# Situation: A 46-year-old man is admitted to the medical ward after 3 days of profuse watery diarrhea and poor oral intake. He has no history of heart or kidney disease. He weighs 64.8 kg; he weighed 67.0 kg at a clinic visit 1 week ago. His blood pressure is 104/66 mmHg lying and 82/54 mmHg sitting, his pulse is 116/min, his oral mucosa is dry, and he has no edema. His admission results are: Serum sodium: 141 mEq/L (135–145 mEq/L) Serum osmolality: 290 mOsm/kg (275–295 mOsm/kg) Hematocrit: 52% (40–50%) Blood urea nitrogen (BUN): 32 mg/dL (7–20 mg/dL) Urine specific gravity: 1.030 How should the nurse classify his fluid imbalance?

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> subject: Nursing Practice III — Care of Clients with Problems in Surgery, Oxygenation, Fluid and Electrolytes, Infectious, Inflammatory and Immunologic Response, Cellular Aberrations

## 문제

Situation: A 46-year-old man is admitted to the medical ward after 3 days of profuse watery diarrhea and poor oral intake. He has no history of heart or kidney disease. He weighs 64.8 kg; he weighed 67.0 kg at a clinic visit 1 week ago. His blood pressure is 104/66 mmHg lying and 82/54 mmHg sitting, his pulse is 116/min, his oral mucosa is dry, and he has no edema.

His admission results are:
Serum sodium: 141 mEq/L (135–145 mEq/L)
Serum osmolality: 290 mOsm/kg (275–295 mOsm/kg)
Hematocrit: 52% (40–50%)
Blood urea nitrogen (BUN): 32 mg/dL (7–20 mg/dL)
Urine specific gravity: 1.030
How should the nurse classify his fluid imbalance?

## 보기

1. Isotonic volume deficit, with water and sodium lost in proportion **✔ 정답**
2. Cellular dehydration, with water drawn out of cells by hypertonic serum
3. Volume deficit in which more sodium than water has been lost from the body
4. Third spacing, with plasma shifted from the vessels into the tissues

**정답: 1**

## 해설

His serum sodium and osmolality are normal, so the fluid he lost had about the same tonicity as plasma: an isotonic (extracellular) volume deficit. The high hematocrit, high BUN, and concentrated urine show hemoconcentration, and the 2.2 kg weight loss shows that the fluid has left the body. Cellular dehydration needs a hypertonic serum (sodium above 145 mEq/L), and a sodium-losing deficit lowers the serum sodium.

## 심화 해설

Classification of the fluid imbalance

The correct classification is isotonic volume deficit. In this condition, water and sodium are lost from the extracellular compartment in approximately the same proportion as they exist in plasma, so the remaining extracellular fluid keeps a normal tonicity. The patient’s serum sodium of 141 mEq/L and serum osmolality of 290 mOsm/kg both fall within their reference ranges, which is the defining laboratory picture for isotonic loss rather than a hypertonic or hypotonic disturbance [2][3].

Because the lost fluid has the same tonicity as plasma, there is no osmotic gradient to shift water between the intracellular and extracellular compartments. The cells therefore do not shrink or swell; the entire volume deficit is borne by the extracellular space, including the intravascular volume. This explains why the patient shows signs of intravascular depletion—tachycardia at 116/min, a postural blood pressure drop from 104/66 mmHg lying to 82/54 mmHg sitting, and dry oral mucosa—without any evidence of cellular dehydration.

