Circulatory shock, commonly known as shock, is a life-threatening medical condition of low blood perfusion to tissues resulting in cellular injury and inadequate tissue function. The typical signs of shock are low blood pressure, rapid heart rate, and signs of poor end-organ perfusion (i.e.: low urine output, confusion, or loss of consciousness). The shock index (SI), defined as heart rate divided by systolic blood pressure, is an accurate diagnostic measure that is more useful than hypotension and tachycardia in isolation. Blood pressure may not be a reliable sign for shock, as there are times when a person is in circulatory shock but has a stable blood pressure. Circulatory shock is not related to the emotional state of shock. Circulatory shock is a life-threatening medical emergency and one of the most common causes of death for critically ill people. Shock can have a variety of effects, all with similar outcomes, but all relate to a problem with the body's circulatory system. For example, shock may lead to hypoxemia (a lack of oxygen in arterial blood) or cardiac and/or respiratory arrest. One of the key dangers of shock is that it progresses by a positive feedback mechanism. Poor blood supply leads to cellular damage, which in turn triggers tissues around the body to become inflamed and inhibit perfusion around the body. Because of this, immediate treatment of shock is critical to the survival of the sufferer.