title: Hemodynamics of fetal breathing movements: the inferior vena cava.
abstract: OBJECTIVE
  Based on the hypothesis that fetal breathing movements (FBM) enhance sections of the circulation to meet the needs of gas transport, we studied the effects of FBM on the fetal inferior vena cava (IVC), which transports blood with the lowest oxygen saturation in the fetal body.
  
  
  METHODS
  One-hundred and ten women with low-risk singleton pregnancies were included in a longitudinal study during the second half of pregnancy. Inner diameter, peak systolic velocity and time-averaged maximum blood velocity were measured in the IVC below the ductus venosus outlet during rest and FBM. Volume flow and pressure gradient were estimated in 55 observations of forced inspiratory movements at 36 weeks of gestation. The results are presented as mean and 95% CI of the mean.
  
  
  RESULTS
  Based on 585 observations obtained during fetal rest and FBM, we found no difference in diameter, 0.42 (95% CI, 0.41-0.43) cm vs. 0.41 (95% CI, 0.39-0.42) cm, respectively, apart from during high-amplitude inspiratory movement, when the diameter was 0.15 (95% CI, 0.13-0.17) cm. The peak systolic velocity was different during rest and FBM, 34.0 (95% CI, 32.7-35.3) cm/s vs. 81.5 (95% CI, 76.2-87.5) cm/s, respectively, and correspondingly for time-averaged maximum velocity, 19.7 (95% CI, 18.9-20.5) cm/s vs. 37.2 (95% CI, 34.9-39.9) cm/s, respectively. Forced inspiratory movements at 36 weeks significantly reduced flow in the IVC compared with rest, 63.6 (95% CI, 44.4-88.1) mL/min vs. 186.0 (95% CI, 142.8-238.1) mL/min, respectively. The pressure gradient increased 14-fold during forced inspiration, from 0.64 to 8.76 mmHg.
  
  
  CONCLUSIONS
  High-amplitude fetal inspiration substantially constricts the abdominal IVC and creates a negative pressure in the chest. The IVC constriction withholds abdominal blood, thus temporarily giving way to other flows.
authors.affiliation: Department of Clinical Medicine, University of Bergen, Bergen, Norway. miriamnyberg@gmail.com
authors.forename: M K
authors.initials: MK
authors.lastname: Nyberg
authors.forename: S L
authors.initials: SL
authors.lastname: Johnsen
authors.forename: S
authors.initials: S
authors.lastname: Rasmussen
authors.forename: T
authors.initials: T
authors.lastname: Kiserud
mesh_terms: D000293:Adolescent; D000328:Adult; D005260:Female; D005318:Fetal Heart / Q000000981:diagnostic imaging / Q000502:physiology*; D005324:Fetal Movement; D005333:Fetus / Q000098:blood supply / Q000502:physiology*; D006439:Hemodynamics; D006801:Humans; D007231:Infant, Newborn; D008137:Longitudinal Studies; D008297:Male; D011247:Pregnancy; D015656:Respiratory Mechanics / Q000502:physiology*; D016216:Ultrasonography, Prenatal / Q000379:methods*; D014682:Vena Cava, Inferior / Q000000981:diagnostic imaging / Q000196:embryology / Q000502:physiology*; D055815:Young Adult