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The role of crystalloid preloading to prevent hypotension associated with spinal anesthesia in parturients during cesarean section has been challenged. Direct measurement of blood volume should provide insight regarding the volume-expanding effects. The aim of the current study was to clarify the effects of volume preload with either crystalloid or colloid solution on the changes in blood volume of parturients undergoing spinal anesthesia for cesarean section.Thirty-six healthy parturients scheduled for elective cesarean section during spinal anesthesia were allocated randomly to one of three groups receiving 1.5 l lactated Ringer's solution (LR; n = 12), 0.5 l hydroxyethylstarch solution, 6% (0.5 l HES; n = 12), and 1.0 l hydroxyethylstarch solution, 6% (1.0 l HES; n = 12), respectively. Blood volume and cardiac output were measured before and after volume preloading with indocyanine green (ICG), and the indocyanine green blood concentrations were monitored by noninvasive pulse spectrophotometry.After volume preload, the blood volume significantly increased in all three groups (P< 0.01). The volume of infused solution remaining in the vascular space in the LR, 0.5-l HES, and 1.0-l HES groups were 0.43 ± 0.20 l, 0.54 ± 0.14 l, and 1.03 ± 0.21 l, respectively, corresponding to 28% of lactated Ringer's solution and 100% of hydroxyethylstarch solution infused. Significant increases in cardiac output were observed in the 0.5-l and 1.0-l HES groups (P< 0.01). A significant correlation between the percentage increase in blood volume and that of cardiac output was observed by volume preloading (r2 = 0.838;P< 0.001). The incidence of hypotension was 75% for the LR group, 58% for the 0.5-l HES group, and 17% for the 1.0-l HES group, respectively.The incidence of hypotension developed in the 1.0-l HES group was significantly lower than that in the LR and 0.5-l HES groups, showing that greater volume expansion results in less hypotension. This result indicates that the augmentation of blood volume with preloading, regardless of the fluid used, must be large enough to result in a significant increase in cardiac output for effective prevention of hypotension.