Effects of crystalloid and colloid preload on blood volume in the parturient undergoing spinal anesthesia for elective cesarean section

Background: The role of crystalloid preloading to prevent hypotension assoc iated with spinal anesthesia in parturients during cesarean section has bee n challenged. Direct measurement of blood volume should provide insight reg arding the volume-expanding effects. The aim of the current study was to cl arify the effects of volume preload with either crystalloid or colloid solu tion on the changes in blood volume of parturients undergoing spinal anesth esia for cesarean section. Methods: Thirty-six healthy parturients scheduled for elective cesarean sec tion during spinal anesthesia were allocated randomly to one of three group s receiving 1.5 1 lactated Ringer's solution (LR; n = 12), 0.51 hydroxyethy lstarch solution, 6% (0.5 1 HES; n = 12), and 1.01 hydroxyethylstarch solut ion, 6% (1.0 1 HES; n = 12), respectively. Blood volume and cardiac output were measured before and after volume preloading with indocyanine green (IC G), and the indocyanine green blood concentrations were monitored by noninv asive pulse spectrophotometry. Results: After volume preload, the blood volume significantly increased in all three groups (P < 0.01). The volume of infused solution remaining in th e vascular space in the LR, 0.5-1 HES, and 1.0-1 HES groups were 0.43 +/- 0 .20 1, 0.54 +/- 0.141, and 1.03 +/- 0.21 1, respectively, corresponding to 28% of lactated Ringer's solution and 100% of hydroxyethylstarch solution i nfused. Significant increases in cardiac output were observed in the 0.5-1 and 1.0-1 HES groups (P < 0.01). A significant correlation bem-een the perc entage increase in blood volume and that of cardiac output was observed by volume preloading (r(2) = 0.838; P < 0.001). The incidence of hypotension w as 75% for the LR group, 58% for the 0.5-1 HES group, and 17% for the 1.0-1 HES group, respectively. Conclusions: The incidence of hypotension developed in the 1.0-1 HES group was significantly lon-er than that in the LR and 0.5-1 HES groups, showing that greater volume expansion results in less hypotension, This result indi cates 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.