To determine if late (at or beyond postnatal day 14) mild permissive hypercapnia is associated with reduction in apnea, bradycardia, and hypoxemic episodes and with improved stability of oxygenation.

Preterm infants who remain intubated or on nasal mechanical ventilation (IMV) at 14 days postnatal age, meet blood gas criteria, have transcutaneous carbondioxide (TcCO2) monitoring with TcCO2 values that trend and correlate appropriately with PaCO2, and are not judged too unstable by Attending neonatologist will qualify.

The investigators will use the data from the 96 hours of intensive multiparametric physiologic monitoring at 2 weeks postnatal age.

The first 24 hours of data collection will be the baseline data. Over the next 72 hours, the investigators will evaluate 3 interventions in a cross-over manner with the initial intervention randomly assigned: Intervention 1 (24-48h of data), Intervention 2 (48-72h of data) and Intervention 3 (72-96h of data).

Inclusion Criteria:

1. Inborn infants weighing 401-1,000 grams on admission and/or 22w 0/7d to 28w 6/7d (<29 weeks) inclusive completed weeks of gestation
2. Infants eligible for full care and resuscitation as necessary, and surviving beyond 24 h of age
3. Enrollment in main study protocol (Aim 1 of PreVENT Apnea) at <1 week post-natal age
4. Informed consent from parent/guardian
5. This study will enroll the subset of infants from Aim 1 who are still intubated or on nasal IMV at 2 weeks postnatal age, meet blood gas criteria (arterial or capillary arterialized blood gas values done q12-24h, as most infants do not have an arterial line at 2 weeks: pH >7.25, PaCO2 >40 mm Hg), have TcCO2 monitoring with TcCO2 values that trend and correlate appropriately with PaCO2, and are not judged too unstable by the Attending neonatologist.

Exclusion Criteria:

1. Refusal or withdrawal of consent
2. Major congenital malformations (e.g., not including patent ductus arteriosus, small hernia)