suboptimal enrollment after 18 months.

Studies in preterm infants have shown that adding volume guarantee (VG) to synchronized modes of ventilation is not only feasible but also advantageous for providing more constant and desirable mechanical breath size. This ideally minimizes ventilator-induced lung injury due to barotrauma and volutrauma. To date, only one recent study has investigated the relative advantages of combining VG with different modes of synchronized mechanical ventilation in clinically stable, preterm infants that were mechanically ventilated at an average age of one month. We aim to further evaluate the effects of PSV+VG versus SIMV+VG ventilation in very low birth weight infants within the first three to five days of life. Our hypothesis is that in very low birth weight infants requiring mechanical ventilation in the first three to five days of life, PSV+VG will allow for more stable physiologic and ventilatory parameters compared to SIMV+VG. The primary endpoints are a reduction in respiratory rate and average mean airway pressure in the PSV+VG group compared to the SIMV+VG group.

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 (PSV+VG or SIMV+VG). Mechanical ventilation will be provided using the Dräeger Babylog 8000Ò. Once on study, infants will receive each ventilation mode (PSV+VG and SIMV+VG) 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 cc/kg, 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 infant's 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.

No laboratory tests will be obtained for this study. However, blood gases may be obtained by the NICU care team during the study per their daily care plan, and subsequent ventilator changes may be made by the NICU care team if necessary to correct an undesired blood gas value.

All data will be obtained using the pre-existing equipment in place to care for the patient prior to this study. This equipment includes the mechanical ventilator, endotracheal tube, cardio-respiratory monitor, and blood pressure monitor.

The key measurements during this study are ventilator variables and physiologic vital signs. Ventilator variables include respiratory rate, actual PIP, mean airway pressure, FiO2, expiratory tidal volume, minute ventilation, and actual I-time. Vital signs include heart rate, oxygen saturation, and blood pressure. Ventilator variables will be continuously downloaded from the Dräeger Babylog 8000Ò using BabyviewÒ software. Heart rate and oxygen saturation data will be downloaded from the MassimoÒ pulse oximeter. Blood pressure measurements will be documented every ten minutes if the infant has an umbilical artery catheter (UAC) or percutaneous arterial line (PAL) in place. If the infant does not have an UAC or PAL in place prior to the study, blood pressure measurements will be obtained every thirty minutes via manual blood pressure cuff.

Inclusion Criteria:

• Infants will be recruited based on the following criteria: weight ≤1500 grams and need for mechanical ventilation at 72 - 120 hours of age. Recruitment will continue until 16 patients have completed the study

Exclusion Criteria:

• Exclusion criteria include significant endotracheal tube leak (>30% leak does not allow for accurate measurement of ventilator data), use of narcotic drip medication, use of paralytic medication, use of inhaled nitric oxide, fraction of inspired oxygen (FiO2) >70% at the time of enrollment, or significant congenital defects of any organ system (e.g. cardiac, respiratory, CNS).

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