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Nonlinearity of intratidal airway resistance


Acute respiratory distress syndrome is a disease associated with high mortality. Understanding the interdependence of ventilator settings and respiratory mechanics is crucial for further developments of protective lung ventilation. Up to now, the nonlinearity of compliance has mainly been the focus of interest. We hypothesized that airway resistance also changes intratidally. Therefore, this study was performed to analyze the dependence of resistance on tidal gas volume.


After induction of anesthesia and tracheotomy, the lungs of 14 surfactant-depleted piglets were ventilated at zero end-expiratory pressure with three different tidal volumes (8, 12, 16 ml/kg) in a randomized order. In addition, baseline measurements (12 ml/kg) were performed before saline lavage. Before any change of the ventilator settings a recruitment maneuver was performed. The nonlinear intratidal airway resistance was analyzed using the SLICE method [1].


Figure 1 shows the intratidal resistance before lavage (grey) and after surfactant depletion (black) plotted against the alveolar pressure. Each curve in the diagram represents the intratidal course of resistance for one ventilator setting. Resistance is increased after surfactant depletion and is intratidally declining before and after lavage.

figure 1

Figure 1


The analysis of resistance shows a dependence on intratidal volume. The nonlinear course of intratidal resistance can be interpreted as a volume-related caliber effect leading to an increase of cross-sectional area of the large and small airways.


  1. Guttmann J, Eberhard L, Fabry B, et al.: Determination of volume-dependent respiratory system mechanics in mechanically ventilated patients using the new slice method. Technol Health Care 1994, 2: 175-191.

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Knorpp, H., Stahl, C., Schumann, S. et al. Nonlinearity of intratidal airway resistance. Crit Care 11 (Suppl 2), P193 (2007).

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