Abstract: Disclosed are methods and apparatus for the detection of inadvertent endobronchial intubation, esophageal intubation and other potentially dangerous complications arising from invasive monitoring, endotracheal intubation and mechanical ventilation during surgery. Methods of the present invention generally require producing a mixed electrial signal comprising the combination of a first electrical signal and a second electrical signal, the first electrical signal being representative of the sounds emanating from a first region of the patients body and the second electrical signal being representative of the sounds emanating from a second region of the patients body. The mixed electrical signal is converted into a display signal, and then the display signal is monitored while reducing the contribution of said first electrical signal in said mixed signal to a value substantialy less than the contribution of said second signal in said mixed signal.