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

Pohlman, Mark C. MD; McCallister, Kathryn E. BS; Schweickert, William D. MD; Pohlman, Anne S. MSN; Nigos, Celerina P. BSN; Krishnan, Jerry A. MD, PhD; Charbeneau, Jeff T. MS; Gehlbach, Brian K. MD; Kress, John P. MD; Hall, Jesse B. MD

doi: 10.1097/CCM.0b013e31818b308b
Clinical Investigations

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.

From the Section of Pulmonary and Critical Care Medicine, Departments of Medicine (MCP, KEM, ASP, CPN, JAK, BKG, JPK, JBH) and Health Studies (JAK, JTC), University of Chicago, Chicago, IL; and Section of Pulmonary and Critical Care Medicine, Department of Medicine (WDS), University of Pennsylvania, Philadelphia, PA.

This research was funded in part by a NIH training grant HL07605.

The authors have not disclosed any potential conflicts of interest.

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© 2008 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins