The Cardiac Index/Oxygen Extraction Diagram to Assess Hemodynamic Status

Abstract

Recent insights into the relationship between oxygen uptake (VO₂) and oxygen delivery (DO₂) have been helpful not only to better understand the pathophysiologic alterations implicated in the development of acute circulatory failure but also to apply the appropriate therapeutic interventions to limit organ failure. Unfortunately, the clinical applications of these concepts have been limited by several factors. First, the calculations of VO₂ and DO₂ are too complex for routine use at the bedside of the acutely ill. Second, the analysis of the relationship between VO₂ and DO₂ is subject to a problem of mathematical coupling of data if the thermodilution cardiac output, the hemoglobin concentration and the arterial oxygen saturation (SaO₂) are used to calculate both parameters [1]. The use of metabolic carts to determine VO₂ from the expired gas analysis could avoid this problem. However, this equipment is not widely available in the everyday practice, because it is expensive, sophisticated, and time consuming. In addition, the use of this technique is difficult in the spontaneously ventilating patient and its accuracy limited when high oxygen inspiratory fractions are administered. Third, although supranormal values for VO₂ and DO₂ have been associated with reduced morbidity and mortality rates [2], optimal values can hardly be recommended for all acutely ill patients, in view of the large variability of the oxygen demand in these patients. Hence, we sought to define a relatively simple diagram, based on experimental and clinical studies, that could be used routinely at the bedside of the critically ill to facilitate the hemodynamic evaluation and to assess the effects of therapy.