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, signs of poor end-organ perfusion (i.e.: low urine output, confusion, or loss of consciousness), and weak pulses. 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. Under normal conditions, a number between 0.5 and 0.8 is typically seen. Should that number increase, so does suspicion of an underlying state of shock. Blood pressure alone 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 results in an inflammatory response to increase blood flow to the affected area. This is normally very useful to match up supply with tissue demand for nutrients. However, if enough tissue causes this, it will deprive vital nutrients from other parts of the body. Additionally, the ability of the circulatory system to meet this increase in demand causes saturation. A major result of which is that other parts of the body begin to respond in a similar way, thus exacerbating the problem. Due to these chain of events, immediate treatment of shock is critical to survival.

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