We performed a prospective, randomized trial to evaluate the impact of volume targeted intermittent mandatory ventilation (IMV), synchronized IMV(SIMV), and fully synchronized assist/control (A/C) during the 6 hours following initial S administration. We randomized 30 premature newborns(28±3 weeks, 1.19±0.49 kg) with respiratory distress syndrome(RDS; data from 27 analyzed) to IMV (n=10), SIMV (n=9), or A/C (n=8) using time-cycled, pressure limited, volume targeted ventilation (Dräger Babylog). After baseline measurements, all infants received 4 mL/kg of S(Survanta®). Ventilator settings were: peak inspiratory pressure to maintain tidal volume (Vt; mL/kg) of 5-7; inspiratory time for zero end-inspiratory flow; PEEP 4-6 cm H2O; FiO2 to keep saturations 88-96%; rate of 40 for IMV and SIMV, and backup rate of 40 for A/C. Target pH was 7.25-7.45; PaCO2 35-50 mmHg. Arterial blood gases, respiratory rate (RR; breaths/minute), Vt, minute ventilation (Ve; L/kg), pulmonary mechanics (PM; VenTrak®), heart rate, and blood pressure were recorded prior to S administration and at 1, 3, and 6 hours. PM, RR, Ve and Vt were recorded continuously for five minutes at data collection times. We report mean values for this five minute period. Oxygenation was assessed using the oxygenation index (OI). Vt consistency was assessed using coefficent of variation(V; SD/mean × 100). ANOVA detected differences between groups; post hoc testing used Student-Newman-Keuls. Results: OI improved similarly in all groups by one hour after S administration. Hemodynamics were not different between groups. Patients ventilated using A/C, a fully-synchronized mode, had lower RR with more consistent Vt at similar Ve.Conclusion:: These data suggest physiologic advantages to A/C during initial treatment of RDS. Table legend: means shown. *p<0.05 A/C vs IMV, SIMV; **p<0.05 OI pre-dose vs 1,3,6 hours in all groups

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