Volume Delivered During Recruitment Maneuver Predicts Lung Stress in Acute Respiratory Distress Syndrome

Abstract

Objective: Global lung stress varies considerably with low tidal volume ventilation for acute respiratory distress syndrome. High stress despite low tidal volumes may worsen lung injury and increase risk of death. No widely available parameter exists to assess global lung stress. We aimed to determine whether the volume delivered during a recruitment maneuver (V(RM)) is inversely associated with lung stress and mortality in acute respiratory distress syndrome.

Design: Substudy of an acute respiratory distress syndrome clinical trial on esophageal pressure-guided positive end-expiratory pressure titration.

Setting: U.S. academic medical center.

Patients: Forty-two patients with acute respiratory distress syndrome in whom airflow, airway pressure, and esophageal pressure were recorded during the recruitment maneuver.

Interventions: A single recruitment maneuver was performed before initiating protocol-directed ventilator management. Recruitment maneuvers consisted of a 30-second breath hold at 40 cm H2O airway pressure under heavy sedation or paralysis. V(RM) was calculated by integrating the flow-time waveform during the maneuver. End-inspiratory stress was defined as the transpulmonary (airway minus esophageal) pressure during end-inspiratory pause of a tidal breath and tidal stress as the transpulmonary pressure difference between end-inspiratory and end-expiratory pauses.

Measurements and main results: V(RM) ranged between 7.4 and 34.7 mL/kg predicted body weight. Lower V(RM) predicted high end-inspiratory and tidal lung stress (end-inspiratory: β = -0.449; 95% CI, -0.664 to -0.234; p < 0.001; tidal: β = -0.267; 95% CI, -0.423 to -0.111; p = 0.001). After adjusting for PaO2/FIO2 and either driving pressure, tidal volume, or plateau pressure and positive end-expiratory pressure, V(RM) remained independently associated with both end-inspiratory and tidal stress. In unadjusted analysis, low V(RM) predicted increased risk of death (odds ratio, 0.85; 95% CI, 0.72-1.00; p = 0.026). V(RM) remained significantly associated with mortality after adjusting for study arm (odds ratio, 0.84; 95% CI, 0.71-1.00; p = 0.022).

Conclusions: Low V(RM) independently predicts high lung stress and may predict risk of death in patients with acute respiratory distress syndrome.

Figure 1

Depiction of the calculation of V_RM. The RM entailed a 30-second breath hold at 40 cmH₂O airway pressure under heavy sedation or paralysis. The preset PEEP level represents the starting airway pressure, with associated resting end-expiratory lung volume, immediately prior to the RM. Airflow was recorded continuously during the RM via an inline Fleisch pneumotachograph. The flow-time waveform was integrated over the duration of the RM (shaded area under the curve) to calculate the insufflation volume, V_RM.

Figure 2

V_RM and predicted inspiratory capacity. Box plots illustrate the median and interquartile range (boxes), mean (diamond), and maximum and minimum values (whiskers). V_RM was significantly lower than predicted inspiratory capacity (mean difference 29.5 ± 8.6 mL/kg PBW; p < .001). When measured beginning from resting lung volume at PEEP, V_RM is analogous to the inspiratory capacity of the ARDS “baby lung.”

Figure 3

Prediction of global lung stress. Left: End-inspiratory lung stress, calculated as the transpulmonary pressure during end-inspiratory pause of a tidal breath. Right: Tidal lung stress, calculated as the transpulmonary pressure difference between end-inspiratory and end-expiratory pauses during a tidal breath. Unlike tidal stress, end-inspiratory stress additionally accounts for the stress already present before tidal inflation—the stress on the lung from its end-expiratory volume at PEEP—which may differ substantially depending on the preset PEEP, chest wall characteristics, and PaO₂/FIO₂. Regression coefficients represent unadjusted association between each predictor and lung stress.

Figure 4

Global lung stress and V_T/V_RM. The ratio of the tidal volume (V_T) to the volume delivered during a recruitment maneuver (V_RM) defines the degree of tidal distension relative to maximum insufflation volume. V_RM was measured beginning from the resting lung volume at end-expiration. Left: End-inspiratory lung stress, calculated as the transpulmonary pressure during end-inspiratory pause of a tidal breath. Right: Tidal lung stress, calculated as the transpulmonary pressure difference between end-inspiratory and end-expiratory pauses of a tidal breath.