Effect of Tidal Volume Size and Its Delivery Mode on Patient-Ventilator Dyssynchrony

Although increasingly recommended, compliance with low Vt ventilation remains suboptimal. Dyssynchrony induced by low Vts may be a reason for it.

To determine the effect of Vt size, and of the ventilator mode used for its delivery (volume vs. pressure control), on the magnitude of patient-ventilator dyssynchrony in patients with or at risk for acute respiratory distress syndrome.

Nineteen mechanically ventilated patients underwent six consecutive ventilatory conditions: three on volume assist-control (VC) mode, each with set Vt of 6, 7.5, and 9 ml/kg, and three on adaptive pressure-control (APC) mode, with those same set Vts and matching inspiratory times. Triggering, cycling, and flow dyssynchronies were identified by inspection of airway flow and pressure tracings. A dyssynchrony index (DI) was calculated as the total number of dyssynchronies divided by the sum of ventilator cycles and ineffective triggering events, expressed as percentage. A severe DI was calculated including only double triggering and severe flow dyssynchronies.

Under VC mode, the median (interquartile range) DIs were 100% (22-100%) at set Vt of 6 ml/kg, and 78% (7-100) at 7.5 ml/kg, both higher than 25% (0-45%) at 9 ml/kg (P = 0.02 and 0.01, respectively). Severe DI was higher at each reduction of Vt size. Under APC mode, compared with VC, DIs were lower at set Vt of 6 and 7.5 ml/kg (P = 0.004 for both). Changing from VC to APC resulted in an increase in exhaled Vt ≥ 1 ml/kg predicted body weight in a minority of patients.

Lower Vts during VC ventilation result in higher patient-ventilator dyssynchrony in most patients with or at risk for acute respiratory distress syndrome. The use of APC mode is an option to reduce dyssynchrony, but it requires careful monitoring to avoid larger-than-target delivered volumes.