Background: Physicians have been utilizing respiratory status and arterial blood gases measurements of the patient for determining the need of noninvasive ventilation (NIV). There is actually no clear and accurate index to assess the appropriate time of using NIV. We hypothesized that diaphragmatic ultrasound in combination with arterial blood gases test can accurately predict when the patients require sequential NIV.Objective: To investigate the feasibility of using the new VOX (velocity

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... ation) index, combining the diaphragmatic ultrasound and oxygen pressure analyses, as a reliable assessment tool for sequential NIV in intensive care unit (ICU) patients. Method: A prospective study including patients admitted to ICU of People's Liberation Army General Hospital First Medical Center Respiratory Department between August 1st 2018 and March 31st 2020. Patients received either continuous low-flow oxygen therapy or sequential NIV based on the physician judgement. The diaphragm movement distance, contraction time, and velocity were recorded and the E-T (excursion-time) index, VOX index, and arterial blood gases were compared. Result: The velocity of the diaphragmatic contraction (VD) was higher (3.29 vs. 1.89, P< 0.001) and P/F ratio was lower (161 vs. 264, P< 0.001) in patients receiving NIV. In predicting the need of utilizing NIV, VOX index showed the largest area under curve of ROC (AUC= 0.97) which was significantly greater than the AUC of VD and P/F ratio.Conclusion: VOX index serves as an accurate indicator of predicting the time of using NIV for ICU patients. It is recommended to switch from invasive ventilation to NIV when VOX< 109.89.