Beta blockers reduce myocardial oxygen demand and electrical excitability, and counter arrhyth- mogenic effects of sympathetic stimulation. Angiotensin-converting enzyme inhibitors and angiotensin-receptor blockers decrease preload and afterload, decreasing myocardial oxygen demand, blocking the formation of angioten- sin II, and slowing the progression of ventricular remodeling and fibrosis. Mineralocorticoid recep- tor antagonists limit potassium loss, decrease fibrosis, and increase the myocardial uptake of RCTs in patients with HFrEF have consistently demonstrated that chronic therapy with beta blockers reduces all-cause mortality, VA, and Three beta blockers (ie, biso- prolol, carvedilol, sustained-release metoprolol suc- cinate) have been proven to reduce mortality in patients with current or prior symptoms of HFrEF without beta-blocker contraindications. Angiotensin- converting enzyme inhibition also reduces mortality Angiotensin-receptor blockers added to angiotensin-converting enzyme inhibitor showed additional benefit to angiotensin-converting enzyme inhibitors in someS5.2-10 but not other RCTs.S5.2-8,S5.2-11 Therapy with the mineralocorticoid-receptor antago- nists, spironolactone and eplerenone, have also dem- onstrated reductions in both all-cause mortality and Recent studies of the angiotensin receptor-neprilysin inhibitor (sacubitril/valsartan) versus angiotensin-converting enzyme inhibitor demonstrated a reduction in SCD and cardiac mortality.S5.2-14 5.3. Defibrillators for Treatment of VA and SCD See Sections 7, 10.2, 10.3, 10.8, and 10.9. Defibrillation is highly effective in terminating life- threatening VA. This therapy can be delivered by a transvenous ICD, a subcutaneous implantable cardio- verter-defibrillator, a wearable cardioverter-defibrillator or an external defibrillator. These devices monitor the heart rhythm continuously and deliver therapy in re- sponse to a tachycardia that meets preprogrammed detection rates and arrhythmia duration. The vast ma- jority of transvenous ICDs are implanted in the subcla- vicular area under fluoroscopy guidance. subcutaneous implantable cardioverter-defibrillators are implanted in the left side of the chest over the sixth rib between the left midaxillary and left anterior axillary lines. 