POTASSIUM PHOSPHATE

General

High Alert Medication: This medication bears a heightened risk of causing significant patient harm when it is used in error.

Pronunciation:
poe-tass-ee-um foss-fate

monobasic potassium phosphate

Trade Name(s)

• K-Phos

potassium phosphate and sodium phosphate

Trade Name(s)

• K-Phos Neutral

potassium phosphate

Ther. Class.

antiurolithics

mineral and electrolyte replacements/supplements

Indications

• Treatment and prevention of phosphate depletion in patients who are unable to ingest adequate dietary potassium.
• Adjunct therapy of urinary tract infections with methenamine hippurate or mandelate (potassium and sodium phosphates or monobasic potassium phosphate).
• Prevention of calcium urinary stones (potassium and sodium phosphates or monobasic potassium phosphate).
• Phosphate salts of potassium may be used in hypokalemic patients with metabolic acidosis or coexisting phosphorus deficiency.

Action

• Phosphate is present in bone and is involved in energy transfer and carbohydrate metabolism.
• Serves as a buffer for the excretion of hydrogen ions by the kidney.
• Dibasic potassium phosphate is converted in renal tubules to monobasic salt by hydrogen ions, resulting in urinary acidification.
• Acidification of urine is required for methenamine hippurate or mandelate to be active as a urinary anti-infective.
• Acidification of urine increases solubility of calcium, decreasing calcium stone formation.

Therapeutic Effect(s):

• Replacement of phosphorus in deficiency states.
• Urinary acidification.
• Increased efficacy of methenamine.
• Decreased formation of calcium urinary tract stones.

Pharmacokinetics

Absorption: Well absorbed following oral administration. Vitamin D promotes GI absorption of phosphates.

Distribution: Phosphates enter extracellular fluids and are then actively transported to sites of action.

Metabolism and Excretion: Excreted mainly (>90%) by the kidneys.

Half-life: Unknown.

TIME/ACTION PROFILE (effects on serum phosphate concentrations)

Contraindication/Precautions

Contraindicated in:

• Hyperkalemia;
• Hyperphosphatemia;
• Hypocalcemia;
• Severe renal impairment;
• Untreated Addison's disease;
• Severe tissue trauma;
• Hyperkalemic familial periodic paralysis.

Use Cautiously in:

• Hyperparathyroidism;
• Cardiac disease;
• Renal impairment.

Adverse Reactions/Side Effects

Related to hyperphosphatemia, unless otherwise indicated

CV: ARRHYTHMIAS, CARDIAC ARREST, ECG changes (absent P waves, widening of the QRS complex with biphasic curve), hypotension hyperkalemia: ARRHYTHMIAS, ECG changes (prolonged PR interval, ST segment depression, tall-tented T waves)

GI: diarrhea, abdominal pain, nausea, vomiting

F and E: hyperkalemia, hyperphosphatemia, hypocalcemia, hypomagnesemia

Local: irritation at IV site, phlebitis

MS: hyperkalemia: muscle cramps;  hypercalcemia: tremors

Neuro: flaccid paralysis, heaviness of legs, paresthesias, confusion, listlessness, weakness

* CAPITALS indicate life-threatening.
Underline indicate most frequent.

Interactions

Drug-Drug

• Concurrent use of  potassium-sparing diuretics,  ACE inhibitors, or  angiotensin II receptor antagonists  may result in hyperkalemia.
• Concurrent administration of  calcium-  or aluminum-containing compounds ↓ absorption of phosphates by formation of insoluble complexes.
•  Vitamin D  ↑ the absorption of phosphates.

Drug-Food:

Oxalates (in spinach and rhubarb) and phytates (in bran and whole grains) may ↓ the absorption of phosphates by binding them in the GI tract.

Route/Dosage

Monobasic Potassium Phosphate

PO (Adults and Children  >4 yr): 1 g (7.4 mmol) in water 4 times daily.

PO (Children  <4 yr): 200 mg (6.4 mmol) in water 4 times daily.

