To assess the changes in (regional) lung volume and gas exchange during minimally invasive surfactant therapy (MIST) in preterm infants with respiratory distress syndrome.

In this prospective observational study, infants requiring a fraction of inspired oxygen (FiO₂) ≥0.30 during nasal continuous positive airway pressure of 6 cmH₂O were eligible for MIST. Surfactant (160-240 mg/kg) was administered in supine position in 1-3 minutes via an umbilical catheter placed 2 cm below the vocal cords. Changes in end-expiratory lung volume (EELV), tidal volume, and its distribution were recorded continuously with electrical impedance tomography before and up to 60 minutes after MIST. Changes in transcutaneous oxygen saturation (SpO₂) and partial carbon dioxide pressure, FiO₂, respiratory rate, and minute ventilation were recorded.

A total of 16 preterm infants were included. One patient did not finish study protocol because of severe apnea 10 minutes after MIST. In the remaining infants (gestational age 29.8 ± 2.8 weeks, body weight 1545 ± 481 g) EELV showed a rapid and sustained increase, starting in the dependent lung regions, followed by the nondependent regions approximately 5 minutes later. Oxygenation, expressed as the SpO₂/FiO₂ ratio, increased from 233 (IQR 206-257) to 418 (IQR 356-446) after 60 minutes (P < .001). This change was significantly correlated with the change in EELV (ρ = 0.70, P < .01). Tidal volume and minute volume decreased significantly after MIST, but transcutaneous partial carbon dioxide pressure was comparable with pre-MIST values. Ventilation distribution remained unchanged.

MIST results in a rapid and homogeneous increase in EELV, which is associated with an improvement in oxygenation.