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Gas exchange principles in the critically ill 

Gas exchange principles in the critically ill
Chapter:
Gas exchange principles in the critically ill
Author(s):

Peter D. Wagner

DOI:
10.1093/med/9780199600830.003.0075
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date: 22 February 2020

Pulmonary gas exchange in the critically-ill patient is almost always impaired. The reasons are usually multiple and complex, with causes both internal and external to the lung. To understand the relative contributions of these many factors in a given patient requires an understanding of the basic principles of gas exchange. This becomes even more important when a patient’s condition changes quickly, which happens commonly in the critically ill. The purpose of this chapter is to lay out those principles and discuss the several causes of arterial hypoxaemia (and hypercapnia) on the basis of those principles. The key principle that governs gas exchange (in the steady state) is conservation of mass, and writing straightforward mass transport equations that express this principle allows gas exchange to be analysed quantitatively. This chapter is intended to serve as a guide to support appropriate application of tests of pulmonary gas exchange, which are laid out in the chapter that immediately follows it.

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