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Ineffective triggering (IT) is the most common manifestation of patient–ventilator asynchrony in mechanically ventilated patients. IT in the expiratory phase (ITE) accounts for the majority of IT and is associated with characteristic features of flow and airway pressure deflection, caused by ineffective effort from the patient. The purpose of this study was to quantify the characteristics of flow and airway pressure deflections of ITE and, using a computerized algorithm, to evaluate their usefulness in the detection of ITEs.Prospective, clinical study.Medical intensive care unit in a 1,000-bed university hospital.A total of 14 mechanically ventilated adult patients with patient–ventilator asynchrony.None.We analyzed 5,899 breaths and found that 1,831 were ITEs. The average values for maximum flow deflection (Fdef) and maximum airway pressure deflection (Pdef) in ITEs were 13.94 ± 8.0 L/min and 1.91 ± 0.97 cm H2O. With a starting value of 0.1 L/min for Fdef and 0.01 cm H2O for Pdef, the area under the receiver operating characteristics curve of Fdef and Pdef for the detection of ITEs was 0.98 and 0.97, respectively. Sensitivity and specificity for the detection of ITEs were 91.5% and 96.2% for Fdef, respectively, for a cutoff value of 5.45 L/min, and 93.3% and 92.9% for Pdef, for a cutoff value of 0.45 cm H2O.We conclude that accurately detecting and quantifying ITEs is feasible using a computerized algorithm based on Fdef and Pdef. Such a computerized estimation of patient–ventilator interaction might be helpful for adjusting ventilator settings in an intensive care unit.