Supporting hemodynamics: What should we target? What treatments should we use?

Abstract

Assessment and monitoring of hemodynamics is a cornerstone in critically ill patients as hemodynamic alteration may become life-threatening in a few minutes. Defi ning normal values in critically ill patients is not easy, because 'normality' is usually referred to healthy subjects at rest. Defi ning 'adequate' hemodynamics is easier, which embeds whatever pressure and fl ow set is suffi cient to maintain the aerobic metabolism. We will refer to the unifying hypothesis proposed by Schrier several years ago. Accordingly, the alteration of three independent variables - heart (contractility and rate), vascular tone and intravascular volume - may lead to underfi lling of the arterial tree, associated with reduced (as during myocardial infarction or hemorrhage) or expanded (sepsis or cirrhosis) plasma volume. The underfi lling is sensed by the arterial baroreceptors, which activate primarily the sympathetic nervous system and renin-angiotensin-aldosterone system, as well as vasopressin, to restore the arterial fi lling by increasing the vascular tone and retaining sodium and water. Under 'normal' conditions, therefore, 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 underfi lling occurs, the mechanisms are activated (sodium and water retention) - associated with low central venous oxygen saturation (ScvO2) if underfi lling is caused by low fl ow/hypovolemia, or with normal/high ScvO2 if associated with high fl ow/hypervolemia. Although the correction of hemodynamics should be towards the correction of the independent determinants, the usual therapy performed is volume infusion. An accepted target is ScvO2 > 70%, although this ignores the arterial underfi lling associated with volume expansion/high fl ow. For largevolume resuscitation the worst solution is normal saline solution (chloride load, strong ion diff erence = 0, acidosis). To avoid changes in acid-base equilibrium the strong ion diff erence of the infused solution should be equal to the baseline bicarbonate concentration.