Angiotensin-converting-enzyme inhibitors (ACE inhibitors) are a class of medication used primarily for the treatment of high blood pressure and heart failure. They work by causing relaxation of blood vessels as well as a decrease in blood volume, which leads to lower blood pressure and decreased oxygen demand from the heart. ACE inhibitors inhibit the activity of angiotensin-converting enzyme, an important component of the renin–angiotensin system which converts angiotensin I to angiotensin II, and hydrolyses bradykinin. Therefore, ACE inhibitors decrease the formation of angiotensin II, a vasoconstrictor, and increase the level of bradykinin, a peptide vasodilator. This combination is synergistic in lowering blood pressure. As a result of inhibiting the ACE enzyme in the bradykinin system, the ACE inhibitor drugs allow for increased levels of bradykinin which would normally be degraded. Bradykinin produces prostaglandin. This mechanism can explain the two most common side effects seen with ACE Inhibitors: angioedema and cough. Frequently prescribed ACE inhibitors include benazepril, zofenopril, perindopril, trandolapril, captopril, enalapril, lisinopril, and ramipril. Medical use ACE inhibitors were initially approved for the treatment of hypertension and can be used alone or in combination with other anti-hypertensive medications. Later, they were found useful for other cardiovascular and kidney diseases including: Acute myocardial infarction (heart attack) Heart failure (left ventricular systolic dysfunction) Kidney complications of diabetes mellitus (diabetic nephropathy) by means of decreasing the blood pressure and increasing perfusion in glomerular arterioles. In treating high blood pressure, ACE inhibitors are often the first drug choice, particularly when diabetes is present, but age can lead to different choices and it is common to need more than one drug to obtain the desired improvement. There are fixed-dose combination drugs, such as ACE inhibitor and thiazide combinations. ACE inhibitors have also been used in chronic kidney failure and kidney involvement in systemic sclerosis (hardening of tissues, as scleroderma renal crisis). In those with stable coronary artery disease, but no heart failure, benefits are similar to other usual treatments. In 2012, there was a meta-analysis published in the BMJ that described the protective role of ACE inhibitors in reducing the risk of pneumonia when compared to ARBs (Angiotensin II Receptor Blockers). The authors found a decreased risk in patients with previous stroke (54% risk reduction), with heart failure (37% risk reduction), and of Asian descent (43% risk reduction vs 54% risk reduction in non-Asian population). However, no reduced pneumonia related mortality was observed. Other ACE