We performed a prospective study in 28 critically ill patients to document variations in oxygen consumption and oxygen transport that occur spontaneously, without any experimental interventions designed to change DO2. Each study consisted of from three to five sets of measurements, including thermal dilution cardiac output and blood gases. The interval between any two measurements ranged from 20 to 60 minutes. It was concluded that changes in VO2 and DO2 can occur spontaneously in patients with normal lactates, even when there are no experimental interventions. These spontaneous changes in VO2 and DO2 make it extremely difficult to interpret studies that demonstrate that VO2 is transport-limited. The changes in DO2 can likely be explained by changes in oxygen demand.