RESUMO
To stratify groups of infants by gestational age and identify the lower limit of normal (LLN) of the cross-sectional area of peripheral muscles as well as diaphragmatic excursion and thickness and parasternal activation during spontaneous respiration in full-term and preterm newborns. A cross-sectional study was conducted at a neonatal unit. Preterm newborns (PTNBs) with gestational age of 28 to 366/7 weeks and full-term newborns (NBs) with gestational age of 37 to 416/7 weeks, clinically and hemodynamically stable, breathing ambient air with no signs of respiratory distress were included. NBs on oxygen therapy, those with known genetic syndromes, malformations of the nervous system and those having undergone surgery were excluded. Ultrasound was performed over the rectus femoris, tibialis anterior, and biceps brachii muscles (cross-sectional area) and diaphragm (thickness and excursion). One hundred twenty NBs were stratified based on gestational age (< 30 weeks [n = 25]; 31 to 35 weeks [n = 51]; 37 to 41 6/7 weeks [n = 44]). Significant differences were found in the cross-sectional area of the rectus femoris muscles and thigh circumference of NBs > 37 weeks compared to the other 2 groups (p < 0.05). Differences were found in diaphragm thickness and excursion between the groups with gestational age > 37 weeks and < 30 weeks. Greater activation of the parasternal muscles was found in the PTNBs. With regard to LLN, significant differences were found between the groups with gestational age > 37 weeks and < 30 weeks for all variables analyzed and between the group with gestational age > 37 weeks and other 2 groups for the cross-sectional area of the rectus femoris, diaphragmatic excursion, and diaphragm contraction velocity. Differences were found among the groups in the size of peripheral muscles as well as diaphragm thickness and excursion. Moreover, greater activation of the parasternal muscle was found in NBs with gestational age < 30 weeks. This study establishes normal values of ultrasound measures for full-term and preterm newborns.