Effective plasma levels vary depending on the medical indication. For congestive heart failure, levels between 0.5 and 1.0 ng/ml are recommended. This recommendation is based on "post hoc" analysis of prospective trials, suggesting higher levels may be associated with increased mortality rates. For heart rate control (atrial fibrillation), plasma levels are less defined and are generally titrated to a goal heart rate. Typically, digoxin levels are considered therapeutic for heart rate control between 0.5 and 2.0 ng/ml (or 0.6 and 2.6 nmol/l). In suspected toxicity or ineffectiveness, digoxin levels should be monitored. Plasma potassium levels also need to be closely controlled (see side effects, below).