To ensure that tissues arc well oxygenated, oxygen supply and demand are now targets of therapy for the critically ill patient. This chapter reviews the physiologic determinants of oxygen supply, how it is threatened by respiratory or cardiac dysfunction or by hemorrhaged or anemic states, and how it can be assessed in individual patients. Activities and conditions that increase tissue oxygen demand arc examined so that clinicians can identify those patients whose oxygen demands may be excessive and should be controlled. Failure of tissues to consume enough oxygen is explained in patients with critically low delivery or with the maldistributed blood flow state seen in sepsis. The monitoring of mixed venous oxygen saturation is examined as a method of tracking the threats to supply/demand balance and of guiding treatment that can support the adequate oxygenation of tissue

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