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

Objective To explore the association between PaCO.sub.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.sub.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.sub.2 >32 mmHg. However, it increased to 48% (237/494) in those with PaCO.sub.2 [less than or equal to] 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.sub.2 >32 mmHg were set as reference. In patients with PaCO.sub.2 [less than or equal to] 32 mmHg, one unit increment of PaCO.sub.2 was associated with 5% reduction of NIV failure. However, it did not associate with NIV failure in patients with PaCO.sub.2 >32 mmHg. Conclusions PaCO.sub.2 and NIV failure was nonlinear relationship. The inflection point was 32 mmHg. Below the inflection point, lower PaCO.sub.2 was associated with higher NIV failure. However, it did not associate with NIV failure above this point. Keywords: Noninvasive ventilation, PaCO.sub.2, Restricted cubic splines, Hypoxemic respiratory failure