Hypophosphatemia: Muscle weakness

Phosphate is very important as a source of chemical energy as it forms ATP in all cells. When phosphate is low, muscles do not function well, in particular the respiratory muscles (including the diaphragm). Low serum phosphate does not necessary mean low intracellular phosphate; therefore, it is always important to clinically assess the patient for physical manifestations of hypophosphatemia. Also of note, hypophosphatemia causes a leftward shift of the oxyhemoglobin curve (increased affinity for oxygen: less oxygen delivery to tissues) and also increases RBC fragility. For both of these reasons, patients who are hypophosphatemic may be slow to wean from mechanical ventilation. Another concern in the ICU setting is that hypophosphatemia can mimic critical care myopathy. Muscle weakness secondary to hypophosphatemia is reversible and if the patient’s phosphate stores are repleted, normal muscle function will return. Other manifestations of hypophosphatemia include:

• MSK: rhabdomyolysis, osteopenia, osteomalacia
• CV: cardiomyopathy, arrhythmias
• Pulm: respiratory failure, failure to wean
• Neuro: delirium, seizures, encephalopathy, hallucinations
• Heme/Metabolic: impaired O₂ delivery, hemolysis, leukocyte dysfunction, metabolic acidosis, glucose intolerance