BACKGROUND: During weaning of synchronized intermittent mandatory rate in preterm infants, the spontaneous breaths must overcome the resistance of the endotracheal tube and the disease-induced respiratory loads. Pressure Support (PS) can be used as an adjunct to synchronized intermittent mandatory ventilation (SIMV) to partially unload the spontaneous breaths. OBJECTIVE: To evaluate the effects of two levels of PS as an adjunct to SIMV on gas exchange and breathing effort during an acute reduction in SIMV rate in preterm infants. METHODS: In all, 15 infants (birth weight 793 +/- 217 g, gestational age 26.4 +/- 1.5 weeks, postnatal age 15 +/- 16 days). Ventilatory support consisted of SIMV with peak inspiratory pressure (PTP) 16.3 +/- 1.3 cmH(2)O, positive end-expiratory pressure (PEEP) 4.3 +/- 0.6 cmH(2)O, and fraction of inspired oxygen (FiO(2)) 0.26 +/- 0.06. Infants were studied during four 30-minute periods: Two baseline SIMV periods and two periods of SIMV plus PS, in random order. During SIMV + PS, SIMV rate was lowered by 10 breaths per minute (b/minute) and PS was set at 3 and 6 cmH(2)O (SIMV+PS3 and SIMV + PS6, respectively). RESULTS: SIMV rate was reduced during SIMV + PS from 21.4 +/- 6.6 to 11.4 +/- 6.6 b/minute. Arterial oxygen saturation, transcutaneous carbon dioxide tension and FiO(2) remained unchanged. Minute ventilation, total respiratory rate and mean airway pressure were higher during SIMV + PS. Per-breath inspiratory effort was lower during SIMV + PS and this was more striking during SIMV + PS6. Spontaneous inspiratory effort per minute increased during SIMV + PS3, but this increase was averted during SIMV + PS6. CONCLUSION: Assistance of the spontaneous breaths with pressure support maintained gas exchange. PS of 6 cm H(2)O prevented an increase in breathing effort during an acute 50% reduction in SIMV rate.