Abstract
Limiting tidal volume (TV) in ARDS patients may not be achieved once assisted modes are used. We systematically investigated on the bench effect of assisted modes on size and variability of TV at different breathing frequencies (BF), patient effort and ARDS severity. We used V500 Infinity ICU ventilator connected to ASL 5000 lung model mimicking mild, moderate and severe ARDS. Thirteen assisted modes were tested falling into three categories, namely volume controlled ventilation with mandatory minute ventilation (VCV-MMV), pressure-controlled ventilation (PCV) including airway pressure release ventilation (APRV) and pressure support ventilation (PSV). Two levels of BF and effort were tested for each ARDS severity in each mode. Median (first-third quartiles) TV was compared across modes using non parametric tests. Probability of TV > 6 ml/kg ideal body weight was assessed by binomial regression and expressed as odds ratio (OR) with 95% confidence intervals. TV variability was measured from coefficient of variation. TV distribution over all BF, effort and ARDS categories significantly differed across modes (P 6ml/kg was significantly increased with spontaneous breaths assisted by PSV (OR 19.36 [12.37-30.65]) and significantly reduced in APRV (OR 0.44 [0.26-0.72]) and PSV with guaranteed volume mode. Coefficient of variation of TV was greater for low BF and VCV-MMV and PCV modes. Risk of TV > 6ml/kg was significantly reduced in APRV and PSV with guaranteed volume mode. APRV had the highest variability.
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