<qa>  What is the standard deviation of pCO2 change reported in another study?Prolonged mechanical ventilation is frequently needed to treat respiratory insufficiency and apnea in very low birthweight VLBW, 1500 g preterm infants. Endotracheal intubation and mechanical ventilation carry many risks including potentially fatal air leaks and ventilator-associated pneumonias. Less-invasive methods of respiratory support are needed to minimize these risks while supporting the convalescing preterm infant. We propose to test the effectiveness of nasal high-frequency ventilation NHFV in stable neonates with mild respiratory acidosis who are dependent on nasal continuous positive airway pressure for respiratory support CPAP. Nasal high frequency ventilation may be effective in decreasing rates of reintubation for apnea or respiratory insufficiency in VLBW infants. We will enroll 60 stable VLBW infants who are currently being treated with nasal CPAP and who have mild respiratory acidosis. Twenty patients will be assigned to each ventilator under investigation. Nasal high frequency ventilation will be applied at the same mean airway pressure as the patients previous CPAP support. The amplitude on nasal high frequency ventilation will be adjusted to achieve adequate shaking of the chest wall. Blood gas measurements, transcutaneous continuous pCO2 monitoring, continuous pulse oximetry, and chest x-rays will be used to assess safety and efficacy. Study power has been calculated to detect a difference drop or rise in partial pressure of CO2 pCO2 equal to two-thirds of the standard deviation of pCO2 change reported in another study van der Hoeven et al., 1998, which is a clinically relevant difference. 