1 in response to the review article by Nimmagadda et al2 about preoxygenation and my accompanying editorial.3 They are correct to remind us that a clinician should never make a clinical decision based only on a value from a single monitor. Like all anesthesia monitors (eg, oxygen saturation, blood pressure, and heart rate), end-tidal oxygenation (EtO2) readings can be misleading due to artifacts. Some of the artifacts that cause falsely elevated EtO2readings are similar to those that cause false end-tidal carbon dioxide readings (eg, dead space and disconnection). However, with EtO2, these artifacts will tend to give a false high reading, and with end-tidal carbon dioxide readings, they will give a false low reading. Any clinical decision, such as when to begin induction of anesthesia, must be based on a combined observation of the patient and all monitors.