Diagnosis of Hypermagnesemia Serum magnesium concentrations > 2.6 mg / dL (> 1.05 mmol / L) At serum magnesium concentrations of 6 to 12 mg / dL (2.5 to 5 mmol / L), the ECG shows prolongation of the PR interval, widening of the QRS complex, and increased T-wave amplitude. Deep tendon reflexes disappear as the serum magnesium concentration approaches 12 mg / dL (5.0 mmol / L); hypotension, respiratory depression, and narcosis develop with increasing hypermagnesemia.