Excessive tidal volume from breath stacking during lung-protective ventilation for acute lung injury.

Rationale: Low tidal volume ventilation strategies for patients with respiratory failure from acute lung injury may lead to breath stacking and higher volumes than intended.
Objective: To determine frequency, risk factors, and volume of stacked breaths during low tidal volume ventilation for acute lung injury.
Design, Setting, and Patients: Prospective cohort study of mechanically ventilated patients with acute lung injury (enrolled from August 2006 through May 2007) treated with low tidal volume ventilation in a medical intensive care unit at an academic tertiary care hospital.
Interventions: Patients were ventilated with low tidal volumes using the Acute Respiratory Distress Syndrome Network protocol for acute lung injury. Continuous flow-time and pressure-time waveforms were recorded. The frequency, risk factors, and volume of stacked breaths were determined. Sedation depth was monitored using Richmond agitation sedation scale.
Measurements and Main Results: Twenty patients were enrolled and studied for a mean 3.3 +/- 1.7 days. The median (interquartile range) Richmond agitation sedation scale was -4 (-5, -3). Inter-rater agreement for identifying stacked breaths was high (kappa 0.99, 95% confidence interval 0.98-0.99). Stacked breaths occurred at a mean 2.3 +/- 3.5 per minute and resulted in median volumes of 10.1 (8.8-10.7) mL/kg predicted body weight, which was 1.62 (1.44-1.82) times the set tidal volume. Stacked breaths were significantly less common with higher set tidal volumes (relative risk 0.4 for 1 mL/kg predicted body weight increase in tidal volume, 95% confidence interval 0.23-0.90).
Conclusion: Stacked breaths occur frequently in low tidal volume ventilation despite deep sedation and result in volumes substantially above the set tidal volume. Set tidal volume has a strong influence on frequency of stacked breaths.