Does an expanding fetal abdominal mass produce pulmonary hypoplasia?

Oligohydramnios and associated fetal complications

During intrauterine life the fetus is bathed in amniotic fluid which provides a low resistance space for free movement and a buffer against external trauma. This fluid is produced in early pregnancy largely as a maternal dialysate, then as a fetal transudate. Fetal urine is the most important source of amniotic fluid after 16 weeks gestation. The control of amniotic fluid is complex and poorly understood; it arises from secondary partitioning of water within the fetoplacental extracellular space and reflects fetal fluid balance.

Congenital malformations of the lung

Congenital malformations of the lung are a frequent cause of abnormal chest radiographs in neonates and young children as well as a common cause for recurrent pneumonia in older patients. Based on the mode of presentation and clinical findings along with an understanding of lung embryology, one can develop a thorough differential diagnosis that allows for the evaluation of the lesion in question. Appropriate management of the patient can then be undertaken with a minimum of diagnostic studies and without any delay in treatment.

Neonatal pulmonary hypoplasia: an autopsy study of 25 cases

Pulmonary hypoplasia (PH) is defined as defective or incomplete development of lungs that are immature for gestational age. A prospective study was done to establish practical criteria for the diagnosis of pulmonary hypoplasia and to determine the relative frequency of pulmonary hypoplasia and its associated congenital malformations. Postmortem, formalin-inflated, routinely processed lungs were examined for fresh lung weight, fixed lung volume, radial alveolar count, and tissue maturity. Of these, lung volume was found to be the least useful. However, no single parameter was adequate for diagnosis in every case. Using the remaining three in conjunction, reliable diagnostic criteria were established. In this series of 113 cases, 22% had pulmonary hypoplasia as the primary cause of death, of which one-third had no associated congenital malformations to account for the development of pulmonary hypoplasia. It was found that premature rupture of membranes (PROM) for as short as 5 days could lead to fatal PH. It is concluded that PH is a common problem in neonatal autopsies and can be diagnosed in the average laboratory. Recognizing PH is important because once it has developed, intrauterine intervention in cases of oligohydramnios and postnatal treatment with surfactant inhalation cannot influence the outcome. It remains to be seen whether early fluid replacement in PROM will prevent development of PH.

Fetal breathing movements in oligohydramnios are not increased by aminoinfusion

OBJECTIVE

To determine whether fetal breathing movements (FBM) in pregnancies with oligohydramnios change with restoration of amniotic fluid volume.

DESIGN

A prospective experimental study.

SETTING

Fetal Medicine Unit, tertiary referral hospital.

SUBJECTS

16 women with singleton pregnancies complicated by severe oligohydramnios.

INTERVENTIONS

Restoration of amniotic fluid volume by transabdominal amnioinfusion. Controls comprised pregnancies in which infused fluid leaked vaginally, so that oligohydramnios was not corrected.

MAIN OUTCOME MEASURES

Change in total breathing movements, change in FBM incidence derived from 40 min recordings immediately before and after amnioinfusion.

RESULTS

There was no significant difference in the change in total breathing movements or in the change in incidence of FBM between the 10 pregnancies in which amniotic fluid volume was restored, and the other six in which fluid leaked after infusion and volume was not restored. In both groups, there was no significant change with infusion in number of FBM (mean change -72, 95% CI -218 to +74 in the fluid-retained group and -64, 95% CI -273 to +145 in the fluid-leaked group) and incidence of FBM (median change -2.5%, range -27 to +10 in the fluid retained group and -4.5%, range -34 to +15 in the fluid-leaked group).

CONCLUSIONS

This study suggests that restitution of amniotic fluid volume in human pregnancies complicated by severe oligohydramnios does not acutely alter the incidence of FBM. These data support an increasing literature suggesting that impairment of fetal breathing is not the mechanism for oligohydramnios-related pulmonary hypoplasia.

Mimicking low amniotic pressure by chronic pharyngeal drainage does not impair lung development in fetal sheep

OBJECTIVE

The etiology of oligohydramnios-related pulmonary hypoplasia is not understood but is known to involve chronic lung liquid loss. We tested the hypothesis that low amniotic pressure in oligohydramnios disturbs the normal tracheal-amniotic pressure gradient to increase lung liquid loss and impair lung development.

STUDY DESIGN

Chronic pharyngeal catheterization with drainage to the exterior was used in 15 fetal sheep to mimic reduced amniotic pressure at the upper airway in the presence of normal amniotic fluid volume.

