To the Editor:—
I congratulate Dr. Ueyama et al. 1for their work regarding the effects of crystalloid and colloid loading in parturients undergoing spinal anesthesia. However, the relevance of this work—and many other studies concerning fluid loading—to fetal well-being is not entirely clear. Studies consistently have shown a beneficial effect of volume loading on maternal blood pressure or the response to the induction of spinal or epidural anesthesia. Moreover, the importance of maternal blood pressure on the fetus is unclear. For example, our work has shown that changes in cardiac output produced by volume loading may correlate better with uterine blood flow than does blood pressure. 2
Profound hypotension clearly can compromise uterine perfusion, but the influence of lesser changes or of various interventions used to influence blood pressure is unknown. In our opinion, the issue of volume loading may be centered on the wrong parameters. A far better end point would be something such as uterine blood flow or perhaps umbilical blood flow. In the past, these could only be assessed in animals. 3However, Doppler methods now permit such measurements. In fact, the current availability of color Doppler makes it possible to measure not only flow velocities, but also absolute flow in selected vessels. To measure the effectiveness of therapeutic interventions in obstetric anesthesia practice, future studies should take advantage of this new technology, rather than being limited to parameters such as blood pressure, blood volume, and cardiac output.