There is a hypertensive emergency when severe hypertension is associated with acute end-organ damage. Examples of such cases are hypertensive encephalopathy, acute pulmonary edema and aortic dissection. In many cases, administration of an intravenous sodium nitroprusside injection has an immediate effect. In lesser urgent cases, oral parenteral drugs such as captopril, clonidine, labetalol and prazosin may be given. When compared with sodium nitroprusside, these drugs have a delayed onset of action by several minutes. Nitrates such as nitro vasodilators (nitroprusside or nitroglycerin) provide nitric oxide to induce nitro vasodilation of both the veins and the arteries to activate calcium-sensitive potassium channels in the cell membrane.