Expiratory muscle activity in preterm babies.

The activity of the right external abdominal oblique muscle, in relation to respiration, in newborn babies from 25 to 34 weeks' gestation was studied, using surface electrodes. Babies without respiratory difficulties showed no activity associated with respiration in these muscles. Grunting babies with respiratory disease showed pronounced activity during expiration. This activity continued when the babies were endotracheally intubated; if the babies were ventilated it sometimes disturbed the tidal ventilation delivered by the machines. When the baby expired during a ventilator inflation, so that gas came up the endotracheal tube against the inflating pressure, the abdominal oblique muscles contracted in 98% of the breaths.