Potassium Phosphate and Sodium Phosphate

PO (Adults and Children  >4 yr): 250–500 mg (8 mmol) 4 times daily.

Potassium Phosphate

IV (Adults): 10 mmol phosphorus/day as an infusion.

IV Infants: 1.5–2 mmol phosphorus/day as an infusion.

Availability (generic available)

Monobasic Potassium Phosphate

Tablets for oral solution: 500 mg (contains 114 mg or 3.7 mmol phosphorus)

Potassium Phosphate and Sodium Phosphate

Capsules for oral solution: 1.45 g (contains 1.45 g or 8 mmol phosphorus)

Potassium Phosphate

Solution for injection: 93 mg (3 mmol) phosphorus/mL

Premixed infusion: 15 mmol/250 mL 0.9% NaCl

Assessment

• Assess patient for signs and symptoms of hypokalemia (weakness, fatigue, arrhythmias, presence of U waves on ECG, polyuria, polydipsia), hypophosphatemia (anorexia, weakness, decreased reflexes, bone pain, confusion, blood dyscrasias), and hypocalcemia (tetany, muscle cramps, Chvostek's sign, Trousseau's sign, arrhythmias, QTc interval prolongation) during therapy.
• Monitor BP, HR, and ECG prior to and continuously during IV therapy.
• Monitor intake and output ratios and daily weight. Report significant discrepancies.

Lab Test Considerations:

Monitor serum phosphate, potassium, magnesium, and calcium levels prior to and periodically during therapy. ↑ phosphate levels may cause hypocalcemia. Hypomagnesemia may occur.

• Monitor renal function prior to and periodically during therapy.
• Monitor urinary pH in patients receiving potassium phosphate as a urinary acidifier.

Symptoms of toxicity are those of hyperkalemia (fatigue, muscle weakness, paresthesia, confusion, dyspnea, peaked T waves, depressed ST segments, prolonged QTc interval, widened QRS complexes, loss of P waves, cardiac arrhythmias) and hyperphosphatemia or hypocalcemia (paresthesia, muscle twitching, laryngospasm, colic, cardiac arrhythmias, Chvostek's or Trousseau's sign).

• Treatment includes discontinuation of infusion, calcium replacement, and ↓ serum potassium levels (dextrose and insulin to facilitate passage of potassium into cells, sodium polystyrene as an exchange resin, and/or dialysis in patients with renal impairment).

Implementation

• High Alert: Too rapid or bolus IV administration of potassium has resulted in fatalities. See IV administration guidelines below.
• PO Dissolve tablets in a full glass of water. Open capsules and mixed thoroughly in ⅓ cup water each. Allow mixture to stand for 2–5 min to ensure that it is fully dissolved.
  • Administer after meals to minimize gastric irritation and laxative effect.
  • Do not administer simultaneously with antacids containing aluminum, magnesium, or calcium.
  • Do not administer IM.

Patient/Family Teaching

• Explain the purpose and side effects of potassium phosphate to patient. If a dose is missed, take as soon as remembered unless within 1–2 hr of the next dose. Explain that tablets and capsules should not be swallowed whole. Tablets should be dissolved in water; capsules should be opened and contents mixed in water. Do not stop taking drug without consulting health care professional. Advise patient to read  Medication Guide  before starting and periodically during therapy in case of changes.
• Advise patient of the importance of maintaining a high fluid intake (drinking ≥8-oz of water each hr) to prevent kidney stones.
• Instruct the patient to report diarrhea, weakness, fatigue, palpitations, trouble breathing, muscle cramps, numbness, tingling, or tremors promptly.
• Rep:  Advise females of reproductive potential to notify health care professional immediately if pregnancy is planned or suspected or if breastfeeding.

Evaluation/Desired Outcomes

• Prevention and correction of serum phosphate and potassium deficiencies.
• Maintenance of acidic urine.
• Decrease in urine calcium, which prevents formation of renal calculi.