TOC  |  Lytes  

HYPOPHOSPHATEMIA                           Causes |  RX                                                                        

The normal range is 3.0-4.5 mg/dl
SX:
muscle weakness or paralysis, respiratory failure, congestive cardiomyopathy, hypotension, rhabdomyolysis, cardiac arrest; also WBC  & platelet dysfunction; hyperventilation, dysarthria, irritability, apprehensive, confusion, obtundation, convulsive seizures, coma; paresthesias, numbness

Causes of Hypophosphatemia:

A. Intracellular shifts
1. Respiratory alkalosis: sepsis, hepatic coma, throtoxicosis, heat stroke. 2. Metabolic alkalosis
3. Administration of glucose, insulin, corticosteroids, epinephrine, lactate. 4. Recovery from hypothermia

B. Deficiency of phosphorus from decreased intestinal absorption
1. Starvation or malabsorption. 2. Vit. D deficiency. 3. Antacid administration. 4. Chronic alcoholism

C. Increased phosphorus urinary excretion.
1. Hyperparathyroidism. 2. Diuretic therapy. 3. Renal tubular defects: Fanconi's syndrome
4. Volume expansion. 5. Hypomagnesemia. 6. Aldosteronism

D. Undetermined mechanism 
1. Acute gout. 2. Hypokalemia. 3. Carcinoma

E. Miscellaneous
1. Pharmacological binding of phosphate in the gut. 2. Thermal burns. 3. Hyperalimentation
4. Nutritional recovery syndrome. 5. Diabetic ketoacidosis. 6. Alcoholic withdrawal   

Causes of severe hypophosphatemia (<1 mg/dl)

• Alcohol abuse and withdrawal
• Respiratory alkalosis
• Malabsorption
• Oral phosphate binders (aluminum-containing antacids)
• Refeeding after malnutrition
• Hyperalimentation
• Severe burns
• Therapy of diabetic ketoacidosis

Moderate hypophosphatemia (1.0-2.5 mg/dl)
is usually asymptomatic and requires no therapy except correction of the underlying cause.
Persistent hypophosphatemia should be treated with oral phosphate supplements, 0.5-1.0 g of  elemental phosphorus PO bid-tid. Preparations include

• Neutra-Phos (250 mg elemental phosphorus and 7 mEq Na & K/capsule) 2-4 cap bid PO in water
• Neutra-Phos K (250 mg elemental phosphorus and 14 mEq K/capsule) 2-4 cap bid PO in water
• Fleet Phospho-Soda (815 mg phosphorus and 33 mEq sodium per 5 ml) is an alternative oral agent.

Serum phosphorus, calcium, and creatinine should be measured daily as the dose is adjusted. Side effects include diarrhea, which is often dose-limiting, and nausea.

Severe hypophosphatemia (<1 mg/dl)
may require IV infusion phosphate therapy 0.08-0.16 mmol/kg (elemental phosphorus, 2.5-5.0 mg/kg) in 500 ml 0.45% saline is given IV over 6 hours. when associated with serious clinical manifestations.
IV preparations include

• K-phosphate (1.5 mEq potassium per mmol phosphate)
• Na-phosphate (1.3 mEq sodium per mmol phosphate).

If hypotension occurs, the infusion rate should be slowed. Further doses should be based on symptoms and on the serum calcium, phosphorus, and potassium levels, which should be measured q6h. IV infusion should be stopped when the serum phosphorus level is greater than 1.5 mg/dl or when oral therapy is possible. Because of the need to replenish intracellular stores, 24-36 hours of phosphate infusion may be required.

Extreme care must be used to avoid hyperphosphatemia, which may cause hypocalcemia, ectopic soft-tissue calcification, renal failure, hypotension, and death. In renal failure, IV phosphate should be given only if absolutely necessary. Hypophosphatemic patients frequently are hypokalemic and hypomagnesemic, and these disorders must be corrected as well.

IV phosphate should not be used in the treatment of diabetic ketoacidosis, unless there is evidence of pre-existing phosphate depletion from another cause.

(Conversion equations for phosphate therapy are as follows: 1 mmol phosphate = 31 mg phosphorus; 1 mg phosphorus = 0.032 mmol phosphate.)

Ref:
Washington Manual of Medical Therapeutics, 29th ed., 1998
J of Critical Illness Nov. 1997, Vol.12, No.11 - Gregory W. Rutecki, etc.