The Significance of Umbilical Vein Doppler Changes during Fetal Hiccups

> Objective: The physiology of hiccups is a sharp inspiratory gasp against a closed glottis, causing a sudden sharp fall in intrathoracic pressure. This would be expected to cause an increase in venous return to the chest; however, we noticed that umbilical venous Doppler waveforms indicated a cessation of flow during hiccups. We observed other hiccuping fetuses to ascertain the range of response of the fetal umbilical vein Doppler waveforms and derive an explanation for them. Methods: Fetuses who were hiccuping at the time of their antepartum testing had observations of umbilical vein flow made, and representative recordings of the trace were obtained. Results: Ten Doppler traces were obtained from fetuses during hiccuping. In every case there was a brief fall of the umbilical vein Doppler waveform to base line, indicating an arrest of flow. Conclusions: Fetal hiccups are associated with arrest of flow in the umbilical vein as demonstrated by Doppler waveforms. This is contrary to predictions based on previously demonstrated fetal physiology of hiccups. We speculate that the findings result from obstruction of the venous return by a combination of the contracting diaphragm at the level of the inferior vena cava as it passes through the diaphragm, by raised intra-abdominal pressure from the descending diaphragm, and possibly from kinking of the umbilical vein at the umbilical ring.