Expulsion of liquid from the fetal lung during labour in sheep

Abstract

Effective gas exchange after birth requires clearance of most of the liquid filling the lung during gestation. To date the focus has been on active Na⁺ transport from lung lumen to interstitium, but Na⁺ transport begins only close to delivery, making it an unlikely mechanism for clearing the bulk of fetal lung liquid. We hypothesised that fetal trunk muscle contractions, known to occur in labour, are involved in lung liquid clearance. We measured maternal uterine contractions, fetal tracheal flow directly and fetal electromyograms in thoracic and abdominal muscles. During labour in five fetal sheep, brief flow pulses were observed in the trachea, most of which expelled a small volume of lung liquid. Tracheal flow pulses were associated with fetal muscle contractions 89% of the time, which were associated on 91% of occasions with uterine contractions. Our results suggest that liquid contained in the fetal lung is cleared before and during labour as a result of fetal muscular effort, perhaps stimulated by uterine contractions.

Introduction

The fetal lung is distended by a large volume of liquid that is continually produced by active secretion of Cl⁻ ions across the pulmonary epithelium into the future airspace (Olver and Strang, 1974, Barker and Olver, 2002, Olver et al., 2004), whereas the postnatal lung contains just enough liquid to form an extremely thin lining layer over the alveolar surface (Walters, 2002), and epithelial ion transport is dominated by active Na⁺ transport out of the lung lumen (Basset et al., 1987, Rutschman et al., 1993, Goodman et al., 1987, Matthay et al., 1996). Although the mechanisms of lung liquid clearance at birth have not yet been amenable to study in the human fetus and neonate, with the bulk of our understanding deriving from animal studies, there is considerable clinical evidence that lung liquid clearance near the time of birth is essential in the human infant. Thus, failure of liquid removal in the perinatal period has been implicated in the pathogenesis of respiratory distress in the newborn (Hales et al., 1993, Morrison et al., 1995, Gowen et al., 1988; van den Berg et al., 2001), and the more serious respiratory distress syndrome has been shown to be associated with failure of Na⁺ absorption in the respiratory epithelium (Barker et al., 1997).

There are two potential alveolar liquid clearance pathways that have been extensively studied. Aquaporins are water selective pores that mediate water transport in fetal, newborn and adult lungs (Carter et al., 1997, King et al., 1996, Folkesson et al., 1994). While aquaporin gene expression has been shown to increase in the rodent lung at birth (Umenishi et al., 1996, Yasui et al., 1997), knockout models survive after delivery without apparent respiratory distress (Bai et al., 1999, Ma et al., 1997, Ma et al., 2000), indicating that aquaporins have no essential role in the perinatal lung. By contrast, the pulmonary epithelial sodium channel (ENaC) is essential to postnatal respiratory adaptation, in that respiratory function is severely or fatally compromised when the ENaC is disabled, either pharmacologically (O’Brodovich et al., 1990), or genetically (Hummler et al., 1996). Additional findings that the expression of ENaC genes increases just before term (O’Brodovich et al., 1993, Tchepichev et al., 1995, Talbot et al., 1999), and that absorption of liquid via the ENaC is triggered by the rise in circulating adrenaline in labour (Brown et al., 1983, Olver et al., 1986, Finley et al., 1998), are in keeping with a consensus that trans-epithelial Na⁺ transport is the chief mechanism of water clearance in the perinatal period (O’Brodovich, 1997, Jain and Eaton, 2006, Bland, 2001, Barker and Olver, 2002).

Notwithstanding strong evidence that absorption across the pulmonary epithelium can occur at a rapid rate near term in the sheep (Brown et al., 1983), rat (Folkesson et al., 2002) and guinea-pig (Finley et al., 1998), net absorption of liquid during spontaneous labour was reported to occur in the sheep fetus only an hour or two before delivery (Brown et al., 1983) or in only half the sheep fetuses studied (Chapman et al., 1994). As a result, if absorption via Na⁺ transport were the principal mechanism clearing liquid from the lung before birth, many or all fetuses would deliver with their lung containing most of the liquid that filled it in late gestation. However, measurements show that just before delivery the fetal sheep lung contains just 25% of the large volume of liquid that filled it 1 week earlier (Bland et al., 1982, Berger et al., 1998). How this impressive lung liquid clearance is achieved is currently unknown, but the process has been shown to begin in advance of labour (Dickson et al., 1986, Pfister et al., 2001) and, importantly, to occur across a time period in which the pulmonary epithelium continues to secrete liquid (Pfister et al., 2001).

We hypothesized that fetal lung liquid is expelled through the trachea during labour and that the driving mechanism is fetal thoracic and abdominal muscle contractions that raise pressure in the lung. In order to test this hypothesis we performed studies in which we recorded the activity of trunk muscles, and made continuous measurements of lung liquid flow in the trachea in late gestation and again during spontaneous labour in the fetal sheep. To assess a possible maternal role in clearance of liquid from the fetal lung, we also monitored uterine muscle activity and amniotic fluid pressure.