What is the most likely cause of numbness around the mouth, carpopedal spasms, and a prolonged QT interval in a postoperative patient who received multiple units of blood, considering electrolyte imbalances such as hypocalcemia (low calcium levels), hypomagnesemia (low magnesium levels), hypokalemia (low potassium levels), or hyponatremia (low sodium levels)?

Hypocalcemia is the Most Likely Cause of the Patient's Symptoms

The reduced level of calcium is most likely responsible for the patient's symptoms of perioral numbness, carpopedal spasms, and prolonged QT interval following massive blood transfusion during abdominal surgery.

Pathophysiology and Clinical Presentation

Massive blood transfusion (6 units in this case) is a well-established cause of hypocalcemia due to:

• Citrate in blood products chelating calcium 1
• Rapid transfusion overwhelming the liver's ability to metabolize citrate 1
• Resulting in acute reduction in ionized calcium levels

The classic clinical manifestations observed in this patient are textbook presentations of hypocalcemia:

1. Neuromuscular manifestations:

   • Perioral numbness (classic symptom of hypocalcemia)
   • Carpopedal spasms (pathognomonic for hypocalcemia)

2. Cardiac manifestations:

   • Prolonged QT interval on ECG (characteristic finding in hypocalcemia) 2, 1

Differential Diagnosis

While considering other electrolyte disturbances:

• Hypomagnesemia (Option A): Can cause QT prolongation and is often associated with hypocalcemia, but doesn't typically cause carpopedal spasms as a primary manifestation 3
• Hypokalemia (Option B): Causes U wave prominence rather than QT prolongation, and doesn't typically cause perioral numbness or carpopedal spasms 2
• Hyponatremia (Option D): Typically causes neurological symptoms like confusion and seizures, not perioral numbness or carpopedal spasms 2

Evidence Supporting Hypocalcemia

The European Society of Cardiology guidelines specifically state: "Hypocalcaemia (less than 7·5 mg·dl⁻¹) usually produces a distinctive lengthening of the ST segment" 2. This directly correlates with the ECG finding of prolonged QT interval in this patient.

The Critical Care Society guidelines emphasize that trauma patients and those receiving massive transfusion commonly experience hypocalcemia, which requires close monitoring and replacement 1.

Management Approach

1. Immediate treatment:

   • Administer IV calcium (calcium chloride preferred in emergency situations) 1, 4
   • Calcium chloride contains more elemental calcium than calcium gluconate and is preferred in emergency situations 1
   • Administer slowly with ECG monitoring, not exceeding 200 mg/minute 4

2. Monitoring:

   • Check ionized calcium levels
   • Monitor ECG for normalization of QT interval
   • Check magnesium and potassium levels (as deficiencies often coexist) 5

3. Prevention in massive transfusion:

   • Prophylactic calcium administration may be considered in patients receiving multiple units of blood products 1

Important Considerations

• Hypocalcemia following massive transfusion is often transient but can be severe and life-threatening if not promptly recognized and treated 6
• Concurrent hypomagnesemia should be ruled out, as it can make hypocalcemia refractory to treatment 5
• Rapid correction of hypocalcemia is necessary to prevent progression to more severe manifestations like seizures or cardiac arrhythmias 7

In conclusion, the constellation of perioral numbness, carpopedal spasms, and prolonged QT interval following massive blood transfusion is most consistent with hypocalcemia, making calcium (Option C) the correct answer.