The laboratory findings support hemoconcentration. The hematocrit is elevated at 52%, the BUN is 32 mg/dL, and the urine specific gravity is concentrated at 1.030. These changes occur because water has been lost from the vascular space, leaving red cells and solutes more concentrated. The 2.2 kg weight loss over one week confirms that fluid has actually left the body, not merely shifted into a third space [2][4].

| Classification | Serum sodium | Serum osmolality | Primary fluid shift | Typical causes |
| --- | --- | --- | --- | --- |
| Isotonic volume deficit | Normal (135–145 mEq/L) | Normal (275–295 mOsm/kg) | Extracellular loss only; no cellular shift | Diarrhea, vomiting, hemorrhage, burns |
| Hypertonic dehydration | Elevated (above 145 mEq/L) | Elevated (above 300 mOsm/kg) | Water moves out of cells into extracellular space | Inadequate water intake, diabetes insipidus, excessive sweating |
| Hypotonic volume deficit | Low (below 135 mEq/L) | Low (below 275 mOsm/kg) | Water shifts into cells; extracellular volume further depleted | Adrenal insufficiency, diuretic overuse, salt-wasting states |

Watch out! Hyperosmolar dehydration is defined by a serum osmolality above 300 mOsm/kg, often with hypernatremia and an elevated BUN/creatinine ratio . This patient’s osmolality is 290 mOsm/kg, so option 2 (cellular dehydration) is ruled out. Cellular dehydration requires a hypertonic serum that draws water out of cells; here the serum is isotonic, so no such osmotic gradient exists [3].

Key point! A sodium-losing deficit, in which more sodium than water is lost, would lower the serum sodium below 135 mEq/L. This patient’s sodium is 141 mEq/L, so option 3 is incorrect. Third spacing (option 4) shifts plasma from vessels into tissues or body cavities, but it does not remove fluid from the body; the patient’s 2.2 kg weight loss and concentrated urine indicate true external fluid loss, not internal redistribution [2][4].

In clinical practice, isotonic volume deficit from profuse diarrhea is managed with isotonic crystalloid replacement because the goal is to restore extracellular volume without creating an osmotic imbalance. The normal serum sodium and osmolality at presentation mean the replacement fluid should match the tonicity of what was lost, which is why isotonic solutions such as 0.9% sodium chloride or lactated Ringer’s are appropriate initial choices [4].References (research sources)

- [2]Preventing and managing dehydration.Research articleSuhayda R, Walton JC (2002)

- [3]Diagnosis and management of sodium disorders: hyponatremia and hypernatremia.Research articleBraun MM, Barstow CH, Pyzocha NJ (2015)

- [4]Fluid resuscitation in critical care.Research articleO'Neill D, Perrin D (2002)

## 임상 시나리오

Isotonic Volume Deficit: Bedside RecognitionClassifying fluid loss by serum sodium and osmolality
When a patient loses fluid and the serum sodium and serum osmolality remain normal, the loss is isotonic—water and sodium left the body in proportion to plasma. The entire deficit is in the extracellular compartment, so expect signs of intravascular depletion: tachycardia, postural hypotension, and dry mucosa.

Confirm that fluid has actually left the body, not just shifted. Look for weight loss (here 2.2 kg in one week) and hemoconcentration: elevated hematocrit (52%), elevated BUN (32 mg/dL), and concentrated urine (specific gravity 1.030).

CautionDo not label this as cellular dehydration. Cellular dehydration requires a hypertonic serum (sodium above 145 mEq/L) to pull water out of cells. A sodium-losing deficit would lower the serum sodium. Normal sodium and osmolality point specifically to isotonic loss.

## 핵심 개념

- **Isotonic volume deficit** — Loss of water and sodium in proportion to plasma, leaving serum sodium and osmolality normal; the volume deficit is confined to the extracellular compartment.
- **Hemoconcentration** — Elevated hematocrit, BUN, and urine specific gravity due to reduced plasma water, concentrating red cells and solutes in the vascular space.
- **Serum osmolality** — A measure of solute concentration in serum (normal 275–295 mOsm/kg); normal values in volume loss suggest isotonic fluid loss.
- **Postural hypotension** — A drop in blood pressure when moving from lying to sitting or standing (here 104/66 to 82/54 mmHg), indicating intravascular volume depletion.
- **Third spacing** — Shift of plasma from vessels into interstitial or body cavities; it does not reduce total body fluid or cause weight loss.

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