We should avoid the term “fluid overload”

Using the right word or phrase to describe a specific pathologic process/patient diagnosis and/or status is important, not only within the intensive care unit team, but also when we communicate with external consultants. This is not just a question of semantics. Using incorrect terms can lead to misunderstanding and even to incorrect therapeutic decisions. For example, it is not uncommon to see clinicians examining an edematous patient, saying that the patient has “fluid overload” or “hypervolemia” or both and proposing fluid restriction and/or diuretics as the logical strategy, when often during the acute phases of resuscitation from circulatory shock this approach may be inappropriate. A fundamental determinant of cardiac output and its ability to vary in response to changing metabolic demands is the body’s effective circulating blood volume. Total circulating blood volume is distributed throughout the circulatory system into vessels that can initially be filled without changing their distending pressures because they merely alter their conformation rather than stretch, rather like a large empty balloon that will not have any measurable distending pressure when air is initially blown into it. Once filled beyond this volume, however, wherein conformational changes alone cannot accept more air without requiring the balloon’s walls to stretch, the distending pressure will start to increase. The volume in the balloon below this point is called its unstressed volume. In the body, this unstressed volume accounts for between 60 and 70% of the total circulating blood volume [1]. As illustrated in Fig. 1, the amount of unstressed volume can be rapidly and dynamically changed by changing blood flow distribution, increasing surrounding tissue pressure, or increasing venomotor tone [2]. The effective circulating blood volume reflects the proportion of blood volume above the unstressed volume and is essentially independent of arterial pressure, because arterial pressure is dissipated across the high resistance arterioles, analogous to a “vascular waterfall”. The stressed blood volume relative to the venous compliance defines the mean circulatory filling pressure, which is the back pressure to venous flow from the body back to the right ventricle. The primary reason for giving fluids during resuscitation is to increase the stressed circulatory blood volume, thus causing mean circulatory filling pressure to rise. If the heart can handle this increased driving pressure, then venous return increases causing cardiac output to rise. If the effective circulating blood volume is low, then mean circulatory filling pressure is also low, and if the effective circulating blood volume is high, then mean circulatory filling pressure is also high. Accordingly, hypovolemia refers to a low intravascular blood volume, wherein mean systemic filling pressure is low and there is a net flux of water and electrolytes out of the interstitial and tissue spaces into the blood stream. This tends to support blood volume but with decreased tissue rigidity and loss of vascular reserve. At the opposite end of the spectrum, hypervolemia refers to an excessive blood volume, wherein mean systemic circulatory pressure is high, causing a net fluid loss into the interstitium resulting in some edema formation. The causes of hypervolemia can be complex and include renal failure, congestive heart failure, or liver failure, especially when these conditions are associated with overzealous fluid administration. Hypervolemia is also sometimes associated with hyponatremia, especially in patients with cirrhosis, renal failure, or heart failure, reflecting a defect in free water excretion. Unfortunately, hypervolemia is not a condition with which the body is prepared to cope, as all stress states are usually associated with hypovolemia. This is why clinicians often need to intervene in the management of hypervolemia with the use of diuretics or ultrafiltration. In essence, the body defends bleeding and hypovolemia. Hypervolemia is not a priority. Nevertheless, in the perioperative setting, both hypovolemia and hypervolemia are associated with several * Correspondence: jlvincent@intensive.org Department of Intensive Care, Erasme University Hospital, Université Libre de Bruxelles, Route De Lennik 808, 1070 Brussels, Belgium Full list of author information is available at the end of the article