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Prevention of adverse drug events caused by potassium phosphate injection

From the March 13, 1996 issue

PROBLEM: A 71 year-old patient presented to the emergency department with atrial fibrillation. A phosphate level measured 0.9 mg/dL (normal serum inorganic phosphate in adults is 3 to 4.5 mg/dL). A nonspecific dose of potassium phosphate was prescribed. It was ordered as "one amp of potassium phosphate". The hospital pharmacy did not provide 24 hour service, so a nursing supervisor retrieved a 15 mL "single dose" vial of potassium phosphate from the pharmacy. Since the vial and pharmacy storage bin both cautioned that dilution was necessary, the nurse diluted the drug in 15 mL normal saline and eventually administered intravenously at a rate of 1 mL/min. The patient suffered a cardiac arrest within 4 minutes. Resuscitation was successful.

This patient received 8.8 mEq of potassium and 6 mmol of phosphorus in 4 minutes! Had the entire syringe been administered he would have received a total of 45 mmol of phosphorus and 66 mEq of potassium!!

SAFE PRACTICE RECOMMENDATION: Concentrated potassium phosphate injection is a combination of monobasic and dibasic salt, the ratio of which is dependent on pH. To avoid confusion it is generally prescribed in terms of millimoles of phosphorus rather than amount of potassium phosphate. Unfortunately, this may lead one to overlook the accompanying amount of potassium. Each mL contains 3 mmol of phosphorus and 4.4 mEq of potassium. It is very important that the rate of injection and total amount of phosphate be guided by this. The patient's ECG and electrolytes must also be monitored. Sodium phosphate injection is also available. Similarly, when this form of phosphate is used for replenishment, the amount of sodium must also be considered.

When using phosphate injection for electrolyte replenishment, each institution should have specific guidelines established based on the level of inorganic phosphate and other clinical factors such as age, renal function, reason for use, etc. Phosphorus doses of 0.08 mmol/kg to 0.32 mmol/kg have been recommended for patients with normal renal function. This amount is infused over 12 hours and repeated every 12 hours until serum phosphate is greater than 2 mg/dL (Vanetta JB, Andress DL, Whang R et al. High dose intravenous phosphorus therapy for severe complicated hypophosphatemia. Southern Med J 1983;76:1424-26).

It is also important to dilute phosphate appropriately, and administer it slowly. This recommendation stems from several reports of severe complications related to the rate of administration. Shackney et al (Shackney S, Hasson J. Precipitous fall in serum calcium, hypotension, and acute renal failure after intravenous phosphate therapy. Ann Intern Med 1967; 66: 906-16) described the development of oliguria, hypocalcemia, hypotension, and death in two patients, one of whom received 100 mmol in 3 hours and the other 50 mmol in 1 hour. The first patient received 0.56 mmol/min; the second received 0.83 mmol/min. The patient above received 1.5 mmol/min. In general, the minimum amount of time that phosphate injection should be administered is over six hours (Stoff JS. Phosphate homeostasis and hypophosphatemia. Am J Med 1982;72:489-95).

There were other factors that gave rise to this serious adverse drug event. Ordering the dose as "one amp" is dangerous since it is available in 5, 10, 15, 30 and 50 mL sizes. The drug should only be ordered specifically in millimoles of phosphorus. Also, depending on the size and scope of service of a hospital it should be considered a standard of practice that either the hospital have 24 hour/day availability of pharmacy service or that a carefully determined supply of needed medications be available in a suitable after hours storage location. Finally, the designation "single dose vial" is considered ambiguous by some practitioners. USP currently has under consideration a proposal to use the term "single use vial" as an alternative for these and other vials designated "single dose vial". [D,N,P,Q]

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