M. Sydow

Objectives: It is not clear whether the mechanical properties of the respiratory system assessed under the dynamic condition of mechanical ventilation are equivalent to those assessed under static conditions. We hypothesized that the analyses of dynamic and static respiratory mechanics provide different information in acute respiratory failure. Design: Prospective multiple-center study. Setting: Intensive care units of eight German university hospitals. Patients: A total of 28 patients with acute lung injury and acute respiratory distress syndrome. Interventions: None. Measurements: Dynamic respiratory mechanics were determined during ongoing mechanical ventilation with an incremental positive end-expiratory pressure (PEEP) protocol with PEEP steps of 2 cm H2O every ten breaths. Static respiratory mechanics were determined using a low-flow inflation. Main Results: The dynamic compliance was lower than the static compliance. The difference between dynamic and static compliance was dependent on alveolar pressure. At an alveolar pressure of 25 cm H2O, dynamic compliance was 29.8 (17.1) mL/cm H2O and static compliance was 59.6 (39.8) mL/cm H2O (median [interquartile range], p < .05). End-inspiratory volumes during the incremental PEEP trial coincided with the static pressure–volume curve, whereas end-expiratory volumes significantly exceeded the static pressure–volume curve. The differences could be attributed to PEEP-related recruitment, accounting for 40.8% (10.3%) of the total volume gain of 1964 (1449) mL during the incremental PEEP trial. Recruited volume per PEEP step increased from 6.4 (46) mL at zero end-expiratory pressure to 145 (91) mL at a PEEP of 20 cm H2O (p < .001). Dynamic compliance decreased at low alveolar pressure while recruitment simultaneously increased. Static mechanics did not allow this differentiation. The decrease in static compliance occurred at higher alveolar pressures compared with the dynamic analysis. Conclusions: Exploiting dynamic respiratory mechanics during incremental PEEP, both compliance and recruitment can be assessed simultaneously. Based on these findings, application of dynamic respiratory mechanics as a diagnostic tool in ventilated patients should be more appropriate than using static pressure–volume curves.

A total of 18 patients with acute lung injury (ALI) were sequentially ventilated with two different modes of mechanical ventilation, each applied for a period of 24 h: (1) volume-controlled inverse ratio ventilation (VC-IRV), (2) airway pressure release ventilation (APRV). The individual sequence of both ventilatory modes was randomized. Ventilatory minute volume was adjusted for a PaCO2 of 35 to 45 mm Hg at the beginning of the study during the first ventilatory mode and then kept constant within preset limits. Hemodynamic variables were stable and similar during the 24-h periods of VC-IRV and APRV as well. Despite the lower sedation and spontaneous breathing during APRV, oxygen uptake was similar during both ventilatory modes. During the 24-h period of VC-IRV there was no relevant change of either airway pressures, alveolo-arterial O2 tension difference (AaDO2)/fraction of inspired oxygen (FIO2) or venous admixture. In contrast, peak airway pressures (Pawmax) during APRV were significantly lower (about 30%; p < 0.01), and decreased further within 24 h (p < 0.05). During APRV AaDO2/FIO2 and venous admixture improved significantly with time after more than 8 h (AaDO2/FIO2: 487 versus 414 mm Hg; p < 0.01; venous admixture: 20.6 versus 13.9%; p < 0.01; medians of onset versus end). The improvement was significantly different between both ventilatory modes (p < 0.01). We conclude that this indicates a progressive alveolar recruitment over time during ventilation with APRV.(ABSTRACT TRUNCATED AT 250 WORDS)