Cardiac output monitoring: throw it out… or keep it?

Throw it out! During circulatory failure, the variable that really matters is not CO but oxygen delivery and ultimately tissue oxygenation. However, a given value of CO does not necessarily inform on tissue oxygenation. Firstly, CO depends much on global oxygen demand; thus an apparently high CO may be inadequate (high oxygen demand), whereas a low CO might perfectly fit requirements (low oxygen demand). Secondly, CO is not the only determinant of oxygen delivery. For instance, even if fluid increases CO, the oxygen delivery to the tissues may increase to a smaller extent due to the inherent haemodilution [1]. Assessing global tissue oxygenation, through clinical examination, lactate, and central venous oxygenation (ScvO2), should be much more relevant than monitoring CO. Thirdly, tissue oxygen supply primarily depends on microcirculation. Under physiological conditions, CO and microcirculatory flows are coupled. However, in some circumstances, the most typical being sepsis, the regulation of microcirculatory flows is impaired and “coherence” between the macroand microcirculation is lost [2]. In this regard, increasing CO cannot guarantee any parallel correction of microcirculatory abnormalities [3]. As an illustration, a clinical study showed that oxygen consumption improves in only half of patients in whom fluid infusion increases CO [1]. When deciding to continue fluid infusion or not, are not effects on tissue oxygenation more important than CO? To justify its monitoring, CO should at least be a target for haemodynamic resuscitation which, unlike arterial pressure, is not actually the case. Also, the arterial pulse pressure is physiologically related to stroke volume. Rather than using costly and often invasive CO monitoring, could we not simply measure arterial pressure along with the above-mentioned tissue oxygenation variables? Finally, studies repeatedly show that using CO monitoring devices during shock does not improve outcome [4]. All these arguments may discourage us from monitoring CO in critically ill patients...