Shock is the state of insufficient blood flow to the tissues of the body as a result of problems with the circulatory system.[1][2] Initial symptoms of shock may include weakness, fast heart rate, fast breathing, sweating, anxiety, and increased thirst.[1] This may be followed by confusion, unconsciousness, or cardiac arrest, as complications worsen.[1]
Shock is divided into four main types based on the underlying cause: hypovolemic, cardiogenic, obstructive, and distributive shock.[2] Hypovolemic shock, also known as low volume shock, may be from bleeding, diarrhea, or vomiting.[1] Cardiogenic shock may be due to a heart attack or cardiac contusion.[1] Obstructive shock may be due to cardiac tamponade or a tension pneumothorax.[1] Distributive shock may be due to sepsis, anaphylaxis, injury to the upper spinal cord, or certain overdoses.[1][4]
The diagnosis is generally based on a combination of symptoms, physical examination, and laboratory tests.[2] A decreased pulse pressure (systolic blood pressure minus diastolic blood pressure) or a fast heart rate raises concerns.[1] The heart rate divided by systolic blood pressure, known as the shock index (SI), of greater than 0.8 supports the diagnosis more than low blood pressure or a fast heart rate in isolation.[5][6]
Treatment of shock is based on the likely underlying cause.[2] An open airway and sufficient breathing should be established.[2] Any ongoing bleeding should be stopped, which may require surgery or embolization.[2] Intravenous fluid, such as Ringer's lactate or packed red blood cells, is often given.[2] Efforts to maintain a normal body temperature are also important.[2] Vasopressors may be useful in certain cases.[2] Shock is both common and has a high risk of death.[3] In the United States about 1.2 million people present to the emergency room each year with shock and their risk of death is between 20 and 50%.[3]
The presentation of shock is variable, with some people having only minimal symptoms such as confusion and weakness.[7] While the general signs for all types of shock are low blood pressure, decreased urine output, and confusion, these may not always be present.[7] While a fast heart rate is common, in those on β-blockers, those who are athletic, and in 30% of cases of those with shock due to intra abdominal bleeding, heart rate may be normal or slow.[8] Specific subtypes of shock may have additional symptoms.
Dry mucous membrane, reduced skin turgor, prolonged capillary refill time, weak peripheral pulses, and cold extremities can be early signs of shock.[9]
Hypovolemic shock is the most common type of shock and is caused by insufficient circulating volume.[7] The most common cause of hypovolemic shock is hemorrhage (internal or external); however, vomiting and diarrhea are more common causes in children.[10] Other causes include burns, as well as excess urine loss due to diabetic ketoacidosis and diabetes insipidus.[10]
Signs and symptoms of hypovolemic shock include:
The severity of hemorrhagic shock can be graded on a 1–4 scale on the physical signs. The shock index (heart rate divided by systolic blood pressure) is a stronger predictor of the impact of blood loss than heart rate and blood pressure alone.[5] This relationship has not been well established in pregnancy-related bleeding.[12]
Cardiogenic shock is caused by the failure of the heart to pump effectively.[7] This can be due to damage to the heart muscle, most often from a large myocardial infarction. Other causes of cardiogenic shock include dysrhythmias, cardiomyopathy/myocarditis, congestive heart failure (CHF), myocardial contusion, or valvular heart disease problems.[10]
Symptoms of cardiogenic shock include:
Obstructive shock is a form of shock associated with physical obstruction of the great vessels of the systemic or pulmonary circulation.[13] Several conditions can result in this form of shock.
Many of the signs of obstructive shock are similar to cardiogenic shock, however treatments differ. Symptoms of obstructive shock include:
Distributive shock is low blood pressure due to a dilation of blood vessels within the body.[7][17] This can be caused by systemic infection (septic shock), a severe allergic reaction (anaphylaxis), or spinal cord injury (neurogenic shock).
Although not officially classified as a subcategory of shock, many endocrinological disturbances in their severe form can result in shock.[citation needed]
Shock is a common end point of many medical conditions.[10] Shock triggered by a serious allergic reaction is known as anaphylactic shock, shock triggered by severe dehydration or blood loss is known as hypovolemic shock, shock caused by sepsis is known as septic shock, etc. Shock itself is a life-threatening condition as a result of compromised body circulation.[22] It can be divided into four main types based on the underlying cause: hypovolemic, distributive, cardiogenic, and obstructive.[23] A few additional classifications are occasionally used, such as endocrinologic shock.[10]
There are four stages of shock. Shock is a complex and continuous condition, and there is no sudden transition from one stage to the next.[24] At a cellular level, shock is the process of oxygen demand becoming greater than oxygen supply.[7]
One of the key dangers of shock is that it progresses by a positive feedback loop. Poor blood supply leads to cellular damage, which results in an inflammatory response to increase blood flow to the affected area. Normally, this causes the blood supply level to match with tissue demand for nutrients. However, if there is enough increased demand in some areas, it can deprive other areas of sufficient supply, which then start demanding more. This then leads to an ever escalating cascade.
As such, shock is a runaway condition of homeostatic failure, where the usual corrective mechanisms relating to oxygenation of the body no longer function in a stable way. When it occurs, immediate treatment is critical in order to return an individual's metabolism into a stable, self-correcting trajectory. Otherwise the condition can become increasingly difficult to correct, surprisingly quickly, and then progress to a fatal outcome. In the particular case of anaphylactic shock, progression to death might take just a few minutes.[6]
During the Initial stage (Stage 1), the state of hypoperfusion causes hypoxia. Due to the lack of oxygen, the cells perform lactic acid fermentation. Since oxygen, the terminal electron acceptor in the electron transport chain, is not abundant, this slows down entry of pyruvate into the Krebs cycle, resulting in its accumulation. The accumulating pyruvate is converted to lactate (lactic acid) by lactate dehydrogenase. The accumulating lactate causes lactic acidosis.
The Compensatory stage (Stage 2) is characterised by the body employing physiological mechanisms, including neural, hormonal, and bio-chemical mechanisms, in an attempt to reverse the condition. As a result of the acidosis, the person will begin to hyperventilate in order to rid the body of carbon dioxide (CO2) since it indirectly acts to acidify the blood; the body attempts to return to acid–base homeostasis by removing that acidifying agent. The baroreceptors in the arteries detect the hypotension resulting from large amounts of blood being redirected to distant tissues, and cause the release of epinephrine and norepinephrine. Norepinephrine causes predominately vasoconstriction with a mild increase in heart rate, whereas epinephrine predominately causes an increase in heart rate with a small effect on the vascular tone; the combined effect results in an increase in blood pressure. The renin–angiotensin axis is activated, and arginine vasopressin (anti-diuretic hormone) is released to conserve fluid by reducing its excretion via the renal system. These hormones cause the vasoconstriction of the kidneys, gastrointestinal tract, and other organs to divert blood to the heart, lungs and brain. The lack of blood