Final answer:
A rapid air return into the esophageal detector device after ET tube placement is indicative of correct tracheal placement, as the trachea has rigid walls that would allow for this to occur.
Step-by-step explanation:
When using an esophageal detector device to assess the placement of an endotracheal (ET) tube during intubation in a cardiac arrest patient, a rapid air return into the device would typically indicate correct tracheal placement. The device is designed to help differentiate between tracheal and esophageal intubation by detecting the presence of negative pressure. If the ET tube was incorrectly placed in the esophagus, the device would not exhibit rapid air return due to the structure of the esophageal walls which do not recoil like tracheal rings. Hence, the observation of rapid air return suggests that the air is entering from an open and rigid structure like the trachea, which is consistent with a successful intubation.