When evaluating a patient for PDT, a thorough history and physical examination will identify anatomic contraindications, including previous difficult tracheal intubation, morbid obesity, obscure cervical anatomy, goiter, short thick neck, previous neck surgery (especially tracheotomy), cervical infection, facial or cervical trauma/fractures, halo traction, or known presence of subglottic stenosis. Physiologic contraindications to PDT include hemodynamic instability, requirement of FiO2 > 0.60, a positive end-expiratory pressure (PEEP) > 10 cm H2O, or uncontrolled coagulopathy. Cervical deformity, previous radiation therapy, edema, or tumor can also make tracheal cannulation difficult and increase the risk of morbidity. The need for emergency control of the airway is an absolute contraindication to PDT.