ULTRASOUND IN PULMONARY HYPOPLASIA

The fetal thorax grows at a regular rate from 16 to 40 weeks, resulting in a linear correlation between gestational age and thoracic size.

1. Normal thoracic circumference
2. Normal thoracic length
3. Ratios between abdominal circumference, head circumference, BPD and femur length remain constant with a high correlation coefficient in normal pregnancy (1,2).

Thoracic circumference / Abdominal circumference is normally >0.8 after 20 weeks. There is a high positive predictive value for pulmonary hypoplasia using chest circumference measurements (3).

ULTRASOUND

• Ultrasound (coarsening of echotexture / progressive increase in sound transmission) is a poor predictor of lung maturity (4,5).
• Unilateral and involve one or more lung segments/
• Unilateral involvement of the entire lung.
• Bilateral (Potters Sequence)

Pleural effusion – no pulmonary hypoplasia	Pleural effusion – pulmonary hypoplasia
Bilateral pulmonary hypoplasia

• Ratio between biventricular outer dimension and the thoracic diameter.
• Constant throughout the second and third trimester.
• Biventricular outer dimension = transverse diameter of the heart at the level of the atrioventricular valves measured at end diastole, increases linearly with advancing age.
• An increased cardiothoracic ratio may result from cardiomegaly without pulmonary hypoplasia or small lungs with a normal heart. One therefore needs to determine which organ system is abnormal in size (6).
• Deviation of cardiac position or axis (7).
• May result from either an intrathoracic mass or an abnormality of the thoracic cage. Both conditions have a high incidence of pulmonary hypoplasia.
• Fetal breathing.
• The implications of fetal breathing activity remains controversial with respect to the prenatal diagnosis of pulmonary hypoplasia and the prediction of fetal outcome.

REFERENCES

1. Fong K, Ohlsson A, Zalev A. Fetal thoracic circumference: A prospective cross sectional study with real time ultrasound. Am J Obstet Gynecol 1988;158:1154-1160.
2. Nimrod C, Nicholson S, Davies D et.al. Pulmonary hypoplasia testing in clinical obstetrics. Am J Obstet Gynecol 1988;158:277-280.
3. D'Alton M, Mercer B, Riddick E et.al. Serial thoracic versus abdominal circumference ratios for the prediction of pulmonary hypoplasia in premature rupture of membranes remote from term. Am J Obstet Gynecol 1992;166:658-663.
4. Fried AM, Loh RK, Umer MA et.al. Echogenicity of fetal lung: Relation to fetal age and maturity. AJR 1985;145:591-594.
5. Cayea PD, Grant DC, Coublet PM et.al. Prediction of fetal lung maturity: Inaccuracy of study using conventional ultrasound instruments. Radiology 1985;155:473-475.
6. DeVore GR, Horenstein J, Platt LD. Fetal echocardiography: Assessment of cardiothoracic disproportion - A new technique for the diagnosis of thoracic hypoplasia. Am J Obstet Gynecol 1986;155:1066-1074.
7. Comstock CH. Normal fetal heart axis and position. Obstet Gynecol 1987;70:255-259.