Disclosed herein are pharmaceutical compositions and methods for inhibiting oxidative stress in a subject having atrial or ventricular arrhythmias, ventricular failure or heart failure. The methods include administering an effective amount of a NOX2 inhibitor agent to the subject, wherein said administering is under conditions such that a level of oxidative stress in myocardial tissue is reduced or eliminated. The pharmaceutical compositions include a NOX2 inhibitor agent.