Fetal splanchnic arteries. A longitudinal study and hemodynamic relations to common Doppler parameters

Abstract

Although the arterial splanchnic circulation is known to be dynamic and of clinical interest during postnatal life, little is known about its prenatal development and function. We hypothesised that current ultrasound technology allows standardised techniques and reference ranges to be established and the dynamics of this section of the fetal circulation to be studied. Aims: The aims of the study were 1) to establish reproducible ultrasound techniques for assessing the fetal celiac, hepatic, splenic and superior mesenteric arteries and longitudinal reference ranges for flow velocity and pulsatility index, 2) to address the hemodynamic relationships between these arteries and the umbilical liver perfusion under physiological conditions and how these arteries are related to the cerebral and umbilical circulation and 3) to establish longitudinal reference ranges for the middle cerebral artery and the cerebroplacental ratio. Material and methods: We recruited 27 women with low-risk pregnancies to the pilot study establishing insonation techniques and a further 161 for a longitudinal study, all after we obtained written consent. In the longitudinal study, we scheduled the participants for ultrasound examinations 3–5 times during the second half of pregnancy, each session lasting a maximum of 1 hour. The Doppler assessment included the middle cerebral and umbilical artery, the celiac, hepatic, splenic and superior mesenteric arteries, the left portal vein, the umbilical vein and the ductus venosus. We carried out fetal biometry each time. The ductus venosus peak velocity represented the port-caval pressure gradient, and the velocity in the left portal vein reflected the distribution of umbilical blood within the fetal liver. Means and centiles were constructed using multi-level modelling. We assessed the relationships between splanchnic arteries using Pearson’s correlation coefficients, and the relationship with the umbilical liver perfusion and the pulsatility index of the cerebral and umbilical arteries was assessed using deviance statistics. Results: We established the interrogation of the left branch as our standard for the Doppler assessment of the hepatic artery since this technique minimised interference from neighbouring vessels and established longitudinal reference ranges. We showed that low port-caval pressure was associated with low impedance in the hepatic artery, supporting the assumption that the hepatic artery buffer response also operates in the fetus (Article I). We also established longitudinal reference ranges for the celiac and splenic arteries and provided terms for conditional ranges for repeat measurements. Splenic and celiac arteries showed compensatory mechanisms supporting the portal perfusion of the fetal liver: low umbilical perfusion and port-caval pressure were linked to splenic and celiac artery vasodilation (Article II). Along the same lines, we established longitudinal reference ranges for the superior mesenteric artery and demonstrated a link to the port-caval pressure and venous liver perfusion (Article III). However, there was limited correlation between the pulsatility index of the branches from the celiac artery and the pulsatility index of the superior mesenteric artery, suggesting largely independent local regulation (Articles II and III). The longitudinal reference ranges established for the middle cerebral artery and cerebroplacental ratio differed from those derived from cross-sectional data (Article IV). Further, the conditional ranges assigned for serial measurements are narrower and shifted compared with the ranges for the entire population (Articles II–IV). The covariation of the umbilical and cerebral arteries with the celiac, splenic and superior mesenteric arteries indicates additional common determinants for these circuits (Articles II and III). Conclusions: We provide longitudinal reference ranges for the upper splanchnic and middle cerebral arteries and the cerebroplacental ratio and terms for conditional mean and ranges suitable for serial measurements. The development of these ranges indicates increased perfusion of the splanchnic tissues towards the end of pregnancy. Our study suggests that the hepatic artery buffer response is operating in the fetus and that the splenic and superior mesenteric arteries, by feeding the portal vein through a distended vasculature, also support the fetal liver perfusion. Although locally these splanchnic arteries operate independently, they also seem to be under common influence with the cerebral and umbilical circulation.