Most critically ill patients are admitted with obvious life-threatening imbalance between oxygen supply and demand. Advances in the care of critically ill patients, including appropriate use of mechanical ventilatory support, have prolonged survival of these patients. However, many patients who survive the initial insult develop progressive multiple organ system failure. Acute lung injury and acute respiratory distress syndrome (ARDS) play pivotal roles in most cases of multiple system organ failure. Multiple system organ failure is now the commonest cause of death in critically ill patients.<sup>5 Beal A.L., Cerra F.B. Multiple organ failure syndrome in the 1990s. Systemic inflammatory response and organ dysfunction JAMA 1994 ;  271 : 226-233

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The mechanisms responsible for the development of multiple system organ failure after resuscitation of critically ill patients are unknown. Incomplete reversal of tissue hypoxia and continued occult tissue hypoxia could be mechanisms of multiple system organ failure. One reason that occult tissue hypoxia has been suggested as a mechanism of multiple system organ failure is the concept of pathologic dependence of oxygen consumption on oxygen delivery in critical illness. Numerous studies suggested that pathologic dependence of oxygen consumption on oxygen delivery is found in critically ill patients, including patients who have sepsis, ARDS, and hepatic failure. However, controversy has emerged because recent studies have not found pathologic dependence of oxygen consumption on oxygen delivery in critically ill patients.

As a result, clinicians are faced with controversy when considering pathologic dependence of oxygen consumption on oxygen delivery as a mechanism of multiple system organ failure, and they face therapeutic dilemmas when managing oxygen delivery and consumption of critically ill patients. In addition, management of oxygen delivery and consumption is central to comprehensive assessment and management of patients who require mechanical ventilation.

This article is designed to address the controversy regarding oxygen delivery and consumption first by reviewing the physiology of oxygen delivery and consumption in order to understand the methods used to determine oxygen delivery and consumption. Second, the problem of mathematical coupling of shared measurement errors in the calculation of oxygen delivery and oxygen consumption is reviewed. Third, we review studies of critically ill patients to determine whether pathologic dependence of oxygen consumption on oxygen delivery occurs in critical illness. Then, randomized control trials of so-called supernormal oxygen delivery in critically ill patients are summarized to assess whether there is clinical trial evidence to support use of supernormal levels of oxygen delivery in critically ill patients. We also discuss the beneficial and adverse effects of mechanical ventilation on oxygen delivery and consumption. Finally, practical recommendations are made regarding clinical evaluation of oxygen delivery and oxygen consumption and regarding methods to increase oxygen delivery and decrease oxygen demand.