Effects of intermittent mandatory ventilation on respiratory timing in preterm infants.

Twenty preterm infants (25-36 weeks' gestation) were studied during intermittent mandatory ventilation (IMV) at rates of 10-34 inflations/min. Airway pressure and abdominal capsule signals were recorded at varying postnatal ages. Spontaneous interbreath interval (IBI), inspiratory (Ti) and expiratory (Te) duration were measured over 100-1000 ventilator cycles. Baseline Te was 0.48 s (+/- 0.129) and increased to 0.65 s (+/- 0.182) when associated with a mechanical inflation. Baseline Ti was 0.34 s (+/- 0.062) and increased to 0.38 s (+/- 0.081) with inflation. IBI increased from 0.82 s (+/- 0.161) to 1.03 s (+/- 0.201) with inflation. 1:1 entrainment (phase-locking) was observed at rates of ventilation below the spontaneous respiratory rate but spontaneous inspiration and mechanical inflation were always out of phase. A linear relationship was noted between the prolongation of IBI and the timing of inflation within the spontaneous respiratory cycle during IMV. This relationship could be used to estimate the range of rates of mechanical inflation capable of inducing 1:1 entrainment.