PaCO2 is nonlinearly associated with NIV failure in patients with hypoxemic respiratory failure

Objective To explore the association between PaCO 2 and noninvasive ventilation (NIV) failure in patients with hypoxemic respiratory failure. Methods A retrospective study was performed in a respiratory ICU of a teaching hospital. Patients admitted to ICU between 2011 and 2019 were screened. We enrolled the patients with hypoxemic respiratory failure. However, patients who used NIV due to acute-on-chronic respiratory failure or heart failure were excluded. Data before the use of NIV were collected. Requirement of intubation was defined as NIV failure. Results A total of 1029 patients were enrolled in final analysis. The rate of NIV failure was 45% (461/1029). A nonlinear relationship between PaCO 2 and NIV failure was found by restricted cubic splines ( p = 0.03). The inflection point was 32 mmHg. The rate of NIV failure was 42% (224/535) in patients with PaCO 2 >32 mmHg. However, it increased to 48% (237/494) in those with PaCO 2 ≤ 32 mmHg. The crude and adjusted hazard ratio (HR) for NIV failure was 1.36 (95%CI:1.13–1.64) and 1.23(1.01–1.49), respectively, if the patients with PaCO 2 >32 mmHg were set as reference. In patients with PaCO 2 ≤ 32 mmHg, one unit increment of PaCO 2 was associated with 5% reduction of NIV failure. However, it did not associate with NIV failure in patients with PaCO 2 >32 mmHg. Conclusions PaCO 2 and NIV failure was nonlinear relationship. The inflection point was 32 mmHg. Below the inflection point, lower PaCO 2 was associated with higher NIV failure. However, it did not associate with NIV failure above this point.