The ultimate goal of therapeutic intervention in a critically ill patient is to maintain oxygen homeostasis where delivery of oxygen to the cells is greater than, or at least equal to, the oxygen demand of the cells. Oxygen demand varies from organ to organ. Total body oxygen demand is the sum of all oxygen required by all tissues and organs for aerobic cellular function. Oxygen consumption (V̇O2) is the quantity of oxygen actually used by the cells. V̇O2 may be calculated if the values of cardiac output (CO), hemoglobin concentration, and arterial and venous oxygen saturations (SaO2 and SV̄O2, respectively) are known. Under normal circumstances, the quantities of oxygen demanded and oxygen consumed are equal, but in situations of inadequate oxygen delivery, oxygen demand may not be satisfied and the quantity of oxygen actually consumed will be governed by the quantity delivered. This then may result in an oxygen deficit and, ultimately, cellular death. This article discusses the principles of oxygen homeostasis, techniques for measuring V̇O2, CO, and SV̄O2, and the relevance of these principles and techniques to clinical practice.
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