Intravenous (IV)fluids are one of the most frequently prescribed treatments in hospitals. The use of IV fluids in some resuscitation situations, such as trauma hemorrhage, has good evidence in their favor. In emergency situations such as shock, its usefulness is not so well documented, although water treatment is recommended in numerous guides and reviews.

For example, guidelines for acute kidney injury advise to identify and correct hypovolaemia through ‘adequate’ fluid replacement and international guidelines for sepsis indicate that the use of IV ‘fluids is the cornerstone of modern treatment’.

Despite this, there is no evidence to determine the indications, dose and speed of administration of IV fluids, as well as the use of IV fluids during the course of a disease. Physicians usually rely only on their clinical acuity to indicate fluid and electrolyte replacement, coinciding with the ‘science of uncertainty and the art of probability’ described by Sir William Osler.

The usual practice is to use the clinical picture of hypovolemia and hypervolemia to indicate when to start and when to stop treatment. None of these characteristics is well described; none is specific to the state of the volemia and many are difficult to evaluate.

A systematic review of 30 studies found that the clinical picture (including hypotension and tachycardia) was not a reliable prognostic factor for hypovolemia.

The same can be said about the clinical picture of hypervolemia, present on many occasions. Although it could be diagnosed accurately, hypervolemia due to excess fluid represents an iatrogenic overdose and should not be used as a marker to suspend water administration. A recent study in Zambia endorses this.

Hypotensive septic adults were randomized to a usual treatment group (IV fluids determined by the treating physician) or to a protocol for sepsis (intensive IV fluid delivery only limited by clinical signs of hypervolaemia, together with vasopressors and blood transfusion if necessary).

The randomized controlled study FEAST (Fluid Expansion As Supportive Therapy) with African children with severe febrile illness observed that bolus fluid administration was associated with three more deaths per 100 patients, in relation to the administration of maintenance fluids alone.

A new analysis of this study suggested that excess mortality was linked more to cardiovascular collapse than to respiratory failure due to pulmonary edema. No quality studies on the subject were conducted in developed countries, although observational studies found associations between positive water balance and increased mortality in inflammatory disorders and acute kidney injury.

Variations in the physiology and pathology of patients influence the assessment of water status. Aging impacts on organic systems.

A decrease in the response of the heart rate to stressors is observed, which is related to a reduction in the response capacity of the β-adrenoceptors to the adrenergic stimuli, dampening the tachycardia characteristic of hypovolemia. This variability is further accentuated in the presence of concomitant diseases.

Medications used to treat chronic diseases can also alter the balance of fluids by direct action, for example collateral effects such as diarrhea and vomiting.

The evaluation of liquids is more complex even in the presence of diseases and acute injuries. For example, sepsis affects the cardiovascular system in multiple ways. Hypovolemia is due to direct loss of fluids, including insensible losses due to fever. Sepsis has a direct inhibitory effect on cardiac function: it reduces contractility and limits cardiac output.

Vasodilatation decreases systemic vascular resistance and, if severe, causes hypotension. Sepsis also damages the endothelial glycocalyx, a network of proteoglycans and glycoproteins that covers the luminal vascular endothelium and allows the loss of fluids into the extracellular compartment.

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