Inclusion Criteria for all infants:  Gestational age < 32+0 weeks Postmenstrual age between 28+0 and 36+6 weeks.  Additional inclusion criteria for Groups 1 and 2:  Off any respiratory support and breathing in-room air Less than 3 clinically significant cardiorespiratory events per calendar day  Additional inclusion criteria for Group 3:  On the bubble CPAP device with the binasal prongs interface Receiving CPAP levels of 5 to 7 cm H2O with gas flows not exceeding 10L/min At least 3 clinically significant cardiorespiratory events per calendar day  Exclusion Criteria:  Major known congenital abnormalities Known congenital heart disorders Known neuromuscular disease Known diaphragmatic paralysis or a diagnosed phrenic nerve injury History of esophageal perforation in the 7 days preceding the study History of pneumothorax requiring chest tube insertion in the 7 days preceding the study Receiving inotropes, narcotics, or sedative agents at the time of study recording