potassium acid phosphate

(poe-tass-ee-um as—id fos-fate) potassiumacidphosphate,

K-Phos Original

(trade name)

Classification

Therapeutic: antiurolithics
Pregnancy Category: C

Indications

Adjunct therapy of urinary tract infections with methenamine hippurate or mandelate.Prevention of calcium urinary stones.

Action

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 effects

Urinary acidification.Increased efficacy of methenamine.Decreased formation of calcium urinary tract stones.

Pharmacokinetics

Absorption: Well absorbed following oral administration.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

ROUTE	ONSET	PEAK	DURATION
PO	unknown	unknown	unknown

Contraindications/Precautions

Contraindicated in: Hyperkalemia; Hyperphosphatemia; Hypocalcemia; Hypomagnesemia; Severe renal impairment.Use Cautiously in: Metabolic alkalosis; Cardiac disease; Renal impairment; Children (safety and effectiveness not established).

Adverse Reactions/Side Effects

Related to hyperphosphatemia, unless otherwise indicated

Central nervous system

confusion
listlessness
weakness

Cardiovascular

arrhythmias (life-threatening)
cardiac arrest (life-threatening)
ECG changes (absent P waves, widening of the QRS complex with biphasic curve)
hypotension

hyperkalemia:

arrhythmias (life-threatening)
bradycardia
ECG changes (prolonged PR interval, ST segment depression, peaked T waves)

Gastrointestinal

abdominal pain (most frequent)
diarrhea (most frequent)
flatulence (most frequent)
nausea (most frequent)
vomiting (most frequent)

Fluid and Electrolyte

hyperkalemia
hyperphosphatemia
hypocalcemia
hypomagnesemia

Musculoskeletal

muscle cramps
tremors

Neurologic

flaccid paralysis
heaviness of legs
paresthesias

Interactions

Drug-Drug interaction

Concurrent use of potassium-sparingdiuretics,ACE inhibitors, angiotensin receptor blockers, or aldosterone antagonists may result in hyperkalemia.Concurrent administration of calcium- or aluminum- containing compounds or sucralfate decreases absorption of phosphates by formation of insoluble complexes.Oxalates (in berries, nuts, chocolate, beans, celery, and tomatoes) and phytates (in bran and whole grains) may decrease the absorption of phosphates by binding them in the GI tract.

Route/Dosage

Oral (Adults) 1 g (7.4 mmol) in 6–8 oz of water 4 times daily with meals and at bedtime.

Availability

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

Nursing implications

Nursing assessment

Monitor intake and output ratios and daily weight. Report significant discrepancies.
Lab Test Considerations: Monitor serum phosphate, potassium, and calcium levels prior to and periodically throughout therapy. Increased phosphate may cause hypocalcemia.
- Monitor renal function prior to and periodically throughout course of therapy.
- Monitor urinary pH when using as a urinary acidifier.

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

Treatment includes discontinuation of infusion, calcium replacement, and lowering serum potassium (dextrose and insulin, sodium bicarbonate, or albuterol to facilitate passage of potassium into cells, sodium polystyrene sulfonate as an exchange resin, and/or dialysis in patients with impaired renal function).

Potential Nursing Diagnoses

Imbalanced nutrition: less than body requirements (Indications)

Implementation

Oral: Tablets should be dissolved in 6–8 oz of water. Allow mixture to stand for 2–5 min to ensure that it is fully dissolved.
- Medication should be administered with meals to minimize gastric irritation and risk of diarrhea.
- Do not administer simultaneously with antacids containing aluminum, magnesium, or calcium.

Patient/Family Teaching

Explain to patient purpose of the medication and the need to take as directed. If a dose is missed, it should be taken as soon as remembered unless within 1–2 hr of the next dose. Explain that the tablets should not be swallowed whole. Tablets should be dissolved in water.
Advise patient of the importance of maintaining a high fluid intake (drinking at least one 8-oz glass of water each hr) to prevent kidney stones.
Instruct the patient to report diarrhea, weakness, fatigue, muscle cramps, or tremors promptly.

Evaluation/Desired Outcomes

Maintenance of acid urine.
Decrease in urine calcium, which prevents formation of renal calculi.