Severe Protracted Hypophosphatemia in a Patient with Persistent Vegetative State on Long-Term Assisted Respiratory Support

Objective: Unusual or unexpected effect of treatment Background: Phosphorous is an essential component of cell structure and physiology, and is required for energy conservation and expenditure. Severe hypophosphatemia can lead to profound dysfunction and injury affecting most organs and can be life-threatening. It can also compromise weaning of mechanically ventilated patients. Long-term assisted ventilatory care in ambulatory or inpatient settings is an expanding medical service for patients with various forms of persistent or progressive incapacitating diseases. Hypophosphatemia, caused by respiratory alkalosis, has been reported in critical-care settings, but its occurrence in medically stable patients requiring long-term respiratory support has not been thoroughly investigated. Case Report: We report the case of a ventilated patient in a chronic vegetative state displaying progressive hypophosphatemia spanning over 3 months, with plasma levels gradually declining to 0.8 mg/dL. Evaluation did not reveal conditions leading to diminished phosphate absorption or enhanced urinary phosphate excretion, but it identified respiratory alkalosis related to a recent increase in target minute-volume ventilation in the adaptive support ventilation (ASV) mode as the cause of hypophosphatemia. Despite the very low plasma phosphate level, the patient was asymptomatic, probably because this type of hypophosphatemia may not represent physiologically significant intracellular phosphate depletion. The respiratory alkalosis resolved upon decreasing the target minute-volume ventilation settings, and serum phosphate was normalized. Conclusions: Since blood gases are not routinely monitored in respiratory and hemodynamically stable patients on long-term respiratory support, hypophosphatemia may herald the development of significant respiratory alkalosis. Assessment of acid-base balance is thus warranted in patients receiving long-term ventilation, especially in those developing hypophosphatemia.