Dehydration: Decreased extracellular fluid, causes

Extracellular fluid ensures blood circulation and the transport of oxygen, nutrients, and hormones to tissues. A decrease in extracellular fluid volume, clinically manifested as hypovolemia, leads to decreased organ perfusion and can rapidly progress to shock, acute renal dysfunction, and death. It is important to distinguish hypovolemia from dehydration, which primarily describes a total water deficit with a predominantly intracellular deficit. These conditions often coexist but require different treatment approaches. [1]

Classic causes of extracellular fluid volume loss include gastrointestinal losses, renal losses due to diuresis, blood loss, and plasma loss into the third space due to inflammation and burns. In practice, mixed causes are encountered, where fluid redistribution, venous congestion, and insufficient effective arterial volume are combined against a background of inflammation. [2]

Early diagnosis relies on a combination of clinical features, laboratory markers, and bedside instrumental methods, including ultrasound assessment of the inferior vena cava, dynamic preload tests, and integrated ultrasound venous overload scales, which allows for safe dosing of fluid therapy and avoidance of both fluid deficiency and overload. [3]

This material presents current data on the epidemiology, causes and risk factors, pathogenesis, symptoms, forms and stages, complications, principles of diagnosis and differential diagnosis, as well as modern approaches to treatment, prevention, and prognosis. The choice of solutions, initial resuscitation volumes, oral rehydration, pediatric features, and management of blood loss are discussed separately. [4]

Epidemiology

The prevalence of hypovolemia is high in emergency and intensive care units (EDs) and is often associated with infections, bleeding, surgery, diuretic therapy, and gastrointestinal losses. In the elderly, the incidence is higher due to a decreased sense of thirst, polymorbidity, and polypharmacy. [5]

In the sepsis population, adherence to the recommended initial crystalloid volume is associated with reduced mortality, although the quality of evidence is low and the optimal dose and infusion rate remain debated. This highlights the need for dynamic assessment of the response to fluid resuscitation. [6]

In patients with heart failure and cirrhosis, a paradox is observed: the total fluid volume is increased, but the effective arterial volume is reduced, which masks the true infusion needs and requires particularly careful selection of tactics. [7]

Hypernatremia and hyponatremia often accompany extracellular fluid volume deficits and worsen outcomes. Proper sodium correction is only possible with consideration of concomitant hypovolemia and potassium deficiency. [8]

Reasons

Extrarenal losses include diarrhea, vomiting, fistulas, profuse sweating, burns, and third-space pancreatitis. In these conditions, water and electrolytes are primarily lost from the extracellular compartment.[9]

Renal losses are caused by diuretics, osmotic diuresis due to hyperglycemia, salt losses due to tubulopathies, and insufficient reabsorption of sodium and water in a number of nephropathies. Clinically, they are accompanied by increased sodium excretion and a decrease in the relative density of urine. [10]

Acute blood loss leads to a rapid reduction in circulating blood volume with the risk of hemorrhagic shock. Here, limiting diagnostic time, early control of the bleeding source, and massive transfusion protocols become paramount. [11]

Systemic inflammation and endothelial dysfunction impair microcirculation and lymphatic return, contributing to the coexistence of hypovolemia, hypoalbuminemia, and peripheral edema, which complicates clinical assessment.[12]

Risk factors

Old age, chronic kidney disease, diabetes mellitus, and the use of diuretics and renin-angiotensin system inhibitors increase the risk of extracellular fluid volume depletion during intercurrent illnesses. [13]

Hyperthermia, physical exertion in a hot climate, lack of access to water, and gastrointestinal infections with diarrhea and vomiting quickly create a volume deficit, especially in children and the elderly. [14]

Septic conditions and pancreatitis, due to severe inflammation and capillary leakage, increase fluid requirements initially, but subsequent fluid overload worsens respiratory mechanics and outcomes. This necessitates early transition to fluid restriction strategies after perfusion stabilization. [15]

Blood loss during trauma and surgery creates a distinct risk profile where early hemostatic protocols, antifibrinolytics, and targeted use of blood components are important.[16]

Pathogenesis

The key factor is a reduction in effective arterial volume and preload, which reduces stroke volume and cardiac output. Compensation includes tachycardia, vasoconstriction, and activation of the renin-angiotensin-aldosterone system, but prolonged hypoperfusion leads to tissue hypoxia and organ dysfunction. [17]

Sodium and water losses reduce the volume of the extracellular compartment, while dehydration with free water deficiency leads predominantly to intracellular dehydration. In practice, these patterns overlap, so treatment must consider both volume and osmolarity. [18]

Systemic inflammation alters microcirculatory permeability and lymphatic drainage, causing a discrepancy between the infused volume and effective tissue perfusion. This explains the phenomenon of fluid "release" into the interstitium and the need for dynamic methods for assessing the infusion response. [19]

In the kidneys, hypoperfusion enhances tubular reabsorption of urea, increasing the urea-to-creatinine ratio, and reduces fractional excretion of sodium, which is used in the diagnosis of prerenal acute renal dysfunction with a number of caveats. [20]

