Acute stressor states are linked to neurohormonal activation that includes the adrenergic nervous system. Elevations in circulating epinephrine and norepinephrine unmask an interdependency that exists between K+ and Mg2+ based on their regulation of a large number of Mg2+-dependent Na+-K+-ATPase pumps present in skeletal muscle. The hyperadrenergic state accounts for a sudden translocation of cations into muscle with the rapid appearance of hypokalemia and hypomagnesemia. The resultant hypokalemia and hypomagnesemia will cause a delay in myocardial repolarization and electrocardiographic QTc prolongation raising the propensity for supraventricular and ventricular arrhythmias. In this review, we focus on the interdependency between K+ and Mg2+, which is clinically relevant to acute hyperadrenergic stressor states found in patients admitted to intensive care units.