bubble_chart Overview

Surgical patients are most prone to this type of dehydration; water and sodium are lost proportionally, so serum sodium remains within the normal range, and extracellular fluid osmotic pressure is also maintained normally. It leads to a rapid reduction in extracellular fluid volume (including circulating blood volume). Since the lost fluid is isotonic, the osmotic pressure of the extracellular fluid remains largely unchanged. Initially, intracellular fluid does not shift into the extracellular space to compensate for the loss of extracellular fluid, so the intracellular fluid volume does not change. However, if this fluid loss persists for an extended period, intracellular fluid will gradually move outward and be lost along with the extracellular fluid, eventually leading to cellular dehydration.

bubble_chart Etiology

Common causes of disease include: a. Acute loss of digestive fluids, such as severe vomiting, intestinal fistula, etc.; b. Internal fluid shift, lost in infected areas or soft tissues, such as abdominal infections, intestinal obstruction, burns, etc. The lost fluid is essentially similar in composition to extracellular fluid.

bubble_chart Clinical Manifestations

The patient does not feel thirsty and exhibits symptoms such as oliguria, anorexia, nausea, lack of strength, dry tongue, sunken eyeballs, dry and loose skin, etc. If excessive fluid loss occurs within a short period, reaching more than 5% of body weight—equivalent to 25% of extracellular fluid loss—the patient may show signs of insufficient blood volume, including thready and rapid pulse, cold and clammy extremities, unstable or decreased blood pressure. If fluid loss continues to reach 6–7% of body weight, equivalent to 30–35% of extracellular fluid loss, the shock condition becomes very severe and is often accompanied by metabolic acidosis. If the patient's fluid loss is primarily gastric juice, the substantial loss of Cl− may lead to metabolic alkalosis, presenting with some clinical manifestations of alkalosis.

bubble_chart Diagnosis

The diagnosis mainly relies on medical history and clinical manifestations. It is essential to inquire in detail about fluid loss, including the daily volume of fluid loss, the duration of the loss, and the characteristics of the lost fluid. Measuring extracellular fluid volume and serum sodium levels helps assess the extent of dehydration and sodium loss. Serum Na⁺

and Cl^- generally show no significant decrease, and plasma osmolality remains within the normal range. Urine specific gravity is elevated. Increased red blood cell count, hemoglobin levels, and hematocrit indicate hemoconcentration. If necessary, blood gas analysis can be performed to determine whether there is an acid-base imbalance.

bubble_chart Treatment Measures

First, address the causes of isotonic dehydration as much as possible to reduce the loss of water and sodium. To compensate for the reduction in extracellular fluid volume, isotonic saline or balanced salt solutions are generally used to restore blood volume as quickly as possible. Estimate the volume of fluid loss based on symptoms such as rapid, thready pulse and decreased blood pressure. For a fluid loss equivalent to 5% of body weight, approximately 3.0L of the aforementioned solution can be rapidly infused (calculated for a 60kg body weight) to restore blood volume. Alternatively, the required fluid volume can be calculated based on hematocrit.

Additionally, the daily requirement should also be replenished, typically 2.0L of water and 4.5g of sodium.

Isotonic saline contains Na⁺ and Cl^- at 154mmol/L each, while serum Na⁺ and Cl^- concentrations are 142mmol/L and 103mmol/L, respectively. Compared to serum, isotonic saline has a Cl^- concentration that is 50mmol/L higher. In cases of grade III dehydration or shock, reduced renal blood flow impairs chloride excretion. If large amounts of isotonic saline are administered intravenously, there is a risk of elevated blood Cl^-, leading to hyperchloremic acidosis. Therefore, using isotonic saline to treat dehydration has some limitations. Balanced salt solutions have electrolyte concentrations similar to those of plasma, making them more physiologically appropriate for treating dehydration. They help avoid excessive Cl^- intake and can assist in correcting acidosis. Furthermore, after correcting dehydration, potassium excretion increases, and K⁺ concentration may be diluted and reduced due to the expanded extracellular fluid volume. Therefore, attention should be paid to the occurrence of hypokalemia. Potassium chloride supplementation is generally recommended once urine output reaches 40ml/h.