The pulmonary physician and critical care. 6. Oxygen transport: the relation between oxygen delivery and consumption.

Abstract

The unifying concept of oxygen transport has until recently been neglected by both car-diologists and respiratory physicians. With the increasing part played by both disciplines in the care of the critically ill, however, this attitude is changing. The primary function of the heart and lungs is to generate a flow of oxygenated blood to the tissues to sustain aerobic metabolism. The main requirements of this system are that it should be energy efficient, so that unnecessary cardiorespiratory work is avoided, and that it should be sensitive to the fluctuating demands of cellullar metabolism. Secondly, metabolic demand and distribution should be matched regionally at rest, during exercise, and in different disease states. Thirdly, oxygen should be able to pass efficien-tly across the extravascular tissue matrix. The mechanisms controlling oxygen distribution are incompletely understood, but are almost certainly important in determining clinical out-come in the critically ill patient.' Although the relation between oxygen delivery (Do2) and consumption (Vo2) has not been clearly established, these variables are often measured to define a population of critically ill patients in whom Vo2 is limited by Do2, the state of so called "pathological supply dependency."2 During recent years many of the publications on critical care, and indeed practice in leading intensive care units, have emphasised the importance of raising Do2 to "supranormal" levels in an attempt to satisfy the increased metabolic demands of these patients. This practice has been justified by the observation that increased DO2 improves oxygen debt and outcome in postoperative surgical patients requiring intensive care.3 No one would dispute that restoring blood volume to improve DO2 in the severely hypovolaemic patient must be beneficial. Controlled trials, however, examining the influence of such strategies on clinical outcome in patients with more complex conditions, suffering from sepsis, cardiovascular collapse, and hypoxic hypox-aemia, have produced conflicting data. Perhaps the concept of global oxygen delivery has failed to emphasise the importance of the regional distribution of blood flow, particularly to the splanchnic and renal vascular beds, which may be more important in determining clinical outcome.' This article reviews current ideas about the