* risk of airway obstruction or even asphyxia * need for emergent intubation after admission * respiratory failure caused by cardiogenic pulmonary edema * unable to implement timed prone and repositioning due to any cause * injury or wound on the ventral body surface affecting the prone position * unstable fracture of cervical vertebra and spine * glaucoma or other sharp increases in intraocular pressure * intracranial hypertension caused by traumatic brain injury etc. * significantly high risk of pulmonary embolism * acute hemorrhagic disease * respiratory rate \>40 breaths/min, with significant dyspnea * transcutaneous oxygen saturation can not be continuously monitored * hemodynamic instability requiring vasoactive drugs (systolic blood pressure \<90 mmhg or mean arterial pressure \<65 mmhg despite adequate volume resuscitation) * awareness disorder or inability to accept instructions, communication barrier with the nursing team, inability to use language or pager to call for help * difficulty or limitation in autonomous movement, inability to adjust the position without assistance from others * body mass index \> 37 kg/m2

* risk of airway obstruction or even asphyxia * need for emergent intubation after admission * respiratory failure caused by cardiogenic pulmonary edema * unable to implement timed prone and repositioning due to any cause * injury or wound on the ventral body surface affecting the prone position * unstable fracture of cervical vertebra and spine * glaucoma or other sharp increases in intraocular pressure * intracranial hypertension caused by traumatic brain injury etc. * significantly high risk of pulmonary embolism * acute hemorrhagic disease * respiratory rate \>40 breaths/min, with significant dyspnea * transcutaneous oxygen saturation can not be continuously monitored * hemodynamic instability requiring vasoactive drugs (systolic blood pressure \<90 mmhg or mean arterial pressure \<65 mmhg despite adequate volume resuscitation) * awareness disorder or inability to accept instructions, communication barrier with the nursing team, inability to use language or pager to call for help * difficulty or limitation in autonomous movement, inability to adjust the position without assistance from others * body mass index \> 37 kg/m2

risk of airway obstruction or even asphyxia need for emergent intubation after admission respiratory failure caused by cardiogenic pulmonary edema unable to implement timed prone and repositioning due to any cause injury or wound on the ventral body surface affecting the prone position unstable fracture of cervical vertebra and spine glaucoma or other sharp increases in intraocular pressure intracranial hypertension caused by traumatic brain injury etc. significantly high risk of pulmonary embolism acute hemorrhagic disease respiratory rate >40 breaths/min, with significant dyspnea transcutaneous oxygen saturation can not be continuously monitored hemodynamic instability requiring vasoactive drugs (systolic blood pressure <90 mmhg or mean arterial pressure <65 mmhg despite adequate volume resuscitation) awareness disorder or inability to accept instructions, communication barrier with the nursing team, inability to use language or pager to call for help difficulty or limitation in autonomous movement, inability to adjust the position without assistance from others body mass index > 37 kg/m2

risk of airway obstruction or even asphyxia need for emergent intubation after admission respiratory failure caused by cardiogenic pulmonary edema unable to implement timed prone and repositioning due to any cause injury or wound on the ventral body surface affecting the prone position unstable fracture of cervical vertebra and spine glaucoma or other sharp increases in intraocular pressure intracranial hypertension caused by traumatic brain injury etc. significantly high risk of pulmonary embolism acute hemorrhagic disease respiratory rate >40 breaths/min, with significant dyspnea transcutaneous oxygen saturation can not be continuously monitored hemodynamic instability requiring vasoactive drugs (systolic blood pressure <90 mmhg or mean arterial pressure <65 mmhg despite adequate volume resuscitation) awareness disorder or inability to accept instructions, communication barrier with the nursing team, inability to use language or pager to call for help difficulty or limitation in autonomous movement, inability to adjust the position without assistance from others body mass index > 37 kg/m2