Fluids, Electrolytes & IV Tonicity

Tonicity in one idea

Water follows solute. An isotonic solution such as 0.9% sodium chloride or lactated Ringer's has the same effective osmolality as plasma, so it stays in the extracellular space and expands circulating volume — the first choice for hypovolemia and hemorrhage.

A hypotonic solution such as 0.45% sodium chloride is dilute, so water moves into cells. It rehydrates cells in hypernatremia and cellular dehydration but is contraindicated where cerebral swelling is possible.

A hypertonic solution such as 3% sodium chloride is concentrated, so it draws water out of cells into the vascular space. It is used for symptomatic hyponatremia and to reduce intracranial pressure, always slowly and always on a pump.

Potassium

Normal potassium is 3.5–5.0 mEq/L. Hypokalemia produces muscle weakness, flat or inverted T waves, U waves and increased digoxin toxicity risk. Hyperkalemia produces peaked T waves, a widening QRS, muscle weakness and eventually cardiac arrest.

Replacement is always diluted and always infused. Maximum peripheral rate is generally 10 mEq per hour, and the client should be on cardiac monitoring for faster central rates.

Sodium, calcium and magnesium

Hyponatremia below 135 mEq/L produces confusion, headache and seizures; correct slowly to avoid osmotic demyelination. Hypernatremia above 145 mEq/L produces thirst, dry mucous membranes and restlessness.

Hypocalcemia produces tetany with positive Chvostek and Trousseau signs; hypercalcemia produces lethargy, constipation and kidney stones. Magnesium generally moves with calcium in its effects — low magnesium causes hyperreflexia and torsades, high magnesium causes loss of deep tendon reflexes and respiratory depression.

• Loss of the patellar reflex during a magnesium infusion → stop the infusion, prepare calcium gluconate
• Peaked T waves → check the potassium immediately
• Positive Trousseau sign on blood-pressure cuff inflation → hypocalcemia