Hypophosphatemia

The diaphragm is the organ that matters. Without phosphate there is no ATP, and the respiratory muscles - the diaphragm first - stop working. Layer on a leftward shift of the oxyhemoglobin curve and increased RBC fragility and the patient is delivering less oxygen while working harder to breathe. This is the patient who fails to wean from the ventilator.

The weakness is reversible, so replete before you extubate. Muscle function returns fully once phosphate stores are restored. Check a phosphate on any patient who is unexpectedly weak or failing a spontaneous breathing trial - in the ICU it mimics critical care myopathy and gets misattributed. Serum phosphate can be normal while intracellular stores are low, so read the patient too: grip strength, sustained head lift, tidal volume, negative inspiratory force.

Refeeding syndrome is a perioperative event, not just an ICU one. Starting nutrition in a malnourished patient drives glucose and insulin, which pull phosphate into cells to build ATP, and phosphate falls off a cliff - usually alongside hypomagnesemia and hypokalemia. It produces impaired myocardial contractility and cardiovascular collapse, respiratory failure, rhabdomyolysis, seizures, delirium, and death. Replete phosphate, potassium, and magnesium before TPN is started, and advance parenteral regimens slowly over the first few days for exactly this reason.

Below 1 mg/dL, delay the elective case. Phosphate under 1.0 mg/dL is associated with hemolytic anemia, heart failure, tachypnea, seizures, and death. Levels under 2.5 mg/dL are reported in 17% to 28% of critically ill patients, so this is a common finding, not an exotic one.

Do not hyperventilate. Extreme hyperventilation to a PCO2 below 20 mmHg can drop phosphate below 0.32 mmol/L, and acute respiratory alkalosis is described as the most common cause of marked hypophosphatemia in hospitalized patients. Set the ventilator to normocapnia rather than a reflexive high minute ventilation.

Cardiac and neurologic effects. Hypophosphatemia causes cardiomyopathy and arrhythmias - get a 12-lead when the level is severe. Neurologically it produces delirium, seizures, encephalopathy, and hallucinations, worth remembering before attributing a confused PACU patient to residual anesthetic.

What travels with it. Rhabdomyolysis, hemolysis, leukocyte dysfunction, metabolic acidosis, and glucose intolerance all track with low phosphate. Hungry bone syndrome after correction of hyperparathyroidism drops it as osteopenic bone reclaims calcium and phosphate.

Phosphate is the backbone of ATP, so a low level is an energy-supply failure in every cell that needs one. Adult hypophosphatemia is a serum phosphate under 2.5 mg/dL against a normal 3 to 4.5 mg/dL; children run higher, around 4 to 7 mg/dL.

Three mechanisms produce it. Poor intake or absorption - chronic diarrhea, and aluminum or magnesium antacids that bind phosphate into non-absorbable salts. Renal wasting - hyperparathyroidism, Fanconi syndrome, glucosuria, alcohol, acetazolamide and thiazides, and dialysis. And, most commonly in the hospital, a shift of phosphate into cells: refeeding, TPN, insulin and glucose loads, and acute respiratory alkalosis, where a rising pH stimulates phosphofructokinase and glycolysis consumes intracellular phosphate. It is present in about 5% of patients overall but up to 80% in alcoholism, DKA, and sepsis.