In Reply:—
We thank Dr. Marcus for the interest in our work and the comments. In parturients with normal uteroplacental circulation, uterine blood flow is the major determinant of oxygen transport to the fetus. The increase in uterine blood flow after colloid preload suggested by Dr. Marcus is interesting. Our results suggested that colloid preloading resulted in marked increases in maternal cardiac output without significant change in blood pressure. Therefore, the increase in uterine blood flow may be associated with an increase in maternal cardiac output.
Doppler ultrasonography is the most commonly used method to measure uterine blood flow in humans. The velocity of the blood flow is one of the representative parameters measured with Doppler ultrasonography. However, the reliability of the measurements is controversial because shape of the waveform is affected by many factors, such as maternal cardiac output, elasticity of the vessel wall, outflow impedance, and blood viscosity. Calculation of blood flow necessitates precise determination of mean velocity, angle of insonation, and vessel diameter. Although angle of the insonation and blood velocity are determined correctly, accurate measurement of small-vessel diameters, such as the diameter of a uterine artery, is difficult. It is also not clear whether the blood flow reflects the functional placental perfusion because some portion of blood flow is shunted to the myometrium. Therefore, extrapolation of measurements from the Doppler ultrasonographic method must be done with caution.
As Dr. Marcus pointed out, an understanding of the uteroplacental blood flow is essential in obstetric anesthesia practice. However, the goal of our anesthetic management is to maintain fetal oxygenation, not uterine artery blood flow. We think that, in the future, fetal oxygenation monitoring would show the effect of preloading on fetal oxygenation.
