 After informed consent is obtained, each patient will be started on the study between 72 to 96 hours of age. Each infant will be assigned via block randomization to an initial ventilation mode PSVVG or SIMVVG. Mechanical ventilation will be provided using the Dräeger Babylog 8000Ò. Once on study, infants will receive each ventilation mode PSVVG and SIMVVG twice, administered in alternating ninety minute blocks. Ventilator management in each mode will be standardized as follows: ventilator rate of 30 breaths per minute, volume guarantee of 4-6 cckg, inspiratory time of 0.3-0.35 seconds, and positive end-expiratory pressure of 5-7 cmH2O. Peak inspiratory pressure PIP will be set at 15-20 above the PIP needed to achieve the set tidal volume. FiO2 will be maintained to keep oxygen saturation in the range of 85-96. The FiO2 will only be adjusted if the infants oxygen saturation is persistently outside of the target range. End tidal carbon dioxide levels will be obtained at two times 30 minutes and 90 minutes during each study block. 