Abstract 1868

The strength of the diaphragm depends on its cross sectional area and geometry. We previously found that both parameters correlate with height, and weight in healthy adults and children (AJRCCM.155:1323). Estimates of diaphragm strength based on diaphragm dimensions have not been reported for infants. We expected to find similar relationships among diaphragm and thoracic dimensions, height and weight in the infant as were found in the adult. We measured diaphragm thickness (tdi) with ultrasound and rib cage anteroposterior and transverse diameters with calipers in 16 healthy term infants [8 males and 8 females, birth and study weights 3.3±0.2 kg (mean±SEM), gestational age 38.8±0.3 weeks, and postnatal age 3±1 days ]. Studies were performed one hour post prandial during quiet sleep. The sleep state was assessed by behavioral criteria. The cross sectional area of the diaphragm (CSAdi), the area of the thorax spanned by the diaphragm (Athor), and the ratio (CSAdi/Athor) were then calculated. We found significant (p<0.02) relationships between tdi and crown-heel length (R=0.58) and body weight (R=0.58). The ratio CSAdi/Athor (a predictor of maximal diaphragm strength) was significantly greater in infants than that previously reported by us for adults (0.8%±0.03 vs 0.06±0.01, p<0.01). Diaphragm thickness did not correlate with gestational age between 38 and 42 weeks (p>0.05). We conclude that, as in adults, in the term infant diaphragm dimensions scale with body length and weight. However, the predicted maximal pressures in infants are much greater than those in adults. We speculate that this finding may reflect the need to generate sufficient diaphragmatic pressure during the first breath after birth to overcome the elastic load of the lung.