A hypertensive crisis is a medical emergency where blood pressure is very high (over 120 mg diastolic - the low reading) and is combined with the impairment of an organ system. This puts the patient at immediate risk of stroke or permanent organ damage. In such situations, the patient's blood pressure must be lowered slowly with anti-hypertensive medication. The three systems that are most at risk are the nervous system, the cardio-vascular system and the kidneys. Immediate treatment is usually with intravenous sodium nitroprusside, which is very fast acting. However, other drugs may be used if it is unavailable, although those drugs often take several minutes to take effect. After initial treatment, the blood pressure must be reduced slowly to normal over a period of several hours.
There can be several causes of a hypertensive crisis, such as sudden vasoconstriction, damage to the cells lining the blood vessels, necrosis of the arteries that feed the capilliaries, a buildup of platelets, or a breakdown of the body's ability to regulate its own blood pressure. It can also be caused by a self-sustaning loop of ischemia which causes the blood vessels to constrict, which convinces the body it needs to constrict blood vessels further.