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To determine whether high rates of ineffective triggering within the first 24 hrs of mechanical ventilation (MV) are associated with longer MV duration and shorter ventilator-free survival (VFS).Prospective cohort study.Medical intensive care unit (ICU) at an academic medical center.Sixty patients requiring invasive MV.None.Patients had pressure-time and flow-time waveforms recorded for 10 mins within the first 24 hrs of MV initiation. Ineffective triggering index (ITI) was calculated by dividing the number of ineffectively triggered breaths by the total number of breaths (triggered and ineffectively triggered).A priori, patients were classified into ITI >=10% or ITI <10%. Patient demographics, MV reason, codiagnosis of chronic obstructive pulmonary disease (COPD), sedation levels, and ventilator parameters were recorded.Sixteen of 60 patients had ITI >=10%. The two groups had similar characteristics, including COPD frequency and ventilation parameters, except that patients with ITI >=10% were more likely to have pressured triggered breaths (56% vs. 16%,p= .003) and had a higher intrinsic respiratory rate (22 breaths/min vs. 18,p= .03), but the set ventilator rate was the same in both groups (9 breaths/min vs. 9,p= .78). Multivariable analyses adjusting for pressure triggering also demonstrated that ITI >=10% was an independent predictor of longer MV duration (10 days vs. 4,p= .0004) and shorter VFS (14 days vs. 21,p= .03). Patients with ITI >=10% had a longer ICU length of stay (8 days vs. 4,p= .01) and hospital length of stay (21 days vs. 8,p= .03). Mortality was the same in the two groups, but patients with ITI >=10% were less likely to be discharged home (44% vs. 73%,p= .04).Ineffective triggering is a common problem early in the course of MV and is associated with increased morbidity, including longer MV duration, shorter VFS, longer length of stay, and lower likelihood of home discharge.