We thank Drs Benumof and Herway for their comments and acknowledge Dr Benumof’s many contributions to the science and practice of preoxygenation. They are correct in stating that both a poor mask seal and a low-tidal volume ventilation can result in a falsely high end-tidal oxygen (EtO2) value despite poor preoxygenation. However, these situations should not occur if: (1) the EtO2 measurements are interpreted accurately; and (2) the preoxygenation technique is performed correctly. A fundamental principle in interpreting the end-tidal gas measurements is that the tidal volume is sufficient to displace the alveolar dead space. Obviously, if the tidal volume is too small or the patient is apneic, the monitor will display erroneous values. The importance of a sealed system during preoxygenation has been thoroughly addressed in research reports, editorials, and book chapters.1,2 Clinical evidence for a sealed system is an adequate movement of the reservoir bag during inspiration and expiration and a normal capnographic tracing that permits measurement of inspired and end-tidal carbon dioxide (EtCO2).1,2 As stated in our review, “the technique should be performed correctly ...” and “the absence of a normal capnographic tracing and a lower than expected EtCO2 and EtO2 should alert the anesthesiologist to the presence of leaks in the anesthetic circuit.”3 Providing that the EtO2 measurements are accurately interpreted and preoxygenation is properly performed, an EtO2 value ≥90% is a reliable noninvasive indicator of its efficacy.