[A case of inspiratory unidirectional value malfunction detected by an abnormal capnogram pattern]

We report a case of substantial rebreathing which occurred during the thoracoscopic bulla resection despite a normal FICO2 reading on capnogram. Early detection of inspiratory unidirectional valve malfunction as the cause of the rebreathing was only possible by careful observation of the capnogram pattern. During the course of one lung anesthesia, a rising end-tidal CO2 concentration (FETCO2) and a change in the pattern of the capnogram were observed. However, the lowest FICO2 value in the late phase of inspiration stayed below the alarm limit setting of rebreathing. For this reason, the capnograph failed to identify the beginning of inspiration and perceived the high FICO2 value in early inspiration as part of expiratory plateau. In case of the inspiratory valve malfunction, it is possible that analysis of the capnogram reveals underestimated FICO2.