Final answer:
An endotracheal tube should be placed when an infant does not breathe independently post-delivery, particularly in premature infants who may lack sufficient pulmonary surfactant necessary for proper lung function.
Step-by-step explanation:
An endotracheal tube should be placed during an infant resuscitation when the infant does not breathe on their own after delivery, especially in cases where the infant may suffer from respiratory distress syndrome (RDS). Such a scenario is more likely in premature birth around or before 26 weeks of gestation, where the production of pulmonary surfactant is insufficient.
This surfactant is necessary for reducing surface tension in the alveoli and for proper lung inflation. Intubation ensures that the airway remains open, and when connected to a ventilator, it mechanically assists the breathing process until the infant can breathe spontaneously or until proper lung development occurs. In the event of spontaneous breathing, application of nasal continuous positive airway pressure (CPAP) may be sufficient without the need for intubation.