The simple tool uses the anatomical differences between the oesophagus and trachea to confirm correct ET tube placement. If there is negative pressure, the oesophagus has been intubated; if air can be aspirated, correct tracheal intubation has been achieved.
If the tube is in the soft, unsupported oesophagus, the walls will collapse around the end of the endotracheal tube, creating negative pressure if you attempt to aspirate air. If the tube is in the rigid trachea, air can easily be aspirated
