Electrolyte Imbalances: Sodium, Potassium, Calcium, Magnesium

Sodium Imbalance

Sodium: primary cation in ECF

• Transport through cells by sodium-potassium pump
• Secreted into mucous and other secretions

Hyponatremia

Plasma sodium below 135 mEq/L

Causes

• Losses from excessive sweating, vomiting, diarrhea
• Certain diuretic drugs with low salt diet
• Hormonal imbalances (low aldosterone, high ADH)
• Excessive water intake

Effects

• Low sodium
• Decreases osmotic pressure in ECF

Hypernatremia

Plasma sodium above 145 mEq/L

Causes

• Insufficient ADH
• Loss of thirst mechanism
• Watery diarrhea
• Prolonged periods of rapid respiration
• Ingesting large amounts of sodium without water balance

Effects

• Weakness, headache
• Dry, rough mucous membranes
• Increased thirst
• Difficulty swallowing
• Cerebral edema: leads to seizures

Potassium Imbalance

Potassium: primary cation in ICF

• Insulin moves potassium from blood to cells
• Secreted by urine
• Level influenced by acid/base balance

Hypokalemia

Plasma potassium below 3.5 mEq/L

Causes

• Losses from excessive diarrhea
• Certain diuretic drugs or insulin
• Excessive aldosterone
• Decreased dietary intake

Hyperkalemia

Plasma potassium above 5 mEq/L

Causes

• Renal failure
• Deficient in aldosterone
• "Potassium-sparing" diuretics
• Leakage of intracellular potassium into extracellular fluids
• Displacement of potassium from cells by severe acidosis

Effects of Potassium Imbalance

1. Cardiac dysrhythmias
   • Hyperkalemia may progress to cardiac arrest
2. Interference with neuromuscular function
3. Paresthesia - "pins and needles"
4. Decreased digestive tract motility

Calcium Imbalance

Calcium: important extracellular cation

• PTH (raises Ca) & calcitonin (lowers Ca) control balance
• Vitamin D promotes calcium reabsorption from intestine (calcitriol activated in kidneys)

1. Protein bound: bound to albumin (40%)
2. Complexed: chelated with citrate, phosphate, sulfate (10%)
3. Ionized: free (50%) *only type detected by PTH

Hypocalcemia

Plasma calcium concentration below 4.3 mEq/L

Causes

• Hypoparathyroidism: PTH and vitamin D reabsorb in nephron
• Deficient serum albumin
• Renal failure: abnormal loss from the kidney

Effects

• Increased excitability of nerve membranes
• Weak heart contractions

Hypercalcemia

Plasma calcium concentration above 5.3 mEq/L

Causes

• Uncontrolled release of calcium ions from bones
• Hyperparathyroidism
• Increased calcium intake

Effects

• Depressed neuromuscular activity
• Decreased neural excitability
• Increased strength in cardiac contractions

Cardiac Effects and Magnesium

Effects of hyperkalemia on cardiac excitability and contractility may cause prolonged depolarization and impair repolarization

• Decrease excitability cause cardiac arrest

Severe hyperkalemia: EKG

• Wide QRS, longer PR
• Tall narrow T wave

Magnesium Imbalance

Magnesium: intracellular ion

Hypomagnesemia

Below 1.8 mg/dl

• Occurs with alcoholism
• Neuromuscular: athetoid/ choreiform movements, tremor
• Cardiovascular: hypertension, cardiac dysrhythmias

Hypermagnesemia

Above 3.0 mg/dl

• Occurs with renal failure
• Depresses neuromuscular function
• Depresses reflexes