Phosphate - Hypophosphataemia

Background

Hypophosphataemia is defined as a serum phosphate of lower than 0.8mmol/L (normal range 0.8 to 1.5mmol/L). It occurs chronically due to increased losses and there can be an acute form due to refeeding or recovery which is potentially life threatening.

Chronic hypophosphatemia usually develops because too much phosphate is excreted. Causes include the following:

• Hyperparathyroidism
• Chronic diarrhea
• Alcoholism
• Use of diuretics for a long time
• Use of large amounts of aluminum - containing antacids for a long time
• Use of large amounts of theophylline (not used routinely in Australasia now)

The phosphate level in blood can suddenly fall dangerously low in people recovering from the following conditions because the body uses large amounts of phosphate during recovery, refeeding syndrome:

• Severe undernutrition (including anorexia/starvation)
• Diabetic ketoacidosis
• Respiratory alkalosis
• Severe alcoholism
• Severe burns

This sudden drop in phosphate level may result in an irregular heart rhythm and even death.

Assessment

History and examination

Symptoms and signs:

• Muscle dysfunction and weakness. This occurs in major muscles, but also may manifest as diplopia, low cardiac output, dysphagia, and respiratory depression due to respiratory muscle weakness.
• Mental status changes. This may range from irritability to gross confusion, delirium, and coma.
• White cell dysfunction, causing worsening of infections.
• Instability of cell membranes due to low ATP levels: this may cause rhabdomyolysis with increased CPK, and also hemolytic anemia.
• Increased affinity for oxygen in the blood caused by decreased production of 2,3BPG.
• Large pulp chambers in their teeth.

Management

Treatment

Standard intravenous preparations of potassium phosphate are available and are routinely used in malnourished patients and alcoholics when the deficit is severe or predicted to be so. Supplementation is used rarely though and should be with consultation with clinicians experienced in its use. There are significant potential risks.

Parenteral phosphate administration is more likely to have complications such as hypocalcemia, tetany, and hypotension. Other complications are metastatic calcification, hyperkalemia associated with potassium-containing supplements, volume excess, hypernatremia, metabolic acidosis and hyperphosphatemia.

The management of patients with hypophosphatemia can be divided into various subgroups based on the severity of the hypophosphatemia and the need for ventilation, as follows:

• Moderate Hypophosphataemia (0.5-0.8 mmol/L) usually resolves when the cause is treated or stopped, so replacement is rarely needed.

• Severe Hypophosphataemia (<0.5mmol/L)
  • Acute with normal kidney function:
    • potassium dihydrogen phosphate 13.6% 2-10mmol elemental phosphate/hr iv for 4hrs.
    • Measure Ca and Phos concentration hourly. Monitor cardiac and kidney function during the infusion. Replacement often usually required until the cause has been treated.
    • Commencing treatment should be discussed with ongoing medical treatment team.

• Milk has high phosphate content and regular consumption can reduced the dose requirement for replacement

Reference

ETG