Mathematical reappraisal of the congenital cystic adenomatoid malformation volume ratio.

OBJECTIVE

The CCAM (CPAM) volume ratio (CVR), an ultrasound index to predict hydrops or neonatal demise in fetuses affected with CPAM, is presumed to be independent of gestational age. A CVR of 1.6 has been proposed as predictive of hydrops. The purpose of our study was to assess the fundamental mathematical concept of the CVR.

STUDY

DESIGN

Using published formulas, the right fetal lung volume was used as a surrogate of a theoretical CPAM involving the entire right lung. The head circumference was estimated using the previously published formula by Chitty et al. The ratio of right lung volume by the head circumference was assumed to be equivalent to the CVR, or CVR'. The relative increase in right lung volume necessary to produce a CVR' of 1.6 cm2 was assessed along gestational age.

RESULTS

The CVR' increased with gestational age. The relative increase in lung volume necessary to produce a CVR' of 1.6 cm2 was smaller with advancing gestational age.

CONCLUSIONS

The CVR increases with gestational age because it results from the division of two parameters of different dimensions, with a resulting quadratic equation. Thus, the size of the CPAM necessary to produce a CVR greater than 1.6 cm2 decreases with gestational age. Therefore, the use of a fixed value of the CVR, e.g. 1.6 cm2, to predict fetal hydrops or neonatal demise, is inappropriate. A different ultrasound index that is independent of gestational age should be used in the counseling of patients with CPAM.