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CRITICAL CARE AND TRAUMA

Functional Residual Capacity and Respiratory Mechanics as Indicators of Aeration and Collapse in Experimental Lung Injury

Christian Rylander, MD^*,, Marieann Högman, PhD, Gaetano Perchiazzi, MD^,, Anders Magnusson, PhD^||, and Göran Hedenstierna, PhD

*Department of Anesthesia, Sahlgrenska University Hospital, Gothenburg, Sweden, the Department of Medical Sciences, Clinical Physiology, Uppsala University Hospital, Uppsala, Sweden, the Department of Medical Cell Biology, Integrative Physiology, Uppsala University, Uppsala, Sweden, the Department of Emergency and Transplantation, Bari University Hospital, Bari, Italy, and the ||Department of Radiology, Uppsala University Hospital, Uppsala, Sweden

Address correspondence and reprint requests to Göran Hedenstierna, MD, PhD, Department of Medical Sciences, Clinical Physiology, Uppsala University Hospital, S-751 85 Uppsala, Sweden. Address email to goran.hedenstierna{at}thorax.uas.lul.se

Increased functional residual capacity (FRC) and compliance are two desirable, but seldom measured, effects of positive end-expiratory pressure (PEEP) in mechanically ventilated patients. To assess how these variables reflect the morphological lung perturbations during the evolution of acute lung injury and the morphological changes from altered PEEP, we correlated measurements of FRC and respiratory system mechanics to the degree of lung aeration and consolidation on computed tomography (CT). We used a porcine oleic acid model with FRC determinations by sulfur hexafluoride washin-washout and respiratory system mechanics measured during an inspiratory hold maneuver. Within the first hour, during constant volume-controlled ventilation with PEEP 5 cm H₂O, FRC decreased by 45% ± 15% (P = 0.005) and compliance decreased by 35% ± 12% (P = 0.005). Resistance increased by 60% ± 62% (P = 0.005). Only the FRC changes correlated significantly to the decreased aeration (R² = 0.56; P = 0.01) and the increased consolidation (R² = 0.43; P = 0.04) on CT. When the PEEP was changed to either 10 or 0 cm H₂O, there were larger changes in FRC than in compliance. We conclude that, in our model, FRC was a more sensitive indicator of PEEP-induced aeration and recruitment of lung tissue and that FRC may be a useful adjunct to PaO₂ monitoring.

IMPLICATIONS: Lung injury was quantified on computed tomography and related to monitored values of functional residual capacity and mechanical properties of the respiratory system. We found the functional residual capacity to be a more sensitive marker of the lung perturbations than the compliance. It might be of value to include functional residual capacity in the monitoring of acute lung injury.

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