In a larger sense, this article might be used as evidence that pulse oximetry can sometimes conceal more than it reveals. No one can deny that the emergence of pulse oximetry as “the fifth vital sign” represents a boon to caregivers in most situations. Unfortunately, it is possible to become excessively, even slavishly, dependent on the digital readout displayed on the face of the pulse oximeter, to the exclusion of information supplied to us by other methods. Allow me to illustrate this point by describing a case drawn from my own experience. Several years ago, a patient in the ICU of a hospital that shall remain nameless was assigned to me. The practitioner from whom I received report noted that an arterial blood gas determination had not been performed, notwithstanding the fact that this was standard practice in that ICU. The patient, a 33-year-old woman, had been intubated and received ventilation for treatment of a drug overdose. An indwelling arterial line had not been placed, because it was anticipated that the patient’s course of mechanical ventilation would be brief. The senior resident who was supervising the clinical team opined that a percutaneous arterial puncture was unnecessary, owing to the fact that the pulse oximeter readings were consistently > 95%. By the time that an analysis was finally performed, some 36 h after the initial arterial blood gas, the Paco2 was reported to be in the teens! This prompted the attending physician to roundly scold her resident, to reduce the ventilator respiratory rate by 50%, and to order a repeat arterial blood gas approximately 2 h later. Lo and behold, the Paco2 appearing on the subsequent report was virtually identical. At some length, it became obvious to us what had occurred here. Because the patient had been hyperventilated for a protracted time period, carbon dioxide had been washed out of body stores, which are capacious (approximately 28 L in an adult patient of normal size). With the subsequent onset of hypoventilation, the Paco2 did not rise promptly as we had expected. Instead, the hypoventilatory state elicited a gradual replenishment of the carbon dioxide in the patient’s whole-body stores. Finally, some 7 h later, the Paco2 rose to the mid-forties. I must admit that this case taught all of us in attendance that day a valuable lesson about the kinetics of carbon dioxide excretion. Nevertheless, it might certainly have been better for that patient if excessive confidence had not been placed in the pulse oximeter. In the final analysis (no pun intended), the article by Fu and coworkers teaches us that clinical tools are only as powerful as the judgement of the practitioner(s) applying and observing them.