Pathophysiology of Fluid and Electrolyte Balance

Fluid Volume Imbalances

Hypovolemia (Fluid Volume Deficit)

Hypovolemia is an isotonic loss of water and electrolytes in proportion, meaning the sodium concentration remains relatively normal. Common causes include hemorrhage, vomiting, diarrhea, excessive sweating, burns, or diuretic use.

Clinical Manifestations:

• Cardiovascular: Tachycardia, weak/thready pulse, hypotension, and orthostatic dizziness.
• Integumentary/Physical: Flat neck veins, delayed capillary refill, cool skin, and poor skin turgor.
• Renal: Decreased urine output ($<30\text{ mL/hr}$) and highly concentrated urine with increased specific gravity.
• Laboratory Findings: Hemoconcentration, characterized by increased Hematocrit (Hct) and Blood Urea Nitrogen (BUN).

Note on Dehydration: While often used interchangeably with hypovolemia, dehydration specifically refers to the loss of water in excess of sodium, which can lead to hypernatremia and cell shrinkage.

Hypervolemia (Fluid Volume Excess)

Hypervolemia occurs when there is an excess of IV fluids, or when the body retains sodium and water due to conditions like heart failure, renal failure, or excess aldosterone.

Clinical Manifestations:

• Cardiovascular: Bounding pulses, hypertension, and Jugular Venous Distension (JVD).
• Respiratory: Crackles in the lungs, dyspnea (shortness of breath), and cough.
• Integumentary: Pitting edema and puffy eyelids.
• Laboratory Findings: Hemodilution, characterized by decreased Hct, decreased BUN, and dilute urine (low specific gravity).

Electrolyte Imbalances

Sodium ($\text{Na}^+$)

Sodium is the primary extracellular cation ($135\text{--}145\text{ mEq/L}$) and is the chief regulator of ECF volume.

• Hyponatremia ($<135\text{ mEq/L}$): This causes water to move into the cells, leading to cellular swelling. Symptoms include headache, confusion, nausea, and in severe cases, seizures.
• Hypernatremia ($>145\text{ mEq/L}$): This causes water to move out of the cells, leading to cellular shrinkage. Symptoms include intense thirst, dry/sticky mouth, agitation, and seizures.

Potassium ($\text{K}^+$)

Potassium is the primary intracellular cation ($3.5\text{--}5.0\text{ mEq/L}$) and is critical for maintaining cardiac rhythm and muscle contraction.

• Hypokalemia ($<3.5\text{ mEq/L}$): Often caused by loop/thiazide diuretics, vomiting, or alkalosis.

  • Symptoms: Muscle weakness, cramps, constipation, and decreased reflexes.
  • ECG Changes: Presence of U-waves.
  • Safety Alert: Low potassium levels significantly increase the risk of digoxin toxicity.

• Hyperkalemia ($>5.0\text{ mEq/L}$): Often caused by renal failure, acidosis, or K-sparing diuretics (e.g., spironolactone).

  • Symptoms: Muscle cramping, diarrhea, weakness, and potential paralysis.
  • ECG Changes: Peaked T-waves and wide QRS complexes, which can progress to cardiac arrest.

⚠️ Critical Clinical Safety Note

Potassium Administration: Intravenous potassium must NEVER be given via IV push; this can cause immediate cardiac arrest. It must always be diluted and administered via an infusion pump.

Monitoring Output: Before administering potassium replacement, always check urine output. If urine output is $<30\text{ mL/hr}$, hold the potassium and notify the provider, as the patient may be unable to excrete the excess, leading to life-threatening hyperkalemia.

Summary Table of Key Lab Values