Symptoms

Early signs include thirst, general weakness, dizziness, tachycardia, decreased urine output, and dry mucous membranes. In the elderly, the classic signs are less pronounced, often dominated by confusion, falls, and decreased exercise tolerance. [21]

Orthostatic hypotension with a drop in systolic pressure of at least 20 mmHg or diastolic pressure of at least 10 mmHg within 3 minutes of standing is an important clinical marker of volume depletion and impaired autonomic regulation. [22]

As the condition progresses, signs of hypoperfusion appear: cold extremities, increased capillary refill, oliguria, and impaired consciousness. In severe cases, signs of shock develop with hypotension, narrow pulse pressure, and lactic acidosis. [23]

In children, attention is paid to the retraction of the fontanelle, the absence of tears, severe dryness of the mucous membranes, and changes in respiratory pattern. These signs help assess the degree of deficiency and determine the appropriate rehydration strategy. [24]

Forms and stages

Extracellular fluid volume deficits are classified as mild, moderate, and severe based on a combination of clinical, hemodynamic, laboratory, and ultrasound findings. A dynamic reassessment of the stage is essential after each therapeutic intervention. [25]

In cases of blood loss, a clinical classification of hemorrhagic shock is used, divided into 4 classes based on blood loss volume, heart rate, blood pressure, state of consciousness, and urine output. This scale guides the choice of hemostatic resuscitation tactics. [26]

In sepsis, initial fluid loading is combined with early response assessment and transition to restrictive strategies in patients without evidence of perfusion deficit using dynamic preload sensitivity tests. [27]

In pediatrics, isotonic solutions are recommended for maintenance infusions, which reduces the risk of hyponatremia compared with hypotonic solutions, especially in hospitalized children. [28]

Complications and consequences

Insufficient volume correction leads to acute renal dysfunction, intestinal ischemia, myocardial damage, and impaired consciousness. The risk is particularly high in patients with concomitant heart failure, chronic kidney disease, and the elderly. [29]

Over-infusion of fluids after initial stabilization increases the risk of pulmonary edema, acute lung injury syndrome, and prolongation of mechanical ventilation. The balance between fluid deficit and overload requires dynamic assessment and early restriction of infusions. [30]

The choice of solution influences the risk of renal outcomes. Several randomized trials and meta-analyses have shown signals favoring balanced crystalloids for composites of adverse renal events, although large studies have not always confirmed a mortality benefit. [31]

In children, the use of hypotonic maintenance infusions is associated with iatrogenic hyponatremia and neurological complications, which justifies the transition to isotonic regimens. [32]

Diagnostics

Clinical assessment begins with vital signs, orthostatic testing, capillary refill time, skin examination, and urine output assessment. In the elderly, classic signs are less reliable and should be supplemented by laboratory testing and ultrasound. [33]

Laboratory markers include a complete blood count with hemoconcentration, biochemistry with creatinine and urea dynamics, the urea-to-creatinine ratio, blood gases, and lactate. Interpretation of the urea-to-creatinine ratio and calculation of the fractional excretion of sodium and urea help identify the prerenal profile, but have limitations in severely ill patients and those receiving diuretics. [34]

Bedside ultrasound assesses the diameter and collapse of the inferior vena cava and, using advanced venous congestion protocols, analyzes hepatic, portal, and intrarenal venous flow, helping to avoid fluid overload. This method should not be used in isolation; its data are interpreted in a clinical context. [35]

Dynamic preload sensitivity tests, such as the recumbent leg raise with continuous cardiac output monitoring, can predict fluid response without fluid administration. This is preferable to static measurements in unstable conditions. [36]

Table 1. Clinical signs of decreased extracellular fluid volume

Sign	Mild degree	Moderate degree	Severe degree
Pulse	Up to 100 per minute	100-120 per minute	More than 120 per minute
Blood pressure	Norm	The downward trend	Hypotension
Orthostatic test	Positive may be absent	Often positive	Expressly positive
Capillary filling	Up to 2 seconds	2-3 seconds	More than 3 seconds

Table 2. Laboratory clues of the prerenal profile

Indicator	Typical direction	Comment
Urea to creatinine ratio	More than 20 to 1	Less reliable in severely ill patients
Fractional excretion of sodium	Less than 1 percent	Restrictions for diuretics and chronic kidney disease
Fractional excretion of urea	Less than 35 percent	Does not exceed fractional excretion of sodium in most situations

Table 3. Ultrasonic indicators of volumetric status

Parameter	Meaning	Interpretation
Inferior vena cava diameter less than 2 centimeters with marked collapsibility	Often indicates low central venous pressure	Hypovolemia is possible given the clinical picture.
The diameter of the inferior vena cava is more than 2 centimeters without collapsibility	Increased pressure in the right atrium	Limit infusion and assess venous congestion
VExUS venous congestion scale	Combined evaluation of inferior vena cava and venous Dopplerograms	Helps avoid over-infusions

Table 4. Classification of hemorrhagic shock

Class	Loss of blood volume	Heart rate	Blood pressure	Diuresis
I	Less than 15 percent	Up to 100 per minute	Norm	Norm
II	15-30 percent	More than 100 per minute	The downward trend	Reduced
III	30-40 percent	More than 120 per minute	Reduced	Less than 30 milliliters per hour
IV	More than 40 percent	Significantly more than 120 per minute	Sharply reduced	Minimal or none

