Background: Over-assisted mechanical ventilation(MV) is associated with respiratory muscle disuse atrophy. Meanwhile, under-assisted MV causes P-SILI or respiratory muscle injuries. And both cause poor outcomes.

Aims: To prove the association between the degree of respiratory effort measured by P_0.1, predicted P_mus and predicted ?transpulmonary pressure(?P_L) with ventilator-free days and mortality at 28 days.

Methods: We conducted a prospective observational study in respiratory failure patients who required invasive MV.

Results: 80 patients with respiratory failure were included and categorized into 3 groups regarding P_0.1 and predicted P_mus, 2 groups regarding the predicted ?P_L. The ventilator-free days at 28 days(28-VFDs) were significantly longer in the P_0.1 group between 1.5-3.5 cmH₂O than the group those < 1.5  and > 3.5 cmH₂O (24 [19-25], 15 [0-24]and 14 [0-26] days respectively, P=0.008). Likewise, the predicted P_mus group between 5-10 cmH₂O showed significantly longer ventilator-free days than those of predicted P_mus < 5 and > 10 cmH₂O (24 [14-26] , 13 [0-23] and 23 [8-25] days respectively, P=0.045). The predicted ?P_L ? 20 cmH₂O group had significantly longer ventilator-free days than those of predicted ?P_L > 20 cmH₂O (24 [15-26] vs. 17 [0-24] days, P=0.007). The association with significantly lower 28-day mortality was found in groups of P_0.1 between 1.5-3.5 cmH₂O(P=0.039) and predicted ?P_L ? 20 cmH₂O (P=0.022).

Conclusions: The degree of respiratory effort measured by P_0.1, predicted P_mus and ?P_L was associated with 28-VFDs. In addition,the P_0.1 and predicted ?P_L were associated with 28-day mortality.