Isotonic dehydration refers to a condition characterized by a reduction in extracellular fluid with normal serum sodium levels. Its defining feature is the reduction in blood volume, while serum sodium concentration and plasma osmolality remain within the normal range.
Etiology
Isotonic dehydration occurs when there is a significant loss of isotonic fluid, leading to decreased blood volume. In the short term, this condition remains isotonic. Common clinical causes include:
- Acute loss of digestive fluids, such as through external fistulas, excessive vomiting, or diarrhea.
- Fluid retention in infected or soft tissue areas, such as intra-abdominal or retroperitoneal infections and bowel obstruction.
- Rapid removal of large amounts of pleural effusion or ascitic fluid, as well as extensive burns, which result in significant fluid loss.
Without timely intervention, isotonic dehydration may progress to hypertonic dehydration due to continued water loss through insensible evaporation or respiration. On the other hand, excessive replenishment with hypotonic fluid can lead to hypotonic dehydration and hyponatremia.
Clinical Manifestations
Symptoms of isotonic dehydration can include nausea, loss of appetite, fatigue, and reduced urine output, though thirst is typically absent. Physical signs may include dry tongue, sunken eyes, dry and loose skin. When fluid loss reaches 5% of body weight within a short period, symptoms such as rapid and weak pulse, cold and clammy extremities, and unstable or decreased blood pressure may develop. As fluid loss increases to 6%–7% of body weight, more severe signs of shock may occur.
Diagnosis
The diagnosis of isotonic dehydration is often supported by a history of significant loss of digestive or other body fluids, along with clinical symptoms. Laboratory findings may include a marked increase in red blood cell count, hemoglobin concentration, and hematocrit. Serum sodium and chloride levels generally remain within the normal range, while urine specific gravity is elevated.
Treatment
Treatment of the underlying cause is crucial, as resolving the primary condition typically allows for easy correction of the dehydration. Isotonic dehydration is managed with intravenous administration of balanced salt solutions or isotonic saline to restore blood volume promptly. This approach is particularly important in cases where signs of hypovolemia, such as rapid and weak pulse or decreased blood pressure, are present, as rapid infusion is often required to restore adequate blood volume.
During rapid intravenous infusion, cardiac function should be closely monitored. Parameters such as heart rate, central venous pressure, or pulmonary artery wedge pressure may be assessed. Balanced salt solutions are commonly used in the treatment of isotonic dehydration. Following correction of the water deficit, potassium excretion may increase, and the expansion of extracellular fluid volume may dilute serum potassium levels. Therefore, caution is necessary to prevent hypokalemia.