Diaphragm dimensions of the healthy term infant

Human neonatal diaphragm development has not been extensively studied. Previous work in children and adults suggests that diaphragm thickness (t(di)) is in scale with body size such that maximal transdiaphragmatic pressure (P(dimax)) remains relatively constant. Such assessments have not been made in healthy term infants. This study was designed to evaluate the relationships among t(di), body dimensions and P(dimax) in healthy term infants.

It was hypothesized that in healthy term infants 1) t(di) is positively correlated with body size and 2) calculated P(dimax) is independent of body weight and length. Fifteen clinically stable term infants (8 males and 7 females) were recruited [birthweight (BW), 3.3 +/- 0.7 kg, (mean +/- SD); head circumference (HC), 33.7 +/- 2 cm; body length (BL) 50 +/- 3 cm; gestational age (GA) 39 +/- 1 wk; and postnatal age 1.7 +/- 0.8 day]. Ultrasound was used to visualize the diaphragm at the level of the zone of apposition and measure t(di). Standard techniques were used to measure the anthropometric dimensions of the rib cage. P(dimax) was calculated using the piston-in-cylinder model of diaphragm function.

Significant correlations were found among t(di) and BW (R = 0.58), BL (R = 0.58) and HC (R = 0.65) but not between GA (R = 0.20). Larger infants tended to have thicker diaphragms and larger cross-sectional areas of the lower rib cage (A(ZAP)). For the group, calculated P(dimax) was independent of either body weight or length and was greater than that calculated for adults.

It is concluded that diaphragm mass in healthy term infants is proportional to body size, whereas calcuated P(dimax) is independent of body size. Since calculated P(dimax) is greater than that predicted for adults, there may be perinatal diaphragm strengthening. This may assist the infant in generating sufficient pressure to overcome the enormous elastic and resistive loads imposed during perinatal pulmonary transition.