Spinal anesthesia for cesarean delivery with dilute solutions of plain bupivacaine: the relationship between infused volume and spread.

In a prospective open study, 40 women received spinal anesthesia for cesarean delivery induced with 15 mg glucose-free bupivacaine administered in dilutions of 0.5% (3 ml), 0.25% (6 ml), 0.125% (12 ml) and 0.083% (18 ml). The solutions were infused over 30 minutes while the women lay in the right lateral decubitus. No clinical or statistical difference in the final level of block was observed between the groups. All the solutions behaved as if slightly hypobaric with the development of essentially unilateral blocks during the infusion period. At the end of the infusion period when the women were turned from the lateral decubitus to a supine tilted position, there was a sudden rise in the levels of block on both sides of the body such that, by ten minutes after turning, there was no discernible difference between the two sides. The final block levels attained were within the expected range for conventional spinal anesthesia using 2.5-3.0-ml glucose-free 0.5% bupivacaine. The findings suggest that the subarachnoid spread of isobaric/hypobaric spinal anesthetic drugs is a complex process unlikely to be explained by reference to simple models of the spinal curves.