This patient's heart rhythm is consistent with Torsades de Points (TdP), which can be precipitated by hypomagnesemia.
TdP is polymorphic ventricular tachycardia with increasing and decreasing amplitude. Conditions that predispose patients to TdP include hypomagnesemia, hypokalemia, hypocalcemia, long QT interval. First-line treatment includes a magnesium (magnesium sulfate) bolus and sometimes exogenously increasing the heart rate (i.e isoproterenol or pacing). Defibrillation is indicated in patients who are hemodynamically unstable. While some episodes of TdP can be deadly, most cases resolve spontaneously.
Roden notes that prolongation of the QT length is one of the most common reasons for withdrawing medications from public availability. He notes that the risk of TdP is not proportional to the length of the QT, but the risk overall is increased with a QT > 500 ms. The risk for TdP with a long QT is also increased in the setting of bradycardia.
Priori et al. conducted a prospective trial to determine the risk of TdP in patients with congenital long QT syndrome who are treated with beta blockers. They find that beta blocker treatment increases the risk of TdP in this population, likely because of the increased incidence of bradycardia.
Figure A depicts a cardiac tracing of TdP.
Answers 1, 2: These electrolytes are not known to influence the risk of TdP.
Answer 3: Hypokalemia is risk factor for TdP.
Answer 4: Hypocalcemia is a risk factor for TdP.
Roden DM. Drug-induced prolongation of the QT interval. N Engl J Med. March 4, 2004;350:1013-22.
PMID:14999113 (Link to Abstract)
Priori SG, Napolitano C, Schwartz PJ, Grillo M, Bloise R, Ronchetti E, Moncalvo C, Tulipani C, Veia A, Bottelli G, Nastoli J. Association of long QT syndrome loci and cardiac events among patients treated with beta-blockers. JAMA. 2004 Sep 15;292(11):1341-4.
PMID:15367556 (Link to Abstract)