A 23-year-old man presents to the emergency department after he was rescued while hiking in Alaska. The patient was found on a mountain trail non-responsive. His temperature is 91°F (32.8°C), blood pressure is 103/61 mmHg, pulse is 60/min, respirations are 11/min, and oxygen saturation is 97% on room air. The patient is placed under a warm air circulator, and his extremities are bathed in warm water. An ECG is performed as seen in Figure A. Dark bullae are noted on the patient's extremities. His mental status rapidly improves with warming. Laboratory studies are ordered and are pending. Which of the following is most likely to be found in this patient?
Hypocalcemia
Hypophosphatemia
Hypomagnesemia
Red blood cells on urine microscopy
Hypocalcemia
This patient is presenting after being found outside with hypothermia and frostbite (dark bullae noted on the extremities in the setting of being out in the cold). Frostbite can lead to rhabdomyolysis, which may cause hyperkalemia (causing ECG changes including peaked T waves and QRS widening as seen in this patient) and hypocalcemia (the most common electrolyte finding in frostbite).

Frostbite occurs when there is an extensive freezing injury to tissues and can cause benign swelling, erythema, and pain, all the way to insensate extremities with necrosis and permanent dysfunction. More severe forms of frostbite can cause rhabdomyolysis or breakdown of the muscles. Breakdown of the muscle cells leads to leakage of the intracellular contents. Common electrolyte derangements include hyperkalemia and hypophosphatemia. Hypocalcemia is the most common electrolyte derangement in rhabdomyolysis. It occurs due to the deposition of calcium in the necrotic muscle cells and the sequestration of calcium in the exposed sarcoplasmic reticulum. Note that hypocalcemia requires no treatment unless it is causing symptoms or ECG changes (such as QT prolongation). The management of rhabdomyolysis is centered on fluid administration to protect the kidney from myoglobin which is released from damaged muscles and is nephrotoxic.

Zimmerman and Shen review rhabdomyolysis. They note that rhabdomyolysis can be commonly associated with myoglobinuria, hyperkalemia, hypocalcemia, and hyperphosphatemia. They suggest that an elevated creatine kinase level is indicated to support the diagnosis.

Figure/Illustration A is an ECG demonstrating peaked T waves (blue arrows) which are associated with hyperkalemia.

Incorrect Answers:
Answer A: Elevated troponin may be seen in rhabdomyolysis. This is a non-cardiac cause of an elevated troponin; however, this is less common than hypocalcemia in rhabdomyolysis and is neither sensitive nor specific.

Answer C: Hypomagnesemia is not a common electrolyte disturbance seen in rhabdomyolysis; however, hypomagnesemia may cause hypocalcemia (from impaired PTH release) and hypokalemia (from renal wasting of potassium). Note that this mechanism is separate from rhabdomyolysis.

Answer D: Hypophosphatemia is common in refeeding syndrome and malnourished patients (such as those with alcohol use disorder) and presents with diffuse muscle weakness. Rather, hyperphosphatemia would be seen in rhabdomyolysis as it is released from damaged muscle cells.

Answer E: Red blood cells on urine microscopy would be seen in a nephritic syndrome in addition to red blood cell casts. Note that the urine dipstick will be positive for red blood cells in rhabdomyolysis due to cross-reactivity with myoglobin; however, there are minimal/no red blood cells actually present.

Bullet Summary:
Frostbite can result in rhabdomyolysis which commonly causes hyperkalemia, hyperphosphatemia, and hypocalcemia (the most common derangement).