Normal values and choosing targets in ICU

Assessment and monitoring of hemodynamics is a cornerstone in critically ill patients as hemodynamic alteration may become lifetreatening in few minutes. It is not easy to define which are the normal values in critically ill patients, as “normality” is usually referred to healthy subjects at rest. It is easier to define what is “adequate” hemodynamic, which embeds whatever pressure and flow set sufficient to maintain the aerobic metabolism. We will refer to the unifying hypothesis proposed by Schrier several years ago. Accordingly, the alteration of 3 independent variables: heart (contractility and rate), vascular tone and intravascular volume, may lead to under-filling of the arterial tree, associated to reduced (as during myocardial infarction or hemorrhage) or expanded plasma volume (sepsis or cirrhosis). The under-filling is sensed by the arterial baroreceptors, which activate primarily the sympathetic nervous system and renin-angiotensin-aldosterone, as well as vasopressin, to restore the arterial filling by increasing the vascular tone and retaining sodium and water. Therefore, in “normal conditions”, the homeostatic system is not activated and water/sodium excretion, heart rate and oxygen extraction are in the range found in normal subjects. When arterial under-filling occurs the mechanisms are activated (sodium and water retention) associated to low central venous oxygen saturation (ScvO2), if under-filling is caused by low-flow/hypovolemia, or normal/high ScvO2, if associated to high flow/hypervolemia. Although the correction of hemodynamics should be towards the correction of the independent determinants, the usual therapy performed is the fluid volume infusion. An accepted target is ScvO2 > 70%, although this ignores the arterial under-filling associated to volume expansion/high flow. For large fluid volume resuscitation the worst solution is the normal saline solution (chloride load, strong ion difference equal to 0, acidosis). To avoid changes in acid-base equilibrium the strong ion difference of the infused solution should be equal to the baseline bicarbonate concentration.