Background: Although the cerebrospinal fluid (CSF) is the pathway of anesthetic delivery and the diluent for neuraxially administered drugs, little is known about its volume, including variability among individuals, longitudinal distribution, or influence of body habitus. Models made to investigate subarachnoid anesthetic distribution lack valid dimensions. CSF volume was measured in volunteers, and the effect of obesity and abdominal compression on CSF volume was evaluated using magnetic resonance imaging.
Methods: Low thoracic and lumbosacral axial magnetic resonance images of 25 healthy volunteers were obtained at 8-mm intervals by fast spin-echo sequence, which highlights CSF. A repeat image series was performed in 15 subjects during external abdominal compression. In two subjects, images were obtained without compression for the entire vertebral column. Dural sac and spinal cord areas were determined in a blinded fashion for each image using video/digital analysis. Area of the sac minus area of the cord constituted area of CSF and roots ("CSF/root"); this area multiplied by 8 mm resulted in CSF/root volume per section.
Results: There is great interindividual variability in CSF/root volume. From the T11-T12 disc to the sacral terminus of the dural sac, the mean volume for all subjects is 49.9 +/- 12.1 ml (mean +/- SD; range 28.0-81.1 ml). This volume was significantly less in relatively obese subjects (42.9 +/- 9.5 ml) than in nonobese subjects (53.5 +/- 12.9 ml). Abdominal compression decreased CSF/root volume by 3.6 +/- 3.2 ml. Sections through intervertebral foramina showed the biggest decrease with abdominal compression, with a lesser change in sections with veins and no change in the absence of these anatomic features. Total vertebral CSF/root volume in two subjects was 94.84 and 120.01 ml, respectively.
Conclusions: CSF volume is widely variable between individuals. The decreased CSF volume that results from increased abdominal pressure, such as with obesity or pregnancy, may produce more extensive neuraxial blockade through diminished dilution of anesthetic. The mechanism by which increased abdominal pressure decreases CSF volume is probably inward movement of soft tissue in the intervertebral foramen, which displaces CSF.