Although many drugs and substances can affect the ECG, most do so subtly. Certain compounds, however, leave a characteristic mark. Digitalis causes classic depression and coving of the ST segment. Toxic doses primarily affect AV conduction, resulting in various degrees of heart block. Many class I antiarrhythmics cause classic QT-interval prolongation. Toxic quantities of these and other antiarrhythmics can cause intraventricular conduction abnormalities and QRS prolongation. Some substances, such as cocaine, cause shortening of both the PR and QT intervals, and their adrenergic effect results in sinus tachycardia. Toxic doses of cocaine can result in ventricular tachycardia and/or fibrillation. Conversely, beta adrenergic blockers, such as propranolol, cause sinus bradycardia as a most common ECG side effect.