A 50-year-old female called for an ambulance 30 min after the onset of a “squeezing” sensation in the substernal region of her chest. Paramedics found her to be in moderate distress due to the pain. She was immediately placed on 15 liters of oxygen by non-rebreather mask (NRBM). En route to the hospital, a peripheral intravenous (i.v.) line was placed, and the patient was given one spray of sublingual nitroglycerin every 5 min (for a total of three sprays), 4 mg of morphine sulfate i.v., and 81 mg of aspirin p.o. Upon arrival in the Emergency Department (ED), the patient’s chest pain was still present without improvement. The vital signs were: temperature 36.7°C, heart rate 86 beats/min, respiratory rate 20 breaths/min, and blood pressure 148/72 torr. The oxygen saturation was 99% while on oxygen. A 12-lead electrocardiogram (EKG) was performed (Figure 1). Pulmonary embolism was considered to be in the differential diagnosis. Approximately 5 min after the first EKG, the patient’s oxygen was discontinued to obtain a room air arterial blood gas sample. Eight minutes later, a second 12-lead EKG (Figure 2) was performed; it showed evidence of an acute inferior wall infarction. Immediately after the second EKG was performed and approximately 9 min after the oxygen was removed, it was replaced (15 L by NRBM). Precisely 3 min after the oxygen was replaced, a third 12-lead EKG was performed (Figure 3) showing near elimination of the ST segment elevations. Other than oxygen, the patient received only morphine sulfate (4 mg i.v.) between the second and third EKGs. The patient’s chest pain persisted. Approximately 45 min after this patient’s arrival in the ED, she was given thrombolytic therapy. Complete relief of her chest pain occurred shortly thereafter (Figure 4). The cardiac enzyme studies later confirmed that she had suffered an acute myocardial infarction. This set of EKGs illustrates the importance of providing oxygen to all patients who suffer from acute chest pain that might be due to acute myocardial infarction. The literature supports the use of oxygen in this setting, but the benefit of 100% versus 40% inspired oxygen has not been proven. (1–6)