*2.10.3. Fetal lung volumes*

Normal lung area develops four times more than the head circumference between week 12 and 32; therefore, the LHR needs to be adjusted according to gestational age. The effect of gestational age on LHR can be minimized by expressing the observed LHR as a ratio to the

The lung-to-head circumference ratio (LHR) is a sonographic measure proposed to identify fetuses with congenital diaphragmatic hernia (CDH) that have a poor prognosis. The lung area contralateral to the CDH is measured at the level of *four-chamber view* by manual tracing of the lung which appears to be the most reproducible way of measuring the lung area [30].

The observed LHR may be expressed as a percentage of the expected mean for gestational age

There has been proposed a four-step stratification of fetuses based on their observed/expected

• Fetuses with an O/E LHR <15% have extreme pulmonary hypoplasia and there are no sur-

• Fetuses with an O/E LHR between 15 and 25% have severe pulmonary hypoplasia and survival prediction is about 20%, those with the liver down having better prognosis than

• Fetuses with an O/E LHR between 26 and 35% and those with an O/E LHR between 36 and 45% but with the liver up have moderate hypoplasia. They have an expected survival rate

• Fetuses with an O/E LHR between 36 and 45% with the liver down and those with an O/E LHR > 45% have mild hypoplasia and have better prognosis, >75% survival rate [33, 34] (**Table 7**).

• Prenatal prediction of postnatal outcome in CDH patients is based on ultrasound measurement of contralateral

• The ratio of observed to predicted lung/head ratio is the most validated predictor for postnatal outcome; the

• Fetal tracheal occlusion has been shown safe and feasible by minimally invasive means.

The product is divided by the head circumference (HC) to obtain the LHR.

as the observed/expected lung-to-head ratio O/E LHR [31, 32].

between 30 and 60%, which depends on the lung size.

lung size and magnetic resonance total lung volumetry.

liver position is also predictive.

**Table 7.** Key points.

(O/E) LHR wherein it was also taken into account their liver position.

The lung area to head circumference ratio (LHR) = lung area/head circumference.

expected mean LHR for that gestational age.

200 Congenital Anomalies - From the Embryo to the Neonate

*2.10.1. The lung area to head circumference ratio*

*2.10.2. Observed/expected lung-to-head ratio*

those with liver up in the thorax.

vivors reported.

O/E LHR = (observed LHR/expected LHR) × 100

On fetal MR imaging there are several methods described for estimation of fetal pulmonary hypoplasia. All of these methods include the measuring of total fetal lung volume (TFLV). MR volumetry is considered the most accurate method for measuring ipsilateral lung and, therefore, for best estimation of fetal lung volumes, compared to three-dimensional ultrasound [35].

#### *2.10.4. Fetal intervention*

Because CDH is an anomaly with poor prognosis that can be identified in the prenatal period, it is justified to perform an intervention which improves lung function. Di Fiore and Wilson studied the concept of triggering lung growth by tracheal occlusion [36]. This was observed in an experiment of nature occurring in fetuses with congenital high airway obstruction syndrome (CHAOS), who display impressive lung growth. Through pregnancy, fetal lungs secrete fluid during their development, which creates a positive pressure under the glottis. Fluid secretion of the lung and a cyclical pressure change are essential for development and growth of the lung [37]. Fluid secretion increases the pressure in airways but during fetal breathing movements, the pressure gradient is normalized as glottis opens. This phenomenon creates cyclical periods of tissue stretch, which are important for an optimal balance between growth and differentiation [38]. Intrauterine tracheal occlusion acts like a closed glottis trapping the fluid causing subsequently increased tissue stretch, which triggers lung growth. The tracheal occlusion decreases the number of type II alveolocytes and lowers surfactant expression in occluded lungs [39–42]. This unwanted effect may be improved by either removing the tracheal occlusion before birth and/or by antenatal steroid administration [43, 44]. Experimentally, in-utero reversal of occlusion (clinically translated as a plug-unplug sequence) achieved morphologically better lung maturation [45].
