Capnographic identification of end-expiratory flow limitation

Patients with end-expiratory flow limitation (eEFL) demonstrate a terminal rise in capnography slope. The high slope could represent phase 5, a phenomenon described for single breath N₂ tests but previously unreported during capnography. This study evaluated 6 healthy subjects exhaling from total lung capacity to residual volume at several set constant rates. We measured the volumes of flow limitation (V_FL) and phase 5 (V_P5) for CO₂ and N₂. A distinct phase 5 occurred shortly after eEFL for both gases_. Increased expiratory flow rate resulted in parallel increases in V_FL and V_P5. The two quantities differed on average by the volume of dead space. These data suggest that phase 5 on capnography identifies eEFL with a small delay resulting from transit of expired gas through dead space. Following phase 5 by volumetric capnography could be useful for monitoring anesthetized patients, who in some circumstances may have lung volumes close to residual volume. eEFL could be treated with lung volume-increasing maneuvers, such as positive end-expiratory pressure.