Variability of Tidal Volume in Patient-Triggered Mechanical Ventilation in ARDS

Limiting tidal volume (V_T) in patients with ARDS may not be achieved once patient-triggered breaths occur. Furthermore, ICU ventilators offer numerous patient-triggered modes that work differently across brands. We systematically investigated, using a bench model, the effect of patient-triggered modes on the size and variability of V_T at different breathing frequencies (f), patient effort, and ARDS severity.

We used a V500 Infinity ICU ventilator connected to an ASL 5000 lung model whose compliance was mimicking mild, moderate, and severe ARDS. Thirteen patient-triggered modes were tested, falling into 3 categories, namely volume control ventilation with mandatory minute ventilation; pressure control ventilation, including airway pressure release ventilation (APRV); and pressure support ventilation. Two levels of f and effort were tested for each ARDS severity in each mode. Median (first-third quartiles) V_T was compared across modes using non-parametric tests. The probability of V_T > 6 mL/kg ideal body weight was assessed by binomial regression and expressed as the odds ratio (OR) with 95% CI. V_T variability was measured from the coefficient of variation.

V_T distribution over all f, effort, and ARDS categories significantly differed across modes (P < .001, Kruskal-Wallis test). V_T was significantly greater with pressure support (OR 420 mL, 95% CI 332-527 mL) than with any other mode except for variable pressure support level. Risk for V_T to be > 6 mL/kg was significantly increased with spontaneous breaths patient-triggered by pressure support (OR 19.36, 95% CI 12.37-30.65) and significantly reduced in APRV (OR 0.44, 95% CI 0.26-0.72) and pressure support with guaranteed volume mode. The risk increased with increasing effort and decreasing f. Coefficient of variation of V_T was greater for low f and volume control-mandatory minute ventilation and pressure control modes. APRV had the greatest within-mode variability.

Risk of V_T > 6 mL/kg was significantly reduced in APRV and pressure support with guaranteed volume mode. APRV had the highest variability. Pressure support with guaranteed volume could be tested in patients with ARDS.