The paper underlines the importance of continuous SvO2 monitoring by means of the insertion of a modified Swan-Ganz optic fibre catheter into the pulmonary artery. Following an analysis of the factors which regulate O2 transport and ATP formation, together with metabolism self-regulating mechanisms which intervene in periods of stress to guarantee cellular metabolic activity, the Authors underline the importance of SvO2 as an indicator of the balance between O2 demand and supply. Real-time SvO2 monitoring is therefore considered a valuable and early marker of rapidly degenerating biological processes in critical patients.