the absence of sugar all spinal anaesthetic solutions measured were hypobaric except for lidocaine CO2 and meperidine, both of which were hyperbaric.