The cardiothoracic ratio in newborn infants.

In order to provide improved techniques and standards for measuring radiographic heart size in infants, cardiothoracic ratios (C/T) using four different thoracic measures were calculated from anteroposterior supine chest radiographs of 175 normal infants who were a week or less in age, with gestational ages of 26.5-44 weeks, and birthweights of 765-4540 g. The lowest variation in C/T was observed when the thoracic measurement was either the chest width at the inner margins of the eighth ribs (normal range, 43.2%-56.5%) or the maximum internal chest width (normal range, 42.7%-56.4%); by these methods, a value of 57% or greater may be considered enlarged. Average C/T was above normal in 131 infants with respiratory distress syndrome and in 105 infants with various other parenchymal abnormalities. The presence of assisted ventilation did not significantly affect C/T measured as described above. In evaluating level of inspiration by the position of the right hemidiaphragm, posterior rib count proved more predictive of heart size than anterior rib count; the normal inspiratory range was 6.6-9.4 posterior ribs.