Table 5. Pediatric guidelines for maintenance infusions

Age	Recommended tonicity	Comments
From 28 days to 18 years	Isotonic solutions with added potassium and glucose	Reduced risk of hyponatremia compared with hypotonic solutions

Differential diagnosis

It is important to distinguish true extracellular fluid volume depletion from situations with low effective arterial volume in heart failure and cirrhosis, where total water is elevated but perfusion is reduced. In such cases, excessive infusion worsens edema and respiration. [37]

Decreased extracellular fluid volume should be differentiated from pure intracellular dehydration due to inadequate water intake and hypernatremia, where the priority is osmolarity control and slow sodium correction.[38]

In sepsis, hypotension may be due to vasodilation with normal preload. In such cases, dynamic preload sensitivity testing and ultrasound assessment help avoid unnecessary infusion. [39]

In patients on diuretics, laboratory indices such as fractional excretion of sodium and fractional excretion of urea reduce diagnostic accuracy. The decision is based on a comprehensive assessment of clinical symptoms, creatinine dynamics, ultrasound, and response to experimental interventions. [40]

Treatment

Primary resuscitation in hypoperfusion. In patients with sepsis-induced hypoperfusion or septic shock, it is recommended to initiate crystalloid infusion at a rate of at least 30 milliliters per kilogram for the first 3 hours, with subsequent individualization based on dynamic criteria. The quality of evidence is moderate-low, so continuous reassessment of perfusion is necessary. [41]

Solution choice. Balanced crystalloids have been shown to reduce the incidence of adverse renal outcomes compared with normal saline in several studies in the critically ill and in the emergency department, although the effects on mortality are inconclusive. Preference for balanced solutions is warranted in patients with a high risk of renal complications. [42]

Dynamic guidance. Use leg raises and other dynamic maneuvers with continuous cardiac output monitoring, inferior vena cava assessment, and, if needed, advanced ultrasound venous congestion protocols to titrate volumes and initiate vasoactive support in a timely manner. [43]

Oral rehydration for mild to moderate cases. Oral rehydration solutions with reduced osmolarity contain approximately 75 millimoles per liter of sodium and 75 millimoles per liter of glucose with an osmolarity of approximately 245 milliosmoles per liter and are effective in most patients, including children, in the absence of intractable vomiting and impaired consciousness. [44]

Special situations. In acute blood loss, the primary focus is control of the bleeding source, early antifibrinolytic therapy as indicated, and massive transfusion protocols with component selection. Crystalloids are administered in doses to maintain perfusion until oxygen delivery is restored. [45]

Table 6. Selection of infusion solution when the volume of extracellular fluid decreases

Situation	First line solution	Notes
Sepsis with hypoperfusion	Balanced crystalloid	Evaluation of response by dynamic tests
Hypochloremic alkalosis after vomiting	Saline solution	Corrects chloride deficiency
Hypernatremia with hypovolemia	Hypotonic solution after initial stabilization with isotonic solution	Slow sodium correction
Blood loss	Component transfusion and limited crystalloids	Priority to hemostasis and blood

Prevention

Prevention of extracellular fluid volume deficit includes prompt replacement of losses during diarrhea and vomiting using low-osmolarity oral rehydration solutions and teaching patients how to rehydrate at home. This is especially important for children and the elderly. [46]

In hospital, unnecessary administration of hypotonic maintenance fluids to children should be avoided, infusion plans should be promptly reviewed in patients at risk of overload, and dynamic methods for assessing fluid response should be used to minimize complications. [47]

Forecast

With early recognition and adequate treatment, the prognosis is favorable, with rapid restoration of perfusion and organ function. Delayed treatment increases the risk of acute renal dysfunction, the need for vasopressor support, and prolonged hospitalization. [48]

In patients with sepsis, adherence to initial fluid guidelines and the use of dynamic criteria for subsequent guidance are associated with reduced mortality and respiratory complications, although further studies are needed to optimize volumes and rates.[49]

FAQ

• How quickly and how much fluid should be administered for sepsis?

It is recommended to start with at least 30 milliliters per kilogram of crystalloid in the first 3 hours, then use dynamic tests and signs of perfusion to guide further titration and decision on vasopressors.[50]

• Which solutions are preferred?

Balanced crystalloids show signs of reducing the risk of renal complications compared with saline in critically ill patients, but the impact on mortality is controversial. The choice depends on the clinical context and concomitant electrolyte disturbances. [51]

• Is it always possible to rely on fractional excretion of sodium and urea?

These indices are useful as part of a comprehensive assessment, but their accuracy is reduced in patients on diuretics and with concomitant chronic kidney disease. The decision should be made based on a combination of clinical, laboratory, ultrasound, and test results. [52]

• What is the difference between hypovolemia and dehydration?

Hypovolemia describes a decrease in intravascular volume, while dehydration describes a deficit in total water with a predominantly intracellular contribution. In practice, these conditions often coexist, so both volume and osmolarity must be considered when choosing a strategy. [53]