Fetal swallowing: correlation of electromyography and esophageal fluid flow

Patterns of fetal drinking behavior were quantified in five singleton fetuses (128 +/- 1 days gestation) by computer analysis of laryngeal-esophageal electromyography (EMG) and thoracic esophageal fluid flow. Esophageal flow was noted to be bidirectional with an average antegrade flow-to-retrograde flow ratio of 4.4 +/- 1.5. Retrograde esophageal flow coincided with diaphragmatic contractions, suggesting that inspiratory gastroesophageal pressure gradients may be operative. The regurgitated fluid was returned to the fetal rumen by thoracic esophageal contractions. Significant net fluid intake was noted during periods of primarily unidirectional antegrade esophageal flow. Fetuses swallowed an average of 35 +/- 9 ml/h during the 12-h study, extrapolating to an average daily volume of 840 +/- 224 ml. An EMG "propagated swallow" representing coordinated contractions of the thyrohyoid, nuchal esophagus, and thoracic esophagus averaged 43 +/- 3 swallows/h and was highly correlated with net esophageal flow. Nearly 60% of the total fluid intake occurred during "bouts" of propagated swallows, which represented only 8% of the study period. Bouts averaged 11.7 +/- 0.4 propagated swallows, lasted 2.1 +/- 0.2 min, and occurred every 27.6 +/- 1.7 min. Although EMG swallowing activity was similar, there were significant differences among fetuses in net esophageal fluid flow and volume per propagated swallow. We speculate that fetal fluid intake is dependent on the availability and physical properties of the swallowed fluid, whereas swallowing activity is primarily related to cortical maturation and thirst stimulation.