Direct cardiorespiratory measurements and the use of optimum values to guide therapy have been associated with improved survival in a number of conditions causing critical illness. Increasingly sophisticated monitoring that provides more accurate and reproducible assessment of the cardiorespiratory system at the bedside is pivotal to this better outcome. The inclusion of fibreoptic filaments and fast response thermistors in pulmonary artery catheters makes possible the continuous monitoring of mixed venous oxygen saturation and the measurement of right ventricular ejection fraction. Although the place of measurement and manipulation of these variables in critically ill patients has yet to be fully defined, clinical studies are promising. We discuss some practical aspects of the use of these measurements and some potential clinical applications. Additionally, some of the studies in which the use of these measurements is increasing our knowledge of the pathophysiology of critical illness and contributing to improved management of critically ill patients, are highlighted.