Perfusion and Oxygenation

Shock occurs when tissue oxygen delivery is inadequate to meet metabolic demands, and cellular dysfunction results. Since a primary goal of treating shock is elimination of cellular hypoxia, it logically follows that detecting and treating shock would best be monitored by measuring the state of tissue perfusion and cellular oxygenation. To this end, many devices that have the capability of monitoring tissue perfusion and oxygenation have been developed. However, to date, none of these devices has gained widespread acceptance in clinical practice. Why is this? This chapter will outline underlying principles of tissue perfusion and oxygenation and review the complexities of making clinically useful measurements with existing monitoring approaches. There are multiple components of the circulation that contribute to cellular oxygenation, each of which is related to monitoring of tissue perfusion and oxygenation. As shown in Figure 19.1, tissue perfusion is determined by cardiac output, the distribution of cardiac output to regional tissue beds, and the state of the microcirculation. Tissue oxygenation is determined by perfusion as well as by arterial oxygenation, nutritional blood flow, and cellular extraction of oxygen. This is a complex system, which is highly dynamic: Alteration of any component has physiologic impact upon other components. Moreover, there is enormous heterogeneity within the circulation, both between organs and within organs. Hence tissue perfusion and oxygenation is never uniform between organs, nor even in particular tissue beds. Nonetheless, despite these complexities, there are several principles that allow useful monitoring to occur: