The relation between cardiac index and oxygen extraction in acutely ill patients

The relation between cardiac index and oxygen extraction (O2ER) can represent a useful basis for the interpretation of hemodynamic parameters in the acutely ill patient. We prospectively collected cardiac index and paired arterial and mixed venous blood gas values in 79 acutely ill patients who were successively monitored invasively during their ICU course. For each patient, the degree of cardiac failure score and sepsis was scored at time of each measurement. A total of 122 observations were reported on a cardiac index/O2ER diagram, on which, based on physiologic data obtained during physical exercise, a line of reference was included representing equal increases in cardiac index and O2ER from normal values at rest. Patients with established heart failure fell below the line of reference. However, patients without cardiac failure or with compensated cardiac failure during adrenergic therapy fell close to the line of reference. Sepsis was associated with elevated oxygen delivery (Do2) and oxygen consumption (Vo2). Although O2ER was lower in the presence than in the absence of septic shock, patients with sepsis fell close to the line of reference. Among these patients with sepsis, O2ER was greater in patients with established heart failure than without heart failure (37.6 +/- 11.7 percent vs 21.5 +/- 4.4 percent, p < 0.01). The relationship between cardiac index and O2ER is much simpler than the relationship between Vo2 and Do2 and it avoids problems of mathematical coupling. It also allows us to refer to physiologic adaptation to changes in oxygen demand. This diagram may be useful at the bedside to facilitate the interpretation of cardiac index and to guide therapy.