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CRITICAL CARE AND TRAUMA

Changes in Functional Residual Capacity During Weaning from Mechanical Ventilation: A Pilot Study

From the Department of Anesthesiology, University of Lübeck, Lübeck, Germany.

Address correspondence to Dr. Hermann Heinze, Department of Anesthesiology, University of Lübeck, Ratzeburger Allee 160, 23538 Lübeck, Germany. Address e-mail to hermann.heinze{at}uk-sh.de.

BACKGROUND: Reduction of high positive end-expiratory pressure levels and pressure support ventilation (PSV) are frequently used before tracheal extubation in critically ill patients, but the impact of PSV on functional residual capacity (FRC) is unknown. In this study, we sought to detect the changes of FRC and pulmonary function during a weaning protocol in patients ventilated after cardiac surgery.

METHODS: The LUFU system (Dräger Medical, Lübeck, Germany) estimates FRC by oxygen washout, a variant of multiple breath nitrogen washout, using a sidestream O₂ analyzer. Postoperative cardiac surgery patients were initially ventilated using biphasic positive airway pressure ventilation (BiPAP) with a positive end-expiratory pressure of 10 mbar. The upper pressure limit was adjusted to deliver a tidal volume of 6–8 mL/kg (BIPAP 10). After 30 min, the upper and lower pressure limits were both reduced by 3 mbar (BIPAP 7). When spontaneous breathing efforts were detected, ventilation mode was switched to continuous positive airway pressure (CPAP) with PSV using the former lower pressure limit as the CPAP level and the corresponding pressure support of the former BIPAP adjustment (CPAP 7_1). Measurements were repeated after 30 min (CPAP 7_2).

RESULTS: Ten patients were studied. FRC decreased (BIPAP 10: 3.6 [1.0] L; BIPAP 7: 3.1 [0.9] L; CPAP 7_1: 2.9 [0.9] L; CPAP 7_2: 2.7 [0.6] L [Mean (sd)]; MANOVA: P = 0.017), as did PF ratio (BIPAP 10: 420 [114] mm Hg; BIPAP 7: 405 [110] mm Hg; CPAP 7_1: 353 [70] mm Hg; CPAP 7_2: 340 [70] mm Hg [Mean (sd)]; MANOVA: P = 0.045). Paco₂ did not change significantly over time (P = 0.221).

CONCLUSION: Decreasing FRC during the weaning process after cardiac surgery may, at least in part, be explained by alveolar derecruitment. Whether this variable could help guide a weaning protocol has to be studied further.