Spinal Hypotension Associated with Cesarean Section Will Preload Ever Work?

Recent years have seen a move away from large-volume intravenous hydration as prophylaxis against post–spinal hypotension. Although controversy still exists, there is accumulating evidence that crystalloid solutions are particularly ineffective in preventing hypotension after extensive sympathetic blockade associated with spinal anesthesia. One reason crystalloid solutions may not be effective is their short intravascular half-life. In this issue of ANESTHESIOLOGY, Ueyama et al. provide data to support the limited and transient effect of 1,500 ml (more than 20 ml/kg) crystalloid on blood volume and cardiac output compared with hydroxyethyl starch 6% (HES). They suggest that to effectively prevent hypotension associated with obstetric spinal anesthesia by intravenous volume administration, augmentation of blood volume must be large enough to result in a significant increase in cardiac output. The widespread prophylactic use of crystalloid solutions followed work in gravid ewes that showed the value of rapid intravenous fluid administration in partly restoring uterine blood flow after spinal hypotension. Blood pressure usually is maintained in the face of vasodilation (other than that caused by central nerve blockade) by a reflexive increase in cardiac output. However, in the presence of spinal induced venodilation, venous return is reduced to an extent that cardiac output cannot increase and is often reduced. The result is severe hypotension with reduced uteroplacental perfusion. Therefore, efforts were made to increase cardiac preload before instituting spinal blockade in the hope of preventing subsequent hypotension; the concept of intravenous “preload” was born. Initial clinical studies of crystalloid prehydration in obstetric spinal anesthesia appeared to be promising, 6,7 but the results were clouded by issues such as the presence of labor (which decreases the incidence of hypotension), aortocaval compression, and study design. Although intravenous fluid preload became an established part of anesthetic practice, it soon became apparent that it was not completely effective in preventing hypotension. However, it was assumed that, because the theory was sound, more preload was required in some patients. Hence, large volumes (up to 30 ml/kg) were increasingly used or studied. Nevertheless, the problem remained. A number of studies have shown either no or a clinically insignificant reduction in the incidence of spinal hypotension after crystalloid preload, leading some to abandon it altogether and others to remove it from routine mandatory practice. The use of colloid solutions for spinal preloading met with mixed results. Gelatin solutions may be ineffective and have not achieved the degree of success seen with modified starch solutions. This may be a result of the difference in intravascular half-life between the two, supporting the argument used to explain crystalloid failure. However, does this necessarily relate to the concept of preload? The indocyanine green dilution method used by Ueyama et al. to estimate blood volume and cardiac output does not permit a simultaneous assessment of cardiac preload. Nevertheless, similar modest increases in blood volume produced by 1,500 ml lactated Ringer’s and 500 ml HES (8 and 10% respectively) were associated with modest increases in cardiac output (11 and 14%), without a significant reduction in the incidence of subsequent hypotension. Doubling the increase in blood volume (to 19%) by use of 1 l HES more than tripled the associated increase in cardiac output (to 43%). Because heart rate did not change substantially, the change must have been a result of an increase in stroke volume. Because there was no change in arterial pressure associated with volume administration, this implies a decrease in vascular resistance. Increases in cardiac output occur with normovolemic hemodilution. An increase in cardiac output of 29% may be seen after isovolumic reduction of hematocrit to 33%. It is likely that hypervolemic hemodilution would produce an even greater increase in cardiac output via a number of possible mechanisms, This Editorial View accompanies the following article: Ueyama H, Yan-Ling H, Hironobu T, Mashimo T, Yoshiya I: Effects of crystalloid and colloid preload on blood volume in the parturient undergoing spinal anesthesia for elective cesarean section. ANESTHESIOLOGY 1999; 91:1571–6. r