* unable to consent; * unable to prone; * indication for immediate endotracheal intubation and mechanical ventilation; * contraindication to prone positioning (severe obesity, abdominal wound, pregnancy, unstable pelvic/spinal lesions, vomiting, etc.); * comfort care or imminent expectation of death.

* unable to consent; * unable to prone; * indication for immediate endotracheal intubation and mechanical ventilation; * contraindication to prone positioning (severe obesity, abdominal wound, pregnancy, unstable pelvic/spinal lesions, vomiting, etc.); * comfort care or imminent expectation of death.

- unable to consent; - unable to prone; - indication for immediate endotracheal intubation and mechanical ventilation; - contraindication to prone positioning (severe obesity, abdominal wound, pregnancy, unstable pelvic/spinal lesions, vomiting, etc.); - comfort care or imminent expectation of death.

- unable to consent; - unable to prone; - indication for immediate endotracheal intubation and mechanical ventilation; - contraindication to prone positioning (severe obesity, abdominal wound, pregnancy, unstable pelvic/spinal lesions, vomiting, etc.); - comfort care or imminent expectation of death.