Fluid and Electrolyte Disorders

Questions and Answers

What is the normal total body water (TBW) for men?

70%

55%

50%

60% (correct)

What is the major cation in the extracellular fluid (ECF)?

Potassium (K)

Chloride (Cl)

Sodium (Na) (correct)

Calcium (Ca)

The normal osmolality of body fluids ranges from _____ mOsm/kg water.

285-295

What does hyponatremia indicate?

Reduced serum sodium concentration (< 135 mEq/L) Signup and view all the answers

Hyponatremia is only a concern for elderly patients.

False Signup and view all the answers

What is a common complication due to rapid correction of hyponatremia?

Neurological damage Signup and view all the answers

Match the following fluid and electrolyte disorders with their corresponding definitions:

Hypovolemia = Sodium deficit due to dehydration Hyponatremia = Relative water excess Hypervolemia = Sodium excess Hypernatremia = Relative water deficit Signup and view all the answers

What is the essential criterion for diagnosing SIADH?

Hypoosmolar hyponatremia with urinary sodium > 30 mmol/L Signup and view all the answers

A plasma sodium concentration of less than _____ mEq/L indicates hyponatremia.

135 Signup and view all the answers

Study Notes

Composition of Fluid Compartments & Total Body Water

Average fluid composition in the body is 55% - 60%.
Total Body Water (TBW) percentages by demographic:
- Men: 60% of body weight
- Women: 55% of body weight
- Pediatrics: 70% of body weight
- Geriatrics: 50% of body weight

Distribution of Water

Major cation in intracellular fluid (ICF): potassium (K).
Major cation in extracellular fluid (ECF): sodium (Na).
Sodium-potassium pump regulates cation concentration difference between ICF and ECF.
Hypothalamus controls TBW and sodium management.
Osmoreceptors stimulate ADH secretion from the posterior pituitary gland.
ADH increases water reabsorption by upregulating AQP2 channels in renal collecting ducts.

Plasma Osmolality

Defined as the number of osmoles per kilogram of water (mOsm/kg).
Normal osmolality of body fluids: 285-295 mOsm/kg.
Glucose conversion from mg/dL to mmol/L requires division by 18.

Introduction to Sodium and Water Disorders

Hyponatremia: low plasma sodium concentration (< 135 mEq/L).
Hypernatremia: high plasma sodium concentration, resulting from too much sodium or too little water.
Dysvolemia: sodium disorder; dysnatremia: water/tonicity disorder.
Hypovolemia: sodium deficit (dehydration); hypervolemia: sodium excess.

Hyponatremia Overview

Common, especially among the elderly (15-20% inpatient, 7-11% outpatient).
Associated risks: cognitive decline, falls, fractures, gait instability, mortality, kidney stones.
Management complications can arise from hyponatremia itself or from rapid correction attempts.

Classification of Hyponatremia

Mild: serum sodium between 130 and 135 mmol/L.
Moderate: serum sodium between 125 and 129 mmol/L.
Severe: profound drops leading to significant clinical manifestations.

SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion)

Characterized by hypoosmolar hyponatremia (plasma osmolality < 100 mOsm/kg).
Essential criteria include:
- Urinary sodium > 30 mmol/L with normal dietary intake.
- Absence of pituitary, thyroid, adrenal, or renal insufficiency.
- Lack of diuretic use.
Supplemental criteria include:
- Low serum uric acid and normal serum urea/creatinine levels.
- Failure to correct hyponatremia with saline infusion, improvement through fluid restriction.

Pseudo-hyperkalemia

Defined as an artificially elevated plasma potassium concentration due to cellular movement.
Contributing factors:
- Prolonged tourniquet use or repeated fist clenching causing acidosis.
- Delay in processing blood samples leading to hemolysis and potassium leakage.
- Leukocytosis and thrombocytosis can also affect test results.
Importance of evaluating renal function and reviewing medication lists.