Phosphate

ADMINISTRATION ROUTES:

IV, PO

ALTERNATIVE NAMES:

Potassium dihydrogen phosphate, Phosphate Sandoz

ICU INDICATIONS:

1. Hypophosphataemia (adult serum phosphate < 0.3 mmol/L, paediatric serum phosphate < 0.6 mmol/L)

PRESENTATION AND ADMINISTRATION:

IV:

10 mL vial containing 1 mmol/mL potassium & 1 mmol/mL phosphate

For central administration, dilute required dose in 100 mL of compatible IV fluid. For peripheral administration dilute required dose in 500 mL or 1000 mL compatible IV fluid.

Central administration is preferred. For either route, administer at maximum rate of 20 mmol per hour.

Discard any solution not used within 24 hours of preparation. Do not use solution that is cloudy or shows precipitate.

Compatible with the following IV fluids: 0.9% sodium chloride, 5% glucose, Glucose and sodium chloride.

Store at room temperature.

PO:

Phosphate Sandoz Effervescent tablets (when dissolved in 16 mL water, contain 1 mmol/mL phosphate & 0.2 mmol/mL potassium)

DOSAGE:

IV:

10 - 20 mmol over 1 hour

Repeat at 12 hour intervals until serum phosphate > 0.3 mmol/L

PO:

Dose according to requirements and response.

Oral phosphate replacement is often ineffective in the ICU setting, may cause diarrhoea, and is generally not indicated.

DOSAGE IN RENAL FAILURE AND RENAL REPLACEMENT THERAPY:

Dose in renal impairment

Dose in renal replacement therapy

Note: Phosphate replacement is not usually appropriate in the setting of renal failure unless the patient is receiving renal replacement therapy.

DOSAGE IN PAEDIATRICS:

IV:

0.15 - 0.33 mmol/kg over 6 hours

Repeat at 6 hour intervals until serum phosphate > 0.6 mmol/L

Infusion rate must not exceed 0.2 mmol/kg/hr

CLINICAL PHARMACOLOGY:

The terms serum phosphate and serum phosphorus are often used interchangeably. It is the phosphorus content of anionic plasma phosphate PO4(3-) that is measured. Phosphorus in the form of organic and inorganic phosphate has a variety of important biochemical functions in the body and is involved in many significant metabolic and enzyme reactions in almost all organs and tissues. It exerts a modifying influence on the steady state of calcium levels, a buffering effect on acid-base equilibrium and a primary role in the renal excretion of hydrogen ion. Around 85% of phophate is in bones & teeth, 14% in cells, and <1% in extracelular fluid. Phosphate balance is closely related to calcium homeostasis.

Clinical effects of hypophosphataemia may differ with age. The lower normal limit in early infancy (1.5 mmol/L) falls to adult equivalent (0.8 mmol/L) by adolescence. In children, severe acute hypophosphataemia (< 0.5 mmol/L) may be associated with weakness, lethargy & paraesthesia. Levels < 0.3 mmol/L should be considered a life-threatening emergency.

CONTRAINDICATIONS:

1. Hyperphosphataemia
2. Hyperkalaemia

WARNINGS:

Refeeding syndrome:

Refeeding syndrome (caused by an endogenous insulin surge triggered by carbohydrate intake) may manifest as significant hypophosphataemia. This generally occurs within 24-72 hours after starting nutrition in high risk patients. Risk factors include anorexia, cachexia, alcoholism, malabsorption, or dysphagia. Diuretic use (causing loss of Mg, K & thiamine) may also predispose. Symptoms may manifest as arrhythmias or tachycardias, weakness, Wernicke's encephalopathy, delirium or coma, or abdominal pain. Hypophosphataemia is the primary diagnostic abnormality. Hypokalaemia, hypomagnesaemia, and hyperglycaemia may also be seen. If refeeding syndrome is suspected, hypophosphataemia should be aggressively treated, along with correction of other abnormal electrolytes.

Other electrolyte abnormalities:

To avoid potassium or phosphate toxicity, infuse solutions containing potassium phosphates slowly. In patients with severe renal or adrenal insufficiency, administration of potassium phosphate injection may cause hyperkalaemia. Infusing high concentrations of phosphate may cause hypocalcaemia, and calcium levels should be monitored.

PRECAUTIONS:

General:

Phosphate replacement therapy with potassium phosphate should be guided primarily by serum phosphate levels and the limits imposed by the accompanying potassium ion.

Laboratory Tests:

No tests in addition to routine ICU tests are required

Drug/Laboratory Test Interactions:

None known

IMPORTANT DRUG INTERACTIONS IN ICU:

None of note

ADVERSE REACTIONS:

Hyperphosphataemia, hyperkalaemia, hypomagnesaemia, hypocalcaemia.

Rapid administration or overdose increases risk of arrhythmias or cardiac arrest.