SESSION TITLE: Mechanical Ventilation and Respiratory Failure Posters
SESSION TYPE: Original Investigation Poster
PRESENTED ON: Wednesday, October 28, 2015 at 01:30 PM - 02:30 PM
PURPOSE: In a previous study, we described how tidal volume measurements during pressure control ventilation may be underestimated in circumstances that include: presence of an active exhalation valve, inspiratory/expiratory effort during the set inspiratory time, and inspiratory time set longer than duration of inspiratory effort. We have called this phenomenon “Volume Error”. Volume Error occurs as a consequence of flow reversal during the mechanical inspiratory phase. Flow reversal can occur either due to inspiratory muscle relaxation in the presence of large Ti or due to expiratory muscle activity. Flow reversal results in underestimation of the delivered tidal volume by an amount equal to the volume lost through the active exhalation valve during the mechanical inspiratory phase. The magnitude of error is large enough to affect mortality, given the association between the size of the tidal volume and the risk of ventilator associated lung injury (VALI). Volume Error may thus lead to unsafe ventilator settings, defined as true VT > 8 mL/kg when displayed VT is £ 8 mL/kg. The purpose of this study was to estimate the probability of a patient being exposed to unsafe ventilator settings due to volume error using a sample of patient data.
METHODS: Patient data were used to estimate VT and Pmus variability using a mathematical simulation of PCV with active inspiratory effort. The simulator was used to predict volume error (displayed VT - true VT) and when unsafe conditions might arise.
RESULTS: Of 21 simulated patients, 11 (52%) had unsafe ventilator settings at inspiratory times between 1.0 and 1.4 seconds. In every case, increasing inspiratory time had the effect of decreasing displayed VT and hence increasing volume error. Six of the 21 simulated patients (19%) had volume error that ranged from -1.2 to -3.2 mL/kg (-13 to -28%).
CONCLUSIONS: In pressure targeted modes, mechanical ventilators equipped with active exhalation valve may not display the true delivered inspiratory tidal volume.
CLINICAL IMPLICATIONS: Generalized to the population of adult patients for whom volume error could occur, more than half might experience unsafe ventilator settings at some point in the course of ventilation.
DISCLOSURE: The following authors have nothing to disclose: Madhu Sasidhar, Robert Chatburn
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