Assessments of fetal swallowed volume: tracer disappearance versus esophageal flow

We sought to compare fetal swallowed volume determinations simultaneously by two techniques--amniotic fluid (AF) tracer disappearance and esophageal flow probe measurements.

Six ovine fetuses (129 +/- 1 days) were chronically prepared with a thoracic esophageal flow probe and vascular and two AF catheters. 125I-labeled albumin was injected into the AF cavity, and samples were withdrawn at timed intervals for 8 hours. The AF volume was calculated by time-0 extrapolation of the semilog of 125I disappearance. Tracer-determined swallowed volume was calculated as the product of AF volume and the slope of isotope disappearance. Flow probe measurement of swallowed volume was determined by computer integration of calibrated flow probe velocity recordings.

The AF volume averaged 805 +/- 168 mL. The isotope disappearance rate from the AF was 2.8 +/- 0.4%/hour. Average tracer-determined swallowed volume (547 +/- 113 mL/day) was greater than flow probe volume (366 +/- 81 mL/day), although these values were not significantly different. However, when corrected for estimated swallowed lung fluid, tracer-determined volume was significantly greater than flow probe volume (P < .05).

Ovine fetal swallowed volume determinations by AF tracer techniques are greater than those determined by esophageal flow probe measurements.