Depending on the circumstances, the term 'hypotension' may carry different meanings. Hypotension is classically used to describe a clinical scenario where the measured blood pressure is insufficient to support organ perfusion. It can also be viewed as the key clinical sign of 'shock'—a syndrome describing a situation in which tissue O2 needs and availability are mismatched, i.e. either tissue O2 availability is insufficient to match O 2 needs (anoxic, anemic, or circulatory anoxia), or the tissues are unable to utilize available O 2 (histiotoxic anoxia) (Table,!). The result of this mismatch between O2 need and availability is cellular ischemia, a situation characterized by an intracellular O2 deficit. This is followed by cellular damage that ranges, depending on the severity and duration of ischemia, from reversible alterations of cellular metabolic function to irreversible cell death. Arterial blood pressure is used as a surrogate marker of the adequacy of systemic perfusion and tissue O2 delivery. For the purpose of this chapter, hypotension is discussed in the context of understanding the pathophysiology of both inadequate systemic perfusion and inadequate tissue O2 delivery.

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