Relationship between lung-to-head ratio and lung volume in normal fetuses and fetuses with diaphragmatic hernia

To examine the relationship between the fetal lung area and lung area to head circumference ratio (LHR) and lung volume by three-dimensional (3D) ultrasonography in normal fetuses and in fetuses with unilateral congenital diaphragmatic hernia (CDH).

In 64 fetuses with CDH at median 26 (range, 20-32) weeks of gestation the contralateral lung volume, lung area and LHR were measured and the values were compared to those of 650 normal fetuses at 12-32 weeks. In the normal fetuses both lungs were assessed but in the 64 fetuses with CDH only the contralateral lung was measured because the ipsilateral lung could be visualized adequately in only 40 (62.5%) of the cases. Regression analysis was used to assess the significance of the association between lung volume and lung area or LHR. In the fetuses with CDH, the observed to expected ratios for lung area and LHR were calculated. The expected lung area and LHR were the normal median for a given lung volume. The significance of the differences between the observed to expected lung area and LHR in fetuses with CDH and normal fetuses was determined. In the 64 fetuses with CDH and in 64 normal fetuses, matched for gestational age, the lung length between the apex and the superior aspect of the diaphragm dome was also recorded.

In normal fetuses the median lung area and LHR in both the left and right lungs increased significantly with lung volume. In the fetuses with CDH, the lung area and lung volume for gestation were substantially lower than in normal fetuses and the ratios of observed to expected lung area and LHR for a given lung volume were significantly lower than the respective values in normal fetuses. Additionally, the mean lung length was 13% greater and the mean lung area was 44% smaller than the respective values in the normal controls matched for gestational age.

The finding of a significant association between LHR and lung volume has validated the use of LHR in the assessment of lung growth. However, the study has also demonstrated that in fetuses with CDH, LHR underestimates the actual lung volume, because the herniated viscera cause a greater lateral, rather than vertical, compression of the contralateral lung.