RESULTS

Pharyngeal pressures relative to amniotic pressures were negative in all drained fetuses (mean +/- SE -3.0 +/- 0.6 mm Hg), in contrast to positive pressures in controls (0.7 +/- 0.1 mm Hg). There was no significant difference in lung weight or deoxyribonucleic acid relative to body weight, or in lung morphometry, between 10 fetuses drained for 10 to 21 days and their control cotwins.

CONCLUSION

Mimicking low amniotic pressure in the upper airway by chronic fetal pharyngeal drainage does not impair lung development in fetal sheep.

Fetal breathing during chronic lung liquid loss leading to pulmonary hypoplasia

The mechanism of oligohydramnios-related pulmonary hypoplasia (PH) is not understood, but is known to involve chronic loss of lung liquid. In order to investigate the recent suggestion that fetal breathing movements are absent or diminished in oligohydramnios-related PH, we made continuous recordings of fetal breathing movements (FBM) during chronic lung drainage in tracheostomized fetal sheep. FBM remained present throughout the drainage period. There was no significant difference in the number or duration of FBM epochs in tracheostomized fetuses compared to controls, although there was a slight reduction in incidence between 126 and 133 days (P = 0.02). PH in the fetuses undergoing chronic lung liquid drainage was confirmed by significant reductions in lung weight, volume, DNA, and total airspace, and in altered maturation. This study is consistent with other data suggesting that inhibition of FBM is not the primary mechanism of oligohydramnios-related pulmonary hypoplasia.

Low amniotic pressure in oligohydramnios--is this the cause of pulmonary hypoplasia?

The mechanism by which oligohydramnios produces lung hypoplasia is not understood. The current theory that extrinsic compression of the fetal thorax causes hypoplasia, either by inhibiting breathing movements or by squeezing out lung liquid, is not supported by observational or experimental data, or by our finding of decreased amniotic pressure around the fetus in oligohydramnios. We hypothesize that lung hypoplasia results from excess loss of lung liquid because of a reduction in amniotic pressure, and hence an increase in the alveolar-amniotic pressure gradient. The magnitude of this increased pressure gradient is calculated to exceed the small standing tracheal pressure; thus low amniotic pressure overcomes the normal laryngeal retentive mechanisms and allows a larger quantity of lung liquid to escape. In the prevention of pulmonary hypoplasia, a role is suggested for the instillation of artificial amniotic fluid to restore normal amniotic pressure.

Pulmonary development in the fetal lamb with severe bladder outlet obstruction and oligohydramnios: a morphometric study

The lethal feature of male newborns with severe bladder outlet obstruction and oligohydramnios is pulmonary hypoplasia. We report a fetal lamb model of bladder outlet obstruction created in the early second trimester, and morphometric analysis of the profound pulmonary hypoplasia that resulted. At 55 to 65 days of gestation 8 consecutive male fetuses underwent surgical clipping of the urethra and urachus near the abdominal wall. A sham operation was performed in 1 female fetus. The lambs were delivered by cesarean section 10 weeks later. The right lungs were immediately fixed at a distending pressure of 25 cm. water. The results of the morphometric studies were compared to control animals from our earlier study of normal fetal lamb lung development; slides for both studies were scored in a blind, randomized fashion by 1 of us (S. G. D.). Although mean body weights for the groups were similar, mean right lung volume in the operated lambs was 63.8 cM.3 compared to 158 cM.3 in the control animals and 137 cM.3 in the sham operated fetus (p less than 0.0005). Relative volume of respiratory tissue was 87 per cent in the operated animals, and 92 per cent in the controls and the sham operated fetus (p less than 0.0005). Microscopic morphometry revealed an increased relative volume of inter-alveolar tissue in the experimental animals (35 compared to 23 per cent in the normal lambs, p less than 0.05), with a corresponding decrease in relative airspace volume. Alveolar numerical density and alveolar surface density were not statistically different between the groups, although total alveolar number and surface area were substantially decreased in the operated lambs due to the reduced mean lung volume. Mean alveolar wall thickness was increased at 3.6 microM. in the operated group versus 2.1 microM. in the normal group (p less than 0.0005) and 2.8 microM. in the sham operated fetus. Thus, the lung in the lamb with severe bladder outlet obstruction is hypoplastic and immature.

Surgical emergencies after prenatal treatment for intra-abdominal abnormality

Emergency surgery was required in three babies who had been treated prenatally after ultrasonic detection of gastrointestinal abnormality. There are at present no indications for prenatal intervention in such babies.