 Volume Guarantee ventilation is a mode of ventilation in which ventilator measures the exhaled tidal volume of each ventilator breath and automatically adjusts the peak inspiratory pressure to deliver the set tidal volume. We have previously shown that Volume Guarantee reduced the severity and duration of hypoxemia episodes during a period of 2 hours in preterm infants The study hypothesis is that Volume Guarantee ventilation will decrease the duration of episodes of hypoxemia when compared to conventional mechanical ventilation during routine clinical conditions and over longer periods of time. 