Hyperosmolar Dehydration: Mechanisms
This condition develops when water losses from the body exceed electrolyte losses. Consequently, the osmotic pressure in all body fluid compartments increases, resulting in generalized hypohydration that affects both the intracellular and extracellular spaces.
At the cellular level, the pathogenesis is driven by the elevated osmotic pressure in the interstitium, which literally "pulls" water out of cells. Water diffuses outward, intracellular solute concentration increases, and the cell volume decreases. The main causes include:
- Inadequate water intake ("dry fasting");
- Fever and hyperthermia with profuse sweating (sweat is hypoosmotic);
- Polyuria in diabetes insipidus or uncompensated diabetes mellitus;
- Prolonged mechanical ventilation with dry gas mixtures or ingestion of seawater;
- Impaired regulatory mechanisms due to antidiuretic hormone (ADH) deficiency.
Clinical Presentation of Hyperosmolar Deficit
Dehydration leads to severe hemodynamic and metabolic shifts. The circulating blood volume decreases, hematocrit rises, resulting in increased blood viscosity and systemic microcirculatory dysfunction.
Acid-base disturbances frequently manifest as acidosis secondary to hypoxia. Neurological findings include psychomotor agitation, anxiety, apprehension, and altered consciousness progressing to coma. The primary subjective symptom is intense, unquenchable thirst driven by cellular dehydration.
Isoosmolar Dehydration: Characteristics
In this type of fluid disorder, there is an equivalent loss of water and solutes; therefore, the osmotic pressure between fluid compartments remains unchanged. The extracellular compartment bears the brunt of the deficit.
Main causes of isoosmolar deficit:
- Acute massive hemorrhage (initial stage);
- Repetitive profuse vomiting or diarrhea;
- Plasma loss (plasmarrhea) in severe burns;
- Excessive use of high-ceiling diuretics causing polyuria.
Decreased plasma volume lowers capillary hydrostatic pressure, rapidly drawing fluid from the interstitium into the vascular bed (autohemodilution). Intracellular fluid volume remains unchanged due to the absence of an osmotic gradient.
Consequences and Regulatory Responses
Isoosmolar fluid loss leads to a drop in effective circulating blood volume (EBV), increased blood viscosity, and severe hemodynamic disturbances. Depending on the underlying cause (diarrhea or vomiting), acid-base balance shifts toward acidosis or alkalosis, accompanied by marked hypoxia.
The body responds to hypovolemia through compensatory stages:
- Initial stage: Loss of 2–3% of extracellular fluid proceeds with normal hemodynamics and renal sodium retention.
- Pronounced stage: An 8–10% drop in EBV is detected by baroreceptors, signaling the brain via cranial nerves and stimulating ADH synthesis.
- Additional mechanisms: Thirst is triggered by hypovolemia, angiotensin II, and osmotic stimuli, alongside activation of the renin-angiotensin-aldosterone system (RAAS).