Think Like a Nephrologist: Water and Salt Disorders

Key Concepts

• Grid Format for Analysis:
  • Total body salt on x-axis: Refers to extracellular fluid volume.
  • Total body water on y-axis.

Total Body Salt

• Volume Overload (Excess Salt):
  • Detected via peripheral edema, high blood pressure, pulmonary edema.
• Volume Depletion (Salt Loss):
  • Detected via hypotension, orthostasis (dizziness/weakness upon standing), lack of edema.

Total Body Water

• Dehydration (Water Loss):
  • Serum sodium concentration > 145 mEq/L indicates dehydration.
• Hyponatremia (Excess Water):
  • Serum sodium concentration < 135 mEq/L indicates excess water.

Measuring Salt and Water Balance

• Salt Balance:
  • Assessed through extracellular fluid volume, blood pressure, presence/absence of edema.
• Water Balance:
  • Determined by plasma osmolarity, serum sodium concentration.

Effect on Cells

• Normal water balance results in normal cell size.
• Dehydration causes cells to shrink.
• Excess water causes cells to bloat.

Disorders by Quadrant

Hypovolemia and Dehydration

• Common Admission:
  • Patients with salt loss (e.g., diarrhea) and inadequate water access, often found in nursing facilities.
• Treatment:
  • Correct volume depletion with isotonic fluids, then address dehydration with D5 water.

Hypovolemia and Hyponatremia

• Scenario:
  • Salt loss with excess water intake (e.g., post-diarrhea).
• Treatment:
  • Correct hypovolemia using crystalloids like normal saline.

Hypervolemia and Hyponatremia

• Common in:
  • Patients with cirrhosis or heart failure due to low perfusion.
• Treatment:
  • Use of diuretics to remove excess salt and water. May require AVP antagonists.

Hypervolemia and Hypernatremia

• Rare Scenario:
  • Often seen in heart failure patients on loop diuretics with fluid restriction.
• Treatment:
  • Administer water (D5) while continuing diuretics.

Management Strategies

• Hypervolemia Patients:
  • Salt restriction (2g/day).
  • Water restriction for those with hyponatremia.

Additional Conditions

• Pure Hypervolemia/Hyponatremia:
  • Can occur without changes in body water.
  • SIADH as example of pure hyponatremia.

Conclusion

• Graph Format:
  • Helps separate and clarify water and salt disorders, aiding in diagnosis and treatment strategy formation.